March 21, 2008

Now is an excellent time to evaluate stands, check soil moisture and fertilizer prices, evaluate your marketing plan or commitments and determine if additional N will be needed for your winter wheat. Wheat prices have increased from $4-$5 a bushel last spring to over $12 a bushel this winter. If you didn't buy N fertilizer last fall or early winter, you'll note that those prices also have doubled since last spring.

Spring N Management

Nitrogen rates are most accurate when based on soil tests for residual nitrate to a three-foot depth (taken now unless you have a soil test from last fall). NebGuide 1460, Fertilizing Winter Wheat I: Nitrogen, Potassium, and Micronutrients, indicates that if you do not have a soil sample, use a base level of 9 ppm nitrate-N as an average. This may be a suggested level for wheat planted after fallow. For wheat planted after an adequately fertilized previous crop, a more accurate level would be 3 to 5 ppm nitrate-N (shaded area in Table 1). Table 1 shows recommend N rates that reflect a range of current price ranges for wheat and nitrogen.

We normally assume a nitrogen credit following soybean of 45 to 50 lbs N per acre which should be sufficient for much of the dryland (non-irrigated) wheat. However, research on wheat following soybean in plots at North Platte found that nitrogen fertilization was necessary. Soybean is an excellent scavenger of soil nitrate. Residual nitrate levels following soybean are usually in the 2 to 4 ppm nitrate-N range. Nitrogen fixed by the soybean does mineralize, but it usually is released too late to benefit the current winter wheat crop. Wheat was fertilized with N (30 to 70 lbs N per acre) depending on yield potential for dryland to irrigated cropping systems. If no N was applied, the wheat showed N deficiency plus low yields and protein. Soil sampling the wheat stubble for the next crop showed elevated nitrate levels, so N rates could be reduced for that crop.

For irrigated wheat, current guidelines suggest adding 1.5 pounds of N per bushel above the 75 bushels per acre yield level if you have consistently produced above that level. See the March 7 CropWatch for current N research on irrigated wheat. A web-based Excel calculator for N recommendation for wheat is at http://soilfertility.unl.edu. The NebGuide can also be used to calculate N recommendations.

Nitrogen Sources

Nitrogen Solutions

Determining viable stand is equally important for weed control decisions which may be important when considering N fertilization. If nitrogen solution-herbicide combinations are used, they need to be applied early for many broadleaf weed problems. Early N allows more time for distribution into the root zone, but the herbicide application may be too early to control some weeds. Later applications may be optimum for some weeds but may cause problems with plant injury because of the herbicide-fertilizer combination.

Urea

Urea (46-0-0) is a good choice for spring topdressing if applied early. The cooler temperatures and the higher probability of precipitation help assure a lower potential for N volatilization loss. Several products that can reduce N volatilization also are available (see the March 7 CropWatch and previous CropWatch articles on managing urea-based fertilizers).

Gary Hergert
Extension Soils Specialist
Panhandle REC, Scottsbluff