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Soil reaction is a very important chemical property of the soil. It is measured in terms of pH. The soils having pH 7 are neutral in reaction and those below or above 7 are acidic or alkaline respectively. In neutral soils, satisfactory plant growth can be expected, provided all other agronomic practices are followed properly. But neither acid soils nor saline soils are suitable for most of the crops except a few tolerant ones. Acidity or salinity acts as a limiting factor in crop production.

When the concentration of some elements which are naturally acidic in reaction, goes too high in the soil, soils become acid and their pH goes down below 7. These elements are basically hydrogen (H), aluminium (Al) and Iron (Fe). In fact, last two elements are not directly responsible for soil acidity but they have a tremendous potential to generate hydrogen ions (H+) in the soil  which are direct cause of acidity (ion is the minutes possible and electrically charged form of an element). The more the number of hydrogen ions in a solution, the greater will be its acidity and the lesser pH count. Therefore, the soils having hydrogen ions concentration above a reasonable limit or pH below 7 are termed as acid soils.

Extent of acid soils in India:

Acid soils constitute about 30 % of the total cultivable area in India. These soils are formed due to drastic weathering under hot humid climate and heavy precipitation. Based on general estimations, nearly 25 million hectares of land is having pH below 5.5 and 23 million hectares fall under the pH range of 5.6 - 6.5. In all those tracts of the country where rainfall and temperature are high, acid soils are predominantly found. North-east region has the largest stretches of acid soils followed by neighboring states of West Bengal, Bihar and Orissa. In the coastal region of Kerala, high rainfall and temperature have contributed to the development of acid soils.

Causes of Acid Soil Formation:

  • High precipitation leading to leaching loss of basic cations from soil.
  • Excessive use of water or keeping the field submerged for a long time accompanied by improper drainage may lead to the development of acidity in the soil.
  • Continuous use of acid forming fertilizers (eg. ammonium sulphate, ammonium chloride, etc) for years may also lead to considerable reduction in soil pH.
  • Continual removal of crop residue from field and no or little addition of organic matter (e.g. FYM, compost, vermin compost etc.).

Effects of soil acidity on crop:

  • Reduction in the amount of nutrients being recycled by soil micro-organisms (e.g. nitrogen supply may be reduced) as the growth of microorganisms is affected by the soil acidity.
  • Induced deficiencies of calcium, magnesium, sulphur, boron and molybdenum.
  • Limited ability of plants to use subsoil moisture.
  • Aluminium, manganese, iron may reach in toxic levels.
  • Acidity itself may cause damage to root hairs and affects moisture and nutrient uptake
  • Affect N-fixation by legumes.
  • Imbalance in microbial population, soil fungi predominates over bacteria.

Farmer’s practices for minimizing soil acidification:

  • Matching nitrogen fertiliser inputs to crop demand.
  • Using forms of nitrogen fertilizer that cause less acidification.
  • Efficiently irrigating the area to minimise leaching.
  • Sowing early after fallow to ensure more rapid utilisation of available nitrogen.
  • Growing deep-rooting perennial species to take up nitrogen from greater depths.
  • Regularly applying lime to counter the acidification inherent in the agricultural system
  • Growing acid tolerant crops or crop varieties more tolerant of acid soils. Sugar cane and bananas are examples of acid tolerant crops.

Management of soil acidity:

Management of acid soils should aim at realization of production potential either by addition of amendments or by manipulation of agricultural practices to derive optimum crop yield under acidic conditions. Liming can significantly improves the physical, chemical and biological properties of soil. There can be significant improvement in yield due to increased availability of several plant nutrients. In addition to liming, the present thinking on acid soil management includes integration of nutrient management practices with in situ soil moisture conservation technology, agro-forestry using a system approach of crop production to meet the food and nutritional security of the farmer. No doubt, liming technology is cost effective and can increase the crop yield. Residue incorporation, minimum tillage, conservation agricultural practices can also increase the crop productivity in acid soil regions.

Table: Calcium Carbonate (CaCO3,) equivalent of some important liming materials

Sl.No.

Liming material (100 kg)

CaCO3 equiv. (kg)

1

Calcium oxide (CaO)

179

2

Calcium hydroxide, (burnt lime)

139

3

Dolomite

109

4

Limestone (CaCO3)

100

5

Marl

70-90

6

Blast furnace slag

70-90

7

Electric furnace slag

65-80

8

Basic slag

60-70

Among the naturally occurring lime sources, calcite and dolomite are the most important. Since calcite and dolomite have industrial use, its application in agriculture is not economical. To reduce the cost of liming, several industrial wastes such as paper mill sludge from paper mills, basic slag from steel industry, press mud from sugar mills using carbonate process can successfully be used in acid soil regions of India as amendments, which are eco-friendly. Liming material must be locally available, properly ground and should have high neutralizing value and low cost for use by small and marginal farmers.

Lime Requirement:

Soil testing for pH is essential for finding out approximately correct dose of liming material. The table provides pH value of the soil and recommended dose of pure calcium carbonate to be used in an acre or hectare of land.

Table: Lime requirement to bring the soil to Indicated pH

pH of soil

buffer suspension

Lime required to bring the soil to indicated pH ( in tones per acre of pure calcium carbonate i.e.CaCO3)

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

3.7

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

8.6

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

References:

ICAR (2010). Degraded and wastelands of India: status and spatial distribution.

Jena, D. (2013). Potential of industrial by-products in ameliorating acid soils for sustainable crop production.

Maji, A.K., Obi Reddy, G.P. and Meshram, S. (2008). Acid soil map of India. Annual Report 2008. NBSS&LUP, Nagpur, India.


Authors:

Surajit Mondal1, Santosh Kumar2, Prem K. Sundaram3 and S. K. Dwivedi4

1Scientist (Soil Physics), 2Scientist (Plant Breeding), 3Scientist (Farm Machinery), 4Scientist (Plant Physiology)

ICAR Research Complex for Eastern Region, Patna - 800014

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