Concise Notes on Soil Acidity

Soil Acidity

Soil acidity is defined by the relative concentration of hydronium ions in the soil solution.

  • The concentration of hydrogen ions quantitatively measures soil acidity.
  • Aluminum ions indirectly contribute through dissolution in water: Al^{3+} + 2H2O \rightleftharpoons Al(OH)2^+ + 3H^+

Effective Cation Exchange Capacity (ECEC)

  • The sum of cations on the soil's ion exchange surface.
  • Indicates the soil's nutrient retention capacity; higher ECEC means higher retention.
  • Soils with higher clay and organic matter content tend to have higher ECEC.
  • Important ECEC-based indicators include exchangeable aluminum, base saturation, and exchangeable sodium percentages.

Auto-ionization of Water and pH Scale

Water self-ionizes: 2H2O (l) \rightarrow H3O^+ (aq) + OH^- (aq)

  • Acidic: $[H_3O^+] > [OH^-]$
  • Basic: $[H_3O^+] < [OH^-]$
  • At equilibrium (25°C): $[H_3O^+] = [OH^-] = 10^{-7} mol/l$
  • Equilibrium constant: Kw = [H^+][OH^-] = 10^{-14}, pKw = pH + pOH = 14
  • pH is the negative logarithm of hydrogen ions concentration: pH = -log[H^+], [H^+] = 10^{-pH}, pOH = -log[OH^-]

The pH scale ranges from 0 to 14; 0-6 is acidic, 8-14 is alkaline, and 7 is neutral.

Pools of Soil Acidity

  1. Active acidity: [H^+] ions in the soil solution.
  2. Exchangeable or reserve acidity: Acidity from exchangeable aluminum and hydrogen ions on cation exchange sites.
  3. Non-exchangeable acidity: Hydrogen ions held within the soil complex, not affecting soil pH.
  • Total soil acidity = active + exchangeable + residual acidity.

Causes of Soil Acidity

  • Rainfall and leaching remove basic cations.
  • Acidic parent material (e.g., granite).
  • Organic matter decay produces organic acids and CO_2.
  • Harvesting high-yielding crops removes basic cations.
  • Nitrification of ammonium produces hydrogen ions.

Effects of Soil Acidity on Nutrient Availability

  • Ideal soil pH for most crops: 6.5 to 7.5.
  • Phosphorus (P) availability is significantly affected; it reacts with aluminum (Al) and iron (Fe) in acidic soils to form less soluble compounds (e.g., AlPO4) and with Calcium (Ca) and Magnesium (Mg) in alkaline soils (e.g., Ca3(PO4)2).
  • Molybdenum (Mo) becomes less available in acid soils.
  • Micronutrients (Cu, Fe, Zn, Mn) become less available in alkaline soils.
  • Aluminum toxicity inhibits root elongation.

Managing Acidic Soils

  • Adaptive measures: growing crops adapted to lower pH values.
  • Liming: Applying alkaline materials to raise soil pH.
  • Lime removes hydrogen ions from the CEC and replaces them with calcium or magnesium.
  • Major types of liming materials: carbonates, oxides, and hydroxides of calcium and magnesium; examples: limestone (CaCO3), hydrated lime (Ca(OH)2), quick lime (CaO), dolomite (CaMg(CO3)2).
  • Liquid lime advantages: faster nutrient absorption, better uniformity, quicker reaction.

Benefits of Liming

  • Raises soil pH.
  • Improves soil structure.
  • Accelerates organic matter decomposition.
  • Eliminates aluminum and manganese toxicity.
  • Increases availability of phosphorus, molybdenum, calcium, and magnesium.
  • Improves conditions for microbiological activities.

Overliming can reduce phosphorus availability and induce micronutrient deficiencies; soil analysis is important.

Lime Requirement

Amount of lime needed to raise soil pH to a desired value.

Factors:

  • Required pH change.
  • Soil buffer capacity.
  • Amount of soil to neutralize.
  • Lime chemical composition.
  • Lime fineness.

Methods of determination:

  • Soil titration with calcium hydroxide.
  • Soil incubation with lime.
  • Aluminum saturation.

Quality of Agricultural Lime

Factors:

  • Neutralizing Value (NV or CCE): The quantity of acid a lime can neutralize compared to pure calcium carbonate.
  • Particle Size Distribution: Finer particles react faster; larger particles have longer-lasting effects.
  • Chemical Reactivity: Depends on mineralogical composition; calcite is more reactive than dolomite.

Effective Neutralizing Value (ENV) = NV x FF (fineness factor).

Example: Neutralizing value of CaO is 178.6%.