
%0 Journal Article
%A Mears, PT 
%A Havilah, EJ 
%A Price, GH 
%A Darnell, RE 
%A McLennan, DJ 
%A Wilson, C 
%T 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
%J Australian Journal of Experimental Agriculture
%D 1993/07/01
%V 33
%N 4
%P 425-434
%K 
%R https://doi.org/10.1071/EA9930425
%U https://www.publish.csiro.au/paper/EA9930425 
%X 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. 
