By Steve Culman, PhD, Assistant Professor and State Specialist in Soil Fertility, School of Environment and Natural Resources, Ohio State University

A new fact sheet from Ohio State University highlights eight practices that can be used to reduce edge of field P losses while maintaining soils for increased crop production. Summarized below, the entire fact sheet can be found here.

Photo: Department of Food, Agricultural and Biological Engineering, The Ohio State University.

1. Control erosion

2. Identify surface inlets to tile and use appropriate practices to reduce surface losses

3. Consider ground and weather conditions prior to application of fertilizer and manure

4. Take a representative soil test

5. Use soil test as screening tool to meet crop production and water quality goals

6. STP value of 40 PPM Mehlich III or less:

To reduce risk of crop yield losses:

  • A STP 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 STP to critical level are available.
  • The maintenance STP range is 20–40 PPM. 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.

To reduce P loss risk following runoff-producing rainfall:

  • 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 percent chance of more than 1 inch of rainfall.
  • Time organic nutrient applications so predicted rainfall 24 hours after application is less than a 50 percent chance of more than 0.5 inches of rainfall.
  • Subsurface placement reduces losses over surface application. 

7. If you have an STP value above 40 PPM Mehlich III:

Response to fertilizer with STP greater than 40 PPM is highly unlikely, therefore:

  • 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.

The risk of P loss increases with STP values over 40 PPM, with greater risk after 150 PPM, therefore:

  • Do not apply additional fertilizer: due to lack of economic return.
  • 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