Soil Nitrogen and Fall Wheat N Nutrition

Wheat Science Newsletter - August 27, 2019 Volume 23, Issue 4

Edwin Ritchey—Extension Soil Specialist
John Grove—Soil Fertility Research
Josh McGrath—Extension Soil Specialist

Most winter wheat grown in Kentucky follows corn – sometimes soybean. The residual nitrogen (N) fertilizer remaining and/or that N released from crop residue is typically sufficient to meet wheat’s fall N requirement, prior to dormancy. But how does one know if sufficient N is present to meet the wheat N requirement?

A soil test is recommended every 1-2 years, depending on crop rotation and knowledge of field history. When dis-cussing soil tests, we usually refer to phosphorus (P), potassium (K), and soil pH. Soil tests provide the field’s cur-rent nutrient status and provide guidance on soil fertility needs of the next crop(s). Soil tests for N are typically not recommended due to the transient nature of soil N soil and the lack of soil test N correlation/calibration for wheat.

In the spring, for non-manured fields, the majority of the N applied the previous year is assumed to have been utilized by that crop or lost to the environment via denitrification of N gases, leaching, or in runoff. This is why spring residual N tests are not necessary or useful in our climate. How-ever, wheat starts growth in the early fall, prior to fall/winter N losses and there is considerable potential for residual N recovery by the crop. That N recovery can lead to substantial fall growth and greater lodging susceptibility, especially if a high rate of N is also applied to the wheat in the spring. This happened to the 2013 winter wheat crop following the low yielding 2012 corn crop. A fall 2019 soil N test may help wheat growers, providing some guidance about a field’s fall soil N status.

The 2019 growing season was wetter than average for most parts of the state, but there were some dry areas, too. Corn yields will likely be very good in many areas, with predictably little residual N remaining. This is a situation where some additional N might be added at or near wheat seeding. In other areas corn yields will be below average to average, whether due to dry weather or possibly soil compaction, and residual N levels are less predictable. These are the fields for which a residual soil N test might be in-formative.

Residual soil testing for inorganic N can provide some valuable insights. Soil inorganic N comes in two forms, ammonium and nitrate, both of which are plant-available. Nitrate is mobile in the soil but ammonium is not. Ammonium is at highest soil concentrations closest to the fertilizer application event, but will quickly convert to nitrate. At the end of the season, the majority of residual soil inorganic N should be nitrate. The presence of organic N sources, such as manure or a killed legume cover crop, will result in higher levels of residual soil ammonium in the soil.

The University of Kentucky Regulatory Services offers a residual soil nitrate N (not ammonium N) test at Princeton, but the Princeton lab is being impacted by the construction/renovation associated with the Grain and Forage Center of Excellence and is unable to offer the test this fall. Many private soil testing labs will conduct soil nitrate and ammonium tests upon request. Getting the right test will depend mostly on the form of N applied to the previous crop. If manure wasn’t applied, or a legume cover crop wasn’t terminated, then a nitrate test will probably be sufficient. However, testing for ammonium should provide additional information if manure or a legume cover crop were used.

Sample the soil to a depth of 12 inches in the middle be-tween the corn rows (at 15 inches from the row in corn grown at a 30-inch row spacing. The deeper depth is used to capture any nitrate N that might have moved into the lower root zone during the season, but would still be avail-able to a fall seeded wheat crop. The number of cores should be sufficient to adequately represent the field or field area being sampled. Remember that the results provided from the lab are only as good as the sample submitted for analyses.

The results will be reported from most labs in parts per million (ppm). Every 6 inches of soil weighs approximately 2 million lb, so 12 inches of soil will weigh approximately 4 million lb. To convert your sample result from ppm to lb/acre, multiply by 4. For example, if a 12 inch soil sample has 11 ppm nitrate N, this is equivalent to 44 lb nitrate N/acre (11 x 4 = 44). Samples that have 30-40 lb nitrate N/acre present in the fall at seeding have adequate N for growth and early tillering and no additional fall N is need-ed. Wheat fields or field areas with sample results exceeding 25 ppm nitrate N (100 lb N/acre) should be monitored carefully so that later N management does not contribute to excessive lodging potential next spring.

Knowing the ammonium concentration can provide additional information, especially when manure or a legume cover crop was used. Most of the N should have converted to nitrate by seeding time, but under certain situations there may still be some in the ammonium form. Make the same conversion as for nitrate from ppm to lb/A and add the two forms together. For example, if the 12 inch sample with 11 ppm nitrate N also has 5 ppm ammonium N, this is equivalent to 64 lb inorganic N/acre [(11 x 4) + (5 x 4) = 64].

What is the best way to provide additional N if little residual N remains? If the soil test recommends P, MAP (10-52-0) or DAP (18-46-0) provide 10 or 18 lb of N for every 100 lb of fertilizer added, respectively. Urea or other forms of N can also be used when the N need exceeds that provided by added P fertilizer.

Remember that sufficient, but not excessive, residual N is present for fall wheat growth most years. Years/areas with high corn yield and/or an exceptionally wet season are those that might favor application of some fall N for wheat. Also of concern are years/areas where the corn yield is lower than expected and where residual inorganic N may be high. Just because corn yield was lower or higher than normal, or the season was drier or wetter than normal doesn’t automatically indicate whether we know enough to decide on the need for fall N for wheat. A residual soil N test should help with that decision.