Illinois corn: Assessing need for supplemental nitrogen
The corn crop in Illinois is off to a good start in many fields, and in most areas is in the V5 to V8 growth stages, just starting its rapid growth phase. On average, the crop under good conditions will add some 200 lb of dry matter per acre per day over the next 80 days or so. It will take up 3-4 lb per day of nitrogen before pollination, after which the N uptake rate will slow.
The spring of 2014 has not been a wet one overall in Illinois. But as usual, rainfall has been very unevenly distributed; some areas have received 6 inches or more over the past month. In the wetter areas, getting sidedressed N applied has been challenging, and some who applied the full amount of N are concerned about how much might have been lost.
Low temperatures through the winter and into early April helped preserve fall-applied N and the small amount of residual N left after last year’s big crop. April and May temperatures were normal to a little above normal. Maximum soil temperatures 4 inches deep under bare ground reached the mid-70s by mid-May, and into the 80s during the warm periods in late May and early June.
Soil temperatures in the 70s and 80s increase activity of soil microbes, both those responsible for mineralization (release of plant-available N from soil organic matter) and those that convert ammonium (NH4+) to nitrate (NO3-).
click image to zoom Table 1 has results from six Illinois fields sampled for N in May by Dan Schaefer of the Illinois Council for Best Management Practices (C-BMP) under the N-Watch® program. These results confirm that N loss was not been excessive by mid-May, and also that much of the fall-applied N was in the nitrate form by May. The fact that more N was recovered than had been applied is not unusual; mineralization and carryover N contribute to the amount that’s there.
The table shows amounts of N recovered from Illinois fields following application of a variety of N forms, rates, and timings. Sampling took place in early to mid-May 2014. Data from Dan Schaefer.
While many soils are moist or even wet, the threat of N loss is far higher where water has stood, or is standing, than where water didn’t stand for more than an hour or two. The heavy downpour that brought 3+ inches of rain to parts east central Illinois on May 21 left a lot of standing water, but by 12 hours later, much of it had run off the fields. This indicates two things: 1) rainfall rate exceeded the infiltration rate, so less water entered the soil than fell on the soil in most places of many fields; and 2) standing water affected a relatively small percentage of the soil surface.