Influence of rock fertilisers on soil microorganisms in an organic pasture

[Truncated abstract] It is often assumed that under organic farming systems the contribution of biological processes to overall fertility is increased, but this is not necessarily the case in organic pasture-grain systems in Australia. In these systems, the biomass and activity of microbial communities in soil is often limited by the inherently low chemical and physical fertility of soil and low inputs of organic matter. To harness the beneficial effects of biological processes on soil fertility, organic pasture-grain systems in Australian need to identify specific management practices that improve chemical and physical conditions in soil for beneficial microorganisms or increase inputs of plant residues. These management practices also need to be permitted by certification standards for organic farming and be suited to soil types and climate. Rock fertilisers have the potential to influence microbial communities in soil in organic pasture-grain systems in Australia. Rock fertilisers may influence the microbial community in soil by altering the chemical and physical conditions in soil, including soil pH, nutrient availability and soil texture, and by altering plant biomass. The unifying hypothesis investigated in this thesis is that rock fertilisers influence microbial communities in soil by altering the chemical and physical conditions in soil. This thesis presents findings showing that rock fertilisers alter the structure of microbial communities in soil due to their influence on the chemical and physical properties of soil. These findings indentify basic properties of the mineral matrix of soil (elemental composition and particle size) as factors driving the structure and diversity of microbial communities in soil. A three-year field experiment was performed on a permanent pasture site in the south-west of Western Australia. This study tested the hypothesis that applying rock fertilisers to the pasture in the transition phase to organic farming would increase the microbial biomass and activity of soil and that this would be associated with changes in chemical fertility of soil. Pasture was fertilised with 2 rates of rock fertilisers (0.6 and 0.3 t ha-1) or soluble fertiliser...