Total Carbon, Organic Carbon, and Organic Matter

Chapter 34

D. W. NELSON, University of Nebraska, Lincoln, Nebraska

L. E. SOMMERS, Colorado State University, Fort Collins, Colorado

GENERAL INFORMATION

This chapter is an updated, revised version of the material contained in Chapter 29, in Volume 2, of Methods of Soil Analysis, 2nd edition (Nelson & Sommers, 1982). Much of the material presented in the original chapter has been modified or replaced by more modem procedures and recent literature pertaining to the methods has been included. In addition, the total C section has been modified to include the latest information on automated instruments for analysis of C.

Total C in soils is the sum of both organic and inorganic C. Organic C is present in the soil organic matter fraction, whereas inorganic C is largely found in carbonate minerals. Not all soils contain inorganic C because of dissolution during soil formation of carbonate minerals originally present in parent material. However, organic C is present in all agricultural soils. In soils formed from cal­careous parent material under arid conditions, it is not unusual for the inorganic C concentration to exceed the amount of organic C present.

Organic C is contained in the soil organic fraction, which consists of the cells of microorganisms, plant and animal residues at various stages of decom­position, stable "humus" synthesized from residues, and highly carbonized com­pounds such as charcoal, graphite and coal (elemental forms of C). Organic C in soil may be estimated as the difference between total C and inorganic C. Organ­ic C can be determined directly by total C procedures after removal of inorganic C or by rapid dichromate, oxidation-titration techniques. In the absence of inor­ganic C, a total C analysis can be used to determine organic C and recover all forms of organic C in soils. However, organic C methods based on dichromate oxidation recover variable proportions of elemental C (e.g., charcoal) and, in some procedures, variable amounts of organic C contained in "humus."

Copyright © 1996 Soil Science Society of America and American Society of Agronomy, 677 S. Segoe Rd., Madison, WI 53711, USA. Methods of Soil Analysis. Part 3. Chemical Methods-SSSA Book Series no. 5.

961

Published 1996

962 NELSON & SOMMERS

Calcite and dolomite are the principal carbonate minerals present in soil, and most inorganic C is associated with these compounds. However, in some alkali soils, significant amounts of inorganic C may be present in soluble car­bonate and bicarbonate salts. The amounts of soluble carbonates present in soil may be determined by procedures outlined in Chapter 15 (Loeppert & Suarez, 1996), and a variety of methods for the estimation of total inorganic C in soils are presented in Chapter 15 (Loeppert & Suarez, 1996).

Total C analysis of soil involves conversion of all forms of C in soils to car­bon dioxide (C02) by wet or dry combustion and subsequent quantitation of evolved CO2 by gravimetric, titrimetric, volumetric, spectrophotometric, or gas chromatographic techniques. Dry combustion is conducted by heating (-I000°C) a soil-catalyst mixture in a resistance furnace or induction furnace in a stream of O2 or COrfree air, followed by quantitation of evolved CO2. Wet combustion is normally carried out by boiling a soil sample with a mixture of potassium dichro­mate (K2Cr207)' sulfuric acid (H2S04), and phosphoric acid (H3P04) in a closed system flushed with a stream of COrfree air and absorbing evolved CO2 in a tared weighting bulb filled with Ascarite (Arthur H. Thomas Co., Philadelphia, PA) (Allison, 1960). Alternatively, wet combustion may be carried out in a Van Slyke-Neil apparatus and evolved CO2 estimated by manometric procedures (Bremner, 1949). 1\vo dry combustion and one wet combustion procedures for total C analysis are described in this chapter.

Soil organic matter has been defmed as the organic fraction of soil, includ­ing plant, animal, and microbial residues, fresh and at all stages of decomposi­tion, and the relatively resistant soil humus (SSSA, 1979). Soil organic matter is normally restricted to only those organic materials that accompany soil particles through a 2-mm sieve. It is difficult to quantitatively estimate the amount of organic matter present in a soil. Procedures used in the past involve determina­tion of the change in weight of a soil sample resulting from destruction of organ­ic compounds by H20 2 treatment or by ignition at high temperature. Both tech­niques are subject to error. The H20 2 method does not quantitatively remove organic matter and the ignition method gives an overestimate because both inor­ganic and organic constituents in soils during ignition can be minimized by removing aluminosilicates with hydrofluoric acid (HF)/hydrochloric acid (HCI) prior to heating or ignition at temperatures that decompose organic matter with­out appreciable dehydroxylation of inorganic materials. Alternatively, the organ­ic matter content of a soil may be estimated by multiplying the organic C con­centration by a constant factor based on the percentage of C in organic matter. Published organic C-organic matter conversion factors for surface soils have var­ied from 1.724 to 2.0. The appropriate factor must be determined experimentally for each soil by independent analysis of organic matter and organic C. Although neither the direct determination of organic matter nor the calculation of organic matter content is completely accurate, the most useful procedures currently avail­able are described in this chapter. Because of the problem associated with deter­mining the organic matter content of a soil, it is strongly suggested that investi-

CARBON AND ORGANIC MATIER 963

gators determine and report the organic C content as an index of the organic mat­ter in a soil.

TOTAL CARBON

Introduction

Analytical procedures used for determining total C in soils must quantify both inorganic and organic forms. In humid regions where extensive leaching of the soil profile has occurred, organic C will be the predominant form present. In arid or semiarid regions, carbonate minerals (e.g., calcite, dolomite) along with soluble carbonate salts may constitute a significant percentage of the total C.

Dry combustion and wet combustion are the two basic approaches used to quantify total C in soils. In both instances, the CO2 liberated from organic and inorganic C is determined through spectrophotometric, volumetric, titrimetric, gravimetric, or conductimetric techniques. An apparatus for performing total C analysis by dry combustion can be fabricated from conventional laboratory glassware and a medium-temperature (-1000°C) resistance furnace. Dry com­bustion procedures using either high-temperature (> 1500°C) or induction fur­naces are found in commercially available, automated total C analyzers. The major~ty of dry combustion methods employ gravimetric determination .of CO2 although titrimetric techniques also are described. Wet combustion methods for total C employ a strong oxidant, such as K2Cr207, in an acid digestion mixture for quantitative oxidation of organic C and dissolution of carbonate minerals. A comparison of principles, advantages, and disadvantages of commonly used methods for total C determination is given in Table 34-1.

The developments in instrumental methods in recent years should be assessed before choosing a procedure for determining total C in soils. The major­ity of instruments are automated versions of primarily dry combustion proce­dures. The relative advantages and disadvantages of manual and instrumental methods should be considered before initiating total C analysis. From a cost standpoint, manual procedures can be set up, in many cases, with apparatus already present in most laboratories; however, they are time-consuming and tedious and require use of careful analytical technique. In contrast, instruments are costly, typically greater than $20 000, but they are capable of analyzing a large number of samples with minimal variability due to operator error. Nearly all commercial units are available with autosamplers and computer interfaces to aid in data acquisition and handling. In addition, several commercial units enable the simultaneous determination of elements (C, H, N, or S).

The methods presented for total C are essentially identical to those pro­posed by Allison et ai. (1965) in the first edition of Methods of Soil Analysis (Black et aI., 1965) and subsequently updated in the second edition (Nelson & Sommers, 1982). Much of the text presented is used with only minor alterations to update the equipment available and the literature cited. A brief description has

Tab

le 3

4-1.

Com

pari

son

of

met

hodo

logi

es u

sed

for

dete

rmin

atio

n o

f to

tal

C in

soi

ls.

Met

hod

Pri

ncip

le

CO

2 de

term

inat

ion

Adv

anta

ges

Dry

com

bust

ion

(res

ista

nce

furn

ace)

D

ry c

ombu

stio

n (i

nduc

tion

fu

rnac

e)

Dry

com

bust

ion

(aut

omat

ed

met

hods

)

Wet

com

bust

ion

(com

bust

ion

trai

n)

Sam

ple

is m

ixed

wit

h C

uO a

nd h

eate

d to

-lO

OO

°C i

n a

stre

am o

f O

2 to

co

nver

t al

l C

in

sam

ple

to C

O2

Sam

ple

is m

ixed

wit

h F

e o

r ac

cele

ra­

tors

and

rap

idly

hea

ted

to >

165

0°C

in

str

eam

of

O2

to c

onve

rt a

ll C

in

to C

O2•

Sam

ple

is m

ixed

wit

h ca

taly

sts

or a

c­ce

lera

tors

and

hea

ted

wit

h re

sist

ance

or

indu

ctio

n fu

rnac

e in

a s

trea

m o

f O

2 to

con

vert

all

C i

n sa

mpl

e to

CO

2 S

ampl

e is

hea

ted

wit

h K

2Cr2

07-H

2S04

-H

3P0

4 m

ixtu

re i

n a

CO

rfre

e ai

r st

ream

to

conv

ert

all

C i

n sa

mpl

e to

CO

2

Gra

vim

etri

c,

titr

imet

ric

Gra

vim

etri

c,

titr

imet

ric

Ref

eren

ce m

etho

d w

idel

y us

ed i

n ot

her

disc

ipli

nes,

var

iabl

e sa

mpl

ing

size

s,

vari

able

sam

ple

size

R

apid

com

bust

ion,

hig

h te

mpe

ratu

re

ensu

res

conv

ersi

on o

f al

l C

to

CO

2

The

rmal

con

duct

ivity

, R

apid

and

sim

ple,

goo

d ac

cura

cy a

nd

cond

uctim

etri

c, i

n-pr

ecis

ion

frar

ed d

etec

tor,

gr

avim

etri

c T

itrim

etri

c, g

ravi

­m

etri

c E

quip

men

t re

adil

y av

aila

ble,

goo

d ac

­cu

racy

, ea

sily

ada

pted

to

anal

ysis

of

solu

tion

s, t

itri

met

ric

anal

ysis

of

CO

2 le

ss s

ubje

ct t

o op

erat

or e

rror

Dis

adva

ntag

es

Tim

e-co

nsum

ing,

lea

kfre

e, O

2 sw

eep

trai

n is

req

uire

d, s

low

rel

ease

of

CO

2 fr

om a

lkal

ine

eart

h ca

rbon

ates

. L

eakf

ree

O2

swee

p tr

ain

is r

equi

red,

in

duct

ion

furn

ace

is e

xpen

sive

.

Exp

ensi

ve e

quip

men

t, s

low

rel

ease

of

CO

2 fr

om a

lkal

ine

eart

h ca

rbon

ates

w

ith

resi

stan

ce f

urna

ce

Tim

e-co

nsum

ing,

gra

vim

etri

c de

ter­

min

atio

n o

f C

O2

requ

ires

car

eful

an

alyt

ical

tec

hniq

ues,

tit

rim

etri

c de

term

inat

ion

of

CO

2 le

ss p

reci

se.

t ~ Z

~

C"I.l I

CARBON AND ORGANIC MATTER 965

Fig. 34-1. Block diagram of dry combustion trains. Option A is based on Allison (1965). Option B is adapted from Rabenhorst (1988).

to update the equipment available and the literature cited. A brief description has been added on the principles employed in selected commercially available total C analyzers.

Total Carbon by Dry Combustion

Introduction

The dry combustion method is based on oxidation of organic C and ther­mal decomposition of carbonate minerals in a medium-temperature resistance furnace. The CO2 liberated is commonly trapped in a suitable reagent and deter­mined titrimetrically or gravimetrically. Spectrophotometric, volumetric or con­ductimetric procedures are used to determine CO2 in some commercial instru­ments. Alternatively, the CO2 released can be reduced to CH4 and quantitated with a gas chromatograph fitted with a flame ionization detector (Geiger & Hardy, 1971). The following description of a medium-temperature dry combus­tion is that presented by Allison et al. (1965).

Principles

In the dry combustion procedures described here, the sample is burned in a stream of purified O2 and CO2 in the effluent gas stream is absorbed by Ascarite or some other suitable absorbent and weighed. Other absorbable gases formed during combustion are removed from the O2 stream before they reach the CO2 absorption bulb. A typical combustion train is comprised of 10 basic ele­ments as diagrammed in Fig. 34-1. The make-up of the individual elements are modifications (AOAC, 1975, p. 924-926; Chemists U.S. Steel Corp., 1938, p. 40-54; Salter, 1916; Winters & Smith, 1929) of those recommended by Fleming (1914) for the rapid determination of C in Fe and steel.

NELSON & SOMMERS

The O2 supply (commercial compressed O2) is first scrubbed by passage through a train consisting of concentrated H2S04 to remove ammonia (NH3) and hydrocarbons, an absorbent such as soda lime to remove CO2, and anhydrous magnesium perchlorate Mg(CI04)z to remove water vapor. The rate of O2 flow is controlled by a needle valve and is measured by a flow meter.

A furnace provides the heat necessary for combustion of the organic C to CO2 and for decomposition of carbonates. In a resistance furnace, the sample is heated by radiation, conduction, and convection in a tube surrounded by heating elements made of high-resistance materials such as Nichrome (in medium-tem­perature models) or silicon carbide (in high-temperature models). In an induction furnace, the source of energy is high-frequency electromagnetic radiation. Fer­rous metals and certain other materials can be heated to high temperatures by electromagnetic induction if enough energy is present. Materials such as soil that do not heat by induction can be heated indirectly by radiation, conduction, and convection from susceptors (materials that do heat) in the induction field. The susceptor may take the form of iron or tin chips that are mixed with the sample to be burned, or a radiator [e.g., the Pt cage (Simons et aI., 1955), quartz­enclosed carbon crucible (Allison et aI., 1965)] that will surround a crucible con­taining the sample to be burned.

The type of furnace determines the packing of the combustion tube. With medium-temperature furnaces, cupric oxide (CuO) or another accelerator is mixed with the soil to aid in combustion of the organic matter and elemental C. With high-temperature furnaces, the organic and elemental C is generally oxi­dized to CO2 by gaseous O2 without special assistance. When medium-tempera­ture furnaces are used, catalysts must be included in the combustion tube at the rear of the heated zone to ensure essentially complete oxidation of carbon monoxide (CO) or other volatile C compounds. Platinized asbestos or CuO wire may be used as a catalyst. However, with any type of furnace, some CO may pass through. A low-temperature (-250°C) catalyst furnace, with catalyst supplied by the manufacturer, may follow the main combustion tube to convert any CO to CO2•

Medium-temperature combustion is not entirely satisfactory for soils con­taining alkaline-earth carbonates because these minerals release CO2 slowly at 950°C (C02 may not be released completely in 30 min). High-temperature com­bustion, on the other hand, causes rapid and quantitative release of CO2 from both Na2C03 and alkaline-earth carbonates.

The gas stream leaving the furnace is freed of particulate matter by a dust trap in the exit end of the combustion tube. The removal of nitrogen oxides, sul­fur oxides, and halogen gases can be effected in several ways. Activated Mn02 appears satisfactory as a dry absorber for the oxides of Nand S and the halogens (Robertson et aI., 1958). To protect the catalysts in the catalyst furnace from being poisoned by these substances, a trap of activated Mn02 must be inserted at the combustion tube outlet. Liquid absorbers for these interfering gases include solutions of H2S04-Cr03, Ag2S04, and KI but they are not recommended for insertion ahead of the catalyst furnace. Most of the water vapor formed during combustion is removed by a concentrated H2S04 tower immediately following the catalyst furnace. The little water vapor passing through is trapped by a tower

CARBON AND ORGANIC MATIER 967

of anhydrous Mg(CI04h next in line. The CO2 is finally absorbed in a suitable bulb containing Ascarite or other absorbent backed by anhydrous Mg(CI04h to ensure that water vapor pressure is the same in exit gas as in entering gas.

Rabenhorst (1988) evaluated many of the parameters involved in deter­mining organic and inorganic C in soils by dry combustion using gravimetric determination of C collected in an absorption bulb. He concluded that sequential combustion of the same sample at 575°C for 15 min followed by combustion at 1000°C for 10 min would quantitatively recover organic and inorganic C, respec­tively, from soils. He also described a simplified analytical apparatus that con­sisted of the following components: (i) compressed O2; (ii) concentrated H2S04;

(iii) Ascarite; (iv) Mg(CI04h; (v) quartz or ceramic combustion tube to hold the sample boat and containing CuO wire and glass wool at distal end; (vi) Drierite; (vii) Mg(CI04)2 and; (viii) Nesbitt bulb containing layers of glass wool, ascarite and Mg(CI04)2. This simplified gas handling train is listed as Alternative C (see "Alternative Arrangements" below; Fig. 34-1) in the following. method and should be considered if dry combustion will be used for total C analysis.

Medium-Temperature Resistance Furnace Method

Special Apparatusl

1. Oxygen cylinder and pressure regulator (A). 2. Oxygen purifying train consisting of concentrated H2S04 for removal

of NH3 and hydrocarbons, Ascarite for removal of CO2 and acid gases, and anhydrous Mg(CI04)2 for removal of water vapor (B).

3. Flow indicator and needle valve for O2 control (C). 4. Furnace unit (D): (a) Resistance furnace equipped with temperature

controller and indicator (Lindberg multiple-unit combustion-tube fur­nace or equivalent) for operation at 900 to 1000°C; (b) Sample insert­er (LEC02 no. 501-062, Alpha3 AR-061 or equivalent); (c) Combus­tion tube, 2.5-cm diam. by 75 cm (zircon ceramic or equivalent).

5. Dust trap (LECO no. 501-010 or equivalent) inserted in the exit end of the combustion tube (E).

6. Sulfur trap filled with activated Mn02 (LECO no. 503-033 or equiva­lent) (F).

7. Catalyst furnace and tube (LECO no. 507-010 or equivalent) (G). 8. Gas scrubber (H): (a) Sulfuric acid tower to absorb most of the water

vapor and to prolong the life of the anhydrous Mg(CI04h trap that fol­lows (especially desirable when combusting organic soils and other organic materials; (b) Water vapor trap filled with anhydrous Mg(CI04h (LECO no. 598-157 or equivalent).

9. Carbon dioxide absorption tube, a Nesbitt, Fleming, or Turner bulb packed with an indicating CO2 absorbent and anhydrous Mg(CI04)(I). The bulb contains from bottom to top: (i) glass wool, (ii) 3-cm layer

1 Capital letters in parentheses refer to units in Fig. 34-1. 2 LECO Corporation, 3000 Lakeview Ave., SI. Joseph, MI 49085-2396. 3 Alpha Resources, Inc., 3090 Johnson Rd., Stevensville, MI 49127-0199.

968 NELSON & SOMMERS

of 8- to 14-mesh (1.4-2.36 mm) absorbent (e.g., Ascarite), (iii) 2-em layer of 14- to 20-mesh (0.85-1.4 mm) absorbent, (iv) l-cm layer of anhydrous Mg(CI04)z, and (v) glass wool (also described in "Special Apparatus" under "Wet Combustion Method").

10. Bubbler trap to seal the train from the atmosphere and indicate flow of exit gas (J).

11. Alternative arrangements; (i) The O2 purifying train (B) and the flow indicator (C) are available as a combined unit (LECO no. 516-000); (b) The scrubber-absorption train described in "Special Apparatus" under "Wet Combustion Method" (Units F-K) can substitute for units H, I, and J (Items 8-10); ( c) Items 4-8 can be replaced as follows. Item 4, pack combustion with 7 to 10 cm of cupric oxide wire followed by plug of glass wool; Items 5 to 7 are deleted; Item 8, replace with by trap of Drierite followed by trap of anhydrous Mg(CI04)z.

12. Accessory items: (i) Combustion boats, ceramic (Alundum, zircon, etc.), (LECO no. 528-053 or equivalent); (b) Boat puller with eye shield (LECO no. 501-062 or equivalent); (c) Combustion tube clean­ing brush (LECO no. 501-082 or equivalent); (d) Plastic tubing (Tygon or equivalent) for connecting components. (Rubber is permeable to CO2.) Any deposits due to malfunctions can readily be observed in transparent tubing); (e) Analytical balance (Mettler H31AR, Mettler Instrument, Hightstown, NJ; or equivalent) on grounded metal plate.

Reagents

1. Oxygen gas. 2. Sulfuric acid, concentrated. 3. Manganese dioxide (Mn02), activated (LECO no. 501-060 or equiva­

lent). 4. Platinized asbestos, 5% Pt (J. T. Baker Chemical Co.; no. 0922,

Philipsburg, NJ; or equivalent); or CuO, wire or granular, low in C for combustion tube catalyst.

5. Cupric oxide powder, low in C, to serve as an accelerator when mixed with soil in the boat.

6. Alundum or Sinderite or equivalent, refractory grade, 60- or 90-mesh (165-250 ~m) size, C free.

7. Anhydrous magnesium perchlorate (Anhydrone, Dehydrite, etc.). 8. Carbon dioxide absorbent, indicating, 14- to 20-mesh (0.85-1.4 mm)

size [Ascarite, Caroxide (Fisher Scientific, Pittsburgh, PA), Indicarb (Fisher Scientific, Pittsburgh, PA), or Mikhobite (G. Frederick Smith Chemical Co., Columbus, OH).

9. Standard C source [dextrose (CJI1206) or benzoic acid (C2H60 2) of reagent-grade or primary-standard quality].

Procedure

Loosely pack the combustion tube with a 7.5-em core of platinized asbestos so that it will come within the exit end of the heated zone of the furnace. Alter-

CARBON AND ORGANIC MATTER 969

natively, use coarsely granular CuO, held in position by two plugs of asbestos fiber packed no more tightly than is necessary to hold the CuO in place.

Bring the furnace to 950°C to lO00°C. Connect the train, and sweep the apparatus with O2 at the rate of 100 mL/min for 10 min. Remove and weigh the CO2 absorption bulb. Repeat this step until the CO2 absorption bulb has attained a weight constant to ±0.2 mg. Replace the CO2 absorption bulb in the train, and introduce well within the heated zone of the combustion tube a ceramic boat con­taining 1.0 g of finely divided CuO. Admit O2 and continue the flow at 100 mL/min for 10 min. Close the stopcocks on the CO2 absorption bulb, disconnect the bulb, and weigh it. The increase in weight of the bulb represents the blank. Remove the boat from the combustion tube. Repeat the determination until a reproducible blank is obtained. With a properly prepared train and high-quality reagents, the blank should be negligible, i.e., within limits of weighing error.

Grind soil to be analyzed for C to pass through a 100- or 140-mesh sieve (0.149- or 0.105-mm openings). Mix 1.000 g of mineral soil of known water con­tent with 1.0 g of finely divided CuO in a combustion boat, cover the mixture lightly (-2 mm) with Alumdum or Sinderite, and follow the procedure used for the blanks. For soils high in organic matter, use a 0.500-g sample. The increase in weight, corrected for blank, should represent the CO2 from the sample. The calculation is as follows

[g CO2, sample] - [g CO2, blank] Total C, % = x 0.2727 x 100 [1 ]

g water-free soil

Comments

Cleanliness is proverbial in the C laboratory, especially when one is deal­ing with low C samples or doing work of the highest order of accuracy. In such instances, freedom from dust, dirt, or fumes is essential. It is advisable habitual­ly to ignite all boats at -900°C before use and to store boats out of contact with the atmosphere. Handling boats and covers only with tongs is considered good practice.

Some experience is usually necessary before reproducible weights of CO2

absorption bulbs can be obtained. Thermal equilibrium with the atmosphere is important. The surface must be kept clean and free of static charge. The follow­ing points may be helpful:

1. Before each weighing, the absorption bulb should be wiped slowly with a lint-free paper tissue, such as Kimwipes or napkin stock. Paper is superior to cloth. Rapid wiping builds up the static charge .Handling the absorption bulb with clean cotton gloves may be helpful.

2. After the CO2 absorption bulb has been wiped, it should be touched to a grounded plate to remove the static charge before weighing. If the bulb is in temperature equilibrium with the atmosphere, repeated weighing should agree within ±0.2 mg. The position of the absorption bulb on the balance pan may be critical. With some balances, accura­cy is dependent on careful centering of the objects to be weighed.

970 NELSON & SOMMERS

3. The CO2 absorption bulb is brought to constant weight by inserting it in the combustion train and operating the train as a blank. Cold bulbs characteristically gain several milligrams on blank runs before attain­ing a weight constant to ±0.2 mg. If a steady gain in weight is obtained in repeated trials with the bulb properly handled and placed on the bal­ance pan, one or more of the absorbers in the train may not be func­tioning properly.

Fine grinding «150 !lm or <100 mesh) of soil samples is especially impor­tant in obtaining reproducible results in determinations of C. Only from finely ground, homogeneous samples, can uniform small subsamples be drawn. Poor replication often can be traced to poor sampling. Fine grinding is a task at best. Where many samples are to be processed, a mechanical mortar and pestle unit, a high-speed impact shaker, or a ball mill will save much time and labor.

When C in soil extracts or other liquids is determined, the sample may be evaporated and dried, preferably under vacuum at 60°C in porcelain or nickel boats of 5- or 10-mL capacity. Liquids will slowly seep through the usual grade of ceramic boats. Porcelain boats are short lived, even at 950°C. Unglazed boats may be rendered leakproof by treating with a glazing mixture and firing in a lab­oratory furnace (Lindbeck & Young, 1964). An alternative method for analyzing soil extracts involves filling ceramic combustion boats with siliceous earth, i.e., Kieselgur (Tiessen et aI., 1981). Since Kieselguhr will absorb about four times its weight in liquid, multiple 3-mL samples of a soil extract can be added to the boat if water is evaporated on a hot plate for 10 min at 80°C between additions.

Combustion tubes eventually develop fine cracks in the hottest region and need to be replaced. Erratic results are one indication of a cracked combustion tube. After every 50 or 100 analyses, the tube should be tested for leaks under operating pressure by stoppering the exit and observing if O2 passes the H2S04

tower in the purifying train. Combustion tube life is prolonged if, during con­templated daily use, the furnace is kept on continuously.

A standard C source, such as analytical reagent or primary standard-quali­ty glucose or benzoic acid, should be run from time to time to check the appara­tus. The organic standard should be diluted and covered with Alundum or Sin­derite to prevent explosion.

Explosive combustion will blow stoppers, and even the boat, from the com­bustion tube. After an explosion, it is essential to bum off the C deposits from the cooler areas inside the tube before additional analyses are made.

A two-tube furnace is advantageous even though only one tube is used rou­tinely. The second tube serves as a reserve in the event the other cracks during a series of analyses or when C deposits resulting from an explosion must be burned out.

The insert dust trap should be cleaned and refilled with glass wool after 40 or 50 determinations or more frequently if deposits appear in the exit tubing.

The Mn02 used to remove S02 from the combustion products before they enter the catalyst furnace should be changed after about 50 determinations or before all the granules appear gray or agglomerated. Peterson (1962) pointed out that the accumulation of combustion reaction products on the Mn02 changes its CO2 absorption-desorption pattern so that longer flushing times must be used, as

CARBON AND ORGANIC MAITER 971

in the gasometric determination of C. Peterson recommended a specially pre­pared Pb02 as a substitute for Mn02 to increase the efficiency of S02 removal but no evidence was presented concerning its capacity to absorb nitrogen oxides or the halogens.

Air-dry samples are preferred to oven-dry samples, because oven drying may result in lower values for total C in some soil samples.

A slight pressure in the combustion tube will be noticed when the stopper is removed after a determination. If this pressure becomes pronounced, it indi­cates increased resistance to gas flow in the S trap or in the water vapor trap. These traps should be examined and repacked or replaced as necessary.

Temperature> 1OOO°C must be avoided. Heating elements will be subject to burnout, and fusion of CuO is likely to occur and cause slagging and tube rupture on cooling. Attention to this is especially important if a temperature controller is not used.

A supply of boats can be rendered C free by preliminary ignition in a muf­fle furnace at 850 to 900°C. These ignited boats should be kept in dust-free stor­age until used.

High-Temperature Induction Furnace Method

Special ApparatusS

1. Oxygen cylinder and regulator (A). 2. Oxygen purifying train (B). 3. Flow meter and needle valve (C). 4. Furnace unit: (i) Induction (high-frequency) furnace (Alpha no. AR-

521 or equivalent) for operation at 1400 to 1600C; (b) Combustion tube (LECO no. 550-122 or equivalent); and (c) Crucible (LECO no. 528-018) with cover.

S. Dust trap for induction furnace, external (LECO no. 501-0lD or equiv­alent).

6. Sulfur trap filled with activated Mn02 (LECO no. 503-033 or equiva-lent).

7. Catalyst furnace and tube (G). 8. Gas scrubber (H). 9. Carbon dioxide absorption tube (I).

10. Bubble trap (1). 11. Alternative arrangements: (a) The O2 purifying train (B) and the flow

indicator (C) are available as a combined unit (LECO no. 516-000); (b) The induction furnace actually is a complex unit that combines the fur­nace (D), dust trap (E), Strap (F), and catalyst furnace (G) (Items 4-5 above), in a single unit; and (c) Various combinations of the 10 basic elements that comprise the train (Fig. 34-1) are available under vari­ous trade names such as Leco. The output of the furnaces (resistance or induction) can be put through a water vapor trap (H, a single V-tube filled with anhydrone) into a CO2 absorption tube (I). No exact com-

5 Capital letters in parentheses refer to units in Fig. 34-1.

972 NELSON & SOMMERS

bination of units is prescribed here since many suitable combinations are possible.

12. Analytical balance (Mettler H31AR or equivalent).

Reagents 1. Reagents 1, 2, 3, 7, 8, and 9 described in "Reagents" under "Medium-

Temperature Resistance Furnace Method." 2. Tin metal accelerator (LECO no. 501-076 or equivalent). 3. Iron chip accelerator, C free (LECO no. 501-077 or equivalent). 4. Tin-coated copper accelerator (LECO no. 501-263 or equivalent). 5. Scoop for adding 1 g of accelerators (LECO no. 503-032 or equiva­

lent).

Procedure Using Iron, Tin, and Tin-coated Copper Accelerators Weigh the CO2 absorption bulb on the analytical balance, insert the bulb in

the train, and open the stopcocks. Set the O2 flow at the rate of 1.5 Umin. Place an empty crucible in the induction furnace. Fire the furnace, following the man­ufacturer's instructions, for 5 min. At the end of the combustion period, remove the crucible, tum off the O2 flow, and then close off and remove the CO2 absorp­tion bulb. Weigh the CO2 absorption bulb (see "Comments" under "Medium­Temperature resistance Furnace Method"). Repeat this process until a blank reproducible ±0.2 mg is obtained. Alternatively, ignite two or more blanks (cru­cible containing 1 scoop each of iron chip, tin and tin-coated copper accelerators) until a blank reproducible to ±0.2 mg is obtained.

Transfer a 0.5000-g sample of soil that passes through a 100- or 140-mesh (106-150 /-lm) sieve to a crucible. Add one scoop of tin metal accelerator, one scoop of iron chip accelerator, and one scoop of tin--coated copper accelerator, and cover the crucible. Insert the covered crucible in the induction furnace. Set the O2 flow at a rate of 1.5 Umin. Fire the furnace according to a manufacturer's instructions. At the end of the combustion period, remove the crucible, tum off the flow of O2, and close off, remove and weight the CO2 absorption bulb (see "Comments" under "Medium-Temperature Resistance Furnace Method"). The increase in weight of the CO2 absorption bulb, after correction for the blank, should be due to CO2 released from the soil sample. Flush the train (without the CO2 absorption bulb) with O2 for about 1 min between successive runs. Deter­mine the blank for the crucible and accelerators using the same procedure.

nt [g COz, sample] - [g COz, blank] Total C, 70 = x 0.2727 x 100 [2]

g water-free soil

Comments

Comments in "Comments" under "Medium-Temperature Resistance Fur­nace Method" on weighing of absorption bulbs and on sample grinding are fully applicable to this procedure. Other comments in "Comments" under "Medium­Temperature Resistance Furnace Method" also are applicable.

Under optimum conditions, the two procedures yield comparable results. Adequately high temperature (> 1650°C) can be developed with the proper addi-

CARBON AND ORGANIC MAITER 973

tions of Sn and Fe, but the temperature maximum is held only briefly. The tem­perature rises steadily until the susceptors melt and fuse with the sample, and thereafter, it falls rapidly. Occasionally this temperature rise and fall occurs before thermal decomposition of C is complete, perhaps because of inadequate contact between the sample and the susceptor material. For most soils, this does not appear to be a major problem since Fe, Sn, and tin-coated copper accelerators (-1 g of each/sample) have been found to yield accurate total C values in a range of calcareous and noncalcareous soils and standard carbonate minerals (Tabatabai & Bremner, 1970).

If the organic matter content of the soil is high, the sample weight should be reduced appropriately. Organic materials can be analyzed by this technique, but sample weights must be reduced to 20 or 30 mg if explosions are to be avoid­ed. Alternatively the organic material in amounts up to 60 mg can be mixed and covered with Alundum or Sinderite as described in "Procedure" under "Medium­Temperature Resistance Furnace Method."

The gravimetric determination of CO2 following combustion with the Leco induction furnace was found by Carr (1973) to yield total C levels comparable with manual wet and dry combustion methods. In addition to gravimetry, an auto­mated CO2 analyzer based on thermal conductivity measurements of the effluent gases was applicable to soil analysis (Tabatabai & Bremner, 1970).

Alternatively, a titrimetric method was developed to allow estimation of both total C and 14C in soil samples amended with 14C compounds (Cheng & Far­row, 1976). A bypass valve and a 12S-mL gas washing bottle (e.g., Corning 31760) are used in place of the CO2 absorption bulb of Fig. 34-1. All CO2

released by combustion is trapped in SO mL ofO.S M NaOH followed by removal of one aliquot for liquid scintillation counting to quantify 14C2 and a second aliquot for titration with standard HCI to determine total C. The total C data obtained were comparable to those obtained by a wet combustion procedure. Recent data also indicates that titrimetric and thermal condl'ctivity methods are comparable for the determination of CO2 (Winter et aI., 1990).

Instrumental Methods

The following section describes representative commercial instruments for determining total C in soils. They were chosen to illustrate the principles involved in instrumenting total C analysis. The inclusion of the following three instruments does not imply that they are superior or inferior to others currently being marketed. As with all instruments, various evaluation procedures should be used to determine and to confirm the validity of data obtained in comparison to accepted, standard methods. Tabatabai and Bremner (1991) described automated instruments available for determining total C in soils as well as which instruments are capable of simultaneous determination of N or S.

Carbo-Erba NA 1500. Carlo-Erba Instruments (Milan, Italy) developed an automated instrument capable of simultaneous determination of C, H, and N in geologic materials, soils, and other environmental samples. The principles involved, development of the instrument and a description of modes of operation are presented by Pella (1990a,b). A sample is placed in a tin sample cup, crimped

974

NiO

Sample in Sn container

Co,Oj Ag

Helium (continuous flow)

Cu

NELSON & SOMMERS

Reference Thermal Conduct­

ivity Sample Detector

Chro atographic colum

J Fig. 34--2. Schematic diagram for Carlo-Erba Model NA 1500 analyzer.

to confine it, and introduced into a quartz reactor. For mineral soils, a typical sam­ple size is 5- to 10 mg, necessitating the use of samples finely ground in a ball mill or similar apparatus. The quartz reactor is maintained at 1050°C with a con­stant flow of He. Flash combustion will occur if a pulse of O2 is injected into the quartz reactor shortly after introduction of the sample. Under these temperature and O2 conditions, the tin is oxidized to SnOz resulting in the temperature increasing to 1700 to 1800°C and the complete combustion of soil organic mat­ter. The combustion products (C02, N oxides, and HzO) are swept by the helium carrier gas through chromium dioxide (Cr02) to catalyze oxidation of organic fragments and C030 4 coated with Ag to remove halogens and sulfur oxides. The gases then flow through a heated Cu (650°C) column to remove excess oxygen, Mg(CI04)z to remove H20 and into a chromatographic column for separation of N2 and COz. The different gases are detected with a thermal conductivity detec­tor. A generalized flow diagram for this instrument is shown in Fig. 34-2.

Schepers et al. (1989) evaluated the NA 1500 coupled with a mass spec­trometer for simultaneous determination of C, N, and 15N in soil and plant mate­rials. The NA 1500 yielded Nand 15N data comparable to that obtained with con­ventional manual methods (Kjeldahl digestion followed by mass spectroscopy analysis). A detailed comparison of C data was not conducted although realistic values for total C in soils and plant materials were obtained. An evaluation of sample preparation methods (soil grindinglWiley mill vs. ball milling) indicated that homogeneous soil and plant samples were essential to reduce analytical vari­ability, especially in view of the typical sample size of 10 mg. Verardo et al. (1990) have described procedures for using the NA 1500 to determine C and N in marine sediments. Due to the analysis of 5 to 10 mg, careful sample prepara­tion and grinding are needed on insure that a representative sample is analyzed. A" with any analytical method, the inclusion of appropriate standards and blanks is essential tinsure valid total C data. The capability of coupling this instrument with a mass spectrometer is a potential benefit as well.

CARBON AND ORGANIC MATfER 975

Leco Instruments. The LECO Corporation (St. Joseph, MI) has marketed instruments for automated analysis of total C in soils and other solid materials for the past several decades.

A description of Leco instruments is presented by Tabatabai and Bremner (1991). Earlier results with the Leco automatic 70-s C analyzer (Tabatabai & Bremner, 1970; Carr, 1973) indicate that reliable soil total C data are obtained using Fe, Sn, and tin-coated copper accelerators in an induction furnace followed by thermal conductivity to quantitate CO2, The newer LECO IR-12 instrument involves combustion of a soil sample in an induction furnace using an O2 atmos­phere followed by passing the gas mixture over a catalyst to convert CO to CO2

and CO2 quantitation with an infrared detector. A related instrument, model DC-12 Duo-Carb, involves combustion of samples mixed with V 205 (vanadium pentoxide) in an induction furnace heated to 1000°C under an O2 atmosphere. The COz produced is measured with a thermal conductivity detector.

In 1993, LECO marketed two instruments for determining total C in soils. Both instruments utilize resistance furnaces to combust samples at >950°C. In the Model CR-412, a sample contained in a ceramic boat is placed in a specially designed horizontal resistance furnace maintained at a constant temperature in the range of 950 to 1400°C under Oz flow. After a delay, Oz is directed onto the sam­ple and carries the COz released through dust and water vapor traps and into an infrared detection system. Merry and Spouncer (1988) evaluated the earlier model CR-12 and found that it gave reasonable soil organic C values when oper­ated at 1200°C. In an evaluation of combustion temperature on C recovery from noncalcareous and calcareous soils, it was found that both inorganic and organic C were recovered between 600°C and 1000°C. Total C determined by the CR-12 and the Allison method (Allison, 1960; see "Wet Combustion Method") were in close agreement for 20 Iowa soils (Yeomans & Bremner, 1991).

Chichester and Chaison (1992) evaluated the CR-12 for determining organ­ic C and inorganic C by combusting samples at 575°C and 1000°C, respectively. They recommended combustion at 575°C for 250 to 360 s to determine organic C followed by combustion at 1000°C for 250 s to determine inorganic C. In addi­tion, the time required for analysis of inorganic C could be reduced from 250 to 60 s by increasing the combustion temperature from 1000°C to 1371°C. In sum­mary, total C could be determined by a single combustion at 1371 °C as found by other investigators.

A second LECO instrument is the model CHN 600 which is capable of simultaneous analysis of C, H, and N. A flow diagram for the CHN 600 is shown in Fig. 34-3. The successor to the CHN 600 is the model CHN 1000. A soil sam­ple «200 mg) is placed in a tin capsule and combusted in a resistance furnace at 950°C using Oz as a carrier gas. The gases formed are scrubbed to remove S gases and equilibrated in a ballast chamber. Mter equilibration, the gas mixture flows through two infrared detectors set to detect COz and HzO. An aliquot of the gas mixture is analyzed for Nz by thermal conductivity after reduction of N oxides and removal of COz and HzO.

The CHN 600 has been used in several soils studies. Total C results from 20 Iowa soils were essentially the same using the CHN 600 and a standard wet­combustion method (Yeomans & Bremner, 1991). A comparison of data obtained

976

Dual Heat Zone Furnace

950C

NELSON & SOMMERS

Exhaust

Fig. 34-3. Schematic diagram for LECO Model CHN 600 analyzer.

by the CHN 600 with a LECO induction furnace instrument and a wet-oxidation method indicated that the CHN 600 was the most precise total C technique (0.014>.12% C) and enabled a technician to perform 90 to 100 analyses in an 8-h d (Sheldrick, 1986). The CHN 600 has been shown to recover 100% of the C in a range of pure organic C compounds [acetanilide (N-acetylaniline), sucrose (C12H220 1), sulfanilic acid (C6H7N03S), and EDTA (ClOH16N20S)] and, as ex­pected, to yield soil organic C values 16 to 59% greater than those obtained by the Walkley-Black method (McGeehan & Naylor, 1988). In general, the CHN 600 has shown to be a reliable and accurate instrument for the determination of total C in soils.

Perkin-Elmer CHN2400. The Perkin-Elmer (Perkin-Elmer Corp., Instru­ment Division, Norwalk, CT) simultaneously measures C, H, and N using the principles employed in the traditional Pregl and Dumas procedures. A sample contained in a platinum boat is oxidized with O2 at -lOOO°C for 2 min in a com­bustion tube in the absence of carrier gas (He) flow. Mter combustion, He flow is initiated and the CO2, H20, and N2 bases produced by combustion are passed over CuO to convert CO to CO2 and silver mesh (silver vanadate on silver wool) to remove S and halogen gases. The gases then flow into a tube maintained at 650°C and packed with copper granules between end plugs of silver wool, where quantitative reduction of N oxides to N2 occurs. The gases are brought to constant pressure and volume in a gas mixing chamber and then allowed to expand into the analyzer portion of the instrument. The analyzer consists of three thermal con­ductivity (TC) detectors connected in series and separated by two traps. The sequence of TC detectors and traps enabling quantification of H, C, and N is as follows:

1. TC detector 1 (output equals total gas composition). 2. Magnesium perchlorate trap to remove H20. 3. TC detector 2 (decrease in output from detector 1 is proportional to H

content). 4. Soda asbestos plus Mg(Cl04)2 trap to remove CO2.

CARBON AND ORGANIC MATTER 977

5. TC detector 3 (decrease in output from Detector 2 is proportional to C content).

6. The remaining gases in the sample are N2•

All operations within the instrument are automatic. Additional work is needed to evaluate this instrument for soil analysis.

The above discussion is an overview of instrumental methods for total C analysis of soils. At present, the LECO instruments have been the most widely used for soil analysis. Several research laboratories have begun using Carlo-Erba instruments for total C analysis of soils. Due to rapid changes in technology and instrumentation, it is essential that manufacturers be contacted for currently avail­able models followed by an evaluation of the instrument.

Total Carbon by Wet Combustion

Introduction

The wet combustion analysis of soils by chromic acid digestion has long been a standard method for determining total C, giving results in good agreement with dry combustion. The main advantages for wet combustion are that the cost of apparatus is but a small fraction of the cost for dry combustion equipment and that the parts needed to assemble the apparatus are standard equipment in most laboratories. The chief disadvantage of the earlier wet combustion procedures (e.g., Heck, 1929) is that they use macro equipment, which is tedious to assem­ble and disassemble, and which occupies considerable bench space more or less permanently. Wet combustion also is used when the special manometric Van Slyke-Neil apparatus (Van Slyke & Folch, 1940; Bremner, 1949) is employed to estimate total C in soils.

The wet combustion method of Allison (1960), described here, embodies important refinements from published procedures, such as simple and effective digestion acid mixture (Clark & Ogg, 1942), a simple purification and absorption train assembled on a small panel (McCready & Hassid, 1942), and a more rapid procedure than formerly used (Heck, 1929; Jackson, 1958, p. 211). The signifi­cant features of this apparatus (Fig. 34--4) are as follows: (i) it can be assembled from simple parts and requires no ground-glass connections, (ii) the small inter­nal volume precludes the necessity for preaeration under most laboratory condi­tions, (iii) it requires only a short period of aeration following digestion, and (iv) the entire assembly (F-K) occupies only a small area. This method is satisfacto­ry for salt-affected soils high in Cl- and also for the dry residues of soil extracts rich in organic matter. A rapid treatment to remove carbonates described in "Pre­treatment Prior to Wet Combustion" permits determination of organic C on the residue of a pretreated calcareous soil. The following description of wet combus­tion methodology was presented by Allison et al. (1965).

Principles

The soil sample is digested in a 60:40 mixture of H2S04 and H3P04 con­taining K2Cr207' The boiling temperature of this mixture, 210°C, is high enough to ensure complete oxidation of carbonaceous matter, yet low enough to prevent

978

c

NELSON & SOMMERS

To Trap I or"

~ Go.-· Ga,- mon

Trop I T2t'OP ~I

MglCIO ) JFiber Qloss . Mods .2 . +

Ascarite- " NoOH

Fibtr Qloss •

K 250 ml Side. arm Nesbitt Erlenmeyer bulb

Fig. 34-4. Diagram of apparatus used to determine C by the wet combustion method. Trap I or II is used for determination of CO2 evolved by gravimetric or titrimetric techniques, respectively (dia­gram is not drawn to scale).

excessive fuming in the condenser. The CO2 evolved is absorbed by a suitable absorbent and weighed, although it may be absorbed in a standard base and titrat­ed.

A combination of fuming H2S04, phosphoric acid (H3P04), iodic acid (HI03) [added to potassium iodate (KI03)], and cr03 has been used for deter­mining C in organic compounds (Van Slyke & Folch, 1940) and in soil (Mc­Cready & Hassid, 1942). The reported advantages of this oxidation mixture are that it vigorously attacks and dehydrates resistant forms of C, thereby reducing boiling time for complete oxidation, and that it facilitates conversion of CO to CO2, Carbon monoxide is often produced when readily oxidizable carbohydrates are present in the sample. Extensive comparisons of the Van Slyke-Folch and the 60:40 H2S04-H2P04 oxidizing mixtures on many soils indicate that the two mix­tures are equally effective in converting total soil C to CO2, The more rapid diges­tion with the Van Slyke-Folch mixture, resulting in a saving of 3 or 4 min per determination, is not sufficient advantage to offset the difficulties of preparing and maintaining a digestion acid that contains fuming H2S04, Moreover it was found that the need for HI03 in the digestion mixture does not exist, which indi­cates that soil organic matter contains little or no active carbohydrate capable of producing CO during digestion (Allison, 1960).

Salt-affected soils frequently contain sufficient Cl- to give errors by wet combustion analysis whether the CO2 is determined titrimetrically (Clark & Ogg, 1942) or gravimetrically (Allison, 1960). When soil high in Cl- is heated in a digestion mixture containing Cr20-r-, chromyl chloride (CrOzCI2) is formed by the following reaction before boiling begins

CARBON AND ORGANIC MATTER 979

The reddish Cr02Cl2 decomposes at about 190°C, releasing free C12, with a color change to pale green. Any Cl2 and traces of undecomposed Cr02CI2 that pass the purification train are retained in the CO2 absorption bulb to give a posi­tive error.

In the methods described, Cl2 interference is prevented by including two traps in the purifying train, one containing KI and one containing silver sulfate (Ag2S04) (Traps F and G in Fig. 34-2). The use of Ag2S04 alone gives protec­tion up to about 0.2% Cl- (Allison, 1960), but its protective value becomes ques­tionable at higher Cl- concentrations, Since KI has a very high capacity to absorb free Cl2 by the reaction

2KI + Cl2 = 2KCl + 212, [4]

the use of a KI trap is recommended for soils high in Cl-. With both traps in the system, Cl- up to 5% of the sample weight does not interfere, provided proper precautions are observed during the early stages of sample digestion. Use of the Ag2S04 trap in conjunction with the KI trap serves to indicate when the latter is exhausted. For soils containing trace or low amounts of Cl-, the carrier stream may flow directly into the Ag2S04 trap.

Wet Combustion Method

The wet combustion method was described by Allison (1960).

Special Apparatus

The apparatus is shown in Fig. 34-4. Assemble the apparatus from the fol­lowing parts: (A) Hoke needle valve: (B) 25-cm high soda-lime tower; (C) 100-mL Kjeldahl flasks to fit a no. 2 stopper; (D) Allihn four-bulb condenser, fitted with a no. 2 stopper at the delivery end; (E) 60-mL open-top separator funnel; (F-H) 25- by 90-mm shell vials with no. 4 stoppers; (I and 1) I5-cm long CaCl2

U-tube; and (K) Nesbitt absorption bulb. Use neoprene stoppers and gum rubber tubing for all connections. Coat all rubber tube connections lightly with silicone lubricant.

Items C through E can be ground-glass joint glassware if desired (Fig. 34-4). All joints are standard-taper 24/40. The following parts are needed: (C) 100-mL round-bottom flask (Coming 4320); (C-I) distilling adapter tube (Com­ing 9421), which contains inlet tube for bubbling C0z-free air into digestion acid mixture; (D) Allihn condenser, -300-mm jacket length (Coming 2480); (E-1) dis­tilling tube with suction side arm (Coming 9420) (side arm is connected to puri­fying traps); (E) graduated separator funnel (Coming 6382A). A heating mantle and rheostat are used to heat the 100-mL digestion flask.

Provide a C0z-free carrier stream by releasing air from an air pressure line through Valve A and passing it through soda-lime Tower B. Connect B in a glass tube 4-mm o.d. that extends downward through Condenser D and dips about 1 cm below the surface of the oxidizing acid in Digestion Flask C. Shorten the stem of Funnel E to a length of about 9 em, and reduce the tip opening of the stem to a diameter of about 2 mm. Adjust the position of the Funnel E to extend into D at

980 NELSON & SOMMERS

least 5 cm below the stopper to avoid contact between oxidizing acid and stop­per. Lubricate Stopcock E with the digestion acid mixture or with syrupy H3P04•

Regular stopcock lubricant should not be used on stopcocks. Assemble the purifying traps, F to J, on a panel to provide stability. Fit the

vials of traps F, G, and H with no. 4 stoppers they have approximately 6 cm of the bottom cut off to provide a tight seal with the vials. Reduce the tip openings of the inflow tubes in F and G, but do not make them smaller than 1 mm in diam­eter, or sealing may occur. Fill traps F and G approximately two-thirds full with 50% KI solution and saturated Ag2S04, respectively. Adjust the inflow tubes so that they extend into the solutions not more than 3.8 cm for Trap F and 1.3 cm for Trap G; otherwise back pressure may develop and cause leaks in the system.

Fill Trap H not more than one-third full with concentrated H2S04, Prepare the inflow tube for H from the barrel of a 5-mL pipette with the tip extending not more than 1.3 cm into the acid (note that Trap H connects directly to Trap I). Place a fiberglass disc in the bottom of the V-tube; and fill the right side, Trap [, with 30-mesh (600 !lm) granular Zn for absorbing any acid fumes that escape past H. Fill the left trap, Trap J, with anhydrous Mg(CI04)2, which absorbs water from the carrier stream containing evolved CO2 before it enters K.

Fill the Nesbitt absorption bulb K with any good, self-indicating absorbent having a high capacity for absorbing CO2, Indicarb and Mikhobite are excellent for this purpose. When filled as shown in Fig. 34--4, the bulb contains succes­sively a 3-cm layer of 8- to 14-mesh (1.4-2.36 mm) absorbent, a 2-cm layer of 14- to 20-mesh (0.85-1.4 mm) absorbent, and a l-cm overlayer of anhydrous Mg(CI04)2, with a wad of glass wool above and below the column.

Reagents

1. Digestion acid mixture: Pour 600 mL of concentrated H2S04 into 400 mL of 85% H3P04, cool the mixture, and store it in a glass-stoppered bottle. Keep the bottle well stoppered to prevent absorption of water vapor.

2. Potassium dichromate, reagent grade. 3. Potassium iodide solution, 50%: Dissolve 100 g of KI in 100 mL of

water. 4. Silver sulfate solution, saturated. 5. Carbon dioxide absorbent, self-indicating, 7- to 14- (1.4-2.8 mm) and

14- to 20-mesh (0.85-1.4 mm) size; Suitable materials are Mikhobite (G. Frederick Smith Chemical Co., Columbus, OH), Caroxite or Indi­earb (Fisher Scientific, Pittsburgh, PA), or Ascarite (Arthur H. Thomas Co., Philadelphia).

6. Soda lime, 8- to 14-mesh size (0.85-1.4 mm). 7. Granular Zn, <30-mesh «600 !lm)size. 8. Anhydrous magnesium perchlorate (Anhydrone, Dehydrite, or equiva­

lent).

Procedure

Place a finely ground soil sample containing 20 to 40 mg of C (usually 0.5-3 g of oven-dry soil) into digestion flask C, and add about 1 g of K2Cr207'

CARBON AND ORGANIC MATTER 981

Wash down the neck of the flask with 3 mL of distilled water, and connect the flask to Condenser D. Weigh the Nesbitt bulb ("Comments" under "Medium­Temperature Resistance Furnace Methods" and the following "Comments" sec­tion), attach it to the system, and immediately open the valve at the top of the bulb. Pour 25 mL of the digestion acid mixture into Funnel E above the con­denser, and cover the funnel with a small beaker. Open StopcockE, allow the acid to flow through D into Flask C, and close the stopcock immediately to prevent loss of CO2, Adjust the air delivery tube that passes through D into C so that its tip extends not more than 1 cm into the acid during digestion.

At this point, tum on the cooling water. Adjust the carrier stream to a flow rate of about 2 bubbles/s, and maintain this rate during digestion. Place the heat­ing mantle around the flask or apply a flame 5 to 6 cm high, and bring the sam­ple to boiling in 3 or 4 min. If Cl- is high, heat the mixture slowly at first, and bring it to boiling in about 5 min. Continue gentle boiling, avoiding excessive frothing, for a total heating period of 10 min. Reduce the rate of healing if visi­ble white fumes of S03 occur above the second bulb of D during digestion.

Remove the heating mantle or flame at the end of the digestion period, and aerate the system for 10 min at the rate of 6 to 8 bubbles/so When aeration is com­plete, shut off the air stream, and disconnect the digestion flask from the con­denser. Close the stopcock on the Nesbitt bulb, and disconnect it from the system. Brush the bulb with camel's hair to remove any lint and dust, and weight it imme­diately. Make a blank determination, using the identical procedure, but without sample. Add four to five glass beads to the blank to prevent bumping. The calcu­lation is as follows

[g CO2, sample] - [g CO2, blank] Total C, % = x 0.2727 x 100 [5]

g water-free soil

Comments

Soil samples should be ground to pass through a sieve with openings 0.5 mm or smaller in diameter. This is necessary to reduce errors due to the presence of occasional fragments of carbonate minerals in a predominantly noncalcareous matrix.

A single analysis, involving all operations from weighing the sample to cal­culation of results, requires 25 min. By using two sets of apparatus, one may ana­lyze two samples concurrently, thereby reducing the overall time required to 15 min per determination, provided the digestion phase of one sample coincides with the aeration phase of the other.

Because CO2 absorption bulbs change weight on standing overnight or for longer periods, it is necessary to bring the bulb to constant weight by the follow­ing procedure before beginning C or blank determinations. Without being weighed, the bulb should be connected to the system, all reagents (but no soil) should be added to the digestion flask, and the apparatus should be operated as directed for sample determinations. After aeration, the bulb should be detached and weighed, and this weight should be used as the initial (constant) weight of the bulb. See "Comments" under "Medium-Temperature Resistance Furnace Meth­ods" for additional comments on care and use of CO2 absorption bulbs.

982 NELSON & SOMMERS

Blank determinations have ranged from 0.8 to 1.2 mg of CO2, for which an average value of 1.0 mg has been used. If blanks are found to be high, preaera­tion may be necessary. The system may be preaerated by placing the digestion flask (containing all materials except the digestion acid) in position for diges­tion, disconnecting the rubber tube between D and F, opening Valve A, and directing a stream of C0z-free air (about 10 bubbles/s) into C and through D for 2 min (spattering of the contents in C must be avoided). The air flow is then read­justed to about 2 bubbles/s, D is connected to F, and the analysis is performed as directed.

The H2S04 in the Trap H should be renewed at the beginning of each day's operation or more often if frothing occurs. The KI solution in Trap F has a high capacity for absorbing Cl-, and the need for its renewal is indicated by the first trace of an AgCI precipitate in Trap G.

The Nesbitt absorption bulb, when filled as described, weighs about 125 g and will absorb about 10 g of CO2, equivalent to about 100 determinations aver­aging 100 mg of CO2 each.

When the apparatus is idle overnight or for longer periods and the Nesbitt bulb is detached, the tube connecting J and K should be clamped off to prevent entrance of water vapor into the desiccant in Trap J.

A titrimetric procedure for CO2 determination is readily adaptable to the above procedure (Fig. 34-4). Replace the Nesbitt bulb with a 250-mL sidearm Erlenmeyer (filtering flask) fitted with a no. 61/2 stopper containing a 22-cm by 14-mm diam. glass tube. This bubble tower should extend to within 0.5 cm of the flask bottom and should be filled with glass beads. Through the glass tube, 25 mL of 1 M KOH should be added, and the soil sample should be oxidized as de­scribed previously. The acid-base indicator Tropaeolin 0 (e.g., Sigma Chemical Co., St. Louis, MO; Aldrich Chemical Co., Milwaukee, WI; Fisher Scientific, Pittsburgh, PA) can be added to the KOH to ensure that sufficient alkalinity remains after trapping the CO2 evolved. After oxidation, the KOH is washed from the bubble tower with distilled water, treated with 5 mL of saturated BaCl2 and several drops of phenolphthalein, and titrated with standard HC!. The data are calculated from

'" tIC Of - m4,lank - mi-sample N 0 6 .0 a ,70 - . X HCl X •

g soIl [6]

The basic principle of the above wet-combustion procedure has been used in developing tubelflask digestion methods for total C analysis. In essence, these methods involve mixing a soil sample, solid K2Cr07, and 3:2 concentrated H2S04:concentrated 85% H3P04 in a sealed vessel containing NaOH to trap CO2 evolved from oxidized organic C and solubilized carbonates. Snyder and Trofy­mow (1984) and Coughtrey et a!. (1986) describe methods using modified culture tubes and Erlenmeyer flasks, respectively. The sample containers are heated in a digestion block or on a hot plate at about 120°C for 2 h followed by diffusion of CO2 into an alkali trap for 12 h. The amount of OH- remaining is determined by titration. These methods also have the advantage of being readily adapted to determination of both 12C and 14C in the same sample. In addition, they are rapid,

CARBON AND ORGANIC MATIER 983

provide comparable data to established methods and use relatively inexpensive equipment.

ORGANIC CARBON

Introduction

Carbon is the chief element present in soil organic matter, comprising from 48 to 58% of the total weight. Therefore, organic C determinations are often used as the basis for organic matter estimates through multiplying the organic C value by a factor. For many years the Van Bemmelen factor of 1.724 was used based on the assumption that organic matter contains 58% organic C. However, a number of studies have shown that the proportion of C in soil organic matter is highly variable for a range of soils and there is no factor appropriate for all soils. If a fac­tor must be selected for converting organic C concentrations of organic matter contents, values of 1.9 and 2.5 for surface and subsoils, respectively, are most appropriate (Broadbent, 1953). The factor varies not only from soil to soil but also between horizons in the same soil. This finding suggests that it is most appropriate to determine and report the organic C in a soil rather than convert the analytically determined organic C value to organic matter content through use of an approximate correction factor.

o.rganic C may be determined by: (i) analysis of a soil for total C and inor­ganic C and subtraction of the inorganic C concentration for the total C content, (ii) a total C determination on the sample after destruction of inorganic C, and (iii) oxidation of organic C compounds by Cr20r- and subsequent determination of unreduced Cr2o.1- by oxidation-reduction titration with Fe2+ or by colorimet­ric methods. Table 34--2 summarizes the principles, advantages, and disadvan­tages of several methods for determination of organic C in soils. All current meth­ods have inherent problems associated with them, and the investigator should use the method most applicable for the soils to be analyzed and the required accura­cy of the results.

In this section, procedures are described for the determination of organic C in both calcareous and noncalcareous soils based on the difference between total C and inorganic C concentrations. Two methods also are given for organic C esti­mations based on destruction of inorganic C compounds prior to total C determi­nations. In addition, two rapid dichromate oxidation procedures are described. The Walkley and Black (1934) method that oxidizes organic C through heat-of­dilution of H2SO.4 is given because it is simple, rapid, widely used, and requires minimal equipment even though the results obtained cannot be considered quan­titative. Many soil testing and soil survey personnel have need for a method that gives an approximate organic C concentration. A tube digestion techniques (Nel­son & Sommers, 1975) that involves extensive heating of the chromic acid-soil mixture is given because it is quantitative, rapid, and represents the best combi­nation of digestion reagents, heating procedure, and titration reagents of the mod­em dichromate methods. Dichromate procedures are widely used in soil investi­gations because of their simplicity and rapidity compared with wet or dry com-

Tab

le 3

4-2.

Com

pari

son

of

met

hodo

logi

es u

sed

for

dete

rmin

atio

n o

f or

gani

c C

in s

oils

.

Met

hod

Pri

ncip

le

Adv

anta

ges

Dif

fere

nce

betw

een

tota

l C

and

ino

r­ga

nic

C

Det

erm

ined

as

tota

l C

af

ter

rem

oval

of

inor

gani

c C

D

ichr

omat

e ox

idat

ion

with

out

exte

rnal

hea

t

Dic

hrom

ate

oxid

atio

n w

ith e

xter

nal

heat

Tot

al C

and

ino

rgan

ic C

are

det

erm

ined

on

sepa

rate

sa

mpl

es:

orga

nic

C =

total

C -

inor

gani

c C

Tot

al C

is d

eter

min

ed i

n so

il sa

mpl

e af

ter

rem

oval

o

f in

orga

nic

C w

ith a

n ac

id p

retr

eatm

ent:

orga

nic

C =

tota

l C

D

ichr

omat

e ox

idiz

es o

rgan

ic C

to C

O2

in a

cid

med

ium

; am

ount

s o

f Cr2

0:r-

redu

ced

is q

uant

i­ta

tive

ly r

elat

ed t

o or

gani

c C

pre

sent

; no

t al

l or

gani

c C

in

sam

ples

is o

xidi

zed

whe

n ex

tern

al

heat

is

omit

ted,

and

a c

orre

ctio

n fa

ctor

is

requ

ired

T

his

is t

he s

ame

as t

he d

ichr

omat

e m

etho

d ab

ove

exce

pt th

at a

ll or

gani

c C

in t

he s

ampl

e is

oxi

dize

d,

and

no c

orre

ctio

n fa

ctor

is

requ

ired

Use

ful

if to

tal

C a

nd

inor

gani

c C

are

rou

­ti

nely

det

erm

ined

A

ccur

ate

if d

olom

ite

is a

bsen

t fr

om s

oil

Ver

y ra

pid

and

sim

ple,

no

spe

cial

equ

ip­

men

t ne

eded

Rap

id a

nd s

impl

e, c

om­

plet

e ox

idat

ion

of

or­

gani

c C

occ

urs

Dis

adva

ntag

es

Tw

o se

para

te a

naly

ses

are

requ

ired

, to

tal

C d

eter

min

atio

n re

quir

es s

peci

al e

quip

men

t, o

rgan

ic C

cal

cula

ted

by

diff

eren

ce h

as s

ome

inhe

rent

err

or

Not

all

dol

omit

e in

soi

l m

ay b

e re

mov

ed b

y ac

id t

reat

­m

ent,

spe

cial

ized

equ

ipm

ent

need

ed.

Inco

mpl

ete

oxid

atio

n o

f or

gani

c C

nec

essi

tate

s us

e o

f a

corr

ecti

on f

acto

rs,

whi

ch o

ften

res

ults

in

erro

neou

s va

lues

; ch

lori

de,

Fe2

+,

and

Mn0

2 in

terf

ere

wit

h m

eth­

od;

it a

ssum

es s

oil

orga

nic

C h

as a

n av

erag

e va

lenc

e o

f 0;

var

iabl

e re

cove

ry o

f C

fro

m c

arbo

nize

d m

ater

ials

C

hlor

ide,

Fe2

+, a

nd M

n02

inte

rfer

e w

ith

met

hod;

som

e sp

ecia

lize

d eq

uipm

ent

is n

eede

d; i

t as

sum

es s

oil

orga

n­ic

C h

as a

n av

erag

e va

lenc

e o

f 0;

var

iabl

e re

cove

ry o

f C

in

carb

oniz

ed m

ater

ials

! e o z Roo ~ i

CARBON AND ORGANIC MATTER 985

bustion. However, the rapid K2Cr207 methods are subject to interference by oxi­dizable or reducible soil constituents such as Cl-, Fe2+, and Mn02'

Organic Carbon as Calculated from Total Carbon Determinations

Methods previously described for total C are basic for many of the proce­dures used to determine organic C in soils. However, soils may contain both organic and inorganic C and, thus, total C analysis procedures recover both forms of C. In noncalcareous soils and soils not recently limed, the total C can be con­sidered to be organic C. With calcareous or recently limed soils, organic C may be estimated as the difference between total C and inorganic C concentrations.

Organic Carbon in Noncalcareous Soils

Prepare soil samples, and conduct a total C determination by dry or wet combustion using titrimetric, gravimetric, volumetric, infrared, or thermal con­ductivity techniques to quantitate evolved CO2 as described in "Total Carbon." Report the total C determined as percentage organic C in the sample (i.e., total C = organic C).

Organic Carbon in Calcareous Soils

Prepare soil samples, and conduct a total C determination on the sample by dry or wet combustion techniques as described in "Total Carbon." Determine inorganic C on a separate sample by one of the quantitative methods described in Chapter 15 (Loeppert & Suarez, 1996). Calculate the percentage organic C in the sample from the relationship

organic C, % = % total C - % inorganic C

Wet and Dry Combustion Techniques for Direct Measurement of Organic Carbon in Calcareous Soils

[7]

In contrast to noncalcareous soils, inorganic C must be removed from cal­careous or recently limed soils before the analysis if wet or dry combustion tech­niques are used to directly measure the organic C present.

Inorganic C is conveniently removed before wet combustion by pretreating the sample contained in a digestion flask with a mixture of dilute H2S04 and fer­rous sulfate (FeS04)' The FeS04 is added to the mixture to minimize oxidation and decarboxylation of organic matter by added H2S04 or by Mn02 present in soil (Allison, 1960). After pretreatment, the digestion flask containing soil is transferred to the combustion train, and a total C determination is carried out as described in "Total Carbon by Wet Combustion."

Inorganic C removal is generally more difficult before determination of organic C by dry combustion techniques. Treatment of soil at room temperature with sulfurous acid (H2S03) followed by heating to remove excess H2S03 is nor­mally used to decompose inorganic C compounds (Piper, 1942, p. 221-222; Bremner, 1949); however, several difficulties are apparent with the procedure.

986 NELSON & SOMMERS

Little destruction of organic matter occurs during room temperature treatment of samples with HZS03, but some decarboxylation is possible as the sample is heat­ed (Bremner, 1949). It is difficult to decide when all inorganic C has been removed and when HZS03 treatment should be discontinued. It is doubtful that dolomite is completely decomposed by the relatively mild HzS03 treatment employed (Allison, 1965). Nommik (1971) suggested that inorganic C may be effectively removed from soil samples by treatment with a metaphosphoric acid solution for 30 min at room temperature and 30 min at 130°C. However, Nom­mik's procedure has not been evaluated with a variety of soils.

Test for Presence of Inorganic Carbon

Place finely ground soil on a spot plate, and moisten with a few drops of water. Add 4 M HCI dropwise to the wetted sample, and observe any efferves­cence. Allow sufficient time for dolomite to react (-5 min). If inorganic C is absent from the soil, proceed with organic C (total C) analysis as per the section on "Total Carbon." If inorganic C is present or the test is not definitive, proceed as described below.

Pretreatment Prior to Wet Combustion

Special Apparatus6

Reagents

1. Digestion reagent for carbonates (HzS04-FeS04): Dissolve 57 mL of concentrated HzS04 and 92 g of ferrous sulfate heptahydrate (FeS04 • 7HzO) in 600 mL of deionized water, cool, and dilute to 1 L.

2. Potassium dichromate, reagent grade, pulverized. 3. Other reagents as described in "Reagents" under "Wet Combustion

Method."

Procedure

Prepare soil samples as described in "Procedures" under "Wet Combustion Method." Transfer a sample of known water content and containing 20 to 40 mg of C (but not more than 2 g of soil) to the flask used for the wet combustion appa­ratus (e.g., a 100-mL Kjeldahl digestion flask or standard taper round bottom). Using 3 mL of the HZS04-FeS04 digestion acid, wash down any soil that adheres to the neck of the flask. Place the flask in a rack or beaker, and allow the sample to digest at room temperature with occasional turning of the flask for at least 20 min or until effervescence appears to cease. Then hold the flask upright over a flame 2 cm high, and boil the contents slowly for 1.5 min to destroy any remain­ing carbonate. Rotate the flask continuously during boiling to avoid excessive frothing. Allow the sample to cool.

Insert a long-stemmed funnel into the flask, and add 2 g of pulverized KZCrZ07. Immediately connect the flask to the reflux condenser (Fig. 34-4), and

6 See the special apparatus listed in "Special Apparatus" under "Wet Combustion Methods."

CARBON AND ORGANIC MATTER 987

proceed with the determination of organic C as directed in "Procedure" under "Wet Combustion Method" beginning with the third sentence.

Report the C present in the pretreated sample as percentage organic C.

Comments

The 3 mL of 1 M (2N) HZS04-5% FeS04 used in this procedure replaces the 3 mL of distilled water used in the total C procedure described in "Procedure" under "Wet Combustion Method." Three mL of this reagent adds 3 millimoles (6 meq) W, which will neutralize 0.3 g of CaC03 (Le., 15% CaC03 in a 2-g soil sample). An appreciable excess of acidity must be present to ensure complete decomposition of carbonates. Rather than using >3 mL of the 1 M reagent for soils containing more than -10% CaC03 equivalent, it is preferable to use 3 mL of a 1.5 M or even a 2 M HZS04-5% FeS04 reagent.

Pretreatment Prior to Dry Combustion

Special Apparatus'

Reagents

1. Sulfurous acid, approximately 5%: Bubble SOz through distilled water until a saturated solution is obtained. Keep the bottle well stoppered to prevent rapid loss of SOz.

2. Sodium hydroxide (NaOH), pellets.

Procedure

Transfer a soil sample that passes through a 100- or 140-mesh (106-150 /lm) sieve (see "Comments" under "Medium-Temperature Resistance Furnace Method") and of known water content to a nonporous combustion boat that has been previously ignited and cooled. Based on an estimate of inorganic C present, treat the sample with an excess of a 5% HZS03 solution. After several hours, remove the water and excess HZS03 by leaving the boat overnight in an evacuat­ed desiccator containing NaOH pellets. Repeat the treatment until evolution ceas­es on addition of HZS03•

Proceed with the determination of organic C by one of the dry combustion methods (see "Medium-Temperature Resistance Furnace Method" or "High­Temperature Induction Furnace Method"). Report the C present in the pretreated samples as percentage organic C.

Organic Carbon in Soil Extracts

Special Apparatus

See the special apparatus listed in "Special Apparatus" under "Wet Com­bustion Method."

7 See the special apparatus listed in "Special Apparatus" under "Medium-Temperature Resistance Furnace Method" and "Special Apparatus" under "High-Temperature Induction Furnace Method."

988 NELSON & SOMMERS

Reagents

See the reagents listed in "Reagents" under "Wet Combustion Method."

Procedure

Place an aliquot of the extract (10 to 50 mL, depending on the organic C content) in a l00-mL Klejdahl digestion flask, and add 1 mL of the H2S04-FeS04 reagent. Immerse the bulb of the flask in boiling water, and direct a stream of dry, dust-free air onto the surface of the liquid in the flask. Reduce the volume of solu­tion in the flask to 3 mL or less. Add five or six glass beads and 1 g of K2Cr207 to the flask, and proceed with the determination of organic C as directed in "Pro­cedure" under "Wet Combustion Method."

Comments

Drying of extracts is best accomplished in l00-mL flasks of the Kjeldahl type. A 2-L beaker conveniently holds four flasks.

Rapid Dichromate Oxidation Techniques

Introduction and Principles

Schollenberger (1927) first proposed that the organic matter in soil may be oxidized by treatment with a hot mixture of K2Cr207 and H2S04 according to Eq. [8].

2 Cr20~- + 3 CO + 16 H+ = 4 cr3+ + 3 CO2 + 8 H20 [8]

After the reaction, the excess Cr20~- is titrated with Fe(~h(S04)2 • 6H20, and the Cr201- reduced during the reaction with soil is assumed to be equivalent to the organic C present in the sample. It must be emphasized that all methods based on determination of Cr201- remaining or cr3+ formed assume that C in soil organic matter has an average valence of zero. Although most dichromate oxida­tion procedures described since the original Schollenberger method have involved chromic acid solutions or mixtures of concentrated H2S04 and aqueous K2Cr207 solutions (Table 34-3), the use of other oxidants has been proposed. Degtjareff (1930) suggested that a mixture of H20 2 and chromic acid be used to oxidize organic matter. However, Walkley and Black (1934) conclusively estab­lished that the addition of H20 2 to chromic acid procedures gave fictitiously high values for organic C because H20 2 reduced Cr201- in acid solution. Edson and Mills (1955) suggested that organic C be oxidized by Cl2 (1 % solution) and resid­ual Cl2 determined colorimetrically by reaction with o-tolidine (C14Hl~2)' The intensity of yellow color was proportional to organic C oxidized. Others have suggested that organic C in aqueous extracts of soil can be determined by oxida­tion with a Mn(III)-pyrophosphate complex (Bartlett & Ross, 1988). The loss of color from Mn(III) is proporti~nal to the amount of organic C oxidized. Tmsley (1950) and Kalembasa and Jenkinson (1973) proposed that the chromic acid mix­ture used to oxidize organic C compounds be 3 and 1.5 M (9 and 4.5 N), respec-

Tab

le 3

4-3.

Cha

ract

eris

tics

of

dich

rom

ate

met

hods

for

det

erm

inin

g or

gani

c C

in

soil

s.

Dig

esti

on r

eage

nt c

once

ntra

tions

Met

hod

K2C

r 20

7 H

2SO

4

N

Scho

llenb

erge

r (1

927)

0.

058

18

Tyu

rin

(193

1)

0.06

6 9

Wal

kley

-Bla

ck (

1934

) 0.

055

12

Ann

e (1

945)

0.

027

11

Tin

sley

(19

50)t

0.

027

7.2

Meb

ius

(196

0)

0.04

5 10

K

alem

basa

& J

enki

nson

(19

73)

0.03

3 9

Nel

son

& S

omm

ers

(197

5)

0.06

6 10

.8

Mod

ifie

d M

ebiu

s:j:

0.03

3 10

.8

Hea

nes

(198

4)

0.05

5 12

Y

eom

ans

& B

rem

ner

(198

8)

0.06

6 10

.8

Cia

vatta

et

al.

(198

98)

0.14

5 10

.2

Soon

& A

bbou

d (1

991)

0.

066

10.8

t R

eage

nts

used

by

Bre

mne

r an

d Je

nkin

son

(196

0a).

:j:

As

desc

ribe

d by

Nel

son

and

Som

mer

s (1

982)

.

H3P

04

3 1.67

Rat

io o

f H

2O

/aci

d

v:v

1.00

0.

50

0.46

0.

67

0.42

0.

067

0.67

0.

67

0.50

0.

67

0.77

0.

67

Dig

esti

on c

ondi

tion

s

Tub

e he

ated

by

flam

e at

175

°C f

or 9

0 s

Fla

sk w

ith

funn

el b

oile

d at

140

°C f

or 5

min

F

lask

with

no

exte

rnal

hea

t, m

ax.

tem

p is

120

°C

Fla

sk w

ith c

onde

nsor

ref

luxe

d at

178

°C f

or 5

min

F

lask

wit

h co

nden

ser

refl

uxed

for

2 h

at

150°

C

Fla

sk w

ith c

onde

nsor

ref

luxe

d fo

r 30

min

at

159°

C

Fla

sk w

ith c

onde

nser

ref

luxe

d fo

r 20

min

at

165°

C

Tub

e he

ated

in

bloc

k at

150

°C f

or 3

0 m

in

Fla

sk w

ith

cond

ense

r re

flux

ed a

t 15

0°C

for

30

min

T

ube

heat

ed i

n bl

ock

at 1

35°C

for

30

min

T

ube

heat

ed i

n bl

ock

at 1

70°C

for

30

min

S

peci

al f

lask

hea

ted

over

fla

me

at 1

160°

C f

or 1

0 m

in

Tub

e he

ated

in

bloc

k at

155

°C f

or 3

0 m

in

Rep

orte

d pr

ecis

ion

CV

,%

1.4-

1.9

8.5

1.6-

4.2

1.3

0.8-

3.1

1.2-

1.8

0.8

3.5

1.0-

3.6

4.1

1.0-

4.4

5.4

2.7

~ o z ~ I::' o ~ (=) ~ tol ::c !

990 NELSON & SOMMERS

tively, with respect to H3P04 (Table 34-3). There is no evidence, however, to sug­gest that oxidation mixtures containing H3P04 are more efficient in oxidizing organic matter than K2Cr20TH2S04 mixtures. The oxidizing mixtures used in most published methods are between 0.0267 and 0.0583 M (0.16 and 0.35 N) in K2Cr207 and 7.5 and 12.5 M (15 and 25 N) in H2S04 (Table 34-3). However, Tyurin (1931) Tinsley (1950), Nelson and Sommers (1975) and Yeomans and Bremner (1988) used an aqueous H2S04-water mixture that was 0.145 M (0.87 N) in K2Cr207. Schollenberger (1927) used concentrated H2S04 (-18 M or -36 N) as the solvent for K2Cr207.

Rapid dichromate oxidation techniques have employed heating times and temperatures that vary from no external heat to extensive boiling of chromic acid mixtures. Schollenberger (1927) suggested that the soil-H2S04-K2Cr207 mixture be heated in a Pyrex test tube over a flame until the solution temperature reached 175°C at which time heating was discontinued. Later investigators realized that the time and temperature of heating were critical and must be standardized to insure that a constant proportion of soil organic matter was oxidized and that a consistent amount of dichromate was thermally decomposed during the digestion. Degtjareff (1930), Tyurin (1931), Schollenberger (1945) and Jackson (1958) sug­gested that the soil-chromic acid mixtures be heated for defined periods (5-10 min) in test tubes submerged in H2S04 or oil baths maintained at prescribed tem­peratures (140-170°C).

Walkley and Black (1934), however, proposed that the heat of dilution of H2S04 (120°C) was satisfactory for oxidizing 75% of the organic C in soils and that a correction factor could be used to account for incomplete digestion. Sever­al investigators have found that an extended period of heating is required to obtain quantitative oxidation of soil organic C by chromic acid (Anne, 1945; Tinsley, 1950; Mebius, 1960; Kalembasa & Jenkinson, 1973; Heanes, 1984). High digestion temperatures (>145°C) lead to thermal decomposition of dichro­mate and resultant high blank values (Tinsley, 1950; Metson et aI., 1979; Heanes, 1984). Clay minerals have been reported to catalyze the thermal decomposition of Cr20,y- (Walkley, 1947) but a recent study has shown little thermal decompo­sition when high clay soils free of organic matter were heated with K2Cr20T H2S04 for 60 min at 125 or 145°C (Heanes, 1984). Digestion temperature is nor­mally regulated by the ratio of water/H2S04 in the mixture (Table 34-3) and the temperature rises as water vapor is lost during heating. Tinsley proposed that cold finger condensers fitted on Erlenmeyer flasks be used to prevent loss of water during digestion, whereas other investigators have used Erlenmeyer flasks fitted with Liebig condensers. Heating times employed in reflux methods have varied from 20 min to 2 h. Ciavatta et ai. (1989) have recently proposed that a chromic acid mixture successfully oxidizes soil organic C when samples are heated in a 200 mL narrow-necked digestion flask by direct flame at 160°C for 10 min. The neck of the flask serves to reflux the digestion reagents.

Nelson and Sommers (1975) proposed that organic C could be determined by heating soil-chromic acid mixtures under reflux in 50 mL Folin-Wu nonpro­tein nitrogen tubes placed in an aluminum block on a hot plate. Heating time and digestion temperature recommended were 30 min and 150°C, respectively. Sub­sequently, a number of other similar tube digestion methods have been proposed

CARBON AND ORGANIC MATIER 991

for estimation of organic C all employing 100-mL tubes. Heanes (1984), Yeo­mans and Bremner (1988), and Soon and Abboud (1991) recommended heating for 30 min at 135°C, 170°C, and 155°C, respectively. Yeomans and Bremner (1988) and Soon and Abboud (1991) specify the same digestion reagents as Nel­son and Sommers (1975), i.e., 5 mL of 0.167 M K2Cr207 and 7.5 mL of concen­trated H2S04, whereas Heanes (1984) recommends 10 mL of 0.167 M K2Cr207 and 20 mL of H2S04, Tube digestion procedures have been reported to yield organic C values that approximate those from dry and wet combustion tech­niques.

Diphenylamine was the first oxidation-reduction indicator used for the titration of excess Cr20j- with Fe2+ (Schollenberger, 1927, 1931, 1945; Allison, 1935). Later studies suggested that the diphenylamine end point could be improved by addition of H3P04, NaF, or HF before titration (Schollenberger, 1931, 1945; Walkley & Black, 1934), and these substances were widely used in dichromate titrations. Peech et al. (1947) established that barium diphenylamine sulfonate (diphenyl-4-sulfonic acid) in combination with H3P04 was as effective and more stable compared with diphenylamine (C12HllN) and has been used as an indicator in other procedures (Tinsley, 1950). Jackson (1958) recommended that o-phenanthroline (C12HgN2) be used as an indicator in Cr20j- titrations because the color change (formation of the complex with Fe2+) occurs at higher oxidation-reduction potential compared with diphenylamine. A mixture of 0-

phenanthroline and H3P04 is normally used to give a good end point; however, the indicator has been successfully used without H3P04 addition. A problem with o-phenanthroline is that the indicator tends to be absorbed by some suspended soil materials, thereby obscuring the color change at the end point. therefore, the diluted chromic acid-soil mixture is often passed through an acid fast filter paper on Buchner funnel before titration. Simakov (1957) proposed that N-phenylan­thranilic acid (C13HnHO) be used as an indicator in Cr20j- titrations with Fe2+.

Mebius (1960) confirmed that N-phenylanthranilic acid gives a very sharp and clean end point and this compound is currently the indicator of choice for Cr20j­titrations.

Other methods of titration not involving oxidation-reduction indicators have been used to estimate reacted Cr20j-. One approach is to add a slight excess of Fe2+ to the Cr20j--H2S04-Soil mixture and then back-titrate the Fe2+ with KMn04 (Smith & Weldon, 1941). In this titration procedure, the only reagent that requires standardization is KMn04 if the same amounts of Fe2+ and Cr20j- are added to both samples and blanks. The end point in the titration of Cr20j- with Fe2+ also may be estimated very accurately by monitoring the oxidation-reduc­tion potential with platinum and calomel electrodes attached to a potentiometer (Raveh & Avnimelech, 1973). The end point of the titration involves a potential change of -400 m V with 0.02 mL of titrant.

The amount of Cr20j- remaining after reaction with soil organic matter also may be estimated by colorimetry after removal of soil by filtration or cen­trifugation (Carolan, 1948). Perrier and Kellogg (1960) proposed that any possi­ble interference of cr3+ in Cr20j- determination be eliminated by dilution and subsequent reaction of excess dichromate with s-diphenylcarbazide (C13H14N40) to yield a violet colored complex with an absorption maxima at 540 nm. Con-

NELSON & SOMMERS

versely, colorimetry has been widely used to determine the amounts of cr3+ formed from the reaction of Cr20.y- with soil (Wilde, 1942; Graham, 1948; Car­olan, 1948; Datta et aI., 19862; Sinha & Prasad, 1970; Sims & Haby, 1971; DeBolt, 1974; Gupta et aI., 1975; Baker, 1976; Heanes, 1984). The green color due to cr3+ is normally quantitated at wavelengths of 590 to 625 nm and the absorbance is usually related to organic matter concentrations in soil by a stan­dard curve prepared from sucrose (Graham, 1948; Sims & Haby, 1971; DeBolt, 1974; Heanes, 1984). Baker (1976) used a probe colorimeter to measure cr3+ absorbance directly in the reaction vessel after centrifugation, thereby avoiding a transfer into a spectrophotometer cuvette. From a comparison of the methods that quantitate Cr20.y- and those that determine cr3+, Metson (1965) concluded that measurement of cr3+ is the preferred procedure.

Dichromate methods that use heat of dilution or minimal heating do not give complete oxidation of organic compounds in soil although the most active forms of organic C are converted to CO2, Walkley and Black (1934) found that on the average about 76% of the organic C in 20 soils was recovered by the heat of dilution procedure, and they proposed that a correction factor of 1.32 be used to account for unrecovered organic C. However, the actual recoveries of organic C from the soils tested varied from 60 to 86%. Schollenberger (1945) reported that the Walkley and Black procedure oxidized an average of 79% (range 70-86%) of organic C in soils he studied. Allison (1960) reviewed available information on the recovery of organic C in a wide variety of soils by the Walk­ley and Black procedure and showed that the average recovery with different groups of soils varied from 63 to 86% and that the correction factor varied from 1.16 to 1.59. Table 34-4 gives data on the correction factor found to be required for the Walkley and Black procedure in investigations carried out during the past 30 yr. Recoveries of organic C by the Walkley and Black technique were highly variable, and the correction factor appropriate for individual soils varied from 1.0 to 2.86. The average correction factor appropriate for a group of soils varied from 1.03 to 1.41. This data clearly show that Cr20~--H2S04 methods that involve minimal heating give variable recovery of organic C from soils. An average cor­rection factor found for a group of soils may be applicable to the "average" soil in the group but will give erroneous values for many soils in the group. There­fore, procedures such as the Walkley and Black should be considered to give approximate or semi-quantitative estimates of organic C in soil because of the lack of an appropriate correction factor for each soil analyzed. If an experimen­tally determined correction factor is not available for a particular groups of soils, the use of 1.3 as the factor appears most reasonable over a range of soils. Meth­ods that involve extensive heating, such as those of Tinsley (1950), Mebius (1960), Nelson and Sommers (1975), Heanes (1964) or Yeomans and Bremner (1988) do not require a correction factor because all of the organic C in the soil is oxidized to CO2. However, methods that involve minimal heating (e.g., Schol­lenberger, 1927; Tyurin, 1931) require a small correction factor (e.g., 1.15) to account for unoxidized organic C.

The rapid dichromate methods are subject to interferences by certain soil constituents that lead to spurious results with some soils (Walkley, 1947). Chlo­ride, ferrous iron and higher oxides of Mn have been shown to undergo oxida-

CARBON AND ORGANIC MAllER 993

Table 34-4. Correction factors for organic C in surface soils not recovered by the Walkley-Black method.

Number Organic C recovery, % Average Origin of soils correction

Reference of samples studies Range Average factor

Tinsley (1950) England 10 77-92 83.6 1.20 Bremner & Jenkinson (1960a) England 15 27-92 84 1.19 Kalembesa & Kenkinson (1973) England & Wales 22 46--80 77 1.30 Orphanos (1973) Cyprus 12 69-79 75' 1.30 Richter et at. (1973) Argentina 12 79-87 83 1.20 Nelson & Sommers (1975) Indiana 10 44-88 79 1.27 Bornemisza et at. (1979) Costa Rica 50 69-81 75 1.33 Rhodes et at. (1981) Sierra Leone 10 93-100 97 1.03 Richardson & Bigler (1982) North Dakota 21 35-91 88 1.33t

1.13* Heanes (1984) Australia 12 85-98 92 1.09 Amacher et at. (1986) Louisiana 179 46--87 71 1.41 Gillman et at. (1986) Queensland 450 65-95 76§ 1.32

8111 1.24 Willet & Beech (1987) Australia 30 60-144 85 1.18 Lowther et at. (1990) Australia 38 74-102 88 1.14 Soon & Abboud (1991) Alberta 39 62-87 71.4 1.40

t Low C samples. * Other samples. § Soils derived from basalt, alluvium, or beach sand. 'II Soils derived from granite or metamorphic rocks.

tion-reduction reactions in chromic acid mixtures leading to incorrect values for organic C. The presence of significant amounts of Fe2+ or Cl- in soil will lead to a positive error, whereas reactive Mn02 in soil samples will result in a negative error and low values for organic C.

Chloride interferes with dichromate methods through the formation of chromyl chloride, as indicated in Eq. [3], which results in consumption of Cr20?-. Chloride interference may be eliminated by washing the soil free of Cl­before analysis or by precipitating the Cl- as AgCl by addition of Ag2S04 to the digestion acid (Walkley, 1947; Quinn & Salomon, 1964; Gupta et aI., 1975). Alternatively, Walkley (1947) found that Eq. [9] may be used to correct organic C values for soils having CUC ratio of ~5.1

organic C in soil (%) = (apparent % C in soil) - (% CU12) [9]

It has recently been reported that Ag2S04 addition did not eliminate Cl­interference in a low temperature tube digestion method and that an assay for Cl­coupled with stoichiometric correction for chromyl chloride loss is necessary for accurate estimates of organic C (Heanes, 1984).

When present in soil, Fe2+ will be oxidized to Fe3+ by Cr20?-, as indicated in Eq. [10], resulting in a positive error in the analysis, i.e., giving high values for organic C content

[10]

994 NELSON & SOMMERS

Appreciable Fe2+ may be present in highly reduced soils, and errors may result when dichromate methods are applied to undried samples of anaerobic soils before drying (Lee, 1939). However, Walkley (1947) found that thorough air-dry­ing of reduced soils before analysis resulted in oxidation of Fe2+ to Fe3+ and accu­rate determination of the organic C present. The amounts of Fe2+ present in well­aerated soils are so small relative to the amounts of organic C present that no detectable interference is likely. Pyrite also is oxidized during treatment of soils with dichromate and samples containing pyrite sulfur concentrations >0.29% result in significant over estimation of organic C by the Walkley and Black method (Willett & Beech, 1987). Metallic iron (FeO) present in soil samples also may lead to positive interferences in dichromate methods, (Allison, 1935; Heanes, 1984). Therefore, care should be taken to ensure that soils are not ground with iron or steel equipment before analysis. The higher oxides of Mn (largely Mn02) compete with Cr20j- for oxidizable substances when heated in an acid medium according to Eq. [11].

[11]

Therefore, any reactive Mn02 present will give a negative error when soils are analyzed by dichromate techniques. Although soils contain substantial amounts of Mn02 and other higher oxides of Mn, Walkley (1947) and Heanes (1984) con­cluded that in most soils the quantity of reactive (reducible) oxides ofMn is small because only the freshly precipitated Mn02 will take part in redox reactions. Even in highly manganiferous soils, only a small fraction of the Mn02 present is able to compete with Cr20j- for oxidation of organic C compounds. Therefore, interference from Mn02 is not thought to be a serious error in the vast majority of soils. In soils with large amounts of reactive Mn02' Walkley (1947) suggested pretreatment of samples with the exact amount of FeS04 necessary to reduce the amount of reactive Mn2 present prior to treatment with K2Cr207 and H2S04,

Other problems associated with dichromate methods involve assumptions about the average oxidation state of organic C in soils (i.e., equivalent weight of C) and recovery of highly reduced forms of organic C from soils. All dichromate methods assume that the organic C in soil has an average oxidation state of zero and an equivalent weight of 3 g per equivalent when reacted with dichromate according to Eq. [8] even though no studies have been conducted to evaluate this assumption. However, the fact that dichromate methods using extensive heating give organic C values similar to those obtained with wet or dry combustion where CO2 is determined directly suggests that this assumption is reasonably correct.

Dichromate methods that involve little or no external heating give very poor recovery of organic C present in carbonized materials (e.g., charcoal, graphite, coal, coke, and soot). For example, Walkley (1947) found that the Walk­ley and Black method recovered only 2 to 11 % of the organic C present in such materials. In a detailed study, Bremner and Jenkinson (1960b) found that the Walkley and Black method gave low recovery (0-57%) of organic C from car­bonized materials, whereas methods involving external heat such as those of Tinsley gave substantial (64-104%) and variable recovery or organic C from such materials. Conversely, Heanes (1984) found that very little organic C in charcoal

CARBON AND ORGANIC MATTER 995

or coke was oxidized by a tube digestion procedure at 135°C. Other investigators have found that the Walkley and Black procedure completely recovers C in weathered coal seams, i.e., coal "blooms" (Kalisz & Sainju, 1991). These con­flicting results suggest that recovery of organic C from carbonized materials is highly dependent upon the characteristics of the materials and digestion condi­tions (i.e., temperature, reagent concentrations). It is appropriate to conclude that dichromate methods cannot be used to quantitatively recover carbonized materi­als from soils or to discriminate between C in carbonized materials and C in soil organic matter because organic C recovery varies with type of carbonized mate­rial and time and temperature of heating of the chromic acid mixture. Therefore, unreliable results for organic C will be obtained if dichromate methods are applied to soils containing significant amounts of carbonized materials. Dry com­bustion methods are most appropriate for soils containing large amounts of ele­mental C.

Walkley-Black Method

The Walkley-Black Method was described by Walkley (1946), Peech et al. (1947) and Greweling and Peech (1960).

Reagents

1. Potassium dichromate, 0.167 M (1 N): Dissolve 49.04 g of reagent­grade K2Cr207 (dried at 105°C) in water, and dilute the resclution to a volume of 1000 mL.

2. Sulfuric acid, concentrated (not less than 96%): If Cl- is present in soil, add AgzS04 to the acid at the rate of 15 g per liter.

3. Phosphoric acid, concentrated. 4. o-Phenanthroline-ferrous complex, 0.025 M: Dissolve 14.85 g of 0-

phenanthroline monohydrate and 6.95 g of ferrous sulfate heptahydrate (FeS04· 7H20) in water. Dilute the solution to a volume of 1000 mL. The o-phenanthroline-ferrous complex is available under the name of Ferroin from the G. Frederick Smith Chemical Co. (Columbus, OH).

5. Barium diphenylamine sulfonate: Prepare a 0.16% aqueous solution. This reagent is an optional substitute for no. 4.

6. Ferrous sulfate heptahydrate (FeS04 • 7HzO) solution, 0.5 M (0.5 N): Dissolve 140 g of reagent-grade FeS04 • 7HzO in water, add 15 mL of concentrated sulfuric acid, cool the solution, and dilute it to a volume of 1000 mL> Standardize this reagent daily by titrating it against 10 mL of 0.167 M (1 N) potassium dichromate, as described below.

Procedure

Grind the soil to pass through a 0.5-mm sieve, avoiding iron or steel mor­tars. Transfer a weighed sample, containing 10 to 25 mg of organic C, but not in excess of 10 g of soil, into a 500-mL wide-mouth Erlenmeyer flask. Add 10 mL of 0.167 M (1 N) KZCrZ07, and swirl the flask gently to disperse the soil in the solution. Then rapidly add 20 mL to concentrated HZS04, directing the stream into the suspension. Immediately swirl the flask gently until soil and reagents are mixed, then more vigorously for a total of 1 min. Allow the flask to stand on an

NELSON & SOMMERS

insulated sheet for about 30 min. Then add 200 mL of water to the flask, and fIl­ter the suspension using an acid resistant fIlter paper (e.g., Whatman 540), if experience shows that the end point of the titration cannot otherwise be clearly discerned. Add three to four drops of o-phenanthroline indicator and titrate the solution with 0.5 M (0.5 N) FeS04. As the end point is approached, the solution takes on a greenish cast and then changes to a dark green. At this point, add the ferrous sulfate heptahydrate drop by drop until the color changes sharply from blue to red (maroon color in reflected light against a white background). Make a blank determination in the same manner, but without soil, to standardize the K2Cr207. Repeat the determination with less soil if >75% of the dichromate is reduced.

Calculate the results according to the following formula, using a correction factor ''/' = 1.30 or a more suitable value found experimentally

...:..(m_4.....;;.;;::18=nk,--_m_Ls...;;;;8=m=ple::.:...)...;.(M.......;;..;Fe_2+.:....) ..:....(0_.0_0-'3)'-('-lO_0....:.,.) x f Organic C, % = -wt. • water-free soil, g

[12]

Comments

The coefficient of variation for the Walkley-Black procedure has been reported to vary between 1.6 and 4.2% (Table 34-3). Ferrous ammonium sulfate also is a suitable titrant for excess Cr20?- in conjunction with the Walkley-Black method. The Smith and Weldon (1941) modification involving complete reduc­tion of Cr20:r- with Fe2+, and subsequent back-titration of excess Fe2+ with Mn04" solution also may be used to estimate unreacted Cr20:r-. Other oxidation­reduction indicators that have provided satisfactory results include barium diphenylamine sulfonate and N-phenylanthranilic acid. The amounts of Cr20?­reduced to Cr3+ by reaction with soil organic matter also may be estimated col­orimetrically or by potentiometric titration with a ferrous ammonium sulfate solu­tion. Grinding samples to <0.2 mm has been shown to reduce sampling errors and the coefficient of variation even when relatively large sample sizes (1 g) are used (Metson et aI., 1979). Heanes (1984) reported that reduction in particle size from 0.5 to 0.15 mm significantly increased recovery of organic C in 12 soils.

Thbe Digestion Method

Special Apparatus

1. Pyrex digestion tubes (lOO mL) sized for block digestor. 2. Block digestor: 40-tube Kjeldahl block digestor supplied by Technicon

Instruments Corp., Tarrytown, NY, or Tecator Inc., Herndon, VA, or equivalent.

Reagents

1. Potassium dichromate solution, 0.167 M (1.0 N)-dissolve 49.025 of K2Cr207 (dried at 140°C) iIi 800 mL of distilled water and dilute the solution with water to a volume of lOoo mL in a volumetric flask. This is the primary standard for the procedure.

CARBON AND ORGANIC MATIER 997

2. Concentrated sulfuric acid-specific gravity 1.84. 3. Ferrous ammonium sulfate solution 0.2 M (0.2 N)-Dissolve 156.8 g

of ferrous ammonium sulfate [Fe(NH4)z(S04)2 • 6H20] in 100 mL of concentrated sulfuric acid and dilute the solution with water to a vol­ume of 2 L in a volumetric flask. This solution must be standardized daily because it undergoes slow oxidation.

4. Indicator solution-Dissolve 0.1 g of N-phenylanthranilic acid and 0.1 g of Na2C03 in 100 mL of distilled water.

Procedure

Weigh an amount of soil air dried and ground to <0.15 mm containing not greater than 8 mg of organic C (usually 100--500 mg) into a clean, dry digestion tube and add 5 mL of 0.167 M (1.0 N) K2Cr207 solution and 7.5 mL of concen­trated H2S04. Place the tube in the digestion block preheated to 150°C for exact­ly 30 min. Remove the digestion tube from the block and allow the samples to cool for 30 min at room temperature. Quantitatively transfer the contents of the tube to a 125-mL Erlenmeyer flask and titrate the sample with 0.2 M (0.2 N) fer­rous ammonium sulfate solution using 0.2 mL of the N-phenylanthranillic acid solution as the indicator. The color change at the end point is from violet to bright green and is very rapid. An illuminated background is recommended for ease in observing the end point and the titration should be performed using a 25-mL burette calibrated at O.l-mL intervals and a variable speed magnetic stirrer and teflon coated stiffing bar.

Each set of soil samples should be analyzed with two unheated reagent blanks and two reagent blanks that are heated at the same time as the samples. The unheated blanks are used to standardize the ferrous ammonium sulfate solu­tion. The difference in titration values between heated and unheated blanks is used to correct all sample titration values for the amount of dichromate consumed by thermal decomposition during the heating process.

Computation of the organic C content of soil is performed as follows: (i) subtract sample titration values (mLsoi/) from the average titration value of the heated (boiled) blank (mL bb), (ii) correct the resulting [mLbb - mL,.oil] value for thermal decomposition of dichromate by dividing the difference in average titra­tion value for unheated and heated blanks by the average titration value for the unheated blank, multiplying the correction factor (normally 0.04-0.08) by the [mLt,b - mL,.oil] value, and adding the product to the [mLt,b - mLsoi1] value (Eq. [13]). The resulting value, labeled "A" is proportional to the amount of organic C present in the soil, (iii) complete the calculation of organic C content using Eq. [14]

where ub is unboiled blank and bb is boiled blank.

( ,A....:.-) -,-(M---,F=e2~+ )-'..(0_.0_O--,3)-O.( 1_0---"-.0) Organic C, % = -

wt. water-free soil, g [14]

998 NELSON & SOMMERS

Comments

The coefficient of variation for the method has been reported as 3.5% (Nel­son & Sommers, 1975). Coefficients of variation reported for other tube digestion methods have ranged from 1.1 to 4.4% (Heanes, 1984; Yeomans & Bremner, 1988; Soon & Abboud, 1991). The precision of the method can be improved by using a computer-aided automatic titration system (Yeomans & Bremner, 1988). Colorimetric analysis of cr3+ also can be used to estimate the amount of dichro­mate that has reacted with organic C during tube digestion (Heanes, 1984; Soon & Abboud, 1991). The potassium dichromate solution is the primary standard for the method and care should be taken in its preparation. This solution is quite sta­ble and may be stored at room temperature indefinitely. The ferrous ammonium sulfate solution oxidizes slowly and thus must be standardized each time it is used. Small particle size reduces the sampling error and increases recovery of organic C. Heanes (1984) found that organic C values increased by about 2% as particle size was reduced from 0.5 to 0.15 mm.

Thermal decomposition of dichromate occurs at temperatures exceeding 136°C (Heanes, 1984) and the degree of decomposition is quite dependent upon the heating conditions. Therefore, it is recommended that the digestion tubes by dry before use to eliminate differences in acid/water ratio and that the heating temperature and time be accurately controlled. A variety of temperatures varying from 135 to 170°C have been recommended for tube digestion methods (Table 34-3). When thermal decomposition of dichromate is accurately taken into account with a heated blank, the four tube digestion methods have quantitatively determined organic C in a variety of soils.

Interferences present in the Walkley-Black procedure also are a problem with tube digestion methods. As a result of extensive heating, the tube digestion methods give complete recovery of organic C from soils and, thus, do not require a factor to account for incomplete oxidation of organic matter. Heanes (1984) reported that little organic C in charcoal and coke was recovered by the tube digestion procedure that he described.

The tube digestion technique can be used to estimate organic C in soil extracts by carrying out the digestion with 1 or 2 mL of extract and 4 or 3 mL of dichromate solution, respectively. It is essential that the acid/water ratio be main­tained at 1.5 in the digest so the volume of dichromate solution must be reduced as the volume of extract is increased. Both heated and unheated blanks should be prepared using the same volume of blank extracting solution and the dichromate solution as that employed for the extracts.

The modified Mebius method described by Nelson and Sommers (1982) is recommended as an accurate and precise dichromate oxidation procedure for those investigators not having access to a block digestor. The major advantage of the tube digestion procedure is the decreased analysis time per sample because of the relatively large number of samples (40) that can be heated at one time.

Comparison of Methods for Determining Organic Carbon

Most studies have shown that very good agreement is obtained when wet combustion, dry combustion, and Van Slyke-Folch (1940) methods are used to

CARBON AND ORGANIC MATTER 999

determine organic C in soils (Bremner & Jenkinson, 1960a; Kalembasa & Jenk­inson, 1973; Nelson & Sommers, 1975). For this reason, wet and dry combustion methods are normally considered to yield absolute values for organic C in soils and other methods are calibrated against the combustion procedures.

A number of studies have been conducted to compare rapid dichromate oxi­dation methods with dry or wet combustion methods (Table 34-5). Many studies have shown that the Walkley-Black method yields variable recovery of organic C from soil, i.e., in some soils >95% of organic C may be oxidized but in other soils <60% of the organic C is converted to CO2• For example, Bremner and Jenkin­son (1960a) showed that for 15 soils the recovery of organic C by the Walkley­Black method (using a correction factor of 1.3) varied from 73 to 119% of wet combustion values. Kalembasa and Jenkinson (1973) observed that with 22 soils the recovery of organic C by the Walkley-Black procedures with correction var­ied from 60 to 122% of wet combustion values although the average recovery by the Walkley-Black method was 102%. Nelson and Sommers (1975) found that the Walkley-Black method with correction gave organic C values for 10 soils that varied from 57 to 114% (average of 102%) of those obtained with wet combus­tion. More recent studies have found that organic C in soils from Sierra Leone and Australia is much more susceptible to oxidation by dichromate in the Walk­ley-Black procedure than was expected, e.g., average uncorrected recoveries of organic C varied from 88 to 97% of wet or dry combustion (Rhodes et ai., 1981; Heanes, 1984; Lowther et ai., 1990). The Walkley-Black method employing a correction factor for unoxidized organic C is not highly accurate for an individ­ual soil, but for a group of soils the average recovery is good when compared with organic C values determined by dry or wet combustion. This is particularly true when the appropriate correction factor has been determined for the group of soils under study. Furthermore, the simplicity and rapidity of the Walkley-Black meth­od, in part, compensate for the lack of accuracy inherent in the procedure.

Dichromate methods that involve limited periods of heating (Schollenberg­er, 1927; Tyurin, 1931) have been shown to yield good recoveries of organic C if appropriate correction factors are applied (Allison, 1935; Crowther, 1935; Smith & Weldon, 1941; Kalembasa & Jenkinson, 1973). The usual correction factors are 1.15 and 1.08 for the Schollenberger and Tyurin methods, respectively. Cia­vatta et al. (1989) found that no correction factor was needed when a modified Schollenberger procedure was used with heating at 160°C for 10 min.

Modem dichromate oxidation methods that involve an extended period of heating, often under reflux, (e.g., Anne, 1945; Tinsley, 1950; Mebius, 1960; Nel­son & Sommers, 1975; Heanes, 1984; Yeomans & Bremner, 1988) give organic C values equivalent to those obtained by dry or wet combustion (Table 34-5). Bremner and Jenkinson (1960a) found that the Tinsley method gave organic C values of 15 soils that varied from 88 to 106% (avg. 101%) of wet combustion values. Kalembasa and Jenkinson (1973) showed that the Tinsley and Mebius methods yielded organic C recoveries from 22 soils that averaged 95 and 94%, respectively, of wet and dry combustion values. Nelson and Sommers (1975) reported that the Mebius procedure recovered from 92 to 110% (avg. 103%) of the organic C found in 10 soils by the wet combustion method. Tube digestion methods have given excellent average recoveries of organic C in soils [Nelson &

Tab

le 3

4-5.

Com

pari

son

of

met

hods

for

det

erm

inin

g or

gani

c C

in s

oils

.

No.

of

Dry

W

et

Wal

kley

&

Mod

ifie

d R

efer

ence

sa

mpl

es

com

bust

ion

com

bust

ion

Sch

olle

nber

gert

B

lack

:j:

Tin

sley

II§

M

ebiu

s

-A

vg.

mg

C/k

g so

il -

Avg

. %

of

wet

or

dry

com

bust

ion

All

ison

(19

35)

71

12.5

C

row

ther

(19

35)

8 37

.6

Sm

ith

& W

eldo

n (1

941)

69

16

.5

Bre

mne

r &

Jen

kins

on (

1960

a)

15

114.

9 N

elso

n &

Som

mer

s (1

975)

10

35

.0

Kal

emba

sa &

Jen

kins

on (

1973

) 22

25

.6

25.6

M

etso

n et

aJ.

(197

9)

7 23

8.7

235.

9 R

hode

s et

aJ.

(198

1)

10

26.0

H

eane

s (1

984)

12

19

.6

Yeo

man

s &

Bre

mne

r (1

988)

15

40

.4

Cia

vatt

a et

aJ.

(198

9)

2 21

.8

Low

ther

et

aJ. (

1990

) 38

12

.4

Soo

n &

Abb

oud

(199

1)

12

21.1

t V

alue

s co

rrec

ted

for

inco

mpl

ete

oxid

atio

n o

f or

gani

c C

usi

ng a

fac

tor

of

1.15

. :j:

Val

ues

corr

ecte

d fo

r in

com

plet

e ox

idat

ion

of

orga

nic

C u

sing

a f

acto

r o

f 1.

30.

§ R

eage

nt c

once

ntra

tion

s as

des

crib

ed b

y B

rem

ner

and

Jenk

inso

n (1

960a

).

'V V

alue

s co

rrec

ted

to i

ncom

plet

e ox

idat

ion

of

orga

nic

C w

ith

a fa

ctor

of

1.08

.

100

100

100

96

101

109

94

102

104

95

109

125

120

99#

92

115 95

tt

96:j:

:j:

# M

etho

d w

as s

imil

ar to

Sch

olle

nber

ger

exce

pt d

iges

tion

rea

gent

con

cent

rati

ons

diff

ered

and

sam

ples

wer

e he

ated

for

10

min

. tt

Dig

esti

on c

arri

ed o

ut i

n 10

0 m

L P

yrex

tub

e.

:j::j:

Dig

esti

on c

arri

ed o

ut i

n 10

0 m

L P

yrex

tub

es i

n he

atin

g bl

ock.

103 94

98

84

Tub

e di

gest

ion

100 99

100 99

99

97

Tyu

rin.

105

~ z ~ Z

Ro

CIl o :: ~

CARBON AND ORGANIC MATIER 1001

Sommers (1975),99%; Heanes (1984),100%, Yeomans & Bremner (1988),99%; Soon & Abboud (1991), 97%].

ORGANIC MATTER

Introduction

The organic matter content influences many soil properties, including (i) the capacity of a soil to supply N, P, and S and trace metals to plans; (ii) infiltra­tion and retention of water, (iii) degree of aggregation and overall structure that affect air and water relationships; (iv) cation exchange capacity; (v) soil color, which in turn affects temperature relationships; and (vi) adsorption or deactiva­tion (or both) of agricultural chemicals. Determination of organic matter content is a routine procedure carried out in soil analysis and testing laboratories through­out the world because of the importance of organic matter in supplying plant available N and deactivating pesticides. However, no completely satisfactory method exists for determination of the matter content of soils. The ignition method described below is a modification of that presented by Ben-Dor and Banin (1959). This method provides a reasonable estimate of organic matter con­centrations in soils but cannot be considered quantitative.

Calculation of Organic Matter Content

The organic matter content of soil may be indirectly estimated through mul­tiplication of the organic C concentration (as determined by procedures outlined in "Organic Carbon") by the ratio of organic matter to organic C commonly found in soils. The organic matter content is normally arrived at by multiplying the organic C concentration by 1.724. However, a number of studies have suggested that this factor is too low for many soils, and consequently the organic matter content is underestimated. For example, Robinson et al. (1929) and Lunt (1931) showed that the correct factor for peats is 1.86 to 1.89. In summarizing much of the early work, Broadbent (1953) concluded that conversion factors of 1.9 and 2.5 would be appropriate for surface soils and subsoils, respectively. Other work­ers have found that factors of 1.9 to 2.0 were satisfactory for surface layers of mineral soils (De Leenheer et aI., 1957; Howard, 1965; Ponomareva & Platniko­va, 1967; Christensen & Malmros, 1982). Loftus (1966) reported that the appro­priate factors for mineral and organic soils in Pennsylvania were 2.2 and 1.8, respectively, whereas, Ranney (1969) found that the organic matter content of Pennsylvania surface soils may be accurately estimated by the equation

organic matter, % = .35 + 1.80 x % organic C [15]

It is evident that estimation of organic matter content from organic C con­centrations is not highly accurate, because the organic C content of organic mat­ter is variable from soil to soil and with depth in the profile. Accurate organic matter content estimates require a knowledge of the factor for the particular soil

1002 NELSON & SOMMERS

studied. However, if an estimate of organic matter content of surface soils must be based on organic C data and no information on the exact factor is available, a factor of two appears to be more universally acceptable.

Direct Estimation of Organic Matter

Principles

To achieve a direct determination of soil organic matter, one must separate it from the inorganic material, which in most soils makes up 90% or more of the weight of the soil. Extraction procedures that bring part of the organic matter into solution while leaving the inorganic material undissolved have only qualitative value, sirice no solvent has been found that will dissolve all or even a major por­tion of the organic fraction. The alternative is to destroy the organic matter, after which the loss in weight of the soil is taken as a measure of the organic content. The requirements of a suitable method are that the treatment used to destroy the organic matter should not destroy or alter the other soil constituents in such a way that their weight is changed and that the organic matter should be quantitatively removed.

The two most commonly used methods for achieving destruction of organ­ic matter are: (i) oxidation of the organic matter with H20 2 and (ii) ignition of the soil at high temperature. The H20 2 method (Robinson, 1927) has serious limita­tions in that the oxidation of organic matter by this reagent is incomplete, and the extent of oxidation varies from one soil to another. This method is therefore unsatisfactory as a means of determining total organic matter of soil, but it can be useful as a means of comparing the readily oxidizable material in different soils. The loss-on-ignition (LOI) method carried out at high temperature gives quanti­tative oxidation of organic matter, but inorganic constituents of the soil, chiefly the hydrated aluminosilicates, lose structural water and carbonate minerals and some hydrated salts are decomposed upon heating. Dehydroxylation and decom­position of inorganic constituents by heating results in weight losses considerably in excess of the actual organic matter content. This problem is particularly pro­nounced with high clay soils containing low amounts of organic matter such as subsoils (Christensen & Malmros, 1982; Howard & Howard, 1990).

Studies have shown, however, that temperatures exceeding 750°C are need­ed to decompose carbonates and that little dehydroxylation of phyllosilicates occurs at temperatures below 450°C (Ball, 1964; Ben-Dor & Banin, 1989). Gibb­site is an exception because this clay mineral has been reported to lose structural water when heated at 300 to 350°C (Ranney, 1969; Gallardo et aI., 1987). Some investigators have ignited soils and attempted to correct LOI values for dehy­droxylation by multiplying a weight loss factor by the clay content of samples (Howard, 1966; Ranney, 1969; Spain et al., 1982). Ranney (1969) also used a low-temperature (100--200°C) ashing procedure (organic matter was oxidized under reduced pressure by activated O2 excited by a radio frequency electromag­netic field) for LOI estimation of organic matter content. He found that some dehydroxylation (1.5% of soil weight) occurred when subsoils were subjected to low-temperature ashing and that the procedure required 5 d for complete removal of organic matter.

CARBON AND ORGANIC MATTER 1003

The need in soil characterization and testing laboratories for a rapid and semiquantitative technique for routine estimation of organic matter content has resulted in development of a number of LOI method (Table 34-6). Environmen­tal hazards associated with use and disposal of Cr has accelerated interest in alter­natives to dichromate oxidation methods for estimation of organic matter (Schulte et aI., 1991). Most investigators have attempted to obtain complete removal of organic matter without dehydroxylating aluminosilicates or decom­posing carbonates by heating samples at or below 450°C for extended periods of time. Excellent correlations have been obtained between LOI values and organic matter content calculated from organic C data (Table 34-6). In Table 34-6, val­ues of regression coefficient "b" below one suggest that some constituent other than organic matter was lost during heating, whereas coefficients greater than one indicate that incomplete removal of organic matter has occurred. In general, low temperature (36(}-375°C) procedures particularly those with short heating times had coefficients greater than one whereas high temperature (>5OO°C) procedures much less than one. These findings suggest that ignition of soils at 400 to 450°C will remove all organic matter and cause minimal dehydroxylation of clay min­erals. A heating time of 8 to 16 h at 400°C results in near maximum weight loss (Ben-Dor & Banin, 1989).

Reflectance and absorption spectroscopy also have been used to estimate the organic matter content of soils. AI-Abbas et al. (1972) showed that relative reflectance at 0.72 to O.80llm was related to the organic matter content of Indiana soils. A curvilinear relationship was obtained between reflectance as measured by

Table 3~. Relationship between soil organic matter content and weight loss-on-ignition (LOI).

Number Ignition conditions Regression coefficientst

Reference of soils Temperature Time b a ,.z

°C h

Ball (1964) 65 375 16 0.916 -0.8 Howard (1965) 36 550 To constant 0.94 -0.96 0.99

weight Ramney (1969) 48 375 28 1.11 +0.35 0.99 Davies (1974) 17 430 24 0.983 -0.64 0.99 Christensen & 85 550 4 0.985 -0.23 0.99

Malmros (1982) Spain et al. (1982) 766 950 0.5 0.796 -0.60 Storer (1984) 215 500 4 0.937 -1.70 0.96 Goldin (1987) 60 600 6 0.810 -1.42 0.86 David (1988) 174 450 12 1.04 -0.03 0.92 Ben-Dor & 91 400 8 0.972 -0.37 0.97

Banin (1989) Howard & 564 550 3 0.840 -1.68 0.98

Howard (1990) Lowther et al. 38 450 16 0.914 0 0.99

(1990) Donkin (1991) 45 450 6 0.568 0 0.98 Schulte et al. (1991) 316 360 2 1.126 -0.38 0.90

t Regression model used was: soil organic matter = (b • LOI) + a, where units are g/100 of soil. Soil organic matter content was estimated as two times organic C concentration. Organic C was deter-mined by Walkley & Black, Tinsley, or dry combustion methods. calculations assume that organic matter was 50% C.

1004 NELSON & SOMMERS

a color difference meter and organic matter content (Page, 1974). Krishnan et al. (1980) found that the organic matter content of Illinois soils was highly correlat­ed with reflectance at wavelengths of 0.624 and 0.564 um. Near infrared diffuse reflectance at wavelengths of 1744,1870 and 2052 nm has been related to organ­ic matter content of Australian soils (Dalal & Henry, 1986). Several investigators have proposed that the organic matter content of soils be estimated by partial extraction with alkaline reagents and determination of humic materials in the extract by absorption spectrometry at 550 to 650 nm (Mehlich, 1984; Strek et aI., 1990; Bowman et aI., 1991). Moore (1985) used absorbance at 330 nm to esti­mate dissolved organic matter in peat water samples. Although each of the tech­niques described above has promise as method for determining organic matter content, none can be recommended at this time because of a lack of evaluation of their usefulness and applicability to a wide range of soils.

Loss-On-Ignition Method

The Loss-On-Ignition Method is a modification of a method described by Ben-Dor and Banin (1989).

Special Apparatus

1. Pyrex beakers or porcelain crucibles (20 mL). 2. Muffle furnace capable of :!:5°C temperature control. 3. Drying oven (105C) with :!:5°C temperature control. 4. Analytical balance capable of weighing :!:0.1 mg.

Procedure

Heat beakers or crucibles in muffle furnace at 400°C for 2 h, cool, and determine tare weight to 0.1 mg. Add 1 to 3 g of air-dried soil ground to <0.4 mrn to a tared beaker and heat at 105°C for 24 h. Cool the beaker in a dessicator over CaCl2 and determine weight of beaker plus sample to 0.1 mg. Obtain weight of oven-dried sample by subtraction. Ignite samples in a muffle furnace at 400°C for 16 h. Cool beakers in a desiccator over CaCl2 and determine weight of beaker plus ignited sample to 0.1 mg. Calculate weight of ignited sample by subtraction. The LOI content of the sample is calculated as

Weight105 - Weight400 LOI, % = x 100

Weight105 [16]

where "weight 105" is weight of soil sample after heating at 105°C and "weight4oo" is weight of soil sample after ignition at 400°C. The organic matter content is assumed to equal the LOI in most surface soils. The LOI can be corrected for de hydroxylation of inorganic constituents through regression analysis. Determine the LOI and the organic C content of representative samples having organic mat­ter levels covering the range expected in soils under study. Regress organic mat­ter content [organic C x 2 (or other suitable correction factor)] on LOI and use the resulting relationship to convert LOI of test samples to organic matter. In many cases the intercept of the regression line (LOI on Yaxis and organic matter

CARBON AND ORGANIC MATTER 1005

onX axis) will be greater than zero indicating weight loss due to dehydroxylation of clay minerals during heating.

Comments

Control of the heating of samples at 105°C is as important as at 400°C. The use of an automatic balance connected to a laboratory computer as an automated data acquisition system will greatly speed up analyses and tend to improve the precision of the method (Storer, 1984). The coefficient of variation for LOI meth­ods applied to soils containing> 1 % organic matter has been estimated to be about 3.3% (Storer, 1984),5.0% (David, 1988),2.0% (Lowther et aI., 1990), and 3.5% (Donkin, 1991). The coefficient of variation tends to decrease as the sample size increases because relative weighing errors become less (Lowther et aI., 1990).

A more accurate estimation of organic matter content of soils may be obtained by using the Rather (1917) method described by Nelson and Sommers (1982). This method involves pretreating the sample with a mixture of HCI and HF to remove hydrated mineral matter and carbonates prior to ignition. This pre­treatment dissolves part of the organic matter so that a correction for the soluble material is necessary. Both weight loss and CO2 evolution are measured during ignition so that the C content of organic matter can be calculated. The method has a coefficient of variation of about 2%. The Rather method is undoubtedly the most accurate available for determination of organic matter since it does not involve the use of an arbitrary conversion factor. The tedious nature of the method coupled with the requirement for pretreatment with HF limits its useful­ness for routine organic matter determinations.

Expression of Soil Organic Matter Content

Due to the difficulty in directly estimating or calculating the amount of organic matter present in soil, it appears that a more appropriate procedure would be to determine and express the soil organic C content as a measure of organic matter. Organic C concentrations in soil may be accurately and precisely mea­sured by a variety of procedures, whereas organic matter content may be only estimated. There would be little confusion in the amounts of organic matter. Fur­thermore, the analyst would be in a position to convert organic C data to organic matter contents using the correction factor deemed most appropriate. On the other hand, when organic matter concentrations are given, they are extremely difficult to convert to organic C values unless the correction factor originally used also is supplied. At the current point in the development of soil science, a uniform sys­tem for expression of the amounts of soil constituents is necessary. Therefore, organic C concentration is preferable to the term soil organic matter content, because the latter is not an appropriate or an accurately measurable entity.

ACKNOWLEDGMENTS

A joint contribution of the University of Nebraska Agricultural Research Division, Journal Series no. 10742, Lincoln, NE 68583-0704; and The Colorado

1006 NELSON & SOMMERS

Agricultural Experiment Station, Colorado State University, Fort Collins, CO 80523. Mention of a trademark or product does not constitute a guarantee or war­ranty of the product by the University of Nebraska or Colorado State University, nor does it imply its approval to the exclusion of other suitable products.

REFERENCES

AI-Abbas, AH., P.I. Swain, and M.F. Baumgardner. 1972. Relating organic matter and clay content to the multispectral radiance of soils. Soil Sci. 114:477-485.

Allison, L.E. 1935. Organic soil carbon by reduction of Cr03' Soil Sci. 40:311-320. Allison, L.E. 1960. Wet-combustion apparatus and procedure for organic and inorganic carbon in soil.

Soil Sci. Soc. Am. Proc. 24:36-40. Allison, L.E. 1965. Organic carbon. p. 1367-1378. In C.A. Black et al. (ed.) Methods of soil analy­

sis. Part 2. Agron. Monogr. 9. ASA, Madison, WI. Allison, L.E., W.E. Bollen, and C.D. Moodie. 1965. Total carbon. p. 1346-1366. In e.A Black et al.

(ed.) Methods of soil analysis. Part 2. Agron. Monogr. 9. ASA, Madison, WI. Amacher, M.e., R.E. Henderson, R.H. Brupbacher, and J.E. Sedberry, Jr. 1986. Dichromate-oxidiz­

able and total organic carbon contents of representative soils of the major soil areas of Louisiana. Commun. Soil Sci. Plant Anal. 17:1019-1032.

Anne, P. 1945. Sur Ie dosage rapide du carbone organique des sols. Ann. Agron. 15:161-172. Association of Official Analytical Chemists. 1975. Official methods of analysis. 12th ed. AOAC,

Washington, De. Baker, K.F. 1976. The determination of organic carbon in soil using a probe-colorimeter. Lab. Pract.

25:82-83. Ball, D.F. 1964. Loss-on-ignition as an estimate of organic matter and organic carbon in noncalcare­

ous soils. J. Soil Sci. 15:84-92. Bartlett, R.I., and D.S. Ross. 1988. Colorimetric determination of oxidizable carbon in acid soil solu­

tions. Soil Sci. Soc. Am. J. 52:1191-1192. Ben-Dor, E., and A. Banin. 1989. Determination of organic matter content in arid-zone soils using a

simple "loss-on-ignition" method. Commun. Soil Sci. Plant Anal. 20:1675-1695. Black, e.A, D.D. Evans, J.L. White, L.E. Ensminger, and F.E. Clark. 1965. Methods of soil analysis.

Part 2. Agron. Monogr. 9. ASA, Madison, WI. Bornemisza, E., M. Constenla, A Alvarado, E.J. Ortega, and AJ. Vasquez. 1979. Organic carbon

determination by the Walkley-Black and dry combustion methods in surface soils and Andept profiles from Costa Rica. Soil Sci. Soc. Am. J. 43:78-83.

Bowman, R.A, W.D. Guenzi, and D.J. Savory. 1991. Spectroscopic method for estimation of soil organic carbon. Soil Sci. Soc. Am. J. 55:563-566.

Bremner, J.M., and D.S. Jenkinson. 1960a. Determination of organic carbon in soil. I. Oxidation by dichromate of organic matter in soil and plant materials. J. Soil Sci. 11 :394-402.

Bremner, J.M., and D.S. Jenkinson. 1960b. Determination of organic carbon in soil. II. Effect of car­bonized materials. J. Soil Sci. 11:403-408.

Bremner, J.M. 1949. Use of the Van Slyke-Neil manometric apparatus for the determination of organ­ic and inorganic carbon in soil and of organic carbon in soil extracts. Analyst (London) 74:492-498.

Broadbent, F.E. 1953. The soil organic fraction. Adv. Agron. 5:153-183. Broadbent, F.E. 1965. Organic matter. p. 1397-1400. In e.A Black et al. (ed.) Methods of soil analy­

sis. Part 2. Agron. Monogr. 9. ASA, Madison, WI. Carolan, R. 1948. Modification of Graham's method for determining soil organic matter by colori­

metric analysis. Soil Sci. 66:241-247. Carr, e.E. 1973. Gravimetric determination of soil carbon using the Leco induction furnace. J. Sci.

Food Agric. 24:1091-1095. Chemists of the United States Steel Corporation. 1938. Sampling and analysis of carbon and alloy

steels. Van Nostrand Reinhold Co., New York. Cheng, H.H., and F.O. Farrow. 1976. Determination of 14C-labeled pesticides in soils by a dry com­

bustion technique. Soil Sci. Soc. Am. J. 40:148-150. Chichester, F.W., and R.F. Chaison, Jr. 1992. Analysis of carbon in calcareous soils using a two tem­

perature dry combustion infrared instrumental procedure. Soil Sci. 153:237-241.

CARBON AND ORGANIC MATIER 1007

Christensen, B.T., and P.A. Malmros. 1982. Loss-on-ignition and carbon content in a beech forest soil profile. Holartic Ecol. 5:376-380.

Ciavatta, c., L.V. Antisari, and P. Sequi. 1989. Determination of organic carbon in soils and fertiliz­ers. Commun. Soil Sci. Plant Anal. 20:759-773.

Clark, N.A., and c.L. Ogg. 1942. A wet-combustion method for determining total carbon in soils. Soil Sci. 53:27-35.

Coughtrey, DJ., DJ. Nancarrow, and D. Jackson. 1986. Extraction of carbon-14 from biological sam­ples by wet oxidation. Commun. Soil Sci. Plant Anal. 17:393-399.

Crowther, E.M. 1935. First report of the organic carbon committee. p. 114--127. In Trans. Third Int. Congr. Soil Sci., Vol. 1. Oxford, England. Thomas Mubry & Co., London.

Dalal, R.C., and R.I. Henry. 1986. Simultaneous determination of moisture, organic carbon and total nitrogen by near infrared reflectance spectrophotometry. Soil Sci. Soc. Am. 1. 50:120--123.

Datta, N.P., M.S. Khera, and T.R. Saini. 1962. A rapid colorimetric procedure for determination of organic carbon in soils. J. Indian Soc. Soil Sci. 10:67-74.

David, M.B. 1988. Use of loss-on-ignition to assess soil organic carbon in forest soils. Commun. Soil Sci. Plant Anal. 19:1593-1599.

Davies, B.E. 1974. Loss-on-ignition as an estimate of soil organic matter. Soil Sci. Soc. Am. Proc. 38:150--151.

De Bolt, D.C. 1974. A high sample volume procedure for the colorimetric determination of soil organ­ic matter. Commun. Soil Sci. Plant Anal. 5:131-137.

De Leenher, L., 1. Van Hove, and M. Van Rurjmbeke. 1957. Determination quantitative de la matiere organique du sol. Pedologie 7:324--347.

Degtjareff, W.T. 1930. Determining soil organic matter by means of hydrogen peroxide and chromic acid. Soil Sci. 29:239-245.

Donkin, MJ. 1991. Loss-on-ignition as an estimator of soil organic carbon in A-horizon forestry soils. Commun. Soil Sci. Plant Anal. 22:233-241.

Edson, S.N., and R.H. Mills. 1955. Colorimetric field test for organic matter in mineral soils. Agric. Food Chern. 852-853.

Fleming, W.R. 1914. Rapid determination of carbon in steel. Iron Age 93:64--66. Gallardo, 1.F., 1. Saavedra, T. Martin-Patino, and A. Milan. 1987. Soil organic matter determinations.

Commun. Soil Sci. Plant Anal. 18:699-707. Geiger, PJ., and J.P. Hardy. 1971. Measurement of organic carbon in arid soils using a hydrogen­

flame ionization detector. Soil Sci. 111:175-181. Gillman, G.P., D.F. Sinclair, and T.A. Beech. 1986. Recovery of organic carbon by the Walkley and

Black procedure in highly weathered soils. Commun. Soil Sci. Plant Anal. 17:885-892. Goldin, A. 1987. Reassessing the use of loss-on-ignition for estimating organic matter content in non­

calcareous soils. Commun. Soil Sci. Plant Anal. 13:1111-1116. Graham, E.R. 1948. Determination of soil organic matter by means of a photoelectric colorimeter.

Soil Sci. 65:181-183. Greweling, T., and M. Peech. 1960. Chemical soil tests. Cornell Univ. Agric. Exp. Stn. Bull. 960. Gupta, U.S., S.M. Gorantiwar, and G.P. Verma. 1975. A new colorimetric procedure for the determi­

nation of soil organic carbon. 1. Indian Sol. Soil Sci. 23:328-331. Heanes, D.L. 1984. Determination of total organic-C in soils by an improved chromic acid digestion

and spectrophotometric procedure. Commun. Soil Sci. Plant Anal. 15:1191-1213. Heck, A.F. 1929. A method for the determining of total carbon and also for the estimation of carbon

dioxide evolved from soils. Soil Sci. 28:225-231. Howard, PJ.A., and D.M. Howard. 1990. Use of organic carbon and loss-on-ignition to estimate soil

organic matter in different soil types and horizons. BioI. Fert. Soils. 9:306-310. Howard, PJ.A. 1966. The carbon-organic matter factor in various soil types. Oikos 15:229-236. Jackson, M.L. 1958. Soil chemical analysis. Prentice-Hall, Inc., Englewood Cliffs, N1. Kalembasa, SJ., and D.S. Jenkinson. 1973. A comparative study of titrimetric and gravimetric meth­

ods for the determination of organic carbon in soil. 1. Sci. Food Agric. 24:1085-1090. Kalisz, PJ., and U.M. Sainju. 1991. Determination of carbon in coal "blooms." Commun. Soil Sci.

Plant Anal. 22:393-398. Krishnan, P., J.D. Alexander, B.J. Butler, and J.W. Hummel. 1980. Reflectance technique for predict­

ing soil organic matter. Soil Sci. Soc. Am. J. 44:1282-1285. Lee, C.K. 1939. The determination of organic matter in paddy soils. The reliability of rapid titration

methods. Ind. Eng. Chern. Anal. Ed. 11:428. Lindbeck, M.R., and J.L. Young. 1964. Glazing technic for leak-proofing combustion boats. Chern.

Anal. 53:18.

1008 NELSON & SOMMERS

Loeppert, R.H., and D.L. Suarez. 1996. Carbonate and gypsum. p. 437-475. In D.L. Sparks et al. (ed.) Methods of soil analysis. Part 3. Chemical methods. SSSA Book Ser. no. 5. SSSA and ASA, Madison, WI.

Loftus, N.S., Jr. 1966. The contribution of extractable humic colloids to the exchange capacity of sur­face soils. Ph.D. diss. Pennsylvania State Univ., Univ. Microfilms, Ann Arbor, MI (Diss. Abstr. 27:2752B).

Lowther, 1.R., PJ. Smethurst, J.e. Carlyle, and E.K.S. Nambiar. 1990. Methods for determining organic carbon in podzolic sands. Commun. Soil Sci. Plant Anal. 21:457-470.

Lunt, H.A 1931. The carbon-organic matter factor in forest soil humus. Soil Sci. 32:27-33. McCready, R.M., and W.T. Hassid. 1942. Semi-micro determination of carbon using the Van Slyke­

Folch oxidation mixture. Ind. Eng. Chern. Anal. Ed. 14:525-526. McGeehan, S.L., and D.V. Naylor. 1988. Automated instrumental analysis of carbon and nitrogen in

plant and soil samples. Commun. Soil Sci. Plant Anal. 19:493-505. Mebius, LJ. 1960. A rapid method for the dttermination of organic carbon in soil. Anal. Chim. Acta

22: 120-121. Mehlich, A 1984. Photometric determination of humic matter in soil, a proposed method. Commun.

Soil Sci. Plant Anal. 15:1417-1422. Merry, R.H., and L.R. Spouncer. 1988. The measurement of carbon in soils using a microprocessor­

controlled resistance furnace. Commun. Soil Sci. Plant Anal. 19:707-720. Metson, AJ. 1956. Methods of chemical analysis for soil survey samples. N.Z. Dep. Sci. Indust.

Resour., Soil Bureau Bull. 12. Melson, AJ., L.e. Blakemore, and D.A Rhodes. 1979. Methods for the determination of soil organ­

ic carbon: A review, and application to New Zealand soils. N.Z. J. Sci. 22:205-228. Mitchell, J. 1932. The origin, nature, and importance of soil organic constituents having base

exchange properties. J. Am. Soc. Agron. 24:256-275. Moore, T.R. 1985. The spectrophotometric determination of dissolved organic carbon in peat waters.

Soil Sc. Soc. Am. 1. 49:1590-1592. Nelson, D.W., and L.E. Sommers. 1975. A rapid and accurate procedure for estimation of organic car­

bon in soil. Proc. Indiana Acad. Sci. 84:456-462. Nelson, D.W., and L.E. Sommers. 1982. Total carbon, organic carbon, and organic matter. p. 539-579.

In AL. Page et aI. (ed.) Methods of soil analysis. Part 2. Agron. Monogr. 9. 2nd ed. ASA and SSSA, Madison, WI.

Nommik, H. 1971. A modified procedure for determination of organic carbon in soils by wet com­bustion. Soil Sci. 11:330-336.

Orphanos, P.1. 1973. On the determination of soil carbon. Plant Soil 39:706-708. Page, N.R. 1974. Estimation of organic matter in Atlantic Coastal Plain soils with a color-difference

meter. Agron. J. 66:652-653. Peech, M., L.A Dean, and J. Reed. 1947. Methods of soil analysis for soil fertility investigation.

USDA Circ. 757. U.S. Gov. Print. Office, Washington, DC. Pella, E. 1990a. Elemental organic analysis. Part 1. Historical developments. Am. Lab. 22:116-125. Pella, E. 1990b. Elemental organic analysis. Part 2. State of the art. Am. Lab. 22:28-32. Perrier, E.R., and M. Kellogg. 1960. Colorimetric determination of soil organic matter. Soil Sci.

90:104-106. Peterson, W.M. 1962. Removal of sulfur fumes by lead dioxide in the combustion method for carbon

in iron and steel. Anal. Chern. 34:575-579. Piper, C.S. 1942. Soil and plant analysis. Intersci., New York. Ponomareva, v.v., and T.A Platnikova. 1967. Data on the degree of intra-molecular oxidation of

humus in various soil groups (problem of the carbon to humus conversion factors). Sov. Soil Sci. 1967(7):924-933.

Quinn, J.G., and M. Salomon. 1964. Chloride interference in the dichromate oxidation of soil hydrolyzates. Soil Sci. Soc. Am. Proc. 28:456.

Rabenhorst, M.e. 1988. Determination of organic and carbonate carbon in calcareous soils using dry combustion. Soil Sci. Soc. Am. J. 52:965-969.

Ranney, R.W. 1969. An organic carbon-organic matter conversion equation for Pennsylvania surface soils. Soil Sci. Soc. Am. Proc. 33:809-811.

Rather, J.B. 1917. An accurate loss on ignition method for determination of organic matter in soils. Arkansas Agric. Exp. Stn. Bull. 140.

Raveh, A., and Y. Avnimelech. 1973. Potentiometric determination of soil organic matter. Soil Sci. Soc. Am. Proc. 36:967.

Rhodes, E.R., P.Y. Kamara, and P.M. Sutton. 1981. Walkley-Black digestion efficiency and relation­ship to loss on ignition for selected Sierra Leone soils. Soil Sci. Soc. Am. J. 45:1132-1135.

CARBON AND ORGANIC MATIER 1009

Richardson, J.L., and R.I. Bigler. 1982. Comparison of Walkley-Black and dry combustion organic carbon determinations in calcareous water-logged North Dakota soils. Commun. Soil Sci. Plant Anal. 13:175-183.

Richter, M., G. Massen, and I. Mizuno. 1973. Total carbon and "oxidizable" organic carbon by the Walkley-Black procedure in some soils of the Argentine Pampa. Agrochimica 17:462-472.

Robertson, G.I., L.M. Jett, and L. Dorfman. 1958. Microdetermination of carbon and hydrogen by a rapid combustion procedure. Anal. Chern. 30:132-135.

Robinson, G.w., W. McClean, and R. Williams. 1929. The determination of organic carbon in soils. J. Agric. Sci. 19:315-324.

Robinson, W.O. 1927. The determination of organic matter in soils by means of hydrogen peroxide. J. Agric. Res. 34:339-356.

Salter, R.M. 1916. A rapid method for the accurate determination of total carbon in soils. Ind. Eng. Chern. 8:637-639.

Schepers, J.S., D.O. Francis, and M.T. Thompson. 1989. Simultaneous determination of total C, total N, and tSN in soil and plant material. Commun. Soil Sci. Plant Anal. 20:949-959.

Schollenberger, CJ. 1927. A rapid approximate method for determining soil organic matter. Soil Sci. 24:65-68.

Schollenberger, CJ. 1931. The determination of soil organic matter. Soil Sci. 31:483-486. Schollenberger, CJ. 1945. Determination of soil organic matter. Soil Sci. 59:53-56. Schulte, E.E., C. Kaufman, and J.B. Peter. 1991. The influence of sample size and heating time on

soil weight loss-on-ignition. Commun. Soil. Sci. Plant Anal. 22:159-168. Sheldrick, B.H. 1986. Test of the Leco CHN-600 determinator for soil carbon and nitrogen analysis.

Can. J. Soil Sci. 66:543-545. Simakov, V.N. 1957. The use of phenylanthranilic acid in the determination of humus by Tyurin's

method. Pochvovedenie 8:72-73. Simons, E.L., J.E. Fagel, and E.W. Balis. 1955. Combustion of tungsten carbide by high frequency

induced radiant heating. Anal. Chern. 27:1123-1125. Sims, J.R., and V.A. Haby. 1971. Simplified colorimetric determination of soil organic matter. Soil

Sci. 112:137-141. Sinha, H., and R.N. Prasad. 1970. A new colorimetric method for the determination of organic car­

bon in soils. J. Indian Soc. Soil Sci. 18:83-87. Smith, H.W., and M.D. Weldon. 1941. A comparison of some methods for the determination of soil

organic matter. Soil Sci. Soc. Am. Proc. 5:177-182. Snyder, J.D., and J.A. Trofymow. 1984. A rapid accurate wet oxidation diffusion procedure for deter­

mining organic and inorganic carbon in plant and soil samples. Commun. Soil Sci. Plant Anal. 15:587-597.

Soil Science Society of America. 1979. Glossary of soil science terms. Rev. ed. SSSA, Madison, WI. Soon, Y.K., and S. Abboud. 1991. A comparison of some methods for soil organic carbon determina­

tion. Commun. Soil Sci. Plant Anal. 22:943-954. Spain, A.v., M.E. Probert, R.E Isbell, and R.D. John. 1982. Loss-on-ignition and the carbon content

of Australian soils. Aus!. 1. Soil Res. 20:147-152. Springer, U., and 1. KIee. 1954. Profung der Leistungfahigkeit von einigen Wichtigeren zur Bestiim­

mung der Kohlenstoffs mittels Chromschwefelsaure sowie Vorschlag einer neuen Schnell­methode. Z. Pflanzenernahr. Dung. Bodenk. 64:1-26.

Storer, D.A. 1984. A simple high sample volume ashing procedure for determination of soil organic matter. Commun. Soil Sci. Plant Anal. 15:759-772.

Strek, HJ., J.J. Dulka, and AJ. Parsells. 1990. Humic matter content vs. organic matter content for making herbicide recommendations. Commun. Soil Sci. Plant Anal. 21:1985-1995.

Tabatabai, M.A., and J.M. Bremner. 1970. Use of the Leco automatic 70-second carbon analyzer for total carbon analysis in soils. Soil Sci. Soc. Am. Proc. 34:608-610.

Tabatabai, M.A., and J.M. Bremner. 1991. Automated instruments for determination of total carbon, nitrogen, and sulfur in soils by combustion techniques. p. 261-286. In K.A. Smith (ed.) Soil analysis. Marcel Dekker, New York.

Takahashi, Y., R.T. Moore, and R.J. Joyce. 1972. Direct determination of organic carbon in water by reductive pyrolysis. Am. Lab. 4:31-38.

Tiessen, H., J.R. Bettany, and J.w.s. Stewart. 1981. An improved method for the determination of carbon in soils and soil extracts by dry combustion. Commun. Soil Sci. Plant Analysis 12:211-218.

TInsley, J. 1950. Determination of organic carbon in soils by dichromate mixtures. p. 161-169. In Trans. 4th Int. Congr. Soil Sci., Vol. 1. Hoitsemo Brothers, Gronignen, the Netherlands.

1010 NELSON & SOMMERS

Tyurin, LV. 1931. A new modification of the volumetric method of determining soil organic matter by means of chromic acid. Pochvovedenie 26:36-47.

Van Slyke, D.D., and I. Folch. 1940. Manometric carbon determination. 1. BioI. Chern. 136:509-541. Verardo, D.I., P.N. Froelich, and A. Mcintyre. 1990. Determination of organic carbon and nitrogen in

marine sediments using the Carlo Erba NA-1500 analyzer. Deep Sea Res. 37:157-165. Walkley, A. 1947. A critical examination of a rapid method for determining organic carbon in soils:

Effect of variations in digestion conditions and of inorganic soil constituents. Soil Sci. 63:251-263.

Walkley, A., and LA. Black. 1934. An examination of the Degtjareff method for determining soil organic matter and a proposed modification of the chromic acid titration method. Soil Sci. 37:29-38.

Wilde, S.A. 1942. Rapid colorimetric determination of soil organic matter. Soil Sci. Soc. Am. Proc. 7:393-394.

Willett, LR, and T.A. Beech. 1987. Determination of organic carbon in pyritic and acid sulfate soils. Commun. Soil Sci. Plant Anal. 18:715-724.

Winter, I.P., E.G. Gregorich, RP. Voroney, and RG. Kachanoski. 1990. Comparison of two sample oxidation methods for quantitative measurement of 12C and 14C in plant and soil. Can. I. Soil Sck. 70:525-529.

Winters, E., and RS. Smith. 1929. Determination of total carbon in soils. Ind. Eng. Chern. Anal. Ed. 1:202-203.

Yeomans, I.C., and I.M. Bremner. 1988. A rapid and precise method for routine determination of organic carbon in soils. Commun. Soil Sci. Plant Anal. 19:1467-1476.

Yeomans, I.C., and I.M. Bremner. 1991. Carbon and nitrogen analysis of soils by automated com­bustion techniques. Commun. Soil Sci. Plant Anal. 22:843-850.