Evaluation of soluble phosphorus loading from manure-applied fields under various spreading strategies

A simple model was developed and applied to a dairy farm in the New York City (NYC) water supply watershed to evaluate the effectiveness of various manure spreading strategies for reducing non-point source, soluble phosphorus (SP) pollution. Phosphorus from manure-spread fields is recognized as one of the important non-point source pollutants in the region and there is acute interest in developing economically viable water quality management practices. The NYC watershed initiative, i.e. the Watershed Agriculture Program (WAP), mandated that water quality management practices would be scientifically justifiable based on the best information available (Walter and Walter, 1999). Thus, this project was carried-out to evaluate manure-handling strategies based on the currently available information. The model for predicting SP loading to perennial streams via surface runoff was developed by combining a mechanistic hydrological model with an empirical relationship for SP concentration in runoff. This study showed that, in the short term, because of soil P accumulation associated with a history of dairy farming, the maximum possible reduction in SP loading to perennial streams is about 50%. Exporting all manure from the NYC watersheds attains this. Utilizing the concept of hydrologically sensitive areas (Walter et al. 2000), this study suggests possible SP loading reductions of 25% with all manure remaining on-farm. This study supports and emphasizes the finding by Walter et al. (2000) that the timing and location of manure spreading strongly influences SP transport.