Ammonium Effects on Phosphorus Absorption through pH Changes and Phosphorus Precipitation at the Soil‐Root Interface¹

Abstract

Abstract The addition of NH 4 + 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 33 P labelled monocalcium phosphate (MCP), MCP + K 2 SO 4 or MCP + (NH 4 ) 2 SO 4 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 K 2 SO 4 and tripled by (NH 4 ) 2 SO 4 . 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 + (NH 4 ) 2 SO 4 . 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 + K 2 SO 4 treatments. The pH of the soil‐root interface was 0.6 units lower in the MCP + (NH 4 ) 2 SO 4 than in the MCP treatment. The higher ratio of H 2 PO 4 ‐ /HPO 4 = 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 (NH 4 ) 2 SO 4 .