Soil Test P value greater than 120 PPM (or P Risk Assessment of "High" to "Very High")

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.

As STP values increase, the higher soil solution P concentrations are exposed to water moving offsite in tile or surface flows. Fields at 120 PPM (240 lbs/A) Mehlich III or higher tend to have annual edge-of-field Dissolved Reactive Phosphorus (DRP) losses of 2 to 5 lbs per acre. Tile water DRP concentrations will be 0.08 to 1.5 PPM. Target DRP concentrations for receiving water such as lakes are often around 0.05 PPM. Fields with STP >120 PPM likely have a history of manure/biosolids application or were planted to high-value crops, such as vegetables, with higher STP needs.

The best management practice on fields with STP values exceeding 120 PPP Mehlich III is to apply no additional phosphorus, drawing down STP over time. Drawing down STP happens slowly. Annual STP changes from crop harvest with out nutrient additions will only fall 2-4 PPM. There is no agronomic benefit associated with maintaining STP greater than 50 PPM. Individual livestock operations may have the need to apply manure to fields greater than 120 PPM due to weather conditions affecting fields available for manure application or limited storage. Some crop benefits of occasional manure application due to other nutrient benefits from manure that enhance yield and/or improve overall nutrient utilization are observed. These applications need to consider the risk of edge-of-field phosphorus losses along with any benefits.

Farmers and other professionals should closely watch soil test reports for elevated STP levels. If elevated phosphorus levels exist, the field should be evaluated for potential phosphorus contributions to see what BMPs would benefit water quality. The best tool to evaluate phosphorus losses is collecting water samples for analysis. Water sampling results can identify if water retention or filtering practices at edge-of-field would be cost effective. Some potential water management and filtering practices are highlighted at the end of this section

Fertilizer rate determination and application practices

  • Apply no additional applications of P when STP > 200 PPM. Limited applications of manure at 5-% of crop removal can be made between 120 PPM and 200 PPM and the trend of lower STP levels is identified in future soil test results. Reductions in STP happen slowly. At current crop yields, nutrient removed will only be expected to move 2-4 PPM annually. Any additional reduction in STP above this crop removal is likely approximate losses in offsite water movement.  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. Where past practices or inherent soil characteristics can lead to variable nutrient levels across a field, using precision soil sampling. Soil Sampling to Develop Nutrient Recommendations Developing a Strategy for Precision Soil Sampling
  • Identify lower testing field for application of organic nutrients.
  • Use variable rate application where spatial variation results in a range of suggested nutrient application rates to avoid applications if only areas of the field are at these elevated soil test levels.
  • Monitor beginning and ending soil test levels, production and fertilizer added for adaptive fertilizer management.

Before planning manure or other organic nutrient sources are applied to field > 120 PPM consult:

Assessing Nutrient Loss Risk in Ohio (NRCS)

Overview of Assessing Nutrient Loss Risk in Ohio criteria for organic nutrient applications at "High" and "Very High" P loss risk  assessments is outlined below:

Recommendations for both "High" and" Very High" risk:

  • Sensitive area avoidance: Follow setback and vegetative recommendations (Table 3 of Assessment Document)
  • Timing of application: Spring application should be made only if applied to a growing crop or cover crop
  • Erosion: Sheet and rill erosion managed at < Soil Loss Tolerance and no active concentrated flow erosion (gullies)

Specific recommendations based on risk:

STP  Mehlich III (PPM)

Phosphorus Loss Risk Assessment Rating

Rate of application

STP Management Strategy Ground cover at Application


Higher Risk

< 50% of P2O5 for planned rotation, limited to an annual application of no more than 125 lbs P2O5.

Short term P application to facilitate changes to manure storage or land access.

Monitor STP. Application rate should result in lowering STP. Do not continue to build STP.

If Surface Applied >50% ground cover or a growing crop or cover crop.

Incorporation is required if minimum ground cover not met for surface application.


Very High Risk


No P application

Exception for dilute water applied through irrigation. Annual rate lesser of <50% crop removal or 35 pounds P2O5 annual

Drawdown STP overtime

If Surface Applied >50% ground cover or a growing crop or cover crop.

Incorporation is required if minimum ground cover not met for surface application.


Best Management Pratices