Organic manure phosphorus accumulation, mobility and management

Organic manures are an important source of P which can make a significant economic contribution to farm fertilizer policies. In the region of 119 000 tonnes of P are returned annually to UK agricultural land in the form of manures collected and handled on farms, with an estimated 66 000 tonnes of P applied to tillage land and 53 000 tonnes to grassland. Previous research on the utilization of manure P has tended to indicate a lower efficiency compared to inorganic fertilizer P in the season following application, but in the longer term manure and fertilizer P can be regarded as equivalent. Failure to adequately account for manure P additions to the land may result in soil enrichment which could increase the agricultural contribution to eutrophication, as a result of surface runoff or leaching. Recent research has indicated that the current guidelines for minimizing runoff losses following the land spreading of manures are generally soundly based. However, there is a need for further research where manures are applied to cracking clay soils with underdrainage, and where rainfall soon after slurry application can increase surface runoff. The careful cycling of manures within a properly devised fertilizer plan should minimize the risk of unnecessary soil P enrichment and subsequent leaching losses by restricting topsoil extractable P levels to less than 70 mgl(-1).