J. Gerwing, R. Gelderman, A Bly, and B. Berg

There is increasing concern about the effects of nitrogen fertilizer on the environment, especially ground water quality. This concern has been intensified by reports of NO₃ - N of greater than10 ppm in several locations in eastern South Dakota, especially where aquifers are shallow and soils are very coarse. In some instances, nitrogen fertilizer moving below the root zone has been implicated.

This nitrogen management experiment was established to study the effects of N rates in a corn-soybean rotation on nitrogen movement below the root zone. In most situations in South Dakota, if nitrogen moves below the root zone it stays there and only rarely moves back up. Therefore, once out of reach of crop roots, NO₃ - N has the potential to move down to the groundwater with percolating water during wet periods.

This nitrogen management experiment was established on the SE South Dakota Experiment Farm near Beresford in 1988. It is located on an Egan silty clay loam soil. Egan soils are well drained soils formed in silty drift over glacial till.

Corn was planted on the site in even numbered years since 1988 and soybeans was planted in the odd numbered years. The rates and timing of nitrogen fertilizer applied to the corn in 2000 are listed in Table 1. The treatments included a check (no N), the recommended rate applied in fall, spring or split between spring and 7 leaf stage and 200 and 400 lb rates spring applied regardless of the previous soil test. These treatments were applied to the same plots each year that corn was planted in the rotation. The recommended rate, was adjusted according to the NO₃ - N soil test level and for credit given to the previous years’ soybeans (1 lb N credit for 1 bushel beans). The recommended nitrogen rate was 123, 62, 90, 95, 95, 110, and 125 lb/A respectively for the even numbered years 1988 through 2000. Nitrogen was broadcast as urea and immediately incorporated by tillage except for the fall application was not incorporated until the following spring. The June portion of the split application was surface broadcast ammonium nitrate. Ammonium nitrate was used for this treatment to prevent volatilization losses.

Phosphorus, potassium and pH soil test levels at the site are 8 and 245 ppm and 5.9 respectively. A randomized complete block design was used on this experiment with four replications. Plot size was 15 by 65 feet.

Corn was planted on April 28, 2000 in 30-inch rows. The site had been disced just prior to planting. Plots were harvested with a field combine. Soil samples were taken to a depth of 6 feet in 1-foot increments on Oct. 10, 2000. Four cores were taken per plot and replicates combined for nitrate analysis. Only the 0, spring recommended (125 lbs), 200 and 400 lb/A N treatments were soil sampled.

Corn yields were limited to 50 bu/a (Table 2) due to drought conditions in early and late summer. Mid summer precipitation (Table 3) was nearly normal but was unable to compensate for the lack of subsoil moisture caused by low rainfall in the late summer and fall of 1999. August through December 1999 rainfall totaled only 1.6 inches. Yields were not influenced by nitrogen rate or timing, likely due to the low demand for N because of the low yield.

Nitrate soil test levels for samples taken in the fall of 1999 and 2000 are reported in table 4. Residual nitrate in the top foot of soil increased with increasing rate of N. The O N treatment had residual levels of 22 lb/a in the top foot while the 400 lb N rate residual was 162 lbs. Nitrate levels in the second foot of soil in the 400 lb treatment was only 30 lb/a, indicating nitrate leaching below the top foot did not occur during the 2000 growing season. The lack of leaching was due to low rainfall.

These plots will be rotated back to soybeans in 2001 and soil sampled in the fall to determine the amount and location of residual soil nitrate. Corn and soybean yields and soil tests from previous years of this study can be found in the SE Farm Progress Reports and in the Plant Science Department Soil/Water Science Research Annual Reports.