Nitrogen fertilization is important on first-year second generation almond trees following whole orchard recycling
Brent Holtz, Ph.D, County Director and Farm Advisor, University of California Cooperative Extension in San Joaquin County, baholtz@ucdavis.edu.
Mae Culumber, Ph.D, Farm Advisor, University of California Cooperative Extension in Fresno County, cmculumber@ucanr.edu.
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 could be a sustainable method of tree removal that can enhance both air and soil quality. Before air quality restrictions, orchard removal meant pushing trees into a large pile and burning them. But when air quality regulations were implemented in 2002 under the Clean Air Act, old orchards were ground up with a tub 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 (more from forest waste) and reduced the price they pay for the debris. Thus, tree fruit and nut growers, who wish to remove dead trees and old orchards, need an alternative method of orchard removal that is also sustainable.
When the woody debris of ground trees is burned in a co-generation plant, carbon (C) stored in the wood is lost from the orchard system. A recycled orchard returns approximately 30-60 tons of wood chips per acre depending on the previous orchards tree size, spacing density, and the varieties. The large quantity of woody debris contains an estimated 30,000 to 60,000 lbs of organic C. There are benefits and tradeoffs associated with returning this large volume of C into the soil prior to replanting. 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 and may reduce the incidence of replant disease. However, the higher carbon to N ratios of organic amendments like wood chips can decrease the availability of applied N fertilizers. Consequently, growers may need to apply fertilizer N at rates greater than what is normally recommended for trees in their first leaf. Another concern is that the woody debris may be so large that it interferes with normal soil preparation and orchard management practices. If whole-orchard recycling can be managed so that it does not reduce the availability of nutrients for new trees, does not worsen replant disease, or interfere with harvest, and enhances long term soil health and nutrition, then growers will be more likely to adopt grinding and incorporating as an alternative to burning the woody debris from their orchards.
In our initial orchard grinding trial established in 2008 at the Kearney Research and Extension Center, stone fruit trees were recycled at 30 tons per acre using the Iron Wolf (a 50-ton rock crusher), and compared to field burning and incorporating the ash. The second orchard was replanted to almond and ultimately, greater yields, significantly more soil nutrients, organic matter, and total carbon were observed in the grind treatment when compared to the burn. Leaf petiole analysis also revealed higher nutrient levels in trees growing in the grind treatment, thus proving in the long term, the high levels of organic matter from the recycled orchard did not stunt replanted trees. Later studies at Kearney found 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. Based on the positive results from this trial and the closure of co-generation plants, we estimate almond growers have chipped and incorporated more than 20,000 acres since 2015.
With the adoption of whole orchard recycling, eight additional research trials were established throughout California to further evaluate the impacts of whole orchard recycling on tree health and soil quality. Initial observations in the new trials revealed that our nitrogen recommendations for first year almond trees needed revision, following whole orchard recycling, as reduced shoot growth in second-generation orchards was often observed in early spring after replanting. As a consequence, nitrogen applications were increased to address the likely imbalance between the carbon to nitrogen levels in the soil (often referred to as the C:N ratio).
In a previous study conducted by David Doll, UC Farm Advisor in Merced County, he determined that first year almond trees grew best when given between 3-4 ounces of actual N (25-35 lbs N/acre) in their first growing season. The applications are typically spread out so that no more than one ounce of actual N is applied per tree per application. This recommendation may not be enough following WOR, especially if 40-60 tons of wood chips are incorporated back into the soil. In 2017, working in Louie Tallerico’s recycled orchard in Manteca, we tripled David’s recommendation after we noticed reduced shoot growth, and applied 11 ounces of N per tree (approximately 100 pounds N per acre). Trees that had initially showed reduced shoot growth responded nicely to the additional nitrogen. Another factor in need of study is the unaccounted reduced efficiency of applying nitrogen through a double-line drip system, where only an estimated 20% of the emitters reach the trees early in the first-year. As the trees mature the double-line drip system will obviously become more efficient delivering nitrogen to trees with larger root systems.
In 2018, we put out a nitrogen trial in Jeff Warkentin’s first-year orchard in Parlier to see if we could determine more accurately the nitrogen requirements of first year almond trees after whole orchard recycling. In order to more precisely apply the nitrogen, triple 15 granular fertilizer was hand applied to each tree. We put out five treatment rates with 5 tree replicates, in a Latin Square designed experiment. Nitrogen rates of 0.0, 0.40, 0.60, 0.80, and 1.0 ounce of nitrogen per tree were applied once per month, for five months, from March through July. After five months, each treatment received 0.0, 2.0, 3.0, 4.0, and 5.0 ounces of additional nitrogen per tree. These applications were in addition to the Jeff’s fertigation through the drip system at a rate of 1.73 ounces of N applied monthly from April to August (with the exception of May when a 2.5 ounce application was made). We expect that the grower applied nitrogen was not all immediately available because of the emitter spacing and the limited range of the small tree roots, especially in the first year.
Our first nitrogen application in March seemed to have an almost immediate impact. Considerable precipitation in March effectively dissolved the granular nitrogen, and differences in shoot growth were detected between treatments soon after (pictures 1 & 2-trees with and without nitrogen treatments in March). Leaf analysis showed that our nitrogen treatments early in the season seemed to have a greater impact on nitrogen tissue levels than applications later in the season (figure 1-leaf analysis May-September). Trunk diameter data showed that we did not receive any additional benefit for applying more than 4.0 ounces of actual nitrogen per season in addition to what the grower applied (figure 2-trunk diameters from monthly applications of N).
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, he too observed an increase in shoot growth early in the spring with early nitrogen applications. It may be that we can use less nitrogen more efficiently if we apply it earlier in the growing season or at planting time. We will attempt to study early nitrogen efficiency in more detail in future trials. But at this point in our research, we would recommend to growers that they apply at least 6-8 ounces of actual nitrogen per tree (50-70 lbs N/acre) in the first year of tree growth following whole orchard recycling. And that early applications may be more important than applications later in the season. Remember that nitrogen applications should be spread out so that no more than one ounce of actual nitrogen is applied per tree per application in the first year of tree growth.
In our WOR trials, we did not have to apply additional nitrogen in the second year of our studies to achieve the tree-growth we expected. We hypothesize that in the first season after WOR, the wood chips, and the microorganisms decomposing them, compete for the nitrogen applied to first-year trees. The wood chips and soil microorganisms may also bind and immobilize excess nitrogen that may otherwise leach through the soil profile where young tree roots have not yet expanded in the spaces between trees. 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 the wood chips were analyzed for their nutrient contents, which averaged 0.31% nitrogen, 0.20% potassium, 0.60% calcium, and 50% carbon. Returning 64 tons of wood chips to the soil per acre provides 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.
The team of researchers studying whole orchard recycling has increased in the last couple of years because of funding opportunities and the need to find an alternative to orchard waste burning in co-generation facilities. The Accelerated Innovation Management (AIM) program of the Almond Board of California (ABC), which emphasizes stewardship of resources, sustainability, and production efficiency, has funded our whole-orchard recycling project to compare whole-orchard recycling with conventional orchard residue removal and burning in a co-generation facility, to refine the life cycle assessment (LCA) model for the evaluation of carbon dynamics, to quantify the physical, chemical, and biological soil properties that result from whole-orchard recycling, and to assess the effects on the growth, health, nutrition, and water use of the replanted orchard.
In 2016, the California Department of Food and Agriculture (CDFA) ranked increasing soil organic matter a funding priority as world leaders were discussing the capture of carbon in agricultural soils as a formal part of the United Nations Climate Change Conference Agreement. Dr. Amélie CM 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 the ABC to study the “Influence of Whole Orchard Recycling on greenhouse gas emissions (GHG) and Soil Health in a Newly Established Almond Orchard.” Our team of researchers also includes Dr. Greg Browne, a plant pathologist with the USDA-ARS in UC Davis, Dr. Andreas Westphal, a nematologist at UC Riverside, Dr. Elias Marvinney and Dr. Emad Jahanzad, post-doctorial scientists studying at UC Davis, Dr. Suduan Gao, a soil scientist with the USDA-ARS in Fresno, and Dr. Amisha Tashee Poret-Peterson, a microbiologist with the USDA-ARS in UC Davis. Our team of UC Farm Advisors includes and Dr. Mohammad Yaghmour, in Kern County, Dr. Phoebe Gordon, in Madera County, Dr. Franz Niederholzer, in Colusa, Sutter, and Yuba Counties, David Doll, in Merced County, and Luke Milliron, in Butte, Glenn and Tehama Counties. We hope that this team will demonstrate the success of whole-orchard recycling and provide scientific evidence to support legislation that allows growers to receive carbon credits for recycling their orchards into the soil. These carbon credits would encourage sustainable agriculture and help compensate growers for the expenses they incur when adopting whole-orchard recycling.
Whole orchard recycling 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 their wood waste. Now growers can expect to pay from $600-700 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 $300-400 per acre to spread their wood chips evenly back onto the soil surface. Typically, after spreading, growers will follow their normal replant program of deep ripping, stubble disking, and soil fumigation. To off-set these expenses we have observed about a 1,000-pound kernel increase per acre from trees growing where the previous orchard was recycled after 8 seasons in our original trial at Kearney. The San Joaquin Valley Air Pollution Control District (SJVAD) has recently approved a program that will reward growers with funding from $300-600 per acre up to $60,000 per year to implement whole orchard recycling. For more information on these incentive programs, contact Jacob Whitson with SJVAD at 559-230-5800 or at Jacob.Whitson@ValleyAir.org.
A big thank you to Louie Tallerico and Jeff Warkentin for letting us experiment with nitrogen rates in their orchards.
Picture 1-photo of a control tree that received no nitrogen in March
Picture 2-photo of a tree that received 0.8 oz of Nitrogen in March
(try to put these pictures side by side in the article)
Figure 1. Percent Nitrogen in leaf tissue from May-September.
Figure 2. Trunk diameters of first year almond trees given additional monthly applications of nitrogen.