Soil Acidity: Nature and Effects

After reading this article you will learn about the nature and effects of soil acidity.

Nature of Soil Acidity (Classification of Soil Acidity):

There are three types of soil acidity according to the presence of ions:

(i) Active Acidity:

H⁺ ions remain freely present in soil solution and produce limited acidity. The active acidity is a measure of the H⁺ ion activity in the soil solution at any given time.

However, the quantity of H⁺ ions owing to active acidity is very small compared to the quantity in the exchange and residual acidity forms. Even though the concentration of hydrogen ions owing to active acidity is extremely small, it is important because this is environment to which plants and microbes are exposed.

(ii) Exchangeable Acidity:

In very acidic soils, exchangeable hydrogen and aluminium ions are present in large quantities. But the quantity of easily exchangeable hydrogen and aluminium ions are in quite less quantity in moderately acid soils. In this type of acidity higher amount of limestone is required to neutralize the acid soil in comparison to active acidity.

(iii) Residual Acidity:

Residual acidity is that which remains in the soil after active and exchangeable acidity has been neutralized. Residual hydrogen and aluminium atoms that are found in non-exchangeable forms by organic matter and silicate clay.

The residual acidity is commonly far greater than either active or exchangeable acidity. Limestone requirement is very high (1000 to 100,000 times greater than active and exchangeable acidity) to neutralize the residual acidity in the soil.

Effects of Soil Acidity on Plants:

The effects of soil acidity may be direct or indirect.

Direct Effects:

(a) Toxic effect of H ions on root tissues;

(b) Influence of soil acidity on the permeability of the plant membranes for cations ; and

(c) Disturbance in the balance between basic and acidic constituents through the roots,

(d) Aluminium (Al) has toxic effect on plants. Al ³⁺ ions precipitate nucleic acid by forming strong complexes. Al is bound to the P in DNA which inhibits cell division by interfering with nucleic acid replication. Al also limits both rooting depth and degree of root branching besides increasing susceptibility to drought.

Indirect Effects:

(a) Adverse effects on the availability of various nutrients i.e., phosphorus, copper, zinc etc.,

(b) High solubility and availability of elements like aluminium, manganese and iron in toxic amount due to high soil acidity,

(c) Beneficial activities of soil microorganisms adversely affected.

These activities are:

(i) Decomposition of organic matter

(ii) Nitrification

(iii) Nitrogen-fixation

(d) Prevalence of plant diseases.