Pasture production and changes in phosphorus and sulphur status in irrigated pastures receiving long-term applications of superphosphate fertiliser

Abstract

A grazing experiment on aLismore stony silt loam in mid Canterbury spanning a 34-year period has demonstrated the importance of both phosphorus (P) and sulphur (S) fertilisers for the development and maintenance of irrigated ryegrassclover pastures, especially during spring and summer growth periods. In the initial stage of pasture development, the absence of P or S fertiliser inputs can lead to a severe reduction in pasture production and a deterioration of the pasture sward towards weeds and weed grasses at the expense of high quality ryegrass and clover species. Superphosphate applied at rates of 21–24 kg/ha per year is sufficient to satisfy phosphate maintenance requirements for irrigated pastures. However, the S content in superphosphate applied at these rates exceeds pasture S requirements. Spring application of sulphatecontaining phosphatic fertilisers which aims to provide P at a maintenance rate of 21–24 kg P/ha per year is also sufficient for pasture S requirements if these fertilisers have a ratio of S to P content of about 0.8: 1. A short-term application of superphosphate for 6 years even at rates well above the maintenance P and S requirements was unable to safeguard pastures against yield reduction or pasture deterioration towards weeds and weed grasses even in the first growing season after superphosphate was discontinued. Superphosphate applications over several years, however, can build up soil P and S reserves which may contribute significantly to plants. Under this situation, the residual value of previous applications may be able to maintain pasture production and high quality herbage species without any reduction for a longer period than that observed under short-term applications. Since soil organic S as well as organic and inorganic P can accumulate under grazed pastures, especially after prolonged annual applications of superphosphate, the contribution of these reserves to pastures should be considered in conjunction with soil phosphate and sulphate tests when assessing pasture requirements for these two nutrients.