To save time and cost, some wheat producers may be thinking about adding a little extra nitrogen (N) as urea or UAN to their phosphorus fertilizer through the drill with the seed. This would either be in addition to, or instead of, any preplant N applications.
While a minimum preplant N application of 20 to 40 lbs N per acre is often desirable, especially in no-till production systems, it is important to avoid placing urea containing fertilizers in direct seed contact. We suggest that NO urea or UAN solution be placed in contact with the seed. If the fertilizer N applied at seeding will be separated from seed by 1 inch or more, urea-containing fertilizers can be safely used.
Methods of early-season nitrogen applications
If the starter fertilizer can’t be “spiked” with urea to add extra N, how can the necessary 20 to 40 pounds of N be applied? Subsurface banding (knifing) of N as either anhydrous ammonia, liquid UAN, or dry product will result in the greatest N use efficiency by the wheat crop. This is especially true for no-till wheat production.
If knifed N applications are not used, the next best application method would be surface banding (dribbling) of UAN solution in streams on 15- to 18-inch centers. Broadcasting urea, ammonium nitrate, or UAN applications are not generally as efficient as subsurface banding, but they are often the best choice due to equipment, logistics, or weed management considerations. Broadcast applications of N will have the most consistent performance if followed by light incorporation, precipitation, or irrigation.
Direct seed placement of nitrogen
When placing starter fertilizer in direct contact with wheat seed, producers should use the following guidelines:
Suggested Maximum Rates of Fertilizer to be Applied Directly With Wheat Seed
Pounds N + K2O (No urea or UAN)
Medium to Fine
Sandy or Dry
The problem with placing urea-containing fertilizer with the seed is that urea is initially converted to ammonia and may be toxic to plant roots if the wheat seed is placed in direct contact with the fertilizer. Producers may hear of someone who has placed urea in direct seed contact and seemed to have no problems, but there are also many cases where urea-containing N fertilizers have injured the developing seedling and reduced or delayed emergence significantly. The risk of injury is greater in drier soils, at higher soil pH levels, and at higher N rates. High soil pH favors a higher concentration of ammonia as compared to ammonium as urea hydrolyzes. There is significant risk associated with placing urea-containing fertilizers in direct seed contact.
The chart below shows how soil texture affected the level of wheat germination when urea-N was applied with the seed in a K-State greenhouse study. The wheat was well watered in this study, but urea-N placed with the seed still reduced germination, especially in the sandy soil. The readings shown below were taken after 10 days. With the high rates of urea used in this study, it is possible that more damage to the seedlings would occur with time as the urea continues to hydrolyze into ammonia.
Figure 1. Greenhouse study evaluating the effect on germination of urea placed with the seed. Plant counts 10 days after planting. Source: Dorivar Ruiz Diaz, K-State Research and Extension.
Field studies have also shown reduced wheat stands due to in-furrow placement of urea. Across 5 site-years in western Kansas the placment of urea in-furrow has resulted in decreased stands at spring greenup compared to the control (Figure 2). In the figures below, ESN is a controlled-release, coated form of urea- N. NBPT is a urease inhibitor designed to reduce surface-applied urea-N losses from ammonia volatilization.
Figure 2. Wheat stands at spring green-up as affected by nitrogen application with different placements and sources. Average of 5 site-years. Source: Lucas Haag, K-State Research and Extension.
The stand reduction becomes especially noticable at higher rates of N. One of the challenges in understanding the risk of seedling injury is that the magnitude of injury varies by field conditions and year. In some years very little stand reduction may be evident, even at higher rates of N; while in other years, stand reductions (and the associated impact on yield) is very evident. As an example, at Tribune in 2017, reductions in stand caused by urea placement with seed, and the effect on yield were quite evident (Figures 3 and 4).
Figure 3. Wheat stands at spring green-up as affected by nitrogen application with different placements and sources. Average of 2 locations in 2017. Source: Lucas Haag, K-State Research and Extension.
Figure 4. Wheat grain yield as affected by nitrogen application with different placements and sources. Average of 2 locations in 2017. Source: Lucas Haag, K-State Research and Extension.
Stands were reduced 32 and 63% compared to the control when 30 and 60 lbs of N as urea were applied in-furrow (Figure 3). This resulted in yield reductions of 14 and 40%, respectively (Figure 4).
If you’d like to apply extra N directly in the seed furrow, one option is to use a controlled-release form of N, such as ESN. As shown in Figure 4, at N application rates of 30 lbs/acre and less, where ESN-N was applied in-furrow, wheat yields were essentially the same as where the N was applied pre-plant -- and higher compared to the same amount of N applied as urea. At the highest rate of application in the study, 60 lbs/acre, even ESN resulted in stand and grain yield reductions.
Air seeders that place the starter fertilizer and seed in a band an inch or two wide, rather than a narrow seed slot, provide some margin of safety because the concentration of the fertilizer and seed is lower in these diffuse bands. In this scenario, adding a little extra urea containing N fertilizers to the starter less likely to injure the seed - but it is still a risk.
Lucas Haag, Northwest Area Crops and Soils Specialist
Alan Schlegel, Agronomist-in-Charge, Southwest Research-Extension Center, Tribune
Dorivar Ruiz Diaz, Nutrient Management Specialist