By Daniel Hudson, UVM Extension Agronomist
Farmers and consultants across the state have begun the process of determining the appropriate amounts of sidedress nitrogen to apply to corn fields. Many farmers are particularly interested given the extreme and varied weather that we have had so far this year. Unfortunately, at this point many of the pre-sidedress nitrate-test (PSNT) results are indicating lower levels of soil nitrate than one would expect – given the amount of manure applied.
This does not mean that an amount should be chosen out of thin air. While I am sure that there are many fields where the N from manure is sufficient, sidedress-N recommendations from the small set of PSNT reports that I have seen have ranged from 30 to 140lb/ac. Given that sidedress nitrogen costs about $1/lb-applied (this may be an old number), the difference in cost between 30 and 140 lb/ac application is $110+/ac. If you guess low, you will lose yield. If you guess high, it will be a massive waste of money. Don’t guess. If you need help collecting PSNTs or if you are interested in using the new Adapt-N tool, please email or call 802-535-7922.
Gone, hiding, or just slow to show up?
The pre-sidedress nitrate test (PSNT) is a snapshot of the current concentration of nitrate (NO3–) in the top 12” of soil from a given location. As originally designed however, the sidedress N recommendation you get with the PSNT report is based on the understanding that what is in the soil right now is also related to what will be released later. Research has shown that the PSNT under-recommends sidedress-N only 1% of the time and over-recommends 30 – 40% of the time.
Given that the PSNT levels were low for the fields tested so far, I asked the lab to also run the ammonium analysis on the samples – in order to see if there was anything about to become nitrate (i.e., ‘in the pipeline’). While I was hoping for the opposite, the ammonium concentration was uniformly low among these samples. On one hand, this is not surprising because the conversion from ammonium to nitrate is fairly rapid in warm soils unless the pH is very low. On the other hand, I have seen much higher ammonium levels in other circumstances. It is possible that the cool/dry weather in May made it so that much of the manure has not mineralized yet, but it is inadvisable to make your sidedress decisions on that ‘hope.’ Rainy weather at the end of May and in the first half of June was favorable for leaching and in some cases denitrification.
In addition to taking PSNTs from various farms, we are also working with farmers to use the Adapt-N program from Cornell, which I have written about previously (see a very short video on the subject here). If it works as expected, it will probably become a convenient and accurate replacement for the PSNT and certainly better than choosing a sidedress rate based on no information.