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 elevated soil solution levels are exposed to loss through tile or surface water with water movement. Edge-of-field testing has shown Dissolved Reactive Phosphorus (DRP) concentrations of 0.02 to 0.4 PPM and 0.1 to 2 lbs of DRP load per acre. Agronomic crop needs are met by current soil-available nutrients and additional nutrient are not needed.
Continued phosphorus application at this STP level on livestock farms would be due to the use of application timing to use manure-nutrient content to replace purchased fertilizer, limited land base, or emergency application due to weather or storage capacity concerns.
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.
Fertilizer rate determination and application practices
- Applying additional commercial fertilizer sourced P does not show an economic return when STP is between 50 and 120 PPM. Manure maybe used but at overall rates that drawdown STP levels. Soil test results should be monitored to document the reduction in STP. Changes 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 estimated crop removal is likely the approximate losses occuring into offsite water movement.
- 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
- Develop a nutrient management plan that reviews soil test levels, yield goals, and the 4R’s of 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 plus evaluate field nitrogen and phosphorus risk of loss potential.
- Identify lower testing field for application of organic nutrients if at all possible
- Apply no more than a 2 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 974 Fertilizer Calculator for Ohio Spreadsheet
- Use variable rate application where spatial variation results in a range of suggested nutrient application rates
- Monitor beginning and ending soil test levels, production and fertilizer added for adaptive fertilizer management.
- 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
- Before planning manure or other organic nutrient sources are applied to field with 50-120 PPM Mehlich III soil test consult:
Assessing Nutrient Loss Risk in Ohio
Overview of Assessing Nutrient Loss Risk in Ohio criteria for organic nutrient applications at "Moderate" P loss risk assessments is outlined below:
STP Mehlich III Phosphorus Loss Risk Assessment Rating |
Rate of application |
STP Management Strategy | Ground Cover at Application | Sensitive area avoidance | Timing of application | Erosion |
50-120 PPM Moderate |
< P removal for annual or multiple year crop rotation with no more than 250 lb P2O5/ac applied annually |
Adaptive P Management with plan Drawdown of STP over time | >30% or Growing Crop/Cover Crop |
Follow setback and vegetative recommendations (Table 3 of Assessment Document) | Spring application should be made only if applied to a growing crop or cover crop | Sheet and rill erosion managed at < Soil Loss Tolerance and no active concentrated flow erosion (gullies) |
- 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
- Field sites at 50-120 PPM STP should be evaluated for additional BMP practices.