Soil Nitrogen and N Management Following the 2016 Crop

The 2016 growing season has been a very good one for corn in Illinois, with the November yield estimate of 202 bushels per acre, slightly higher than our previous best of 200 bushels per acre in 2014.

In sharp contrast to the wet June of 2015, most of Illinois received below-normal rainfall in June 2016, with parts of western and southwestern Illinois receiving less than an inch for the month. With May rainfall close to normal across the state in 2016, the wet soils and N loss conditions that we saw over most of Illinois in 2015 never materialized in 2016.

2016 soil nitrogen and N response

In our N-tracking project funded by the fertilizer assessment (Nutrient Research & Education Council), we apply 200 lb. of nitrogen per acre as fall-applied ammonia, early spring-applied ammonia, fall ammonia plus spring-split UAN, or spring-split UAN. We then sample during the spring to see how much N remains in the top 2 feet of soil.

Averaged over three locations in 2015, sampling in mid-June and at tasseling recovered about 70 and 42 percent, respectively, of the amount of N applied as fertilizer. In 2016, we found a little more N than this –about 76 percent in mid-June and 47 percent at tasseling. Grain yield levels didn’t differ greatly between the two years, but more of the N needed by the crop was taken up by tasseling in 2016 compared to 2015. Yields were similar in both years, though, so having more N taken up by tasseling did not clearly lead to higher yields.

Warm soils and wet weather in December 2015 caused a lot of concern about loss of fall-applied N in 2016. We did not, however, find lower amounts of soil N following fall N applications than we found following spring applications in 2016. But there were some differences between the two years in how much of the N recovered was in the ammonium form (coming directly from ammonia application) and how much was nitrate. Nitrate can move in the soil and be lost.

In both 2015 and 2016, about 80 percent of the N recovered in early May following fall ammonia application was nitrate. Following spring ammonia application, 59 percent of the nitrogen was nitrate in early May in 2015, while in 2016 only 39 percent was nitrate. By early June, when N uptake by plants began, 80 to 90 percent of the recovered N was nitrate in both years, regardless of timing of application.

It’s clear from these numbers that applying ammonia in the fall versus spring did not have much effect on how well the nitrogen was protected by remaining in the ammonium form, at least by the time N uptake began. This suggests that N loss is tied more closely to conditions during N uptake (June) than to N fertilizer timing, although sidedressing ammonia, which we did not do in this study, would have increased the amount present as ammonium.

Most N rate trials are showing considerably less N needed in 2016 compared to 2015. This year, we’re often seeing yields leveling off at N rates of only 140 to 160 lb. N per acre, at yield levels between 200 and 250 bushels per acre. We think that this reflects both the large supply of N from mineralization of soil organic matter under the favorable conditions of May and June this year, and also the drier June weather this year that limited N loss and root damage. Another sign of a good supply of N from the soil was the delay in development of N deficiency symptoms in corn grown without fertilizer N. It was common in 2016 to see unfertilized corn in our trials remain dark green into mid-June, when the crop was 3 feet tall or taller.

Nitrogen in the soil this fall

One might expect that less N loss might lead to higher amounts of N left over at the end of the season. We aren’t seeing this in most cases. Dan Schaefer of IFCA took soil samples at the time of crop maturity at a number of on-farm sites, nearly all of these showed soil N levels of less than 6 or 7 ppm, which we consider to be baseline levels. We’re also finding low soil N levels in samples taken following harvest in our N-tracking trials. The only place we’re seeing elevated levels is at N rates considerably higher than those needed to maximize yield. Using too much N is never a good idea, and that was especially true in 2016.

Those who added N late in late vegetative stages this year in addition to normal rates applied earlier might well have ended up with more soil N than usual after harvest. A vigorous cover crop like cereal rye will take some of this up. But the low levels of soil N in fields with normal rates of fertilizer N may not have enough N to produce vigorous cover crop growth. Trying to “prime” cover crop growth by applying fertilizer N this fall will increase N uptake, but may not necessarily increase net uptake after subtracting the amount applied.

Despite slow cooling at the start of November this year, soil temperatures are now below 50 degrees over most of northern and central Illinois, and ammonia application is underway. Soils are mostly in good shape for this, but application should be delayed in fields with wet surface soils .

Nitrogen management for the 2017 crop

We can think of no good reason to adjust N rates, unless planned rates are substantially higher that the 175 (155 in northern Illinois) and 210 lb. N per acre or so calculated by the N rate calculator for corn following soybeans and corn following corn, respectively, in the region of Illinois where fall N is used. If planned rates are higher than this, a downward adjustment is in order. We never know what the spring will bring, but it makes more sense to react to loss conditions if they occur that to apply more N “just on case.”

If the plan is to apply some N in the spring after fall application, consider subtracting that amount from the fall application rate in order not to exceed the suggested rate. If 2017 is like 2016 with regard to N nutrition, using more than suggested rates will nothing to increase yields, but will increase both economic and environmental costs.

There’s been a lot of talk in recent years about how CEC “limits” the amount of N that should be applied to a given soil; the maximum amount commonly mentioned is 10 lb. N per unit of CEC. CEC is a measure of the concentration of negatively-charged exchange sites in a soil. Soils with more clay and more organic matter have higher CECs; silt loam or silty clay loam soils with 3 or so percent organic matter typically have CEC values between 20 and 40. That’s more than enough to “hold” the right amount of N.

There’s no sound basis for considering CEC a measure of “N-holding capacity,” any more than to consider it as potash- or calcium-holding capacity. One unit of CEC in the topsoil will hold 360 lb. of ammonium, so a soil with a CEC of 20 would have only 2.5% of its exchange sites occupied by ammonium if it were all on the exchange sites. Ammonia applied to soil dissolves instantly in soil water, then converts quickly to ammonium. Like any positively-charged ion (cation) in the soil solution, some ammonium ions quickly occupy exchange sites, and some stays in soil solution. Little if any ammonium does not move down in most soils, regardless of the amount applied (within reason.) A possible exception is light, sandy soils with very low CEC values. Nitrogen should not be applied in the fall in such soils.

So there’s no reason to be concerned about CEC values when it comes to applying N on the vast majority of Illinois soils. Instead, base N rates on the N rate calculator, which is based on research showing what rate can be expected to maximize profit. And then apply N responsibly in order to minimize N loss and to keep N for the crop.