MAXIMIZING AGRICULTURE GROWTH
By S.M. ALAM NIA, Tandojam
July 28 - Aug 03 , 2003
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 mha.
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 the underground.
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.