By Steve Culman, Elizabeth Dayton, Kevin King and Greg LaBarge
Looking at current field research and the literature available on the topic of minimizing losses of phosphorus at the edge of the field, the following recommendations are a starting point to maximizing productivity while minimizing environmental impacts on water quality.
Phosphorous Rate, Application and Timing
Avoid overloading soils. Soil test and follow tri-state fertilizer recommendations. Where soil test levels are above 40 ppm Bray P1 or 58 ppm Mehlich III-ICP, do not apply additional phosphorus in the corn-soybean rotation. These soil test levels require no additional fertilizer, according to the Tri-State Fertilizer recommendations. Fertilizing soils testing above these levels increases risk of phosphorus in runoff and tile drainage.
Avoid winter application. Eliminate surface application of manure or fertilizer to frozen or snow-covered fields. Frozen ground is ground that is frozen to the degree that tillage is not possible. Surface-applied manure or fertilizer is subject to runoff events that may occur before the ground thaws and allows nutrients to bind to soil.
Avoid surface application of fertilizer/manure. Surface applications of phosphorus are subject to higher loss if runoff producing rainfall events happen close to application. Placement of nutrient below the surface of the soil reduces loss. New placement tools or strategies need to be implemented that place phosphorus below the surface with minimal soil disturbance. Until these tools become available, use banded application.
Farm and Field Features
Minimize erosion. Appropriate conservation practices should be implemented to minimize erosion. Maintain 30% cover as crop residue/cover crop. Filter strips, grassed waterways and water diversion structures are appropriate tools.
Slow the movement of water. Surface water flows from fields directed to tile via standpipes should be converted to blind inlets. As risk loss potential increases for a field consideration should be given for edge of field treatments which control water movement or treat water as it is leaving the site. Drainage-water-management-control structures in ditch treatments, such as two-stage ditches and other stream practices, can reduce loading.
Know your field’s risk. Soil test phosphorus. Field proximity to water and soil hydrologic class impacts edge-of-field losses of phosphorus. The NRCS Ohio Phosphorus Risk index provides a risk of loss index and should be used as part of the development of a Nutrient Management Plan to assess the individual field risk.
Strive to build soil quality. Soil condition is a mitigating factor. Increasing the water infiltration by reducing compaction and improving soil structure increases water retention, nutrient cycling, crop rooting capacity and crop yield.