Seasonal variation of phosphorus in soil and pasture

Availability of soil phosphate to pasture plants, as indicated by rate of growth and phosphorus concentration in herbage dry matter, is typically greater in spring than in autumn and winter. The hypothesis that this apparent higher phosphate status of soils under pasture in spring is due to a build-up of available phosphate during the period of slow growth in winter has been examined by measurement of pasture and soil properties at monthly intervals over 14 months on seven widely differing soils in the Hawke's Bay and Wellington districts. The experimental areas were on developed pastures under rotational grazing. Rate of pasture growth, botanical composition of pasture, and concentration of phosphorus in the pasture species all followed the normal seasonal pattern. Anion-exchangeable phosphate, concentration of phosphate in O.O1M CaCl 2 , and phosphate retention showed significant changes over the summer, but there was little variation over the winterspring period to indicate a build-up of available inorganic phosphate in the late winter and early spring. It is suggested that the high phosphorus status of soils under pasture in spring is due to release of phosphate from organic residues and soil organic matter.