By Greg LaBarge, CPAg/CCA, Ohio State University Extension
A new factsheet highlights eight steps to reducing edge of field P losses while maintain soils for increase crop production. The Phosphorus Nutrient Management for Yield and Reduced P Loss at Edge of Field-AGF-509 (https://ohioline.osu.edu/factsheet/agf-509) highlight practices that can be used to reduce edge of field losses of P. There are eight field specific steps to considered.
• A soil test P value of 20 PPM defines the critical level. At 20 PPM or less, the risk of yield loss increases. Annual in-season P nutrient application recommended. Recommendations to build soil test P value to critical level are available.
• The maintenance STP range is 20 to 40 PPM. The recommended rate of P is equal to P removed in harvested crop. Annual application is not required. Risk of yield loss is low with flexibility to delay application one (or more) growing seasons, if needed.
The greatest risk for event P losses occur when nutrient application is followed by runoff-producing rainfall. Use the following steps to reduce P loss risk at application for water quality.
• Use an agronomic rate of no more than two years’ worth crop removal as recommended in the Tri-State Fertilizer Recommendations.
• Time fertilizer applications so predicted rainfall 12 hours after application is less than a 50% chance of more than 1 inch of rainfall.
• Time organic nutrient applications so predicted rainfall 24 hours after application is less than a 50% chance of more than 0.5 inches of rainfall.
• Subsurface placement reduces losses over surface application.
7. With a soil test P value above 40 PPM Mehlich III, response to fertilizer is highly unlikely for improving crop yields.
• Do not apply additional fertilizer due to lack of economic return.
• Organic applications should be determined using P content from testing the organic source and not exceed P crop removal rate for the next two (and no more than three) crops in the rotation.
• Consider in-crop application to a nitrogen-using crop to reduce purchased N, maximizing economic return to organic application.
• Where in-crop application is not used, incorporate organic nutrients.
In terms of water quality, the risk of P loss increases with STP values over 40 PPM, with greater risk after 150 PPM. Do not apply additional fertilizer due to lack of economic return and do not make organic applications when STP is greater than 150 PPM.
Follow NRCS 590 site and rate criteria between 40 to 150 PPM. Time organic applications so predicted rainfall 24 hours after application is less than a 50 percent chance of more than 0.5 inches of rainfall.
Use the Ohio P risk index, field scale hydrology/water quality models, or monitoring to evaluate site risk for P losses and need for further site BMPs. See step 8.
8. BMPs for high risk P loss sites: Monitor fields for P loss using the Ohio P risk index and field scale hydrologic and water quality models. For sites with predicted high P losses, consider one or more of the following water management practices based on fitness and cost effectiveness for field site:
• Production practices that increase soil organic matter to retain water on site
• Drainage Water Management (NRCS 554)
• Nutrient removal wetlands (NRCS 656)
• P Precipitating Filter
• Saturated Buffer (NRCS 604)
• In-field water storage or detention basins.