PHENOMENON OF SALINE AGRICULTURE
DR. S. M. ALAM & T. HUSAIN
July 14 - 20, 2008
Pakistan is an agricultural country which is fortunate in terms of soil, topography, and climate, which are generally suitable for year round agriculture. Due to the existence of the country in arid and semi-arid climatic regions, artificial irrigation has been practiced for a long lime. Rainfall in the country is very sporadic and the annual rainfall distribution based on per cent of geo-graphical area, which may be classified as: about 68 per cent of the geographical area lies under annual rainfall of 351-500 mm. This leaves geographical area where the annual rainfall exceeds 100 mm.
The total geographical area of Pakistan is 79.61 million hectares of which 20.82 million hectares (about 25 pc) are currently under cultivation. In Pakistan, Punjab occupies 20.6 mha out of which 11.04 mha (about 54pc) are cultivated. The area of Sindh is 14.1 mha of which 5.45 mha (about 19pc) are cultivated. Of Baluchistan 34.7 mha only 1.4 mha (about 4pc) are under cultivation while the area of NWFP comprises of 10.2 mha out of which 1.93 mha (nearly 10 pc) are cultivated. The country has more than 6.30 mha of salt-affected land, which are mainly situated in Indus plain, where about 4.5 mha of land is affected. The province-wise sail-affected areas are: Punjab 2.56 mha; Sindh 3.321 mha; NWFP 0.516 mha; and Baluchistan 0.30 mha. This situation is resulting in a national economic loss of millions of rupees annually.
Pakistan is a land of promise and has tremendous development possibilities by virtue of its unique geographical location, fast inquisitional talents of its people, and richness of natural and cultural resources. Most of the land areas of Pakistan are classified as arid to semi-arid because rainfall is not sufficient to grow agricultural crops, forest and fruit plants, vegetables in pastures. Salt affected area out side Indus Basin is 5.6 mha spanned over the hot deserts and some western and eastern areas of Thar.
Thar and Cholisian deserts are parts of the Great Indian Desert and cover the area east of the southern half of the Indus plain. The Thar area is between the Jhelum and Indus River.
Water is unique natural resource in Pakistan. Conservation and management of water supplies is crucial as the demand for water continues to rise because of burgeoning population. In Pakistan agriculture is predominantly irrigated. Water is one of the critical constraints to agricultural production in Pakistan which has blessings of nature such as fertile land, network of rivers, sunshine versatile climate, and vast potential of agricultural production. The country's major agricultural potential is within the smooth plains found naturally on Indus.
River such as Kabul, Jhelum, Chenab, Ravi, and Sutlej have flows in southwardly directions to finally enter into a single stream into Arabian Sea near Karachi. Due to mismanagement of water resources, inadequate drainage systems, poor performance of existing irrigation and drainage systems the agricultural production is far below its potential.
Saline agriculture can be defined as the use of genetic resources such as plants, animal, fish, insects and micro-organisms, and improved agricultural management to obtain profitable use from saline land and irrigation water on a sustainable basis. It is a rich collection of possible systems for the use of saline resources. The components of these systems will vary according to the needs of the farmers and the capabilities of the land and water. Pakistan has some major advantage in the development of saline agricultural system, its greatest advantage is a rich heritage of research that commenced in the 1970s and continues till today. Over the last three decades, there has been outstanding work on the selection of salt tolerant trees, shrubs, grasses and crops that enable increased production from salt-affected land.
Salinity, aridity, and water logging are serious problems for agriculture which contributes towards large scale spread of poverty and social unrest among the citizens. At present, around 6.5 million hectares of land is salt affected and more than 75pc of the tube wells are pumping out brackish waters. There is a problem of drainage in the country due to application of irrigation water from the rivers and canal waters. The emphasis in the present proposal has been to economically utilize the waste lands and brackish ground water for growing salt tolerant plants rather than reclaiming the soil to grow conventional crops. This biological approach involves screening and selection of highly salt tolerant plant species, varieties from the naturally existing germplasm, wide hybridization and other biotechnological techniques, and plants for increased plant establishment and productivity in saline areas. The PAEC agricultural institutes are actively engaged in to reclaim salt affected lands by using bio-drainage technique and planting salt tolerant plants.
Still, there is a need of series of thought to harness salt affected soils. The Pakistan's scientists are among the world leaders to develop the alternate approach of Saline Agriculture, which aims at better use of saline land and saline irrigation water on a sustained basis through the profitable and irrigated use of genetic resources (plants, animals, fish and insects) and improved agricultural practices.
Salinity is a pivotal problem of semi-arid regions where rainfall is insufficient to leach salts out of the root zone. These areas often have high evaporative rates, which can encourage and increase in salt concentration at the soil surface through capillary rise. Salt accumulation in these soils is generally caused by lack of appropriate drainage, combined with seepage from water delivery systems and by inappropriate irrigation management. It is a common knowledge that salinity reduces crop growth and that saline/brackish water with a high Na+/Ca2+ ratio can destroy the soil structure. The presence of a cemented hard pan at varying depths and insufficient precipitation for leaching often adds to the problem. Newly established irrigation projects with improper planning and management practices may also add salt to soils. Historically, soil salinity contributed to the decline of several civilizations. Despite the advanced technologies today, salinization of millions of hectares of land continues to reduce crop productivity severely world-wide.
At present, no continent on our globe is free of salt-affected soils. They are not only distributed in deserts and semi desert regions, but also frequently in fertile alluvial plains, river valleys, and coastal areas close to densely populated areas and irrigation systems. Despite that the properties and attributes of salt-affected soils have been well known for a long time, it is appropriate to give a brief definition of salinity and alkalinity as these soils are seriously affecting the productivity of the land.
As Pakistan is situated in arid and semi-arid climate zones the high evapo- transpiration is the basic cause for salt accumulation on the soil surface.
The average summer temperature goes up to 40oC and the minimum winter temperature remains in between 20 C to 50 C. The annual rainfall varies between 100 mm to 700 mm distributed unevenly throughout the country. Thus, the insufficient rainfall followed by high evaporative demand and with shallow ground water depth enhances the movement of salts towards soil surface. Continuous use of surface irrigation water has also altered the hydrological balance of the land, which generally increases the amount of salts in the soil. Each year, about 120 million tons of salts are added to the land through canal water and brackish under ground water, but only about 1/5th finds its ways to the sea.
The remainder accumulates in the soil and continues to decrease the growth and survival of crops. As the salt concentration increases, die choice of crops becomes limited and one has logo for tolerant plants suited for specific conditions.
Salt in the soil solution decreases the water potential and reduces the amount of water available to plants. Soil salinity and therefore soil water potential values vary with depth in the field. The plant root absorbs water from zones of lower salinity. Saline agriculture usually involves some compromise on yields as even a very salt tolerant species are bound to suffer some yield losses under the adverse conditions. It is hence advisable to observe precautions in such ventures. Saline soils are usually reclaimed by leaching the salts out of the soils through irrigation and drainage systems. Leaching through drainage controls salinity in soils in which levels of sodium (Na) are not excessive. Drainage is the first requirement for managing salinity. Drainage established the salt content of the soil is reduced by leaching water that has salinity level within crop tolerance. Leaching is the process in which extra water is added to a field and allowed to soak through the soil and drain away the underground.
Leaching is commonly done for one time and may not be necessary for a number of years. Leaching can easily be delayed until the season when more water is available. A common method of leaching is to pond the water in basin over the entire field. Sometimes, the excess water is removed by pumping. Permissible depths for ground-water tables vary according to the type of soil being irrigated. Leaching is most efficient, when the flow is unsaturated. Increasing the amount of water per application and the time interval between water applications decreases the total amount of water requirement for leaching. The amount of leaching water that enters the soil by surface flooding determines how much salt is removed from the soil.
For instance, when water is leached through the soil, a surface depth of 6 inches of water for every foot of plant root will leach out 50 percent of the salt. One foot of water for every foot of root zone leaches out 50 percent of the salt. Two feet of water per foot root zone leaches out 90pc of the salt. The upward movements of saline water from shallow water tables can cause salt buildup in the plant root zone. The water table should be at least 4 1/2 to 5 feet below the surface during most of the crop-growing season.