University of Nebraska
1992 Wheat Technology Conference
It has not been many years since little or no nitrogen (N) fertilizer was used on fallow wheat in western Nebraska. It wasn’t until the late 60s that low grain protein indicated that N availability was becoming a major problem limiting grain yields as weed as grain protein in the Panhandle area.
While a general N deficiency exists for high wheat yields, accurately predicting the amount to apply is not easy. It is complicated by precipitation that is often low and erratic and soils that are highly variable in ability to supply N and hold available water. High temperatures and lack of precipitation during grain fill have commonly been observed to decrease yield response to fertilizers. While visual response can be large, grain yields may be unaffected. What is the best technology available for using N fertilizers on winter wheat in the Panhandle?
Soil Sample for Residual nitrate-N: The soil root-zone is the source of any carryover N from previous fertilization of cropping histories as well as any mineralization during fallow. Mineralization during fallow was a primary source of N for wheat until the 1960s. However, after years of cropping and soil erosion losses, relatively little N is usually released during fallow at the present time.
In order to evaluate the N status of your soil, soil samples should be obtained to a depth of 2 to 3 feet for residual nitrate-N analysis. This is the zone of most active root activity. (See current NebGuide G91-1000 on sample numbers and how to take the soil sample.) These samples should be taken prior to seeding or in the early spring before topdressing. Send samples to a laboratory that uses UNL’s recommendations based on deep samples. This is the best you can do to obtain the most accurate N recommendation possible.
When to Apply N: Nitrogen can be applied effectively either prior to seeding or as a spring topdressing. In terms of maximizing N fertilizer effectiveness, spring topdressing is the preferred method. Topdressing allows a later soil sample and most important allows later evaluation of yield potential than fall application. If stands are poor and soil moisture very low, a producer may decide not to invest more money in the crop. This can be a major advantage some years. Spring N applications are also often more effective per pound of N applied than fall application since they are applied closer to the time of actual N uptake and will normally result in higher grain protein than equal rates applied in the fall.
While long term results over 100 experiments shows fall and spring applications equal in increasing wheat yield, fall application sometimes stimulates early growth resulting in increased water use and depressed yields. Spring topdressing seldom if ever depresses wheat yield. While yield depression associated with N application is not usually a major problem, it is more likely to occur in the panhandle than other areas of the state.
Nitrogen can also be applied anytime after harvest during the fallow period. Recent experiments in southwest Nebraska have shown that applying N early in fallow (May) was less effective than application at mid to late August prior to seeding, which was also somewhat less effective than spring application.
Kind of N to Apply: The N source to use is dictated to great extent by a producer’s decision on when he applies the N. Topdressing is generally limited to dry or liquid N forms. Dry N fertilizer includes N ammonium nitrate (33% N) and N urea (45% N), the only commonly available liquid form is Urea-ammonium nitrate (28% N).
The preferred N source of both liquid and dry forms is ammonium nitrate. Both urea and urea-ammonium nitrate are subject to greater volatilization losses when unincorporated than ammonium nitrate. Volatilization loss is N lost to the atmosphere as ammonia gas. High soil pH conditions prevalent in the Panhandle make volatilization N losses more probable than in other areas in the state. However, because of various fertilizer plant production problems, ammonium nitrate many or may not be available. Urea-ammonium nitrate and urea are probably about equal as N fertilizers for topdressing in the spring. Incorporation reduces the probability of volatilization losses greatly.
Many producers want to apply N prior to planting rather than wait until spring. While all forms of N can be used effectively in the fall, ammonia becomes the preferred choice primarily because of its low price. However, this price advantage largely disappears in increased application costs unless the ammonia is applied in combination with normal tillage and rates above about 50 #N/A. Each producer needs to determine costs based on his operation and local ammonia versus dry and liquid N costs. Ammonia is a very effective N source and usually the preferred source for many producers.
Ammonia can also be very effectively applied in the spring as a topdressing, but special application equipment is required. Such equipment is not generally available without special adaptation of various knives and coulters. Ordinary knife applicators have been used successfully in the spring, but is not recommended because severe stand losses can occur depending on soil conditions.
If producers want to apply early in fallow, ammonia has been somewhat more effective than either ammonium nitrate or urea. In general the earlier in fallow it is applied, the more important it probably is to use ammonia. However, as indicated above, N fertilizer effectiveness decreases the longer the time between application and maximum crop uptake occurs.