When the University of Minnesota changed its fertilizer nitrogen (N) recommendations for corn in 2006, few of the research sites in the extensive, seven-state database included irrigated fields with coarse-textured soils. Since these conditions are common in Dakota County, the University of Minnesota established several study sites over seven years with the help and support of local producers, Dakota County, and the Vermillion River Watershed Joint Powers Organization to study the corn yield response to fertilizer nitrogen rates under Dakota County conditions. The objective of this research is to determine the optimum N application rate that maximizes profitability while it minimizes the impact on groundwater. Download the University of Minnesota Dakota County Nitrogen Fertilizer Research for Corn on Irrigated Soils fact sheet.
Dakota County’s irrigated sandy soils are highly productive, but they are also susceptible to leaching. Even the finer-textured loam soils are often underlain by gravel, suggesting that all fertilizer N applications in Dakota County need to be carefully considered. When N is applied above the optimum rate, producers pay for fertilizer the crop doesn’t use and the excess nitrogen may be lost to the environment.
Since 2006, nitrogen rate studies have been conducted on both loamy and sandy soils in Dakota County. In all of the local studies, nitrogen was applied in 30 or 40 pound increments to obtain a yield response curve. The Economic Optimum N Rate (EONR), that rate where the last increment of N is paid for by a resulting yield increase, was determined for each of the studies and compared to the 2006 University recommendations.
While results from the loam soils fit well within the 2006 University fertilizer nitrogen recommendations, the guidelines have come up short on the irrigated sandy soils. As a result, updated recommendations from the University of Minnesota will be released soon and they will be similar to Wisconsin’s guidelines for irrigated sandy soils. The revised optimum N rates for sandy soils will be in the 200 to 220 pounds N per acre range, but unlike Wisconsin’s guidelines, there will be a 30 pound N per acre credit for a previous soybean crop.
Figure 1 demonstrates typical corn yield responses to nitrogen on irrigated loamy (red) and sandy (yellow) soils in Dakota County. As the figure shows, corn yields increased with fertilizer N to a point and then leveled off. The goal of the new recommendations is to find the point on the response curve where the last increment of fertilizer is paid for by a resulting yield increase. This economically optimum N rate (EONR) is represented by the dashed vertical lines on the graph. At current corn prices and fertilizer costs, any nitrogen applied above this point would actually reduce net returns. On the loam soils, EONRs between 150 and 160 pounds N per acre yielded 240 bushels per acre and fell within the “new” guidelines. In contrast, the EONRs on the irrigated sandy soils were approximately 40 pounds per acre greater than for the loam soils. At these sites, EONRs of 200 and 204 yielded 207 and 223 bushels per acre, respectively. Even at these higher rates, the N use efficiencies for irrigated sands were at least as good as the N use efficiencies on the loams, since the soil N contribution on the sandy soils is lower. This is demonstrated by the check yields, where the loamy soils contributed enough soil N to yield 150 bushels per acre, while the sandy sites yielded only 90 bushels per acre where no N had been applied.
Figure 1. Corn grain yields at varying rates of nitrogen fertilizer on irrigated loamy soils in Dakota County, 2007-2010.
Nitrogen (N) stabilizers on irrigated, coarse-textured soils
In addition to defining N rates that maximize returns, studies in 2012 compared a variety of nitrogen stabilizer products against the current best management practice (BMP) of splitting N applications on coarse-textured soils. These products are designed to increase efficiency by either controlling release or altering reactions in the soil to better match N release with crop uptake. They are more expensive than conventional fertilizers and any benefits they provide depend on a number of factors. The nitrogen treatments in the 2012 study included the following:
- Check – No nitrogen
- U-160 – The conventional fertilizer, urea, was split-applied at planting and the 6-leaf stage (V6) for a total of 160 pounds N per acre (80 pounds N per acre each application).
- SU-160 – SuperU was applied at planting at 160 pounds N per acre. SuperU contains urease and nitrification inhibitors and works by altering soil reactions. A urease inhibitor slows the enzyme that converts urea to ammonium, thus reducing volatilization losses when fertilizers are left on the soil surface. A nitrification inhibitor slows the soil bacteria that convert ammonium to nitrate, reducing the risk of leaching or denitrification losses.
- ESN-160 – ESN (Environmentally Smart Nitrogen) was applied at planting at 160 pounds N per acre. ESN is a controlled release fertilizer that uses a physical barrier (polymer coating) to slow solubility and nitrogen release.
- ESN/U-160 – A split application of ESN (80 pounds N per acre) was made at planting and followed by urea at V6 (80 pounds per acre).
Figure 2 shows results from the study near Hastings, Minnesota in 2012. While this represents just one year of data, early indications are that the current best management practice of splitting N applications (U_160) performed at least as well as the ESN and better than the SuperU (208 vs. 198 and 175 bushels per acre, respectively). Bars with the same letter are not significantly different from one another (p<0.05).
Figure 2. Corn grain yields of different nitrogen sources at varying times of application, Hastings, 2012.
This study will be repeated again this year, with results available in late 2013. For more information, contact Neith Little at firstname.lastname@example.org
The information given in this publication is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement by the University of Minnesota is implied.
Thanks to Paul Bauer, Greg Stoffel, Brian Murphy, Randy Sorg, George Conzemius and Al Swanson for making this on-farm research possible.