Introduction

Soil acidity is the result of excessive accumulation of H⁺ ions (Hydrogen) over OH^– ions (Hydroxide) in the soil solution. Hence acid soil or acidic soil is rich in H⁺

In other words, acidity means a low percentage of base saturation. It happens when precipitation exceeds the evapotranspiration. Thus here, the leaching is prominent, ultimately causing loss of bases from the soil.

When the process of weathering (that is breaking and dissolving of rocks and minerals into the soil) is drastic, the whole soil profile becomes acidic.

Acid soil: Understanding the basic

• Acid soils are formed under natural conditions as well as artificially by the continuous use of certain fertilizers.
• Soils develop in humid regions, e.g., cool temperatures and moist tropic regions, and are mostly acidic.
• Under these conditions, the alkali and alkaline earth bases that are liberated during weathering are leached out leaving the soil deficient in bases.
• The leaching of bases is the prerequisite for the formation of acid soils which are dominantly found in regions with high rainfall.
• The mean temperature, type of vegetation, parent material, and hydrological conditions also govern the extent of acid soils and the degree of acidity.
• Acid soils mostly occur in almost all the major soil groups except the black soils in India. In India, acid soils are found extensively in the northeastern, eastern, and peninsular regions including Bihar, west Bengal, and major parts of Orissa, and the Himalayan regions.
• There are large areas of acid soil in Assam, Manipur and Tripura. Peaty and marshy soils which are acidic are found in West Bengal, Assam, Kerala, Tamil Nadu, and the tarai regions of Uttar Pradesh.
• Very low pH(4 or less) is an indication of the presence of sulphuric acid. Suh soils are not extensive but are important.

Classification of acid soil in India

Acidic soil level

Low pH and a high proportion of exchangeable hydrogen and aluminum are the main characteristics of acid soil.

Forms of Acidity in Soil:  The few forms of acidity are described below

1. Strong acidity

This is due to the presence of soluble Al and absorbed Al (Aluminum).

2. Moderate acidity

In this case percent base saturation is higher than strong acidity.

3. Weak acidity

It is due to the presence of hydrated Al³⁺ ions and H⁺ ions. Its pH value is 5.2

4. Very weak acidity

This acidity is due to the presence of a COOH group of organic origin. Its pH value is 5.2-6.5.

Natural causes of soil acidity

1. Leaching due to high rainfall

Due to heavy rainfall, soluble base salts are dissolved and removed along with the rainwater, and in the process exchange site becomes saturated with H⁺ ions.

2. Due to crop removal

Crops remove significant amounts of bases from the soil.

3. Soil formed from the acid parent material

Soils developed from acidic parent materials having granite, rhyolite, and sandstone are acidic in nature.

4. Microbial activity

Microbes have been found responsible for many processes like the decomposition of organic residues and nitrification. Such microbial activities result in the formation of acids in soil.

5. Contribution to the environment

When the electricity is discharged into the atmosphere during the rainy season, atmosphere nitrogen (N), Sulphur(S), and oxygen (O) form acidity in the soil.

Human-induced causes

1. Weathering

Plant roots acidify soil by releasing protons and organic acids so as to chemically weather soil minerals.

2. Acid rain /Air pollution

Continuous evolution of greenhouse gasses like methane,  and CO₂ due to various human activities are the primary sources of air pollution, which in turn leads to the formation of acid rain. The acidic materials mixed with the rainwater acidify the soil.

3. Use of chemical fertilizers

Ammonium sulfate and ammonium nitrate fertilizers produce acidity when added to the soil. Ammonium ions of these fertilizers replace calcium and magnesium from the exchange complex and the calcium sulfate so formed is lost due to leaching. In the case of soils having free lime, the acidity increases continuously with the use of acidic fertilizers.

4. Deforestation

It means the clearing of a wide area full of trees. This leads to the loosening of topsoil, rendering the subsoil acidic.

Land Degradation

It means the loss of productive capacity of the soils, which in turn creates the formation of acid soil.

Signs and Symptoms of Acid Soil

Farmers will see evidence of acidic soil by noticing the following signs-

• Reduced yields from acid-sensitive crops and pastures.
• Poor establishment of pastures.
• Failure of perennial pastures to persist.
• Poor legume productivity and nodulation.
• Increased need for fertilizer N in crops following legume-based pastures due to poor nitrogen fixation.
• Reduced tolerance of crops.
• Increased incidence of acid-tolerant weeds.

Impact on plant growth

Soil acidity influences plant growth by the production of organic acid which is produced due to decomposition of organic matter or due to root secretion. The effect of soil acidity may be separated into two groups-

Direct

• Toxicity effects of H+ ions are observed in root tissues.
• The permeability of the plant membranes for cations is affected.
• Balance between basic and acidic constituents through roots is disturbed.
• Enzyme changes occur in plants due to pH changes.

Indirect

• Availability of some nutrients like phosphorus.
• Higher availability of AL, Mn, Fe, Zn, Cu, etc.
• Adverse effects on the beneficial activities of microorganisms.
• Increase in plant diseases.
• Nutrients like Ca, and K become deficient.

Remedies and other techniques

Acid soils can be managed in two ways, viz. either by growing crops suitable for a particular soil pH or by ameliorating the soils through the application of amendments that will counteract soil acidity.

The former is rather risky. Intensive and continuous cropping of arable crops in humid regions will aggravate soil acidity and the use of acid-producing fertilizers will further accentuate the process. Intensive cropping without liming in humid regions will increase soil acidity. The use of acidifying fertilizers will accentuate the process. Urea is better than ammonium sulfate in this regard, but its use over a long period is tantamount to the formation of greater acidity in acid soil or moderate acidity in neutral soil. On acid soils, judicious use of fertilizers and liming in combination is the best practice.

Prevention

The many ways by which acid formation can be prevented in soils are:

• Soil amelioration

Lime has been recognized as an effective soil ameliorant as it reduces ammonium toxicity and increases base saturation. P and Mo availability of acid soils. Liming also increases atmospheric N fixation as well as N mineralization in acid soils through enhanced microbial activity.

• Liming materials

Commercial limestone and dolomite limestone are the most widely used amendments. Carbonates, oxides, and hydroxides of calcium and magnesium are referred to as agricultural lime. Among the naturally occurring lime sources calcitic and dolomitic are important carbonates.

• Lime requirement

Acid soil may be defined as the amount of liming material that must be added to raise the pH to the prescribed value. Shoemaker et al. (1961) buffer method is used for the determination of the lime requirement of an acid soil.

• Crop choice

The selection of crops tolerant to acidity is an effective tool to counter this problem and breeding of such varieties is of specific importance for attaining higher productivity, particularly in areas where liming is not an economic proposition. The crops can be grouped on the basis of their performance in different soil pH ranges.

Sustainable practice for soil health

Slightly acid soils are quite suitable for growing crops without any treatment, but those that are strongly or even moderately acidic do not allow the normal growth of plants. In such cases, it is necessary to neutralize their acidity before crops can be grown successfully. The application of substances containing calcium and magnesium such as lime, limestone, dolomite, etc., is a common procedure for the improvement of these soils.

Q&A

1. What happens when a soil is acidic?

Soil acidity influences plant growth by the production of organic acid which is produced due to decomposition of organic matter or due to root secretion. It toxifies the plants and affects their cell membranes.

2. What causes soil to become acidic?

Acidity in the soil can be caused by various natural or manmade reasons such as

• Leaching due to heavy rainfall
• Soil formed from acid parent material
• Use of acid-forming fertilizers
• Presence of aluminum silicates
• Weathering
• Acid rain

3. What are the signs of acidic soil?

A few signs of acidic soil are

• Reduced yields from acid-sensitive crops and pastures.
• Poor establishment of pastures.
• Failure of perennial pastures to persist.
• Poor legume productivity and nodulation.
• Increased need for fertilizer N in crops following legume-based pastures due to poor nitrogen fixation.

4. What is the best way to acidify soil?

The best possible way to acidify soil is the addition of Sulphur either by natural methods like leaching or by man-made ways like adding fertilizers.

Summary

• Soil is a dynamic natural body developed as a result of the pedogenic process during and after the weathering of rocks.
• Acid soils are formed under natural conditions as well as artificially by the continuous use of certain fertilizers
• Soil acidity is the result of excessive accumulation of H⁺ ions (Hydrogen) over OH^– ions (Hydro-oxide) in the soil solution.
• Acidity in soil can be divided into low, medium, and very high acidity with varying pH.
• There are various causes of acidity in the soil. They are either natural or man-made.
• Soil acidity influences plant growth by the production of organic acid which is produced due to decomposition of organic matter or due to root secretion.
• Acid soils can be managed in two ways, viz. either by growing crops suitable for a particular soil pH or by ameliorating the soils through the application of amendments that will counteract soil acidity.
• The acidity of soil can be reduced by lime treatment.

References

Website reference

• https://acrobat.adobe.com/link/review?uri=urn:aaid:scds:US:2286975b-637a-3425-9be3-917467e35fb5

Book Reference

• T.D Biswas, S.K Mukherjee, Soil Science, McGraw Hill Education Private Limited, New Delhi, 2013.
• Dr. J.A. Daji, A textbook of soil science, Media Promoters and Publishers PVT.LTD, Bombay, 2005.

Written By: Ankita Gangopadhaya