Have you ever questioned the results of the late spring soil nitrate sampling in corn? Have you ever heard someone voice skepticism about the value and reliability of this management tool? If the answers are “yes,” you’re not alone.
Despite the challenges of collecting and interpreting soil nitrate samples, the practice remains a valuable tool to guide farmers when making the decision whether to apply or not apply additional sidedress nitrogen in fields that received the majority of nitrogen as pre-plant applications, say Iowa Soybean Association On-Farm Network® researchers.
One key consideration, says On-Farm Network Director Pat Reeg, is collecting representative soil samples.
Sampling errors can result in mixed, unexpected or inconsistent results that, in turn, provide poor predictability of the soil test. Yet there are strategies for improving the reliability of data collected, he said.
“An analogy would be checking your cholesterol without fasting,” Reeg said. “To obtain accurate and representative results, you need to collect samples using proven methodologies.”
“Collecting representative samples throughout the entire area being evaluated is key to optimizing the use of nitrogen fertilizer as well as time and resources.”
Soil sampling procedures must take into consideration the chance for higher concentrations of nitrate when nitrogen is injected, knifed or banded. This can occur with applications of anhydrous ammonia, (especially spring anhydrous ammonia with a nitrification inhibitor and injected manure).
A good way to show how the injected, knifed or banded nitrogen affect soil nitrate distribution between corn rows is to use the N-WATCH™ template (see photo). Based on the sampling method done May 20, 2015, two cores are collected every 3 inches for a total of 22 cores.
Soil nitrate concentration can be relatively high if all of the cores were collected from the anhydrous ammonia band or relatively low if the ammonia band was avoided (see graph). The N-WATCH template nicely shows how sampling results can be skewed by ammonia band. The spike in nitrate concentration in the band area would decrease with the higher soil moisture and recent rainfall event close to the sidedresssing.
Some skepticism regarding soil nitrate sampling is understandable given the challenges associated with obtaining representative samples. There are numerous factors caused by:
- How commercial fertilizers or livestock manure was applied
- Timing when samples are collected (temporal) due to rapid nitrification of ammonia and movement of nitrate with the water between the corn rows
- Where samples are collected (spatial) due to field variability
Researchers recommend sampling when corn is 6-12 inches tall and collecting within several test areas that are 1-10 acres in size. Sampling should be taken to a depth of at least 12 inches and be comprised of 16-24 cores within three or more representative areas within a field.
Nitrate sampling procedures can be found in ISU extension publication PM-1714 (pdf). Specifics for interpretations of results of the late spring test can be found in this ISA publication "Predicting Reliability of the Late Spring Soil Nitrate Test" (pdf).