Composting Trees to Improve Ag Soil an Alternative to Biomass Burning

– by Tim Hearden, February 23, 2016, Capital Press

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Photo: Almond Board of California

University of California researcher Brent Holtz believes a test plot here could hold the answer to the San Joaquin Valley’s worsening soil quality problems.

On a recent morning, Holtz and others demonstrated a device called an Iron Wolf, which uprooted and ground up whole almond trees and incorporated the woody biomass into the soil.

Holtz, a UC Cooperative Extension farm adviser based in Stockton, Calif., said studies he’s been doing since 2003 have shown that whole-orchard chip incorporation treatments increased organic matter, soil carbon, nutrients and microbial diversity — all to the benefit of new plantings.

“Growers in the southern part of the valley have been seeing sodium levels increasing … especially in the drought years when we have less leaching,” Holtz told growers during the gathering, which was videotaped by the Almond Board of California. “A good point to this orchard grinding is it’s helping alleviate our sodium levels.”

In a new study, Holtz hopes to compare the effects of using the Iron Wolf to recycle an almond orchard to using a large tub grinder, which leaves much finer particles of wood, UC officials said.

The research comes as some cogeneration plants have shut down in recent years, forcing growers to look for alternative ways to deal with tree biomass when they remove old orchards. In addition, growers in recent years have reported an increase in problems with salinity of groundwater, with many saying they expect it to affect the quantity or quality of their harvests.

Holtz began testing woody biomass’ impacts on soil quality with an Almond Board-funded study in 2003, incorporating shredded prunings 1 to 2 inches into the soil. Over time, researchers found higher soil nutrient levels, lower pH and more organic matter in the soil, and the organic matter bound up sodium to the extent that leaves had about half the amount of sodium of other orchards, the board reported.

Holtz also tried planting trees in containers with one-third wood chips and two-thirds soil. Within a couple years, nutrient levels were higher, water infiltration occurred faster and trees were showing less water stress because the wood chips were holding water in the soil, according to the Almond Board.

In 2008, Holtz used the Iron Wolf — a 50-ton rototiller capable of grinding whole trees and incorporating their chips into the soil — to grind up whole stone fruit trees and bury the organic matter in the soil. By the third year, the nutrients were significantly greater where trees had been ground and incorporated.

The machine does have its drawbacks, Holtz said in an interview. For one thing, it left some larger-than-anticipated chunks on the orchard floor, while the tub grinder could be an alternative for growers who could disc the materials into their soil.

Also, the Iron Wolf is expensive, costing about $52,000 a month to rent or $1.2 million to buy, he said. Researchers will also have to make sure they’re not putting tree diseases back into the soil, said Gabriele Ludwig, the Almond Board’s director of sustainability and environmental affairs.

“The flipside of it could be that by having increased organic matter in the soil, we can change the microbial system … and work against the bad things,” said Ludwig, adding that more trials in multiple locations are needed.

Holtz agrees.

“With all this talk about cogen plants closing down and growers won’t have any alternatives for biomass,” Holtz said, “my point is I think there’s a great alternative for that biomass and that’s putting it back into the ground.”

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