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.
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.
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
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.