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

Final report for SW20-912

Project Type: Research and Education
Funds awarded in 2020: $349,807.00
Projected End Date: 12/31/2023
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 was replaced by 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:

August 2020-2021

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. 

August 2021-2022

Across all three field sites, the percent potassium remaining in the different hull/shell amendment sources decreased over time, indicating potassium released into soil solution. Corresponding substantial increases in soil exchangeable potassium were found at all three sites, regardless of irrigation type. The potassium solubilization curves from these field sites resemble similar potassium release curves from other crop residues as described in the review paper, Andrews et al. 2021.  Ms. Andrews is currently working with a statistician to develop a model for estimating percent potassium released form these amendments as a direct function of water applied (irrigation and rainfall).  Potassium release is characterized by a dramatic reduction in K within the first 3 inches of applied water, followed by a more gradual reduction over time. Overall, around 30-50% of initial K was released within the first 3 inches of water, and then around 80-95% of total K was released by the following summer. Generally, from Fall until the following summer, shells decomposed around 20-30% total, the shell-based compost at Bullseye decomposed by around 50%, hull/shell mix decomposed around 50%, hulls decomposed around 60%. When allowed to remain on the soil surface under catch frame harvest, the hull/shell mix at Westwind decomposed by 45% after 365 days. 

At Crown Nut Company, there were no statistically significant differences in July leaf values in 2020 or 2021. There were no signs of any potential nitrogen immobilization in the soil impacting July leaf N values. Average leaf K was slightly higher for the amended trees in July 2021, though nonsignificant. Amendments maintained yield in 2020 and 2021 as there were no differences in average dry kernel lb/ac or average % crackout. While a small percentage of amendment materials were found in yield samples, the grower/processor reported this did not interfere with processing. Hulls, mix, and shells released an estimated 103-135 total lb/ac of potassium at approximately $52-68/ac value. The grower reported that potassium sulfate fertilizer costs around $200 for 400 lb/ac, whereas the amendment materials are free for them. 

At Bullseye, from Fall 2020 application until the following summer, the shells released approximately 80% of total K while compost released approximately 60%. However, dry mass remaining was approximately 70% for shells and 50% for compost, illustrating that potassium release is not strongly limited by decomposition rate. The shells released a higher fraction of initial potassium, and the compost decomposed relatively more than the shells. Potassium release corresponded with significant increases in soil exchangeable potassium under both amendments from around one month (1.85 inches of water) following application onward. All July leaf nutrients were in the optimum range, and there were no significant differences between treatments. Average leaf potassium under shells was slightly higher, though nonsignificant. Shells and compost maintained yield as there were no significant differences in average dry kernel lb/ac or average percent crackout. 

At Westwind, the hull/shell mix amendment applied in Fall 2020 released around 95% of total initial potassium by June. This corresponded with significantly higher soil exchangeable potassium particularly in winter and spring. After one year, the amendment decomposed only by around half of its dry weight. Average July leaf nutrients in 2021 showed statistically significantly higher leaf potassium and lower leaf magnesium under the amendments. However, all macronutrients fell well within the recommended range, except nitrogen which was similarly deficient in both amended and control trees. Stem water potential was significantly less negative for amendment trees when measured six days after irrigation events in April, June, and July, but not when measured only 3 days after irrigation. This suggests that the amendment can reduce tree drought stress particularly during the later period of an irrigation cycle. Collaboration with irrigation scientists using Electrical Resistivity Tomography imaging indicated that the amendment provided a mulching effect by increasing irrigation infiltration rate and reducing evaporation from the soil surface. Average kernel lb/ac yield and average percent crackout were both slightly higher for amended catch frame harvested trees, though not statistically significant. Issues with the catch frame equipment illustrated the importance of working with effective equipment that is adjusted appropriately for almond trees and experienced operators who can ensure efficacy in the future.

At Westwind, baseline microbial biomass carbon in Fall 2020 was similar across all treatment rows. By Fall 2021 we saw significant (p<0.001) treatment effects. Control rows had the least microbial biomass carbon, with the unamended off-ground harvest rows having slightly more. The rows which had received the almond hull and shell amendment had the most microbial biomass carbon, with the amended off-ground harvest rows having even more than their amended on-ground harvest counterparts. Statistical analysis showed that when amendment and harvest type were analyzed separately for Fall 2021, the effect of amendment was highly significant (p<0.0001) while the effect of harvest type was not yet significant (p=0.1486). In the intermediate timepoint, Spring 2021, the reverse order of treatments was observed, with the control rows having the highest microbial biomass carbon and the amended rows having the lowest, though the treatment effect at this timepoint was not statistically significant. This intermediate step is likely the effect of temperature, with the amendment acting as a mulch to increase soil moisture retention and create cooler soil temperatures, especially in the spring. By the fall, however, the hypothesized treatment effects had emerged.

At Westwind, samples were taken for 16S and ITS shotgun sequencing at three time points and stored at -80 C. However, after contracting with a sequencing laboratory, we were told that they were unable to extract enough DNA to move forward with sequencing. Instead, microbial community composition was assessed using PLFA (phospholipid fatty acid analysis) in both the amendment organic layer and the top 0-10cm soil. In October 2021, the 1 year old amendment layer contained around double the microbial biomass compared to the 1 week old amendment layer. The original amendment had higher total bacteria and total fungi biomass, specifically higher actinomycete, saprophyte, mycorrhizae, and protozoan biomass, with a higher ratio of fungi to bacteria. Compared to the control soil, the soil microbial community under the combined amendment and catch frame treatments contained higher average total biomass, including both higher total bacteria and fungi biomass, specifically higher actinomycete, mycorrhizal, and protozoan biomass with a very low fungi to bacteria ratio. The PLFA data indicates that the amendment layer is full of microbial biomass, can support higher trophic levels such as protozoa, and is building a microbial food web over time. Compared to control soil, the soil with the amendment and catch frame contained higher soil microbial biomass after one year and Principal Components Analysis indicated that higher bacterial biomass in particular distinguished it from the control soil. There was a slight increase in arbuscular mycorrhizal biomass under the amendment as well. 


Dr. Andrews successfully defended their thesis in December 2022 and now serves as the speciality crop advisor for UC cooperative extension for Napa, Sonoma and Marin counties. Dr. Andrew's departure gave way to Ms. Sydney Cho to join the team and carry out remaining research and education tasks including supporting the Almond Board of California workshop in December 2022 and a field day at Westwind Farms in April 2023. Two mauscripts have been published in Plant and Soil and Applied Soil Ecology with Dr. Andrews as the lead author. All information products have been developed for this project and uploaded to the project management system. 

Participation Summary
3 Producers participating in research

Research Outcomes

Recommendations for sustainable agricultural production and future research:

Study results included clear recommendation and practical applications for each participating farmer producer. These recommendations are derived from the factsheets located in the information products section of this project report. Below we break them down by farm:


The hull/shell amendment increased K cycling, decomposed rapidly, improved water dynamics during dry periods by acting as a mulch, and increased root biomass. Maintaining the amendment with off-ground harvest maximized K benefits, established a microbially-rich organic layer on the soil surface, and increased microbial biomass in the soil beneath it. Beneficial microbial functional groups included saprophytes and arbuscular mycorrhizae.

Crown Nut

Hull/shell amendments increased K supply by releasing K rapidly under water application, increasing soil XK in the first 2 months, and increasing tree K status in the third year after 2 applications. This practice did not influence yield at this site where K was already sufficient. Almond hulls/shells can be recycled as a convenient K-rich organic matter amendment. This practice improves orchard K cycling and can help reduce reliance on K fertilizer. Application rates can be chosen to increase K status in K-deficient orchards, or to replace K removed in fruit at harvest to maintain sufficient K while avoiding Mg competition. Growers can monitor soil XK and July leaf K levels to understand orchard effects in different soil types and management approaches.


Fresh shells and composted shells both released K rapidly under water application, increased soil XK in the first 2 months, and slightly increased tree K status after two applications. While the compost slightly improved the soil microbial community, fresh shells significantly increased the microbial biomass of many beneficial functional groups including saprophytes and arbuscular mycorrhizae. After a year, lower soil exchangeable sodium was found under both shells and compost in the upper 0-10 cm soil. Growers can use both materials to improve K cycling and reduce reliance on K fertilizers. Fresh shells can be used to increase soil microbial biomass and promote beneficial microbial groups.

These recommendations were also reported in the UC extension article and the Western SARE podcast located in the information products section.

New Grants

This work stimulated a series of new grants examining related topics in orchard organic matter management and offground harvest listed here:

CDFA Fertilizer Research and Education Program – Improving nitrogen and potassium management in almond orchards with hulls and shells as a soil amendment and offground harvest.

Foundation for Food and Agriculture Research – Advancing knowledge and adoption of climate-smart practices in semi-arid perennial cropping systems.

Western Sustainable Agriculture Research and Education Program – Integrating cover crops and organic matter amendments for whole orchard regenerative management.

Almond Board of California – Multi-scale evaluation of stacked regenerative and climate smart practices in almond systems.

Almond Board of California – Yield prediction for resource management and yield optimization in almond

Research articles submitted or in preparation

A number of collaborations arose from the study plots supported by this grant including the following manuscripts not found in information products:

Hartman LH, EM Andrews, ACM Gaudin, PH Brown and SDS Khalsa (Submitted) Evaluation of almond hull and shell amendment across soil health management practices. Soil Systems

Kisekka I, SR Peddinti, D Vanella, EM Andrews, PH Brown and SDS Khalsa (Submitted). Organic Soil Amendment Effects on Soil Hydrology in an Almond Orchard Evaluated Using Time-Lapse Electrical Resistivity Tomography. Agricultural Water Management

5 Grants received that built upon this project
5 New working collaborations

Education and Outreach

6 Curricula, factsheets or educational tools
3 Journal articles
3 On-farm demonstrations
1 Online trainings
1 Published press articles, newsletters
4 Tours
1 Webinars / talks / presentations
2 Workshop field days

Participation Summary:

45 Farmers participated
100 Ag professionals participated
Education and outreach methods and analyses:

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 in the form of a workshop.

This workshop was conducted at the 2022 Almond Board of California annual conference in Sacramento, California. Conference participants enjoyed a poster session including those developed by this project and other related projects. This poster session was followed by in-person surveys on two specific advanced practices - use of almonds hulls as a soil amendment and offground harvest. A total of 33 participants completed the survey with key insights into the potential of these for wider adoption by the almond industry. Those results are reported as the information product "WSARE Workshop Results".

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 industries will be an essential partnership for future sustainability of California agriculture.

A field day was conducted in April 2023 at Westwind Farms in Woodland, California. Field day participants enjoyed a poster session including those developed by this project and other related projects. This poster session was followed by a field tour to observe ongoing field plots with and without almond hulls and shells and offground harvest. This tour was followed in-person surveys. A total of 18 participants completed the survey with key insights into the potential of these for wider adoption by the almond industry. Those results are reported as the information production "WSARE Field Day Results".

3 Farmers changed or adopted a practice

Education and Outreach Outcomes

Recommendations for education and outreach:

Adoption of best orchard management practices by California growers is a necessary step to reduce nutrient loading to groundwater, reduce dust from harvest, to improve soil health and to maintain economically viable orchard systems, while satisfying the needs for sustainability. However, there has been inadequate information on linking science-based results on practices such as almond hull and shell amendments and offground harvest with adoption. Research over from this project identified many promising outcome that can improve nutrient management, soil organic matter, tree water use  and maintain economically viable orchard systems. Results include availability of potassium from hulls and shells, improved soil health, maintained tree performance and the enabling of undisturbed mulch layers from ongoing offground harvest. Given the progress in the research of these practices, there was enough of an understanding to address growers regarding practice adoption. Our outreach and education events included field days, a grower workshop, online presentations, tours and inviting in new researchers and extension agents leading to multiple new grants in this space. Even though COVID restrictions limited in-person opportunities and staff availability to further engage with producers, we developed multiple information products for ongoing engagement over time including blog posts, podcasts, factsheets and educational results in addition to peer-reviewed research papers. Our recommendation will help guide practice, policy, investment and incentives necessary to meet agricultural and environmental challenges for orchards in California.


50 Producers reported gaining knowledge, attitude, skills and/or awareness as a result of the project
Non-producer stakeholders reported changes in knowledge, attitudes, skills and/or awareness as a result of project outreach
100 General public
5 Students
20 Ag Service Providers
Key areas taught:
  • Compost Feedstocks
  • Use of almond hulls & shells as organic matter amendments
  • Offground harvest
  • Potassium fertilizer
  • Tree water use and soil water infiltration
  • Weed suppression
Key changes:
  • Nutrient inputs to help reduce fertilizer use

  • Soil water retention and irrigation conservation

  • Improvements in soil health

  • Lower food safety risk for organic matter

  • Weed suppression

  • Dust reduction

Information Products

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.