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Iowa State University Extension Phosphorus and Potassium Soil-Test Interpretation Guidelines Have Changed

Antonio Mallarino, Professor and Nutrient Management Extension Specialist, Department of Agronomy, Iowa State University
March 1, 2023

Guidelines for soil-test phosphorus (P) and potassium (K) interpretations and application rates were last updated in 2013. Since then, significant field research was conducted to assure that nutrient management guidelines are kept current. The new response data, increased crop yield levels in Iowa, and better awareness of usually large spatial soil-test variability in production fields indicated a need for adjusting the boundaries of soil-test interpretation categories and suggested nutrient application rates. The updated publication PM 1688 has been posted for free download at the Iowa State University Extension Publications Store and the Soil Fertility websites. This is a brief overview of major changes made and what remains unchanged based on the available research data.

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