Successful sequestration is dependent on planting sites, as the best soils for sequestration are those that are currently low in carbon. The varied results displayed in the graph highlights this fact. For the UK, successful sequestration is expected for arable land over most of England and Wales, with unsuccessful sequestration expected in parts of Scotland, due to already carbon rich soils (existing woodland) plus lower yields. Soils already rich in carbon includes peatland and mature forest. Grassland can also be carbon rich, however Milner et al. argue that the most successful carbon sequestration in the UK takes place below improved grasslands. The bottom chart displays the estimated yield necessary to achieve CO2 negativity for different levels of existing soil carbon saturation. The higher the yield, the more likely CO2 negativity becomes.