How good is the evidence to support investment in soil protection?

We review options for assembling evidence to support investment in protection from soil degradation by erosion, decline in organic matter, compaction and contamination. Convincing rationales for investment in soil resources need evidence about the benefits of improved soil status and the efficacy of possible actions. Changes in the bulk properties of soil over large areas are difficult to measure within the timescales of policy reviews, so soil monitoring rarely supports timely policy assessment. In addition, there are gaps in our understanding of the impacts of changes in soil properties at landscape scales. There is adequate quantitative evidence about the effects of land use change on soil erosion and soil organic matter (SOM) but rather less for compaction. Quantitative predictions of the effects at above field-scale on soil properties from altered land management within a given land use are quite limited, but can be described qualitatively. The effects of controls on soil contamination at regional scales are predictable from mass flows. There are accepted narratives about the benefits of reduced erosion and compaction, and enhanced SOM on levels of services and goods, including agricultural productivity and hydrology. However, quantitative models for estimating these benefits require more development. The impacts on greenhouse gas emissions of changes in soil organic carbon levels can be quantified and the impacts of contamination interpreted using threshold levels for protection of food and the environment. Overall, there is an urgent requirement for better quantitative models to predict soil-based policy outcomes from altered land use and especially land management.