Soil Amendments

Introduction

• Soil amendments are materials added to soil mainly to improve soil properties rather than to supply nutrients directly.
• They play a crucial role in reclaiming problem soils, improving soil structure, regulating soil pH, and enhancing water and nutrient availability.
• Soil amendments are an essential component of sustainable agriculture and Integrated Nutrient Management (INM).

Definition: Soil amendments are substances applied to soil to improve its physical, chemical, and biological properties, thereby creating a favorable environment for plant growth.

Objectives of Soil Amendments

• Correct soil problems such as acidity, salinity, and sodicity to make soils suitable for crop growth.
• Improve soil structure, aggregation, porosity, aeration, and water infiltration.
• Regulate and maintain optimum soil pH, creating a favorable environment for roots and microbes.
• Increase availability of essential nutrients by reducing fixation and toxicity effects
• Improve fertilizer use efficiency (FUE) by enhancing nutrient retention and uptake.
• Reclaim degraded, alkali, acidic, and salt-affected soils for productive agriculture.
• Reduce soil compaction and crusting, especially in sodic soils.
• Promote better root growth and microbial activity.
• Maintain long-term soil fertility and sustainable productivity.

A) Chemical Soil Amendments: Chemical soil amendments are inorganic materials used primarily for pH correction and reclamation of problem soils. They do not act as fertilizers but improve soil conditions for better nutrient utilization.

i) Liming Materials (for Acid Soils)

Examples

• Lime (Calcium carbonate – CaCO₃)
  • Dolomite [Calcium magnesium carbonate – CaMg(CO₃)₂]
  • Quick lime (Calcium oxide – CaO)
  • Slaked lime (Calcium hydroxide – Ca(OH)₂)

Functions / Effects

• Neutralize soil acidity by reacting with hydrogen ions (H⁺).
  • Increase soil pH to an optimum range for crop growth.
  • Reduce aluminium (Al³⁺) and manganese (Mn²⁺) toxicity, which restrict root growth in acidic soils.
  • Improve availability of phosphorus (P) by reducing fixation.
  • Supply calcium (Ca) and magnesium (Mg) (especially dolomite).
  • Increase availability of molybdenum (Mo), important for nitrogen fixation.
  • Enhance microbial activity, including nitrogen-fixing and decomposer microbes.
  • Improve soil aggregation and tilth, aiding better root penetration.
  • Increase fertilizer efficiency, especially of nitrogen and phosphorus fertilizers.

ii) Gypsum (for Sodic / Alkali Soils)

Chemical formula: Calcium sulphate dihydrate (CaSO₄·2H₂O)

Functions / Effects

• Supplies calcium (Ca²⁺) to the soil.
  • Calcium replaces exchangeable sodium (Na⁺) from soil colloids.
  • Displaced sodium is leached out with irrigation or rainwater.
  • Improves soil structure and aggregation by flocculating clay particles.
  • Reduces soil dispersion and surface crusting.
  • Increases soil permeability, infiltration, and drainage.
  • Improves root growth and aeration in sodic soils.
  • Enhances water movement and reduces waterlogging.
  • Helps reclaim alkali (sodic) soils without significantly changing soil pH.
  • Supplies sulphur (S), an essential secondary nutrient.
  • Improves response of crops to fertilizers in reclaimed soils.

iii) Acid-Forming Amendments (for Alkaline Soils)

Acid-forming amendments are used to reduce soil pH in alkaline and calcareous soils where high pH limits nutrient availability. Examples: Elemental sulphur (S), Pyrite (Iron disulphide – FeS₂)

Functions / Effects

• Lower soil pH through microbial oxidation processes.
  • Elemental sulphur is oxidized by Thiobacillus bacteria to form sulphuric acid (H₂SO₄).
  • Pyrite undergoes oxidation producing sulphuric acid and iron compounds.
  • Reduce alkalinity and make soil environment favorable for crop growth.
  • Increase availability of phosphorus (P) by reducing calcium phosphate fixation.
  • Improve availability of iron (Fe), zinc (Zn), manganese (Mn), and copper (Cu) in alkaline soils.
  • Correct iron chlorosis commonly observed in calcareous soils.
  • Enhance efficiency of applied fertilizers in high-pH soils.
  • Particularly useful in calcareous, sodic-calcareous, and alkaline soils.

Organic Soil Amendments

Organic soil amendments are natural organic materials added to soil to improve its physical, chemical, and biological properties.

Examples

• Farmyard manure (FYM)
  • Compost
  • Vermicompost
  • Green manure
  • Crop residues
  • Press mud (sugar industry by-product)

Functions / Effects

• Improve soil structure and aggregation, making soil friable and well-aerated.
  • Increase soil organic matter and cation exchange capacity (CEC).
  • Enhance water-holding capacity, especially in sandy soils.
  • Improve soil porosity, infiltration, and drainage.
  • Increase microbial population and activity, stimulating nutrient transformations.
  • Promote mineralization of nutrients, making them available to crops.
  • Reduce nutrient losses through leaching and volatilization.
  • Improve availability of macro-, secondary, and micronutrients.
  • Buffer soil pH and reduce adverse effects of acidity or alkalinity.
  • Improve soil biological health and long-term soil fertility.
  • Support sustainable agriculture by recycling farm and industrial wastes.

B) Physical Soil Amendments

Physical soil amendments are materials added to soil to modify soil texture, structure, porosity, and water movement.

Examples

• Sand – used in heavy clay soils
  • Clay or silt – used in sandy soils
  • Biochar – carbon-rich material produced from biomass

Functions

• Improve soil aeration by increasing pore spaces.
  • Enhance drainage in heavy and compact soils.
  • Reduce soil compaction and crust formation.
  • Improve moisture retention in light (sandy) soils.
  • Biochar increases water-holding capacity and CEC.
  • Improve root penetration and overall soil tilth.
  • Help in long-term improvement of soil physical condition.

c) Biological Soil Amendments

• Biological soil amendments contain living organisms that improve soil fertility through biological processes.
• Examples: Biofertilizers (Rhizobium, Azotobacter, PSB,  Mycorrhiza, Microbial inoculants

Functions

• Improve nutrient solubilization (especially phosphorus).
  • Enhance biological nitrogen fixation.
  • Increase nutrient mobilization in soil.
  • Improve root growth and root surface area.
  • Enhance uptake of P, Zn, Fe, and other micronutrients.
  • Improve soil microbial diversity and activity.
  • Reduce dependence on chemical fertilizers.

Role of Soil Amendments in Problem Soils

Advantages of Soil Amendments

• Improve soil physical properties (structure, porosity, drainage).
  • Improve soil chemical properties (pH regulation, nutrient availability).
  • Improve soil biological activity and microbial health.
  • Increase fertilizer use efficiency (FUE).
  • Help in reclamation of problem soils.
  • Improve crop yield and quality.
  • Maintain long-term soil fertility.
  • Support sustainable and eco-friendly agriculture.

Limitations of Soil Amendments

• Do not supply nutrients in large or immediate quantities.
  • Require correct dose, timing, and method of application.
  • Effects are slow and long-term, not immediate.
  • Many amendments are bulky and difficult to transport.
  • Require soil testing for effective use.
  • Improper use may give poor or uneven results.