Management of acid soil in NE Hill region for sustainable crop production

P Debnath, SK Pattannaik, D Sah, AK Pandey *

In India, most of the acid soils are sedentary and are found in hilly terrains. Generally, these are common on hill tops, hill slopes, terrace and un-terraced uplands, medium lands and valley. The acid soils (pH < 6.5) in India occupy approximately 90 million ha of the geographical area. Acidic soils below pH 5.5 occupy around 25 million ha of arable land.

Such type of problematic soils is mainly concentrated in North Eastern region of India. In North East India more than 80 per cent of land is under hilly terrain having soil pH less than 5.5. The toxicity of soil Al has been recognized as one of the important factors for limiting the productivity of crops on acid soil. Low productivity of this soil has been attributed to low availability of P, Ca, Mg, and Mo and toxicity of Al, Fe, and Mn.

Acid soils are bases unsaturated and consequently do not sustain the crop productivity even with application of inorganic fertilizer. Therefore, satisfactory crop yield is not obtained unless these soils are properly amended with lime. Rock phosphate, FYM and vermicompost are main sources of nutrient for increasing the nutrient supply capacity in acid soils.

The farmers of the region in general are hill farmers, they have common apathy towards the use of inorganic fertilizer. High rainfall and climate situation augment to produce a huge amount of biomass. Conversion of these organic residues may lead to sustain the production in acid oils of this state.

Problems associated with acid soils

The acid soil has a pH of less than 5.6 and usually below pH 5.0. The low soil pH is associated with a number of soil chemical and biological characteristics that manifest themselves as the component of the problem of acid soil syndrome. These components may adversely affect plant growth. The following specific problems are associated with acid soil.

1. Aluminium toxicity
2. Manganese toxicity
3. Molybdenum deficiency
4. Legume nodulation failure
5. Heavy infestation of plant diseases
6. Calcium and magnesium deficiency
7. Reduction of soil biological activity
8. Reduction of phosphorus availability
9. Bacterial growth is inhibited by Al and Fe toxicity.

Liming

Liming improves the base status, inactivates Fe, Mn and Al in soil danger of toxicity of micronutrient. Liming stimulates microbiological activity in soil, helping mineralization of organic N and fixation of atmospheric nitrogen. Application of lime at 10 per cent of lime requirement every year placed in furrows has been found effective.

For the soils of East Khasi hills, addition of lime at 25 per cent of lime requirement was good enough from the economic point of view. Addition of lime at 25 percent of lime requirement neutralized the exchangeable Al of Alfisols, Inceptisols and Ultisols.

Despite the effectiveness of the naturally occurring calcitic and dolomitic limestone as amendments for acid soil, profitability of their use is often doubtful. This is so because lime requirement are high and leaching losses are also high.

Lime stone reserve of NE India
Sl. No. States Reserves (million tonnes)
1. Arunachal Pradesh 1503
2 Assam 135
3. Manipur 4.6
4 Meghalaya 4665
5 Nagaland 450
6 Sikkim 2.1

Quality characterization of liming materials

The quality of agriculture limestone in neutralizing soil acidity is to a large extend governed by its Ca and Mg content, particle size, moisture content, neutralizing value and cost, etc. Natural limestones must be crushed before they are mixed with the soil.

Limestones crushed to pass a 100 mesh sieve are almost as reactive as the oxide and hydroxide, while a limestone crushed to pass a 20 mesh can be taken considerably longer to exert its full effects on soil pH. This is because the reactivity of a liming material seen to be related to its active surface area.

Effectiveness of limestone

Sl. No. Limestone passing through Efficiency rating
1 60-100 mesh sieve 100
2 20-60 mesh sieve 60
3 8-10 mesh sieve 20
Source: Das, D.K. 2011

The chemical effectiveness of an agriculture limestone is evaluated on the basis of its CaCO3 equivalence. This is defined as the acid neutralizing capacity of the liming material expressed as weight percent of CaCo3.

The relative capacity of particular liming material to neutralize a certain amount of H+ in the soil is its neutralizing value. All the liming materials for this purpose are composed against pure CaCo3 which is known to have a neutralizing value of 100 (this is also its molecular weight). The smaller the materials required neutralizing a definite quantity of H+, the higher is its neutralizing value.

Relative neutralizing power of different forms of lime

Form of lime Molecular weight Neutralizing value percent Pound equivalent to 1 Ton of pure CaCo3
Calcium carbonate 100 100 2000
Magnesium carbonate 84 119 1680
Calcium hydroxide 74 135 1480
Magnesium hydroxide 58 172 1160
Calcium oxide 56 178 1120
Magnesium oxide 40 250 800
Source: Das, D.K. 2011

Technology of lime use
When to apply lime

Generally lime may be applied any time during the year. However, crop rotation, farming system and the form of lime used will be the deciding factors. When quick reacting hydrated lime is used, it should be added to low rate and may be applied at the time of planting.

In cereal legume rotation apply lime before cereal. When potato is taken in rotation, liming should follow it to keep the soil organism under control. If a farmer fails to set limestone applied before planting a crop, stop dressing of limestone should be considered as an emergency measure.

How to apply lime

The principal requirement of any method of applying lime is that it should be distributed evenly and should be thoroughly mixed with soil. The only way of mixing the lime thoroughly with the soil is by tillage operations. When large application are used, split application are recommended, half before ploughing and rest after ploughing, usually to 10-15 cm depth.

Frequency of liming

The residual effects of liming are usually expected to last for 5-7 years. An annual application of 200-500 kg lime/ha/year has been reported to be adequate to maintain the level of calcium and magnesium in the soil under continuous cropping while keeping a check on the release of exchangeable aluminium.

Lime requirement (ton per acre) for different pH targets

Measured pH of soil buffer suspension Lime requirement in to per acre of pure CaCO3 for different pH targets
pH 6.0 pH 6.4 pH 6.8
4.8 9.1 10.6 12.4
4.9 8.6 10.1 11.8
5.0 8.2 9.6 11.2
5.1 7.8 9.1 10.6
5.2 7.4 8.6 10.0
5.3 6.9 8.2 9.4
5.4 6.5 7.7 8.9
5.5 6.0 7.2 8.3
5.6 5.6 6.7 7.1
5.7 5.2 6.2 7.1
5.8 4.8 5.7 6.5
5.9 4.4 5.2 6.0
6.0 3.9 4.7 5.4
6.1 3.5 4.2 4.8
6.2 3.1 3.7 4.2
6.3 2.7 3.2 3.7
6.4 2.3 2.7 3.1
6.5 1.8 2.2 2.5
6.6 1.4 1.7 1.9
6.7 1.0 1.2 1.4

Industrial wastes as amendments for acid soil

The choice of amendment for acid soils very much depends on the availability of lime source and its cost. Industrial wastes such as steel mill slag, blast furnace slag, lime sludge from paper mills, cement kiln wastes, precipitated calcium carbonate etc., have been used as amendment. Lime sludge from paper mills contain 65-68% CaCo3, 2% R2O3 (sesquioxide), 1% free CaO and 1.5% free alkali.

The paper mills located in the acid soil regions of Assam, Nagaland, West Bengal, Orissa, Madhya Pradesh and Andhra Pradesh produce around 187000 tons of lime sludge annually which could be used for amending acid soils.

Good grade limestone suitable for industrial use cannot possibly be recommended as agricultural lime. Poor grade lime with comparative high silica content could be used as an amendment. But poor grade limestone are not marketed, thus, the only choice is to depend on the industrial wastes.

Since most acid soils are less productive due to low fertility, low water retentivity and poor physicochemical condition, the resource poor farmers of acid soil region cannot afford commercial limestone. Therefore, it is often said that soil acidity and poverty are synonymous.


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