ISSUE 7 June 17, 2004
WATER AND NITROGEN
Some areas of the state have received or will receive more than their share of precipitation. A loam soil will hold about 2 inches of plant available water per foot. With four inches of rain on "dry" soil, water will move at least two feet into the soil if it all soaks in. Additional rainfall will move water even deeper. Nitrogen that was applied last fall or before planting this spring, organic matter mineralization, and whatever the soil test showed as residual nitrate will move when water moves into the soil. Young seedlings are particularly affected, since their root systems are shallow and downward growth might also be hampered by temporarily perched water tables. If the nitrogen is deeper than roots can get it, nitrogen deficiency would likely result. If the nitrogen is still only a foot or two below the surface, most crops will eventually catch up to it, however, if the nitrogen is deeper than a foot, the plant may struggle to achieve a satisfactory yield goal if the crop is one that makes yield decisions early in the season, such as small grains. Corn and sunflower are more forgiving.
Another process that affects nitrogen availability is denitrification. Denitrification is the transformation of nitrate to nitrogen gases by a number of soil bacteria. These bacteria are most active when the soil is saturated with water. Denitrification does not affect newly applied ammonium-form N; only nitrate. Therefore, a recent sidedress ammonia application or cultivated urea application would not be affected. However, the nitrate side of 28% liquid N would be affected. In the Red River Valley, this process appears to be very fast. Some measurements in the corn belt have given an N loss of 50% nitrate within a few days.
In our site-specific field studies during wet years in the valley, depressional areas commonly test extremely low at the end of the year, even when crops in those areas drowned out and crop removal was zero. In contrast, denitrification does not appear to be as great a factor west of the valley. There is surely some denitrification going on, because a number of Canadian studies have documented that it proceeds in drier soils under seasonal flooding, but our site-specific studies in Valley City and Mandan show that even at the end of a wet season, the drowned-out areas still test high in nitrate. It appears that leaching is the primary means of loss west of the Valley, whereas in the Valley the losses may be mostly due to denitrification with standing water, at least in the heavier soils.
To determine the N status of a wet field, take a dual approach. Soil samples, separated out into 0-6 inch, 6-12 inch, and 12-24 inch, will help to determine where the nitrate is located. Plant analysis will help identify if the plants are yellow because of lack of N or S, or just beat up because of the rain. A caution- soil samples for nitrate are not going to be helpful if N was applied recently. The soil test only identifies nitrate and not ammonium-N. Tracking ammonium-N is difficult if the N was applied in a band. I would not recommend trying to determine how much is left. Perhaps waiting a week - 10 days and then sampling for nitrate and tracking crop N status would be a better idea.
In small grains, supplemental N must be applied as soon as possible once the fields are dry enough to run on. To contribute to yield, the application should be made prior to jointing. In corn and most other row crops, the application can be made over a longer period of time, but in corn, the application is best made before the 9-leaf stage.
Ext. Soil Specialist