Silber, A., Institute of Soil, Water and Environmental Sciences, Agricultural Research Organization, Volcani Center, P.O. Box 6, Bet Dagan 50250, Israel

NH4 concentrations in soil-solutions are commonly low even under irrigation with a high NH4-N concentration as a result of rapid adsorption on the surface sites of the solid phases and nitrification reactions that diminish the NH4 content. In contrast, the NH4 concentrations in soilless media that have low buffering capacity and are subjected to frequent fertigation events may be high. Consequently, in soilless culture the impact of varying irrigation NH4 concentration on rhizosphere pH and on plant physiological processes may become crucial and therefore, controlling the irrigation NH4 concentration is of considerable importance. The objective of the present study was to study the effect of nitrogen source on two important factors associated with NH 4 nutrition of soilless-grown plants: (i) pH modification and improvement of nutrient availability; and (ii) Interaction between NH 4 concentrations and irrigation frequency.