Available phosphorus, sulfur, potassium, and other cations in a long-term grazing experiment in south-western Victoria

Relationships between amounts of superphosphate applied to pasture and stocking rate on available nutrient status were assessed from 1979 to 2000 on a chromosol derived from basalt at Hamilton, Victoria. The pastures were stocked with sheep at low, medium, or high grazing pressures in factorial combination with 6 levels of superphosphate. Overall annual amounts of single superphosphate (8.8% P, 11% S, 19% Ca) applied ranged from 0.4 to 36 kg P/ha. Potassium chloride (KCl) was applied to all plots periodically. Average stocking rates ranged from 7 to 19 dry sheep equivalents (DSE)/ha. The nutrient status of plots was monitored by sampling the soil to a depth of 10 cm, avoiding the areas used by the sheep for camping. In 1994, samples of topsoil (0-5 cm and 5-10 cm) were taken from the camp areas and non-camp areas in each plot in order to assess 'plant-available' P, S, and extractable cations. Changes in P and S with depth to 80 cm in high and low grazing pressure treatments were also assessed. The influence of cumulative P applied on Olsen P varied with time and grazing pressure. During the first 12-15 years, the Olsen P of high grazing pressure plots was greater than that of low grazing pressure plots, but subsequently the reverse has been the case. Plant-available S also increased where more superphosphate had been applied. Levels were greater than 7 mg S/kg soil except at the lowest level of superphosphate, indicating that forms of P fertiliser with less S could be used here. After initial applications of KCl, this fertiliser was not applied for 8 years. During this time the K status fell from 270 to 120 mg K/kg soil, just above where plant responses to K are likely, emphasising the need to monitor the K status of productive pastures on these soils. Levels of exchangeable Mg2+ fell as more superphosphate was used, whereas levels of Ca2+ rose. The Olsen P of soil deeper than 10 cm was always less than 5 mg P/kg soil. P supply to deep-rooted pasture plants may thus be limiting when the topsoil is dry. These results, together with published assessments of animal production at this site, show that for pastures fertilised annually, the Olsen P associated with the most profit varied from 7 mg P/kg soil at 7 ewes/ha (10.5 DSE/ha), to 14 mg P/kg soil at 18 ewes/ha.