Crop cultivation on problem soils in Pakistan - Soil serves as a medium for plant growth and development. However, raising crops on problem soils such as water logged, salt affected, wind and water eroded is a matter of great concern. Water logging is a state of soil when water table reaches in the root zone or rises to such an extent that growing crops become impossible. Under arid and semi-arid conditions, water logging causes salinity and imposes oxygen stress on plant growth and development.
Water logging is persistent problem in areas where lands are heavily irrigated. Major causes for water logging comprise poor natural drainage due to lower slope of one foot per mile, imperfect vertical drainage in fine textured soils, excessive irrigation and existence of impermeable hard pan in soil resulting in perched water table.
Important solutions to water logging include the installation of tube wells in the waterlogged areas and planting hydrophytes which absorb and transpire water in much higher quantities and their growth rate is also fast. Planting trees like white leadtree and mulberry along canals and water courses will help to maintain the underground water levels and will also provide some income. To add to this, planting trees like acacia, mulberry, Erythrina, popular species, red silk-cotton, and castor etc. will not only reclaim water logged soils but also will provide income to the farmers and help solve the fuel and timber problems at national level. Rice is the crop that is adapted to standing water conditions and the soils with water logging problem can be placed under rice crop.
Salinity is another problem hampering plant growth and development from germination up to harvesting. Three major categories of salt affected soils include saline soils, saline sodic soils and sodic soils. Saline soils are characterized by presence of excess chlorides and sulphates of calcium, magnesium and sodium having electric conductivity (EC) of more than 4 deci Siemens per meter (dS m-1) at 25 ◦C, pH less than 8.5 and exchangeable sodium percentage less than 15. On the other hand, saline sodic soils contain the excessive soluble salts as well as exchangeable sodium and possess electrical conductivity of more than 4 dS m-1, exchangeable sodium percentage of more than 15, and pH either more or less than 8.5. Sodic soils contain excess amounts of exchangeable sodium to adversely affect plant growth but no soluble sulphates and chlorides of calcium and magnesium with electric conductivity of less than 4 dS m-1, pH between 8.5 and 10 and exchangeable sodium percentage of more than 15.
Water logging and salinity are inter-linked as a decrease in the depth of water level within five feet inhibits root growth and causes yields of all major crops to decline rapidly. The salinity causes 25 per cent reduction in potential production of major crops as presence of excess amount of soluble salts reduced uptake of nutrients and moisture leading to incomplete vegetative and reproductive growth.
Weathering of parent rocks, high evaporation, saline underground water, transfer of salts from salt affected areas to normal areas and use of industrial water for irrigation are important reasons of salinity. One liter canal water contains about 150 milligram salts. On the other, one liter of ground water contains more than 1000 milligram salts. To reclaim saline soils, establishment of adequate drainage system, using low salt content irrigation water, leveling of land, dividing field into small plots and suitable crop rotation are essential measures.
Leveling of land allows uniform water distribution and leaching of salts from the root zone. Soaking irrigation before cultivation is also recommended for this purpose. Employing organic matter in the form of farm yard manure, green manure, crop residues improves leaching of salts. Heavy irrigation at least once a year is preferable to move the salts out of the root zone. More frequent irrigations, alternate use of canal irrigation water with tube well water, changing water supply according to salt tolerance level of plant, using more seed rate, sowing on ridges, continuous sowing, sowing cover crops, cultivating salt tolerant crops and using appropriate type of fertilizer are some other agronomic measures for improving productivity of salt affected soils.
Salt sensitivity or salt tolerance varies considerably depending upon plant species and plant growth stages. Relatively cotton and sugar beet are more salt sensitive at germination stage than at later developmental stages.
Incorporating amendments like gypsum and sulfuric acid into saline soils followed by heavy irrigation leaches the replaced sodium and soluble salts out of the root zone. Gypsum is cheaper source of calcium in the country. When gypsum added into the soil, sodium leaches out of the root zone. Importantly, sulfuric acid may also be used in place of gypsum in calcareous sodic soil.
Wheat, cotton, sugarcane, rice, barley, oil seeds, berseem, lucerne, oats, mash, lentil, kallar grass, sugar beat, etc. are salt tolerant crops and can be grown depending upon level of salinity.
Erosion is another problem facing the farming sector. About 18 percent of the total area is affected by erosion in the country. Water erosion is more dangerous compared to the wind erosion. Removal of soil by wind erosion or water erosion and subsequent incorporation of underlying material by tillage reduces the organic matter. The contents of P. Zn, Fe and Mn decrease considerably as severity of erosion increases. The physical properties such as water holding capacity and infiltration capacity are also reduced due to soil erosion which consequently further enhances the process. The soil structure and water holding capacity can be improved by green manuring, whereas heavy manuring reduces the soil erodibility up to 5 times. Soil erodibility can be reduced by establishing wind breaks, growing cover crops, bund making, mulching, contour farming, terracing, strip cultivation, planting forest trees, and stabilizing vegetation cover, because root system of plants penetrates into the soil and binds its particles in proper place.
Agricultural production can be augmented many folds if problem soils are brought under cultivation ultimately to become self sufficient to feed over 15 billion people in the country. Reclamation of problem soils will be helpful to lift living standards of farmers by bringing more area under cultivation, improving nutrient and water use efficiency and boosting yields per unit area.
Keywords: Salt affected soils, salinity, crop growth, soil reclamation