Some amounts of the salts are always present in soil. When concentration of these salts are low, they are not harmful but with the increase in the salt content the plant growth is adversely affected which in turn decrease the productivity.
In India, salt affected soils are estimated to cover 7 m ha area. These types of soils are mainly confined to the arid and semi- arid regions.
Salt affected soils have been classified in to three categories viz. Saline soils, Saline alkali soils and alkali or sodic soils.
Saline soils:
Soils containing excess of neutral soluble salts dominated by chlorides and sulphates so as to affect plant growth. High osmotic pressure of soil solution hinders the water uptake by roots of plants.
These soils are characterized by EC more than 4.0 dS m-1 , pH less than 8.5 and exchangeable sodium percentage (ESP) less than 15.0 .
These soils are characterized by saline efflorescence or white encrustation of salts at the surface. In India, these soils are known as ‘reh’ and in others as ‘thur’.
Management:
Removal of excess salt to a desired level in the rooting zone is the basic principle in the reclamation of saline soils.
Leaching with water of good quality and adequate drainage are the two essential components.
The addition of organic matter improves physical conditions of the soil and increase the water holding capacity, hence keeps the salt in diluted form.
Rice is considered a satisfactory crop during initial year of reclamation. After rice, growing of legume is suitable for the production. Cultivation of salt tolerant crops (Table-1) is the other management practices.
These soils are characterized by EC less than 4.0 dSm-1, pH more than 8.5, ESP more than 15.0.
The soils has sufficient sodium saturation and alkalinity to adversely affect plant growth and crop productivity. Carbonates of the sodium are dominant salt.
Due to high ESP, the soils are dispersed and deflocculated.
When dispersed and dissolved organic matter is deposited on the surface, alkali soils gives dark brown- black appearance ( black alkali soils).
Impeded drainage condition, low infiltration rate and low hydraulic conductivity are the most significant physical properties.
High pH of the soils lowers the availability of the plant nutrient elements except molybdenum.
In some parts of India, they are called Usar and in others Kallar. In India, these soils are mainly distributed in arid and semi-arid regions of Punjab, Uttar Pradesh, Bihar and Rajsthan.
Management:
Deep plowing, land leveling and bunding is necessary to bring the soil in good physical condition and to make the soil pervious. Heavy irrigation is applied after the addition of gypsum to facilitate the leaching of soluble salts of Na.
Use of amendments and adequate leaching are prerequisites for any reclamation measure. Several amendments such as- Gypsum, S,H2SO4, CaCl2, FeSO4, iron pyrites are available but gypsum have been the most popular because of low cost and easy availability.
The addition of gypsum results in formation of Na2SO4 in the soil which is highly soluble and leached down by heavy irrigation.
The above reaction is reversible therefore sodium sulphate formed needs to be removed by leaching.
The fertilizers which have acidic residual effect should be used. The preferential order is NH3 >NH4Cl > (NH4)2SO4 > Urea.
The addition of organic matter is always good in improving soil physical conditions viz. aeration, water holding capacity, infiltration, pulverization.
It is advisable to grow tolerant and medium tolerant crops.
Tolerant crops: Rice, Sugarbeet, Dhanicha
Medium tolerant crops: Wheat, Barely, Oats, Millets
Sensitive crops: Legume, Maize, Ground nut
Saline – alkali soils:
These soils are characterized by EC more than 4.0 dSm-1, pH more than 8.5, ESP more than 15.0.
Such soils have characteristics of both saline and alkali soils. Therefore, soils showing high salinity and ESP should be reclaimed for both but first for salinity and later for excessive exchangeable Na.
Acid Soils:
The leaching of bases is the main cause of the formation of acid soils though parent acidic rock is also contributing factor.
These soils are high in exchangeable Al3+ and H+ with a pH value less than 5.5 and respond to lime application.
The adverse effect of acid soils on plant growth is mainly related to presence of aluminum, manganese and iron in toxic concentration, deficiency of calcium and magnesium, nutrient imbalance and microbial imbalance.
The aluminium toxicity has multiple effects, of which the inhibition of root growth is perhaps the most important.
The manganese (below pH 5.0) toxicity interferes with plant metabolism.
The acid soils are generally low in available phosphorus and have high P fixation capacity. The status of available micronutrient elements, except molybdenum, is generally adequate in these soils.
The population of bacteria and actinomycetes are lower, and those of fungi are higher.
The acid soils cover a large area of about 47 mha in the states of Assam, Tripura, Mnipur, W.Bengal, Bihar, Orissa, Karnataka, Tamil Nadu, Himachal Pradesh and Kerala.
Management:
Acid soils can be managed in two ways viz. either by growing crops suitable for a particular soil pH or by ameliorating the soils through the application of amendments which will conteract soilacidity.
The acid soils are made more suitable for agriculture use by liming which raises the pH.
The common practice of liming is to apply ground limestone to soil. When liming materials (Table-2) are added to the soil they get hydrolysed and release hydroxyl ions which neutralize hydrogen ions (cause of acidity). This process can be represented by the following equations:
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