The objective of this study was to test the hypothesis that reduction of pH at the soil‐root interface when NH4+ ions were absorbed is the cause of the increased absorption of P in the presence of NH4+.The influence of addition of (NH4)2SO4 or KNO3 to an MCP band on P absorption by corn was determined in a greenhouse study involving four soils ranging in pH from 4.2 to 8.2. The in‐situ activity of the corn plants was determined periodically during the growth, and the dry weight of tops and roots in the fertilizer zone as well as the nutrient content of the tops were determined after 17 days.Application of (NH4)2SO4 with MCP was found to increase fertilizer P uptake when compared to MCP alone on soils of pH 8.2, 7.4 and 5.5. No difference was measured on a soil of pH 4.2. Accumulations of Ca and P were found on the root surface of the plants grown in the alkaline soils. Ca, Fe and P was found on the root surface in a soil with a pH of 5.5. No root surface accumulation was found at a soil pH of 4.2. Fertilizer P uptake at 9 days was greatest where NH4+ and least where NO3‐ accompanied the P. Changes in the soil‐root interface pH and resulting changes in the H2PO4‐/HPO4= ratio at the root surface which are modified by the inherent soil pH are implicated in these findings.