As nearly every aspect of orchard management has been optimized, growers are turning their attention below ground.
Efforts to improve soil function are being explored across the industry, but low-cost practices like mow and blow and mulch are especially important strategies for smaller farms that can’t afford to invest in all new systems.
“It’s an untapped area of improvement on our farms,” said Oregon cherry grower Mike Omeg, during a tour of his orchard in The Dalles in March. “It’s difficult for us to compete with these really large farms on efficiency. We can’t prune our way out of that problem, and you can’t just plant new orchards unless you have $35,000 an acre. Improving the soil is something we can do.”
Although it’s invisible below the soil surface, there’s just as much biomass below ground as above — a second half of the orchard that’s often ignored.
Cultivating a strong root system and a diverse community of beneficial organisms — from pest predators to mycorrhizae fungi that make roots more efficient — will lead to healthier trees and better fruit, according to experts who spoke at a cherry soil health day hosted by Oregon State University Extension in March.
“It’s there to work for you if you encourage it, feed it and give it a good home,” said David Granatstein, sustainable agriculture specialist for Washington State University. But often, modern orchard management works against healthy, active soils by limiting root diversity and compacting soil structure, he said.
While it’s typical to test soil composition, for nutrients or organic matter, it’s the structure of the soil that matters most, said Gary Johnson, crop consultant with Wilbur Ellis. Structure that encourages fine root growth can dramatically enhance water and nutrient efficiency.
“When you start to dig, you’ll be shocked to find how rare functional feeder roots are in our orchard rows,” Johnson said. “If you want to grow better trees, grow more roots. If you want to grow better quality fruit, enhance the soil structure.”
Low hanging fruit
Once you start thinking about managing your orchard below ground as well as above, it’s easy to see simple opportunities for improvement, Omeg told a group of cherry growers.
He’s been experimenting with soil health improvements for years, and he was happy to share what is benefiting his bottom line.
“If you are running water and not fertigating, you are losing that opportunity to stimulate the soil biology that feeds the trees,” Omeg said. “If you are not mowing and blowing, you are losing an opportunity to put mulch from the alleyway into the tree row.”
He sees another easy opportunity in pruning wood. He shakes his head now talking about how he paid a crew to haul away that wood — “a carbon source ready to rock and roll for you” — for years.
Now, he grinds the prunings into mulch to recycle the nutrients.
The process is accomplished with two flail mowers: a big one that mulches wood up to 6 inches in diameter and a smaller one with an auger that moves the new mulch into the tree rows as it drives by.
Omeg said the new equipment was paid for in just the first year by not hauling away the wood. Then, he gets the financial benefit of how the mulch improves the soil, which improves the tree’s health.
“Cherry trees like to absorb something that used to be a cherry tree,” Omeg said. The mulch “is made up of that balance of nutrients we work so hard to generate in our trees.”
That’s intuitive, but also backed up by research that shows it’s not just the levels of nutrients that matter for fruit quality, it’s the ratios, Johnson said. And prunings, because they are mostly new growth, have just the right ratio of nutrients you need for tree growth.
“Prunings are higher in nutrients than other wood chips. It’s a great resource to stimulate soil biology and recycle nutrients,” Granatstein said. “The soil organisms literally pull it into the soil.”
Most growers try to incorporate compost into their soils to boost organic matter, but recent studies have shown that natural mulches on the surface provide extra benefits, he said.
Cherry trees evolved in forests, and mulch acts like the litter on the forest floor, encouraging beneficial fungi and feeder roots and helping to keep the soil moist and cool.
It’s expensive to haul in enough wood chips to cover an orchard, but Granatstein said that every bit helps and by repurposing prunings, mowing and blowing, or just hauling in mulch for underperforming blocks, growers are finding ways to work it into the bottom line.
But mulch isn’t magic either. Omeg said that it doesn’t reduce his need for herbicides or irrigation, as some organic growers contend.
“But I’m not putting this on to control weeds. I’ve got oodles of herbicides. I’m putting this on to stimulate soil biology and earn more money by producing larger, firmer cherries,” he said.
The benefits of the mulch can be boosted further with a variety of other supplements to feed the soil microbes, such as boron and liquid fish fertilizer, delivered through fertigation.
Since some calcium products can pose a hazard to irrigation lines — err in the concentrations and you can create concrete — Omeg prefers to apply lime in solution with an old bullet sprayer aimed at the fresh mulch piled in the tree rows.
Grow your own soil
A diverse, healthy soil ecosystem also needs a diverse food supply — that’s the premise behind cover crops.
But despite the appeal of growing your own fertilizer, it’s hard to successfully grow crops like clover or alfalfa in the alleyways of working orchards, Granatstein said. Not much besides grass is tough enough for regular traffic from tractors and ATVs.
However, in young orchards that require less traffic, Omeg is successfully using cover crops.
In one new planting on a slope overlooking the Columbia River, Krymsk 6 rootstock trees, planted in May 2016, are growing on berms, surrounded by the remains of kale, mustard and radish plants, squashed by just-melted snow. This year, Omeg plans to install a UFO training system.
The site has been farmed for over 100 years and the soils are tired, drained of micronutrients by generations of growers, Omeg said.
To ameliorate that, he turns to a diverse mix of cover crops, even applying fertilizer to boost the cover crop’s growth, so it can boost the soil. Then, he brings in cows to graze down the cover crops and directly deliver their own fertilizer.
These soil health strategies were accidentally put to the test when it turned out that fumigation contractors couldn’t properly treat the entire block, due to the hilly terrain.
That put Omeg on alert for replant disease, but he hasn’t seen any consequences yet, a major benefit he attributes to his cover crop, cows and healthy soil.
“We couldn’t see any difference between where we fumigate and where we didn’t,” Omeg said. “In the past, we’ve seen it night and day, but here you can’t see it.”
Research backs this up as well. Plant parasitic nematodes that contribute to replant disease are killed off by fumigation, but so are their predators, so the parasitic population often rebounds.
With alternatives like cover crops, mulch and mustard seed meal that encourage predator nematodes and other protective soil microorganisms, the benefits can be longer lasting.
In replant disease management experiments in British Columbia apple orchards, wood chip mulch, mow and blow alfalfa and compost treatments all resulted in better tree growth than fumigation after three years, said Tom Forge, a research scientist for Agri-Food Canada.
“We alleviate replant disease by restructuring the soil, by optimizing soil health to suppress pathogens or having better root health so we can live with them,” Forge said at the workshop. “Alfalfa mulch is better at enhancing predator nematodes, but bark mulch overall enhanced the good bacteria, fungi and other soil suppressive services.”
There’s not much mulch visible on the berms — aimed at preventing wet feet — in Omeg’s new planting, but he strives to keep the soil mulched, both by mowing and blowing the cover crop and by hauling in straw.
But the soil organisms are now so healthy, they are breaking down the mulch and sucking it into the soil at record speed.
“It’s a good sign, but now we’re using up a lot of mulch,” Omeg said.
From the perspective of the soil, that’s a good problem to have. •
– by Kate Prengaman
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