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

A. Bly

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 than 10 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 from 1988-1998 and soybean was planted in the odd number years, 1989-1999. The rates and timing of nitrogen fertilizer applied to the corn in 1998 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, however 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 and 110 lb/A respectively for 1988, 1990, 1992, 1994, 1996, and 1998. Nitrogen was broadcast as urea and immediately incorporated by tillage except for the fall application which was not incorporated until the following spring.

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 feet by 65 feet.

Soybean was planted on May 17, 1999 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. 19, 1999. Four cores were taken per plot and replicates combined for nitrate analysis. Only the 0, spring recommended (110 lbs), 200 and 400 lb/A N treatments were soil sampled.

Nitrate soil test taken in the fall of 1998 showed a residual nitrate level of 308 lb in the top two feet of the 400 lb N treatment for corn (Table 1). Deeper samples indicated almost no N moved below 2 feet. The large amount of surface N should have been available for soybean in 1999. Heavy April and early May rainfall caused concern on whether the nitrogen

leached deeper into the profile so the high N rate plot was resampled on May 18, 1999 to a depth of 5 feet. The results of this sampling showed the top foot of soil, which had 198 lb of NO₃ – N in fall, had only 19 lb remaining (Table 3). The majority of the N had moved to the 3 to 5 foot depths. The total amount of N in the spring sampling was similar to fall, indicating very little N had moved below the 5 foot profile, however it moved down in the profile about 3 feet. Rainfall between the fall and spring sampling was 11.6 inches (Table 3). Rainfall between the 1998 fertilizer application and fall sampling was higher (17.5 inches) but had little effect on N movement since the corn crop was growing during that period, removing water from soil and preventing leaching.

Fall soil sampling showed nitrate levels in the 2 to 4 foot depth were lower than in mid-May indicating further leaching took place (Tables 2 and 3). This likely occurred in June and July when there was 10 inches of rainfall (Table 4). In addition, the soybeans suffered almost complete defoliation due to hail on July 2 which reduced transpiration, increasing the amount of water available for leaching.

Severe hail damage on July 2 likely reduced yields, however, the soybeans still averaged 30 bu/a (Table 5). Yields were not affected by the previous year’s nitrogen treatments. A yield increase was not expected since soybeans normally do not respond to nitrogen when properly nodulated. In addition, the large carryover nitrogen levels that were in some treatments at planting and could have potentially increased yields, had likely been leached below the root zone before soybeans could use it.

These plots will be rotated back to corn in 2000, nitrogen treatments applied 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, 1988-1998.