With warm temperatures and the crop just entering its most rapid growth and nitrogen uptake phase, it seems highly likely that, unless soils start to run out of water in the next two weeks, the crop growing in soils with the normal (nitrogen rate calculator) amount of fertilizer nitrogen will be able to take up most of its nitrogen over the next few weeks with little danger of developing nitrogen deficiency.
In a trial at Urbana in 2015 with 200 lb. of nitrogen applied in April, the crop was at stage V9 and had 45 lb. of nitrogen in the plants on June 12. By tasseling time on July 13, it had 159 lb. nitrogen per acre in the plants. Soil nitrogen between these two dates fell from 240 lb. to 93 lb. per acre, and total (plant plus soil) nitrogen fell from 285 to 242 lb. per acre. Rainfall totaled more than 8 inches between these two dates. Even with the drop in soil nitrogen to a relatively low level (about 6 ppm nitrate-N and 5 ppm ammonium-N) by pollination, the crop in this treatment yielded 235 bushels per acre.
At the estimated 1 lb. of nitrogen taken up for each bushel of yield, the 2015 crop would have taken up about a third of its nitrogen after tasseling. Given the low amount of soil nitrogen at tasseling, this additional nitrogen had to have come from mineralization and, possibly, from nitrogen deeper in the soil profile as the crop drew water up during dry weather late in the season. In any case, it’s clear that low soil nitrogen at tasseling did not result in low yields due to nitrogen deficiency.
It’s premature to draw a strong parallel between the 2015 results and what we might expect this year, but with drier weather, soil nitrogen levels similar to those we saw in 2015, the crop darker green, and a root system that is likely to be somewhat deeper this year, all signs point to the likelihood of less chance for nitrogen loss and deficiency than we saw in 2015. In 2015, yields in most of our trials were high or very high, indicating that nitrogen loss and deficiency were not yield-limiting; exceptions were in fields where root damage form standing water was severe, and crops could not fully recover. While this looked like nitrogen deficiency, adding more nitrogen to such damaged crops often didn’t help very much in 2015.
Despite the dark green color of most Illinois corn fields in mid-June and soil nitrogen numbers that show no shortage, we are continuing to hear about producers and retailers gearing up to apply more nitrogen, including in some fields that have had a full amount of nitrogen applied and where soils have not been saturated this spring. In fields that have already received their full complement of nitrogen, with most or all of the nitrogen applied this spring, there is no clear justification for adding more N.
This does not appear to be one of those years when “just in case” justifies adding more nitrogen fertilizer. It’s highly unlikely that a corn crop that is deep green at knee- to waist-high will experience nitrogen deficiency due to lack of soil nitrogen. When nitrogen deficiency symptoms do develop in late vegetative or reproductive stages, this usually results from the crop’s running short of water to keep photosynthesis going at full speed. What is called “firing” and looks like a shortage of nitrogen is really loss of lower leaf area as the plant dries out. As lower leaves start to shut down they move nitrogen out to younger parts of the plant (including the ear) to keep the plant going as long as possible. Adding more nitrogen neither prevents nor cures this.
If some or all of the nitrogen was applied at modest rates last fall or in early spring in an area that has gotten wet several times since, and if soil nitrogen sampling shows levels of less than 15 or so ppm of nitrate-N in the top foot (2-ft. samples will capture nitrogen that has moved down but aren’t always practical) then adding more nitrogen might be indicated. We can’t accurately estimate the chances that applying more nitrogen will pay its cost, but if the crop is deep green and growing rapidly despite what seem to be low soil nitrogen numbers, that’s a hint that chances of getting a return may not be very high. The crop is always a better indicator of soil nitrogen sufficiency at a given growth stage than are soil nitrogen tests.
For those heading out to apply more nitrogen, remember that applied nitrogen has to get to the roots in order to do any good. If we get average rainfall over the next few weeks, that won’t be a problem. But if it stays dry, nitrogen is likely to stay close to where it lands in or on the soil. Roots pull water from the surface soil first, and there will need to be enough rain to bring soil moisture levels up to activate roots and to move surface-applied nitrogen into the soil before root uptake can resume. Placing nitrogen close to the rows in tall corn was only slightly higher-yielding in our trials last year than applying the same amount at normal sidedress time. The soil is a good reservoir for nitrogen, and so nitrogen applied a month or more before the crop takes it up is usually available. Even in the wet June of 2015 it was neither necessary nor cost-effective to spoon-feed nitrogen to the crop. All signs point to even less benefit to that approach in 2016.
Some people are using slowed-release forms of nitrogen for applications made at or after the normal sidedress time. When nitrogen is applied when crop uptake is close to its maximum, which starts at about the V7 stage, the main risk is that nitrogen won’t be released in time for the plant roots to take it up. Any slowing of the release of nitrogen increases that risk. Uptake of nitrogen remains at a high rate for only about three weeks, and it’s unlikely that nitrogen, especially when applied as urea or ammonium, will convert to nitrate and move out of the rooting zone in the few weeks before nitrogen uptake starts to slow. That’s especially the case now, with dry soils more common than wet soils, and extended wet periods not in the forecast.