Dr. S. M. Alam, Dr. R. Ansari and M. Athar Khan.
Nuclear Institute of Agriculture, Tando Jam, Pakistan
May 08 -21, 2000
Soil salinity and sodicity problems are common in arid and semiarid regions, where rainfall is insufficient to leach salts and excess sodium ions out of the rhizosphere. Nearly 10% of the total land surface is covered with different types of salt-affected soils. At present, there are nearly 954 million hectares of saline soils on the earth's surface. All these salt affected soils are distributed throughout the world. More than 80 million hectares of such soils are in Africa, 50 million hectares in Europe, 357 million hectares in Australasia, nearly 147 million hectares in Central, North and South America. Similarly, a large bulk of about 320 million hectares and land in South and South East Asia is under the grip of salinity.
The total geographical areas of Pakistan is 80.0 million hectares or 197.0 million acres, with a very good canal irrigated system of about 62,400 km long and mainly confined to Indus plain covering an area of 19.43 million hectares (48 million acres). The salt affected soils are mainly situated in this plain. In Pakistan, about 6.30 million hectares of land are salt-affected and of which 1.89 hectare is saline, 1.85 million hectare is permeable saline-sodic, 1.02 million hectare is impermeable saline-sodic and 0.028 million hectare is sodic in nature. It is estimated that out of 1.89 million hectares saline patches, 0.45 million hectares present in Punjab, 0.94 million hectares in Sindh and 0.5 million hectares in NWFP. Out of 19.3 mha area available for farming, irrigated agriculture is practised on about 16 mha. The irrigation water is mainly supplied through the worlds largest canal system arranged through dams. Intensive and continuous use of surface irrigation has altered the hydrological balance of the irrigated areas, specially Indus basin. The substantial rise in the water table has caused salinity and water logging in large areas of Sindh, Punjab, NWFP and Balochistan. The magnitude of the problem can be gauged from the fact that the area of productive land was being damaged by salinity at a rate of about 40000 hectares annually.
Causes of alt accumulation:
The location of Pakistan is in arid and semi-arid climatic zones. Generally high evapo-transpiration in semi-arid and arid zones is the basic cause for salt accumulation on the soil surface. The average summer temperature is about 40°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.
Salinity is an important problem affecting irrigated agriculture of Pakistan. 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. Each year about 120 million tonnes of salts are added to the land in canal water and brackish under ground water. Only about 1/5th of this salt finds its ways to the sea. The remainder accumulates in the soil, it continues to decrease the growth and survival of crops.
The soils with electrical conductivity of less than 4 dSm-1 are generally considered as salts free, where almost all crops can be grown. As the salt concentration increases, the choice becomes limited and one has to go for tolerant plants suited for specific conditions.
Improving salt-affected soils:
Saline agriculture almost 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. A common method of leaching is to pond the water in basins over the entire field. Sometimes the excess water is removed by pumping from wells. Permissible depths for ground-water tables vary according to the type of soil being irrigated. The amount of leaching water that enters the soil by surface flooding determines how much salt is removed from the soil. 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 per cent of the salt. One foot of water for every foot of root zone leaches out 80 per cent of the salt. Two feet of water per foot root zone leaches out 90 per cent of the salt.
The leaching of salts through irrigation however, requires extreme care as this should not add to the underground water table. Tube wells are generally sunk to get rid of such shallow water tables without which leaching may not be advisable. The upward movements of saline water from shallow water tables can cause salt build up in the plant root zone. A water table should be at least 41/2 to 5 feet below the surface during most of the crop growing season.
Adding chemical amendments:
Most saline soils need chemical amendments to restore their productivity. Many suitable amendments are available, gypsum, sulphur and sulphuric acid are the most common, but application of acid needs special care due to its corrosive action. Application of acid is effective in reclaiming saline soils by lowering ESP (exchangeable sodium percentage ), SAR (sodium adsorption ratio ), pH. Scrapping of salt layer, land leveling, deep ploughing, sub-soiling, sanding (i.e. adding sand), flushing with good quality and enough quantity of water and drainage also help in improving soil condition. For better results in cultivating saline soils, higher seed rate for higher plant population, crop rotation, proper choice of crops, sufficient amount of NPK fertilizers, avoiding urea and using ammonium sulphate as an N source is advised.
Green manuring through leguminous crops and application of farm yard manure not only provide organic matter and other nutrient, but also make the soil porous for aeration and moisture absorption and enhance soil microorganisms, thus improving the overall condition of the soil. Similarly replacement of sodium-ions by calcium using gypsum helps in mitigating the adverse conditions.
It should also be stressed that lands, specially the saline ones should never be left fallow for extended periods, because in uncultivated lands there is a constant upward flux of water, which keeps evaporating and leaves behind the salts on the surface. Presence of vegetation of any kind helps to minimize this development.
Modern research has identified of more than 1500 plant species that have high levels of tolerance to saline soils, these are called halophytes. Some of these are able to withstand salt concentrations in excess of those found in sea-water. These plants (trees, shrubs and salt tolerant grasses and herbs) are a major resource that can be used in the development of agricultural systems for salt affected soils. In addition, there are opportunities to increase the salt tolerance of existing crops using conventional plant breeding and molecular biological approaches.
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. Furthermore, at least some of these plants are able to lower local water-tables, improving the condition of the land, and acting as 'biological pumps'.
Growing of suitable crops at ECe, 0-15 dSm-1 - a moderately saline soil.
If the average Electrical Conductivity (ECe) of a plot is 0-15 dSm-1 (9600 ppm), which is fairly high and may have serious effects on the growth of plants, but the soil is of loamy texture, the underground water table is not shallow i.e. at 8-10 ft some good quality irrigation water is available, it makes the conditions less hostile for plant sustenance. A number of species may be grown here as given below. Good quality irrigation water should be used during the initial establishment phase and for periodical leaching of salts from soil surface. Under proper management, this can be a cost effective venture.
i) Cereals - Paddy rice (Oryza sativa), Sugar cane, Oat, Wheat (Triticum and durum), Aegilops, Triticale, Sorghum(Sorghum bicolor L.), Barley(Hordeum vulgare L.), Corn (Zea maiz L.), Pearl millet (Penasitum thypdodeum L.), Rye
ii) Oilseed: Rape, Canola, Mustard
iii) Vegetables: Spinach, Sugar beet, Red beet, Tomato, Carrot etc.
iv) Fodder and Forage: Guar, Dhancha (Sesbania sesban L.), Berseem (Trifolium alexandrinum), Lucerne (Medicago sativa L.), Melilotus species, i.e. Sweet clover, Honey clover, Indian clover, White clover
v) Fibre: Cotton, Sunhemp, Kenaf vi) Fruits: Fig, Grape, Pomegranate, Zizyphus (Zizyphus jujuba)
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 (given below) can grow, provided under ground water table is not shallow. The establishment and sustenance of these plant species will be improved with periodical use of good quality irrigation water or in areas with regular rains.
i) Fruits: Date palm, Wild date palm and Coconut
ii) Grass: Kallar grass (Leptochloa fusca), Orchard grass, Bermuda grass, Rhodes grass (Chloris guyana), Para grass, Tall wheat grass (Agropyron elongatum), Rye grass, Sudan grass
iii) Woody species: Jojoba (Simmondsia chinensis L.), Guava (Psidium guajava L.), Jujube (Zizyphus mauritiana L), Mesquite (Prosopis species), Mangroves, Acacias (nilotica, ampliceps, stenophylla, machonochieana), Atriplex species, Mustard tree (Salvadora persica)
iv) Miscellaneous: Life plant (Bryophyllum pinnatum), Aloe (Aloe perfolia),
Dodonaea (Dodonaea viscosa), Periwinkle (Vinca rosea), Purslane (Portulacea oleraceae), Reed plant (Saccharum arundinacea), Bottle palm, Cactus, China rose, Drumstick tree, Wild banana (Agave americana), Wild cherry (Withania somnifera), Senna (Cassia angustifolia)
It transpires from the foregoing that 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 minimized 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 are likely to be increased. 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 to reduce the imports of agricultural commodities to a great extent. Leaching with only good quality water or by rain reduces shock and improves growth. Use of much to conserve moisture, planting on ridges, light but more frequent irrigation often helps under such conditions. Out of about 5000 crops that are cultivated throughout the world, a few can survive with water that contained more than 1% 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 Government should encourage the 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. Because for the last ten years the production of agricultural crops have become stagnant, rather it is towards deteriorating side. This situation is of course inforcing the government to import wheat and other cereal products from foreign countries. This is causing expenditure of huge amount and thus affecting the economy of the country. A true, honesty and real efforts by the population will of course InshaAllah bring the situation at a recoverable stage and this will help to get rid of such turmoil situation of the agricultural scenario of the country.