v1.3 (2026-08-23) · Standard Corn-Belt fertility recommendations from Iowa State University Extension. Categories: Very Low / Low / Optimum / High / Very High. P/K are well-supported; S/Zn are documented but secondary; Mg/B are not in PM 1688 — use Tri-State or Penn State for those. This is a soil-fertility recommendation source; it is intentionally NOT a nutrient management plan (NMP) and does not encode state P-Index, NRCS 590, or manure application limits.
Build-up to target. Zone Forge computes this source's supported nutrient rates in-app and cites the published method. Its published lime method is computed in-app too.
Yes — Zone Forge's calculators are free and need no signup. This page shows the states and crops Iowa State University Extension — PM 1688: A General Guide for Crop Nutrient and Limestone Recommendations in Iowa covers so you can verify the source behind your recommendation.
Browse the full Coverage Explorer — every state, crop and source — or see the science →