Kat Jarvis-Shean, Orchard Systems Advisor UCCE Sacramento-Solano-Yolo
In a lean year like this with high fertilizer prices, you may be thinking about cashing in on some stored nitrogen in your walnut trees. We know that trees have much more nutrient storage capacity than annual crops, but how much of this can be relied upon to meet nitrogen demand, and when might it not be appropriate? A series of trials in the late 1980s can help inform this decision-making process. At three different sites, UC researchers had two sets of trees – both had previously received the grower’s normal nitrogen application, but then they stopped applying any nitrogen to one set while continuing to apply adequate nitrogen to the other set. They watched July leaf nitrogen levels and yield to see when those two metrics would respond to the lack of applied nitrogen. At one site with 8th leaf Serrs, leaf nitrogen and yield dropped significantly the year after nitrogen was withheld. At another site with 10th leaf Hartleys and a third site with other mature Hartleys, it took three years for leaf nitrogen to drop significantly and four years for yield to drop. In the Serr block, yields rebounded to be indistinguishable the year after nitrogen was restored. In the Hartley orchards, it took two years for yield to rebound to the levels of continuously fertilized trees.
What can we learn from all this? Sensitivity to a lack of nitrogen will vary from orchard to orchard. The Serr site was a sandy loam, while the Hartley sites were a loamy sand and a silt loam, meaning the Hartley sites probably had more nitrogen buffering in their soils. Given that it’s hard to know how much soil volume trees at a specific site may be mining for nitrogen, leaf nitrogen content is a much more reliable way to guide decisions. How much nitrogen you can cut back will depend on your leaf nitrogen content in previous years, and how much free nitrogen you may already be applying if you’re irrigating with groundwater. Leaf nitrogen in the range of 2.5-2.7% is considered sufficient. Shooting from the hip a bit, I’d say if last year’s leaf levels were above 2.9%, you may be able to get by applying roughly half the crop’s nitrogen demand this year in large, mature trees without driving your trees into the insufficient range. This would especially be true if you’re already meeting a large portion (likely 25% or more) of your nitrogen demand with nitrates in your groundwater. Remember that for every 1 ppm of nitrate in your water, there’s 0.6 lb of nitrogen in an acre-foot of applied water. If your report shows a different form, 1 ppm nitrate-nitrogen, that changes to 2.7 lb for every acre-foot. The closer last year’s leaf nitrogen was to 2.7% and/or the less nitrate in your groundwater, the more caution I would urge in decreasing nitrogen applications.
The way to get the most benefit from a lower rate of nitrogen would be to use the same number of applications but with a lower rate each time. This is relatively easy with a fertigation system. However, if you’re broadcasting, you might be tempted to decrease the cost of application, not just the cost of your inputs, by applying your nitrogen in fewer doses. Strongly consider the cost of one round of broadcasting in person hours and fuel (likely $5-10/acre depending on your operation), versus the loss in use efficiency when concentrating your application (ballpark ~30% efficiency with one dose, 40-50% with two doses, 60-70% with 3-4 doses). If you save $5/acre by applying nitrogen with one less dose, but lose 20-30% of that nitrogen below the rootzone because of lower uptake efficiency, are you coming out ahead? Every orchard’s economics will be different. If you do feel the need to limit application to fewer doses, be very careful with the duration of the irrigation sets that follow those fewer doses, running shorter sets to help keep that nitrogen in the rootzone, and available to the tree, for as long as possible. Given all the stored moisture in our rootzones this spring, recent research indicates you shouldn’t need to apply much irrigation before June. Just enough to move nitrogen into the soil profile.
All of the above is pertinent to mature trees. Younger trees are more sensitive to reduced nitrogen application because they have less biomass in which nitrogen from previous years may be stored. Forcing deficiency on young trees impacts growth of future nut-bearing branches, delaying future yield potential. Older trees will have more tissue in which nitrogen may have been stored from previous years, so will be more resilient to decreased nitrogen applications, making them better candidates for decreased nitrogen application during cost-cutting times.
Whatever approach you use to decrease nitrogen input cost, the one thing I wouldn’t skimp on would be leaf sampling. Both Hartley sites saw a decrease in leaf N the year before decreased yield. If you farm multiple blocks, at minimum, continue to collect samples from a few blocks that represent similar ages, yields, management and growing conditions.