Accounting for N, 220 bu/ac corn has 213 lb N in above ground biomass (148 lb N in grain and 65 lb N in the corn stover) with 126 to 141 lbs N coming from fertilizer. In other words, 25% to 33% (or more) of the fertilizer-N (47 to 62 lb N) isn’t utilized by the crop. The mean recovery for fertilizer-N in grain + stover averages only 67% for corn after corn and 75% for corn after soybean even with best N and irrigation practices, such as side-dressing fertilizer at V6. Some nitrogen just gets lostĬould this much N really bypass the crop roots and make it into the aquifer? Using the University of Nebraska recommendations for soils and yields in the BGMA, 188 lbs N/ac is a good target rate. Therefore, 17 ppm nitrate-N x 2.72 x 12 ac-ft water = 578 lbs of nitrate-N contamination in the aquifer under an acre of BGMA. So, if all the water from 1 acre of the 50-foot thick aquifer was pumped aboveground, this would yield an acre-sized pool about 12 feet deep (25% of 50 feet). The upper 50 feet of the BGMA aquifer is contaminated with high nitrate water (Figure 1), and the aquifer matrix is water-filled sand and gravel (25% water-filled space). This represents 17 x 2.72 = 46.2 lb N/ac-ft water. For example, the average groundwater nitrate-N for the Bazile Groundwater Management Area (BGMA) in north central Nebraska is 17 ppm with considerable variation by location and depth (Figure 1). Therefore, nitrate N pounds applied in 1 ac-ft is the concentration in ppm multiplied by 2.72, and the annual lbs N from irrigation would be calculated by the total ac-ft applied. One ac-ft is 325,851 gallons or about 2,720,000 lb water (one gallon of water weighs 8.34 lb). How much nitrogen is in the aquifer?Ī commonly accepted practice for estimating nitrate-N applied in 1 ac-ft of irrigation water is to multiply nitrate-N concentration in parts per million (ppm) by 2.72. Here, we explore this tool by going through commonly-used conversions and calculations for N supply and nitrate-N leaching. Nitrogen budgeting can be used in developing a framework for N management so that crop production can continue with minimal impact on water quality. Nitrogen (N) budgeting, where accounting principles are applied to measured quantities of individual N sources, is one tool for understanding how long-term fertilizer-N use and irrigation contributes to this problem. This problem gained recognition in Nebraska during the 1960’s as associated with fertilizer-N use and irrigation practices, and even now remains largely unresolved because of the complexity and scale of land use. Increasing nitrate-N concentrations in groundwater has become a national issue. Krienke, University of Nebraska Department of Agronomy and Horticulture, Lincoln, NE
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Snow, University of Nebraska Water Sciences Laboratory, Lincoln, NEīrian T. Miller, USDA-ARS, Agroecosystem Management Research Unit, Lincoln, NEĭaniel D.