Causes of salt accumulation
The location of Pakistan is in arid and semi-arid climatic zones. The average summer temperature is about 45°C and the minimum winter temperature remains between 2°C to 5°C. The annual rainfall varies between 100 mm to 700 mm throughout the country. The evaporation rate is generally very high and exceeds that of precipitation. Thus, the insufficient rainfall followed by high evaporative demand and shallow ground water depth, enhances the movement of salts towards soil surface.
In Pakistan salinity is an important problem affecting irrigated agriculture. Improper irrigation practices and lack of drainage have generally led to accumulation of salts in the soil in concentrations, which are harmful to the crops. There is a major imbalance in the amount of salt entering and leaving the soil in Pakistan.
Salt affected soils are also caused by excess accumulation of salts, typically most pronounced at the soil surface. Salts can be transported to the soil surface by capillary transport from a salt laden water table and then accumulate due to evaporation. They can also be concentrated in soils due to human activity, for example the use of potassium as fertilizer, which can form sylvite, a naturally occurring salt. As soil salinity increases, salt effects can result in degradation of soils and vegetation.
Salt is a natural element of soils and water. The ions responsible for salinization are: Na+, K+, Ca2+, Mg2+ and Cl-. As the Na+ (sodium) predominates, soils can become sodic. Sodic soils present particular challenges because they tend to have very poor structure which limits or prevents water infiltration and drainage.
Way to improving salt-affected soils
Saline agriculture always involves some compromise on yields as even a very salt tolerant species is bound to suffer some yield losses under the adverse conditions. It is hence advisable to observe precautions in such ventures. Saline soils may be improved by leaching the salts from the root zone. Leaching is the process in which extra water is added to a field and allowed to soak through the soil and drain away underground.
Modern research has identified of more than 1500 plant species that have high levels of tolerance to saline soils, these are called halophytes (salt-loving plants). Some of these are able to withstand salt concentrations in excess of those found in sea-water. These plants are Paddy rice , Sugar cane, Oat, Wheat, Aegilops, Triticale, Sorghum, Barley, Corn, Pearl millet, Rye, Oilseed, Rape, Canola, Mustard Vegetables: Spinach, Sugar beet, Red beet, Tomato, and Carrot etc a major resource that can be used in the development of agricultural systems for salt affected soils.
The research that has been conducted in Pakistan over the last decade shows that there is a wide range of plant species with varying levels of salt tolerance that can be used in saline agriculture like Date palm, Eucalyptus, Wild date palm, Danka, Pomegranate, Zizyphus, Kallar grass, Bermuda grass, Rhodes grass and Coconut.
Growing of suitable crops at ECe, 15-20 dSm-1 – a highly saline soil. At high salinity (ECe 15-20 dSm-l, 9600-12800 ppm), with coarse textured soil, where good quality irrigation water is not available, the growth of majority of plants will be restricted and only some salt tolerant plants includes Bottle palm, Cactus, Periwinkle, China rose, Drumstick tree, Wild banana, Wild cherry, Purslane and Reed plant can grow, provided under ground water table is not shallow.
Intensive cultivation on such soils without using gypsum has made them compact, hindering the penetration of roots to deeper layers. The physio-chemical conditions of such soils can be improved through better soil management practices. Use of gypsum, being a cash input, needs to be encouraged through advancing credit to the farmers and ensure its timely supply at convenient farm locations, at subsidized rates.
Soil samples should be taken after leaching the site for analysis to determine whether salinity level is suitable for planting. Later, the land may be irrigated again to get rid of some of the sodium sulphate that is formed in the chemical reaction but still carbonate remains as a calcium salt.
Salts are most efficiently leached from the soil profile under higher frequency irrigation (shorter irrigation intervals). Keeping soil moisture levels higher between irrigation events effectively dilutes salt concentrations in the root zone, thereby reducing the salinity hazard.
The writers are associated with the Department of Plant Pathology, University of Agriculture Faisalabad, Pakistan. They can be reached at <email@example.com>