Acid Soils: Formation, Characteristics and Lime Requirement

After reading this article you will learn about:- 1. Formation of Acid Soils 2. Characteristics of Acid Soils 3. Control 4. Lime Requirement.

Formation of Acid Soils:

Soil microorganisms and plant roots respire to form carbon dioxide, which is also produced when organic matter decomposes. This carbon dioxide reacts with water to form carbonic acid that reacts with the insoluble primary minerals present in the soil as well as the clay and humic micelle.

Suppose the clay and humic micelle have absorbed Ca⁺⁺, Mg⁺⁺, K⁺, Na⁺ and H⁺ ions in the ratio of 60:15:5:10:10.

Carbonic acid reacts with clay micelle as shown below:

These soluble bicarbonates of calcium, magnesium, potassium and sodium are washed down by high rainfall in humid regions. So the above reversible reactions proceed in the forward direction.

This means that basic cations are gradually released from the primary minerals and clay and humic micelle in the form of soluble bicarbonates, which are gradually washed down by high rainfall in the humid regions where the soils ultimately become strongly acidic in reaction.

Characteristics of Acid Soils Harmful for Plant Growth:

Acids soils are formed when the basic elements like calcium, magnesium, potassium etc. have been washed down. So, strongly acidic soils are always deficient soils are in calcium, magnesium and potassium. Highly leached acid soils are also deficient in sulphur and boron.

Strongly acidic soils contain excessive amounts of aluminum, iron and manganese in the soluble form, which is very harmful for crop growth. Acid soils may also contain toxic amounts of copper and zinc.

Phosphate, if present as inorganic orthophosphate ions, H₂PO₄, precipitated as insoluble hydroxyl phosphates as shown below:

Strongly acidic soils are therefore deficient in available phosphorus and also available molybdenum because it reacts with the silicates and iron and aluminum compounds. Bacteria and actinomycetes cannot function properly in strongly acidic soils, where fungi dominate the soil microorganisms.

Control of Soil Acidity:

Soil acidity is controlled by adding lime to the soil.

The different forms of lime are as follows:

Calcium oxide, CaO, called quick lime or burnt lime because limestone CaCO₃ is burnt to produce it.

Calcium hydroxide Ca(OH)₂ is called the hydroxide of lime. It is formed when quick lime reacts with water.

CaO + H.OH à Ca(OH)₂

Calcium carbonate or carbonate of lime occurs under natural conditions.

When applied to the acid soil, the soil acidity is neutralized as shown in the following equation:

Lime first neutralizes the active acidity as shown in Equation (i). The Ca⁺⁺ ion replaces the H⁺ ion from the micelle to restore the original level of active acidity as shown in Equation (ii) So reversible reactions proceed in the forward direction. In this way soil acidity is ultimately controlled.

Lime Requirement of Acid Soils:

Take 100gms of finely powered soil in a series of beakers, to which powdered limestone is added at an increasing rate of 440, 880, 1320…kg/ha, which is equivalent to 20,40,60,80…mg/100gms. The limestone powder is thoroughly mixed with soils after they are, moistened.

The soils are incubated for about a week to allow time for the limestone powder to neutralize the soil acidity. The pH of the soil in each of the beakers is then measured. The amount of limestone powder added to the soil in kg/ha to raise the soil pH to seven is the lime requirement of the soil. It is 5280 kg/ha Fig. 7.9.