Most winter wheat grown in Kentucky follows corn - sometimes soybean. The residual nitrogen (N) fertilizer remaining and/or the 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 discussing soil tests, we usually refer to phosphorus (P), potassium (K), and soil pH. Soil tests provide the field's current nutrient status and provide guidance on soil fertility needs of the next crop(s). Soil tests for N are typically not ran on a normal soil test due to the transient nature of N in the soil but can be requested for an additional fee through your local Extension Office.
In the spring, for non-manured fields, the majority of the N applied in the previous years is assumed to have been utilized by that crop or lost to the environment via denitrification, leaching, or in runoff. This is why spring residual N tests are not necessary or useful in our climate.
However, wheat starts growth in the early fall, prior to the fall and winter N losses. 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 it be 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 informative. Residual soil testing for inorganic N can provide some valuable insights.
The University of Kentucky Regulatory Services offers a residual soil nitrate N (not ammonium N) test at Princeton. 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 between 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 available 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.
Remember that sufficient, but not excessive, residual N is present for fall wheat growth most years. Years and 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 and 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.
For more information contact the Pulaski County Extension office. Information gathered from "Soil Nitrogen and Fall Wheat Nutrition By Edwin Ritchey, John Grove, Josh McGrath".