Using Soil Tests Phosphorus Results to Identify Agronomic and Conservation Needs

“What are the right decisions for phosphorus management in crop production that reduce water quality impacts?” is a common question I have from farmers looking to improve yield yet are concerned about downstream water quality impacts of phosphorus.

A representative agronomic soil test has long been an essential tool for sound agronomic nutrient management decisions. That same agronomic test result can be a useful indicator for identify fields where additional conservation practices might improve water quality. Fields with Soil Test Phosphorus (STP) levels two to three times higher than the agronomic need result in increased phosphorus losses measured in edge of field water quality monitoring. 

As soil test results are reviewed this fall, consider keeping a list of fields in three categories based on STP levels that define risk of yield loss for the corn/soybean rotation and risk of increased water quality impacts.

  1. Less than 20 PPM Mehlich 3 STP (or 30 PPM if wheat/alfalfa in the rotation)
  2. Between 20-40 PPM (or 30-50 PPM if wheat/alfalfa are in the rotation)
  3. Greater than 50 PPM

More detailed agronomic and water quality management considerations based on these three categories are described next.

  1. Soil Test Phosphorous (STP) values are less than 20 PPM Mehlich 3 in corn/soybean rotation or 30 PPM where wheat and alfalfa are in rotation.

          CROP YIELD—Reduce risk of crop yield losses with nutrient application.

  • A STP of 20 PPM in corn/soybean (or 30 PPM where wheat/alfalfa are in the crop rotation) defines the critical STP level. STP less than critical level result in increased risk of yield loss.
  • Make banded (or bulk incorporated) fertilizer application prior to or at crop planting reduces risk of yield loss.
  • Consider applying organic nutrients (manure or biosolids).
  • Consider in-crop application of manures to a nitrogen-using crop to reduce purchased N, maximizing economic return to organic application.
  • Consider build up recommendation to increase to build STP to critical level.

          WATER QUALITY—The greatest risk for event P losses occurs when nutrient application is followed by runoff-producing rainfall. Use the following steps to reduce P loss risk at application:

  • Use an agronomic rate of no more than two years’ worth crop removal as recommended in the Tri-State Fertilizer Recommendations.
  • Subsurface placement reduces losses over surface application. 
  • 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.
  1. Soil Test Phosphorous (STP) values are in a range of 20-40 PPM Mehlich 3 in corn/soybean rotation or 30-50 PPM where wheat and alfalfa are in rotation.

          CROP YIELD— Risk of yield loss is low with flexibility to delay fertilizer application one (or more) growing seasons, if needed.

  • The maintenance STP range is 20–40 PPM for corn/soybeans or 30-50 PPM for wheat/alfalfa.
  • The recommended rate of P is equal to P removed in harvested crop. Annual application is not required.
  • Consider applying organic nutrients.
  • Consider in-crop application of manures to a nitrogen-using crop to reduce purchased N, maximizing economic return to organic application.

          WATER QUALITY—The greatest risk of P losses occurs when nutrient application is followed by runoff-producing rainfall. Use the following steps to reduce P loss risk at application:

  • Use an agronomic rate of no more than two years’ worth crop removal as recommended in the Tri-State Fertilizer Recommendations.
  • Subsurface placement reduces losses over surface application. 
  • 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.
  1. Soil Test Phosphorous (STP) values are above 50 PPM Mehlich 3.

          CROP YIELD—Response to fertilizer with STP greater than 40 PPM corn/soybean (or 50 PPM with wheat and alfalfa in rotation) is unlikely.

          WATER QUALITY—The risk of P loss increases at edge of field as STP values increase as STP values 100-150 PPM.

  • Do not apply additional fertilizer due to lack of economic return.
  • Follow NRCS 590 site and rate criteria between 40 to 150 PPM.
  • Do not make organic applications when STP is greater than 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.

STP is a good initial screening tool to identify fields where working with a conservation planner might reduce P losses. The conservation planner can confirm that the high risk actual could result in high losses or that the risk is actually lowered by other factors that affect P losses such as management already implemented or inherent field characteristics.

References:

Tri-state Fertilizer Recommendations (2020)  https://go.osu.edu/fert-recs

Phosphorus Nutrient Management for Yield and Reduced P Loss at Edge of Field AGF-509 https://ohioline.osu.edu/factsheet/agf-509

Identifying Critical Concerns and Agricultural Best Management Practices https://agbmps.osu.edu/

 

Crop Observation and Recommendation Network

C.O.R.N. Newsletter is a summary of crop observations, related information, and appropriate recommendations for Ohio crop producers and industry. C.O.R.N. Newsletter is produced by the Ohio State University Extension Agronomy Team, state specialists at The Ohio State University and the Ohio Agricultural Research and Development Center (OARDC). C.O.R.N. Newsletter questions are directed to Extension and OARDC state specialists and associates at Ohio State.