Despite the advanced technologies available today, salinization and water logging of million of hectares of land continues

By Dr. S.M. ALAM
April 28 -May 04, 2003

Soil salinity and water logging are the two major problems in arid and semi-arid regions, where rainfall is insufficient to leach out the salts and the excess sodium ions down and out of the root zone. These areas often have high evaporation rates, which can encourage an increase in salt concentration at the soil surface through capillary rise. The presence of a cemented hardpan 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 salts to soils. Historically, soil salinity contributed to the decline of several ancient civilization. Despite the advanced technologies available today, salinization and water logging of million of hectares of land continues to reduce the agricultural crop productivity severely throughout the world including Pakistan.

Of the approximately 13 billion hectares total land on earth, about 1 billion are affected by salinity/sodicity. According to a report, saline/sodic soils cover about 26% of the world's cultivated land. Incidentally, most of the developing and under-developed countries of south and southeast Asia, Africa and South America are the worst affected by this menace. More than 80 million hectares of such soils are in Africa, 50 million hectares in Europe, 357 million hectares in Australasia, nearly 147 mha. in Central, North and South America. Similarly, a large bulk of about 320 million hectares of land in South and South East Asia is under the grip of salinity. It shows that no continent on our planet is free from salt-affected soils.

The geographical area of Pakistan is 79.61 million hectares of which 19.82 million hectares (about 25%) are presently under cultivation. In Pakistan, Punjab occupies 20.6 mha out of which 11.04 mba (about 55%) are cultivated. The area of Sindh is 14.1 mha, of which 5.45 mha (about 40%) are cultivated. Of Balochistan's 34.7 mha only 1.4 mha (about 4%) are cultivated, while the area of NWFP comprises of 10.2 mha, out of which 1.93 mha (only 10%) are cultivated.

The country has more than 5.727 mha of salt-affected land, which are mainly situated in Indus plain, where about 4.2 mha of land is affected. The province-wise salt-affected areas are:

Punjab 2.56 mha; Sindh 2.321 mha; NWFP 0.516 mha and Balochistan 0.304 mha. This situation is resulting in a national economic loss of millions of rupees annually. As Pakistan lies in arid and semi-arid climatic zones, the high evapotranspiration is the main basic cause for salt accumulation on the soil surface. The average summer temperature goes up to 45C and the minimum winter temperature remains between 2C to 5C. 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 groundwater 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 tonnes of salts are added to the soil through canal water and brackish under ground water, but only about 1/5th finds its ways to the sea. The remaining accumulates in the soil and continues to rapidly decrease the growth and survival of crops. As the salt concentration increases, the choice of crops becomes limited and one has to go for tolerant plants suited for specific conditions.

Saline agriculture usually 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 therefore advisable to observe serious precautions in such conditions. Saline soils are usually reclaimed by leaching the salts out of the soils through irrigation and drainage systems. Leaching is the process in which extra fresh water is added to a field and allowed to soak rapidly 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. Permissible depths for groundwater 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. 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 80 percent of the salt. Two feet of water per foot root zone leaches out 90 percent of the salt. The upward movements of saline water from shallow water tables can cause salt build up in the plant root zone. The water table should be at least 41/2 to 5 feet below the surface during most of the crop growing season.

Scrapping of salt layer of the soil, land leveling, deep ploughing, sub-soiling, sanding, flushing with good quality water and drainage, method of irrigation also helps to large extent in improving soil condition. In many cases, chemical amendments may be needed to restore soil 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. These chemicals amendments remove a large proportion of sodium and result in a marked improvement in 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 and using ammonium sulphate as an N source is advised.

Green manuring through leguminous crops and application of FYM, not only provide organic matter and other nutrients, but also make the soil porous for aeration, moisture absorption as well as enhance soil micro-organisms, thus improving the overall conditions of the soil. It should also be kept in mind that lands; especially the saline ones should never be left fallow for extended periods. Presence of vegetation of any kind helps to minimize this development. Reclamation of such soils is not easy and may be costly. If, there may not be enough irrigation water available for leaching, the laying of tile drains and sinking of tubewell may not be within the reach of every farmer, sufficient quantity of ameliorants may not be readily available. Even under the most efficient system and management, it may not be possible to remove completely of its excess salt.

UTILIZATION OF SALINE LANDS: Modern research has identified more than 1500 plant species that have high levels of tolerance to saline soils. Some of these are able to withstand salt concentrations in excess of those found in seawater. 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. Furthermore, some of these plants are able to lower local water tables, improving the condition of the land, and acting as a form of " biological drainage". In addition, there are opportunities to increase the salt tolerance of existing crops using conventional plant breeding and molecular biological approaches.

Crops vary widely in their tolerance to salts. Sugarbeet, cotton and barley can tolerate up to 10 times as much salt as most clover, beans and fruit trees. Scientists have successfully grown a number of crops on marginal saline lands. It is pointed out that if the average electrical conductivity of a plot is 0-15 dSm-1 (9600 ppm), which is high and may have serious effects on the growth of plants, but the soil is loamy in texture, underground watertable is at 8-10 ft and some good quality irrigation water is also available, the conditions become less hostile for plants. A number of species may be grown in such conditions. Good quality water should be used during the initial phase and for leaching of salts from soil surface.

CEREALS: Certain cultivars of paddy rice; sugarcane, oat, wheat, aegilops, triticale, sorghum, barley, pearl millet and rye; Oil seed: rape, canola, mustard; Vegetables: spinach, sugarbeet, red beet; Fodder and forage: guar, dhancha, berseem, lucerne; Fibre: cotton, sunhemp, kenaf; Fruits: fig, grape, pomegranate, zizyphus.

If, the salinity of a plot ranges between 15-20 dS/m with coarse textured soil and the only source of irrigation is bad quality underground water (EC 10-15 dS/m) present at a shallow depth, the growth of majority of the plants will be restricted under such highly adverse conditions and only some salt tolerant plants can be grown. Suitable plants are given below:

FRUITS: date palm, wild date palm and coconut; Grass: karnal or kallar grass, orchard grass, bermuda grass, rhodes grass, para grass, tall wheat grass, rye grass, sudan grass; Woody species: jojoba, guava, jujube, mesquite, mangroves, acacias, atriplex species, mustard tree; Miscellaneous trees: life plant, aloe, dodonaea, periwinkle, purslane, reed plant, bottle palm, cactus, china rose, drumstick tree, wild banana, wild cherry, senna. These plants are also effective in reclamation of salt-affected soils.

CONCLUSIONS: Saline agriculture can be a profitable and usable under proper management and by observing suitable effective measures. Initial establishment is crucial for growth and hence, stress should be lowered at this stage. If the plant survives the shock at seeding stage, the chances of its subsequent survival and growth are likely to be averted. Leaching with only good quality and quantity of water or by rain generally reduces the shock and improves the growth. Use of mulch to conserve moisture, planting on ridges and more frequent irrigation often helps under such conditions. Proper choice of crops, and frequent crop rotation are also helpful in gainful utilization of salt-affected soils, which are prevailing in large areas of the country.