Use of Almond Hull and Shell as Organic Matter Amendments in Advanced Orchard Management

Progress report for SW20-912

Project Type: Research and Education
Funds awarded in 2020: $349,807.00
Projected End Date: 07/31/2023
Host Institution Award ID: G126-21-W7899
Grant Recipient: University of California Davis
Region: Western
State: California
Principal Investigator:
Dr. Sat Darshan Khalsa
University of California Davis
Dr. Patrick Brown
University of California Davis
Dr. Amelie Gaudin
University of California, Davis
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Project Information


Standard almond harvest in California utilizes an on-ground approach which requires multiple machinery passes to shake trees, sweep nuts into row alleys after drying, and collect the crop. This relies on fruit contact with a bare orchard floor, limiting application of organic matter amendments (OMA). This high protein crop requires high nutrient inputs. Inefficient fertilizer use can lead to nutrient leaching, groundwater contamination, and financial losses. Almond huller/processors face management decisions concerning increasing hull and shell biomass. Harvest machinery can disturb topsoil and create dust. Accessible strategies are needed to improve soil health, optimize crop nutrient use, mitigate environmental issues, and optimize hull and shell value.

This project addresses the need for advanced nutrient and harvest management through orchard trials using food-safe OMA to improve soil health and provide nutrients. This creative solution will produce results that guide recommendations using hulls and shells as OMA (HS OMA) to address critical issues currently limiting orchard sustainability. This research will examine complementary off-ground catch frame harvesters to minimize soil disturbance and promote nutrient mineralization.

Research questions will investigate effects of HS OMA management strategies on soil health, plant nutrition, economic value, and environmental impacts:

1. How does almond HS OMA application affect components of soil health?

2. How does HS OMA decomposition and nutrient mineralization impact plant-available nutrients, crop nutrition, and yield?

3. What are the nutrient values, application costs, and associated potential yield effects from HS OMA in different production contexts?

4. What are the broad environmental implications of HS OMA practices?

Research components will utilize randomized complete block design (RCBD) at three sites to evaluate the effects of HS OMA. At one site, a split-plot design will be added to RCBD treatments to compare catch frame harvest to conventional on-ground harvest. Each site will be a case study, independent in location and distinct in design. Outreach and collaboration will share findings among growers, crop advisors, Extension educators, and the public. At on-site field days, growers will discuss practices and guide orchard walk-throughs, prompting dialogue and farmer-to-farmer education. Researchers will present results using posters and engaging take-home materials. A final workshop will integrate feedback and present aggregated data. Researchers will creatively deliver outreach using a decentralized social media approach, engaging existing platforms with partners to provide content through active media outlets such as blog posts, podcast episodes, and videos. At conferences, researchers will present findings and promote social media outlets. Pre/post surveys will assess educational outcomes at events and a stakeholder-developed survey will assess barriers to practice adoption.

This project will promote change in the almond industry by integrating self-generated OMA with nutrient management to improve almond orchard soil health and nutrient management. Assessing benefits and barriers to adoption will ensure research directly assists growers. Results will inform recommendations to improve the sustainability of the almond industry and lay groundwork for future studies. Recommendations will be widely accessible and a variety of management options will allow growers to tailor practices to unique regional contexts.

Project Objectives:

Soil Health Objective: Assess impacts of almond HS OMA and advanced orchard management strategies on soil health, nutrient mineralization over time, and soil microbial activity related to HS OMA decomposition. Monitor and evaluate changes in soil health measures in three separate case study orchards. Assess broad potential environmental implications of HS OMA and advanced orchard management strategies by evaluating potential changes in soil health and nutrient retention.

Plant Nutrition Objective: Assess the impacts of almond HS OMA and advanced orchard management on almond tree nutrient uptake and crop productivity. Monitor and evaluate HS OMA decomposition over time. Measure changes in crop nutrition by analyzing July leaf samples annually. Analyze annual yield data across treatments. Assess findings in plant nutrient use to characterize crop nutrient uptake from OMA.

Economic Objective: Assess the economic value of HS OMA inputs. Quantify the nutrient value of potassium in HS OMA and compare to potassium content in fertilizer sources. Compare costs of HS OMA practices to current fertilizer costs. Use annual yield data to assess potential economic benefits from HS OMA effects. Document and compare costs of HS OMA and fertilizer application and labor at each site. Document and compare the costs of on-ground and off-ground harvest at Tracy field site.

Educational Objective: Aggregate research data in engaging outreach documents and media that characterizes HS OMA management strategies to facilitate grower adoption. Demonstrate and discuss potential costs and benefits through presentations at conferences, three field days at trial sites, and a final workshop. Prompt discussion among growers and advisors to improve research. Use surveys to identify learning outcomes at all events. During the first year, use an online stakeholder-developed survey to assess potential barriers to adoption for conventional and organic almond growers. Integrate survey findings into outreach activities.



Click linked name(s) to expand/collapse or show everyone's info
  • Helen Andrews (Researcher)
  • Dr. Anthony Fulford (Educator)
  • Dr. Brent Holtz (Educator)
  • Dr. Katharine Jarvis-Shean (Educator)
  • Kirk Pumphrey - Producer
  • Kristin Jacobs - Producer
  • Amanjit Sandu - Producer



Research question 1: Can almond hulls and shells be used as a soil amendment over almond tree roots to supply potassium for crop uptake? (Tracy field site)

Hypothesis 1: Surface-applied hulls, shells, and a mix of the two materials can provide mineralized potassium over time for crop uptake. These materials contain different concentrations of potassium and other nutrients and likely release nutrients at different rates. Increased soil exchangeable potassium under amendments could improve tree nutrient status over time as measured in July leaf nutrient values.

Research question 2: How does the application of a hull/shell mix affect soil health and crop performance with and without off-ground harvest in an almond orchard? (Woodland field site)

Hypothesis 2: Microbiological soil health variables most likely to improve under amendments and off-ground harvest (reduced soil disturbance) are increased SOC content, microbial biomass, and a more stable microbial community composition dominated by fungal organisms. Chemical soil health variables most likely to improve are CEC, higher nutrient availability, and potentially pH over time. Soil physical improvements under the combined amendments and off-ground harvest may include improved aggregate stability and water retention by year 3. Crop performance variables most likely to improve under these treatments are improved nutrient uptake, reduced drought stress, and improved crop yield.

Research question 3: What are the effects of almond shell amendments vs. composted shells on soil fertility and water conservation in an almond orchard? Specifically, what are the differences in soil nutrient availability and tree water status over time, and could potential improvements enable savings in nutrient and water inputs? (Davis field site)

Hypothesis 3: The shell amendment will likely provide a mulching effect as a physical barrier on the soil surface to reduce tree water stress particularly during summer months. Shells will mineralize potassium, calcium, and other nutrients and could improve soil organic matter content, CEC, and pH in the long term. The shell-based compost (originally 70% shells, 30% manure) will provide higher amounts of plant essential nutrients which will be reflected in soil and plant nutrient concentrations, but may not provide the same benefits against tree water stress as the shell amendment. 

Materials and methods:

Methods all field sites

  • Soil: baseline soil fertility immediately prior to annual applications in the Fall. Soil sampling at 0-10cm, 10-20cm, 20-30cm, and 30-60cm. Analyze for exchangeable potassium, nitrate-N, Olsen phosphate, sodium, calcium, magnesium, CEC, percent soil organic matter, and pH.
  • Soil exchangeable potassium in the top 0-10cm at intervals following application. 
  • Water: ongoing rainfall and evapotranspiration records from the CIMIS database. Ongoing irrigation applied in acre-inches provided by grower records or water meter readings. 
  • Plant: decomposition rates using mesh litter bags to measure mass loss over time. Percent K remaining in samples of hull/shell materials over time. 
  • Plant: July leaf nutrient concentrations: sample 20 clusters of 5 leaves per sample tree. 3 sample trees per treatment row. 
  • Plant: yield across treatment plots including dry kernel lb/ac using weigh wagon, percent crackout.
  • Plant: annual trunk circumferences measured in January.
  • Plant: stem water potential at key time points in the season to assess drought stress using a pressure chamber.

Additional Methods for Hypothesis 2 (Woodland field site)

  • Biomass carbon and nitrogen using fumigation extraction, biodiversity using 16S and ITS and analyzing with microbiome explorer R package, broad group community composition using PLFA. 
  • Soil physical characteristics in Fall 2022: aggregate stability using a rainfall simulator, water retention using a Hyprop, and bulk density using a metal ring and mallet.
Research results and discussion:

Data that is being collected from all field sites will be analyzed this Fall with further results anticipated by the end of 2021. All plans outlined in the original submitted timeline chart have been fulfilled since August 2020 with the exception of implementation of catch frame harvest at the Woodland site last Fall and survey distribution. The catch frame harvest treatment will be implemented in August 2021 and the survey will be distributed at the Almond Conference 2021. 

Participation Summary
3 Farmers participating in research


Educational approach:

August 2020 - current

Survey: Collaborators created a stakeholder developed survey to assess grower perceptions of management practices, barriers to adoption, and areas for research improvement. This survey will help characterize contextual factors related to stakeholder-identified benefits and barriers to adoption. The survey positions research within a larger defined orchard management framework. Survey variables relate the practices of using hull/shell organic matter amendments and off-ground harvest to crop system considerations such as insect and pathogen management, soil fertility changes, nutrient availability, economic factors, equipment, etc. This survey uses a Likert scale through Qualtrics. It was developed with extensive feedback from an advisory committee consisting of project advisors, producers, Extension specialists, and other researchers. The next steps are to further refine survey content and move it to the Qualtrics platform. It will be distributed to producers and crop advisors at the Almond Board Conference in December 2021. 

Social Media: Ms. Andrews wrote a blog post draft for the Sac Valley Orchards website under the direction of Dr. Katherine Jarvis-Shean. Project advisors and collaborating graduate students have provided feedback. General preliminary data will be added to this blog post before 2022. The post will be published in the Spring of 2022 following Dr. Jarvis-Shean's suggestion.  In the next year, further social media collaborations will be developed with the Almond Doctor and the UCD Fruit and Nuts Research and Information Center. 

Presentations: Dr. Khalsa gave a talk during a virtual field day discussing dairy manure compost using different feedstocks including almond byproducts such as hulls and shells. Ms. Andrews submitted a poster to the American Society for Horticultural Science discussing organic amendments and potassium cycling. 

Educational & Outreach Activities

1 Webinars / talks / presentations

Participation Summary:

15 Farmers
60 Ag professionals participated
Education/outreach description:

To date, our almond grower collaborators hosting the three field trials and their respective agricultural consultants have provided feedback about critical areas of crop system management to consider during the creation of the stakeholder-developed survey. They commented on barriers to adoption, motivating factors, and other implementation considerations. Their input has been integrated into the survey draft which will be distributed widely to growers at the Almond Board Conference 2021.

Furthermore, Dr. Khalsa gave a talk on March 18th about soil amendments in orchards with a focus on dairy manure compost. The virtual field day hosted by another UC Davis collaborator and supported by the California Dairy Research Board and the Almond Board of California was attended by 65 participants, approximately 12 farmers and 53 agricultural professionals. Discussion ensued about the use of almond byproducts like hulls and shells and how they are used as dairy feed for the high quality hulls, but also how lower quality shells can be added to dairy manure compost. The synergy between the dairy and almond industires will be an essential partnership for future sustainability of California agriculture.

Learning Outcomes

Key areas taught:
  • Compost Feedstocks

Project Outcomes

3 Farmers changed or adopted a practice
1 New working collaboration
Project outcomes:

The three farmers hosting field trials have applied additional almond hulls and shells as amendments to multiple areas and orchards outside the WSARE field plots. Generally, they view this practice as a method to provide nutrient inputs, improve soil health, and buffer the soil surface against soil evaporative losses. Growers and crop advisors have described conversations with neighbor growers about this practice. All growers in this study obtain hull/shell amendments free of charge from local processors. Anticipated changes in COVID restrictions over the next year will hopefully enable more in-person farmer-to-farmer discussions. Data obtained from soil and crop measurements during this first year will be used to inform future recommendations supporting environmental and economic sustainability related to soil health, nutrient and water management.  

Success stories:

A small almond grower near Woodland, CA is using almond hulls and shells as an organic matter amendment to improve components of soil health, provide organic nutrient inputs, and ultimately to improve crop performance and yield. After the hull/shell material was applied in October 2020 at 8 tons/acre across the berms and alleys of test plots, he applied a layer of compost across the entire orchard as a best management practice. After winter rains, cover crops grew successfully through these amendments in the Spring. Amendments provide a nutrient-rich mulch over tree roots during critical periods of crop nutrient and water uptake in the Spring and summer. In August, standard on-ground harvest will be compared to catch-frame harvest as a soil health treatment. Researchers are collecting data on soil nutrients and fertility, soil physical characteristics, soil microbiology, nutrient status, crop performance and yield from August 2020-2023. The grower will use findings from this field experiment to tailor this practice to meet his production and crop system goals. 

A grower located near Tracy, CA is comparing almond shells, hulls, and a mix of these materials to better understand potassium release in an almond orchard field site. His operation is vertically integrated with a processing facility located one mile from the orchard plot. In this field trial, he uses a compost spreader to concentrate these materials over tree roots in the rows. He hopes to substitute a portion of organically-sourced potassium from hulls and shells for the necessary potassium fertilizer to reduce fertilizer expenditures. He also applies these materials in orchard alleys close to the processor to reduce soil cracking and dust generation.

An orchard manager outside of Davis, CA is comparing a fresh almond shell amendment to a shell-based compost to evaluate effects on soil fertility and potential water conservation. Her operation has been generating a compost comprised of 70% almond shells and 30% from local sources and applying in orchards in recent years. She is interested in the differences between these two materials and the potential nutrient benefits provided by the compost process. Data will help her understand effects of these materials on soil and crop functioning. 

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.