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Book Review: Principles of Plant Nutrition.

aspects of soil physical properties, and presents the results of current studies in Asia, Africa and Latin America, supplemented by relevant papers from Australia, North America and Europe. They are grouped in nine chapters covering soil structure, soil physical properties and land use, soil/water/temperature/ plant relationships, root growth and soil and water conservation. The importance of the subject as a whole, and of the book in particular, is reflected in a reminder by R Lal that, to maintain present nutritional levels for the world population, the rate of new land development may be as much as 610 million hectares per annum, with 120 million hectares suggested for the tropical areas of Africa and Latin America. A sombre picture of the hazards facing such development is drawn in a paper by K A Edwards showing that in Kenya one water supply reservoir of 1470 X 103 cubic metres capacity lost more than 50% of its storage capacity in sixteen years due to silting, with incoming water carrying sediment loads of 1,500 tons/kmf/per annum. He points out that the rate of erosion from cultivated fields in the most densely populated parts of Kenya may be as much as 5,000-6,000 tons/km'i/per annum, and that this is too high for low efficiency subsistence agriculture to survive without drastic falls in yields. Soil erosion is further considered in papers on laboratory and field techniques studying soil aggregation, water infiltration and run-off characteristics, the use of rainfall simulators, and the effects of soil conditioners. Mulch and minimum or zero-tillage systems of farming are considered by a number of authors as offering effective soil conservation on the majority of tropical soils. On the other hand, R Nicou and J L Chopart report that, on the sandy soils of the dry tropical zone of Senegal, cultivation to 20 ern greatly improves growth of rice, millet, sorghum and groundnuts, by improving porosity and increasing the depths of rooting, both leading to better water and nutrient utilization. The theme of water use is taken up by D Hillel, who denounces such concepts as "field capacity", "permanent wilting points" and others as anachronistic. Rather, he calls attention to the "SPAC" (soil/plant/atmosphere continuum) dynamic concept, going on to assert the value of trickle and drip irrigation systems as preferred alternatives to traditional flood systems. In related work, S Valet shows by use of neutron moisture meter techniques, that windblown terrace soils of the River Niger can be cultivated more intensively than traditionally supposed, with the assistance of sprinkler and trickle irrigation techniques. Several papers deal with lysimeter studies and, in particular, with the transient flooding so often the result of high intensity rain storms. In more applied work, J E Mansfield considers that, in northern Nigeria, climate is the main determinant of the type of crop permissible and management level of yield obtained, with soil only having any significance in extreme cases. He goes on to describe a computer-based system of land capability classification. F N Muchena discusses the wider aspects of land quality, using a multidisciplinary approach, and illustrates the system by examples drawn from the Kenya Soil Survey. There is, thus, something for everyone _ advanced student, research worker and planner. It offers a stimulating review of a technical aspect of critical importance to land development in the tropics, and should do much to strengthen international standardization in this field. The reviewer feels that a very important omission is the almost complete lack of comment on the value of tree crops in the tropics. There is one interesting paper from Malaysia on rooting of oil palms, but only passing references in one or two papers to their value in the environment as a whole. Let us hope that a future conference and publication will redress the balance. The price of the present volume is high, but should not deter professional workers and librarians from acquiring such a valuable addition to the literature on tropical agriculture. G A WA TSDN