Nitrogen, corn production, and groundwater quality in Minnesota’s irrigated sands

Farm Forum

Corn in irrigated coarse-textured soils can be very productive with nitrogen applications, but excess nitrogen can increase groundwater contamination. To understand how much nitrogen is needed to optimize corn production and minimize the environmental impact of nitrogen fertilizers, a four-year study was done at the Rosholt Farm in Pope County and in Dakota County farmers’ fields.

Urea nitrogen rates ranged from 0 to 280 lb N/acre in 40 lb increments with half of the rate applied pre-plant and the other half at V4. Single pre-plant applications of enhanced efficiency fertilizers: ESN (a polymer coated urea), a blend of urea and ESN, and SuperU (urea with nitrification and urease inhibitors) were also studied. Lysimeters installed below the root-zone and water-balance calculations along with drain gauges were used to quantify the nitrate concentration and the amount of water and nitrate moving pass the root-zone.

The University of Minnesota recently revised their Maximum Return to Nitrogen Rate (MRTN) guidelines for irrigated continuous corn grown in coarse-textured soils: The new guideline suggests a MRTN of 209 lb N/acre at a nitrogen price to corn value ratio of 0.1. The MRTN for continuous corn in the four-year study was very similar to the new guidelines. For corn after soybean the MRTN was 164 lb N/acre. A split-application of urea was as good as or better than a single pre-plant application with enhanced efficiency fertilizers. Averaged across all years, the split-urea application increased corn grain yield by 5.4% compared to the averaged corn grain yield of enhanced efficiency fertilizers.

In May-June excess precipitation – while the crop is small and not taking large amounts of water or nitrogen—caused most of the nitrogen loss. During that time, 75% of the total leaching and 73% of the total nitrate-nitrogen load below the root-zone occurred. During July-August nitrate concentrations in the soil-water were large, but even though substantial water (precipitation and irrigation) was applied, little or no nitrate leaching occurred as the crop was actively using water and nitrogen.

At the MRTN, season-long nitrate-nitrogen load was 77 lb of nitrate-nitrogen/acre for continuous corn and 95 lb of nitrate-nitrogen/acre for corn after soybean. In the soybean phase of the corn-soybean rotation, where no nitrogen was applied, the season-long nitrate-nitrogen load was 47 lb of nitrate-nitrogen/acre. In continuous corn, reducing the MRTN by 20% reduced corn grain yield by 4% and nitrate-nitrogen load by 9%, and a 25% reduction in MRTN resulted in an additional 2% reduction for both, while no significant reduction occurred for corn after soybean. Similar nitrate-nitrogen load occurred with enhanced efficiency fertilizers as with the split-application of urea. After four years of no nitrogen application in the check plot, we measured 9 to 20 parts per million nitrate and load of 19 to 46 lb nitrate-nitrogen/acre.

Key points

• In irrigated sandy soils, the large rate of nitrogen application needed for economic optimum yield is best accomplished by splitting the application.

• The amount of nitrogen applied in May-June, when potential for loss is greatest, should be minimized.

• During July-August irrigation management is critical to supply adequate amounts of water to the crop without creating excess water supply that could result in nitrate leaching.

• It is difficult to meet drinking water quality standards in these corn cropping systems.

• Using enhance efficiency nitrogen fertilizers will not be enough to reduce groundwater nitrate loading.

• Reducing nitrogen rates to sub-optimum levels will result in reduced corn grain yield and relatively minimal reduction in nitrate leaching towards achieve pristine water quality goals.