The total geographical area of Pakistan is 79.61
million hectares of which 20.82 million hectares (25%) are currently
under cultivation. Among the four provinces, Punjab occupies 20.6 mha
out of which 11.04 mha are cultivated. The area of Sindh is l4.1 mha,
of which 5.45 mba are cultivated. Of Balochistan's 34.7 mha only 1.4
mha are cultivated, while the area of NWFP comprises of 10.2 mha, out
of which 1.93 mha are cultivated. The country has more than 6.30 mha
of salt-affected land. The province-wise salt-affected areas are:
Punjab 2.56 mha; Sindh 2.321 mha; NWFP 0.516 mha and Balochistan 0.304
Pakistan is a land of promise and tremendous
development possibilities by virtue of its unique geographical
location, fast acquisitional talents of its people, and richness of
natural and cultural resources. Most of the land area of Pakistan is
classified as arid to semi-arid, because rainfall is not sufficient to
grow agricultural crops, forest and fruit plants, vegetables and
pastures. The culturable area of Pakistan is about 35.4 mha, forest
land 3.5 mha, culturable waste 8.6 mha, cultivated area 23.3 mha,
waterlogged and salt-affected area in the Indus Basin is 6.8 mha.
Salt-affected area outside Indus Basin is 5.6 mha.
Pakistan agriculture is predominantly irrigated.
Water is one of the most limiting constraint for agricultural
production in Pakistan. Pakistan is blessed by nature with fertile
lands, extensive network of rivers, sunshine, versatile climate, and
vast potential of agricultural production. The country's major
agricultural areas lie within the smooth plains formed naturally by
mighty Indus River with its several tributaries such as Kabul, Jhelum,
Chenab, Ravi and Sutlej, which flow in southwardly directions, finally
enter into a single stream and flowing into Arabian Sea. Due to
mismanagement of water resources, inadequate drainage systems, poor
performance of existing irrigation and drainage systems, the
agricultural production is far below its potential.
Salinity, sodicity, aridity and water logging are
serious problems of agriculture in Pakistan, which contribute towards
large scale spread of poverty and social unrest among the populations.
There is a problem of drainage in the country due to application of
irrigation water from the rivers and canal waters. This biological
approach involves screening and selection of highly salt tolerant
plant species/varieties from the naturally existing germplasm or from
these developed through breeding, wide hybridization and other
biotechnological techniques, and then introducing the selected plants
for increased plant establishment and productivity in saline areas.
Salinity is a vital problem in arid and semi-arid
regions, where rainfall is inadequate to leach out salts out of the
root zone. These areas often have high evaporative rates, which can
encourage an increase in salt concentration at the soil surface
through capillary rise. Salt accumulation in these soils is generally
caused by lack of appropriate drainage and inappropriate irrigation
management. It is a common knowledge that salinity reduces crop growth
and that saline water with a high Na+/Ca2+ ratio can destroy the soil
structure. Tie presence of a cemented hard pan at varying depths and
insufficient precipitation for leaching often adds to the problem.
Historically, soil salinity caused to the decline of several ancient
civilizations. Despite the advanced technologies available today,
salinization of millions of hectares of land continues to reduce crop
productivity severely worldwide.
As Pakistan is situated in arid and semi-arid
climatic zones, the high evapotranspiration is the basic cause for
salt accumulation on the soil surface. The average summer temperature
goes up to 40°C and the minimum winter temperature remains between 2°C
to 5°C. 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 ground
water depth, enhances the movement of salts towards soil surface. Most
irrigation waters contain more salts than they are removed by the
crops, so that continued irrigation without leaching progressively
salinized the land. 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 tons of salts are added to the land through canal water and
brackish under ground water, but only about 1/5th finds its ways to
the sea. The remainder accumulates in the soil and continues to
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 soils are usually reclaimed by leaching the
salts through irrigation and drainage systems. Leaching through
drainage controls salinity in soils. Drainage is the first acquirement
for managing salinity. Drainage established the salt content of the
soil is reduced by leaching with water that has a salinity level
within crop tolerance. Leaching is the process in which extra water is
added to a field and allowed to soak through the soil and drain away
Leaching is commonly done at each irrigation, but
may not be necessary for a number of years. 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. Leaching is most efficient, when the flow is unsaturated.
Increasing the amount of water per application and the time interval
between water application decreases the total amount of water
requirement for leaching. 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% of the salt. The water table should be at least 4-1/2
to 5 feet below the surface during most of the crop growing season.
Drainage is an essential element of managing or reclaiming a soil in
which the permeability has been reduced by sodium (Na). The
reclamation of a Na-affected soil requires more than leaching with a
good quality water. The first step is to supply calcium (Ca) source to
displace sodium Na from the exchange, and then Na may be leached.
Permeability may be decreased by application of water low in salinity
before Na has been replaced by Ca.
For reclamation of Na-affected soil, the most
common Ca source is gypsum (CaSO4, 2H2O), which is widely available
and costs less than other Ca sources. Other possible amendments are
CaC12. 2H2O, elemental Sulphur (S), sulphuric acid (H2SO4) etc.
Generally, hydrated (CaC12) is expensive and not commonly used for
reclamation of salt-affected soils. Gypsum and elemental S are solids
and must be incorporated into the soil by plowing, which may also
facilitate reclamation. The soluble amendments CaC12. 2H2O and H2SO4
give better results. Calcium chloride may be applied in the irrigation
water, but H2SO4 requires special handling and should not come in
contact with concrete or metal. Elemental sulphur (S) and sulphuric
acid react with CaCO3 in the soil to produce Ca for reclamation.
Elemental S is microbially oxidized giving the product of H2SO4.
Application of acid needs special care due to its corrosive action.
These chemical amendments remove a large proportion of Na+ from the
soil columns profiles and result in a marked improvement in soil
condition. For better results in cultivating saline soils, higher seed
rates for higher plant population, crop rotation, proper choice of
crops, species sufficient amount of NPK, avoiding use of urea and
using ammonium sulphate as an N source is practically and favourably
advised. Scrapping of salt layer, land leveling, deep ploughing,
sub-soiling, sanding, flushing with good quality and quantity water
and drainage, method of irrigation also helps in improving soil
condition. In many cases, chemical amendments may also be needed to
restore soil productivity.
Green manuring and the application of farm yard
manure (FYM) not only provide organic matter and other plant
nutrients, but also make the soil porous for aeration and moisture
absorption and increase soil microorganisms, thus improving the
overall conditions of the soil. Reclamation of such soils is not easy
and may be costly. There may not be enough irrigation water available
for leaching, the excess salts, the laying of tile drains and sinking
of tube wells may not be within the reach of every farmer, sufficient
quantity of ameliorates may not be readily available or may be too
costly to procure and transport and this may not be advisable without
expert technical assistance. Moreover, even under the most efficient
system and management, it may not be possible to get rid the soil
completely of its excess salt, which may any way, rebuild in the
absence of proper precaution.
Modern research has identified more than fifteen
hundred 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, salt
tolerant grasses and herbs) are a major resource that can be used in
the development of agricultural systems for salt-affected lands.
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.
Different 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. Research workers have
successfully grown a number of crops on marginal saline lands. This
can be a positive and cost-effective. Cereals: certain cultivars of
paddy rice (Oryza sativa L.), sugar cane (Saccharum officinarum L.),
oat (Avena sativa L.), wheat (Triticum aestivam L.), aegilops,
triticale, sorghum (Sorghum bicolor L.), barley (Hordeum vulgare L.),
corn (Zea mays L.), pearl millet, rye (Secale cereale L.); Oilseed:
rape (Brassica napus L.), canola (Brassica campestris L.), mustard;
Vegetables: spinach, sugar beet (Beta vulgaris L.), red beet; fodder
and forage: Guar (Cyamopsis tetragonoloba L. Taub), dhancha (Sesbania
sesban L.), berseem (trifolium alexandrinum), lucerne (Medicago sativa
L.), sweet, honey, Indian and white clovers; fibre: cotton (Gossypium
hirsutum L.), sunhemp, kenaf (Hibiscus cannabinus L.); Fruits: fig
grape, pomegranate, zizyphus (Zizyphus jujuba); Fruits: date palm,
wild date palm and coconut; grass: Karnal or Kallar grass (Leptochloa
fusca), orchard grass, bermuda grass, rhodes grass (Chloris guyana),
para grass, tall wheat grass (Agropyron elongatum), rye grass, sudan
grass; woody species: jojoba (Simmondsia chinensis L.), guava (Psidium
guava L.), jujube (Zizyphus mauritiana L.), mesquite (Prosopis
species), mangroves, acacias (nilotica, ampliceps, stenophylla,
machonochieana), atriplex species, mustard tree (Salvadora persica);
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) These plants are
also effective in reclamation of salt-affected soils.
To further improve this agricultural phenomenon,
proper choice of crops, and frequent crop rotation are helpful in
gainful utilization of salt-affected soils. Drainage and leaching are
the principal components of maintaining soil productivity in irrigated
agriculture. Traditionally, irrigation has been supplied far in excess
of plant requirements to ensure that plants were not water stressed
and that the soil was not salinized.