Tillage Impacts on Edge-of-Field Phosphorus Loss.

Minimizing phosphorus loss from agricultural fields is key to productivity and protection of water resources.    Often, no-till practices are promoted as a best management practice to reduce phosphorus losses.  While no-till practices can reduce losses of sediment bound phosphorus, they can also lead to phosphorus stratification within the soil profile and consequently higher dissolved phosphorus in runoff.  Eighty-six site years of edge-of-field surface runoff data from the Discovery Farms programs in Wisconsin and Minnesota was used to analyze phosphorus losses across multiple agricultural fields and tillage systems.  Discovery Farms is a farmer led effort to gather field-scale information to quantify the impact of a variety of farming enterprises.  The mission of the program is to collect water quality information under real-world conditions and provide practical, credible, and site-specific information to support better farm management decisions.  In Wisconsin and Minnesota, there are two main time periods for phosphorus loss: during snowmelt and early spring through crop canopy.  The drivers of phosphorus loss during those two time periods are different.  During snowmelt, most of the phosphorus loss is in the dissolved form and fields with stratified soil phosphorus levels generally have higher losses. On average, dissolved phosphorus is 67% of total P loss during this time period.  During early spring to crop canopy, most of the phosphorus loss is in the particulate form and fields with high soil losses have higher phosphorus losses.  Minimizing phosphorus loss from agricultural fields is a balance between appropriate tillage to minimize the movement of soil bound phosphorus and incorporation of fertilizer phosphorus to reduce phosphorus stratification in the soil and consequently phosphorus losses during snowmelt.  Finding ways to incorporate tillage practices into a farming system or rotation can be beneficial in reducing phosphorus impacts to surface waters.