Common Soil Parameters

What are the common parameters of soil that we could test?

The 3 soil attributes

Physical attributes

Texture, structure, etc

Biological attributes

Organic content

Chemical attributes

pH, conductivity, cations, anions etc

Physical Properties

Colour

described by using a Munsell Colour Chart

describe colour in terms of Hue, Value, and Chroma.

A typical designation is 10YR 5/2. This is the symbol of a greyish brown colour with 10YR hue, 5 value, and 2 chroma

The symbol components are always arranged in this sequence.

Soil Colour Determination

Hue - colour of pigment that must be mixed with black and white (or the proper shade of grey) to produce the colour to be matched

Soils range in hue from red (R) through yellow-red (YR) to yellow (Y) with some spots of green-yellow (GY) or even green (G).

0-10 prefixes divide the space between the absolute hues

eg 7.5YR

value - the square root of the percentage of light reflected

soils range from 2 to 8

Soil Colour Determination

Chroma the amount of pigment that must be mixed with the proper

value of grey to produce the particular colour

pure grey colours have 0 chroma

increasing brightness is indicated by chroma’s up to about 8 in soils

Soil Texture

Soil Texture is the most commonly used descriptor of a soil.

Texture is a property of the fine earth (<2mm) fraction that depends on the particle‑size distribution.

Particle size in this fraction varies from 2mm diameter down to less than 0.1mm

The distribution of particles over this size range influences many important soil properties such as ease of cultivation and water‑holding characteristics.

Soil Texture

Soil is allocated to a textural class, depending on its content of sand‑, silt‑ and clay sized particles.

In the field, determined subjectively from the feel of a moist soil molded between the fingers and thumb

This is because the particle‑size distribution influences the mechanical properties of the material.

Class Exercise 1

Finger assessment of soil texture for mineral soils

Soil Structure

Described as weak, moderate, or strong depending on how distinct the peds? are.

Fine, medium, or coarse depending on the size of the peds.

Shape of the peds - platy, granular, blocky, columnar, or prismatic,

Settling rate lab. method

for the silt‑size particles plus some of the coarse clay, rate of settling is proportional to the square of the

diameter of the particles v = 6000d2

v is the settling velocity in cm/minute d the particle diameter in mm

two sizes needed for determining soil texture; limits between: sand and silt (0.05 mm) and silt and clay (0.002 mm)

Example 8.1 Calculate how far a 0.05 mm diameter particle would

travel in one minute.

v = 6000 x 0.052

= 15 cm/min

in 1 minute, particles of this size would have travelled 15 cm

Exercise 8.2 Would larger particles than 0.05 mm travel more or

less than 15 cm in this time? More

What does this mean about the sand fraction? It would sink faster

Calculate how long a 0.002 mm diameter particle would take to travel 1 cm.

v = 0.024 cm/min 1 cm in 41.7 minutes

Applying this in the lab a uniform suspension of soil in water is allowed to

settle for one minute a sample is then withdrawn from a depth of 15 cm will contain no particles larger than 0.05 mm

diameter larger particles will have settled beyond that depth

even if they started at the surface the sample will contain silt and clay only any particles smaller that had sunk from this level

will be replaced from above repeat at a depth appropriate for 0.002 cm eg 3 cm after 2 hours

Exercise 8.3 answers

a) 40 x 2.04 g = 81.6 gb) 40 x 0.40 g = 16 gc) 81.6-16 = 65.6 gd) Silt 66.8%, clay 16.3%, sand 16.9%e) silt loam

Soil Water The amount of water present in the soil at any one time is

most commonly expressed as a percentage of the oven‑dry weight of the soil

Percentage is commonly determined for the field capacity of the soil and for air‑dry soil.

These two percentages are used as limits to classify the water present in a soil after a rain as gravitational, capillary, and hygroscopic

Soil Conductivity

Determined quantitatively by a conductivity meter.

Standard water:soil mix

An important parameter when assessing salinity.

Used to estimate the concentration of soluble salts in the soil (Na+, Mg2+ and Ca2+ , Cl-, SO4

2- and HCO3-).

Soluble fertiliser may also contribute K+, NH4+ and

NO3-.

High EC is undesirable for most plants.

Stone Content

affect soil fertility by taking up space

reduce ability of a given volume of soil to hold water and nutrients

a hindrance to cultivation

measured by eye in the field

by separation and weighing in the lab

Soil pH

Tested with a calibrated pH meter.

Soil solutions are well buffered – do you remember what this term means?

By using a standard, valid comparisons between soils can be made (absolute values are difficult to interpret)

Soil pH

Measurements can be made in the surface layer of a moist soil provided sufficient water is present to make liquid contact between the electrodes.

Under some circumstances where buffering capacity is not adequate, soils maybe suspended in 0.1M KCl for pH determination

There are many ways by which the pH of a soil can be measured – refer to handbook

Calcium Carbonate Content - Field Method

Field estimate of CaCO3 content is based on the reaction of soil with dilute acid giving both visible and audible effects.

Method is only approximate and not sensitive to differences in CaCO3 contents above 10%.

Biological Attributes

Organic content

determined by oxidative digestion (dichromate)

measured by: back titration of excess dichromate colorimetry of green product

Chemical Attributes

Nitrogen wet ashing with conc. sulfuric formation of ammonium ions converted to ammonia (NaOH) steam distillation collection in boric acid titration with std HCl known as the Kjeldahl method – standard for N doesn’t pick up NO2/NO3 need conversion by initial reduction

Phosphorus availability vs total a major issue for P various extraction solution to assess availability, eg

Olsen 0.5 NaHCO3 analysis by:

ICP XRF colorimetry

Micronutrients can exist in soils in:

water‑soluble exchangeable adsorbed complexed secondary clay minerals insoluble oxides primary minerals

extraction with chelating DTPA and ICP analysis

Sodium absorption ratio (SAR) predictor of salinity measure Ca, Mg & Na

in mmole/L use eqn or nomogram

2

])Mg[]Ca([

]Na[SAR

22

Exercise 8.4 A soil is tested for

leachable Na, Ca and Mg, and the results are (in mmole/L: 15, 3 and 2 respectively. What is the SAR?

5.9

223

15

Cation exchange capacity (CEC) relies in the removal of ions with a concentrated

solution of an ionic substance intended to drive off the adsorbed ions

techniques used to analyse the released ions include: titration with EDTA – Ca & Mg flame AAS – Na, K ICP emission - all Kjeldahl N analysis – all adsorbed ions are

replaced by NH4, which are then released by excess K; the ammonium is then analysed

some methods use approximations and correction factors to achieve quick result

pH buffering capacity adding known amounts of acid (as HCl) or alkali (as

NaOH or lime) to soils allowing a equilibrium period before measurement of

pH graph of amount added (per kg of soil) vs pH plotted buffer capacity is the slope of the graph quoted as an amount of acid or alkali (typically

millimoles H+ or g CaCO3) per kg of soil per pH unit.

Pesticides residues left in soils from pesticides or their by-

products

analysed by GC-MS for sensitivity and ease of identification