P. BERNARD TINKER is Director of Terrestrial and Freshwater Sciences with the Natural Environment Research Council, Swindon, England, having previously been Deputy Director and Head of the Soils Division at Rothamsted Experimental Station, Harpenden, England. Until 1977 he was Professor and Head of the Department of Plant Sciences at Leeds University, and before this Lecturer in Soil Science at Oxford University. Dr. Tinker has published widely in the soil sciences and plant nutrition literature. He is co-author of Solute Movemnt in the Soil-Root System and has edited books on plant nutrition and mycorrhizas.ANDRE LAUCHLI is Professor of Plant Nutrition and Chairman of the Department of Land, Air and Water Resources of the University of California at Davis. Before coming to Davis in 1979, he held a number of faculty positions including Professor of Plant Physiology at the Veterinary University, Hannover, and Associate Professor of Botany at the Technical University, Darmstadt, both of which are in the Federal Republic of Germany.