The nutritive values of food crops and factors which may modify them are subjects of increasing interest. In recent years vitamin and mineral constituents of plants have been used to evaluate some aspects of nutritive value, and importance of a variable supply of minerals from plant sources in animal nutrition has been discussed by Maynard (15). During summers of 1941 and 1942, extensive experiments have been conducted at this laboratory on effects of varying relative proportions of macro-nutrients1 supplied to tomato plants on vitamin content of tomato fruits (6, 11). Although variations in mineral constituents of tomato fruit are of relatively minor nutritional importance (10), studies relative to mineral composition were made to aid in an understanding of physiological processes involved in mineral absorption by plants. In soil studies of mineral nutrition of plants, Jenny and Ayers (12) have clearly distinguished two aspects of problem : 1. Soil colloidal phenomena as they are influenced by fertilization and as they in turn influence composition of soil solutions ; and 2, absorption and accumulation of nutrients by plants from soil solution. For convenience, they have termed second aspect the biological accumulation phase by root hair cells. More exactly this aspect might include localized retention of ions by living system. Since cytoplasmic constituents of any organism are changing continuously, chemical analysis of any organ at any given interval can only be empirical. Investigations with artificial culture solutions of known composition have been profitable in studying those empirical relationships involved in ionic absorption by plants. Such factors of nutrient composition as presence of essential ions, relative concentration of constituent ions, osmotic concentration of solution, reaction of medium, and total quantity of nutrient available to plant can be varied over wide ranges under carefully controlled conditions. It is purpose of this paper to present data concerned primarily with mineral composition of tomato plants as it is influenced by composition of nutrient medium with special attention given to ionic interaction. In many investigations of effect of changes in nutrient supply on plant composition, nutrient solutions of fixed composition were utilized to ? The term macro-nutrients as used in this paper refers to six nutrients (Ca, K, Mg, N03, S04, and P04) in contrast to micro-nutrients (Cu, Mn, B, Zn, Mo, and Fe). The distinction between these two groups of nutrients is based upon fact that former group is needed by plants in much greater amounts than latter group.