Iowa State urges late-spring nitrate testing as residual soil nitrogen runs high and variable

Iowa State University is urging corn growers to run the Late Spring Nitrate Test (LSNT) before sidedressing this season, after statewide soil sampling found residual nitrogen levels running high and unusually variable from field to field — a combination that could let many farmers cut nitrogen rates and costs.

Sampling by the Iowa Nitrogen Initiative found that in the top two feet of soil, fields that had received no fertilizer carried a median 28 lbs of nitrogen per acre, most of it available to this year’s crop, the university’s extension agronomists said in a May 29 advisory. Median LSNT readings were 4 ppm, ranging from 0 ppm at the 25th percentile to 10 ppm at the 75th, with the top quartile testing 11–25 ppm.

That spread has real cost implications. Across the middle half of fields, the variation alone implies in-season nitrogen rates can be cut by more than a third — roughly 55 lbs N per acre — in higher-testing fields versus lower-testing ones, with larger savings in the top quartile.

The advisory noted that the figures likely understate nitrate that will be available by sidedress timing, as warming soils drive further nitrogen mineralization. Above-average spring rainfall can also push nitrate into the second foot of soil, where it is not captured by the test but remains available to the crop.

With nitrogen input costs still elevated, the guidance underscores soil testing as a low-cost lever for trimming fertilizer spend without sacrificing yield. Iowa State pointed growers to its LSNT decision tool and the Iowa Nitrogen Initiative’s N-FACT model to tailor rates ahead of sidedressing.

Source: Iowa State University