Chemical Properties of Soils: Soil Reaction, Cation Exchange Capacity, Base Saturation, and Redox Potentials

Soil reaction is one of the master variables governing chemical, biological, and physical properties of soils. It refers to the degree of acidity or alkalinity of the soil mass, quantitatively expressed as the negative logarithm of hydrogen ion activity (pH = -\log₁₀[H^+]).

• Active Acidity: Refers to the concentration of hydrogen (H^+) and aluminum (Al³⁺) ions present in the soil solution at any given moment. It is measured directly using a glass electrode pH meter.
• Reserve (Potential) Acidity: Represents the quantity of exchangeable H^+ and Al³⁺ ions held electrostatically on the colloidal exchange complex (clays and humus). It is vastly larger than active acidity and acts as a buffer.
• Buffering Capacity: The ability of a soil to resist sudden changes in pH upon the addition of acidic or basic amendments. Fine-textured soils rich in 2:1 layer silicates and organic matter possess high buffering capacities, whereas coarse-textured sandy soils have low buffering capacities.

Cation Exchange Capacity (CEC) is the capacity of a soil to adsorb and exchange positively charged ions (Ca²⁺, Mg²⁺, K^+, Na^+, NH_4^+, H^+, Al³⁺). It is expressed in centimoles of charge per kilogram of dry soil (cmol(p^+)/kg or traditionally meq/100g).

• Origin of Charges: Isomorphic substitution within clay mineral crystal lattices generates permanent negative charges (e.g., substitution of Mg²⁺ for Al³⁺ in octahedral sheets, or Al³⁺ for Si⁴⁺ in tetrahedral sheets). pH-dependent charges arise from the dissociation of functional groups (carboxyl and phenolic hydroxyls) on soil organic matter and exposed edges of silicate clays.
• CEC Values of Colloids: Humus (100-300\;cmol/kg), Vermiculite (100-150\;cmol/kg), Smectite (80-120\;cmol/kg), Illite (20-40\;cmol/kg), Kaolinite (3-15\;cmol/kg).
• Anion Exchange Capacity (AEC): The capacity of a soil to hold and exchange negatively charged anions (NO_3^-, Cl^-, SO_4^2-, H_2PO_4^-). It is prominent in highly weathered tropical soils (Oxisols and Ultisols) rich in sesquioxides (iron and aluminum oxides/hydroxides) under acidic pH conditions where protonation creates positive charges.

Base saturation is a vital chemical fertility index that denotes the proportion of the soil CEC occupied by basic cations (Ca²⁺, Mg²⁺, K^+, Na^+) rather than acidic cations (H^+, Al³⁺).

• Formula: Base Saturation (%) = ( Sum of Basic CationsCEC ) × 100.
• Agronomic Significance: Soils with high base saturation (>80%) are typically neutral to alkaline and fertile. Soils with low base saturation (<50%) are acidic, heavily leached, and prone to aluminum toxicity and calcium/magnesium deficiencies.
• Exchangeable Sodium Percentage (ESP): Measures the sodicity hazard of soils, calculated as ESP = ( Exchangeable Na^+CEC ) × 100. Soils with an ESP > 15 are classified as sodic (alkaline) soils, characterized by structural dispersion and poor hydraulic conductivity.

Redox potential (Eh), measured in millivolts (mV), reflects the intensity of oxidation or reduction reactions in soils, governed by the availability of electron acceptors.

• Aerobic Soils: High Eh (+400 mV to +700 mV). Oxygen serves as the primary electron acceptor. Organic matter decomposes rapidly; nitrogen exists as nitrate (NO_3^-); iron is present in oxidized ferric (Fe³⁺) forms.
• Submerged (Paddy) Soils: Low Eh (dropping below +300 mV down to -300 mV). Oxygen is rapidly depleted by microorganisms. Sequential reduction of alternative electron acceptors occurs: NO_3^- is reduced to N_2 gas (denitrification), Mn⁴⁺ is reduced to soluble Mn²⁺, Fe³⁺ is reduced to mobile Fe²⁺ (causing gleying), and sulfates are reduced to hydrogen sulfide (H_2S).
• Nutrient Availability Dynamics: Submergence increases the availability of phosphorus due to the dissolution of iron phosphate complexes upon reduction of Fe³⁺ to Fe²⁺, but can lead to zinc deficiency due to the precipitation of zinc as frankinite or zinc sulfide.

Salt-affected soils are categorized based on three chemical parameters: Electrical Conductivity of the saturation extract (EC_e), soil pH, and Exchangeable Sodium Percentage (ESP).

• Saline Soils: EC_e > 4\;dS/m, pH < 8.5, ESP < 15. Dominated by neutral soluble salts of chlorides and sulfates of sodium, calcium, and magnesium. White alkali crusts are common.
• Sodic (Sodified/Alkaline) Soils: EC_e < 4\;dS/m, pH > 8.5, ESP > 15. Dominated by sodium carbonate and bicarbonate, leading to high hydroxyl ion concentrations and dispersion of soil colloids (black alkali).
• Saline-Sodic Soils: EC_e > 4\;dS/m, pH < 8.5, ESP > 15. Properties are intermediate; if leached without calcium amendments, they convert into sodic soils. Reclamation requires gypsum (CaSO_4 · 2H_2O) for sodic soils to replace exchangeable sodium with calcium.