Dissolution of milled-silicate rock fertilisers in the soil

Abstract

Dissolution of dry-milled basalt, dolerite, gneiss, and K-feldspar added to 23 soils has been related to milling time and soil properties. The rocks milled for 10, 60, and 120 min were mixed with 23 type of soils from south-western Australia at a rock/soil ratio of 1/100 (≈10 t/ha), wetted to 110% field capacity, then incubated at 20°C for 2 and 10 months. Measurements of cations extractable by 1 M CH₃COONH₄ at pH 7 indicate that substantial dissolution occurred in the soil and that milling increased dissolution. After 10 months of incubation, about 18% of Ca and Mg had dissolved from basalt and dolerite milled for 120 min and 40% of Na and K from gneiss and K-feldspar milled for 120 min. Some dissolution occurred with 1 h extraction of non-incubated rock–soil mixtures and these released elements are considered to be readily available plant nutrients. The silicate rocks had minor effects on soil pH and EC of soil-rock mixtures (i.e. increased pH by ≤0.5 unit and EC by ≤127 μS/cm in 1 : 5 water extracts). The large increases in silicate rock dissolution in soils due to milling indicate that milled basalt and dolerite may be used as Ca and Mg fertilisers, and K-feldspar as a K fertiliser. Further research is needed to identify soils and plants for which the application of the silicate rock fertilisers will be most beneficial.