ABSTRACT
Worldwide, but especially in the drylands that cover about 40% of the earth’s land surface, natural resources have been diminishing because of increased demands for food and fi ber. Much of this production in drylands comes from subsistence pastoralism (Grice and Hodgkinson 2002). This sort of pressure is on-going and in some places is accelerating, and results from the ever-increasing human population. Inevitably, marginal and long-neglected natural resources have to be re-assessed in preparation for utilization. Not less than 400 Mha and perhaps as much 900 Mha of land in arid and semi arid regions may be salt-affected from natural and anthropogenic causes. Defi nitive data on the annual worldwide loss of farmland due to salinization and related causes is lacking. However, salinity is unquestionably the most important problem of irrigated agriculture and one-fi fth of the world’s cropland (about 47 Mha) is saltaffected. Reliable estimates suggest that in the Mediterranean region alone, some 15 Mha have become seriously affected by anthropogenic infl uences. In India over 6 Mha, of a national total of only 40 Mha of irrigated farmland, have been made useless by salinity and water logging, while over 40000 ha are abandoned each year. In Pakistan, estimates show that about 0.45 Mha of land in Pakistan (as one example) is being rendered useless by salinity every year. Even in USA and Australia there are large areas that lie waste as a result of salinity (some of it non-irrigated or ‘dryland salinity’). In USA, well over 500,000 ha of Californian soil particularly in the Coachella, Imperial, Sacramento and San Joaquin valleys have been strongly affected and rendered useless by salinity. Overall, the USA reportedly loses 80-120,000 ha of cropland annually due to salt build up in soil or irrigation water. It is estimated that 7-10% of the world land area is salt affected (Dudal and Purnell 1986). Halophytes survive salt concentrations around 200 mM NaCl or more in order to reproduce in such environments and they constitute about 1% of the world’s fl ora (Flowers and Colmer 2008). Halophytes grow in many arid and semi-arid regions around the world and are distributed from coastal areas to mountains and deserts (see PART 1, this volume).
