SUPERPHOSPHATE MAINTAINS SOIL FERTILITY AND BEEF-PRODUCTION ON GRAZEDWHITE CLOVER PASTURES IN THE SUBTROPICS .1. RESIDUAL SOIL-PHOSPHORUS,SULFUR, AND NITROGEN, AND PASTURE GROWTH-RESPONSES

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 (Trifol ium repens) and carpet grass (Axonopus affinis) pasture, to measure lo ng-term changes in soil fertility and pasture production under grazing on a duplex soil in a coastal, subtropical environment. Hereford wean er 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 st eers/ha) for 7 years to 1981. The effects of P and stocking rate durin g 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 stocki ng rate effects on soil fertility. Available soil P (bicarbonate-extra cted) and sulfur (S, phosphate-extracted) responded (P<0.05) linearly over time to rates of maintenance superphosphate. For soil P, this eff ect increased with time and showed seasonal fluctuations, which were d escribed by a model with linear and curvilinear functions. After 9 yea rs, average soil P levels at the 4 rates of applied superphosphate cha nged from 32 to 12, 24, 45, and 91 mg P/kg soil, respectively. A resid ual value function (RVF) was used to predict the residual value of app lied phosphorus over time, which fitted the observed values of availab le soil P for each fertiliser treatment (R2 = 0.99). The soil S respon se to superphosphate remained constant over time. Mean values at the 4 rates of applied superphosphate were 8, 11, 15, and 25 mg/kg. Over th e 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 nei ther was significantly affected by superphosphate application. Total p asture and clover growth measured in cages responded linearly to super phosphate, with maximum (P<0.05) response occurring in spring each yea r from 1972-73 to 1976-77. It was concluded that on this duplex soil w ith 32 mg/kg of bicarbonate-extractable P, omitting superphosphate red uced soil P after 12 months and pasture growth within 18 months. Annua l 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 an d improved growth and seed reserves of white clover in this environmen t.