SUMMARYMeasurements of profiles of ferrous and ferric iron and pH in blocks of reduced soil in contact with planar layers of rice {Oryza sativa L.) roots are reported. Initially 11-d-oId plants were kept in contact with the soil for up to 12 d. Over this period, substantial quantities of iron were transferred towards the root plane, producing a welldefined zone of ferric hydroxide accumulation. The pH in this zone fell by more than two units. The profiles changed with time. The decrease in pH was in part due to protons generated in ferrous iron oxidation, and in part due to protons released from the roots to balance excess intake of cations over anions, N being taken up chiefly as NH/. But the decrease in pH was less than expected from the net acid production in these two processes, possibly because of proton consumption in CO., uptake by the roots. Because of the pH-dependence of soil acidity diffusion, the two sources of acidity greatly reinforce each other. Some implications for nutrient and toxin dynamics are discussed.

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Water scarcity in certain regions of sub-Saharan Africa impacts agricultural production while prolonging dry seasons, and contributing to food insecurity and malnutrition. The objectives of the study were to evaluate the nutritional yield (edible yield × nutritional content) of indigenous leafy vegetables commonly consumed in Africa under water-stressed conditions. Two species of amaranth, two species of African nightshade, and two varieties of Ethiopian kale were examined under environment-controlled water-sufficient and water-deficient conditions. Fresh and dry stem and leaf weights significantly decreased in the three crop groups under the water-deficient treatment. Nutritional values for carotenoids, tocopherols, calcium and zinc showed significant differences at the content level among treatments. Among the 6 crops evaluated, the amaranth species named Amaranthus hypochondriacus and the African nightshade species named Solanum scabrum had the lowest nutrient losses in drought conditions.

Models and experimental studies of the rhizosphere of rice plants growing in anaerobic soil show that two major processes lead to considerable acidification (1-2 pH units) of the rhizosphere over a wide range of root and soil conditions. One is generation of H* in the oxidation of ferrous iron by O2 released from the roots. The other is release of H ÷ from roots to balance excess intake of cations over anions, N being taken up chiefly as NH4 +. CO2 exchange between the roots and soil has a much smaller effect. The zone of root-influence extends a few mm from the root surface. There are substantial differences along the root length and with time. The acidification and oxidation cause increased sorption of NH4 + ions on soil solids, thereby impeding the movement of N to absorbing root surfaces. But they also cause solubilization and enhanced uptake of soil phosphate.