Effects of mixed cropping farming systems on changes in soil properties on the Canterbury Plains

Before European settlement, most of the 750,000 ha of land comprising the Canterbury Plains was under native tussock grassland with pockets of podocarp forest. The dominant land use today is mixed cropping in which cereals and cash crops are grown for 2 to 4 years followed by grass-clover pasture for 2 to 4 years. These cropping rotations are generally too short for either a substantial build-up in soil organic matter under pasture or its breakdown under arable cropping to occur. Nonetheless, there is a cyclic improvement in both N-supplying capacity and soil structure under pasture and a decline in both properties under arable. Large N inputs (e.g., 100-300 kg N ha-1 yr-1) occur through N₂ fixation by the clover component of the pasture, but the potential for leaching losses of nitrate is substantial after the pasture is ploughed in. During the pasture phase there is an increase in temporary soil binding agents in the topsoil layer and consequently an increase in aggregate stability. There is also a change in soil porosity caused by pasture root growth and earthworm activity. In contrast, soil physical properties rapidly deteriorate under the arable phase of the rotation.