Using corn, sorghum, and tomatoes, the rate of PO4 absorption was not directly proportional to the PO4 concentration of the culture solution. Rate of growth increased with increasing PO4 concentrations throughout the growing period. Maximum growth was obtained at 0.5 p.p.m. PO4. Corn and sorghum made a very good growth at 0.2 p.p.m. PO4, whereas tomatoes made a poor growth. Phosphate played a minor role as a buffer material in the plant sap. Corn absorbed PO4 more rapl.dly from acid than from alkaline solutions. There was no appreciable difference in the rate of PO4 absorption from solutions having a reaction of pH 4.0, 5.0, and 6.0.