Acid Sulphate Soils: Formation, Classification and Management

After reading this article you will learn about:- 1. Introduction to Acid Sulphate Soils 2. Formation of Acid Sulphate Soils 3. Classification and Characteristics 4. Management.

Introduction to Acid Sulphate Soils:

Soil with sufficient sulphides (FeS₂ and others) to become strongly acidic when drained and aerated enough for cultivation are termed acid sulphate soils or as the Dutch refer to those soils cat clays. When allowed to develop acidity, these soils are usually more acidic than pH 4.0. Before drainage, such soils may have normal soil pH and are only potential acid sulphate soils.

Generally acid sulphate soils are found in coastal areas where the land is inundated by salt water. In India, acid sulphate soil is, mostly found in Kerala, Odisha, Andhra Pradesh, Tamil Nadu and West Bengal. The area covered under acid sulphate soils in Thailand and India combinedly is about 2 million acres.

Formation of Acid Sulphate Soils:

Lands inundated with waters that contain sulphates, particularly salt waters, accumulate sulphur compounds, which in poorly aerated soils are bacterially reduced to sulphides. Such soils are not usually very acidic when first drained in water.

When the soil is drained and then aerated, the sulphide (S^2-) is oxidised to sulphate (SO₄^2-) by a combination of chemical and bacterial actions, forming sulphuric acid (H₂SO₄). The magnitude of acid development depends on the amount of sulphide present in the soil and the conditions and time of oxidation.

If iron pyrite (FeS₂) is present, the oxidised iron accentuates the acidity but not as much as aluminium in normal acid soils because the iron oxides are less soluble than aluminium oxides and so hydrolyze less.

Reactions Involving the Formation of Acid Sulphate Soils:

Acid sulphate soils form due to oxidation of sulphides in soils. The slow oxidation of mineral sulphides in soils is non-biological until soil pH reaches an acidity of pH 4.0, but the process is not well understood. Below pH 4.0, the bacteria, Thiobacillus ferroxidans are the most active oxidizers and the activity builds up rapidly.

Accelerated by bacteria (Thiobacillus ferroxidans):

Rapid in pH (non-biological):

Classification and Characteristics of Acid Sulphate Soils:

Acid sulphate soils contain a sulphuric horizon which has a pH of the 1: 1 soil: water ratio of less than 3.5, plus some other evidences of sulphide content (Yellow colour). Sulphaquepts, sulphihemists, sulphohemists, and sulphaquents great groups are included in acid sulphate soils.

Such strong acidity in acid sulphate soils results toxicities of aluminium and iron, soluble salts (unless leached), manganese and hydrogen sulphide (H₂S) gas. Hydrogen sulphide (H₂S) often formed in lowland rice soils causing akiochi disease that prevents rice plant roots from absorbing nutrients.

Management of Acid Sulphate Soils:

Management techniques are extremely variable and depend on many specific factors viz. the extent of acid formation, the thickness of the sulphide layer, possibilities of leaching or draining the land etc.

The general approaches for reclamation are suggested below:

(i) Keeping the Area Flooded:

Maintaining the reduced condition of flooded (anaerobic) soil inhibits acid development, which requires oxidation. This solution almost limits the use of the area to rice growing. Unfortunately, droughts occur and can in short time periods cause acidification of these soils. The water used to flood the potential acid sulphate soils often develop acidity and injure crops.

(ii) Controlling Water Table:

If a non-acidifying layer covers the sulphuric horizon, drainage to keep only the sulphuric layer under water (anaerobic) is possible.

(iii) Liming and Leaching:

Liming is the primary way to reclaim any type of acid soil. Acid soil may require 11.45 MT per hectare of lime in a 20-year period whereas acid sulphates soils may require from several metric tons per hectare per year, upto even 224 MT per hectare (100 t/acre) within a 10-year period or less.

If these soils are leached during early years of acidification, lime requirements are lowered. Leaching, however, is difficult because of the high water table commonly found in this type of soil and low permeability of the clay. Sea water is sometimes available for preliminary leaching.