Chips from old trees renourish the soil for new ones

The end of the productive life of a block of almonds offers a unique opportunity to use the old trees to make one last contribution to the health of the soil before replanting.

University of California studies show the feasibility of chipping the old trees, incorporating the chips into the soil and taking a year off before replanting.

"It's a once-in-a-30-year opportunity to add organic matter to your soil," said Brent Holtz, UC Cooperative Extension farm advisor in San Joaquin County. "Pruning might be 1,000 to 2,000 pounds an acre, but it's not like the 60 tons you can get from recycling."

Holtz has been a leading researcher of chipping and keeping first annual pruning material in the orchard. In more recent years, he has suggested adding chipping and keeping entire blocks of trees that are due for replacement.

He appeared at the Nickels Soil Lab Annual Field Day in Arbuckle as growers looked over an area of the research orchard where old, unproductive trees were removed and chipped in December 2016. They were incorporated into the soil before replanting in the spring of 2019.

"The trees are looking great," Holtz said, standing near rows of nonpareil and Carmel pollinator almonds planted at Nickels after the old block was recycled. "I think one of the things that helped is that the trees were taken out in 2017 and they didn't put the new trees in until 2019."

The incorporation of massive amounts of organic material benefits the soil, the environment and the next generation of trees, Holtz said.

"It improves water-holding capacity, infiltration and yield," Holtz said. "Showing how long the carbon stays in the ground gets this qualified as a healthy soils practice."

Although recycling an entire orchard by chipping the wood and incorporating the chips into the soil can work, the challenge is in the details.

The decision to remove in 2016 a 20-acre orchard planted a quarter century earlier at the Arbuckle site gave researchers a chance for an in-depth study of recycling into the ground and impacts on soil quality and growth and productivity of the next trees.

The fate of those old trees has become agronomically and environmentally more important than ever, as many orchards planted near the beginning of the almond boom are due for replacement. The Almond Board of California estimates 50,000 acres of old trees are being replaced every year.

Holtz recommends taking a year off between incorporating the old trees into the soil and replanting to give the soil microbes time to break down the wood chips—so they do not interfere with orchard operations.

When the 20 acres of old trees at the Nickels site were first removed and chipped, the resulting piles of wood were imposing.

"There were three piles of wood chips that were three or four stories tall," Holtz said. "But spread out, it was an inch or two."

After waiting well over a year, the new block was planted in nonpareil almonds and the pollinator variety Carmel at a three-to-one ratio. The pause appears to have helped make the new orchard workable.

"One of the things people worry about is the chips getting in the way," he said. "There is more on the ground here from cover crops and weeds than the chips."

In addition to needing time and fumigation to rid the ground of diseases and nematodes, the new trees also require supplemental fertilizer because otherwise the carbon in the chips can tie up nitrogen and starve the trees.

"The chips are 50% carbon, so we need more nitrogen at first," said Holtz, who has studied how much additional nitrogen the new trees need. "We probably need to put on more than 30 pounds, but we probably don't need to put on 100 pounds."

"Trials show that between 30 and 60 pounds is right," Holtz added. "I think around 45 pounds, but no more than 1 ounce of nitrogen per tree at a time."

He advised applying the nitrogen earlier rather than later to feed the microbes that break down the chips and also break down whatever was living on or in that old wood.

Although it is too early to assess the yield of the trees planted in the chip-enriched ground at Nickels, the early growth of the trees is promising.

Plots were divided into those fumigated or not and between those receiving 25 tons or 52 tons of chips an acre or no chips at all.

"To date, tree growth is generally good across this orchard," Holtz said. "Fumigation did improve tree growth as measured by trunk cross-sectional area. Tree growth in the two different chip rates was greater than in the no-chip treatments. But there was no difference in trunk area between 25 or 52 tons of chips per acre."

The difference in size between the fastest and slowest growing among these young trees is just 10%, as measured by trunk size.

(Bob Johnson is a reporter in Sacramento. He may be contacted at bjohn11135@gmail.com.)