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Manure and litter management currently uses some form of phosphorus (P) indicator to determine the correct amount of fertilizer and/or manure to be applied to a field. In a previous study a modified version of the Soil and Water Assessment Tool (SWAT) that allows manure application rates and area to change dynamically within a simulation as a function of soluble soil P concentration was validated.

In this study the calibrated modified SWAT was used to assess the effect of two different dairy waste application field (WAF) manure application schemes on water quality, soluble soil P concentrations, and WAF area, compared to a 30-year baseline of manure applied to WAFs at the nitrogen (N) rate in the upper North Bosque River watershed (UNBRW) in central Texas.

During a 30-year simulation, application at the P rate reduced total P (TP) from the watershed by over 25 percent compared to the baseline. Both scenarios increased nitrate loading from the watershed substaintially. However, the increase in total nitrogen for each scenario was less than 16 percent. The P rate scenarios kept all WAF soils below the critical 200 ppm soluble soil P concentration (with average soluble soil P concentrations below 100 ppm), while not leading to excessive increases in WAF area nor manure stockpiling.