Nitrogen management takeaways from recent research
- Results indicated walnuts accumulate less nitrogen (N) per ton of harvested nuts than previously thought, but research still needs to estimate how much N is needed to grow the tree itself.
- Evenly dividing N application over May, June, July and August will match N accumulation in the fruit.
- When sampling leaves to assess orchard nitrogen status, sample at least 29 trees, at least 100 feet apart.
With regulations stemming from findings of groundwater nitrate contamination and orchard management expenses on the rise, walnut growers are working to apply their fertilizer efficiently, to reduce nitrate leaching and wasted resources. But much of the information necessary to optimize fertilizer usage has been lacking.
In 2013, a team of UC Davis researchers (Drs DeJong, Brown, Lampinen, Hopmans, Laca & Saa) and UC Cooperative Extension farm advisors (Jarvis-Shean, Buchner, Fulton & Grant), with support from the California Walnut Board and the California Department of Food and Agriculture, began a multi-year project to quantify the timing and scale of walnut orchard nutrient needs on typical soils at today’s high yields with today’s cultivars.
The team is still waiting to collect data on one final big piece of puzzle – how much nitrogen is used to grow the perennial tissue (roots, branches and trunk). But we do have some answer to important nitrogen management questions about use throughout the growing season and how to make leaf sampling more efficient.
How much nitrogen does it take make a ton of walnuts?
Over three years at six production orchards, we found an average of 29 pound of nitrogen in every ton of harvested in-shell nuts. Now we know how much N is in a ton of nuts. We’re still working to figure out how much more is needed for tree growth, so this is not a finalize number for your official Irrigate Lands nitrogen worksheets.
How’d we get our 29 lbs answer? We sampled harvested nuts from six production orchards over three years just before growers harvested. We wanted to capture a broad range of walnut growing conditions, and check for differences in nitrogen use by variety, so we had one Chandler site and one Tulare site each near Hanford, Linden and Red Bluff. At each site we collected nut samples from ten trees each year and analyzed the meat and shell and hulls for nitrogen content.
Total nitrogen accumulation varied from site to site and year to year, from 23-35 lbs per ton of in-shell harvested nuts at 8% moisture. Figure 1 shows this variation by sites and years. Notice that Chandler and Tulare numbers are similar. We found no significant difference in nitrogen accumulation by cultivar. Practically speaking, this means nitrogen does not need to be managed differently for different cultivars.
Observed total N content in one ton of nuts is lower than that observed by UC research in the 1990s by about one quarter. At some sites, some nuts approach as much N accumulation as was previously reported, indicating that our larger sample size may show more of the variability than was captured by the six trees in the older study. Additionally, our figures do not yet include N allocation for perennial tissue growth, though this is expected to be small compared to what is removed compared to nuts. It is likely that the upper range of N use in our sites will overlap with older figures once perennial tissue growth is added to the total nitrogen budget.
When should nitrogen be applied to optimize efficient use?
By sampling walnut fruit each month in six orchards over three years, we found N accumulation in fruit is fairly evenly distributed over the course of the growing season. Practically speaking, evenly dividing N application over the growing season starting one month after leaf out will match N accumulation in the fruit.
Figuring out how much nitrogen to apply at what times of year requires knowing when walnut trees using nitrogen. Previous research had already taught us a few lesson:
- Don’t apply nitrogen in the late fall or during winter. The majority of this nitrogen will turn into nitrate and leach out of the root zone, below where trees can use it.
- Trees use stored nitrogen for the first month of growth, so applications in March or April are not necessary and will be vulnerable to leaching loss.
We went into the project knowing that nitrogen is taken up during the growing season, but how much is used when? To answer this question, we sampled fruit from the same six sites during the same three years every month of the growing season, from May through harvest, and analyzed those fruit for nitrogen content. Based on the weight of the fruit each month and at harvest, we could transform those data into pounds of nitrogen used each month to build what would grow into one ton of walnuts.
Figure 2 shows nitrogen accumulation in the fruit (meat, shell and hull) on a monthly basis, averaged over all sites, cultivars and years. The dark brown bar shows the amount of nitrogen accumulated in previous months. The light brown portion shows the amount of nitrogen added to the fruit during that month.
Notice how the height of the light brown are about equal for each month. This illustrates the fact that nitrogen accumulation in fruit was observed to be evenly distributed over the course of the growing season for all cultivars and sites. This is in keeping with research previously done on almonds in California, and on walnuts in Europe. This indicates an even division of nitrogen application over the growing season may be a simple, straightforward approach to increasing nitrogen use efficiency.
How can leaf sampling be made easier and more reliable?
Using a 30-tree grid in each of the six orchards over three years, our data showed that when sampling leaves to assess orchard nitrogen status, 29 trees should be sampled, each at least 100 feet away from each other, to be within 0.1% of the true orchard N status 95% of the time.
Nitrogen deficiency can reduce tree growth and yield. Applying N in excess of tree demand is inefficient and potentially detrimental. Leaf nitrogen sampling is a great tool for monitoring orchard N status and avoiding deficiencies and excesses.
Researchers have developed “critical values” thresholds to show the sweet spot for leaf N concentration to optimize fertilizer management. By comparing an orchard’s leaf nitrogen concentration against these “critical values”, you can see if an orchard is nitrogen deficient, or has an over-abundance of nitrogen. Previous research found that for walnuts, the nitrogen “sweet spot” is between 2.3-2.7% N. By watching nitrogen levels at the same time of year over many years, trends in nitrogen levels can be seen, to show if management changes are changing the amount of nitrogen orchard trees are taking up from the soil.
But it takes time to collect leaf samples, and it costs money to run those lab analyses. Spending the time to sample a leaf from every tree in orchard would guarantee the lab results truly reflected the orchard’s nitrogen status. But who has the time for that? On the other hand, sampling just a few trees will give some number back on a lab form. But how do you know if those few trees represent the big picture of nitrogen in that orchard?
To minimize time spent sampling, we wanted to figure out the lowest number of trees that could be sampled from an orchard and still capture the orchard’s nitrogen status, but also ensure confidence in the sample results. So, we looked at how much natural variability there was in leaf nitrogen from tree to tree in our six orchards over three years.
From that we can calculate how many trees need to be sampled to come comfortably close to capturing the variability 95% of the time. How close is “comfortable”? How often is “most of the time”? Given the walnut leaf % N sweet spot is 2.3-2.7%, we decided we’d be comfortable with a fudge factor of +/- 0.1%. In other words, we’re OK with lab results showing 0.1% N more or less than what’s really happening in the orchard.
Based on all our samples, statistics found that even in the most variable orchard, sampling 29 trees gave us that +/- 0.1% comfort range 95% of the time. If only 21 trees were sampled, results were only in our “comfortably close” range 90% of the time. With 13 trees sampled, results were in our “comfortably close” range only 80% of the time. This analysis also found trees farther than 100 feet apart were independent.
Practically speaking, this means to assess orchard nitrogen status, sample leaves from at least 29 trees at least 100 feet apart. If there is significant difference in soil texture within an orchard block, even more trees are necessary for the same certainty.
Conclusion
This multi-year, multi-site project, conducted with support from the California Walnut Board and the California Department of Food and Agriculture, has produced a number of results useful for walnut nitrogen management. Harvest results indicated walnuts accumulate less nitrogen per ton of harvested nuts than previously thought. Monthly fruit samples showed evenly dividing N application over May, June, July and August will match fruit N accumulation. Sample grid analysis reveal that when sampling leaves to assess orchard nitrogen status, at least 29 trees 100 feet apart should be sampled to ensure the test results are within 0.1% of the true orchard N status 95% of the time. The research team continues to gather and analyze data to answer additional questions, including how much nitrogen is needed to support tree growth and how May leaf sampling can give an early season picture of an orchard’s nitrogen status. We look forward to sharing these results in the future.
