SOIL NITRATE VARIATIONS UNDER GRASS, SORGHUM AND BARE FALLOW IN SEMIARID KENYA

Topsoil nitrate was measured every few days under grass, sorghum and b are fallow at two sites in lower Embu District, Kenya. Concentrations were highest soon after the start of rain and within a week most nitra te had been lost. Under bare fallow and sorghum, nitrate concentration increased during dry weather and losses occurred when soil water cont ent exceeded 10%. Modelling of soil water movement indicated that ther e was no leaching below the rooting zone, so denitrification may be th e main route for loss of soil nitrogen. Application of nitrogen fertil izer early in the season is not recommended as it would be lost with t he native soil nitrate. The use of phosphorus fertilizer at the phosph orus-deficient site reduced soil nitrate concentration throughout the season and increased nitrogen uptake by sorghum. At the phosphorus-ric h site, vigorous sorghum growth kept the soil nitrate concentration lo w, even though much nitrate was produced and lost in bare fallow. Adeq uate soil phosphorus is considered essential if the natural nitrogen r esources of the soil are to be managed efficiently.