Causes of Soil Acidity Rainfall and Leaching

 Causes of Soil Acidity 

Rainfall and Leaching

In areas with high rainfall, water moves through the soil, washing away essential basic cations like Sodium (Na), Calcium (Ca), Potassium (K), and Magnesium (Mg).

Acid rain (rain containing dissolved acidic gases like sulfur dioxide) further contributes to soil acidity by adding hydrogen ions to the soil.

Acidic Parent Material

The type of rock from which soil is formed influences its pH.

Soils developed from granite tend to be more acidic, while those from limestone or calcareous shales are more alkaline.

Organic Matter Decay

When organic matter decomposes, it releases organic acids.

The carbon dioxide (CO₂) produced during decomposition reacts with water (H₂O) to form carbonic acid (H₂CO₃), which slightly increases soil acidity.

Harvest of High-Yielding Crops

Plants absorb essential nutrients (basic cations) like Ca, Mg, K, and Na from the soil.

When crops are harvested, these nutrients are removed from the field, leading to soil depletion and increased acidity.

Nitrification of Ammonium Fertilizers

When ammonium-based fertilizers (NH₄⁺) are converted into nitrates (NO₃⁻) through the process of nitrification, two hydrogen ions (H⁺) are released into the soil.

This increases soil acidity over time.

Correcting Soil Acidity with Lime

Liming Acidic Soils

Applying lime (calcium-containing materials) is the most common way to neutralize soil acidity. The main liming materials include:

Calcium oxide (CaO) – Also known as quicklime or burnt lime.

Dolomite (CaMg(CO₃)₂) – A natural mineral containing both calcium and magnesium.

Calcium carbonate (CaCO₃) – Also known as agricultural lime or ground limestone.

How Liming Works

Lime reacts with the soil, removing hydrogen ions (H⁺) from the cation exchange complex.

The hydrogen ions are replaced with calcium (Ca²⁺) and magnesium (Mg²⁺), reducing acidity and increasing soil fertility.

Importance of Liming

Improves Soil Structure

Lime helps soil particles bind together, improving aeration and water infiltration.

Accelerates Decomposition and Humus Formation

Liming speeds up the breakdown of organic matter, leading to the formation of humus, which improves soil fertility.

Eliminates Aluminum and Manganese Toxicity

Raising soil pH to 6.5 prevents aluminum (Al³⁺) and manganese (Mn²⁺) from becoming toxic to plants.

Increases Nutrient Availability

Lime enhances the availability of essential nutrients like phosphorus (P), molybdenum (Mo), calcium (Ca), and magnesium (Mg), which are often deficient in acidic soils.

Promotes Microbial Activity

Beneficial microbes responsible for mineralization, nitrification, and nitrogen fixation thrive in neutral to slightly alkaline soils.

Problems Associated with Over-Liming

Reduced Phosphorus Availability

Excess lime can react with phosphorus in the soil, making it less available to plants.

Micronutrient Deficiencies

Over-liming can induce zinc (Zn), boron (B), and manganese (Mn) deficiencies, which are crucial for plant growth.

Summary

Soil acidity is caused by rainfall, parent material, organic matter decomposition, crop harvesting, and ammonium fertilizers.

Liming is the most effective method to correct soil acidity.

Proper liming improves soil structure, nutrient availability, and microbial activity.

However, over-liming can create nutrient imbalances, leading to deficiencies.