ABSTRACT

References ..................................................................................................................................... 52

Soil fertility in systems under arid and semi-arid conditions, hereafter referred to as dry

areas or drylands, is constrained by environmental extremes of hot and cold temperatures,

as well as by low water availability. With some exceptions, these soils have inherently

low fertility, low availability of nitrogen and phosphorus, low water-holding capacity,

high pH, low soil organic matter (ranging from 0.1 to 3%), shallowness, stoniness, and

other specific problems (Matar et al., 1992). These areas are quite widespread, occupying

around 30-40% of the world’s terrestrial surface. Given the vulnerability of these lands

to degradation, it is estimated that some 44 million km

— 34% of the total world’s area,

supporting 2.6 billion people-is at risk from desertification (Eswaran et al., 2001). Hence,

these lands are of great global significance even if their agricultural production potential is

relatively low.