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–). 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.
Table 1. 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.
Where water stands long enough – typically 3 to 4 days at warm temperatures – for the crop to begin to lose some of its green color, that’s a signal that soil oxygen is becoming depleted. Two negative consequences of lack of oxygen are: 1) the start of denitrification (conversion of nitrate to gaseous forms of N) and N loss; and 2) the beginning of root damage, some of which may be permanent. Most of those fields have recovered well.
We can assume that most fertilizer N is by now in the nitrate form, though some of that applied as sidedressed NH3 or (to a lesser extent) as sidedressed UAN may still remain as ammonium. This is typical for mid-June, and it means that the N is subject to denitrification and, in lighter-textured or tile-drained soils, to moving out of the rooting zone.
Measuring N loss from wet soils is not very practical, especially while it’s still wet. Previous work has shown that, at soil temperatures in the 70s, as much as 7 or 8 percent of the nitrate present can be converted to gas and be lost for each day the saturated conditions persist. Conversion rates may be lower than this with lower temperatures deeper in the soil and at night, if some of the N is still in the ammonium form, and if soils still have some oxygen present. So denitrification losses may be less than expected in some fields. And if plants are badly damaged by saturated soils, loss of N may be a smaller problem than the loss of yield potential from plant damage.
It’s rare that whole fields remain saturated for days, so in most fields, the risk of N loss by leaching or through movement out of the field through field tiles is greater than the risk of loss by denitrification. Tiles began running relatively late this spring, which helped keep N in the fields. The first water to reach the tiles came, in many fields, from rainfall before the N was all converted to nitrate, or from before N had been applied, so may have carried relatively little nitrate. But by June, it’s not unusual for water from field tiles to have nitrate-N levels of 10 to 20 parts per million. At 15 ppm N, one acre-inch of water leaving the field carries with it about 3.8 lb of N.
In fields where all of the N has been applied (especially if some was applied in the spring as NH3), where crop color has remained or returned to healthy green, and where there is no standing water now, it’s reasonable to assume that it’s not necessary to add more N. In fields where all of the N has been applied but where water stood long enough to have the crop lose much of its green color, adding supplemental N will increase yield only if plants can grow enough new roots to take advantage of the added N. Chances of such recovery are much greater when the water comes in early vegetative growth like it did this spring (so far, at least) than when it comes later.
In fields that still need sidedressed N, or where plants stood in water to the point of turning pale green but now seem to be recovering, N should be added as soon as practicable. The easiest way to apply N to wet fields is as urea (with a urease inhibitor such as Agrotain® added) applied by air, but that’s also costly. Waiting until high-clearance equipment can get through to apply broadcast urea with urease inhibitor or UAN (also with urease inhibitor, especially if rainfall is likely to be delayed) using drop tubes is usually cheaper, but may require days or weeks to dry out enough.
The amount of supplemental N – that applied to make up expected loss after a full rate was applied – should generally not exceed 50 lb of N or so. If a planned sidedress application is made late, plant uptake will start quickly, reducing the time available for loss. That may allow the rate to be lowered from what had been planned, especially if the planned rate would have brought total N to the higher side of guideline rates.
While it’s good to apply supplemental N (if it’s needed) or planned sidedressed N as soon as we can, the yield cost of further delays depend on the N available to the plant now. The best way to know how much N is available to the crop now is to observe canopy color; as long as leaves remain a reasonable shade of green, the plant is not deficient, or not deficient enough to cost yield as long as final N supply is adequate. In that case, some delay in applying N may not cost any yield. If it turns dry after surface application of N, uptake will be delayed and the risk will increase of having the crop run out of N.
Putting all this in perspective, the 2104 season has not been one of above-average N loss potential, except in areas that were unlucky enough to get big downpours. Remember that mineralization of soil organic matter is contributing substantially to the N supply in the soil now, helping to counter some N loss from tile lines. This is not the case in saturated soils, where mineralization is slowed as denitrification speeds up.
Still, if good rainfall and temperatures continue, the N supply, even if it’s reduced some by loss, is unlikely to limit yield. In fact, most of the highest yields we have seen in several hundred N rate trials over the last 20 years have come at modest N rates. I think this happens because good root systems mean good uptake of water and N, and that conditions that are ideal for yield also tend to be very good for soil N supply.