Soil Science 221 Lecture 2

CEC

total capacity of soil to hold exchangeable cations acting as a vital reservoir for plant nutrients

Importance of CEC

Reservoir of nutrients for plant, indicator of fertility and potential fert, reduction in nutrient losses, affects soil buffering capacity

Soil Colloids

electrically charged surfaces, type and amount determine soils CEC

Crystalline Silicate Clays Colloids

layered structure, dominant in most soils, sjmectite,Illite,

Noncrystalline Silicate Clays Colloids

form from volcanic ash

Iron and aluminum oxides Colloids

important in highly weathered tropical soils

Organic Matter Colloids

non mineral partially decomposed charge is very pH dependent highest CEC of all

Wester Soils CEC are typically occupied by

Ca 50-80, Mg, 10-49

How do Plants influence CEC?

roots push off H+H+ and try to replace it with Ca2+ on the soil colloid

Soil Organic Matter

materials of biological origin in the soil (living mirco organisms, newly incorporated crop residue)

What happens to SOM under cultivation?

it decreases and the soil becomes more compact, it can be reversed over time though

SOM Benefits

source of nutrients, increases water holding capacity, improves soil structure and water infiltration rate, provides microbial habitat, buffers against temp and pH, carbon sequestration

Soil OM is a

colloid and increases cation exchange capacity

Soil pH

measure of acidity or alkalinity of soil, acidic <7, neut. 7, alkaline >7

Why is Soil pH important

control solubility vs insolubility of nutrients, makes toxic elements more available, biological activity, impacts plant health directly

Soil acidity is caused by

anything that forms H+ or consumes OH

Al 3+ in soil acidity

strong tendency to perform hydrolysis, leaves H+ after combing with OH-

3 Acidity Pools: Active

H+ ions in the soil solution

3 Acidity Pools: Exchangeable acidity

H+ and Al + ions on the soil exchange sites

3 Acidity Pools: Residual Acidity

H+ and Al+ ions/compounds that cannot be exchanged

Basic Reservoirs

CaCO3 release OH- when acids are added

Acidic reservoirs

Al and H bound to clay release H+ when bases are added

Buffering Capacity comes from

amount of exchange sites available and the shifting chemical equilibria to counteract pH changes(more exchange sites=more resistance)

Low Buffering Capacity

sandy soils, highly leaches soils low CEc, soils dominated by 1:1 clays, low in organic matter

High Buffering Capacity

clayey soils, soils high in OM, high CEC values, calcareous soils, soils rich in 2:1 clays

What causes strong buffering at a low pH

hydrolysis and precipitation of aluminum compunds

What causes strong buffering at high pH

calcium carbonate by the precipitation and dissolution of calcium carbonate

To raise and lower pH

raise=add liming material, lower=add acid or acid forming material

How does CaCO3 raise soil pH

direction of reaction determines if OH- ions are produced or consumed, adding liming pushes reaction to the right,

Salinity

accumulation of dissolved mineral and salts in soil reduces water uptake and toxicity

Sodicity

proportion of Na+ in soil CEC that in high amounts causes soil dispersion, poor structure, reduced water infiltration

Ag causes of soil salinity

irrigation water (dissolved salts_, low rainfall, high evaporation, fertilizers and amendments, crop removal

Natural Causes of Soil Salinity

groundwater, parent materials, shallow water tables, low lying areas

Soil Salinity can cause

osmotic stress decreases soils water potential, ion toxicity Na+ and Cl- enter plant tissues

Visual Symptoms

stunted growth and reduced yields, leafy yellowing and browning, premature leaf drop, poor seed germination

Soil Sodicity and Plant Growth

dispersion Na+ replaces Ca2+ and Mg2+ on clay exchange sites, clay particles repel each other, soil structure destroyed,

Soil Sodium Na+ status can be expressed as

exchangeable sodium percentage ESP or sodium adsorption ratio (SAR)

When sodium is present the ph can

rise to 10 or 11 because Na2CO3 is more soluble than CaCO3

Reclaiming Saline Soils

add clean water and leach salts from root zone, ensure proper drainage to remove leached salts

Reclaiming Sodic/Saline- Sodic Soils

replace Na+ with soluble Ca2+ usually gypsum, leach displaced Na+ with water improve drainage, multiple treatments over 7 years