When the University of Minnesota changed its fertilizer nitrogen (N) recommendations for corn in 2006, none 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 six 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.
Dakota County’s irrigated sandy soils are highly productive, but they are also susceptible to leaching. Even the finer-textured loamy 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-pound increments from 0 to 210+ pounds N per acre 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.
Results from the trials on loam soils can be seen in Figure 1. The curved lines, color coded by year, show the yield response to nitrogen. The vertical lines represent the economic optimum N rates for each of the three studies. The EONRs ranged from 148 to 160 pounds N per acre over the three years and resulted in yields between 220 to 240 bushels per acre. Nitrogen rates above the EONR do not result in enough of a yield increase to pay for any additional fertilizer and may result in excess N in the environment. University recommendations for these sites would be in the range from 120 to 160 pounds N per acre, so the results from the loam studies support the new guidelines.
Figure 1. Corn grain yields at varying rates of nitrogen fertilizer on irrigated loamy soils in Dakota County, 2007-2009.
On the sandy soils, the yield responses suggest that irrigated corn requires more nitrogen than current University recommendations. Figure 2 is typical of the results seen on sandy soils in Dakota County. The solid curved lines represent the yield responses to nitrogen (207-298 bushels per acre), while the vertical dashed lines represent the N rate where returns over N costs were maximized (EONR).
Figure 2. Corn grain yields at varying rates of nitrogen fertilizer on irrigated sandy soils in Dakota County, 2008-2010.
As Figure 2 demonstrates, EONRs on irrigated sands generally exceeded the new University guidelines by approximately 30 pounds N per acre. However, since the contribution of soil N to yield ((Yield0 /YieldEONR) x 100) was much less for sandy soils than for the loams, the fertilizer N efficiency was at least as high as on the irrigated sands as it was on the loam soils. On average, the check treatment yield (no fertilizer N) on loam soils was 138 bushels per acre, while the check yielded only 94 bushels per acre on the sandy soils where corn followed a non-legume crop.
In 2012, studies continued in Dakota County to determine optimum N rates for corn on irrigated sands. In addition to defining N rates that maximize returns, the studies compared variety of nitrogen stabilizer products and potential leaching losses. Dakota County producers benefit by applying these local results as they strive to maintain productivity while reducing risks to the environment.