Whole-orchard recycling (WOR) involves grinding whole trees into wood chips, spreading the wood chips evenly on the soil surface and incorporating them into the soil before replanting. This approach is a potentially sustainable method of tree removal that could enhance both air and soil quality.
Before air quality restrictions, orchard removal meant pushing trees into large piles and burning them. But when air quality regulations were implemented in 2002 under the Clean Air Act, removed orchards were ground up with a tub or horizontal grinder and the woody debris was hauled to a co-generation plant to generate electricity.
However, since 2015, many biomass co-generation facilities have closed throughout California because utility companies are looking for cleaner sources of energy (solar and wind) and not renewing contracts. The remaining co-generation facilities still open have reduced the amount of wood debris they will accept from orchard waste (and are required to take more forest waste) and reduced the price they pay for the debris. Thus, tree fruit and nut growers who wish to remove dead trees and unproductive old orchards need an alternative method of orchard removal that is sustainable.
A growing team of researchers and growers implementing WOR are investigating the benefits and drawbacks of this practice. Concerns expressed by growers include increased costs of orchard removal, nutrient deficiencies, disease potential and wood debris interfering with orchard practices. But a decade worth of research on WOR points to increased yields, carbon storage, irrigation efficiency and improved soil health.
Furthermore, in 2018, the California almond industry announced its Almond Orchard 2025 Goals, which emphasize the use of innovative, responsible and sustainable growing practices that protect farms of the future. One of the four 2025 goals focuses on finding high-value uses for almond hulls, shells and woody debris, committing to achieve zero waste from orchards by putting everything grown to optimal use. Reaching that goal will require new outlets for almond co-products as well as the implementation of new practices such as WOR.
WOR Impacts on Soil and Tree Nutrition
When the woody debris of ground trees are burned in a co-generation plant, carbon stored in the wood is lost from the orchard system. A recycled almond orchard returns approximately 45 to 80 tons of wood chips per acre depending on the previous orchard’s tree size, spacing density and the varieties planted. The woody debris is approximately 50% organic C, which means approximately 22 to 40 tons of organic C is returned to the soil per acre during recycling.
There are benefits and tradeoffs associated with returning this large volume of C into the soil prior to replanting. New research by Dr. Mae Culumber and Dr. Suduan Gao suggests 20 to 30% of this C can be lost as CO2 in the first year after recycling (See Figure 1). However, CO2 emissions drop substantially after the first season, suggesting that the majority of recycled wood chips may remain in the soil as long-term, slowly degraded sources of organic C.
Organic C, which is the C stored in organic matter, promotes the physical and microbiological properties that influence improved water infiltration, retention and aeration. The enhanced soil structure promotes tree root growth. However, the higher carbon to nitrogen ratios of wood chips can decrease the availability of applied N fertilizers. Consequently, growers may need to apply nitrogen fertilizer at a greater than normal rate.
Another concern is that the woody debris may be so large that it interferes with normal soil preparation and orchard management practices. If WOR can be managed so that it does not reduce the availability of nutrients, worsen replant disease or interfere with harvest, and can enhance long term soil health, nutrition, yield and water retention, then growers will embrace WOR as a viable alternative to burning woody debris in the field or a co-generation facility.
In Dr. Brent Holtz’s first orchard grinding trial, established in 2008 at the UC Kearney Research and Extension Center, stone fruit trees were recycled at a rate of 30 tons per acre using the Iron Wolf (a 50-ton rock crusher) and compared to field burning and incorporating the ash. Second-generation trees were replanted to almond that were observed for more than a decade. Significantly greater yields (See Figure 2), tree circumference (See Figure 3), more soil nutrients and organic matter and total carbon (See Figure 4) were observed in the grind treatment when compared to the burn. After eight harvest seasons, a cumulative 2,000-pound kernel increase per acre was observed from trees growing where the previous orchard was recycled when compared to trees growing where the previous orchard was burned. Leaf petiole analysis also revealed higher nutrient levels in trees growing in the grind treatment when compared to trees in the burn, thus demonstrating that WOR did not stunt replanted trees, but rather increased tree growth and yield.
Later studies at this site showed that WOR increased the soil water infiltration rate and soil moisture retention while decreasing soil compaction and bulk densities. Significantly higher microbial biomass carbon was observed in the WOR treatment while microbial biomass nitrogen was decreased. A deficit irrigation trial established at Kearney provided evidence that trees growing where the previous orchard was recycled showed less water stress (Plos One paper, http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0229588). Based on the positive results from this trial and the closure of co-generation plants, we estimate that more than 40,000 almond acres have been recycled in California since 2015.
With the adoption of WOR, ten additional research trials were established throughout the San Joaquin Valley to further evaluate the impacts of WOR on tree health and soil quality. Initial observations in the new trials revealed that our nitrogen recommendation following WOR was too low, as reduced shoot growth in second-generation orchards was often observed in early spring of their first growing season after replanting. Consequently, nitrogen applications were often increased to address the likely imbalance between the C:N ratio resulting in the soil after WOR.
Nitrogen Trials After WOR
In a previous study conducted by David Doll (The Almond Doctor), he determined that conventionally planted first-year almond trees grew best when given between three to four ounces of actual N per tree (25 to 35 pounds N/acre) in their first growing season. This recommendation may not be enough, however, following WOR, especially if 45 to 80 tons of wood chips are incorporated back into the soil. In 2017, we tripled David’s recommendation by applying 11 ounces of N per tree through the season (approximately 100 pounds N per acre) after the team noticed reduced shoot growth in Louie Tallerico’s recycled almond orchard in Manteca, where 65 tons per acre was recycled. Trees that had initially showed reduced shoot growth responded nicely to the additional nitrogen. However, the team estimates that only 20% of the applied N and water reach the trees early in their first growing season when applied through a double-line drip irrigation system. As the trees mature and their roots expand, the double-line drip system will be more efficient at delivering N and water to trees.
In 2018, the team put out a nitrogen trial in Jeff Warkentin’s first-year orchard in Parlier to see if we could determine more accurately the N requirements of first year almond trees after WOR. In order to more precisely apply N, triple 15 granular fertilizer (15-15-15) was hand-applied to each tree. Five treatment rates were put out with five tree replicates in a Latin Square designed experiment. After five months, each treatment received zero, two, three, four and five ounces of additional nitrogen per tree. These applications were in addition to Warkentin’s fertigation through his double-line drip system at a rate of 1.73 ounces of N applied monthly from April to August. Again, the team expected that the grower-applied nitrogen through the double-line drip system was not immediately available because of emitter spacing and the limited range of the small tree roots.
The first triple 15 application in March seemed to have an almost immediate impact on tree growth. Considerable precipitation in March effectively dissolved the granular nitrogen, and differences in shoot growth were visible between treatments in April. Leaf analysis showed that nitrogen treatments early in the season had a greater impact on nitrogen tissue levels than applications later in the season. Trunk diameter data showed that no additional benefit was received for applying more than four ounces of actual N per season per tree, in addition to what the grower applied. Timing of nitrogen may be more critical early in the growing season after whole orchard recycling. In Dr. Greg Browne’s studies, where he applied nitrogen with whole orchard recycling and anaerobic soil disinfestation (ASD), he too observed an increase in shoot growth early in the spring with early nitrogen applications.
The team hypothesized that less nitrogen might be able to be used more efficiently if applied earlier in the growing season or at planting time. In the 2019 WOR trial at Kearney, 75 tons of wood chips per acre was recycled. One-quarter ounce of N was applied in the form of triple 15 at planting time, and again every two weeks with each irrigation, from April through June. The team then fertigated with one ounce of actual N per tree per month from July to September with UAN32. Button emitters were used, rather than drip irrigation, so that only water and fertilizer was applied at each tree site.
The team ended up applying five total ounces of N per tree or 46.6 pounds N per acre. For the first time in the research trials, a significant increase in trunk diameters from trees growing after orchard recycling when compared to control trees was observed, growing where the previous orchard was not recycled, and given the same amount of nitrogen and phosphorous their first season (Figure 5). There is also evidence from Dr. Greg Browne and Dr. Phoebe Gordon’s trials that phosphorous could be important in first-year tree nutrition after WOR. They found that extra applications of phosphorous and nitrogen, as separate treatments, each improved tree growth, alone and in combination.
Early Nitrogen Recommendations
The team is continuing to study early nitrogen and irrigation efficiency in more detail in a new trial at Kearney, but at this point in the studies, it is recommended to growers that they apply at least 5 to 8 ounces of actual N per tree (45 to 75 pounds N/acre) in the first year of tree growth following WOR. Early applications, starting at planting time, are more important than applications later in the season.
Remember that nitrogen applications should be spread out so that no more than 1 ounce of actual nitrogen is applied per tree per application in the first year of tree growth in order to prevent nitrogen burn. Another advantage of using granular fertilizer applications early in the season is that some growers have applied too much water too early in the season in order to deliver the desired amount of nitrogen and have experienced Phytophthora Root and Crown Rot infections.
In the WOR trials, additional nitrogen did not have to be applied above the normal recommendation in the second or following years of the studies to achieve the desired tree growth expected. The team hypothesizes that in the first season after WOR, the wood chips, and the microorganisms decomposing them, compete for available nitrogen while the C:N ratio has been dramatically increased. The wood chips and soil microorganisms, while competing for the nitrogen, may also prove beneficial by binding and immobilizing excess nitrogen that may have otherwise leached through the soil profile.
As the wood chips decompose, nitrogen should be released slowly and become available for uptake by the trees. Increased nitrogen efficiency may be observed as the wood chips decompose and release bound nitrogen. Samples of wood chips were analyzed for their nutrient contents, which averaged 0.31% nitrogen, 0.20% potassium, 0.60% calcium, and 50% carbon. Thus, in the Manteca trial, where 64 tons of wood chips per acre was recycled, the team added 396 pounds of nitrogen, 768 pounds of calcium, 256 pounds of potassium and 64,000 pounds of carbon per acre. These nutrients will not be immediately available to the next-generation orchard, but as the woody material decomposes and soil organic matter increases, the stored nutrients will be released gradually and naturally.
How to Implement WOR
The first step of the WOR process is to excavate the trees from the ground with as much tree roots and crown as possible. The trees are then carried to a stationary horizontal grinder (or tub grinder) with a front-in loader and ground into wood chips using either two- or four-inch screen sizes that will limit chip size. When practicing WOR, it is recommended that growers request the 2-inch screen, for the team believes the smaller wood chips will be less likely to interfere with orchard floor management activities or harbor potential pathogens. It may take longer to grind the orchard using a 2-inch screen, but the smaller wood chips will be easier to spread and incorporate.
The wood chips are then loaded into modified manure spreaders and spread on the orchard floor. Care should be taken to spread the wood chips evenly over the entire orchard, for wood chips can often be 1-foot thick near the piles and only 1-inch thick in the far corners of the orchard. If the wood chips are not spread evenly, and left too deep in places, they can be difficult to incorporate and pull critical nutrients away from replanted trees. When the wood chips are spread evenly, they are usually only one to two inches thick over the entire soil surface and can be incorporated much more easily.
After spreading the wood chips, most growers will have their orchard deep ripped (5 to 6 feet deep) to break up soil compaction layers, hard pans or to pull up large roots. Deep ripping is usually done while the wood chips are on the soil surface.
After ripping, growers will typically stubble disk their fields in order to smooth out ruts created by the ripper and to incorporate the wood chips. Plowing or roto-tilling has also worked successfully to incorporate wood chips after ripping, and the equipment used typically depends on grower access.
After wood chip incorporation, most growers will fumigate if they have plant parasitic nematodes or suspect replant disease. WOR has not interfered with fumigation efficacy in the initial trials, for the wood chips are usually incorporated in the top six inches of soil while the fumigates are injected at depths between 18 and 24 inches.
After fumigation, the orchard floor is typically prepared for re-planting by disking, leveling and berm building while the new irrigation system is installed. The team strongly suggests that growers take a year off between taking out an orchard and replanting as trying to do all this in a few short months is very difficult and stressful. Fortunately, with potted trees, growers have more flexibility and can replant in the late summer or fall if desired.
Remember that all the wood chips don’t need to be completely incorporated or decomposed in the first year—you have three years before your first harvest of your next generation orchard. Several growers have successfully used a Northwest Tiller (or other forms of cultivation) to further incorporate wood chips and smooth the orchard floor for harvest. Nut harvest samples collected from bank out wagons found very few wood chips when growers used a Northwest Tiller to incorporate wood chips and smooth the orchard floor before their first harvest.
There is some evidence that pathogens such as Ganoderma, a wood decay pathogen, or Armillaria, the Oak Root Rot pathogen, do not survive well after being wood chipped. The 2-inch screen is recommended if these pathogens are present and you recycle, for the pathogens don’t survive as well in smaller pieces of wood. Wood-decomposing mushrooms are often observed in recycled orchards, and it is speculated that they could be inhibitory to pathogens, but this is one of many projects that has yet to be studied.
To date, a disease problem in recycled orchards has not been observed, but there is no guarantee that WOR will control pathogens, and the process could very likely spread contaminated wood chips throughout the orchard. If the Oak Root Rot pathogen is present, fumigating at both deep and shallow depths with chloropicrin before replanting is recommended (Tri-Cal recommendation working with Dr. Adaskaveg.) The team’s research has focused on recycling wood from Prunus species, primarily stone fruit and almond, and the team does not have long-term experience with recycling wood from other tree species such as walnut, pistachio or citrus, and can’t assume that WOR will always give the same positive results under different scenarios.
WOR has been an expensive undertaking for growers who used to get their orchards removed for practically nothing when co-generation facilities were paying nicely for wood waste. Now, growers can expect to pay from $600 to $700 per acre to have their orchard ground up, whether they are keeping the wood chips or not. If growers decide to keep their wood chips and recycle their orchard, they can expect to pay an additional $200 to $400 per acre to spread their wood chips evenly back onto the soil surface.
Incentive opportunities from the San Joaquin Valley Air Pollution Control District (SJVAPCD) and the California Department of Food and Agriculture’s (CDFA) Healthy Soil Initiative have made the cost of WOR more affordable. SJVAPCD has approved a program that will reward growers with funding from $300 to $600 per acre, up to $60,000 per year to implement WOR. For more information on these incentive programs, visit their website at valleyair.org or email firstname.lastname@example.org. In 2019, the CDFA added WOR to their Healthy Soils Initiative, and in 2020 gave 15 growers approximately $60,000 each to recycle their orchards (http://www.cdfa.ca.gov/healthysoils/).
USDA Natural Resource Conservation Service is in the process of developing a practice standard for WOR, so check with your local USDA NRCS for incentives for growers to recycle through their Environmental Quality Incentives Program and Conservation Stewardship Program. In addition, the Almond Board of California is working on developing protocols for private company funding, for many food producing companies have set greenhouse gas reduction goals that they cannot achieve without help from growers.
Continuing WOR Studies
The team of researchers studying WOR has increased in the last couple of years to find alternatives to field and co-generation burning. ABC, which emphasizes stewardship of resources, sustainability and production efficiency, has funded the team’s WOR project to quantify the physical, chemical and biological soil properties on soil health and the effects on tree growth, yield and water use efficiency. In 2016, Dr. Amélie Gaudin was funded by the CDFA to study the “Potential of Whole Orchard Recycling to Build Sustainability and Resilience of Almond Production.” In 2018, Dr. Mae Culumber was funded by the CDFA and ABC to study the “Influence of Whole Orchard Recycling on greenhouse gas emissions (GHG) and Soil Health in a Newly Established Almond Orchard.” In 2019, Dr. Amisha T. Poret-Peterson was funded by the CDFA to study “Optimizing fertilization and irrigation recommendations in a newly planted almond orchard after recycling.”
In 2020, Dr. Mae Culumber was again funded by the CDFA to study the “Regional orchard soil health and greenhouse gas emissions after whole orchard recycling.” We are hoping that Dr. Suduan Gao will receive funding in 2021 from CDFA’s Specialty Crop Block Grant to study “Water and nutrient use efficiency of almonds replanted after whole orchard recycling.” It is hoped that this team effort will demonstrate the success of whole-orchard recycling as a sustainable method of orchard removal and provide the scientific evidence needed to support legislation that allows growers to receive carbon credits for recycling their orchards. Carbon credits would encourage sustainable orchard removal practices, that can sequester significant amounts of carbon and help compensate growers for the huge expenses incurred when orchard recycling.