Fields which have had prolonged standing water or waterlogged areas are probably nitrogen deficient. Standing water provides conditions favorable to bacteria which convert nitrate in the soil to nitrogen gas, which is lost to the atmosphere. Where water does not stand, heavy rainfall will wash nitrate out of the root zone. In either case, there is little or no nitrogen available to the plant. Organic sources of nitrogen (manure,crop residue, etc.) will not generally become available until the soils warm up. After the water has drained, it will be necessary to apply top dress nitrogen to these fields in order to stimulate. When nitrogen fertilizeris applied at the wheat tillering stage, a positive response to growth and yield is produced. The nitrogen will stimulate new root development, quicker growth response and green color.
Ammonium nitrate is the best material for top dressing because it can be moved into the soil with the least amount of rainfall.It is also the most expensive nitrogen source.
Ammonium sulfate may require more than one rainfall event to move it into heavier soils, and so it may need to be applied earlier than the other materials. It has the advantage of supplying sulfur in addition to nitrogen, which is important on many eastside Stanislaus County soils, especially in a high rainfall year. Since a maximum of only 10 lbs of actual sulfur are needed, it works well to blend a little ammonium sulfate with one of the other materials to save on flying costs.
Urea is a form of nitrogen which will move easily into the soil if a rain comes immediately after application, otherwise it converts to ammonia-form fertilizer and will take more rainfall to move, and its movement will be similar to ammonium sulfate. Urea should be used only if rains are anticipated within about a week of spreading, otherwise ithas a tendency to be lost to the atmosphere, especially under warm, wet, windy conditions.
In eighteen of twenty tests conducted in Yolo County by Tom Kearney, urea performed as well as ammonium sulfate. In the remaining two tests, incorporating rains did not come until three weeks after application and by then 50% of the nitrogen in the urea had been lost. In contrast,100% of the ammonium nitrate was still there after three weeks without rain. Kearney found no loss in performance of any of the materials when spread on saturated soils, even if there was standing water on the field.
Fields which have had dairy lagoon water applied should not need supplemental fertilizer because the nitrogen content of straight lagoon water is very high.
November 5, 1999