Implications of ceasing annual superphosphate topdressing applications on pasture production

Abstract

Two field-plot trials were superimposed on irrigated, sheep-grazed pastures on Lismore stony silt loam in Canterbury to investigate residual effects of previous superphosphate applications on dry matter (DM) production and herbage composition. Residual effect of phosphorus (P) in superphosphate was shown to depend upon previous application rate and number of years of superphosphate application. In contrast, residual effect of sulphur (S) in superphosphate was independent of previous application rate if superphosphate had been applied for 25 years at rates at or above the S maintenance rate. Previous applications of superphosphate at the annual rate of 376 kg/ha for 6 years failed to safeguard pasture against yield reduction even in the first year after superphosphate was discontinued. This reduction was attributed equally to both P and S deficiencies. Residual values of both P and S were more substantial in areas where superphosphate had been-applied-at--the-higher rate of 564 kg/ha for 6 years. However, these residual values were also short-lived and DM production declined by 13% in the second year after the cessation of superphosphate application. Different patterns of residual values of superphosphate were recorded on areas where superphosphate had been applied over a longer term of 25 years. Where superphosphate had been applied annually at 188 kg/ha, DM production fell by 10% in the first year after topdressing ceased. This reduction was due entirely to P deficiency affecting clover growth, while S deficiency - in both grass and clover - was not observed until the third year after topdressing ceased. On areas where superphosphate had been applied at a higher rate of 376 kg/ha for a similar period of 25 years, DM production did not decline until the second year. This initial decline was attributed to S deficiency in clover while P deficiency in grass and clover was not apparent until the 4th year after the cessation of superphosphate application. The results were used in a simple model that uses previous topdressing history to predict yield reductions if topdressing is withheld. Keywords residual effects, phosphorus, sulphur, irrigated pasture