Soils 3

What is a colloid in soil?

Organic and inorganic matter with particle size <2 μm and large surface area, including clays, organic matter, and oxides.

What are the sources of soil colloids?

Weathering of silicate rocks and recombination into new mineral structures.

What are the four major types of soil colloids?

Layer silicate clays, Fe and Al oxides, allophane (amorphous materials), and organic matter (humus).

What elements are clays primarily composed of?

Aluminum (Al), Silicon (Si), Iron (Fe), Magnesium (Mg), and Oxygen (O).

What are the two basic crystal units in clays?

Octahedra (Al or Mg surrounded by oxygen/hydroxide) and Tetrahedra (Si surrounded by oxygen).

What is isomorphic substitution in clays?

Replacement of one ion by another of similar size but different charge, creating permanent negative charge.

What is a 1:1 clay mineral? Give an example.

One tetrahedral and one octahedral sheet bonded; example: kaolinite.

What is a 2:1 expanding clay mineral?

Two tetrahedral sheets and one octahedral sheet with large interlayer spaces; example: smectites.

What is a 2:1 semi-expanding clay mineral?

Two tetrahedral sheets and one octahedral sheet with limited expansion; example: vermiculite.

What is a 2:1 non-expanding clay mineral?

Two tetrahedral sheets and one octahedral sheet held tightly by potassium ions; example: mica.

What are Fe and Al oxides in soil?

Secondary minerals like goethite, hematite, and gibbsite, common in weathered soils.

What is organic matter in soils?

Decomposed plant residues containing hydrocarbons and functional groups that contribute to pH-dependent charge.

What are sources of charge in soils?

Isomorphic substitution (permanent charge) and pH-dependent charge (functional groups and broken edges).

How does pH-dependent charge work?

Functional groups accept or release protons depending on pH, affecting the colloid’s charge.

What is Cation Exchange Capacity (CEC)?

The total amount of positive charge a soil can adsorb, measured in cmolc/kg.

What factors affect CEC?

Texture, colloid type, and soil pH.

How does texture affect CEC?

Clay soils have higher CEC than sandy soils.

How does colloid type affect CEC?

Humus > 2:1 clays > 1:1 clays > Fe/Al oxides.

How does pH affect CEC?

Higher pH increases CEC by deprotonating functional groups.

What is cation preference in soil exchange?

Cations with higher charge and smaller hydrated radius are more strongly held (Al3+ > Ca2+ > Mg2+ > K+ = NH4+ > Na+).

What is Base Saturation?

The percentage of CEC occupied by base cations (Ca2+, Mg2+, K+, Na+).

What is Exchangeable Acidity?

The percentage of CEC occupied by acid cations (H+, Al3+).

What is Anion Exchange Capacity (AEC)?

The soil’s ability to adsorb anions, which increases with lower pH.

How can you manipulate CEC to improve fertility?

By adding organic matter, liming to raise pH, or fertilizing with cations.

What is soil pH?

A measure of H+ concentration, where lower pH means higher acidity.

What factors contribute to soil acidity?

Organisms producing organic acids, accumulation of organic matter, carbonic acids, aluminum hydrolysis.

How does aluminum hydrolysis affect soil pH?

Al3+ reacts with water to release H+, increasing acidity.

What factors contribute to soil alkalinity?

Presence of carbonate minerals, weathering releasing base cations, production of base-producing anions.

How does pH affect the exchange complex?

Acidic soils have more acid cations; basic soils have more base cations.

What is buffering in soils?

The resistance of soil pH to change due to residual, exchangeable, and active acidity.

What buffers soil pH?

Aluminum, CEC, and carbonates.

How does human activity change soil pH?

Through chemical fertilizers, acid deposition, irrigation practices, and pyrite oxidation.

How does soil pH affect plant growth?

Affects nutrient availability and toxicity; certain plants prefer acidic or basic soils.

What are saline soils?

Soils with high total salt concentration (EC > 4 dS/m, SAR < 13).

What are sodic soils?

Soils with low total salts but high sodium concentration (EC < 4 dS/m, SAR > 13).

What are saline-sodic soils?

Soils with high total salts and high sodium concentration (EC > 4 dS/m, SAR > 13).

How does salt affect plant growth?

Causes low nutrient availability, water stress, high pH, and potential boron toxicity.

How does sodium affect soil structure?

Causes colloids to disperse, reducing water infiltration and increasing erosion.

What is SAR?

Sodium Adsorption Ratio, measuring sodium relative to calcium and magnesium in soil solution.

What is ESP?

Exchangeable Sodium Percentage, degree to which soil exchange sites are saturated with sodium.

How do you remediate saline soils?

Flush soils with clean water and ensure proper drainage.

How do you remediate sodic soils?

Add gypsum (CaSO4) to replace sodium, then flush with water.