Soil and Plant Analysis

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, and 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 110 and 94 % 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.

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.

pH of soil buffer suspension (Field soil sample)	lime required to bring pH down to indicated level (CaCO3) in tonnes per acre
	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 95 %. A decrease of 70-74% in pH dependent and total acidity was recorded by liming. An average yield improvement of 30 % 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 M ha, in Himachal Pradesh, 0.33 M ha, in Bihar, 2 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.