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Using N-Rich Reference Zones to Inform In-Season Nitrogen Fertilization Practices in California Small Grains By MICHELLE LEINFELDER-MILES | UC Cooperative Extension NICK CLARK | UC Cooperative Extension TAYLOR NELSEN | Department of Plant Sciences, UC Davis TOM GETTS | UC Cooperative Extension KONRAD MATHESIUS | UC Cooperative Extension SARAH LIGHT | UC Cooperative Extension GIULIANO GALDI | UC Cooperative Extension and MARK LUNDY | Department of Plant Sciences, UC Davis Over the last year, a team from UCCE has been working with California small grains growers on practices that can improve nitrogen (N) use efficiency. At demonstration sites, we have implemented practices that UC Grain Cropping Systems Specialist Mark Lundy has been investigating for several years, namely N-rich reference zones, a soil nitrate quick test, handheld reflectance devices and aerial imagery. We demonstrate how to use these tools to manage N fertilizers in small grain crops across variable soil and climatic conditions in the Sacramento Valley, Delta, San Joaquin Valley and Intermountain Region. The demonstrations are funded by the CDFA Fertilizer Research and Education Program and a USDA-NRCS California Conservation Innovation Grant. Our goal is to help growers and consultants learn and implement these practices to guide N fertilization in small grains, thereby increasing crop productivity and N use efficiency while reducing potential for N loss to the environment. What are “N-Rich Reference Zones”? Reference zones are most useful to growers who can apply the majority of their seasonal N budget during or after the tillering stage of growth. Previous work has shown that N fertilizer applied during the season−between the tillering and heading stages of small grain development−results in higher yields, higher protein and increased fertilizer use efficiency compared to preplant applications. The reference zone is a relatively small area within the field where extra N fertilizer is added at the beginning of the season. This extra fertilizer ensures that the reference zone will not be N-limited from planting until an in-season fertilizer decision is made. When a grower is determining whether and how much N fertilizer to add in-season, measurements from both the reference zone and the broader field are compared to understand whether the broader field is sufficient in plant-available N. Fertilizer N Rate and Field Variability Fertilizer N rate and field variability are two important considerations when creating N-rich reference zones. The amount of N to apply in the N-rich zone will depend on several factors such as yield goal, protein goal and when the expected in-season fertilizer application will take place. There should be sufficient N applied to the reference zone at planting to ensure that the plants in the zone are not limited by N at the stages of growth when the in-season fertilizer is applied. Table 1 (see page 9) gives some examples of how much N fertilizer to apply to the N-rich zone for a range of potential yields. It is important to establish the N-rich zones in representative parts of the field. Areas of the field that are unique (i.e. low areas, high areas, gravel strips, etc.) 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 multiple zones to account for these sources of spatial variability if they represent large areas in the field. Soil maps (available from casoilresource.lawr.ucdavis.edu/ soilweb-apps/) and historical aerial imagery can often help in identifying field patterns and good location(s) for reference zones. How and When to Apply the N-Rich Zone Fertilizer A grower can establish N-rich zones during the pre-plant fertilizer application. For example, a grower may apply 50 pounds N per acre across the field and then make another pass or two in the zone to apply an additional 50 to 100 pounds N per acre (depending on what the grower calculates is necessary, as described above.) This method might be most easily adopted by growers. We have observed, however, that if the fertilizer is placed too deep in the soil profile, the N may not be readily available to the seedling crop early in the season because it is below the root zone. Therefore, N-rich zones established by this method may not provide a reliable early-season point of comparison. Instead, we have found that broadcasting urea is the most effective way to establish N-rich zones. At our demonstration sites, we broadcasted urea after tillage or shortly after planting, but always ahead of a storm or irrigation event that could incorporate the fertilizer. Orienting the zones perpendicular to the rows or tractor passes also helps 8 Progressive Crop Consultant March / April 2021
Average Yield (lb/ac, 12% protein) Crop stage of assessment/application N rate (lb/ac) needed in N-rich zone 3000 tillering 65 5000 7000 3000 5000 7000 tillering tillering flag leaf flag leaf flag leaf 110 150 120 200 275 Table 1. Approximate N fertilizer application rates suggested for use in N-rich reference zones based on a range of average yields and two stages of crop growth. Suggested rates ensure that crops within the reference zone are not N-limited when an in-season fertilizer application decision is being made at the crop stage indicated. to capture field variability. When the zones are too narrow and run in the same direction as the field work, it can be hard to differentiate between a field pattern associated with equipment passes and a N effect, particularly early in the season. Monitoring the Field Once the crop begins to grow, the field should be monitored periodically to assess whether the crop is likely to respond to a N fertilizer application. A combination of the soil nitrate quick test (SNQT) and plant reflectance measurements taken from both the N-rich zones and the broader field can indicate when a top-dress fertilizer application may be beneficial. The soil nitrate quick test and plant reflectance measurements complement other important information like current crop growth stage, crop yield and protein goals, and local weather records to inform a site-specific N fertilizer recommendation. The SNQT is a simple and low-cost test that provides a ballpark estimate of the soil nitrate-N concentration in the root zone. Nitrate is a highly plant-available Continued on Page 10 YOU STICK TO GROWING. WE STICK TO YOUR SOIL. Accelerate your younger orchards and extend the viability of your older orchards with PhycoTerra ® . Visit PhycoTerra.com for more details. March / April 2021 www.progressivecrop.com 9
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