Soil Test P value less than 50 PPM (or "Lower" P Risk Assessment)

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.

Tri-State 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

Use of manure

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

Best Management Practices