The significance of drilling date and crop cover with reference to soil erosion by water, with implications for mitigating erosion on agricultural land in South East England
Early drilling of autumn-planted cereals is strongly advised in UK government publications targeted at farmers, in part as a measure to combat soil erosion by water. However, in years when rainfall is heavy in early autumn, this strategy is ineffective. Late drilling of autumn-planted cereals also increases the risk of erosion, but for a different reason: crop cover develops more slowly in cooler weather, resulting in a longer exposure of nearly bare ground. The crucial factor affecting both strategies is the timing of autumn and early winter rainfall. We discuss a conceptual model based on the notion of a ‘window of opportunity’ for erosion, comprising the relationship between drilling date, date of attainment of a sufficiently protective crop cover and the timing of rainfall; variations are presented for different weather conditions and management choices. Of these three factors, only the date of drilling can be chosen by the farmer. The date of attaining a sufficiently protective crop cover can only be predicted approximately. The timing of rainfall cannot be predicted. Thus, erosion control advice to farmers, which is based on choice of date of drilling to minimize erosion during the ‘window of opportunity’, is both difficult to formulate and likely to be ineffective. Sites at risk of erosion need to have better thought-out mitigation measures in place, rather than relying on a fortuitous temporal pattern of autumn and winter rainfall to minimize the risk of erosion.