Untimely cultivation, grazing and forestry operations when the soils are wet give rise to a rapid and marked loss of soil structure and a reduction in soil permeability and aeration owing to the compaction, deformation and consolidation of the topsoil and upper subsoil. This degradation results from the pressure of heavy machinery or treading by stock. Some soils are more at risk than others due to their poor resistance and resilience to compacting forces, poor drainage, limited water holding capacity, or higher rainfall.
Maps showing the risk of soil degradation are derived from a combination of measured data from the National Soils Database (NSD), spatial soil and climate layers, and expert opinion. The soil polygons are taken from the Land Resource Inventory (LRI). For most polygons there are either only a couple of soil measurements, or none at all. In the former case, the small number of measurements means that variability within soil series is unaccounted for. In the latter case, the polygon’s subgroup was used to estimate the likely content of carbon and relevant clay minerals. No account is taken of Land Use, e.g., on carbon content. LRI polygons are generally of 50-200 ha, consequently smaller pockets of different soils will not be depicted. Only a small number of climate stations measure potential evapotranspiration, limiting the New Zealand-wide accuracy of this input. The maps are based on an averaged risk (over a 20-year period), consequently risk in extremely wet seasons is likely to be higher than shown on these maps. The risk model is primarily based on knowledge of soil processes and representative soils, and experience of compaction in the Manawatu. The model output was compared with expert assessments in Canterbury and the Waikato, but it has not yet been evaluated in other parts of New Zealand. Our expectation is that the model is valid for all of New Zealand. For all of these reasons, this map should not be used at a more detailed scale than 1:50,000. It is suitable for a general overview of areas that are likely to be of higher or lower risk, but cannot be used for detailed management decisions at the paddock scale.