Fall Soil Nitrate Testing

Why is soil nitrate part of basic soil test packages at some labs and not at others? The answer is based on the primary geography serviced by the laboratory. While nitrate tests can be added to basic soil tests performed on fall collected soil samples, they may be of little value in some geographies.

Post season nitrate soil tests can be valuable in evaluating nitrogen management plans to determine if excess levels of nitrate remain after the crop is harvested. These results and interpretations are highly impacted by the weather.

In arid regions of the country that receive less then 12-14” of annual rainfall, the retention of meaningful nitrate from one season to the next is common. In regions that receive 20-30” of annual rainfall, very little if any nitrate is carried to the following growing season. This additional rainfall can leach soil nitrate below the root zone or cause soil saturation leading to denitrification.

Elevated rainfall is the reason that in the eastern corn belt crop nitrogen recommendations are not traditionally based nitrate tests unless there is manure applied within the cropping system. The conversion of organic nitrogen or ammonium nitrogen contained in the manure to nitrate can slow nitrate movement and loss that can have an impact on nitrogen application rates. To maximize the effectiveness of this concept in regions of elevated rainfall, soil samples for nitrate analysis should be collected shortly before additional nitrogen is applied in the growing season, not the fall before.

Secondly most nitrate test interpretations are based on a 0-12” or 12-24” soil sample, not the 0-6” or 0-8” samples traditionally collected for fall nutrient, organic matter, and soil pH. While there are versions of nitrate interpretations that convert shallower samples to a 0-12” equivalent, the conversion assumes that the soil concentration of nitrate is uniform through the sampling depth. This is likely not true in most situations. At ALGL we recommend collecting separate samples for nitrate tests to avoid the sample depth issue.

If you have any questions about adding soil nitrate analysis to your routine soil samples, reach out to you ALGL regional agronomist.