Salty ground is generally unsuitable for plant growth. In semi-arid and arid regions, where irrigation is widely practised, there is always a danger that high concentrations of salts may accumulate in the soil and reduce its productivity or even render it completely unproductive. Restricted drainage, caused either by slow permeability or by a high water table, is the principal factor in the formation of saline soils. Because all irrigation waters contain dissolved salts to some extent, nonsaline soils may become saline unless water is applied in addition to that required to replenish water losses by plant transpiration and evaporation, in order to leach out the salt that has accumulated during previous irrigations and due to the addition of fertilizer.
Salinity is regarded as a problem when the concentration of salts reaches 0.2% of the dry weight of a loam soil; most species of plants die when the concentration is around 2%. Salts that most frequently predominate are the carbonates, chlorides and sulphates of sodium, magnesium and calcium, and mixtures of two or more of these salts. Reductions in growth and yield of crops due to saline conditions may go unnoticed. But with higher salt concentrations, plant growth is even more restricted, and the characteristic symptoms (burning and firing of leaves) of saline poisoning are shown. This is effected by the high osmotic pressure of the soil solution and the reduced solubility of some nutrients at higher pH values.
When sodium salts accumulate in the soil, they usually impart undesirable physical characteristics to the soil, such as reduced permeability to water and reduced aeration. This increases the susceptibility of the soil to further salinization. Also many soil chemicals, including the nutrients nitrogen and phosphorus, are more mobile in the soil in the presence of high levels of sodium. This can increase the amount of water pollutants in run-off water and underground drainage.
Egypt is a case in point. Prior to the building of the Aswan Dam, the fields of the Nile delta and floodplain were annually flooded and the ebbing flood flushed away surplus salts. But the modern canal irrigation leaves the salt behind and is gradually poisoning on of the world's leading cotton-producing regions and turning it into desert. Each year, one ton of salt accumulates on each hectare of delta fields. 35% of Egypt's cultivated surface is afflicted by salinity. The country is spending tens of millions of dollars each year laying the largest drainage network in the world in an attempt to flush out the salt.
In Pakistan, 15 million acres of the 37 million acres under irrigation are estimated to be salinized. Of the area earmarked to receive water via China's giant Yangtse Diversion scheme, 2.7 million hectares already suffer from salinization. An FAO study reported that 500,000 acres in Syria, half of the country's irrigated land, are waterlogged or salinized. In Iran, of 16.8 million hectares of arable land, 7.3 million are estimated to be saline. In India, the amount of land devastated by water and salt has been estimated at between 6 million and 10 million hectares, almost a quarter of the 43 million hectares under irrigation. In the US, 25 to 35% of the country's irrigated land suffers from salinity and the problem is getting worse.
The Asia-Pacific region is responsible for around 75 per cent of all human-induced salinization in arid, semi-arid and dry sub-humid areas - the susceptible drylands - of the world. In the mid-1980s, Pakistan, India and China could alone account for about 50 per cent (30 million ha) of the world's irrigated land damaged by salinization. In Pakistan, salt build-up in the soil is known to reduce crop yields by 30 per cent (Worldwatch Institute 1997). Estimates of secondary salinity (dryland and irrigated) in Australia vary from three to nine million hectares (SCARM 1998). This has reduced productivity and sometimes increased erosion in these areas. It was estimated in 2000 that around 5% of Australia's productive lands had been lost to salinization. This is expected to increase seven-fold over the first half of the 21st century.