We believe this can be explained by nitrogen loss through denitrification and surface runoff. Denitrification is simply the loss of nitrogen in a gaseous form to the atmosphere. It is the most common form of loss in heavy soils with a low water infiltration rate. Most of our blackland and transitional greyland soils have water infiltration rates of less than 0.2-inch per hour.

The top few inches of soil quickly become saturated during a heavy rain. Warm, saturated soils create ideal conditions for several facultative anaerobes, which can use the oxygen off the nitrate ion and release the nitrogen back into the atmosphere. This is called “denitrification,” and can at times reach 10 pounds of nitrogen per acre per day.

Heavy water runoff during this time can also carry soluble nitrogen away from the fields and into the waterways. We do not see these types of nitrogen losses in drier years. This probably explains the small difference between the early and late nitrogen applications over a period of years, but possible real differences in wheat yields, year to year.

How can a producer use this information to maximize profits? None of us have the benefit of a “crystal ball” so we cannot determine wet years ahead of time. Over the past 20 years, we have been well served by targeting mid-February to begin topdressing wheat and hope to finish by the first week in March. In research plots, we have often produced as much wheat by topdressing in mid-March (past jointing) as we have in early March.

As for nitrogen rates, 100 pounds of actual nitrogen per acre has been our most cost effective topdressing rate. We have often seen small yield increases with rates in excess of 100 pounds of nitrogen per acre, but the yield increases have seldom offset the cost of the additional nitrogen. We also have not shown any differences in nitrogen sources; dribbled nitrogen solutions (32 percent), ammonium nitrate, and urea have all performed similarly.


Curtis_Jones @tamu-commerce.edu; James_Swart@tamu-commerce.edu