Seasonal and fertilizer effects on the organic cycle and microbial biomass in a hill country soil under pasture

Abstract

A two year investigation of soil phosphorus and the soil organic cycle was carried out on a typical hill country site in the North Island, New Zealand. This included investigation of changes in soil phosphorus, as well as seasonal and fertilizer (superphosphate) effects on soil microbial phosphorus and sulfur, sodium bicarbonate extractable phosphorus and calcium chloride extractable sulfur. No net utilization of soil organic phosphorus occurred when application of phosphate fertilizer was withheld. On the contrary, accumulation of organic phosphorus was found in both fertilized and unfertilized plots. Immobilization of inorganic phosphorus into organic forms appeared to be a significant factor in fertilizer phosphorus requirements at this site. It was also a significant cause of the decline in the soil phosphorus status when no fertilizer was applied. Despite declining pasture production, there were no effects of withholding superphosphate on the soil biological cycle as measured by soil microbial phosphorus and sulfur, total organic phosphorus and sodium bicarbonate extractable organic phosphorus. However, seasonal variations occurred indicating storage and release of phosphorus by the soil organic matter and microbial biomass. Release of phosphorus occurred during periods of rapid pasture growth and could account for phosphorus uptake by the pasture at those times.