Soils 3

Question: What is a colloid in soil?

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

Question: What are the sources of soil colloids?

Answer: Weathering of silicate rocks and recombination into new mineral structures.

Question: What are the four major types of soil colloids?

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

Question: What elements are clays primarily composed of?

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

Question: What are the two basic crystal units in clays?

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

Question: What is isomorphic substitution in clays?

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

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

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

Question: What is a 2:1 expanding clay mineral?

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

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

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

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

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

Question: What are Fe and Al oxides in soil?

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

Question: What is organic matter in soils?

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

Question: What are sources of charge in soils?

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

Question: How does pH-dependent charge work?

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

Question: What is Cation Exchange Capacity (CEC)?

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

Question: What factors affect CEC?

Answer: Texture, colloid type, and soil pH.

Question: How does texture affect CEC?

Answer: Clay soils have higher CEC than sandy soils.

Question: How does colloid type affect CEC?

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

Question: How does pH affect CEC?

Answer: Higher pH increases CEC by deprotonating functional groups.

Question: What is cation preference in soil exchange?

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

Question: What is Base Saturation?

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

Question: What is Exchangeable Acidity?

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

Question: What is Anion Exchange Capacity (AEC)?

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

Question: How can you manipulate CEC to improve fertility?

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

Question: What is soil pH?

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

Question: What factors contribute to soil acidity?

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

Question: How does aluminum hydrolysis affect soil pH?

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

Question: What factors contribute to soil alkalinity?

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

Question: How does pH affect the exchange complex?

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

Question: What is buffering in soils?

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

Question: What buffers soil pH?

Answer: Aluminum, CEC, and carbonates.

Question: How does human activity change soil pH?

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

Question: How does soil pH affect plant growth?

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

Question: What are saline soils?

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

Question: What are sodic soils?

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

Question: What are saline-sodic soils?

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

Question: How does salt affect plant growth?

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

Question: How does sodium affect soil structure?

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

Question: What is SAR?

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

Question: What is ESP?

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

Question: How do you remediate saline soils?

Answer: Flush soils with clean water and ensure proper drainage.

Question: How do you remediate sodic soils?

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