Regional trends in Scottish advisory soil acidity and phosphorus results: significance of management history, land use and soil attributes

Four geographically distinct regions of Scotland are used to describe temporal and spatial trends in soil phosphorus (P) using ca. 180 000 routine advisory samples which included information on previous and next crop. Regional differences in Modified Morgan's extractable P concentrations exist primarily in response to differences in land use and particularly the greater role of livestock in the west compared to arable cropping in the east. No obvious or consistent temporal trend in extractable P was apparent, but a very wide range of soil properties and extractable P concentrations was evident. The current (since 2010) target soil P status is moderate (M) for cereal-based arable rotations and the upper half of moderate (M+) for rotations with potatoes, and more than half the samples fell into this range (with 40% M-). Some ca. 30% of samples fell below this, while 14% had high or very high P status, most noticeably where the previous crop was either potatoes, vegetables or berries. Half (ca. 13 000) of the samples with a low soil P status also had a below optimum pH. An apparent contradiction between the regularly reported long-term declines in agricultural P surplus not being matched by a drop in extractable soil P status is discussed. This 'lag effect' means many Scottish agricultural soils have the capability to buffer change in extractable soil P. We make the first attempt to link advisory P data with information from the National Soil Database of Scotland and explore the value of introducing 'soil type' into the P fertilizer recommendation system.