Superphosphate maintains soil fertility and beef production on grazed white clover pastures in the subtropics. 1. Residual soil phosphorus, sulfur, and nitrogen, and pasture growth responses

Abstract

Phosphorus (P) as superphosphate was applied at 0, 12, 24, and 48 kg P/ha each year from 1972 to 1980 to an established white clover (Trifolium repens) and carpet grass (Axonopus affinis) pasture, to measure long-term changes in soil fertility and pasture production under grazing on a duplex soil in a coastal, subtropical environment. Hereford weaner steers grazed the pasture at 2 stocking rates (1.67 and 2.5 steers/ha) from 1972 to 1974, and at 3 stocking rates (1.67, 2.50, and 3.3 steers/ha) for 7 years to 1981. The effects of P and stocking rate during 1972-76 on pasture and animal production were small and variable, so the study was extended to 1980-81 to measure longer term P and stocking rate effects on soil fertility. Available soil P (bicarbonate-extracted) and sulfur (S, phosphate-extracted) responded (P<0.05) linearly over time to rates of maintenance superphosphate. For soil P, this effect increased with time and showed seasonal fluctuations, which were described by a model with linear and curvilinear functions. After 9 years, average soil P levels at the 4 rates of applied superphosphate changed from 32 to 12, 24, 45, and 91 mg P/kg soil, respectively. A residual value function (RVF) was used to predict the residual value of applied phosphorus over time, which fitted the observed values of available soil P for each fertiliser treatment (R2 = 0.99). The soil S response to superphosphate remained constant over time. Mean values at the 4 rates of applied superphosphate were 8, 11, 15, and 25 mg/kg. Over the 9 years, total soil nitrogen increased (P<0.01) linearly from 0.134 to 0.179% and pH decreased (P<0.05) slightly from 5.7 to 5.2, but neither was significantly affected by superphosphate application. Total pasture and clover growth measured in cages responded linearly to superphosphate, with maximum (P<0.05) response occurring in spring each year from 1972-73 to 1976-77. It was concluded that on this duplex soil with 32 mg/kg of bicarbonate-extractable P, omitting superphosphate reduced soil P after 12 months and pasture growth within 18 months. Annual applications of superphosphate from 125 to 250 kg/ha (12-24 kg P/ha) maintained bicarbonate-extractable soil P in the range 24-45 mg/kg and improved growth and seed reserves of white clover in this environment.