Field amelioration of acidic soils in south-east Queensland. I. Effect of amendments on soil properties

Replicated field trials with rates of lime ranging up to 8 t/ha were conducted at each of 27 sites in south-east Queensland. At 16 of these sites, single rates (2 t/ha) of gypsum or phosphogypsum were also applied. Soil samples (0-10 cm) were collected from each plot and analysed for pH in both water (pH(w)) and 0.01 M CaCl2 (pH(Ca)), for electrical conductivity, exchangeable cations, and extractable Al and Mn. Gypsum application resulted in either a general trend for, or significant (P < 0.05), reductions in pH(w) but had no significant effect on pH(Ca). The relationship between rate of applied lime and soil pH at each site permitted the calculation of pH buffer capacity for a wide range of soil types and properties. The pH increase per t applied lime ranged from 0.14 to 0.82 and from 0.16 to 0.63 for pH measured in water and 0.01 M CaCl2, respectively, reflecting the range in pH buffer capacity which was significantly correlated with organic carbon. Multiple regression indicated that organic carbon and clay significantly contributed to the variation in pH buffer capacity but only around 40% of the variation could be accounted for. The pH values at which Al saturation was reduced to 10% ranged from 4.82 to 6.02 (pH(w)) and from 4.26 to 4.93 (pK(Ca)) and indicated that if neutralising exchangeable Al is the basis for liming, then no single target pH value will be appropriate for all soils. However, the target pH at which Al saturation would be reduced to 10% could be predicted from the initial pH and initial Al saturation. The effective cation exchange capacity (EC:EC) was increased by liming at all sites and the additional exchange capacity was occupied by Ca. This increased Ca saturation was not necessarily at the expense of exchangeable K and Mg, which were significantly (P < 0.05) reduced at only a few sites. The increase in ECEC for a unit increase in pH ranged from 0.5 to 9 cmol(+)/kg and at some sites represented a doubling of the soil's cation exchange capacity.