No abstract

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.

No abstract

Synopsis Animal traffic caused serious compaction of fine textured pasture soils, severely decreased pore‐space open to aeration, and caused a 20% decrease in alfalfa‐brome‐Ladino yields during the first pasturing year on Ontonogan clay loam.

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.