End-of-season stalk nitrate testing can help you measure whether your crop received adequate nitrogen. The test measures the amount of nitrogen returned to the stalk after grain fill. Therefore, if your corn stalk ends the season with adequate nitrate, then you know nitrogen sufficiently flowed from the seed, to grain and back to stalk.
Key Points
- Use stalk nitrate tests to evaluate seasonal nitrogen fertilization rates.
- Take samples soon after physiological maturity (black layer).
- Cut stalk sections between 6 and 14 inches above ground.
- Consider seasonal weather trends and yield when interpreting test levels.
When to Sample
- Sample soon after the black layer has formed in corn kernels. This black layer seals the seed off from the cob at the end of grain fill.
- To determine if the black layer has formed, gently scrape tissue from the base of the kernel to see if a black layer is exposed.
- Corn sampled too early will not have nitrogen returned to the stalk; corn sampled too late, especially after harvest, may have leached nitrate prior to sampling. Both can cause you to underestimate the nitrogen available to their crop.
How to Sample
- Sample 15 stalks per field by collecting 8-inch sections between 6 and 14 inches above the soil level. Hedge clippers are ideal for this activity. It is easiest to cut the stalk at 14 inches, push aside the top of the plant, then cut the stalk at 6 inches.
- Stalk nitrate tests are available from most soil laboratories.
Interpretation
- Low test levels occur when wet weather increases nitrogen loss or yield.
- Excessive levels occur when hot, dry weather limits yield and nitrogen loss.
| PPM NO3 | TEST LEVEL |
|---|---|
| 0-250 | Low |
| 250-700 | Marginal |
| 700-2,000 | Optimal |
| 2,000+ | Excessive |
*Universities have published different NO3 levels as as low, adequate and excessive levels.
