A laboratory study of application of basalt dust to highly weathered soils: effect on soil cation chemistry

Surface (0-10 cm) samples from 7 highly weathered soils in tropical coastal Queensland were incubated for 3 months at room temperature and at field moisture capacity with basalt dust applied in 2 size fractions: <150 <mu>m and 40 mum. The basalt application was mixed at 0, 1, 5, 25, and 50 t/ha to cover situations of moderate applications as well as where the amendment might be banded to achieve high local concentrations. Basalt dust application caused desirable increases in soil pH, reduced the content of exchangeable acidic cations, increased soil cation exchange capacity, and increased the content of base cations in all soils. By determining fundamental surface charge characteristics of these variable charge soils, it was possible to show that the additional base cations released from the basalt dust were present as exchangeable cations, and that the amounts released were controlled by the number of negatively charged sites available, i.e. soil cation exchange capacity. Selected treatments were then subjected to a strong leaching environment to assess the longevity of the effects obtained. Soil properties remained virtually unchanged by the leaching treatment, except that significant amounts of monovalent K and Na were removed. At the higher rates of application, the amounts of base cations released from the basalt were small in comparison with the actual amounts applied, indicating that the amendment could be effective over a considerable period of time.