Soil Reaction

Definition

Soil reaction, commonly referred to as soil pH, is a measure of the acidity or alkalinity of the soil solution. It indicates the concentration of hydrogen ions ($H^+$) present in the soil moisture, determining the chemical environment that influences plant nutrient availability and microbial activity.

Main Content

1. The pH Scale

• The scale ranges from 0 to 14, where pH 7.0 is neutral. Values below 7.0 indicate acidic soil, while values above 7.0 indicate alkaline (basic) soil.
• Soil reaction is measured on a logarithmic scale; a change of one pH unit represents a tenfold change in acidity or alkalinity.

2. Causes of Soil Acidity

• Leaching of basic cations (Calcium, Magnesium, Potassium) by heavy rainfall removes them from the root zone, replacing them with Hydrogen and Aluminum ions.
• The use of nitrogenous fertilizers (like ammonium sulfate) releases H+ ions during the process of nitrification.

3. Causes of Soil Alkalinity

• Accumulation of soluble salts, often found in arid and semi-arid regions where evaporation exceeds precipitation.
• The presence of calcium carbonate (lime) in the parent material or irrigation water increases the pH of the soil.

Working / Process

1. Measurement of Soil pH

• A soil sample is collected from the top 15–20 cm of the profile, dried, and sieved to remove debris.
• The sample is mixed with distilled water or a calcium chloride solution (usually in a 1:2.5 ratio) to create a suspension.

2. Interaction with Soil Colloids

• Soil colloids (clay and organic matter) act as a reservoir for H+ ions, known as reserve acidity.
• The ions in the soil solution are in equilibrium with those adsorbed on the surface of soil particles, creating a buffer system that resists rapid pH changes.

3. Effect on Nutrient Availability

• pH dictates the chemical form of nutrients; for example, at low pH, Phosphorus often reacts with Iron to form insoluble compounds.
• Below is a visual representation of the pH scale and its impact on nutrient solubility:

Acidic (pH < 6)      Neutral (pH 7)      Alkaline (pH > 8)
    |----------------------|----------------------|
    [ Iron/Manganese High ] [ Optimal Availability ] [ Calcium/Magnesium High ]
    [ Phosphorus Low      ] [                      ] [ Micronutrients Low     ]

Advantages / Applications

• Nutrient Management: Proper soil reaction ensures that essential nutrients like Nitrogen, Phosphorus, and Potassium are readily available for plant uptake.
• Crop Selection: Farmers can choose crop varieties that are adapted to specific pH ranges, such as blueberries for acidic soil or alfalfa for neutral-to-alkaline soil.
• Soil Amendment: Understanding soil reaction allows for the application of lime to raise pH in acidic soils or sulfur/gypsum to lower pH in alkaline soils.

Summary

Soil reaction is the quantitative expression of the acidity or alkalinity of soil, primarily driven by the concentration of hydrogen ions. It serves as a master variable that controls the solubility of nutrients, the health of soil microorganisms, and the overall suitability of land for specific agricultural practices.

• pH Scale: The measurement unit for acidity/alkalinity.
• Buffer Capacity: The ability of soil to resist changes in pH.
• Liming: The process of adding alkaline materials to correct soil acidity.