Test zones show farmers grain crops' fertilizer needs

Small-grain growers can monitor crop fertilizer needs during the course of the season by applying large amounts of nitrogen to narrow strips at planting, then comparing the rest of the field to these lush, green areas.

According to University of California specialists, the simplest benefit of the test strip is that it lets farmers see the difference between the area known to have plenty of nitrogen and other, paler zones that might benefit from additional fertilizer.

Farmers can see this difference more precisely using advanced Normalized Difference Red Edge values from drone-mounted cameras, or hand-held devices that provide measurements of vegetative vigor.

UC researchers are offering tips on how to create these narrow, nitrogen-rich strips within grain fields as a practical and economical way of monitoring whether the entire field would benefit from midseason fertilizer applications.

"There should be sufficient nitrogen applied to the reference zone at planting to ensure that the plants in the zone are not limited by nitrogen at the stage of growth when the in-season fertilizer is applied," said Michelle Leinfelder-Miles, a UC Cooperative Extension farm advisor based in Stockton. "The amount of nitrogen to apply in the nitrogen-rich zone will depend on several factors, such as yield goal, protein goal and when the expected in-season fertilizer application will take place."

In general, she said, the higher the anticipated yield and the later in the season the reference zone will be used to help make a fertilizer decision, the more nitrogen farmers should apply at planting.

A wheat grower expecting yields of 3,000 pounds at 12% protein and planning on using the test strip to help make decisions about fertilizer at the tillering stage, for example, would want to apply 65 pounds of nitrogen to the reference area. But a wheat grower anticipating a 7,000-pound yield at 12% protein and planning on an application at the flag-leaf stage would want to apply 275 pounds, to make sure the reference strip is not nitrogen-limited.

Leinfelder-Miles is part of a team of researchers including Cooperative Extension farm advisors Nick Clark, Sarah Light, Konrad Mathesius, Tom Getts and Giuliana Galdi, grain cropping systems specialist Mark Lundy and assistant specialist Taylor Nelson, who are working with farmers on demonstration projects of this technique.

They said the goal of their research is to make it simpler and easier for grain growers to know they have applied enough nitrogen to optimize their crop without wasting money on excess fertilizer.

The researchers presented their suggestions on using the test strip via the UC Small Grains Blog, after moving their presentations from the annual Small Grains Field Day to the internet because of COVID-19. The blog may be found at ucanr.edu/blogs/smallgrains.

Farmers can calculate crop nitrogen need before a midseason application, or can simplify the process by applying to the reference zone some round number of additional pounds of fertilizer that is likely to be more than the crop will take up before that midseason decision is made, the UC specialists said.

"A grower can establish nitrogen-rich zones during the preplant fertilizer application," Leinfelder-Miles said. "For example, a grower may apply 50 pounds of nitrogen per acre across the field, and then make another pass or two in the zone to apply an additional 50 to 100 pounds of nitrogen per acre. This method might be most easily adopted by growers."

She said the reference zone created at planting should include typical areas from throughout the field.

"It is important to establish the nitrogen-rich zones in representative parts of the field," Leinfelder-Miles advised. "Areas of the field that are unique, like low areas, high areas or gravel strips, should be avoided. It is also important that the zones capture field variability. If certain areas have distinct soil types or known patterns of yield or management differences, a grower should establish enough zones to account for these sources of spatial variability if they represent large areas in the field. Soil maps and historical aerial imagery can often help in identifying field patterns and good locations for reference zones."

Once the nitrogen-rich test zones are created, she said, a soil quick test can be done before midseason fertilizer applications to measure the difference in crop-available nitrogen in the rich zone as compared to the rest of the field.

"Reference zones are most useful to growers who apply most of their seasonal nitrogen budget during or after tillering," Leinfelder-Miles said. "The reference zone is a relatively small area within the field where extra nitrogen fertilizer is added at the beginning of the season. This extra fertilizer ensures that the reference zone is not nitrogen-limited between planting and the time when an in-season fertilizer decision is made."

The team of researchers is continuing their field studies to fine-tune the technique under different conditions, and to show farmers throughout the state how it works.

"To demonstrate what we know about this practice across the small-grain-growing regions of the state, our team has helped growers establish nitrogen-rich zones in their fields across a wide range of soil and climatic conditions," Leinfelder-Miles said. "These demonstration sites are in the Sacramento Valley, delta region, San Joaquin Valley and the intermountain region. Our goal is to help growers and consultants learn how to use nitrogen-rich zones to improve small-grain fertilizer use efficiency and reduce negative environmental outcomes."

(Bob Johnson is a reporter in Sacramento. He may be contacted at bjohn11135@gmail.com.)