Effectiveness of single and coastal superposphates applied either in autumn or spring

Phosphorus (P) is known to leach laterally in water flowing during winter o ver pasture growing on flat soils that are shallow sands over slowly permea ble lateritic ironstone gravel or clay soils in the high rainfall (> 800 mm annual average) areas of south-western Australia. The climate is Mediterra nean, with hot dry December to March and cool wet April to November growing season, with excess water flowing over the surface from June to early-Augu st. Fertilizer P is presently applied at about mid-March, near the start of the growing season. Single superphosphate has been applied for many years, which has a good residual value, and so the soils are no longer acutely P deficient. Consequently, a better method may be to apply the fertilizer at mid-August, after waterlogging and P leaching have usually receded, and rad iation and temperature are rising, so pasture growth is increasing. The fie ld experiment reported here was on a shallow sand over lateritic ironstone gravel where lateral leaching of P occurs. The experiment compared from 199 0 to 1994 the effectiveness of single superphosphate (SSP), the fertilizer used at present, and `coastal' superphosphate (CSP), a partially acidulated rock phosphate containing about half the total P and one third the water-s oluble P initially present in SSP. The fertilizers were applied annually ei ther at mid-March or at mid-August. SSP applied at mid-March was the most e ffective treatment studied in the years when pasture plants had emerged bef ore fertilizer was applied at mid-March. This is attributed to pasture plan ts being able to take up P from SSP applied at mid-March before leaching of P occurred, so that relative to SSP applied at mid-March, the other P fert ilizer treatments (CSP applied at mid-march and mid-August, SSP applied at mid-August) were about equally or less effective. However, in years when th e growing season had yet to start before fertilizer was applied at mid-Marc h, then relative to SSP applied at mid-March, the other fertilizer treatmen ts were equally or more effective. This is attributed to extensive leaching of P from SSP applied at mid-March, so that due to P losses from SSP appli ed at mid-March, the other treatments were equally or more effective. It is therefore concluded that profitable pasture production with reduced leachi ng is achieved by applying SSP at mid-March if soils are moist and pasture plants are growing at that time. However, if the soils are dry and no pastu re plants are growing at mid-March, then CSP should be applied at mid-Augus t.