The unsaturated zone is the medium through which pollutants move from the soil surface to groundwater. Polluting substances are subjected to complex physical, chemical and biological transformations while moving through the unsaturated zone and their displacement depends on the transport properties of the water-air porous medium system. Pollution caused by human activities, agriculture, and industry, has brought about a growing interest in the role of the unsaturated zone in groundwater pollution. Due to the complexity and the multidisciplinary nature of the subject, it is being investigated by specialists from various scientific disciplines, such as soil physicists, chemists, biologists, and environmental engineers. This state of affairs has motivated the initiative taken by the Water Quality Commissions of IUPAC (the International Union of Pure and Applied Chemistry) and IAHS (the International Association of Hydrological Sciences) to convene an international workshop, which was organized and hosted by the Institute of Soils and Water of the Agricultural Research Organization in Bet Dagan, Israel in March 1983. The lecturers at the workshop were an invited group of specialists who are engaged in studying the many facets of the unsaturated zone, and the present book is a selection of their presentations. Each chapter of the book relates to a different aspect of the unsaturated zone."

The papers collected in this book were given and discussed at the symposium on "Soil water physics and technology," which was held in Rehovot, Israel, from August 19th-September 4th, 1971. It was sponsored by the International Society of Soil Science (1.S.S.S.) through its Commissions I (soil physics) and VI (soil technology), and organized by the Israeli Soil Science Society. Thanks are due to the Editors for having assembled contributions and discussion remarks into a well-rounded, coherent book. The subjects covered in this book are the theoretical and practical aspects of the following topics: water movement in soils, soil-water interactions, evaporation from soil and plants, water requirements of crops, ion activity and migration in soils, soilwater management and salinity. In as much as these contributions were not solicited, they represent ideas and subjects considered important by the authors and debators. In science, one often finds a gap between basic research and practical application. If reading this book creates a feeling of an apparent lack of balance between theory and practice, this represents the state of our science today, and the thoughtful reader can and will recognize that much remains to be done. W. R. GARDNER T.]. MAR5HAL President, Commission I President, Commission VI 1.5.5.5.

The world population in 1950 was 2. 5 billion and is more than 5 billion today. The agriculture community, however, has responded remarkably well to meeting the increased need for food and fiber. While the population growth during this period averaged almost 2% annually, the production of grain increased at an even faster rate. From 1950 to 1973, grain production increased 3. 1% annually, but slowed to about 2% from 1973 to 1984. There fore, as a whole, the world has more grain per capita available today than ever before. Several countries that were food importers just a few years ago are food exporters today. The world carry-over stocks today are the highest in years. While the major concern just a few years ago was whether the world could produce adequate food, the problem for many countries today is how to export their surpluses. Although the world as a whole has surplus food supplies, there are millions of people without adequate food to exist, and there are additional millions who have a bare subsistence diet at best. The average daily calorie supply for the developed countries is more than 3,300 per person, while the average for the developing countries is only about 2,200. The major global food produc tion problems have shifted from Asia to Africa, where malnutrition, poverty, and starvation are attracting world attention."

The world population in 1930 was 2 billion. It reached 3 billion in 1960, stands at 4. 6 billion today, and is expected to reach 6 billion by the end of the century. The food and fiber needs of such a rapidly increasing population are enormous. One of the most basic resources, perhaps the most basic of all, for meeting those needs is the Soil. There is an urgent need to improve and protect this resource on which the future of mankind directly depends. We must not only learn how to use the soil to furnish our immediate needs, but also ensure that the ability of the soil to sustain food production in the future is unimpaired. This is indeed a mammoth task; a 1977 United Nations survey reported that almost one-fifth of the world's cropland is now being steadily degraded. The diversity of soil makes it necessary for research to be conducted in many locations. There are basic principles, however, that are universal. This series, Advances in Soil Science, presents clear and concise reviews in all areas of soil science for everyone interested in this basic resource and man's influence on it. The purpose of series is to provide a forum for leading scientists to analyze and summarize the available scientific information on a subject, assessing its importance and identifying additional research needs. But most importantly, the contributors will develop principles that have practical applications to both developing and developed agricultures.