Changes in pH and organic carbon were minimal in a long-term field study in the Western District of Victoria

Abstract

Changes in soil pH from a long-term experiment at Hamilton, Vic., associated with time, fertiliser application, and stocking rate were assessed. The pH was measured in a 1 :5 suspension of soil in water (pH_W) from 1980 onwards and in a 1:5 suspension of soil in 0.01 M CaCl₂ (pH_Ca) from 1984 onwards. Topsoils (0–10 cm) were sampled regularly from 1980 to 1999, and the soil profile to a depth of 80 cm in 1994. The site was sown to perennial ryegrass, phalaris, and subterranean clover in 1977. Treatments fertilised with different amounts of superphosphate were grazed by sheep at stocking rates of 7–19 dry sheep equivalents (DSE)/ha. Average applications of phosphorus (P) ranged from 0.5 to 38.7 kg P/ha.year.

The pH_Ca of the topsoil in 1984 was 4.9. It decreased at an average rate of 0.005 pH_Ca or 0.008 pH_W units/year, with little variation due to fertiliser or stocking rate. Measurements in 1994 revealed subtle but statistically significant (P < 0.01) trends in soil pH_Ca that were associated with grazing pressure, inputs of fertiliser, and whether or not areas sampled were used by the sheep for camping. The top 0–5 cm of soil was slightly less acidic (+0.07 pH_Ca units) in the camp areas compared with non-camp areas. Below 5 cm to a depth of 80 cm, camp areas were more acidic (–0.19 pH_Ca units). At the highest stocking rates, heavier applications of superphosphate were associated with greater subsoil acidity: –0.06 pH_Ca units per 100 kg phosphorus (P) applied. There was no relationship at medium stocking rates. At low stocking rates, higher P applications were associated with more alkaline subsoils. Net removal of product from 1979 to 1994 (wool and meat removed from plots and excreta transferred to camps) was estimated to be equivalent to 140–380 kg/ha of lime over this 15-year period. The organic carbon (OC) content of the topsoil did not change over 20 years of records from 1979, and was unaffected by inputs of P. In 1994, the OC content of the 0–5 cm layer of topsoil was greater than the 5–10 cm layer (mean values 5.5% and 3.8% respectively; P < 0.001). The OC content of camp areas was higher than that of non-camp areas, this difference being more pronounced in the 0–5 cm layer (P < 0.01).

It was concluded that the rate of change of pH was slow because of the high pH buffering capacity of the soil, the small amount of alkalinity removed in product, and the generally high perennial grass content of the pastures. Soils shown to be at greatest risk of acidification from this study were those under camp areas, and where high fertiliser rates were applied to pastures with a low perennial grass content. Rotational grazing should diminish these problems by reducing the concentration of excreta in camp areas, and favouring perennials over annuals in both camp and non-camp areas. Inputs of lime may eventually be needed to compensate for the acidifying effect of product removal.