Fall is the perfect time to review the Phosphorus Loss Risk Assessment document for determining P application rates

Fall is a busy time of year for many of us involved in agriculture; from harvest to cover crop seeding, to wheat planting and fertilizer application, we have a ton of tasks to get done in a short window of time. But with the hectic nature of the season comes an opportunity to refresh ourselves with the recommendations for Phosphorus application rates. The NRCS Ohio Nutrient Management Technical Note, “Assessing Nutrient Loss Risk in Ohio” contains information and guidance for determining P application rates based on soil test Phosphorus levels. The document also contains information for determining risk for Nitrogen loss, as well as manure application guidelines for different application methods. The chart below serves as guidance for risk level and P application rate for four classes of soils: Lower risk, Moderate risk, Higher risk, and Very High risk.

It is important to note the third row of the chart, “STP Management Strategy”. This row tells us how we should be managing the levels of P in our soils, whether we should be using our crops to pull excess P out of the soil, or whether our soils have enough storage for a multi-year P application.

According to the Tri-State Fertilizer Recommendations, we should be applying an agronomically-dictated rate of P, or less, if soil test P levels are less than 50 ppm Mehlich-3. If our soil test P levels are between 50 and 120 ppm Mehlich-3, we should be applying at maximum the amount of P that our crops will remove from the soil – whether that is a single or multi-year crop rotation in your operation. At this level, we need to ensure we are seeing a reduction in soil test P levels over time to ensure we are not losing P from our fields. By applying just enough or slightly less than the amount of P that our crop naturally removes from the soil, we will begin to see a reduction in soil test P levels over time as the crop begins to take up P from the soil. Beyond this soil test P range, we start to move into the higher and very high risk soil test P levels.

From 120-200 ppm Mehlich-3 P, we want to focus on removing excess soil test P, by applying, at maximum, a rate less than 50% of the P removal rate for your annual crop or multi-year crop rotation. By applying an even lower rate of chemical fertilizer to our crops, this will ensure the crop pulls more P from the soil, slowly lowering the soil test P level over the next few years.  Fields that fall into the very high risk category, containing over 200 ppm Mehlich-3 P, have a much higher potential to negatively impact water quality through nutrient losses. In this case, we do not want to apply any P at all. At a soil test P level above 200 ppm, there is ample P for the crop to take up and utilize, and a drawdown approach should be utilized to reduce the soil test P level over time.

Because even a small amount of Phosphorus can have a large, negative impact on water quality, it is important to understand soil test P levels in our fields. Regular soil testing can provide a snapshot of how much nutrient is contained in a field, which will not only help protect water quality, but can save producers money. In situations where soil test nutrient levels are high enough to reduce application rates, the bottom line can see quite a benefit as well.

For any questions about changing your N or P application rates, soil testing, or water quality, contact Rachel Cochran, Water Quality Extension Associate for Paulding, Defiance, and Van Wert Counties at (567) 344-5016 or cochran.474@osu.edu, or visit waterqualityextension.osu.edu.

Information taken from the USDA-NRCS Technical Note “Assessing Nutrient Loss Risk in Ohio”, published November 2020.

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