Winter Wheat Nitrogen Management in the 18- to 25-inch Precipitation Zone

Chapter 6-Fertility, No. 15, Spring 1990

Roger Veseth

The need for improved nitrogen (N) fertilizer use efficiency in winter wheat is increasing from both production efficiency and environmental standpoints. Improved fertilizer use efficiency would increase farm profits and reduce the potential for pollution of surface and groundwater. Selection of placement method, timing and rate of applied N fertilizer can influence use efficiency, crop production potential and environmental impacts.

Nitrogen fertilizer rates should be based on soil test results and realistic yield potentials for available water. Timing and placement of N fertilizer are influenced by amount and distribution of precipitation and soil factors. Together these determine N availability to the winter wheat crop and potential for losses to the environment.

Improving N Use Efficiency

Improving N fertilizer use efficiency has been a research focus of several soil scientists in the Pacific Northwest, particularly those involved in the STEEP (Solutions To Environmental and Economic Problems) research program on conservation farming in Idaho, Oregon and Washington.

Two soils researchers who have been addressing this topic in the STEEP effort are Robert Mahler, at the University of Idaho in Moscow, and Fred Koehler, at Washington State University in Pullman. The results of their research, and related research projects in the Northwest, have shown that climatic and soil conditions strongly influence management choices to improve N fertilizer use efficiency, achieve desired yields and reduce N losses.

Mahler and Koehler point out that N fertilizer should be applied before or at seeding of winter wheat in most cropland areas of the Inland Northwest which receive less than about 18 inches of annual precipitation. Research has shown that banding all of the winter wheat N fertilizer requirement below seeding depth before or at seeding time generally results in the higher yields and N use-efficiencies, and consequently less environmental hazard than with broadcast or spring-applied N.

The opposite is true, however, in some areas of northern Idaho which receive more that 25 inches of annual precipitation. In these wetter areas, up to 90 percent of fall-applied N can be leached below the root zone in well-drained soils or lost through denitrification. Denitrification is the microbial conversion of nitrate N to the gaseous form N which can be lost to the atmosphere under waterlogged, anaerobic conditions in poorly-drained soils. This is particularly a problem in

bottomlands with clayey subsoils which restrict downward water movement. Under these conditions, it is best to split N applications between fall and spring, with a majority of the N spring-applied and only sufficient fall applied N to ensure vigorous fall growth and stand establishment.

Strategies for N fertilizer application in winter wheat are less clear for Inland Northwest winter wheat areas which receive from 18 to 25 inches of annual precipitation (intermediate zone). Strategies will also vary in the zone depending on annual precipitation, soil and landscape characteristics and other considerations.

Research Effort

The development of N fertilizer management strategies in this intermediate precipitation zone has been the focus of a cooperative research project by Mahler and Koehler, with assistance from UI soils research associate Larry Lutcher. The project was conducted from 1982 through 1987 at the UI Plant Science Farm near Moscow in a 22-inch annual precipitation zone. Data from the 1984-85 winter wheat crop was eliminated in the summary because dry fall conditions caused poor fall germination and reduced yields compared to the other years. Annual precipitation (Sept. 1 to August31) during the study were 32,26, 19,29 and 20 inches for the 1982-83 through 1986-87 winter wheat seasons, respectively.

Background Information

The research site was a nearly level bottomland location with a deep Latahco silt loam soil. Soil pH (1: 1 saturated paste) was 5.4 in the surface 12 inches. Organic matter content was 3 percent. Previous crops were winter wheat, spring dry pea, spring wheat and fallow in 1982, 1983, 1985 and 1986, respectively. Stephens winter wheat was seeded in mid-to late-September each year with Koehler’s research, double-disk drill which bands dry fertilizer with the seed through the double disk openers in a 12-inch spacing, or 2 inches below the seed with separate hoe openers ahead of the seed openers. Seedbeds were prepared with conventional tillage. Phosphorus and sulfur fertilizer applications based on soil tests were broadcast and incorporated as needed before seeding.

Nitrogen Management

Nitrogen fertilizer application rates followed UI and WSU fertilizer guides for dryland winter wheat. They were based on soil test N available to 4 feet, estimated N release from decomposing soil organic matter during the growing season and estimated N tie-up in microbial decomposition of previous crop residue. Nitrogen fertilizer rates for the 126 bu/acre winter wheat yield goals varied from 119 to 172 pounds N/acre during the 4 years.

Seven of the N fertilizer placement/timing treatment combinations included:

1. All banded below the seed
2. All surface broadcast -fall
3. All surface broadcast -spring
4. 50% banded below the seed and 50% surface broadcast – fall
5. 50% banded below the seed and 50% surface broadcast – spring
6. 25% banded below the seed and 75% surface broadcast – fall
7. 25% banded below the seed and 75% surface broadcast – spring

Broadcast N treatments were either applied immediately after seeding or in April when the plants had developed about four tillers (Zodaks growth stage 24). Treatments with N fertilizer banded with the seed instead of below the seed (similar to below the seed treatments of 1, 4, 5, 6 and 7) were also included. Two N fertilizer sources were evaluated in each fertilizer placement/timing treatment, These were urea (45-O-O, granular) and ammonium nitrate (34-O-O, granular). A total of 25 placement/timing/N source treatments were evaluated in the study, including a non-fertilized control.

Results

Nitrogen Source – When seed-banded treatments are not considered, urea and ammonium nitrate produced similar winter wheat yields. The researchers point out that with the slightly acid soils in the study site, volatilization loss of N (loss as N gas) with broadcast urea N was not a problem and both sources of N were equally efficient. On alkaline soils, however, research has shown that broadcast, non-incorporated applications of urea can result in significant volatilization losses. There was a reduction in yield with seed-placed urea compared to ammonium nitrate (see following section).

Nitrogen with the Seed –Toxicity problems were encountered with both N sources in all seed placement combinations as a result of salt and/or ammonia damage to the seed during germination and emergence. As expected, urea was most damaging when placed with the seed. Average yields with seed-placed urea treatments were only 35 to 55 percent of yields in treatments with seed-placed ammonium nitrate treatments. Treatments with N banded below the seed or broadcast produced average yields 75 to 200 percent higher than in treatments with any seed-placed N, The lowest N rate with the seed provided approximately 35 pounds N/acre, a rate higher than the commonly suggested limit of 15 to 25 pounds N/acre under average conditions. Maximum, safe N application rates for placement with the seed vary with soil water content, temperature, soil texture and N source.

Best Treatments – The researchers point out that yield and N use efficiency are considered the two most important measures for evaluation of the relative success of N fertilizer treatments. They defined N use efficiency in their study as the percentage of inorganic soil N (soil test N + mineralized N released from soil organic matter + applied fertilizer N) found in the above-ground portion of the winter wheat plant at harvest.

Treatments with 25 to 50 percent of the N banded below the seed at planting and the remaining 75 to 50 percent spring broadcast, produced significantly higher yields and N use efficiencies when compared to the average of all other treatments except the control and seed-banded treatments (Table 1). The control and seed-banded treatments were excluded here because they produced unacceptably low yields and N use efficiencies compared to the other treatments.

Both of these split application treatments of below the seed/spring-broadcast resulted in higher yields than with similar application splits with fall broadcasting in 3 of the 4 years. Nitrogen use efficiencies achieved using these spring-broadcast split application were higher than with fall-broadcast splits each year (Table 2).

Although split application treatments of banding below the seed/spring broadcast were more effective than similar splits with fall broadcasting, broadcasting all of the N in the spring significantly reduced yields compared to all fall broadcasting in 3 of the 4 years (Table 3). The researchers point out that this shows the need for a portion of the N fertilizer to be available in the fall to provide adequate fall growth and development. It also allows some N movement deeper in the root zone for use later in crop development.

Table 1. Comparison of winter wheat yields and N use efficiencies of split N fertilizer application treatments of banded below the seed/spring broadcast vs. all other N fertilizer treatments (excluding the control and seed-banded treatments), 1982-87 near Moscow, ID.

Table 2. Comparison of winter wheat N use efficiencies of spring vs. fall broadcasting of N fertilizer, each in split application with fall banding below the seed, 1982-87 near Moscow. ID.

Table 3. Comparison of winter wheat yield with 100 percent spring vs. 100 percent fall broadcast applications of N fertilizer, 1982-87 near Moscow, ID.

Conclusions

The researchers conclude that splitting the winter wheat N fertilizer application between fall and spring would generally provide the highest yield and N use efficiency in the 18- to 25-inch precipitation zone. The actual proportions of the split between spring and fall, and fall placement would depend on the annual precipitation. At the lower end of this precipitation range, they suggest that most of the N be banded below seed depth at or before planting with a small amount broadcast in the spring as a topdressing. At the upper end of this precipitation zone, a smaller portion of the N fertilizer should be applied in the fall, to reduce leaching and denitrification losses overwinter, with the remainder applied in the spring.

Management Cautions!

The researchers stress that, due to limits on the size and complexity of the study, several important aspects of N management for winter wheat production were not addressed in the study. They caution producers to also consider some of the following aspects before making a choice of N fertilizer application method and timing. Differences in N use efficiency and yield potential could result.

Grass Weed Control – Research has shown that broadcast applications of N fertilizer can increase the populations and competitiveness of some grass weeds, such as wild oat and downy brome. Since weed control on the experimental plots was much more intensive than possible under farm production fields, increased weed competition from the different treatments was not a factor, Consequently, they point out that the need to control grass weeds should be considered by producers in their selection of N application methods and timing.

Surface Runoff Losses – Another consideration in choosing the method and timing of N fertilizer applications is the potential for loss in surface runoff. On sloping cropland, precipitation events, resulting in surface runoff after a broadcast application of N fertilizer, could cause significant loss of N in the runoff water, particularly if the fertilizer is not incorporated in the soil. Loss of N would be most severe with intense runoff events after N fertilizer has been broadcast in late winter or early spring on wet or frozen soil.

Feasibility – These experimental results indicate that higher winter wheat yields and N use efficiencies may be achieved by applying part of crop’s N requirement in the spring under the climatic and soil conditions near Moscow. However, it may not always be feasible to apply the desired amount of N fertilizer at the proper time. Wet soil conditions, sloping cropland, equipment capacities and limitations, and costs may limit ground or aerial application of large amounts of N fertilizer in the spring. Sufficient precipitation after broadcast applications in the spring is also needed to effectively move the N into the root zone for plant uptake. Nitrogen “stranded” near the soil surface under dry spring conditions could limit root absorption and yield potential.

Nitrification Inhibitors – The study did not address the influence of nitrification inhibitors which can be used with fall-applied ammonium N fertilizers to help slow the conversion of ammonium N to nitrate N, thereby reducing the potential for N losses by denitrification under wet anaerobic conditions or by leaching. Nitrogen in the ammonium form is immobile in the soil and, unlike nitrate N, is not subject to loss by denitrification or movement below the root zone with soil water. The success of nitrification inhibitors for keeping N fertilizer in the ammonium form varies with climatic conditions, soil properties and other factors. Use of these products could influence the effect of N fertilizer placement and timing selection on N use efficiency and yield potential.