Crop Management Systems Impact on Ground Water Nitrate

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Nitrate is a very soluble molecule that easily moves downward through the soil with water. Careful water and nitrogen fertilizer management is essential to minimize the amount of nitrate reaching the ground water.

Improved water/nutrient management practices developed by the Management Systems Evaluation Area (MSEA) project for the northern cornbelt sand plain can reduce ground water nitrate levels. They help keep nitrogen in the root zone and available for plant uptake. But key to doing this is matching fertilization with crop use, and avoiding having excessive amounts in the soil at times when crop needs are minimal.

Split applications of fertilizer are important to reduce spring leaching. If variable rate technology could be used to apply nitrogen fertilizer to where the most efficient use would occur, even greater reductions might be possible. Unfortunately, areas of similar grain yield potential do not remain similar from year to year on sandy soils, and it's difficult to accurately estimate yield goals for nitrogen applications. Grain yields within test fields varied as much as 15 percent.

Continuous corn requires fertilizer every year, while a corn-soybean rotation only needs fertilizer every second year, for the corn. And soybeans also tend to "scavenge" residual nitrate from soil, reducing nitrate-N leaching even more.

Results and conclusions

In general, the corn-soybean farming system has not adversely affected nitrate-N concentrations in the aquifer at three of the four research sites. The exception was Oaks, North Dakota, where nitrate-N concentrations beneath the corn-soybean rotation increased from 4 ppm in June 1991, to between 35 and 50 ppm in November 1993. This occurred because the site was newly irrigated land, and residual nitrate accumulated during dry years leached rapidly into the ground water.

At the Arena, Wisconsin site, nitrate-N concentrations are actually decreasing over time in both phases of the corn-soybean rotation.

Also at Princeton, nitrate-N concentrations in the aquifer under the potato portion of the potato-sweet corn farming system have increased from 15 ppm to more than 30 ppm. About 10 percent of the nitrogen applied during April and June of 1991, reached the water table by April 1992.

Nitrate-N concentrations were greatest beneath the potato crop because, to accommodate the crop's needs, application rates were 1.4 times greater than on other crops. An initial increase in potato-sweet corn was also a result of excessive nitrogen applied in the starter in 1991 (110 lbs/A vs. the targeted 60 lb/A).

At the Minnesota site, denitrification occurs both in the deeper portion of the aquifer and beneath the wetland. Nitrate-N concentrations in the aquifer across the entire Princeton MSEA typically decreased from 15 to 25 ppm near the water table to less than 0.2 ppm about 25 feet below the water table.

MSEA project research sites

Research sites are located on outwash sand plains in Minnesota, North Dakota, South Dakota and Wisconsin. Cropping systems overlay ground water that, in the spring of 1991, contained median concentrations of 7 ppm nitrate-N in Minnesota, 6 ppm in North Dakota, 29 ppm in South Dakota, and 18 ppm in Wisconsin. The main research site at Princeton, in east central Minnesota, is situated on the 1,700 square mile Anoka Sand Plain. The other sites are Oakes, North Dakota; Aurora, South Dakota and Arena, Wisconsin.

Common to all sites is a ridge tillage corn-soybean rotation using split applications of nitrogen fertilizer, banded herbicide applications and an irrigation scheduling system (checkbook method). Sizes of the corn and soybean management system blocks vary from 4.4 acres at the main site to about 1.5 acres at satellite sites. A "conventional" continuous corn system and a potato-sweet corn rotation (an intensive management and input system) are used for comparisons.

Nitrogen was applied as urea (46-0-0) and/or urea ammonium nitrate (28-0-0) appropriate for realistic yield goals from each site. Except for South Dakota, all corn crops (rotation, continuous, and sweet corn) received three nitrogen applications: a starter followed by two split applications (usually at the six- and eight-leaf stages). Potatoes also received three nitrogen applications: 60-70 lbs N/A in the starter, 50 lbs N/A at drag off and another 50-70 lbs N/A at hilling for a total of around 170 lbs N/A.

Measuring water quality

Nitrate-N concentrations in ground water are measured with multi-level ground water samplers installed at various depths in the aquifer. They are placed in wells located in the center and just beyond both ends ("upgradient" and "downgradient" relative to the direction of ground water flow) of each crop system block. They provide the information needed to evaluate changes in ground water quality. Impacts were also measured indirectly in each field by sampling the soil to a depth of three feet before and during each growing season.

More information?

To obtain other MSEA project fact sheets, or if you have questions about the project, contact:

Bruce Giebink
MSEA Education Coordinator
Department of Soil Science
452 Borlaug Hall
St. Paul MN 55108

phone: 612/625-4749

e-mail: bgiebink@soils.umn.edu

Produced by Communication and Educational Technology Services, University of Minnesota Extension.

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