Fluctuations in soil moisture, and plant uptake of surface applied phosphate

Abstract

The mechanisms leading to decreased plant uptake of surface-applied phosphatic fertilizers during short periods of drying were investigated in controlled glasshouse experiments with subterranean clover. Phosphate and water were added differentially to surface and subsurface layers of reconstituted soil profiles. The surface layers were subjected to brief periods of drying and to partial or complete remoistening at frequencies ranging from daily to fortnightly. Phosphorus uptake from surface applications and shoot yield were proportional to the frequency of remoistening of the soil surface to field capacity, and the response to additional subsurface phosphate diminished as the amount of surface water increased. In treatments where moistening did not achieve field capacity, yield was linearly proportional to the amount of water applied to the surface. Watering the surface to field capacity twice a week led to a 50 percent reduction in phosphorus uptake compared with daily watering. Watering twice as frequently with half the amount each time did not affect plant yield or phosphorus uptake. Thus it appeared that rapid phosphorus uptake occurred only at high moisture contents, and that uptake was proportional to the volume of soil brought close to field capacity and the length of time that it remained moist. This indicates that desorption of soil-adsorbed phosphate and its diffusion to plant roots can occur rapidly only at high moisture contents. Similar results were obtained with different soil types when the extractable phosphate was concentrated near the soil surface.