SALINITY - A GLOBAL THREAT TO FERTILE LANDS
DRS.M.A.KHAN, R.ANSARI & S.M.ALAM,
Research Analyst, PAGE
June 8 - 14, 2009
Salinity is the accumulation of water soluble salts in the soil to a level that has drastic effects on agricultural production, environmental health and economic welfare of a country, whereas technically, a soil is considered saline if the electrical conductivity of its saturation extract (ECe) is above 4 dS m-1. Salt affecting lands occurs in practically all climates, from the humid tropics to the arid regions. Saline soils can be found at different altitudes, from below sea levels such as (around the Dead Sea 1293 feet below sea level; it is entirely devoid of life as it has 32% salinity, the highest salt content of all the seas waters) to mountains rising above 5000 meters, such as the Tibetan Plateau in the China or the rocky mountains in the United States.
The occurrence of saline soils is not limited only to desert conditions the problem has been reported in the tropical belts of Africa and Latin America, and even in the Polar Regions, particularly Antarctica. In fact, there is no continent in the world, which is free from this hazard. Salinity has destroyed the several civilization of the world.
Although salinity has become an alarming problem of agriculture throughout the world, the development of this menace is of a greater magnitude in arid and semi arid areas. This is mainly due to low precipitation and high transpiration causing disturbance in salt balance in the soil. This also renders ground water brackish and affects plant growth adversely. Estimates of saline lands in the world are shown below:
ESTIMATES OF SALINITY SPREAD WORLDWIDE
CONTINENTS SALT AFFECTED LANDS (MHA) Australia 357.33 North & Central Asia 211.69 South America 129.16 South Asia 87.61 Africa 80.61 Europe 50.80 South East Asia 19.98 North America 17.72 Mexico & Central America 1.96 TOTAL 956.86
Salinity limits the productivity of almost all crops and its impact and severity is exacerbated by the activities of man. With the steady increase in population, especially in the under developing countries of the world with concomitant decline in new agricultural lands, the need to tackle these stresses is urgent. About 15% of the total land area of the world has been degraded by soil erosion and physical and chemical degradation including soil salinization and that global food production should increase by at least 38% by the year 2025 and 50% by the year 2050 if food supply to the growing world population is to be maintained at current levels. It is also worth mentioning that most of the suitable lands all over the world have been extensively cultivated and expansion into new areas to increase food production is rarely possible or desirable. Therefore, more efforts are needed to improve the productivity per unit area.
Salt affected lands are common feature of irrigated agriculture, according to FAO and UNESCO estimates. As much as half of all the existing irrigated lands of the world are under the influence of salinization, alkalization and water-logging. Irrigated agriculture without proper precautions of maintaining salt balance, has contributed to secondary salinization and there is much archaeological and historical evidence to show this deterioration of soils. Soil salinization happened in Mesopotamia, where land development was based on irrigation farming 7000 years ago with wheat as the main crop. Over-irrigation with improper management, unsuitable quality of irrigation water and poor soil drainage led to accumulation of salts to the extent that wheat could no longer be grown and had to be replaced by barley. Unchanged cultivation practices led to further salinization of land and water resources reaching such levels that even barley could not grow. This subsequently resulted in down fall of the Sumerian civilization.
The trend continues unabated as in present times also, soils worldwide under irrigated agriculture have deteriorated similarly e.g. Indus in Pakistan, the Ganges system in the north west of India, the head-waters of the Mekong river system in north-east of Thailand and Cambodia, the Huang and associated rivers in the northern Chinese plains, the Colorado in the south west of the USA, the Nile in Egypt and the Murray-Darling catchments in Australia.
SALINITY HAZARD IN PAKISTAN
Pakistan is basically an agricultural country. It is spread over an area of 79.61 million hectares (mha) of which 20 million (16.08 million irrigated and the rest 3.92 rain-fed) is available for farming. 70% of its population living in rural areas is directly dependant on agriculture. The economic prosperity and well being of the rural population depends on successful cultivation of crops like wheat, rice, cotton, sugarcane, maize, pulses, vegetables, oil crops etc.
Annual rainfall in the country varies between 100-700 mm and rain fed agriculture, though unreliable, contributes substantially to overall agricultural production. Irrigated agriculture is more productive, the general shortage of water in recent times and creeping salinity hazard not withstanding. The precipitation being less than evaporation and injudicious use of irrigation water have rendered lands saline in rain-fed and irrigated areas alike. According to a report, about 62% of the canal command area has been badly affected by moderate to severe salinity.
Extensive but mismanaged use of canal water for irrigation in Indus plains caused the ground water table to rise, and due to harsh environmental conditions, this water is evaporated rapidly leaving the salts at or near the soil surface. Consequently, about 6.3 million hectares of salt affected land of the country has become saline, about half of this lie in canal command area, while about 40,000 hectares of agriculture land are becoming saline annually. It is estimated that about 60% of the total agricultural land is suffering with soil salinity or water logging (or both) of different levels.
SALT AFFECTED AREAS IN DIFFERENT REGIONS OF PAKISTAN
PROVINCE AREA SURVEYED SLIGHTLY SALINE MODERATELY SALINE SEVERELY SALINE VERY SEVERELY TOTAL Punjab 20.6 0.472 0.805 0.738 0.652 2.668 Sindh 9.20 0.118 0.325 1.173 0.494 2.110 NWFP & FATA 9.10 0.005 0.026 0.009 0.009 0.049 Balochistan 30.50 0.003 0.075 0.465 0.806 1.3489 PAKISTAN 69.40 0.599 1.230 2.385 1.960 6.174
It should also be remembered that the population of the country is about 170 million at present, which is likely to double by 2020. This increase in population will exert enormous pressure on the already scarce land resources in the country. In order to meet the food demand of increasing population the Government of Pakistan started soil reclamation projects like SCARP, LBOD and RBOD, which have been effective to some extent but are cost intensive and mismanaged. Thus, instead of bringing relief these schemes have worsened the problem.
Another possibility which appears to be more feasible is the development of crop cultivars suitable for the salt affected areas, as also suggested by many workers. This approach involves understanding the response of plants at different growth stages under saline conditions, like reported in different crops such as wheat, soybean, alfalfa, sorghum, rice, maize and cotton. Such studies also provide evidence that salinity tolerance is genetically controlled in plants like genotypes of triticale and in species such as rice, maize, sorghum, cotton, grasses, and forages. These also provide clues to breeders looking for plants of economic importance with improved salt tolerance.