By Dr C. M. Ayyub and Waqas Ahmad
ABOUT 6.30 million hectares in Pakistan are salt-affected. Out of which 1.89 million hectares are saline, 1.85 million permeable saline-sodic, 1.02 million impermeable saline-sodic and 0.028 million hectares are sodic in nature.
Out of the 1.89 million hectares saline patches, 0.45 million hectares exist in Punjab, 0.94 million hectares in Sindh and 0.5 million hectares in NWFP. Out of the 19.3 mha area available for farming, about 16 million hectares are irrigated. The irrigation water is mainly supplied through the canal system and arranged through dams.
Intensive and continuous use of surface irrigation has altered the hydrologic balance of the irrigated areas, especially in the Indus Basin. The substantial rise in water table has caused salinity and water-logging in large areas of Sindh, Punjab, KP and Balochistan. The magnitude of the problem can be gauged from the fact that the productive land is being damaged by salinity at a rate of about 40,000 hectares per annum.
Global constraints on fresh water supplies and the need to dispose of agricultural, municipal, and industrial waste waters have intensified interest in water reuse options. In many instances, the value of water decreases solely because of its higher salt concentration. Although quantitative information on crop salt tolerance exists for over 130 crop species, there are many vegetables which lack definitive data.
Vegetable crops are defined as herbaceous species grown for human consumption in which the edible portions consist of leaves, roots, hypocotyls, stems, petioles, and flower buds.
The salt tolerance of vegetable species is important because the cash value of vegetables is usually high compared to field crops.
In this review some general information is presented on how salinity affects plant growth and development and how different measurements of salinity in solution cultures, sand cultures, and field studies can be reconciled to a common basis.
The general effect of salinity is to reduce the growth rate resulting in smaller leaves, shorter stature, and sometimes fewer leaves. The initial and primary effect of salinity, especially at low to moderate concentrations, is due to its osmotic effects.
Roots are also reduced in length and mass but may become thinner or thicker. Maturity rate may be delayed or advanced depending on species. The degree to which growth is reduced by salinity differs greatly with species and to a lesser extent with varieties within a species. The severity of salinity response is also mediated by environmental interactions such as relative humidity, temperature, radiation and air pollution.
All salinity effects may not be negative; salinity may have some favourable effects of yield, quality, and disease resistance. In spinach, for example, yields may initially increase at low to moderate salinity. Sugar contents increase in carrot and starch content decreases in potatoes as salinity increases, cabbage heads are more solid at low salinity levels, but are less compact as salinity increases.
Important environmental factors that show significant interaction with salinity include temperature, wind, humidity, light, and air pollution. High temperatures and low humidity may decrease crop salt tolerance. Thus, significant reductions in yields will be realised at lower salinities, and yields will decrease more rapidly with increasing salinity under hot, dry conditions.
Salinity slows germination rate and at higher levels reduces germination percentage. At low concentrations the only effect is on germination rate and not total percentage of seeds germinated. Thus reported data are dependent upon the time of observation as well as the germination conditions.
Saline agriculture can be a profitable and practical venture under proper management and by observing suitable precautionary measures. Initial establishment is crucial for subsequent growth and hence stress should be minimised as far as possible at this stage.
Saline agriculture is an economical and effective approach to use unproductive lands for growing various plant and food crops. If the plant survives the shock at seeding/transplanting stage, the chances of its subsequent survival and growth increase. Several salt-tolerant grain, fruit and fodder species have been identified for practising saline agriculture in the country. This approach, if prudently adapted, can help reduce imports of agricultural commodities to a great extent.
Use of much to conserve moisture, planting on ridges, light but more frequent irrigation often helps under such conditions. Out of about 5,000 crops that are cultivated throughout the world, a few can survive with water that contains more than one per cent salt. The world food base can be vastly expanded if the currently cultivated crops can be grown by maintaining proper soil management.
It is also necessary that the government should encourage local agencies by supplying all sorts of facilities for the betterment and reclaiming of salt- affected lands.
At present, it is a burning topic from agricultural point of view as for the last ten years production of agricultural crops has become stagnant rather it is towards deteriorating side. This situation is forcing the government to import vegetables seed and other cereal products incurring in a huge expenditure affecting the country’s economy.
The University of Agriculture, Faisalabad, is carrying out extensive research to improve cultivation of different vegetables under saline conditions. If the research proves to be a success, it will largely contribute to improving vegetable production in salt-effected lands ridding the country of the turmoil in the agricultural sector.