As more cogeneration plants are shutting down, growers are being forced to consider alternative methods for dealing with tree biomass upon orchard removal. The question is: Are there alternative uses for the woody biomass from orchard removals and prunings? One idea is to incorporate that biomass back into the soil; however, that leads to another question: How does the additional organic matter (OM) affect the soil, and more important, how would it affect the health of a subsequent almond orchard planted into those soils?
This was the subject tackled by farm advisor Dr. Brent Holtz, UC Cooperative Extension, San Joaquin County, in a session on soil quality at last December’s Almond Conference. The initial years of this research were funded by Almond Board of California.
Dr. Holtz reported on a shredding-of-prunings trial conducted in Madera County beginning in 2003. At the start of the trial, shredded prunings were left on the orchard floor, leading to chips being picked up at harvest. “Every year, more chips were being picked up and going to the processor,” Holtz said, leading to his decision to shred and incorporate into the top inch or two of soil.
After 11 years of the shredding trial, Holtz found that the soil had:
- More wood-rotting basidiomycetes fungi;
- More bacterial- and fungal-feeding nematodes;
- Increased soil nutrient levels;
- Lower pH; and
- More organic matter and soil carbon.
Significantly, the organic matter in the soil was binding up the sodium such that in a leaf analysis, sometimes half the amount of sodium was present compared to the control.
Higher Wood-Chip Ratio
But Holtz, wondering if more organic matter could be added to the soil, began a new experiment in which he mixed one-third part wood chips and two-thirds part soil placed in 35-gallon containers, and then planted almond trees in them. No fertilizer was added to the containers. After two years, the nutrients in containers with wood chips started to become available to the trees, and all nutrients in the soil were significantly higher in the wood chip–amended containers than in soil in containers with no wood chips. Water infiltration was significantly faster in containers with wood chips, and trees showed less water stress, as the chips were holding water in the soil, according to Holtz. In addition, organic matter increased from less than 1% to 5%, and carbon levels increased from 0.5% to almost 3%.
“This gives us a new vision,” Holtz said. “Can we remove whole orchards and incorporate the organic matter without significantly affecting the next orchard? I don’t want to see these wood chips go to cogeneration or be burned; I’d like to see them returned to the soil. When we remove an orchard, we’re going to take 25 to 30 years’ worth of carbon accumulation and photosynthesis and haul it away, locking out about 30 tons per acre of organic matter from the orchard.”
Iron Wolf
To see if whole orchard incorporation was feasible, Holtz began a project at the UC Kearney Agricultural Research and Extension Center, where a 4-acre stone fruit orchard was about to be removed. The researchers used an Iron Wolf, a 50-ton rototiller that the Forest Service uses to clear fire breaks, to grind the entire fruit trees and incorporate the chips into the soil. The Iron Wolf pushes the trees over, grinds them up, and then reverses direction to incorporate the chips into the soil.
Two treatments were established: The Iron Wolf was used to shred and incorporate whole trees in several plots; in other plots, the trees were burned and the ashes spread on the soil surface. Almond trees planted in both plots were fertilized normally.
In the first year of the study, most of the nutrients were significantly less in the grind plot compared to the burned plot; however, the young trees grew the same in both plots. But these differences became less in the second year; and by the third year, the nutrients were significantly greater where trees had been ground and incorporated, a trend that continued through three more years of the trial.
Beneficial Trends
Leaf analysis showed a trend for significantly increased levels of potassium and a decrease in sodium in the grind plot. Yields tended to be greater in the grind plot compared to the burned plot; however, there was not a significant difference between the methods as of 2014, Holtz explained. The researchers also found that when measuring water stress through stem water potential, the grind plot experienced significantly less stress, evidence that the organic matter is increasing the water-holding capacity of the soil. In addition, they found that bud failure was less in the grind plot.
In essence, the whole orchard chip incorporation treatment resulted in increased organic matter, soil carbon, nutrients and microbial diversity, including beneficial fungi. “Fungal decomposition of the organic matter may be contributing to available nutrient levels, which would be gradually released as the woody aggregates are decomposed,” Holtz said.
“Added to these benefits is evidence of an increased water-holding capacity of the soil, which may be important in holding applied nutrients in the soil, thus preventing leaching into the groundwater,” he said, adding that he is hopeful growers may eventually be able to get carbon credits for incorporating whole orchards.
While this research trial provides a potential solution to the decreasing availability of cogeneration, there is additional work needed about its impacts under variable orchard and soil conditions to prove its feasibility for all California Almond orchards. ABC will continue to invest in research opportunities to prove this technique.
Growers interested in learning more about this research are invited to attend a UC Cooperative Extension field event on Tuesday, Feb. 16, in Chowchilla. Researcher and farm advisor Dr. Brent Holtz will be on hand to discuss his research on the effects of whole orchard recycling on second generation tree growth, soil carbon and fertility, and attendees will see a live demonstration of an Iron Wolf pushing, grinding and incorporating whole almond trees.
