. .

Salt problems in irrigated soils

  1. Closure of coal mines
  2. Import of cotton affecting local prices
  3. Decline in revenue collection
  4. Salt problems in irrigated soils
  5. Changes in corporate and income tax rates

40,000 hectares of cultivable land is lost to waterlogging and salinity

By: Dr. Syed Manzoor Alam and R. Ansari
Nuclear Institute of Agriculture, Tandojam
July 05 - 11,1999

Total geographical area of Pakistan is 79.61 million hectares or 197 million acres. Of this 19.82 million hectares or 48.96 million acres are currently under cultivation which is only 25 per cent of the total area. The salinity and waterlogging problems are very common and tarnished over 6 million hectares land in the country and each year, about 40,000 hectares of irrigated land is lost to this problem. Both, waterlogging and salinity, are interlinked problems and co-exist at most of the places across the country. Salt problems are specially serious in Punjab and Sindh provinces where 3.28 and 3.06 million hectares of land have gone saline. Similarly, in NWFP and Balochistan provinces the salt affected lands are estimated at 0.61 and 0.1 million hectares, respectively.

Origin and cause of salt-build up

Salt comes from the minerals of the earth's crust. Weather decomposes minerals and releases the salt in a soluble form. Humid regions usually have enough rainfall to leach this salt through the soil and into the ground water, which carries it to streams. The streams carry it to the oceans. In arid areas, the rainfall is too scanty to leach the salt out of the soil. The rain is largely dissipated by evaporation and by plant use. Both processes, evaporation and plant use, occur at higher rates in arid regions than they do in humid regions. Scanty rainfall, evaporation and plant use favour salt build-up in arid regions. A harmful build-up, or accumulation, occurs when a field continually receives salt from other locations. The salt is brought into the area by ground water. Irrigation phenomenon often speeds the process. Salt accumulates when water evaporates at the surface or is extracted by plant roots. Both processes separate the salt from the water. Salt is removed from the soil when water moves downward through the root zone and into the sub-soil.

The rapidity with which salt builds up in the root zone is determined by the quality of the irrigation water, the method of irrigation, the type of field drainage and other conditions. All irrigation waters contain dissolved salt. The salt content varies from 0.1 of a tonne to 5 tonnes per acre foot. It is extremely important that irrigation water be tested to determine its quality. A water quality test may alert the farmer for two hazards: the presence of salinity or the formation of a sodium soil. It also can reveal the presence of excessive amounts of bicarbonate. Excess water tends to raise the water table and thus increases drainage problems when the water table rises to within 5 or 6 feet of the surface, ground water and its salt move upward into the root zone and to the soil surface. Adequate drainage keeps the water table from rising and allow the water to flow away before it has chance to rise into the soil zone occupied by the roots of the crops.

Effects on plants and soils

Too much salt in soil affects plants in two ways. It prevents them from getting enough water, even though the soil may be well watered. This results in stunted plants that frequently have a characteristic blue-green colour. If the salt is evenly distributed in a field, all the plants will be stunted. Yields may be reduced as much as 25 per cent. It has a direct tonic effect on plants. A characteristic leaf-burn develops, leaves fall off. Trees may die when harmful amounts of sodium or chloride accumulate. Salt-affected soils are the soils that have been harmed by soluble salts.

Saline soils

Soils that contain too much soluble salts are called saline soils. These soils are generally flocculated — that is, the soil particles are grouped together in clumps. The clumps are crumbly and do not stick together; water and air move freely between them. Crop growth on saline soils is usually poor and spotty because the salt delays or prevents seed germination. Another sign of saline soil is the appearance of a white crust on the surface of the soil.

Sodic soils

Soils that have appreciable amounts of sodium, adsorbed on their individual particles, are called sodic soils. They are not flocculated because the soil particles on which the sodium is adsorbed separate from the flocculated clumps. Water and air cannot move through the soil freely, even though there may be more openings.

Sodic soils often exhibit black surface deposits. Such a soil is sometimes referred to as a "black alkali". The black deposits occur because the sodium dissolves the organic matter in the soil.

Improving salt-affected soils

Saline soils: Saline soils may be improved by leaching. Leaching is the process in which extra water is added to a field and allowed to soak through the soil and drain away underground. Leaching is not effective if the ground-water table is too close to the surface. When water is leached through the soils, a surface depth of six inches of water for every foot of plant root zone 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 of root zone leaches out 90 per cent of the salt. 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.

Sodic soils: Sodic soils may be improved by adding chemical amendments, leaching the soils, and then employing particles that build soil structure. Most sodic soils need chemical amendments to restore their productivity. Many suitable amendments are available like gypsum and sulphur which are the most common.

Some sodic soils contain calcium sulfate and when they are leached the water dissolves the gypsum and free the calcium. This dissolved calcium replaces the absorbed sodium. The sodium salts formed from this reaction can be removed by leaching. Some irrigation water contains appreciable amounts of calcium or magnesium. When such water is applied to sodic soil, an exchange reaction takes place. The sodium is replaced and removed from the soil.

Crop selection: Crops vary widely in their tolerance to salts. Sugar beets, barley and cotton can tolerate upto 10 times as much salt as most clover, beans and fruit trees. Crops such as beets, barley and tall wheat grass are relatively sodium tolerant. Special tillage and irrigation methods are very helpful in reducing the bad effects of salinity on the growth of the plants. Good management practices also include the adoption of special treatments such as adding soil amendments, supplying organic matter and growing of sod crops to maintain soil structure.