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Pushing high-density orchards into peak production in just a few years requires precision nutrition for the trees.

Using frequent soil tests to guide both pre-plant soil amendments and fertigation will drive growth but prevent burning sensitive young trees.

“This is the one chance we get to really affect how the plant is going to be serviced through the nutrition we give it,” said Pete Sackett, operations manager for Wilbur-Ellis. He spoke at the 60th annual International Fruit Tree Association meeting in Wenatchee, Washington, in February. He was joined by Dan Griffith of G.S. Long to talk about best practices for nourishing new orchards.

Before planting, more growers seem to be boosting field fertility with compost instead of manure, due to public perception that manure may be unhealthy, Griffith said.

“The risk is very, very low on tree fruits, but the market is being moved to compost,” he said. “For those of you that use manure, I use about 70 percent of the compost rate I talk about because manure is generally a little hotter.”

If soil tests show less than 40 pounds of nitrogen per acre-foot, phosphorus is below 30 parts per million and total soluble salts are low, Griffith recommends applying 20 tons of cattle compost per broadcast acre. In the middle range, with phosphorus between 30 to 50 ppm and nitrogen about 40 to 80 pounds per acre-foot, he’ll apply 10 tons per acre.

But if salts are high and nitrogen levels are already over 90 pounds per acre-foot, or if phosphorus is over 60 ppm, he recommends skipping the compost entirely.

“The reason the phosphate is important, on the chemical extract side, is if you get too much phosphate, you get lots of heavier metal deficiencies such as zinc and iron,” Griffith said. “It moves painfully slow in the soil, so it can build up too much and you will have problems. I certainly have over the years.”

And if you are using chicken compost instead of cattle compost, Griffith recommends scaling back to a third of the amounts suggested above, because it is so much stronger.

Sackett recommends doing as much as possible to get an orchard set up — soil improved, trellis built, irrigation ready — to minimize the stress on the baby trees when they are planted.

“So when the trees are planted and mudded in, that makes maximum root contact with the soil,” Sackett said. “And for those that don’t have irrigation, there’s a sprayer available like we had when I was a kid, and you just go tree by tree and mud them in.”

After the orchard is planted, if you want to go the fertigation route — as growers with new drip irrigation systems often do — it’s important to work with experienced suppliers to set up the equipment, said Griffith. To be safe, you want a system with check valves and a switch to ensure that if your irrigation pump shuts down, the fertilizer injection shuts off as well, he said.

“Big problems can happen because fertilizer is heavier than water. The next day, because fertilizer is heavier than water, at the lowest point in the mainline you’ll be shooting pure fertilizer and killing trees, which I have done,” he said.

Griffith described some common fertigation rates for new apple trees, adding that in Eastern Washington, cherries and pears burn more easily than apples.

To push growth on nonbearing apple trees, it’s common to apply between 10 to 20 pounds of nitrogen per acre per week, distributed through a three- or four-hour injection time, he said. Running longer can waste fertilizer by watering it too deep into the soil, but running for a shorter window can lead to uneven distribution of the nutrients across the orchard.

“You’ve got to learn for yourself not to irrigate too far below the root zone, whether it’s with a screw auger or some electronic monitoring, otherwise you are wasting your fertilizer,” Griffith said.

He recommends minimizing the risk of mistakes in the field by mixing a “batch tank” of fertilizer — 10 pounds per acre for 10 acres for example — to keep the process consistent.

Once the system is set up, the amount and type of fertilizers applied should be based on regular soil samples. Sackett said he tests the soil weekly when he is running high rates of fertilizer.

If your soils are getting salty, you should switch to the more expensive, low-salt nitrogen sources, he said. Sackett also recommends testing leaf tissue samples to check that the trees are getting the right amount of nutrients from the soil.

But there are also downsides to driving aggressive growth with high levels of fertilizer, including propensity toward winter injury, blind wood and poor fruit quality in some varieties if they are cropped early, according to Sackett and Griffith.

“I have had no luck trying to crop Honeycrisp while trying to grow the tree to fill the space,” Sackett said. “It makes for a large, inedible apple with bitter pit.”

Trees pushed with high levels of nitrogen are also more prone to crown rot, and growers without irrigation need to beware that water stress can cause the trees to burn if the soil is high in salts. Pushing for aggressive growth can also lead to micronutrient deficiencies, such as zinc.

“When we push these baby trees really hard, we cut them off on August 1. Realistically, they are going to be prone to winter injury,” Griffith said.

To harden off the trees to prevent winter injury, they recommend deficit irrigation in the fall. •

– by Kate Prengaman