By Md Tariqul Islam*, NewAge, August 27, 2009
SALINITY affects the physiological processes of plants and it is the most important factor which severely affects crop production. These adverse effects may be attributed to non-availability of water, disturbance in nutrient uptake causing deficiency and ion-toxicity to plants. Salinity and sodicity stresses are ever-present threats to crop yields, especially in countries where irrigation is an essential aid to agriculture. As per FAO/UNESCO soil map of the world, a total of 953 m ha covering about 8 per cent of the land surface is suffering from salinity/sodicity. The salt affected soils are reported to occupy 42.3 per cent of the land area of Australia, 21.0 per cent of Asia, 7.6 per cent of South America, 4.6 per cent of Europe, 3.5 per cent of Africa, 0.9 per cent of North America and 0.7 per cent of Central America. Recent estimates indicate that 6.74 m ha area in India is affected by soil salinity and alkalinity. In Bangladesh, 2.85 million hectares of land are saline affected and there is a great possibility to bring these areas under cultivation with salt tolerant crop varieties and proper reclamation and management.
Saline soils are often recognised by the presence of white salt encrustation on the surface and have predominance of chloride and sulphate of sodium, calcium and magnesium in quantities sufficient to interfere with growth of most crop plants. Soil with neutral soluble salts has saturation paste pH less than 8.5. The electrical conductance of saturation extract of saline soils is more than 4 dSm-1 at 25 degrees Celsius and exchangeable sodium percentage is less than 15. The sodium adsorption ratio of the soil solution is generally less than 15. However, soil salinisation with neutral soluble salts of sodium invariably result in soil solution SAR greater than 15. Such soils are termed saline-sodic.
Parent material, weathering and release of salts, shallow groundwater and capillary rise, indiscriminate use of irrigation waters, ingress of seawater along the coast, salt-laden sands blown by sea winds, lack of natural drainage, etc are the sources and causes of accumulation of salts in soil. On the other hand, continuous accumulation of salts from tidal flooding, upstream withdrawal of sweet water, cyclone and tidal surges, prawn culture, irrigation with saline water, capillary movement of groundwater, etc may cause coastal salinity.
Soils containing excessive salts of sodium carbonate and sodium bicarbonate and having sufficient exchangeable sodium to interfere with growth of most crops plants are called alkali. These have pH of the soil saturated paste more than 8.5, ESP 15 or more and ECe limitless if resulting from salts capable of alkaline hydrolysis.
The alluvium (parent material) rich in plagioclase feldspars under hydrolytic dissolution release high amount of sodium. Weathering of alumino-silicate minerals through carbonation provides solutions of bicarbonates and carbonates of alkali in addition to silica and alumina. The bicarbonates and carbonates migrate with the subterranean and surface waters and accumulate in un-drained areas under arid and semi-arid conditions to form alkali soils.
Rice plants tolerate salinity stress by 3 mechanisms acting upon singly or jointly. These mechanisms are exclusion, dilution, compartmentalisation or maintenance of high potassium-sodium ratio.
Exclusion: It refers to the restricted uptake of sodium ions by tolerant rice varieties.
Dilution effect: Usually the tolerant varieties grow faster than non-tolerant varieties under saline condition. It was experimentally found that lower shoot sodium content in the Pokkali variety is not due to any better control of sodium transport by its roots but is directly attributable to the dilution effect of its rapid vegetative growth.
Compartmentalisation: When a rice plant is exposed to saline condition, the older leaves die due to high amount of sodium accumulation while younger ones remain green and growing. This physiological behaviour of rice plant is called compartmentalisation which is a useful feature of gramineae.
High potassium-sodium ration: It is now established that salt tolerant varieties maintain a higher potassium-sodium ratio compared to that in non-tolerant variety. Relatively higher amount of potassium than sodium ions is probably required in panicles for the protection of growing panicles from the toxic effect of sodium ion. It was experimentally revealed that growing panicles of tolerant genotypes maintain higher potassium-sodium ratio compared the growing panicles of BR11 salinity sensitive variety at reproductive phase.
Strategies to manage coastal salinity: Irrigation with harvested rainwater and cultivation of salt tolerant crop varieties are most useful techniques in many countries.
Strategies to manage sodic soils: The chemical amendment based technology has been developed to reclaim the alkali/sodic soils. Various components of this technology includes; field levelling, bunding, soil sampling to know the sodicity status for working out amendment dose, application of gypsum/pyrite as per requirement of the soil and its mixing in upper 10cm soil, keeping water ponding for 5-7 days, following rice-wheat rotation for the first 3 to 4 years and growing sesbania during summer as green manure crop after wheat harvest in April. By adopting this technology about 1.3 m ha area has been reclaimed in the states of Punjab, Haryana and western UP.
Salt-tolerant varieties: A sizeable part of the salt-affected area in India is in possession of small and marginal farmers who are themselves poor. Under such situations, chemical amendments based reclamation technology without government subsidy is not sustainable. Development of salt tolerant varieties of important field crops is an option of great promise for utilization of such areas, as most of these varieties give significant yield without or with little application of chemical amendments. Several varieties of field crops like rice, wheat and mustard have been developed which have potential to yield reasonable economic return both in high pH alkali soils and also in saline soils. In case of rice, the most promising varieties include CSR10, CSR13, CSR19, CSR23, CSR27, CSR30 and CSR36. These varieties can be cultivated in soils with pH and EC range from 9.4 to 9.8 and 6-11 dS/m.
Reclamation of saline waterlogged soils: Sub-surface drainage technology has been developed to lower the water table in saline waterlogged areas. The system consists of a network of concrete or rigid PVC pipes along with filter installed manually or mechanically at a designed spacing and depth below soil surface to control water table depth by draining excess water and disposing it out of the area by gravity or by pumping from an open well, called sump. The first approximation of the area covered under sub-surface drainage in India.
Alternate land use systems: A sizeable part of the salt-affected soils in India is constituted by the village community lands, lands along the roads, railway tracks and other government lands reserved for specific purposes. Reclamation of such area for crop production is posing problems because of community rights on such land resources. These sites offer ample opportunities to raise salt tolerant trees, bushes and grasses to produce fuel wood, fodder and energy. An alternate technology of raising multipurpose forest tree plantation, fruit trees, agroforestry systems and other high value medicinal and aromatic crops seems quite feasible. Several salt tolerant forest and fruit species have been identified which can be grown in highly sodic and saline soils. The promising forest species include Prosopis juliflora, Acacia nilotica, Tamarix articulata and Casuarina equisetifolia.
Agroforestry: Several grass species have mechanism to tolerate high salt concentration in the root zone soil. Some of these highly tolerant grasses either exclude the absorption of salts from the soil and/or deposit the absorbed/translocated salts at points within the plant system which do not allow them to interfere in metabolic processes. Such grasses have been identified. Grasses like Leptochloa fusca has the potential to yield high biomass even at pH level of 10.4 and more. Similarly, Bricharia mutica is another salt tolerant grass, which can be grown even under prolonged waterlogged and salt situation. Several experiments have been conducted at CSSRI, Karnal and elsewhere to study the performance of these grasses in association with salt tolerant trees like Prosopis juliflora and Acacia nilotica in a unified agroforestry system. A field study conducted at Gudha experimental farm for six years indicated that Leptochloa fusca has the potential to yield about 20 tonnes/ha of green biomass per annum when planted with Prosopis juliflora in a soil having pH of 10.4. Leptochloa grass has a special characterstic that it starts disappearing when sodicity level in the soil decreases. Thus, allowing the regeneration of other moderately salt tolerant grasses and other annuals. The results of this experiment clearly indicated that sodic soils can be reclaimed by growing Prosopis juliflora and Leptochloa fusca for five years. During this period, the surface soil is reclaimed and salt tolerant crops like Berseem (Trifolium alexendrinum), Egyption clover, oats and senji can be grown without the application of amendments.
Medicinal and aromatic crops: A number of medicinal and aromatic crops have been screened for salinity and sodicity tolerance in India. Crops like Isabgol (Plantago ovata) and Matricaria can be successfully cultivated in soils having pH of 9.5 and EC between 8-10 dS m-1. Similarly, dill (Anethum graveolens), a spice crop and Salvadora, a non-edible oil tree can be grown in saltaffected vertisols very successfully. Industrial species like Euphorbia and mulethi (Glycyrrhiza glabra) also have good scope for cultivation in salty environments.
Soil reclamation: Salt tolerant trees and grasses when planted either in association or as sole plantations reclaim the sodic soils over a considerable period of time. The mechanism for sodic soil reclamation by trees involves; a) dissolution of native calcium carbonate present in precipitated form in sodic soils by the biological activity of tree, grass roots, b) addition of leaf litter and turn over of old roots which increase organic carbon in the soil, c) penetration of water into the otherwise impermeable soil through the holes created by the decayed roots which facilitates reaction with CO2 evolved from root respiration and thus producing carbonic acid. This acid, though weak in reaction initiates the process of dissolution of native CaCO3. The free Ca in the soil solution available through this reaction replaces the Na ions on the exchange complex and d) initiation of biological activity in the soil due to improved organic matter contents, moisture and fertility regime. The degree of reclamation depends upon the kind of tree species, planting density, the adopted management practices and fencing provided to the plantation to check encroachment by humans and animals. Several experiments have been conducted in the past to study the reclamation effects of trees on physical, chemical and biological properties of sodic soils. The results of a long-term field experiment after 20 years (planted in 1970) in a soil having pH of 10.3 indicated that Prosopis juliflora has the maximum impact on reducing soil pH and EC and improvement in soil organic carbon and plant nutrients.
Bio-drainage: Bio-drainage refers to a technique of lowering groundwater table in waterlogged areas through the use of raising tree plantations. This technique removes excess soil water through the process of transpiration by trees using solar radiation energy. It is a kind of preventive technique to avoid the development of salinity and water-logging problem in canal command areas. The technique is highly useful when the soils are still in the process of salinisation due to rise in ground water level. However, if the soils are already salinised it has limited scope. Several species of trees have been screened to study their capacity to transpire water from different salinity and water table depths. The most promising species identified for bio-drainage include Eucalyptus, Populus, Casurania and Bambusa. Several programmes are in progress throughout the country to reclaim waterlogged areas in canal commands through bio-drainage.
*Dr Md Tariqul Islam is a senior scientific officer at the Bangladesh Institute of Nuclear Agriculture, Mymensingh
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