Chemical deterioration of soils is a type of soil degradation that involves loss of nutrients or organic matter, salinization, acidification, soil pollution and fertility decline. The removal of nutrients reduces the capacity of soils to support plant growth and crop production and causes acidification. In arid and semi-arid areas problems can arise due to accumulation of salts, which impedes the entry of water in plant roots.
Chemical deterioration of soils is often also due to agricultural over exploitation, relying solely on replenishing nutrient losses through harvesting by artificial chemical fertilizers. Indiscriminate irrigation for getting higher yield has led to water logging; nutrient losses and increased salinization of lands threatened the sustainability of crop yields. The increasing salinization of thousands of hectares of cultivated lands with raised water table in many canal command areas is a serious threat and needs attention failing which a significant area may be rendered unproductive and unfit for cultivation.
In India nearly 145 million hectares of lands are cultivated and a rough estimate indicates 48 million ha of soil is acidic in nature of which 25 m ha shows pH below 5.5 while about 23 m ha has pH between 5.6 and 6.5 (Bhat, et. al. 2007). Acid soils are characterized by low soil pH which varies from strongly acidic (4.5-5.5) to extremely acidic (<4.5), low cation exchange capacity and low base saturation. Acid soils occur in the tropics and subtropics as well as in moderate climate areas while acid sulphate soils occur in the low inundated lands and in the mangrove swamps of coastal region. The acid sulphate soils are highly acidic (pH as low as 3.5), with or without jarosite mottles, having toxic concentration of available Fe and soluble Al, may contain 10 to 40 % organic matter and are very poor in available P content.
Major Production Constraints-
Increased solubility and toxicity of Al, Mn and Fe; deficiency of Ca and Mg; reduced availability of P and Mo and reduced microbial activity with decreasing pH are characteristic features of acid soils. Toxic concentration of Al has been reported in sedentary soil with pH below 5.0 and alluvial soil with pH below 4.5. The expected loss of productivity was estimated due to acidification.
The deficiency of Ca and Mg directly affects plant growth, as both these nutrients are essential for cell wall formation, enzyme activities, stability of cell membranes and carbohydrate metabolism. In acid soils, low P availability to plants due to its fixation by Fe and Al oxides or due to the presence of excess Al is well documented.
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.
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 the atmospheric N fixation as well as N mineralization in acid soils through enhanced microbial activity.
Other management aspects are-
- Growing of acid tolerant crop species and varieties.
- Moderately acidic soils can be managed with sufficient quantity of organic matter. The leaching loss of basic cations can be minimized by improving physical conditions of soils through application of organic wastes and lime. Humified organic matter can tightly bind to Al ions and prevent them from building toxicity in soil solutions.
- For a soil with sub-surface acidity, gypsum or phosphogypsum is useful.
- Lime sludge from paper mills and basic slag can be safely used to ameliorate soil acidity.
- In case of acid sulphate soils, it is better not to disturb the sulphuric horizon and rice cultivation is preferable as it maintains the soils in reduced condition.
Salt Affected Soils:
The salt affected soils occur in the arid and semi-arid regions where evapo-transpiration greatly exceeds precipitation. The accumulated ions causing salinity or alkalinity include sodium, potassium, magnesium, calcium, chlorides, carbonates and bicarbonates. The salt affected soils can be primarily classified as: Saline soil, Sodic soil and Saline-sodic soil
1) Saline Soil:
Major Production Constraints-
Saline soils contain excess neutral soluble salts like chlorides and sulphates of sodium, calcium and magnesium with EC >4 dSm-1, pH<8.2 and ESP<15. These soils are fairly permeable due to well-flocculated soil structure. Osmotic effects of salts, toxic concentrations of soluble ions like Na, Cl, B, and reduced availability of essential nutrients due to competitive uptake affect plant growth in this type of soil. Excess salinity results into delayed germination, poor crop stand, stunted growth and reduced yield.
Leaching: Leaching of soluble salts from the crop root zone is performed to reduce the toxic effects of salinity. Leaching efficiency in saline soils primarily depends upon soil type, depth of applied water, soil profile characteristics, absorption and exchange reactions during leaching, and the initial wetness of the soil. The simplest process is to flood the fields. Some field studies (Verma and Gupta, 1989) have demonstrated that controlled leaching through intermittent irrigation is more effective for removing salts than uncontrolled leaching through continuous ponding.
Drainage: Sub-surface drainage is an effective tool for lowering the water table, removal of excess salts and prevention of secondary salinization. Sub-surface drainage through tile drain decreases salinity and increases crop productivity.
Fertilizer management: Salinity affects the growth and yield of most of the crops. Yet, at a given salinity level, yield can be increased through balanced nutrient management. Nitrogen is the most limiting nutrient for crop production in saline soils which are poor in organic matter and available N content. High salinity reduces urea hydrolysis and nitrification rates due to poor microbial activities. The major losses of surface applied nitrogen are through NH3 volatilization as it is favored by high salt concentration.
Potassium, in general, is adequately available in the salt-affected soils unless the soil is very low in 2: 1 type of clay minerals. Green manures due to its narrow C:N ratio decompose fast and makes N, P and K available quicker than the other organic sources, stimulates microbial and enzymatic activities in the salt-affected soils and also increases hydraulic conductivity and leaching of salts in saline soils (Bandyapadhyay et al. 2001).
Irrigation Management: Efficient water management leads to increased crop yield under saline soil condition. Drip, sprinkler and pitcher irrigation have been found to be more efficient than the conventional flood irrigation method since relatively lesser amount of water is required under these improved methods. Drip and pitcher methods are very useful for saline soils as they add water directly into the root zone at controlled rates and even saline water can be used under these methods without any detrimental effect on crop growth (Bandyopadhyay et al. 2001) owing to dilution of salts at the root zone.
Crop Choice and Planting Techniques:
Yield reduction in saline soils can be minimized through selection of salt tolerant crops and different planting techniques that can reduce the salt stress on plant growth. Planting the seed in the centre of the raised bed/ridge may affect the germination as it is the spot of greatest salt accumulation. A better salinity control can be achieved by using sloping beds with seeds planted on the sloping side just above the water line. Planting the seeds at the bottom of the furrows places the seed in a wet and less saline zone and enhances germination (Minhas, 1994).
2) Sodic Soil:
Major Production Constraints-
The distinguishing characteristics of sodic soils are high exchangeable sodium percent (ESP > 15), ECe less than 4 dS m-1, pH more than 8.2 and presence of higher amount of carbonates and bicarbonates of Na. Some sodic soils are also termed as saline sodic soils as they contain large quantity of soluble salts and ECe is more than 4 dS m-1.These soils, due to high ESP, possess poor physical properties. The high Na content of the soils leads to dispersion of fine clay particles resulting into low permeability, crusting and hardening of the surface soil upon drying. As a result, the aeration, soil water movement and root growth is impeded. Besides, the high Na content is often toxic to many plants, which exhibits poor growth and yield in these types of soils. The soils also have poor aggregate stability. Low organic matter content, toxic concentration of C03 and HC03, poor microbial activity due to unfavorable pH and reduced availability of N, K, Zn, Fe etc. also affect the productivity of these soils.
Management of sodic soils consist of reclamation of soil which involves replacement of exchangeable Na with Ca through different amendments along with improved agro-techniques including crop choice, land shaping and water and fertilizer management.
Due to calcareous nature of most of the sodic soils in the Indo-Gangetic plains, Ca sources like gypsum or acid formers like pyrites are preferred more as amendments although large variations in the efficiency of different sources is reported. Besides choice of a particular amendment depend upon its availability and cost. Application of gypsum reduces soil pH and ESP of the sodic soils and increases the yield of crops.
Other management aspects are-
- Deep ploughing and sub-soiling and also profile inversion can be beneficial to break the hard impermeable layer if any.
- Green manuring, incorporation of crop residues, application of FYM, press mud and other organic materials are helpful in ameliorating such problematic soils.
- Rice is preferred to be grown during reclamation of alkali soils owing to its high tolerance to soil sodicity. The ponding of water for optimum rice growth promotes buildup of pCO2 and leaching of salts resulting from the exchange of Na by Ca ion.
Bandyopadhyay, B.K., Sen, H.S., Maji, B and Yadav, J.S.P. 2001. Saline and Alkali Soils and Their Management. ISCAR Monograph 1.SCAR, CSSRI, WB, 72 pp.
Bhat, J. A., Kundu, M. C., Hazra, G. C., Santra, G. H and Mandal, B. 2010. Rehabilitating acid soil for increasing crop productivity through low-cost liming materials, Science of the Total Environment, 408:4346-4353.
Minhas, P.S. 1994. Use of saline water for irrigation. In: Salinity Management for Sustainable Agricullure (D.L.N Rao, N.T Singh, R .K . Gupta and N .K. Tyagi , Eds.) pp 201-225. CSSRI ,Karnal , India.
Verma , S. K. and Gupta, R.K. 1989. Leaching behaviour of saline clay soil under two modes of water application. Journal of the Indian Society of Soil Science 37, 803-808.
Dhaneshwar Padhan1*, Arbind Kumar Gupta1, Arup Sen1 and Amit Kumar Pradhan2
1Department of Agricultural Chemistry and Soil Science,
Bidhan Chandra Krishi Viswavidyalaya, West Bengal- 741252
2Department of Soil Science and Agricultural Chemistry,
Bihar Agriculture University, Sabour, Bhagalpur, Bihar-813210