Iowa State University Extension agronomists have advanced soil-testing research and revised recommendations for phosphorus, potassium and lime.
“Field research is conducted continuously to assure that nutrient management suggestions are up to date,” says Antonio Mallarino, an Extension agronomist and professor of agronomy. “This research has indicated some recommendations should not be changed, but other recommendations needed significant change to optimize nutrient management in order to improve the profitability and sustainability of crop production.”
Mallarino and John Sawyer, agronomy professor and Extension agronomist, have updated the Extension publication, “A General Guide for Crop Nutrient and Limestone Recommendations in Iowa” (PM 1688). It is available online for free at: https://store.extension.iastate.edu/ItemDetail.aspx?ProductID=5232 
Mallarino says the most significant changes are:
• Include interpretations for the new moist- and slurry-based test for potassium (K);
• Changes to soil-test interpretations categories for K using dried soil samples;
• Adjustments to both crop nutrient concentrations and default crop yields needed to estimate nutrient removal for maintaining soil-test levels in the optimum category; and
• Discontinued using the P and K subsoil categories for interpretations.
Mallarino indicates recent research showed the moist- and slurry-test for K is more reliable at assessing K fertilization needs of crops than the commonly used test based on dried soil samples, even with the improved interpretations for the dry test.
The agronomists maintained many of the current recommendations farmers use to determine soil fertility, including:
• The general concept of phosphorus (P) and potassium (K) recommendations are for long-term profitability and reduced risk of yield loss, by emphasizing crop response-based applications for the very low and low soil test classes, and removal-based maintenance based on estimated crop removal with harvest for the optimum soil test class.
• Interpretation categories for current tests, such as Bray-P1, the colorimetric version of the Mehlich-3 test and the ICP (inductively-coupled plasma).
• Amounts of P and K recommended for grain production in the very low and low soil test interpretation categories.
• The soil pH considered sufficient for crops.
• Interpretations for micronutrients, which currently include only recommendations for zinc (Zn) in corn or sorghum.
Ongoing research studying several micronutrients for corn and soybean has not been completed.