A soil test that includes a value for Soil Test Phosphorus (STP) is useful to discuss both the risk of yield loss in crop management and as an indicator for risk of environmental loss of P.
In crop management, fields with an STP level of less than 20 PPM (40 lbs/A) Mehlich III are at the greatest risk of yield loss due to a deficiency of phosphorus in a corn/soybean rotation. If the crop rotation includes wheat or alfalfa, the STP value with greatest risk of yield loss is STP less than 30 PPM (60 lbs/A) Mehlich III. This STP value where soil test below that value has the greatest risk of yield loss is defined as the “critical level”.
With STP less than the "critical level" an annual application of fertilizer at the crop removal rate to reduce risk of yield reduction. Another consideration with STP less than the "critical level" is to apply additional fertilizer at rates more than crop removal to build up STP to the "critical level" with the benefit being to provide flexibility for future applications.
Soils in the range from above the "critical level" to 50 PPM is called the "maintenance range". To maintain STP in the "maintenance range" periodic application of crop removal rates of fertilizer. A maintenance application can be applied any time during the crop rotation at the discretion of the farmer.
Once STP level reach 50 PPM, no additional fertilizer application is suggested since there is no agronomic benefit for commonly grown grain, oilseed and forage crops.
The chart in Figure 1 represents the described recommendatin framework. More Information can be found in the Tri-State Fertilizer Recommendations for Corn, Soybean, Wheat and Alfalfa, (2020) Bulletin 974 which should be used as a guide for developing phosphorus recommendations.
Figure 1. Tri-State Phosphorous and Potassium Recommendation Framework.
From an environmental perspective, Soil test phosphorus (STP) is a good indicator for the potential risk of phosphorus movement at the edge-of-field into downstream waters. STP is a quick and widely available measure that can be used to identify field level P loss risk potential. Actual measured phosphorus losses from individual fields will be impacted by agronomic management, soil conditions and water movement resulting in higher, or lower, losses than would be predicted from soil test alone.
Fields with STP below 50 PPM (100 lbs/A) Mehlich III will generally produce low levels of phosphorus runoff in water quality sampling. Acute losses of phosphorus can be measured in water samples leaving a site can occur at the time of fertilizer application, especially with surface applications.
Fertilizer rate determination and application practices
- Apply P at crop removal when STP is at "critical level" up to 40-50 PPM. When STP is less than "critical level", apply crop removal rate annually or crop removal rate plus buildup recommendations if increasing STP is desired. Increases in STP happen slowly. For each 20 pounds of P2O5 above crop removal is estimated to increase STP 1 PPM. Tri-State Fertilizer Recommendations for Corn, Soybean, Wheat and Alfalfa, (2020) Bulletin 974
- Soil test once per rotation or at least every four years. A single sample should not represent more than 25 acres. Use variable rate application based on precision soil sampling where past practices or inherent soil characteristics can lead to variable nutrient levels across a field. Soil Sampling to Develop Nutrient Recommendations Developing a Strategy for Precision Soil Sampling
- Develop a nutrient management plan that reviews soil test levels, yield potential, and the 4R nutrient stewardship (the right rate, right placement, right timing, and right source of nutrients to maximize crop yield and minimize environmental impacts) to meet crop nutrient needs. Monitor beginning and ending soil test levels, production and fertilizer added for adaptive fertilizer management.
- Apply no more than a two year rate of nutrient need based on soil test utilizing land grant university recommendations Developing Phosphorus and Potassium Recommendations for Field Crops Tri-State Fertilizer Recommendations for Corn, Soybean, Wheat and Alfalfa, (2020) Bulletin 974Fertilizer Calculator for Ohio Spreadsheet
- Losses from surface applications should be avoided. Consider risk of surface applications of nutrients and potential of rainfall after application
- If nutrient is surface applied, review before application predicted precipitation for the 12 hours after application, if greater than 50% chance of one inch of precipitation will occur then delay application or use exemptions of applied to cover crop or incorporated prior to rainfall. This is a required practice for nitrogen and phosphorus fertilizer application in the Western Lake Erie Basin watershed codes as designated in Ohio Revised Code
- Nutrients from fertilizer or manure should not be applied to frozen or snow-covered soils except where appropriate cover or other exemptions allow. This is a required practice for nitrogen and phosphorus fertilizer application in the Western Lake Erie Basin watershed codes as designated in Ohio Revised Code
Use of manure
- Fields at less than 50 PPM STP are ideal for manure application since agronomic utilization phosphorus and potassium can be maximized for crop need and soil test maintenance. Consider risk of erosion losses that might result from application method used.
- If manure is surface applied, before application review predicted precipitation for the 24 hours after application, if greater than 50% chance of 0.5 inch of precipitation will occur then delay application or use exemptions of applied to cover crop or incorporated prior to rainfall. This is a required practice for nitrogen and phosphorus fertilizer application in the Western Lake Erie Basin watershed codes as designated in Ohio Revised Code
- Nutrients from fertilizer or manure should not be applied to frozen or snow-covered soils except where appropriate cover or other exemptions allow. This is a required practice for nitrogen and phosphorus fertilizer application in the Western Lake Erie Basin watershed codes as designated in Ohio Revised Code
Assessing Nutrient Loss Risk in Ohio
- Evaluate field nitrogen and phosphorus risk of loss potential. A proceedure for assessing field conditions for nutrient loss is provide in the guide linked below:
Assessing Nutrient Loss Risk in Ohio