Soils 345 Final

Soil colloids

Very small particles with extremely high surface area and reactive surface charge controlling nutrient retention and soil chemistry.

Sources of negative charge

Isomorphous substitution and deprotonation of OH groups on colloids.

Organic colloids (humus)

Highly decomposed organic matter with extremely high, pH-dependent CEC.

Humus functional groups

Carboxyl (-COOH), phenolic OH, alcoholic OH — release H⁺, generating negative charge.

CEC of humus

Typically 150-300 cmolc/kg depending on pH.

Phyllosilicates

Layer silicate clays built from Si tetrahedra and Al octahedra.

1:1 clays (kaolinite)

One tetra + one octa layer; low CEC, non-expanding.

2:1 clays (smectite)

Two tetra + one octa layers; very high CEC, major shrink-swell behavior.

2:1 clays (illite)

K⁺-stabilized interlayer; moderate CEC; limited swelling.

2:1:1 clays (chlorite)

Hydroxide interlayer; moderate CEC; limited expansion.

Noncrystalline clays

Allophane and imogolite; short-range order; strong P adsorption.

Allophane

Spherical, highly reactive aluminosilicate colloid; strong phosphate fixation.

Imogolite

Tubular short-range-order mineral with very high surface reactivity.

Fe oxides (goethite)

Common oxide with variable charge; gives yellowish hues.

Hematite

Fe oxide giving red color; common in well-drained, oxidized soils.

Al oxide (gibbsite)

Al hydroxide; abundant in highly weathered soils; variable charge.

Variable charge minerals

Charge becomes positive at low pH and negative at high pH.

Isomorphous substitution

Replacement in mineral crystal lattice generating permanent charge.

Dioctahedral clays

Two of three octahedral sites filled; Al-dominant.

Trioctahedral clays

All octahedral sites filled; Mg/Fe-dominant.

CEC definition

Total exchangeable cation charge soil can hold, cmolc/kg.

Buffered CEC

CEC measured at pH 7; indicates potential CEC.

ECEC

Effective CEC at existing soil pH (includes exchangeable Al³⁺).

Cation selectivity principle

Higher valence and smaller hydrated radius = stronger attraction.

Hydrated radius importance

Larger hydrated radius = weaker sorption; Na⁺ disperses clays.

Flocculation

Ca²⁺ and Mg²⁺ compress diffuse layer and promote aggregation.

Dispersion

Na⁺ expands diffuse layer and causes particles to repel.

Base saturation

Fraction of CEC occupied by Ca²⁺, Mg²⁺, K⁺, Na⁺.

Al saturation

Fraction of ECEC occupied by Al³⁺.

Salt-replaceable acidity

Exchangeable Al³⁺ and H⁺ displaced by neutral salt.

Residual acidity

Structural H⁺ and hydrolyzable Al not displaced by salts.

pH-dependent charge

Increases as pH rises (more deprotonation).

Permanent charge

Due to isomorphous substitution; independent of pH.

Anion exchange capacity (AEC)

Positive surface charge at low pH enables anion retention.

CEC and fertility

Higher CEC soils store more nutrients and resist leaching.

Effect of organic matter

Strong increase in CEC per unit mass compared to clays.

Colloid size

<1 µm particles with enormous specific surface area.

Colloid specific surface area

Controls reactivity, CEC, and OM interactions.

Double layer thickness

Thinner layers (high ionic strength) promote flocculation.

Charge density

Charge per unit area; affects cation affinity.

Cation replacement rule

Exchange occurs on a charge-equivalent basis.

Sodium hazard

High Na⁺ saturation causes clay slaking and dispersion.

Smectite shrink-swell

Large interlayer expansion; problematic for engineering.

Kaolinite properties

Strong hydrogen bonding; stable aggregates; low activity.

Illite interlayer

K⁺ locks tetrahedral sheets; prevents swelling.

Allophane + OM interactions

Forms organo-Al complexes stabilizing aggregates.

P fixation by oxides

Fe/Al oxides form inner-sphere complexes with phosphate.

Weathering degree → colloid type

Advanced weathering → oxides + kaolinite.

Colloid-water interactions

Control retention, mobility, and chemical reactivity.

Cation activity vs concentration

Activity accounts for ionic strength; used in pH definition.

pH formula

pH = -log₁₀(a_H⁺).

pKa meaning

pH at which acid is half dissociated.

Al hydrolysis

Produces 3 H⁺ per Al³⁺ through stepwise reactions.

CEC measurement method

NH₄OAc at pH 7 most common buffered method.

Neutral salt pH behavior

KCl pH lower due to displaced exchangeable acidity.

Major colloid groups

Organic, silicate clays, noncrystalline clays, Fe/Al oxides.

Soil acidity pools

Active, exchangeable (salt replaceable), residual.

Active acidity

H⁺ in soil solution; smallest pool but controls pH reading.

Exchangeable acidity

H⁺ + Al³⁺ on colloids released by salt extraction.

Al toxicity mechanism

Al³⁺ inhibits root elongation and nutrient uptake.

Effect of liming

Precipitates Al as Al(OH)₃ and increases base saturation.

Buffering capacity

Soil's resistance to pH change; high in clay and OM soils.

pH vs nutrient availability

Micronutrient solubility increases at low pH; P becomes less available.

CEC-pH relationship

pH-dependent colloids increase CEC as pH rises.

Sulfide oxidation

FeS₂ + O₂ → sulfuric acid; acid sulfate soils.

Organic acids

Contribute to acidity in forest floors; mobilize metals.

Soluble Al generation

Hydrolysis reactions produce H⁺.

Lime requirement factors

CEC, base saturation, buffering, target pH.

Acidity in conifer soils

Organic acids produce naturally low pH forest floors.

Texture definition

Relative % of sand, silt, clay in fine earth fraction.

Sand properties

Fast drainage; low WHC; large pores.

Silt properties

High available water; intermediate Ksat.

Clay properties

High WHC; low Ksat; small pores.

Textural triangle use

Determine class from % sand-silt-clay.

Loam description

Balanced properties; ideal for roots.

Loam

Balanced properties; ideal for roots.

Structure

Arrangement of particles into aggregates (peds).

Granular structure

Common in A horizons; indicates biological activity.

Blocky structure

B horizons; angular or subangular.

Prismatic structure

Vertically oriented aggregates common in Bt.

Columnar structure

Sodium-affected soils; dispersive.

Platy structure

Horizontal layers restricting flow.

Structure grade

Weak, moderate, strong.

Ped size classes

Very fine to very coarse.

Aggregate formation

Driven by roots, OM, drying-wetting, freeze-thaw.

Aggregate stability

OM + polyvalent cations enhance stability.

Effect of clay mineralogy

Smectites swell; kaolinite stable.

OM's role in structure

Binds particles through polysaccharides.

Texture-structure interaction

Structure compensates for fine texture by forming macropores.

Parent material influence

Loess → silt; till → mixed texture; outwash → sands.

Soil structure & productivity

Granular aggregates increase aeration and root penetration.

Particle density

~2.65 g/cm³ for quartz-dominant soils.

Bulk density

Dry mass / total bulk volume.

Db of surface soils

~1.1-1.4 g/cm³.

Db of subsoils

~1.4-1.8 g/cm³.

Root-limiting Db threshold

>1.6 g/cm³ restricts root growth.

Total pore space formula

TPS = 1 − Db/Dp.

Macropores

Drainage and aeration pores >0.08 mm.

Mesopores

Plant-available water pores.

Micropores

Water retention pores <0.08 mm.