When manure is applied to supply crop nutrients, the application rate is based primarily on the nitrogen (N) content. As a result, the situation arises where manure as applied
contributes more phosphorus (P) to the soil than is required by the crop. This has a potential environmental consequence: runoff and erosion may eutrophy streams and
ponds. Few analytical tools are available to facilitate the establishment of application limits for P on agricultural land. The task is complex because the impact is usually
downstream of the farmland and can be episodic in nature. A modeling approach is best suited in order to integrate all of the factors. The Nutrient Loading Model (NLM) is a simple representation of the environment as a topsoil compartment, a subsoil compartment and a water compartment. Nutrient, both background (historic) and applied, is lost from the topsoil by leaching, erosion and crop removal. In the water compartment, the model includes processes of sedimentation to the stream bed and flushing. When reasonable input parameter values are used, representative of Manitoba farmland, the model indicates that over an average year, P contributions to water result in acceptable water concentrations. However, after severe storm events, the water P concentrations will be markedly higher than the annual average and the potential for impact is greater. Present soil fertility recommendations may lead to decreased soil P with time, but the current guidelines for manure application may result in accumulation of P in the soil. This accumulation coupled with periodic severe storm events has a significant potential to result in impacts such as eutrophication. The NLM can compute estimates of impacts following specific inputs (forward computation), but is intended to provide reverse computation, where P loading limits are defined based on the
criteria for acceptable water concentrations.