The increasing world population is already causing intensive pressure on the most productive areas of the earth's surface. Hopes of improved living standards, if realised, will increase the pressure still further. The need to make better use of less productive areas thus becomes an urgent priority for human endeavour. In this respect, the arid and semi-arid regions of the world, occupying about 11,000 million acres or 4,500 million hectares, roughly one third of the tota11and surface, pose a constant challenge to man's technological skill. Though the possibilities of economic use of the real...