Agricultural Phosphorus in Relation to Its Effect on Water Quality

Keywords: Phosphorus, agriculture, eutrophication, soil critical P values, pathways of P loss.

A number of issues have focussed attention on the use of phosphorus (P) in agriculture in recent decades, including the following. The role that P transported from soil to water plays in producing the adverse effects of eutrophication in surface fresh water bodies. The need to use P efficiently in agriculture because it consumes about 85% of the annual global P production and the world’s resources of P are less than those of the other major plant nutrients. There has been a steady decline in P fertiliser use on many UK arable farms such that since the mid 1990s there has been an increasing negative apparent P balance in many soils growing arable crops.

Current estimates suggest that agriculture contributes a little less than 30% of the P load in most fresh water bodies in England, Wales and Scotland. Here we review the sources of this P and the pathways by which it may be transported from soil to water. We conclude that within river catchments P is moved from discrete sources rather than from all areas (diffuse sources). Much of the P that is carried to rivers is in water moving over the surface of the landscape taking with it eroded soil or organic material, such as slurry, with their associated P. The transport of P from discrete sources makes possible remedial measures that are discussed in detail. Principal among these are not to apply slurry at inappropriate times and not to excessively enrich soil with plant available P. For the latter, a critical value for the soil type and cropping system should be determined and maintained. This will ensure an efficient use of P from both fertilisers and organic manures.

C J Dawson, Chris Dawson and Associates, Strensall, York, UK; and

A E Johnston, Rothamsted Research, Harpenden, UK.

40 pages, 3 figures, 7 tables, 44 references.