Considering New Orchard Replacement Options: Whole Orchard Recycling and Anaerobic Soil Disinfestation

By: Mohammad Yaghmour UCCE Kern and Kings Counties, Brent Holtz UCCE San Joaquin County, and Greg Browne USDA-ARS Davis, CA

With more than a million estimated acres of bearing and non-bearing acres of almonds in California, and tens of thousands of these acres reaching unproductive ages each year, the almond industry is thinking strategically about the process of orchard replacement and how it can best be done, given current and future needs and environmental considerations.  The expected lifespan of an almond orchard is approximately 20 to 25 years, and when growers decide it is time for orchard replacement, several management issues come to the forefront. 

Among the key orchard replacement challenges is what to do with all the wood from the old trees.  Old almond orchards may contain up to 80 tons of woody biomass per acre, depending on tree size and spacing, and burning restrictions and closure of several co-generation plants that previously processed orchard waste has forced some growers to consider new possibilities for dealing with the residues.

Another key challenge in orchard replacement practices is how to manage replant problems.  Growers face restrictions on soil fumigation, changes in rootstock usage, and new orchard residue management practices, etc., all of which can affect management of replant problems.  Among these problems are Prunus replant disease (PRD), which is a specific soilborne microbe-induced suppression of early tree growth and yields (affects successive plantings of stone fruits, including almond); plant parasitic nematodes (several damaging  species that can reduce orchard productivity over its lifetime); aggressive pathogens such as Phytophthora and Armillaria, which cause crown and root rots; and physical and chemical soil problems related to previous crop production (e.g., compaction, salinity, herbicide residues, etc.).

In this article, we highlight two practices, whole orchard recycling (WOR) and anaerobic soil disinfestation (ASD), as potential elements in new orchard replacement strategies.  We consider promising aspects WOR and ASD based on results from trials completed to date and mention newly established WOR and ASD trials. Special reference is made to potential impacts of the WOR and ASD on management of replant problems.  Finally, we mention a few of the important contributions being made by fellow researchers, growers, fumigation specialists, and equipment manufacturers in the new trials.

Whole Orchard Recycling (WOR)

WOR has recently gained traction. In the past, most growers would push the trees of removed orchards into piles and burn them.  This option became less viable due to increased regulations to improve air quality after the 2002 Clean Air Act.  Thereafter, growers started to grind the trees into small wood chips and haul them to co-generation plants to produce energy.  In both previous cases, carbon dioxide (CO2) is released into the atmosphere, and CO2 is considered one of the greenhouse gases that contribute to air pollution.  The number of acres to be replanted is expected to increase significantly in the coming years, and there are limitations for burning the biomass in orchards or sending it to the co-generation plants. 

WOR, i.e., the grinding and incorporation of almond biomass back into the soil, is a sustainable alternative to biomass burning that could improve air and soil quality.  In 2008, an experiment was established at UC Kearney Agricultural Research and Extension (KARE) Center to compare standard tree removal and burning to whole orchard grinding and incorporation into the soil using the “Iron Wolf,” a 100,000-pound rock crusher, that incorporated 30 tons per acre.  Almond trees were replanted into the burn and grind treatments in February 2009.  Effects of the burn and grind treatments on tree growth and soil physical and chemical properties (e.g., water holding capacity, soil organic matter, nitrogen to carbon ratio, and plant nutrients) were evaluated over time.  Initially, soil analysis revealed significantly higher organic matter, electrical conductivity, cation exchange capacity, and higher carbon, calcium, and sodium in the burn treatment, compared to the grind treatment.  It is likely that these responses resulted from the immediate degradation of wood structure that results from burning.  However, after six growing seasons, the grinding treatment actually resulted in higher levels of soil organic matter, electrical conductivity, total carbon, and soil nutrients compared to the burn treatment.  Also, trees had higher yields in the grind treatment compared to the burn treatment (Table 1), and, by the 5th leaf in 2013, petiole analysis exhibited significantly higher macro nutrients content (nitrogen, phosphorus, potassium) in the grinding treatment, except for magnesium, which was significantly lower in grind plots.  In the same way, trees in the grinding treatment accumulated significantly less sodium and more iron and manganese.  The early WOR experiments provided an alternative solution to manage generated tree biomass, and this management practice appears to improve soil physical and chemical qualities available for newly replanted trees. Given the intriguing responses in the initial WOR trial, new trials were designed and established, as previewed below (see “New replant trials…”).

Anaerobic soil disinfestation (ASD)

Although new to perennial crop systems, anaerobic soil disinfestation (ASD) has been researched and used commercially in annual cropping systems in Japan, The Netherlands, and the U.S.  ASD is now being adapted for strawberry production in California and vegetable and floriculture systems in the southeastern U.S.  In annual cropping systems, ASD generally has provided broad-spectrum control of many soilborne pathogens in diverse soils, but less is known about its effectiveness for orchard crops.

Researchers have found that ASD mechanisms may be multiple and complex, including the generation of organic acids, metal ions, volatiles, and microbial population shifts that are lethal or suppressive to soil pests.  ASD is implemented for several weeks and requires readily available carbon substrate(s), moist soil conditions, and coverage with plastic mulch, which raises soil temperature, retains moisture, and retards gas exchange. High soil temperatures favor ASD while low temperatures limit it. 

In four previous trials conducted at KARE during 2013-2016, we found that ASD was as effective as preplant strip fumigation with Telone C35 in controlling PRD and stimulating tree growth (Figure 1) and yield.  Damaging plant parasitic nematodes were not present in the initial four ASD trials at KARE.  Given the technical efficacy of ASD in these “first generation trials”, we established “second generation” trials to optimize the ASD carbon sources and application methods (ASD was expensive as implemented in first generation trials) and test ASD with nematodes as well as with PRD, as previewed below.

New replant trials testing WOR and ASD.

Currently, there are six WOR replant trials throughout major almond producing regions in California to refine the life cycle assessment model for evaluation of carbon dynamics and balance, as well to examine effects of WOR on soil physical, chemical, and biological properties and their impact on replanted trees growth and health.  Also, six new ASD trials were established in the San Joaquin Valley in 2016 to examine economical ASD carbon sources, test streamlined ASD application procedures, and test the practice in soil where PRD and nematodes are both present as replant problems. Amelie Gaudin, Plant Sciences UC Davis; Andreas Westphal, Nematology UC Riverside; and others have joined our efforts in WOR and ASD testing.

Among the new trials, four of the most extensive ones were established at two orchard replant sites in Kern County in collaboration with Wonderful Orchards. Both sites are expected to express PRD, and one of them harbors damaging plant parasitic nematodes. In two of the four trials (one at each orchard), the main treatments are wood chips (WOR) vs. no-chips (residue hauled away).  Wood chips were placed back on the soil surface at the same rate the trees were removed with approximately 39 and 65 tons per acre in the first and second sites, respectively (Figure 2). Embedded in each replicate wood chip and no-chip plot, there were four additional subplot treatments: 1) a non-fumigated control, 2) spot fumigation with Telone C35, 3) strip fumigation with Telone C35, and 4) ASD using ground almond hulls and shells as a substrate and carbon source (Figure 3). Wood chips and ASD material were incorporated in the soil using a stubble disk. However, using a disk to incorporate a high rate of wood chips at the second site was a challenge.  Wood chips and ASD materials were successfully incorporated into the soil using a rototiller from Northwest Tillers (Figure 4).  ASD treatments received irrigation water through drip lines covered with totally impermeable film (TIF) tarp (Figure 5). In the other two trials (one at each orchard) treatments were assigned to test the importance of each ASD component, i.e., carbon substrate, tarp, and soil moisture. Rice bran was used as the carbon source in the additional ASD trials, and, for comparison, treatments of a non-treated control, spot-fumigation, and strip fumigation (Telone C35) were included.    

Finally, each of the four new WOR and ASD trials in Kern County is being replanted with two key rootstocks, Hansen 536 (a peach x almond hybrid) and Nemaguard peach.  Peach x almond and additional hybrids have become popular rootstock choices in the San Joaquin Valley, and it is important to learn more about their response to PRD and plant parasitic nematodes in WOR and ASD contexts, with and without soil fumigation.  Our trials will address some key rootstock questions that surface in relation to WOR, ASD, and management of almond replant problems.

The efforts in executing the trials in Kern County would not have been possible without the collaboration and generous contributions of Wonderful Orchards, TriCal, Inc., the Almond Board of California, the California Department of Food and Agriculture, and the California Department of Pesticide Regulation.  We would also like to thank Northwest Tillers for their kind incorporation of WOR residues in one of the Kern County trials.

Table 1. Almond kernel weight (lbs/acre) for grind verses burn treatments
Year	Grind	Burn	Difference
2011	1,007.3 lbs/ac	925.0 lbs/ac	82.3 lbs/ac
2012	1,618.4 lbs/ac	1,533.1 lbs/ac	85.3 lbs/ac
2013	2,100.6 lbs/ac	1,853.1 lbs/ac	247.5 lbs/ac
2014	2,829.5 lbs/ac	2,331.1 lbs/ac	498.4 lbs/ac
2015	1,599.6 lbs/ac	1,427.1 lbs/ac	172.5 lbs/ac
2016	1,603.2 lbs/ac	1,504.6 lbs/ac	98.6 lbs/ac
Total	10,758.6 lbs/ac	9,574 lbs/ac	1,184.6 lbs/ac

Figure 1. Responses of trees to preplant treatments in one of the initial ASD trials at the UC Kearney Research and Extension Center. Foreground, stunted trees exhibiting PRD in a

non-fumigated control plot; left and right backgrounds, vigorously growing trees in preplant fumigated and preplant ASD plots, respectively.

Figure 2. Dr. Greg Browne inspecting wood chips spread on the soil surface at approximately the same rate the trees were removed (65 tons per acre).

Figure 3. Anaerobic soil disinfestation substrate (ground almond hull and shell) spread on soil surface as a treatment of a total of four treatments applied to wood chip and no-chip plots.

Figure 4. Incorporation of wood chips and ASD substrate (ground almond hull and shell) into the soil using a Northwest Tiller rototiller.

Figure 5. Anaerobic soil disinfestation plot after installing drip lines and placing TIF tarp.