Late spring nitrate test in 2026: An opportunity to reduce nitrogen costs?

Residual soil nitrogen levels last year were relatively high and extremely variable from field to field. That means the Late Spring Nitrate Test (LSNT) can generate a large return on investment for in-season nitrogen decision-making this year. For those making in-season nitrogen applications, there may be significant opportunity for savings on fertilizer inputs.

Spring soil sampling throughout Iowa — conducted by the Iowa Nitrogen Initiative — demonstrates that soil nitrate levels remain highly variable across fields. In the top two feet of soil, the median soil nitrate level in fields that had not received any fertilizer inputs was 28 lbs N/acre, most of which will be directly available to this year's crop. The median LSNT value from these fields was 4 ppm; the 25th percentile was 0 ppm and the 75th percentile was 10 ppm. The top 25% of fields had values from 11-25 ppm!

The practical significance of this variability for in-season nitrogen application is large. Across the middle half of fields sampled — those between the 25th and 75th percentile — LSNT values ranged from 0 to 10 ppm. This difference alone indicates that in-season nitrogen rate can be reduced by more than 1/3, or approximately 55 lbs N/acre, in higher-testing fields compared to lower-testing ones. In the highest quartile where LSNT values were greater than 10, the savings are significantly larger. This represents substantial potential for cost savings — but only if you know where your fields fall.

Moreover, these values likely underestimate what the corresponding LSNT will be for these fields compared to the recommended LSNT sample timing closer to sidedress because warming soils will promote nitrogen mineralization. See CROP 3140. Also, above-average rainfall between April 1 and the time of sampling can move nitrate from the top foot of soil to the second foot of soil where it is not measured by the test but still available to the crop; in this case, CROP 3140 recommends adding 3-5 ppm to your LSNT value.

Iowa State University encourages soil nitrate testing prior to nitrogen fertilizer application. The Late Spring Nitrate Test — N Decision Response Tool can help personalize fertilizer decisions based on your individual risk tolerance and soil test results. Although spring soil nitrate levels are a major factor affecting nitrogen fertilizer needs, other factors are at play; the Iowa Nitrogen Initiative’s N-FACT decision-support tool allows users to explore how planting date, May precipitation, and residual soil nitrate interact to affect optimum rates. For example, later planting and a drier May substantially reduce nitrogen needs. A user manual is available to guide interpretation.