When rocks and minerals undergo weathering process, large quantities of soluble salts are formed. In humid regions, these salts are washed down to the groundwater and to the sea. But in arid and semiarid regions they accumulate in the soil. Excessive irrigation and poor water management are the two chief causes of water logging and salt accumulation. Accumulation of salts in soil leads to unfavourable soil-water-air relationship and affect the crop production.
A. Causes for Salt Accumulation
The following are the main causes which lead to development of salty soils (Salinity or alkalinity).
(i) Arid climate – About 25% of earth surface is arid in which salt accumulation is a common problem. In India, about 25 m.ha are salt affected with different degrees of degradation.
(ii) High subsoil water table – When the water table is within the capillary range, the water containing soluble salts rises to surface. When the water evaporates the salts are deposited as encrustation. It is estimated that in Punjab, annually about 50,000 acres becomes saline because of raising water table.
(iii) Poor drainage – Due to poor drainage, accumulation of water leads to water logging condition, which leads to salt accumulation.
B. Quality of Irrigation Water
Irrigation water containing more than permissible quantities of soluble salts with sodium carbonate and bicarbonates make the soil salty.
Inundation with sea water – In coastal areas, periodical inundation of land by sea water during high tides makes soil salty. Besides, deep bore wells are also the reason for saline soils.
Nature of parent rock minerals – The saline nature of parent rock minerals leads to salt accumulation.
Seepage from canals – The continuous seepage leads to salt accumulation.
C. Classification of Problem Soils
The soil problems can also be divided into: (a) Chemical, (b) Physical.
(i) Soil chemical problems – The salt affected soils can be classified based on their ESP, pH and EC as follows:
- ESP (%)- <15
- EC (m.mhos/cm)- >4
- pH- <8.5
- ESP (%)- >15
- EC (m.mhos/cm)- >4
- ESP (%)- >15
- EC (m.mhos/cm)- <4
- pH- <8.5
Reclamation of saline soil – Leaching or flushing with good quality of water provided, there should be good drainage system should be there to flush water.
Reclamation of alkali soil – By converting exchangeable sodium into soluble salts by adding the following amendments.
• Calcium chloride
• Calcium sulphate (gypsum)
• Sulphuric acid
• Ferrous sulphate
• Aluminium sulphate
Reclamation of saline alkali soil – The reclamation of these soils is similar to that of alkali soils. First step is to remove the exchangeable sodium and then the excess salts and sodium are to be leached out. Commonly salt affected soils are referred as problem soils as indicated above. Further, based on pH value it can also be grouped as acid soils where the pH value is less than 7.
Management practice for chemical problems of soil – Reclamation of saline and alkali soils are not complete unless proper remedial measures are undertaken to restore the soil fertility and structure of the soil. The following are the important management practices to overcome these problems.
• The saline soil can be easily improved with leaching of salts by using of good quality water and by providing good drainage system.
• Application of gypsum would improve the permeability of soil by making good soil aggregates.
• In acidic soil, lime application should be adequate and excessive leaching should be avoided.
• Salt resistant or saline resistant species should be selected for cultivation.
• Application of amendments viz., gypsum and press mud is found to suppress the sodium and chromium content in plant and soil.
• Growing resistant crops like ragi, cotton, barley and rice can be advocated.
• Growing green manure crops like sunn hemp, dhaincha and kolinji can be advocated.
• Growing resistant varieties like COC 771 in sugarcane and CO 43 in rice may be made.
• Adoption of drip irrigation for possible crops is also recommended to overcome soil physical and chemical problems.
• Liberal application of FYM.
• Application of green manure.
• Excess phosphorus application.
• Proper drainage to keep the soil without adverse effect to plant system.
(ii) Soil physical problems – Fluffy soils, ill drained soils, soils with high infiltration rate, soils with shallow depth and encrustation in soil surface are the possible physical problems. Too frequent irrigation in clayey soils with very high water retention results in poor drainage, water logging and crop damage. Excess irrigation and heavy rain create hardening of soil surface in red lateritic soils with high Fe and Al hydroxides and low organic matter. This results in soil crusting. This leads to poor germination, restriction of shoot and root development and slow entry of water into the soil profile.
Management – In light soils, shallow depth of water with more frequency should be adopted. To increase the infiltration rate in clay soil, amending the soil by mixing with coarse textured soil or tank silt at the rate of 50 tones per hectare is advocated. Organic wastes like crop residue, farm waste, coir pith, filter cake etc., at the rate of 20 tones per hectare once in every year can be applied. Poorly drained clay soils can be improved by providing tile drains and trenches intermittently.
To make the soil more permeable and to overcome poor drainage, addition of organic wastes or sandy soil at the rate of 20–50 tons per ha, respectively is advocated. The encrustation problem could be alleviated by incorporating organic matter and adding montmorilonite clay containing silt.
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