Causes of Dissolved Phosphorus Loss on Discovery Farms in Wisconsin and Minnesota.

Runoff of nutrients and sediment in agricultural landscapes continues to be a water quality problem across many areas. While curbing particulate phosphorus (P) loss can be done through conservation practices, controlling dissolved P loss is more difficult. Nutrient stewardship emphasizes the balance of rate, timing, placement and method. In order to further our understanding of phosphorus stewardship, it is important to understand the impacts that timing of application and soil test P levels have on dissolved phosphorus loss to surface waters. The University of Wisconsin Discovery Farms Program and Discovery Farms Minnesota work with farmers in each state to monitoring water quality and assess soil management practices for each farm. Records from 20 monitored fields on 17 different farms have been examined to assess the soil test P levels, phosphorus application timing and dissolved phosphorus loss at the water monitoring stations. Several years of data have shown that manure application shortly before runoff can increase annual phosphorus loss by two to four times. Analyzed data has also determined that soil test values of phosphorus influence dissolved phosphorus loss. For implementation of phosphorus stewardship, farmers and resource managers must understand the importance of controllable factors like soil test phosphorus and application timing.