Soil Alkalinity, Salinity, and Sodicity

Bryant Scharenbroch

Introduction to Soil ScienceACCA & The Morton Arboretum

Laboratory notebooks will be collected on Wednesday, 10/22/08

Alkalinity, salinity, sodicity

Review questions: Chapter 10: 5, 6, and 11

Review article: Savin et al. 2004 (Sarah)

Laboratory exercise: pH and EC

Soil Ecology

Review questions: Chapter 11: 6, 7, and 9

Review article:Gregorich et al. 2006 (Kevin)

Laboratory exercise: CEC

Today Next time

Arid soils

Soils of dry regionsOrders: Aridisols, Entisols, Mollisols, Alfisols, and Vertisols

Suborders: Ustic, Xeric, and Natric (SAR>15)

Water-limited, high pH, carbonate-rich, salt-rich, and Na-rich

Arid region agriculture40% of world’s cultivated land in dry regions, but most dry lands are uncultivated deserts and rangelands

Irrigation only possible on small fraction

Tillage practices (removing unwanted vegetation, fallow periods, etc.) to increase infiltration

Stubble mulch and no-till to reduce evaporation and erosion

Islands of fertility

(serc.carlton.edu)

SOMNutrients

AggregationSurface biopores

infiltration

NPP

RestitutionOrganismsWind traps

Desert pavementThin (single) layer of rock fragments (coarse gravel to cobbles) via wind erosion or shrink-swell and uplifting

>65% surface coverage is considered desert pavement

Provides protection from wind erosion and traps additional soil particles

Increased pavement = decreased infiltration

Impacts plant water availability and salt leaching

Creates water sinks (high infiltration and low runoff) Creates water sources (low infiltration and high runoff)

(USGS)

Microbial crustsThick, dark, jagged coating

Algae, fungi, and cyanobacteria

Fragile

Reduce wind and water erosion

Rough types increase infiltration

Smooth types decrease infiltration

(serc.carlton.edu)

Calcium-rich layers

Cl: Low precipitation

P: Calcareous soil materials (with free Ca carbonates)

Conditions may induce P or micronutrient deficiency

Cementation (e.g., petrocalcic or duripans)

Depth of calcic and gypsichorizons determined by depth of leaching

Stony Gypsisol, Israel, with desert pavement;a massive petrogypsic horizon occurs near to the surface (Yermi-Epipetric Gypsisol)

Calcic horizon in a Mollisol

Saline seeps

Emergence of saline groundwater via cultivation

ReducedEVT

Increasedpercolation

Removal ofdeep-rooted

plants

Watertablerise

Increasedsalts left via E

Alkaline soils

Alkalinity

EVT > PPT

Accumulation of cations (K, Na, Ca, and Mg) released from weathering

Base cations are non-hydrolyzing and do not produce H upon reacting with water like Fe or Al (pH 7) pH > 7 as a result of OH reactions

Hydroxyl generating anions carbonate (CO32-) and bicarbonate (HCO3

-) from calicite of carbonic acid react with water to form OH ions, thus increase pH

Reactions go to left with increasing biological activity and to the right with increasing calcite dissolution

Problems with alkaline soils?

Problems with alkaline soils

1. Nutrient deficiencies2. Clay dispersion

Zn, Cu,Fe, and Mn

Insoluble at high pH

Often requires irrigation and chelation

Systemic or foliar application

Tree Fe deficiencies

http://www.canr.msu.edu/vanburen/ffc14.jpgArborjet

B, Mo,and Cu

Inner-sphere complexation of B increases with pH

Mo availability increases with pH to toxic levels

Molybdenosis in ruminant animals as a result of high Mo and low Cu

P

Dissolved Ca and Mg ions constrain dissolution of P carrying minerals

Soluble P reacts with Ca to form insoluble Ca-P

Fungi, bacteria, and Brassica plants excrete organic acids to dissolve Ca-P

What is CEC of alkaline soils?

CEC of alkaline soils

Higher CEC than acid soils (assuming similar texture and SOM)

2:1 type clays common in alkaline soils have high permanent charge (e.g., smectite)

High pH will stimulate high levels of pH-dependent charge

What is soil clay dispersion?

Soil clay dispersionSlaking and aggregate destructionReduced macroporosityReduced aerationReduced percolationSurface sealing (crusts)

http://www.agric.wa.gov.au/ikmp/images/F05790a.GIF

Soil clay particles can be unattached to one another (dispersed) or clumped together (flocculated) in aggregates. Soil aggregates are cemented clusters of sand, silt, and clay particles.

Dispersed Particles Flocculated Particles

(Jim Walworth, U. Ariz)

Flocculation is important because water moves mostly in large pores between aggregates. Also, plant roots grow mainly between aggregates.

(Jim Walworth, U. Ariz)

In all but the sandiest soils, dispersed clays plug soil pores and impede water infiltration and soil drainage.

(Jim Walworth, U. Ariz)

Most clay particles have a negative electrical charge. Like charges repel, so clay particles repel one another.

Negatively charged clay particle

Negatively charged clay particle

(Jim Walworth, U. Ariz)

A cation is a positively charged molecule. Common soil cations include sodium (Na+), potassium (K+), magnesium (Mg2+), and calcium (Ca2+).

Cations can make clay particles stick together (flocculate).

Negatively charged clay particle

Negatively charged clay particle

++

(Jim Walworth, U. Ariz)

Flocculating Power of CationsCations in water attract water molecules because of their charge, hydrated

Cations with a single charge and large hydrated radii are the poorest flocculators.

Cation Charges per molecule

Hydrated radius (nm)

Relative flocculating power

Sodium 1 0.79

0.53

1.08

0.96

1.0

Potassium 1 1.7

Magnesium 2 27.0

Calcium 2 43.0

Water molecule is polar: (+) on one end, (-) on the other end

(+)

(-)

(+) Hydrated cation +

(Jim Walworth, U. Ariz)

Ca2+ and Mg2+Na+

SAR

EC

Aggregate stability (dispersion and flocculation) depends on the balance (SAR) between (Ca2+ and Mg2+) and Na+ as well as the amount of soluble salts (EC) in the soil.

Flocculated soil

Dispersed soil

++++++

++++

++

+ + +++++

+

+

Lower EC Higher EC

(Jim Walworth, U. Ariz)

Na+

SAR

EC

Soil particles will flocculate if concentrations of (Ca2+ + Mg2+) are increased relative to the concentration of Na+ (SAR is decreased).

Flocculated soil

Dispersed soil

+

++

Ca2+ and Mg2+

++

++++++++

++++

++++++

(Jim Walworth, U. Ariz)

Na+

SAR

EC

Flocculated soil

Dispersed soil

++

+

Ca2+ and Mg2+

++++++

Soil particles will disperse if concentrations of (Ca2+ + Mg2+) are decreased relative to the concentration of Na+ (SAR is increased).

++

+

+

(Jim Walworth, U. Ariz)

Soil particles will flocculate if the amount of soluble salts in the soil is increased (increased EC), even if there is a lot of sodium.

Flocculated soil

Dispersed soil

Na+

SAR

EC

Ca2+ and Mg2+

Lower EC Higher EC

++

++

+

+

+

++

++++++++

++++

++++++

(Jim Walworth, U. Ariz)

Soil particles may disperse if the amount of soluble salts in the soil is decreased (i.e. if EC is decreased).Ca2+ and Mg2+

Na+

SAR

EC

Lower EC

Flocculated soil

Dispersed soil

Higher EC

++++

++

++

+

(Jim Walworth, U. Ariz)

Why are clays in alkaline soils subject to dispersion?

Alkaline clay dispersion

Al and Fe (acid soils), which are strong flocculating/cementing agents are lacking

Monovalent cations (K and Na) are good at dispersion and not leached from alkaline soils

Salinization

What is salinization?

SalinizationProcesses that result in the accumulation of neutral soluble salts

PPT/EVT of 0.75

Low, flat areas and high water tables

Soluble salts (chlorides and sulfates of Ca, Mg, Na, and K) from weathering are moved from wetter to drier areas (up profile and across landscape)

Salts are left as water evaporates

Salinization16 million ha (increasing 10% annually)1/3 of US arid soils are salt impacted

How do salts affect plants?

Salt impacts on plants

Salts lower the osmotic potential of soil solution; thus plants must respond by lowering root osmotic potential

Na, Cl, H3BO4-, and HCO3

- toxicity

Dispersion and puddling leading to reduced aeration and water-logging

Will soluble salts increase or decrease pH?

Soluble saltsNeutral salts (CaSO4, Na2SO4, NaCl, and CaCl2) will tend to lower pH by moderating alkalizing reactions due to the common ion effect

A salt will be less soluble if one of its constituent ions is already present in the solution

More common ions will reduce dissolution of carbonates

Which is more likely to increase salts, over- or under-irrigation?

Which is more likely to increase salts, over- or under-irrigation?Over-irrigation

Input of salts > output of drainage water

Increased EVT and raise water table

90 cm of water applied may deposit 6 Mg ha-1

(3 ton ac-1)

Salinization of the Fertile crescent (SE Iraq): poorly drained and irrigated with water from the Euphrates converted productive land to be barren wastelands

How to measure salinity?

Total dissolved solids

Evaporate water and weigh solids

Irrigation water (5 to 1,000 mg L-1)

Soil extract (500 to 12,000 mg L-1)

EC

Pure water is a poor conductor of electricity, and conductivity increases with salts

Electrical conductivity is an indirect measure of salt

EC > 2 dS m-1 will adversely affect sensitive plants

EC > 4 dS m-1 will adversely affect most plants

Electromagnetic inductionMeasures electrical current in the body of soil, related to EC, thus salt

Magnetic field is generated and generated currents are measured

Measures to depth and without disturbance

(EM38 in Sudduth et al. 2003)

Exchangeable Na percentage

Relates Na on CEC

ESP = exchangeable Na (cmolc kg-1) x 100CEC (cmolc kg-1)

ESP > 15 are associated with deteriorated soil physical properties

Sodium adsorption ratio

Compares Na relative to Ca and Mg

SAR = [Na+] / (0.5[Ca2+] + 0.5[Mg2+])1/2

SAR of 13 = ESP of 15

K could be included with Na

SAR

The ratio of ‘bad’ to ‘good’ flocculators gives an indication of the relative status of these cations:

Na+++

+ + ++

+

Ca2+ and Mg2+++

++++++++++

++

(Jim Walworth, U. Ariz)

Saline soils “white alkali”EC of > 4 dS m-1

ESP of < 15 (SAR < 13)

CEC is with Ca and Mg, not Na

pH is usually below 8.5

Plant growth hindered by salts

Non-Na salts prevent dispersion

Infiltration, aggregate stability, and aeration are not problematic

Referred to as white alkali soils, due to white salt crust

http://sis.agr.gc.ca/cansis/taxa/soil/regosolic/saline_pr_sl.jpg

Saline-sodic soilsEC of > 4 dS m-1

ESP of > 15 (SAR < 13)

Plant growth hindered by salts and Na

Intermediate dispersion, infiltration, aggregate stability, and aeration

But, neutral salts contribute cations that move in close to CEC, thus reducing dispersion

If soils are leached of salts, they will become sodic

L. MacDougal

Sodic soils“black alkali”

EC of < 4 dS m-1

ESP of > 15 (SAR < 13)

pH > 8 to 10 (Na more soluble than Ca carbonate, so high carbonate, bicarbonate, OH in solution)

Plant growth hindered by excess Na, OH, and HCO3

-

Main detriment is the dispersion and subsequent decreased infiltration, aggregate stability, and aeration

SOM will disperse, dissolve, and move up in capillary flow; thus black alkali or slick spot nomeclature

soer.justice.tas.gov.au/2003/image/548/index.php

How to assess structure degradation?

Hydraulic conductivity

Readiness of water movement

KSAT is so low for sodic soils that infiltration rate is zero

Causes of low KSAT of sodic soils

Exchangeable Na increases tendency for aggregates to slake; and slaked particles clog pores

Na increases swelling of expandable 2:1 clays; thus macropores are squeezed shut

High Na and low salt contents leads to dispersion;thus clay particles exist in a gel-like condition

Which soil will be more puddled?

EC of 5 dS m-1 and ESP of 16

EC of 1 dS m-1 and ESP of 16

Puddling

Low salt concentration and high Na content will encourage dispersion and increase puddling

Soil dispersion (high Na, low salt)

Na has a single charge and large hydrated radius; thus Na swarm

Soil dispersion (high Na, low salt)

High salts: cationsstay closer and anions come closer to clay; ionic swarm is compressed; flocculation

Low salts: cationsdiffuse away from clay, anions move from clay; ionic swarm is expanded; dispersion

De-icing saltsImpact is usually temporary as long as drainage is adequate

KCl or sand are better options than NaCl

“halophyte exotic” seaside goldenrod

Reclamation of saline soils?

Reclamation of saline soils

Leach soils with low salt irrigation water

Leaching ratioLeaching ratio indicates minimum amount of water to be leached through wet soil and meet plant EVT needs; it is multiplied by the water added

If EC in water is high (high ECiw) and the plant has a low salt tolerance (low ECdw); it will require more water to leach

LR = ECiw / ECdw

Leaching ratio is figured from a salt balance (Siw + Sp + Sf + Sm = Siw + Sp + Sf + Sm)

(irrigation water, deposition, fertilizers, mineral weathering = drainage water, crop removal, chemical precipitation of carbonates and sulfates)

Reclamation of saline soils

LR is problematic because it does not take into account water table rise, etc.; thus, direct measurements of EC or EM throughout the profile are preferred

Reclamation of saline soils

Deep-rooted vegetation to lower the water table and reduce upward movement of salts

Should we leach saline-sodicsoils for reclammation?

Reclamation of saline-sodic soils

If simply leach a saline-sodic soil, will reduce soluble salts and they will likely become sodic (Na and pH increase)

First reduce Na, then deal with salts

Reducing NaReplace with Ca (gypsum, tons ha-1)

Deep-rooted plants will increase gypsum percolation

Reducing NaS yielding sulfuric acid, changes sodium bicarbonate to leachablesodium sulfate and also decrease pH

Review question 10.5

Review question 10.5Sounds like a sodic soil

Irrigation water likely has SAR indicating high levels of sodium compared to calcium and magnesium; thus stimulating a SAR in the soil solution, a higher pH, and a higher exchangeable Na level

Consequently aggregate stability was likely reduced when colloids become highly sodium-saturated. The high pH and Na levels, along with poor soil structure, result in reduced plant growth

Review question 10.6

Review question 10.6

Additions of gypsum, elemental sulfur or H2SO4 can replace the exchangeable Na+ with Ca2+ or H+ ions, and irrigation water can be used to leach the Na+

ions from the soil

Growth of Na- tolerant plants such as barely, rye and clover can help open up root channels

Review question 10.11

Review question 10.11Change in ESP needed is from 30 to 4%, or a reduction of 26 percent

Calculate amount of Na+ ions to be replaced:

Multiply CEC by change in ESP = 25 cmolc/kg x 26/100 = 6.5 cmolc/kg soil

Calculate gypsum (CaSO4·2H20) needed to replace 6.5 cmolc of Na+ ions

Calculate mass of 1 cmolc gypsum required172 g gypsum x 1 mole x 1 molc = 172 = 0.86 g/cmolc

Mole 2 molc 100 cmolc 200

For 6.5 cmolc/kg: 0.86g/cmolc x 6.5 cmolc = 5.6g CaSO4 2H2O/kg soil

Express gypsum needed for 1 hectare 30 cm deep:

One hectare 15 cm deep weighs 2 x 106 kg5.6g gypsum/kg x 4 x 106 kg/hectare = 22.4 x 106g gypsum/haThis is 22.4 x 103 kg/ha or 22.4 Mg gypsum/ha