Does Maintaining Soil Test Phosphorus Require Application Based on Crop Removal?

October 4, 2023

Early-season phosphorus deficiency on corn at the V3 growth stage in southwestern Minnesota. Photo by University of Minnesota Ex- tension Nutrient Management.

Phosphorus fertilizer recommendations generally follow one of two approaches. In the “build and maintain” approach, farmers measure soil test phosphorous (STP), build P levels to a specific target, and then maintain it by applying P to compensate for what crops take up. Farmers taking the “sufficiency” approach, in contrast, test the soil to determine how much P the crop needs to maximize yield but don’t worry about maintaining specific soil test values.

Researchers examined the fate of applied P and changes in STP values over time for the two approaches. They established various STP levels, monitored annually STP and the amount of P removed in harvested grain, and assessed changes in various P fractions in the soil. At four of six sites, initial STP was maintained with a negative net P (less P was applied than was removed in grain). At those sites, applying removal‐based P rates increased STP over time. There was also a significant correlation between changes in net P and inorganic P fractions. Most change occurred in more plant‐available P fractions, accounting for 66 to 100% of the change in net P. There was little, if any, effect of surplus application of P on the organic P fractions or on less‐available inorganic P fractions.

Adapted from Sims, A. L., Fabrizzi, K. P., Kaiser, D. E., Rosen, C. J., Vetsch, J. A., Strock, J. S., Lamb, J. A., & Farmaha, B. S. (2023). Soil phosphorus balance in Minnesota soils and its effects on soil test phosphorus and soil phosphorus fractions. Soil Science Society of America Journal, 87, 918–931. https://doi.org/10.1002/saj2.20549

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