Cation Exchange and Soil Acidity Principles

Cation Exchange

Process where cations replace each other in soil.

Cation Exchange Capacity

Total exchangeable cations a soil can hold.

Permanent Charge

Charge that remains constant regardless of pH.

pH-Dependent Charge

Charge that varies with changes in pH levels.

Smectite

2:1 clay mineral with constant charge below pH 6.

Kaolinite

1:1 clay mineral with variable charge based on pH.

Cation Selectivity

Preference of soil colloids for certain cations.

Lyotropic Series

Ranking of cations by binding strength to colloids.

Buffered CEC

Maximum CEC measured at pH 7 with NH4+.

Effective CEC

Measured CEC in unbuffered soil, important for acidity.

Soil pH

Negative logarithm of hydrogen ion concentration.

Sources of H+ Ions

Include carbonic acids, organic matter, and acid rain.

Aluminum in Soil Acidity

Increases with lower pH, causes toxicity and hydrolysis.

Active Soil Acidity

H+ ions in soil solution, smallest acidity pool.

Exchangeable Soil Acidity

Acidity in equilibrium with soil solution, includes Al3+.

Residual Soil Acidity

Non-exchangeable acidity bound to colloids, largest pool.

Total Soil Acidity

Sum of active, exchangeable, and residual acidity.

Buffering Capacity

Soil's resistance to pH changes.

Negative Effects of Soil Acidification

Harms plant growth and microbial activity.

Mass Action

Concentration of cations influences exchange direction.

Complementary Cations

Presence of multiple cations affects exchange dynamics.

Hydrolysis of Al3+

Aluminum reacts with water to form Al-oxides.

Soil Colloids

Negatively charged particles that attract cations.

Acidic Soil Conditions

Low pH negatively impacts soil health.

Soil Weathering

Process that alters soil charge characteristics.

Plant Cation Uptake

Plants absorb cations, affecting soil acidity.

Element toxicity

Toxic elements include aluminum, manganese, hydrogen, iron.

Nutrient availability

Acidic soils limit availability of essential nutrients.

Bacterial diversity

Declines significantly at low soil pH levels.

Fungal activity

Predominates in soils with low pH.

Liming materials

Used to raise soil pH by neutralizing acidity.

Soil pH change factors

Include required pH change, buffer capacity, and soil depth.

Saline soils

High soluble salts, low sodium, EC > 4 dS/m.

Sodic soils

High sodium, low salts, degraded soil structure.

Saline-sodic soils

Intermediate effects, high sodium, EC > 4 dS/m.

Total dissolved solids

Mass of salts per volume of water.

Electrical conductivity

Measures current flow through salt solutions.

Exchangeable sodium percentage

Percentage of sodium in soil's cation exchange capacity.

Soil salinity management

Irrigation and leaching to remove excess salts.

Gypsum usage

Replaces sodium with calcium to improve soil structure.

Osmotic stress

High salt concentrations hinder plant water absorption.

Soil structure degradation

Sodium causes particle dispersion, reducing soil quality.

Compaction

Increased by dispersed particles, limiting root growth.

White alkali

Evaporation leaves salt crust on soil surface.

Calcium replacement

Calcium ions displace sodium on cation exchange sites.

Flushing sodium

Leaching sodium from soil after calcium replacement.

Hydraulic conductivity

Rate of water movement through soil.

Buffer capacity

Soil's ability to resist pH changes.

Salinity effects on plants

High salinity leads to reduced growth and yield.

Crusting

Surface hardening due to salt accumulation.

Soil forming factors

Environmental characteristics affecting soil development.

Irrigation with saline water

Contributes to soil salinity in arid regions.

Start of practice questions!

YES

Which soil colloids gets most of its CEC from isomorphic substitution

Smectite

Which soil colloid would contribute the most to high soil CEC, even in low pH (acidic) conditions

Smectite

A) Consider a soil sample that has 40% clay and 3% organic matter. You know from soil survey data that the dominant clay mineral in this soil is likely mica. You also know that at a neutral pH (pH = 7) mica has an average buffered CEC of 30 cmol./kg soil, and organic matter has an average CEC of 200mol/kg soil. Estimate the total CEC of this soil (cmold/kg soil) based on the clay and SOM content.

0.4 x 30= 12
0.3 x 200 =6
12+6=18

B) Imagine that the soil sample above was from a field that has an average soil pH of 6. Would the total CEC you estimated above actually be higher or lower in the field sample at a pH of 6? Why?

Total CEC is lower with a lower pH- So the CEC estimated above would be lower

C) if lime (CaCOs) is applied to the field to raise the soil pH, what would happen to the amount of CEC contributed by the clay and by the organic matter (i.e., increase or decrease)? Which one (the clay or the SOM) would have the greatest change in CEC? Why or why not?

If pH increases CEC will increase
OM Will increase in CEC more with the line due to added minerals
(Not full point s)

Reversibility

Cation exchange reactions are reversible depending on the amount of each ion in soil vs. on colloids.

charge equivalence

This is why it takes three H+ ions to exchange with one A|3+ on soil colloids

Ratio Law

At equilibrium, the ratio of specific ions in the soil solution will be the same as the ratio of these ions on soil colloids.

Anion effects on mass action

An exchange reaction can be prevented from reversing if the released cation either precipitates, volatilizes, or strongly associates with an anion.

Cation selectivity

This principle determines the order of preference of specific cations and how tightly they are held on exchange sites based on hydrated radius and charge.

Complimentary cations

Cations located next to each other on exchange sites can impact each other's ability to exchange with ions in the soil solution.

Saline

Exchange complex dominated by Ca and Mg

Sodic

Disperse clays, degrade aggregates & soil structure

saline-sodic soil

Soluble salts alleviate dispersion from sodium

calcic horizon

Accumulation of calcareous material (e.g., CaCO3)

Acidification

Results from oxidation from ammonium fertilizers

What contributes acidity (increases H+ ions) in soil

Oxidation/decomposition of organic residues
Cation uptake by plant roots
Organic acids

What best describes the role of aluminum in soil acidity

Al3+ ions in solution cause hydrolysis of water molecules then combine with OH to form AL-Oxides

Know how to describe the pool of soil acidity represented in 3 portions

Residual, exchangeable, active

Buffering capacity

Ability of soil to resist changes in pH by moving acid cations to or from the exchangeable/residual pools of acidity

Buffered CEC

Maximum cation exchange capacity in a soil sample when measured at a standardized pH level

Effective CEC

Apparent cation exchange capacity in a soil sample when measured at the actual soil pH level

Electrical conductivity

Total salts in the soil solution measured in a soil: water mixture

Explain why you are more likely to find acidic soils in regions with greater rainfall, and alkaline soils in arid or semi-arid regions with low rainfall. You must specifically address why rainfall amount (climate) causes this difference-don't just list one or two things that cause acidity or alkalinity.

Areas with more rainfall will have more leaching of basic cations, and the production of more H+ ions. In low rainfall areas there will be more basic cations present as leaching is not as prevalent, there is also less evaporation causing the salts to move upward through the soil profile.

Explain why you are also more likely to encounter salinity and/or sodicity issues in arid or semi-arid soils than in regions with more rainfall

In arid or semi-arid regions there is less rainfall meaning less leaching of the soil, which allows for an increase in the salinity of the soil, there is also more evaporation which pulls the salts up in the soil. There is generally poor drainage in these areas as well which allows for increased salts.

Explain the effect of salt concentration on clay dispersion/flocculation.

More tightly bound salt concentrations cause flocculation of clay colloids, while looser bound concentrations allow them to disperse.

Would you use limestone (CaCO3) or gypsum (CaSO4) as a useful amendment if exchanging Na is the only target for reclaiming salt-affected soils? Explain why?

Gypsum would be the best amendment as they replace sodium ions with calcium ions, and it improves soil structure.

Which of the following describes the mechanism by which excess sodium (Nat) in the soil solution can disperse clay and organic matter?

Swarms of loosely attracted Na+ ions prevent colloids from attracting to each other.

Which of the following can cause soils to become salt-affected (e.g., saline, sodic, or saline-sodic)?

High groundwater levels in low elevation
Irrigation in arid/semi-arid areas
Tidal inundation in coastal soils

There is a soil with an EC > 4 dS/m, pH > 8.5, and an exchangeable sodium percentage (ESP) >15 (high sodium saturation). How would you classify this soil for salt status?

Saline Sodic

High total salts (high EC) in the soil inhibits plant water uptake.

True

Soils with high CEC also tend to have high buffering capacity.

True

Soils with high buffering capacity need more lime to alleviate soil acidity.

True

Gypsum (CaSO4) is an amendment used to raise soil pH in alkaline soils

False

Leaching with high-quality irrigation water is an effective remediation method for soils with high EC and low ESP.

True

Larger residue particles decompose faster than smaller particles.

False

Active acidity is the largest pool of acidity in soil.

False

Residues with which of the following qualities will decompose most quickly and result in the most plant-available nitrogen released into the soil? (CH. 12?)

Low lighting and polyphenols, low C/N ratio

Explain what causes nitrate depression in soil during the early stages of decomposition. (CH.12?)

This results when microbes must supplement with N from soil to decompose high C/N residues

Which of the following residues is most likely to cause the greatest nitrate depression in soil during the early stages of decomposition, based on the C/N ratio? (CH 12?)

Wheat straw 80:1

Why are soils with abundant reduced sulfur compounds (which are typically buried deep in the profile or under water) said to have a high "potential acidity"?

When these soils are exposed to oxygen the sulfur compounds in the soil undergo oxidization reactions that produce Sulfuric Acid

Why are sub-soil horizons containing chalky calcium carbonate deposits usually only found in arid or semi-arid soils?

They are found here due to the limited rainfall and soil processes in these climates. There is less leaching, and evaporation encourages salt accumulation in the soil.

What is the primary source of soil organic matter?

The decomposition of plant materials, and other plant residues.

Soil Organic Matter

The organic fraction of the soil that included plant, animal, and microbial residues in various stages of decomposition. Biomass of soil microorganisms. Substances produced by plant roots and other soil organisms.

Decomposition

Breakdown of large organic molecules into smaller simpler components

Carbon/Nitrogen ratio (C/N ratio)

• The ratio of carbon's mass to nitrogen's mass in an organic
fertilizer
• Provides a relative estimate of the decomposition rate
• A higher C/N ratio implies a longer time to decompose the
organic component of the fertilizer, hence, a longer time for
inorganic nutrients be available for use

Nitrate depression

Period results when microbes must supplement with N from soil to decompose high C/N residue