Salinity poses a major threat to agriculture. It can be managed either by physical manipulation of the environment (leaching, drainage, water quality) or genetic manipulation of plants to enhance biological tolerance.
A combination of these two approaches provide an integrated method of adapting the best crop to the best agronomic procedures as attempts are made to extend crop production into saline environment. Salinity exposes plants to an environment with toxic levels of ions and with reduced water availability.
Salinity occurs naturally in arid and semi-arid regions and as water development brings more land into irrigation, the problem expands. The condition is aggravated by poor soil drainage, improper irrigation methods, poor water quality, and insufficient water supply for adequate leaching and insufficient disposal sites for water that leaches salts from the soil.
Problems caused by soil salinity are compounded when a high water-table impedes root development and concentrates salts in the already limited root zone.
Bio-saline agriculture can be defined as the use of genetic resources (plants, animals, fish, insects and micro-organisms) and improved agricultural practices to obtain profitable use from saline land and irrigation water on a sustainable basis. It is a rich collection of possible systems for the use of saline resources. The components of these systems vary according to the needs of farmers and the capabilities of land and water.
Pakistan has some major advantages in the development of saline agricultural systems. Its greatest advantage is a rich heritage of research that commenced in early 1970s and predominantly continues today. Over the last three decades there has been outstanding work on the selection of salt tolerant trees, shrubs, grasses and crops that enable increased production from salt-affected land.
Much of this work has been supported by international funding agencies. However, it remains clear that despite these achievements there have been little change in ‘fields’: adaptation varies with location but is mostly negligible.
Sustainable crop production in Pakistan is threatened by several factors. Some of the most outstanding limitations include soils degradation by excessive salts accumulation and soil deterioration by the use of saline brackish underground waters. This results in complete crop failure in many cases.
Out of 79.6 mha of the total area, about 22.6 mha are cultivated of which two-third (75 per cent) is irrigated through canal and tube-wells. Of the total cultivated land, nearly 6.8 mha are salt affected, whereas salinity and water logging coexist in an area of 1.1 mha. Half of this salt affected area exists as wastelands, whereas 3.9 mha are cultivated to crops with reduced production.
There is a shortage of canal irrigation water especially during crop growth and supplemental irrigation through tube-well which accounts for about 33 MAF water annually. Salt accumulation on productive land contributes significantly to environmental pollution as well as rapid degradation of natural resources.
Bio-saline agriculture technology is an alternative approach for effective utilisation of salt affected soils which involves the cultivation of salt tolerant species/cultivars with genetic traits. Identification of specific characteristics related to salt tolerance provides biological markers useful in selecting salt tolerance crops. Salt-tolerance is the ability of plants to survive under excess salts in the rooting medium without any adverse affects on the growth of plants. This technology gives economic return and provides vegetative covers to soil which reduces evaporation and hence the rate of salinisation. 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, hybridization and other techniques, and then introducing the selected plants for increased plant establishment and productivity in saline areas.
Crop tolerance to salinity ranges widely from the very salt-sensitive bean to the highly tolerant barley and cotton. Plant sensitive to salts during germination seems to tolerate high salinities during vegetative stages. The PAEC Agricultural Institutes are actively engaged to reclaim salt affected lands by using bio-drainage techniques and planting salt tolerant plants.
Still, there is a need of series thought to gainful utilisation of salt affected soils. The Pakistani scientists have developed an alternate approach to ‘Bio-saline Agriculture’, which aims at better use of saline land and saline irrigation water on a sustained basis through the profitable and integrated use of genetic resources (plants, animals, fish and insects) and improved agricultural practices.
Modern research has identified more than 1,500 plant species tolerant to saline soils. Some are able to withstand salt concentrations in excess to 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.
Some of these plants are able to lower water tables, improving the condition of land, and acting as a form of “biological drainage”. In addition, there are opportunities to increase salt tolerance of existing crops using conventional plant breeding and molecular biological approaches.
Crops vary widely in salt tolerance. Sugar beet (Beta vulgaris L.), 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. If the average electrical conductivity of a plot is 0-15 dSm-1(9600 ppm), which is fairly high and may have serious effects on the growth of plants, but if the soil is loamy in texture, underground water table is at 8-10ft and some good quality irrigation water is also available, the conditions become less hostile for plant sustenance.
A number of species may be grown in such conditions as follows. Good quality irrigation water should be used during the initial establishment phase and for periodical leaching of salts from soil surface. Under proper management, this can be a positive and cost-effective venture.
Cereals: Certain cultivars of paddy rice, sugarcane, oat, wheat, sorghum, barley, corn, pearl millet, rye. Oilseed: Rape, canola, mustard. Vegetables: Spinach, sugarbeet, redbeet; fodder and forage. Guar, dhancha, berseem, lucerne, sweet clover, honey clover, Indian clover, white clover. Fibre: cotton, sunhemp, kenaf. Fruits: fig, grape, pomegranate, zizyphus.
The salinity of a plot ranges between 15-20 dSm-1 (9600-12800 ppm), with coarse textured soil and the only source of irrigation is bad quality underground water (EC 10-15 dSm-1) present at a shallow depth, the growth of majority of crop plants is restricted under such conditions and only some salt tolerant plants can be grown.
Plants which can grow under these scenarios are: 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, mustard tree. Miscellaneous: Life plant, aloe, dodonaea, periwinkle, purslane, reed plant, bottle palm, cactus, china rose, drumstick tree, wild banana, wild cherry, and senna. These plants are also effective in reclaiming salt-affected soils.
Salt tolerance ratings cannot provide accurate estimates of actual crop yields which depend on many other growing conditions, including weather, moisture, fertility, soil type, pH, water stress, insects and disease.
Conclusion: Saline agriculture could be a profitable venture under proper management and by observing suitable precautionary measures.
Initial establishment is crucial for subsequent growth and hence, stress should be minimised as far as possible at this stage. If a plant survives the shock at seeding stage, the chances of its subsequent survival and growth are likely to be increased.
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, light but 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. Drainage and leaching are principal elements in maintaining soil productivity in irrigated agriculture.
Plants exposed to saline environments are subjected to water stress and specific ion toxicities. These effects are manifested in a number of physiological and morphological processes and include inorganic ion regulation and maintenance of essential nutrient ions; osmotic adjustment by regulation of organic and inorganic substances; physiological tolerance of cells to toxic ions; and efficient use of metabolic energy.
An understanding of these processes and their involvement in salt tolerance will provide information on the criteria required to select plants to enhance productivity in saline environment.
The study of variant cell lines which differ in their tolerance of normally lethal levels of salt could possibly elucidate the mechanisms involved in dealing with salt and provide possible biochemical markers useful in the selection of salt-tolerant cells and plants.
A major limitation to yield in stress environments such as salinity may be the efficient use of metabolic energy for growth. A better understanding of the efficiency of carbohydrate utilization may lead to improved crop production in stress environments.