ISSUE 4 May 28, 1998
After a heavy downpour, or an extended period of rain, some areas of fields become covered with water. The immediate concern is the ability of the crop to survive being submerged for hours, or even days. But after the water drains off of these areas, the crop may turn yellow. Sometimes, the yellow color is caused simply by the stress of being under water. The crop will usually recover color in a few days with sunny, warm weather. However, sometimes the yellowing may persist.
Persistent yellowing is most often caused by low soil N availability. During flooding, denitrification bacteria are active and may transform nitrate to plant unavailable nitrous gases very quickly. Some research has shown up to 50% N losses with 24 hours of flooded conditions. If there has been denitrification, soil sampling will reveal low levels of nitrate remaining in normal sampling depths.
In sandier soils, nitrate can move very quickly out of the root zone. N may be in the soil, but at depths temporarily unavailable to young roots. Soil sampling to a 2 ft. depth will reveal low levels of N in the 0-6 inch depth and greater levels at the 6-24 inch depth. Crops would be expected to recover and receive adequate N in a mild leaching situation.
Tools to determine the nature of yellowing include plant analysis and soil testing and field assays. To collect plants for plant analysis of small grain from 2-5 leaf stage, collect two separate samples from the above ground leaves from 50 plants in the affected area and 50 plants from a "normal color" area. Obtain a soil sample, divided into a 0-6 inch and 6-24 inch depth, dry, and send along to the testing laboratory for analysis of nitrate. Dry the plants before shipping or delivering to the testing laboratory (The NDSU Soil and Water Characterization Lab does not conduct plant analysis commercially). The analysis will provide a good picture of the nutrient status of the plants.
To perform a field assay, dissolve a small amount of urea in water and drench the soil around some plants in the field. Avoid pouring the fertilizer over the plant leaves to protect against leaf burning. Flag the area and come back in about 24-48 hours. If the problem is N, the plants will have turned green.
Application of top-dressed N in small grains is not easily accomplished. The greatest problem is logistics of obtaining airplanes or ground applicators to apply the fertilizer. Application of high rates of N in a broadcast liquid application is not the preferred method of application because of the burning potential of the fertilizer. If this method is chosen, try to apply the fertilizer when the leaves are wet from dew, on cloudy days, when it is cool, and avoid application to dry leaf tissue on hot sunny days. A better option would be straight-stream nozzles on a ground applicator, with nozzle spacings of about 12-16 inches apart and applied at a slight angle to the row. The best option for crop safety would be granule fertilizer, such as urea or ammonium nitrate applied when leaves are dry. If you apply a urea-based fertilizer, rain within 48 hours is necessary to prevent N volatilization losses.
In row crops, the options are more easily carried out. Side-dressing is very effective until the crops are too tall or closed together to allow field traffic.