The addition of NH₄⁺ ions to a fertilizer P band is known to increase the absorption of P. This study was conducted to further clarify the mechanisms responsible for this phenomenon.

Pellets of ³³P labelled monocalcium phosphate (MCP), MCP + K₂SO₄ or MCP + (NH₄)₂SO₄ were placed 1.25 cm to the side of a corn root tip growing in the soil at the surface of a box with a sloping removable front. Roots in the vicinity of the pellet, and the shoots, were harvested 15 days later.

Fertilizer P concentration in the shoots was doubled by the addition of K₂SO₄ and tripled by (NH₄)₂SO₄. Autoradiographs of the area surrounding the pellets indicated an accumulation of P on the surface of roots in the MCP treatment but not in the MCP + (NH₄)₂SO₄. Autoradiographs of root cross‐sections confirmed this observation. Electron microprobe scans of root cross‐sections indicated a precipitation of Ca and P at the soilroot interface in the MCP and MCP + K₂SO₄ treatments. The pH of the soil‐root interface was 0.6 units lower in the MCP + (NH₄)₂SO₄ than in the MCP treatment. The higher ratio of H₂PO₄^‐/HPO₄⁼ ions at the lower pH is thought to be responsible for the prevention of the precipitation and the increased absorption of P in the presence of (NH₄)₂SO₄.