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Learning objectives

To study the genesis, characteristics and reclamation of acid soils

Soil acidity refers to presence of higher concentration of H [+] concentration in soil solution and at

exchange sites. Soil acidity is a major problem in relation to plant growth and therefore acid soils are called a

problem soil. Acid soils are characterized by low soil pH, which varies form strongly acidic (4.5-­‐5.5) to

extremely acidic (<4.5) and with low base saturation.

Soil acidity is of three kinds viz., active acidity, exchangeable acidity and residual acidity. The

hydrogen ions in the soil solution contribute to active acidity. It may be defined as the acidity developed due

to concentration of H [+] and Al [3+ ] ions in the soil solution. The concentration of hydrogen ion in soil solution

due to active acidity is very small, implying that only a meager amount of lime would be required to

neutralize active acidity. Inspite of smaller concentration, active acidity is important since the plant root and

the microbes around the rhizosphere are influenced by it.

The concentration of exchangeable Al and H ions contribute to exchangeable acidity. It may be

defined as the acidity developed due to adsorbed H and Al ions on soil colloids.

Aluminium hydroxyl ions and H and Al ions present in non – exchangeable form with organic matter

and clay account for the residual acidity.

Total acidity

Active acidity + Exchangeable acidity + Residual acidity

Sources of soil acidity

Leaching due to heavy rainfall

Acid soils are common in all regions where rainfall or precipitation is high enough to leach

appreciable amounts of exchangeable bases from the surface soils and relatively insoluble compounds

of Al and Fe remains in soil. The nature of these compounds is acidic and its oxides and hydroxides react

with water and release hydrogen ions in soil solution and become acidic. Besides, when the soluble

bases are lost, the H [+] ions of the carbonic acid and other acids developed in the soil replace the basic

cations of the colloidal complex. As the soil gets gradually depletes of its exchangeable bases through

constant leaching, it gets desaturated and becomes increasingly acid.

Leachable

Acidic parent material

Some soils have developed from parent materials which are acid such as granite and that may

contribute to some extent soil acidity.

Acid forming fertilizers and soluble salts

The use of ammonium sulphate and ammonium nitrate increases soil acidity. Ammonium ions from

ammonium sulphate when applied to the soil replace calcium ions from the exchange complex and the

calcium sulphate is formed and finally leached out.

2-­‐

leached out

Nitrification NH4

NH4

H

Clay Clay

NH4

H

Acid soil

Besides, basic portion of ammonium sulphate is NH4

and it undergoes biological transformation in

the soil and form acid forming nitrate ions. Similarly, sulphur also produces acid forming sulphate ions

through oxidation. Divalent cations of soluble salts usually have a greater effect on lowering soil pH than

monovalent metal cations.

Humus and other organic acids

During organic matter decomposition, humus, organic acids and different acid slats may also be

produced and also concentration of CO2 increased. The increased concentration of CO2, hydrolysis of

acid salts and various organic acids increased the total acidity of soil.

Aluminosilicate minerals

At low pH values most of the Al is present as the hydrated Aluminium ions, which undergoes

hydrolysis and release hydrogen ions in the soil solution.

Carbon dioxide

Soil containing high concentration of CO2, the pH value of such soil will be low and the soil becomes

acidic. Root activity and metabolism may also serve as sources of CO2, which ultimately helps the soil to

become acidic.

Hydrous oxides

These are mainly oxides of iron and aluminium. Under favourable conditions they undergo stepwise

hydrolysis with the release of hydrogen ions in the soil solution and develop soil acidity.

Production constraints

 Increased solubility and toxicity of Al, Mn and Fe

 Deficiency of Ca and Mg,

 Reduced availability of P and Mo and

 Reduced microbial activity

Management of acid soils

Management of the acid soils should be directed towards enhanced crop productivity either through

addition of amendments to correct the soil abnormalities or by manipulating the agronomic practices

depending upon the climatic and edaphic conditions.

Soil amelioration

Lime has been recognized as an effective soil ameliorant as it reduces Al, Fe and Mn 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

Source of lime material is an important aspect of acid soil management and the economics of

application of different sources need to be given due importance. 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, dolomitic and stromatolitic limestones are

important carbonates. The other liming sources are marl, oyster shells and several industrial wastes like

steel mill slag, blast furnace slag, lime sludge from paper mills, pressmud from sugar mills, cement wastes,

precipitated calcium carbonate, etc were found to be equally effective as ground limestone and are also

cheaper. Considering the efficiency of limestone as 100%, efficiencies of basic slag and dolomite were found

to be and % respectively.

Burnt lime is the oxide of lime or quick lime. Quick lime is produced in large kilns. Its reactions in soil

are much faster compared to those of carbonates.

Addition of water to burnt lime makes hydroxide or hydrated lime (slaked lime), which is more caustic than

burnt lime.

Lime when applied to acidic soils either in the form of oxide, hydroxide or carbonate reacts with

carbon dioxide and water to form bicarbonate.

These liming materials on reaction with soil colloid, replace hydrogen and aluminium ions from the

colloidal phase to soil solution.

Ca [++]

Clay

H [+]

Clay

[++]

Clay +

H [+] + Ca [++]

H [+]

Lime requirement:

Four important factors govern the lime requirement, viz.,

- The

required change in pH

- The

buffer capacity of the soil to be limed

- The

chemical composition of the liming materials used

- The

fineness of the liming materials.

A fine textured acid soil requires much larger quantity of lime than does a sandy soil or a loamy soil

with the same pH value. Calcitic or dolomitic limestone reacts slowly with soil colloids, whereas burnt lime

and hydrated lime react faster and bring about changes in soil pH within a few days.

Lime requirement of an acid soil may be defined as the amount of liming material that must be

added to raise the pH to some prescribed value. Shoemaker et al . (1961) buffer method is used for the

determination of lime requirement of an acid soil. Lime requirement interms of pure CaCO3 can be observed

from the following table.

Lime requirement of an acid soil

pH of soil buffer suspension (Field soil sample)	lime required to bring pH down to indicated level (CaCO ) in tonnes per acre 3	Col3	Col4
pH of soil buffer suspension (Field soil sample)	pH 6.0	pH 6.4	pH 6.8
6.7	1.0	1.2	1.4
6.6	1.4	1.7	1.9
6.5	1.8	2.2	2.5
6.4	2.3	2.7	3.1
6.3	2.7	3.2	3.7
6.2	3.1	3.7	4.2
6.1	3.5	4.2	4.8
6.0	3.9	4.7	5.4
5.9	4.4	5.2	6.0
5.8	4.8	5.7	6.5
5.7	5.2	6.2	7.1
5.6	5.6	6.7	7.7
5.5	6.0	7.2	8.3

5.4	6.5	7.7	8.9
5.3	6.9	8.2	9.4
5.2	7.4	8.4	10.0
5.1	7.8	9.1	10.6
5.0	8.2	9.6	11.2
4.9	8.6	10.1	11.8
4.8	9.1	10.6	12.4

Benefits:

The most conspicuous effect of liming was on the exchangeable acidity, which registered a

decrease up to %. A decrease of 70-­‐74% in pH dependent and total acidity was recorded by

liming. An average yield improvement of % could be obtained.

Crop choice:

Selection of crops tolerant to acidity is an effective tool to counter this soil 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 range.

Relative tolerance of crops to soil acidity

Crops Optimum pH range

Cereals

Maize, sorghum, wheat, barley 6.0-­‐7.5 Millets 5.0-­‐6.5 Rice 4.0-­‐6.0 Oats 5.0-­‐7.7

Legumes

Field beans, soybean, pea, lentil etc. 5.5-­‐7.0 Groundnut 5.3-­‐6.6 Other crops

Sugarcane 6.0-­‐7.5 Cotton 5.0-­‐6.5 Potato 5.0-­‐5.5 Tea 4.0-­‐6.0 Occurrence in India:

95% of soils of Assam and 30% of geographical area of Jammu and Kashmir are acidic. In West

Bengal, 2.2 Mha, in Himachal Pradesh, 0.33 Mha, in Bihar, Mha and all hill soils of erstwhile U.P. come

under acid soils. About 80% of soils in Orissa, 88% in Kerala, 45% in Karnataka and 20% in Maharastra are

acidic. The laterite zone in Tamil Nadu is covered with acid soil and about 40,000 ha are acidic in Andhra

Pradesh.

1. Text book of Soil Science by T.D. Biswas and S.K. Mukherjee. Tata McGraw-­‐Hill Publishing Company

Limited, New Delhi.

2. A text book of Soil Science by D.K. Das. Kalyani Publishers

3. Acid Soils of India and Liming by Mandal, S.C., M.K. Sinha and H. Sinha.

Tech. Bulletin (Agric),

51; ICAR, New Delhi.

4. Soil Acidity and liming by Adams, F. (Ed) (1984). 2 [nd] Edn, American Soiciety of Agronony, Madison,

U.S.A.

Questions to Ponder

1)What ions are the principal sources of soil acidity?

is soil pH altered by fertilizer application?

what is agricultural lime?

4)what are the factors that determine the frequency and rate of liming?

5)What is fluid lime?

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Summary Cheat Sheet

Key Recall Points

• Genesis, Characteristics, is exam-relevant for SSAC122 and objective questions in soil science.
• Use soil-test based interpretation with focus on pH, CEC, and nutrient availability.
• Apply the 4R principle: right source, right rate, right time, and right method.

Exam Traps

• Do not mix up soil fertility concepts with fertilizer quantity alone.
• Numerical and term-based questions often test definitions, units, and threshold values.
• In problem-solving, interpretation must follow soil reaction, crop stage, and management context.

References

3 sources • [1] [2] [3]

[1]

ICAR e-Course: Soil Chemistry, Soil Fertility and Nutrient Management

Official

[2]

Brady and Weil, The Nature and Properties of Soils

Book

[3]

Indian Council of Agricultural Research

Official