EFFECT OF SURFACE APPLICATIONS OF LIME, GYPSUM AND PHOSPHOGYPSUM ON THE ALLEVIATING OF SURFACE AND SUBSURFACE ACIDITY IN A SOIL UNDER PASTURE

Changes in the chemistry of an acidic grey massive earth soil in response to various ameliorant treatments (gypsum and phosphogypsum in the presence or absence of lime) were investigated in a subterranean clover-based pasture in the southern highlands of New South Wales. Lime, gypsum, and phosphogypsum, or lime in combination with gypsum and phosphogypsum were broadcast at 2500 kg ha-1 on the surface of the soil in May 1990. Pasture production was determined and the soil was sampled to 25 cm depth, 6 and 18 months after treatment application. Surface soil pH was increased to 6-1 by the application of lime, gypsum plus lime and phosphogypsum plus lime treatments in the 0-5 cm depth interval, but remained unchanged when gypsum or phosphogypsum was used alone. Calcium chloride extractable aluminium increased down the soil profile under all treatments to 10-15 cm, but was highest in the gypsum treatment at depth. The application of phosphogypsum increased the 0.01 m CaCl2 extractable fluoride in the surface 5 cm from 26 to 43 muM. In contrast, fluoride concentrations were decreased to between 5.3 and 7.3 muM in the lime, gypsum plus lime and phosphogypsum plus lime treatments. Gypsum and phosphogypsum decreased the concentration of Al3+ in solution and on the exchange sites in the 0-5 cm depth interval. However, the effectiveness of the amendments to reduce toxic Al3+ concentrations were confined to the surface 5 cm. The concentration of aluminium and the activity of Al3+ in the 0-5 cm soil layer at both soil samplings were decreased by the amendments. Lime, and gypsum or phosphogypsum in combination with lime, were the most effective treatments for reducing the activity of Al3+. The activity of Al3+ increased with depth in all treatments. The pH and activity of Al3+ measured in the 0.01 m CaCl2 extracts plot near the gibbsite solubility line and suggest that Al solubility was controlled by this mineral. Exchangeable Ca in the 0-5 cm soil layer was significantly increased by the application of lime whereas exchangeable aluminium was decreased by lime, gypsum and phosphogypsum. There was no significant change in exchangeable cations other than Al below the surface 5 cm which suggests limited leaching of lime, gypsum and phosphogypsum in the profile within the 18 month study period. Pasture yield was significantly increased by the addition of lime and was related to the decrease in the activity of Al3+ in the surface soil.