What happens to phosphorus in the soil?

Continual long-term application of manure at levels exceeding crop utilization will increase soil P levels. In many situations, dairy manure application has been based both on material handling demands and crop nutrient supply considerations. Contemporary concerns about the impacts of nutrient management practices on water quality have shifted the focus of applications to balancing for the N needs of the crop in the year of manure application. Minimizing the purchase of commercial fertilizer N and reducing the risk of nitrate leaching have been the basis of these recommendations. However, repeated applications at N-based rates can result in a build up of soil test P above amounts sufficient for optimal crop yields, due to the generally lower ratio of N:P added in dairy manure (5:1) than taken up by major grain and hay crops (8:1).

The difference in this ratio is aggravated by differential availability of manure N and P during the year of application. Dairy manure N is rarely more than 50% as available as fertilizer N, while manure P availability approaches that of fertilizer P. Consequently, even more manure is required to supply the crop N needs, increasing the collateral applications of P. Some nutrient recommendations encourage the application of manure only at rates that will supply the P needed throughout the crop rotation in a field in order to minimizes the build-up of P. This will often require greater investment in manure application to cover more area in a given year and more total area for manure application than for N-based application.