Boron and zinc: the almond postharvest duo
As we settle into the dormant season, the prep for next year gets well underway. This includes meeting the nutritional needs of the tree before it rests so that it’s ready to go come spring.
For almond, boron (B) and zinc (Zn) are the most important nutrient applications for fall, with boron taking the lead. Why? Because they need to already be in the bud at bloom.
Boron at high levels is toxic, but it is an essential nutrient and at low levels can be deficient and limit production. Where needed, adding B can make a significant increase in production. This micronutrient accumulates in the hulls, so when applied before harvest, some of it will get into the tree before harvest, but a lot of the boron ends up in the hulls and is hauled away during harvest. This makes postharvest an important time for boron input.
“Boron is really the key nutrient addition during this time,” said Franz Niederholzer, orchard systems farm advisor with University of California Cooperative Extension. “That postharvest application is really critical.”
Niederholzer champions a fall B spray, but also asks growers to keep special attention on hull boron levels to determine if a fall spray is needed. Particularly in a year such as this, where drought conditions had California growers reaching for groundwater, which can contain significant levels of boron. There are some areas of the state where soils, groundwater and even surface water have high boron content, which has growers fighting the other side of the battle. But boron is more likely to be deficient in fields than it is in excess in many California almond orchards.
A harvest hull sample is the key piece of information needed to determine if fall boron fertilizer should be applied.
“If you’re super high in boron, you’re probably aware of it, but it’s good to have the data to confirm it,” said Niederholzer.
The traditional fall foliar spray recommendation for boron on almond is 0.4 lbs actual B/ac, and has been this way since the mid-1990s. (This also equals about 2 lbs/ac of the popular commercial product Solubor.) Of course, it’s a bit of a sliding scale, where less can be used, but Niederholzer advises against applying anything more than a half pound of actual B maximum per acre in a fall foliar spray.
Research has shown that orchards significantly deficient in boron can increase yield by anywhere from 100-300 lbs. of kernels per acre with a fall boron spray. This might not be the case in drought years because of possible additions via groundwater, but, generally speaking, many growers have a tendency to forget about boron in the fall, when the nutrient’s cost is usually lower.
“If we’re talking about hundreds of pounds of nuts per acre that come out of $5-$10 of material per acre fertilizer application, the ROI is off the chart,” Niederholzer said.
Not only that, but soil-applied boron after harvest doesn’t get into the flower in time to change the picture at bloom, added Niederholzer. It will get in later in the season, but bloom is the most critical time.
Trailing behind boron is a postharvest application of zinc. Unlike boron, zinc doesn’t get exported in the crop nearly as much, though zinc is more of a widespread deficiency risk in California’s almond growing regions. And while there should still be adequate levels of zinc in the tree, and fall is a good application window, this nutrient can still be applied during the growing season. The traditional strategy program is 20 lbs/ac zinc sulfate put on in November, which gets zinc into the tree and can also help with defoliation around normal leaf fall. This could be necessary if disease, such as rust or scab, is present and the winter is dry and natural defoliation slow, and it complements an integrated disease management program.
Both the boron and zinc can go on in October, with zinc sulfate at a rate of 5 lbs/ac, which won’t damage the leaves and allows them to serve as an avenue for entry.
“At the lower zinc rate, the leaves remain healthy and provide a good energy source to the tree until natural leaf drop, delivering maximum carbohydrate storage for next year,” said Niederholzer. “There’s been a trend toward a lower rate of zinc sulfate – save money, save the leaves – in October.”
Phosphorus and potassium
The idea behind applying potassium (K) and, where needed, phosphorus (P) postharvest is that the soil serves as a bank until the nutrients are needed to grow the crop next spring. Growers can put these nutrients on in the fall when there’s relatively more time and the dry material costs are lower than liquid products.
“It’s a logistical economic decision this time of year,” said Niederholzer. “Growers can fertigate with potassium and phosphorus in season or make an ante, if you will, and put some up ahead of time.”
The trick, however, for a fall application – or even a winter application – in almonds in a drought year is water. A cost savings occurs in dry materials because water isn’t being delivered in the form of liquid fertilizer.
But growers do have to have their own water to dissolve the dry material.
“And if you have a dry winter and the wind blows hard, or you put a sweeper through the orchard to move leaves or mummies or anything like that, you run the risk of blowing your expensive fertilizer all over the place,” Niederholzer added.
This is also a less critical issue than boron and zinc. Growers can take leaf samples throughout the growing season to check K and P levels and make the necessary fertigation applications to avoid problematic deficiencies.
Niederholzer was part of a study in which as much as 60 lbs/ac of nitrogen was applied to almonds in September/ October. During the three years of study, there was no change in production the following year, suggesting that there isn’t much benefit to a postharvest nitrogen application.
“The trees don’t have that much room to store it,” Niederholzer said.
The important point is flower bud differentiation, where the buds come to a “fork in the road” during summertime around harvest and become either a flower or vegetative growth, after which growers can’t change the course, but simply sustain the buds that develop.
“Fall potassium, phosphorus or nitrogen really just sustain and allow growers to keep whatever potential is made during the summer and through harvest,” said Niederholzer. “Good nitrogen management in season should eliminate the need for nitrogen post harvest.”
At the heart of any California agricultural commodity is water – how much can a grower put on and what’s in it. The risks of salinity and/or toxic elements are up in many orchards because of the need to apply groundwater, which might not be at the higher standards of surface water. But in years where there is little to no surface water, like recently, the options against using potentially substandard water become extremely limited to nonexistent.
“Growers apply N-P-K fertilizer and feed the crop and adjust application rates as the season goes on,” said Niederholzer. “But the focus for 2021 going into 2022 is really boron. If growers have been using boron religiously, be careful with what’s there and adjust the rates, maybe on how much the hull levels show. Also, the need for a late season salt management or winter irrigation can fit in with the need to apply less expensive dry K and P materials.”
While there aren’t any major changes to fall practices, Niederholzer warns that for orchards that were shorted water this past year, the upcoming crop may not be as good as it would be with adequate levels of good water, which means growers should prepare to match the fertilizer needs to the crop they having coming. Applying the same rate year in and year out can be troubling for growers economically and the trees nutritionally.
“I think that boron is the real focus right now,” said Niederholzer. “If growers are managing everything as well as they can, in a good way, a fall boron is the one thing to really pay attention to because timing is so critical, and growers can catch up with other fertility practices once the season starts.”
Photo: Cooperative Extension orchard systems farm advisor Franz Niederholzer applies boron after harvest. Photo: Franz Niederholzer