Friday, 14 July 2006 - 10:45 AM

Soil Salinity and Associated Nutrient Constraints in Indian Subcontinent.

Abdul Rashid, National Agricultural Research Center, Land Resources Research Program, Park Road, Islamabad, 45500, Pakistan

Accumulation of excessive soluble salts and/or exchangeable sodium (Na+) is a characteristic of arid and semi-arid lands, like many regions in countries of the Indian subcontinent (i.e., India, Pakistan, Bangladesh, Nepal, Bhutan, and Sri Lanka). In the subcontinent, most extensive salt-affected soils occur in two countries, i.e., atleast 9.38 Mha in India and 5.73 Mha in Pakistan. However, the reported extents vary drastically. In India, for example, 7.026.1 Mha have been reported to be saline and sodic. Salt affected soils are spread widely covering the Indo-Gangetic plains, arid regions and coastal areas. With the expansion of irrigation, vast acreage of non-saline soils in the canal command areas have been affected by secondary salinity. The malady continues to increase due to the mismanagement of canal irrigation as well as due to brackish groundwater irrigation. Salt-affected soils differ from the normal soils in many respects and vary from one place to another depending upon topography, climate, hydrology, drainage and land use. Whatever the situation, excessive accumulation of salts in the root zone soil is a serious threat to agricultural productivity. In addition to other soil physiological stresses which decrease crop yields, salinity and sodicity also adversely affect soil fertility. Salt-affected soils generally exist in the hyperthermic temperature regime, conducive for rapid decomposition of organic matter; thus, contain very low organic matter and are poor in fertility. Most salt-affected soils are deficient in nitrogen (N), phosphorus (P), and are medium to high in potassium (K). Zinc (Zn) deficiency is widespread and sulfur (S) and boron (B) deficiencies are becoming important in several areas. Also, in salt-affected soils, B, lithium (Li), fluorine (F), selenium (Se) and molybdenum (Mo) could be toxic. Boron, Li, and F are phyto-toxic, whereas Se and Mo may not adversely affect plant growth but the crops grown on them, particularly forages, may contain excessive concentration of these elements causing health hazard to grazing animals. High alkalinity and/or salinity decreases microbial activity and hence the rate of N mineralization and, thus, necessitate higher rates of N fertilizers. Also, due to the adverse effects of salinity and sodicity on transformations of soil and applied fertilizer N, crops respond to much higher levels of N in these soils compared with normal soils. Green manuring can improve fertilizer N use efficiency by crops. The transformations and availability of applied and soil P and crop responses to P application greatly differ in sodic and saline soils. Available P in the soil increases with increase in EC and pH. During reclamation of sodic soils with gypsum, extractable P declines due to its conversion to less soluble Ca-P compounds. In saline soils, P availability decreases due to higher retention of soluble phosphate, antagonistic effect of Cl- and SO42- on plant absorption of P and restricted root growth. In sodic soils, high Na and low calcium (Ca) result in decreased K uptake by plants. Though saline soils are often medium to high in K, during reclamation, losses of K due to leaching may take place leading to its low availability. Excess salts may also interfere with plant nutrition by affecting nutrient availability, uptake, or their physiological role within the plant. Due to considerable variations in the chemical composition, precipitation-dissolution reactions, adsorption-desorption kinetics and transformation of nutrients, crop responses to applied nutrients vary greatly in saline and sodic soils. In sodic soils, presence of CaCO3 and poor air-relations affect the availability of nutrients. The solubility of nutrient cations decreases due to the dominance of carbonate equilibria. In saline soils, the solubilities of these cations do not decrease and they remain in available forms. The degree of salinity also influences crop response to fertilizer; if the degree of salinity is initially high, crop response to fertilizer is limited. Through integrated plant nutrient supplies (IPNS), diverse nutrient sources can be deployed to sustain crop yields, and reclaim the soil, provided crop residues are available at the farm level. In short, special plant nutrient management strategies are needed for salt-affected soils. Also, salt-tolerant crops/cultivars and cultivation techniques are required for saline agriculture.

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