Fall–applied N continues to be a popular fertilization practice. Fall fertilization saves the grower time in the spring, and spreads out the work for the fertilizer dealership in the fall. Despite these advantages, fall N must be viewed with scrutiny as this N is very susceptible to loss during the long time between application and crop uptake. With N loss in mind the best management of fall-applied N typically involves:
Ammonium sulfate is a potentially safe fall-applied N source over a wide range of production conditions. This advantage is largely because of their lower risk of volatilization losses compared to N fertilizers containing urea, and to their lower risk of leaching or denitrification losses compared to fertilizers containing nitrate-N. An added utility of ammonium sulfate is its ability to be applied in the fall and winter as long as the soil temperature is lower than 50°F and the field slope is less than 5% (according to the University of Illinois Agronomy Handbook). Urea-containing fertilizers, on the other hand, are generally not recommended for fall or winter applications as large potential losses of N can occur (University of Illinois Agronomy Handbook). The cost of fertilizer N, however, may cause growers to ignore these guidelines, and there are increasing reports of fall-applied urea being used.
New urea sources have been developed lately as a means to minimize N losses. Some sources, such as SuperU, incorporate urease and nitrification inhibitors in the formulation. The urease inhibitor reduces the conversion from urea to ammonium-N and the nitrification inhibitor reduces the conversion from ammonium-N to nitrate-N, both decreasing the potential losses of the applied N. Other fertilizer options include the controlled release urea sources, such as ESN, where the urea granules are coated with different materials that slow the release of N.
Considerable research has been conducted on the yield response between fall and spring-applied N with the general conclusion being that more N is required in the fall to maximize yield. Much less research, however, has been done on optimizing the source of fall-fertilizer N, which could be one way to fine-tune the practice. Others are advocating inclusion of urease and/or nitrification inhibitors (i.e. Super-U), or polymer coating of urea granules (i.e. ESN Smart N) as ways of improving urea's efficacy for fall applications. Alternatively, others would favor abolishing the practice of fall-applied N entirely, arguing that the risk of environmental damage from N losses outweighs the convenience of early N applications. Thus, fall-applied N is a management that clearly warrants continued scientific verification, and this need forms the basis of our research.
Our objective was to evaluate the role of N source in the success of fall-applied N for corn production and compare it with the spring application of these sources. The overall goal is to use this information to justify and/or improve fall applications of N. We believe that certain sources and/or formulations of N may have added utility for fall applications, especially in the cooler soil conditions of Northern Illinois.
The project was conducted at the Northern Illinois Agronomy Research Center in DeKalb, IL during the 2006, 2007 and 2008 growing seasons on a Flanagan silt loam soil. This soil had 6.2 pH, 5% OM, 70 lb P/acre and 344 lb K/acre. The factors under study were N application time (fall and spring), N sources (ammonium sulfate, urea, a 50-50 mixture of urea and ammonium sulfate, Super-U (urea plus a nitrification inhibitor and a urease inhibitor), and ESN (controlled release urea), and N rate (0, 50, 100, 150, and 200 lb N/acre). The fall and spring treatments were broadcast in mid-December and mid-April, respectively. Treatments were arranged in a randomized complete block design with four replications. An individual experimental unit consisted of four 21 foot- long rows, spaced 30 inches apart, with all rows receiving the respective N treatment. The entire field was disked in late April and planted within a week with a locally-adapted high yielding hybrid at a seeding rate of 35,000 seeds/acre. The middle two rows of each plot were harvested with a plot combine, and grain yield was expressed as bushel/acre at 15% moisture.
The data was analyzed using the Mixed procedure in SAS. Nitrogen source, rate and application time were considered fixed effects. Year and its interaction with the fixed effects were considered random effects, as well as block. Main effects and interactions were included in the model. The grain yield response to N fertilizer was modeled with a quadratic-plateau function. The significance of the random effects was evaluated with a likelihood-ratio test. Single-degree of freedom contrasts were performed to evaluate pre-planned comparisons between specific treatments, particularly the difference between spring and fall application for sources and application time.
A summary of the weather conditions between January 2006 and November 2008 is presented in Table 1. The 30-year average precipitation from September to December (fall) totaled 11.1 inches; 2006 and 2008 were 2 inches above-average and 2007 was 4 inches below-average. The 30-year average precipitation from March to June totaled 14.7 inches. Precipitation for this period was below-average for 2006 and 2007, and near-average for 2008.
Significance levels for fixed and random effects are presented in Table 2. The only random effect that was significant was Year (p<0.05), but none of the interactions with Year were significant. This result indicates that weather conditions affected corn yield, but it did not affect corn yield response to N rate, time and source or their interactions.
There was a significant effect of N application time on grain yield (p<0.05). On average across N rates, grain yield for spring (229 bu/acre) N application was 9 bu/acre higher than for fall application (220 bu/acre). Moreover, spring application resulted in significantly (p<0.1) higher grain yield than fall-applied N at all N rates (except 0), as determined by contrasts (Fig. 1). In our earlier work (Wessel et al. (2007), we found that the yield difference between December and spring applied N for Northern and Central Illinois was only 6.4 bu/acre when the cumulative rainfall between March and May was less than 10.6 inches, but it increased 0.008 bu/acre per inch in excess of this threshold. Cumulative March-May precipitation was lower than this threshold in 2006 and 2007 and 0.33 inches higher in 2008, which agrees with the yield differences between fall and spring applications that were observed in this study.
Nitrogen application rate also significantly affected corn grain yield (p<0.0001) (Table 2) with unfertilized corn yielding 163 bu/acre compared to 239 bu/acre with spring-applied N. For spring-applied N, yield was maximized with 155 lb N/ac resulting in a 76 bu/ac yield response (Fig. 1), while fall-applied N required 197 lb N/acre to achieve a 71 bu/acre increase to 234 bu/acre. The significantly higher yield with spring application and the lower N rate required to maximize yield (155 vs 197 lb N/acre for spring and fall applied N, respectively) demonstrates that N use is more efficient when the fertilizer is applied closer to crop uptake. Interestingly, the difference in the N rate required to maximize yield was of similar magnitude to the commonly used soybean N credit, which clearly indicates the agronomic relevance of N application time.
There were also significant differences among N sources (p<0.06; Table 2). Corn grain yield was larger with SuperU, than with ammonium sulfate, ammonium sulfate+urea, and ESN, but did not differ from urea alone (Table 3). Grain yield from urea alone also did not differ from the enhanced efficiency urea-sources (SuperU and ESN). Spring application of all sources, except for ammonium sulfate, resulted in significantly higher grain yield than fall application (Table 3).
SuperU resulted in higher corn yield than ESN (p=0.03) when applied in the fall, but not in the spring (p=0.32), while neither of these enhanced efficiency sources resulted in significantly higher corn yields than urea alone at either application time. When applied in the fall, corn yield did not differ from spring for ammonium sulfate, ESN and SuperU, but when applied in the spring both ESN and SuperU resulted in higher yields than ammonium sulfate (p<0.05 and p<0.01, respectively).
The lack of significant differences in grain yield among N sources when applied in the fall can be attributed to the relatively dry conditions during the winter and early spring periods (December to March). In addition, it must be considered that fall fertilization was done in mid-December, with average temperatures of 32°F which would have slowed nitrification. Both environmental conditions resulted in small losses of N fertilizer. Conversely, the average precipitation in April might have promoted some N-fertilizer losses and could explain the slight differences found among N sources when applied in the spring. Considering that the differences observed in our study were detected in an average to dry spring, larger differences among N sources might be expected in years of above average precipitation in the spring, or in other areas of the state with higher soil temperatures in the fall and spring.
Spring application of n increased grain yield by 9 bu/acre compared to fall application, supporting the concept that nitrogen use efficiency increases when the fertilizer is applied closer to crop uptake. Application of SuperU resulted in the highest overall grain yield (228 bu/acre), but it was not significantly different from urea alone (225 bu/acre). Similarly, plots fertilized with urea alone produced similar yields as those fertilized with ammonium sulfate, ammonium sulfate+urea, and ESN.
This three year study suggest that urea alone could be applied in the late fall in Northern Illinois. However, it is uncertain if these same results would occur in Central and Southern Illinois, where N is typically applied earlier and the spring is warmer and more conducive to N losses. For this reason, the study is being repeated at additional locations throughout Illinois in 2009 and 2010, which will allow us to evaluate each of these sources under a wider range of environmental conditions and provide information for the entire state.