With a total crop values of $5 billion, almond (Prunus dulcis) is one of California’s most important crops. Almond production is faced with multiple challenges, many of which are related to soil degradation. Based on the recent Almond Board of California Sustainability Survey, there has been a shift in growers’ soil management practices away from the conventional bare soil management towards maintaining a resident vegetation soil cover throughout the winter. This change was in response to increasing concerns regarding impact of poor soil health on tree productivity, and the increased interest to sequester carbon and reduce the environmental footprint of almond production. Short-term studies conducted in the 1990s showed that cover cropping was compatible with intensive almond production systems. However, this practice was never widely implemented due to remaining concerns regarding possible tradeoffs and lack of cover crop best management practices. To address these uncertainties, my project is first conducting a state-wide growers’ survey to identify barriers which have hampered cover crop adoption in California, and to understand the early adopters’ motivations and management practices. In addition, three replicated, randomized experiments have been established in 2017 in growers’ orchards across California’s soil-climate gradient to assess the impact of two cover crop mixes and termination dates on soil health. The two multi-species cover crop mixes were designed to address key grower objectives (pollinator habitat and soil health). The project is integrated within a larger system-wide initiative in collaboration with other UC Davis research teams evaluating multiple co-benefits and potential tradeoffs of cover cropping on ecosystem services (pollinator health, soil food web support, weed management, parasitic nematode suppression and water conservation). This will provide growers with a comprehensive opportunity cost assessment and support the development and adoption of sustainable soil management practices.
Objective 1. Compile knowledge on the use, management and barriers for the adoption of cover
crops in irrigated almond orchards.
Objective 2. Determine the potential of two cover crop mixes to enhance soil health across a soil-climate gradient.
Objective 3. Determine how cover cropping management can be integrated in almond orchards to address specific soil-related issues.
a. Orchard trial 1. Compaction study. Determine the capacity of cover cropping to restore
and maintain soil quality and structure at a compacted site compared to deep ripping.
b. Orchard trial 2. Termination study. Determine how cover crop termination timing
affects nitrogen fixation, and C:N dynamics in orchard soils.
c. Orchard trial 3. Nitrogen study. Determine the effect of cover cropping on nitrogen
cycling in orchard systems.
Objective 4. Evaluate the performance of each cover crop species mixture through an ecosystem
service assessment to weigh the benefits to tradeoffs of cover cropping.
Objective 5. Extend information and communicate results to growers on the potential of cover
cropping to augment ecosystem services in almond orchards, through workshops and grower field
Objective 6. Contribute to the scientific body of knowledge related to sustainable agriculture by
publishing the outcomes of objectives 1-4 in peer-reviewed journals.
Growers’ Survey (Obj. 1).
Survey design. A questionnaire was designed in 2017 following the Tailored Design Method (Dillman et al., 2009) and using Qualtrics©, a cloud-based survey software. Questions were chosen to meet four main objectives. Firstly, the questions provided insight on currently used floor management practices across the landscape and their respective popularity amongst users. Second, they identified potential barriers respective to either (1) ecosystem tradeoffs, which could offset short-term agronomic productivity or (2) the practicality of integrating cover cropping (CC) operations in almond orchard systems. Thirdly, the survey determined motivators of non-users (perceived benefits) and of current users (perceived benefits and realized benefits following implementation). Finally, the survey determined current gaps in knowledge and information, which producers may need to build their understanding of CC.
Survey respondents. The target group of respondents are individuals involved in the decision-making process of almond farms: owners, managers and/or operators. Respondents of the survey were asked to identify themselves as either users or non-users of cover crops, each of which then followed different questionnaires. Questions addressed to both users and non-users identified perceived benefits and concerns associated with cover cropping. The questionnaire addressed specifically to users included questions relating to their experience with cover crops (types of crops grown, proportion of the land with CC, annual or perennial, winter or spring, single or multi- species mix) and to their management practices (seeding date and date of termination). The definition of users specified in the survey was: “growers having cover crops grown on either part or all of their acreage with a minimum of 1 acre, in at least one growing season in the past five years”. The target group of respondents did not include external farm advisors nor extension specialists.
Survey distribution. An IRB human subject’s approval was obtained for this study for the graduate student and PI. The survey questions were communicated to the Almond Board of California (ABC) and approved for distribution in December 2017. The paper-version of the survey was distributed at the 2017 & 2018 Almond Board of California Annual Conference. An online version was also made available through the project website (https://almondcovercrop.faculty.ucdavis.edu). Paper-versions of the survey were distributed through UCCE Extension Centers at almond-specific growers’ workshops and outreach events. The survey was also communicated through the Western Farm Press and UCCE County Newsletters. Ultimately, the information drawn from this survey will be used to tailor research agendas respective to the evaluation of cover-cropping systems and to bring relevant and applicable information that address growers’ interests.
Growers’ Survey URL: https://almondcovercrop.faculty.ucdavis.edu
Field Study (Obj. 2 & 3).
Cover Crop Design. Two types of cover crop mixes were designed for this project: a Pollinator mix (PM) and a customized Soil Building mix (SM) as indicated in Table 1. The pollinator mix is currently the most popular cover crop mix in almond orchards (Pam Mustard Mix, Project Apis). The PM is composed primarily of flowering mustard species to attract bees and provide a habitat. Species in this mix come to bloom relatively quickly and can provide flowers from November to February attracting bees within the orchard prior to and during almond bloom. The PM has previously been recommended by the Almond Board in 2016 and has gained popularity with almond growers. However, no comprehensive study has been conducted to analyze the performance of this mix, relative to different ecosystem services. The Soil Building mix is designed to address soil-related issues and develop overall soil health whilst limiting water demand. The SM was developed based on the USDA-ARN Cover Crop Chart (v.2.1) evaluation (Johnson et al., 2015). Species were selected from three plant families (brassica, grass and legume families) to provide diverse rooting architectures and nutrient cycling characteristics. Least relative water-consumptive plant species in each family were chosen. White mustard is kept as a standard between the PM and SM, to ensure both mixes provide services to bees and to provide biofumigant potential through the production of isothiocyanates by white mustard. The capacity of both mixes with regards to both aspects will be compared. Furthermore, Daikon radish as a tuberous crop will be kept in both mixes to address challenges pertaining to soil compaction and water infiltration, which have been identified as key issues in almond orchards. Finally, the SM plant species were selected based on the relative price of seed ($1.25 per pound or $62.5 per acre). Including native California flowering species nearly tripled the price of the mixes: therefore, for the purpose of this study, native species were not selected. Finally, mix design considered the seed availability throughout the season, and its accessibility in different regions of CA, to limit cover cropping transition costs. Both PM and SM will likely impact soil health, and C and N retention differently. The cover crop mixes will be compared to resident vegetation at each of the sites and to bare, chemically-controlled floor management.
Table 1. Cover crop mix composition (% weight) and seeding rate (lbs./acre)
Field Replicated Trials. The trial is in year 2 of 3. We established three randomized complete block design (RCBD) trials with four replications (3 tree rows x 4 treatments x 4 blocks). A total of three sites have been chosen in three ecoregions of California: Glenn county in the Sacramento Valley, Merced county in the North San Joaquin Valley, and Kern county, in the South San Joaquin Valley. All orchard sites were clay loam to sandy clay loam with 50% Nonpareil on Nemaguard rootstocks with microsprinkler irrigation. Each orchard was a minimum of 4 acres (final range of 8-40 acres) including floor management treatments: winter planted cover crops – a pollinator mix and a soil building mix, winter resident vegetation and chemically-controlled bare soil. Cover crop mixes were seeded following almond harvest in October to November 2017 and reseeded in fall of 2018. A key challenge for bearing orchards was finding compatible seeders to allow for wide seeding of the cover crop whilst avoiding damage to the tree branches. To do so, we used compact seeders of 6-15 feet width and ran several passes. The cover crop was terminated in March at all sites. Resident vegetation plots were mowed and maintained to provide a continuous ground cover. Bare soil plots were chemically-controlled according to conventional practices.
Orchard-specific studies. In line with regional survey results, we designed four orchard-specific cover crop studies to address regional knowledge demands. At our first and southern-most site, we included a ripping treatment to compare its effect on soil health compared to that of cover crops. At our first and second sites, we studied the effect of different cover crop termination times on soil health shifts. At our third and northern-most site, due to considerable regional concerns on frost damage, we decided to focus our study on cover crop effects to heat transfer and frost incidence in the orchard microclimate.
Orchard Site 1. Compaction Study (Obj. 3).
The southern-most site included a ripping treatment to compare this practice’s capacity to remediate soil compaction as compared to cover cropping. Orchard middles were ripped in fall 2017 using a Schmeiser mid-row ripper. Ripping depth was set at approximately 60 cm with a width of 15 cm at approximately 1.5 m from the tree rows. The middles will not be ripped in subsequent years.
Figure 1. Orchard site 1 Experimental design including two cover crop mixes and ripping treatments
Orchard Site 1 & 2. Termination & Nitrogen Study (Obj. 3).
Two termination methods of the cover crop were compared: an “early termination” in late-February to mid-March and a “late termination” in April. The cover crop middles were flail-mowed followed by a chemical-herbicide application approximately 1-week following mowing. Cover crop residues were left at the surface on the orchard middles. Separate cover crop biomass samples were collected prior to each termination and dry weights were measured. The dry samples were ground to the consistency of a fine powder and analyzed for total N and C for each sampling event, to record the composition of the residue returned to the orchard.
Figure 2. Orchard site 2 Experimental design including two cover crop mixes and termination treatments
Orchard Site 3. Frost Study. Due to considerable regional concerns on frost damage, we decided to focus the site-specific study of our northern-most site on cover crop effects on heat transfer and frost incidence in the orchard microclimate. We monitored top-soil temperatures at a 6 cm depth using ECH20 5TE sensors (Meter group) connected to EM50 dataloggers. Soil ECH20 5TE sensors also provided readings of soil volumetric water content (%) and EC throughout the winter. Diurnal orchard temperatures and relative humidity (%) were recorded using HOBO Pro-v2 loggers (Onset corp.) set at 5 cm above the soil surface, 90 cm (3 feet) and 150 cm (5 feet), which corresponded to the mid-tree canopy height. Finally, to monitor cover crop canopy development and growth rates throughout the winter, we used NDVI/PRI spectral reflectance sensors (Decagon corp.).
Figure 3. Orchard site 3 Experimental design including two cover crop mixes and frost-monitoring stations
Soil sampling and analyses. Baseline soil samples were collected prior to cover crop seeding in October 2017. Post-cover crop soil samples were collected in October 2018. For the second year of the trial, soil samples will also be collected during cover crop development in February and within 1-2 weeks of termination at each site. In each plot, samples (0-20 cm) were taken from the center of the middles and at the border of the cover crop nearest to the NonPareil tree row. Baseline samples were analyzed for chemical properties (pH, soluble salts, OM%, Total C, Sum of Cations (CEC), K, Ca, Mg, Na, S, Olsen P, nitrate and ammonium content and total N) and physical properties (texture, bulk density). Post-treatment soil analyses also included microbial biomass carbon (MBC), microbial biomass nitrogen (MBN) and labile C using the PoxC method as well as aggregate size distribution and stability. In-situ measurements of water infiltration were taken using a Decagon Dual-Heal infiltrometer at baseline and post-treatment in October 2018. Soil water retention curves will be obtained using the HYPROP system at 1-2 weeks post-termination in 2019. Samples were also used to assess soil food webs and the beneficial nematodes present by our collaborator.
Cover crop sampling and laboratory analyses. Total cover crop biomass as well as species composition and relative abundance and separate C:N ratio for each species was collected from a 0.25 m2 area within each plot before each mowing event and each termination. Dry biomass was determined after drying at 60°C to a constant weight. The dry samples were ground to the consistency of a fine powder and analyzed for total N and C.
Almond yields. At each experimental site, yield production was collected and weighed per treatment replicate. At the northern-most site, manual polling was conducted to remove all almonds from three individual trees per treatment replicate. Yields were brought to UC Davis, hulled and dried to obtain final kernel dry weights. Mechanical harvesters were used at the other two sites. At both sites, conditioners were used to remove residues from the yields. Yields were piled in rows and left to dry approximately a week prior to hulling and kernel weight measurements.
Ecosystem service assessment (Obj. 4). In parallel to our study’s measurements, a number of other orchard ecosystem variables were assessed including pollinator habitat and visitation, parasitic nematode abundance, beneficial nematode abundance, navel orangeworm (NOW) survival and weed pressure. This study is a part of a larger project designed to provide a full ecosystem assessment of cover crop effects on almond production. For more information on the full-orchard assessment, visit: https://almondcovercrop.faculty.ucdavis.edu/project-summary/research-activities/. A multivariate system analysis will be conducted to weigh benefits to tradeoffs as affected by orchard floor management to define best management practices, which optimize benefits for both the producer and the environment.
Dillman, D.A., Smyth, J.D., & Christian, L.M. (2009) Internet, mail, and mixed-mode surveys: The tailored design method, 3rd ed. Hoboken, NJ: John Wiley & Sons, Inc.
Johnson, H and M. Liebig. 2015. Cover Crop Chart. USDA-ARS Northern Great Plains Research Laboratory. Mandan, MD.
Growers’ survey. Our respondents implemented different floor management practices with 17% of respondents using bare soil and 83% using a vegetative ground cover during all or part of the year (28% winter cover crop, 14% in-season cover crop, 14% perennial cover crop and 28% resident vegetation). However, considering that there are about 94% of CA almond growers who use bare soil, our data is currently skewed towards cover crop adopters. In fact, amongst our respondents, 85% indicated that they have used cover cropping in the last 5 years. All growers reported having soil-related issues in their orchard with poor soil biology and poor water retention being the most important issues. All respondents recognize that cover cropping could provide potential benefits and indicated soil health and pollinator habitat as the most important to them. However, although benefits are recognized, our data indicated that some growers discontinued cover cropping because its performance did not match expected returns. Regarding potential tradeoffs of cover cropping, operational difficulties were identified as a greater concern than agronomic and economic challenges related to cover cropping. Of operational concerns, difficult establishment of the stand was the most important operational concern. Overall, our survey results (n=31) suggested that a lack of best management practices (BMPs) is currently the most important barrier to cover crop adoption in almond. We are in the process of collecting more data and hope to reach a representative sample of 500 growers over the next year.
Cover crop biomass and performance. Our three field sites in order of decreasing seasonal precipitation were as follows: Corning (Tehama County), Merced (Merced County) and Arvin (Kern County). Biomass production was proportional to precipitation amounts by vegetative cover type and the seeded cover crop produced more biomass than the local resident vegetation at all three sites: 312% more dry matter than resident vegetation in Corning, and 180% more in Merced. However, it is important to note that the species composition of the cover crop mixes varied widely amongst regions. The soil mix for example was composed of 60% white mustard in Corning whereas it was composed of 59% ryegrass in Merced. This must be considered when developing seed mixes and selecting cover crop species for a particular purpose in an orchard. Due to differences in species composition, the C:N ratio of the cover crop also varied amongst sites. The soil mix had a lower C:N ratio in Corning and Arvin as compared to the soil mix in Merced. Our results highlight the need to develop region-specific almond cover crop BMPs in California.
Soil Health. The field trial is currently in year 2 of 3. Baseline soil samples at the center of the orchard middles indicated soil organic matter (SOM) contents of 2.94%, 1.79% and 2.55% at Corning, Merced and Arvin, respectively. Approximately six months following the cover crop termination, average SOM in CC plots were 0.14%, 0.07%, and 0.02% greater than in the control, for each respective site. Cover cropped plots had approximately six months of development in the year followed by bare ground to facilitate harvest operations. As such, we could expect mineralization to affect the organic matter gains incurred in cover crop plots. Although CC plots had slightly higher SOM contents, differences were not considerable following the first cover crop season.
At each sampling event, soil was sampled at the center of the middles and at the border of the cover crop, closer to the tree berms. At all sites, we found higher C contents and NO3– and NH4+ content in the middles rather than near the tree berms, at both baseline and post-treatment sampling. While differences in SOM were of approximately 20%, total N differences went up to 2-fold between the two soil areas. This heterogeneity in orchard soil properties may be attributed to residue placement: leaves and branches are conventionally removed from tree berms and stacked in orchard middles. The location of the micro-sprinkler wetting zone may also contribute to soil quality differences in the orchard. As such, the choice of cover crop seeding width may be a key component when designing a cover crop for orchard soil health improvements.
Although some trends were observed in post-treatment physical soil health parameters, no significant differences were found in this first year. Trends toward more aggregation in the form of large, small and microaggregates was found in cover cropped plots compared to the control. However, bulk density differences were minimal amongst treatments. In-situ infiltrometer readings indicated 1.8 times greater infiltration in cover crop plots compared to bare plots. During the season, cover cropping visibly reduced surface water pooling and improved trafficability. However, the measured infiltration rates remained very low regardless of treatment. Biological soil parameter measurements (MBC, MBN and enzyme activities) are still in process. Although some changes in soil parameters were observed in this first year, it is likely that cover cropping effects will only become visible in the 2nd-3rd year of this practice, in line with other cover crop literature.
Almond yields. We did not observe statistically significant yield differences amongst treatments in 2018. Compared to bare soil, yields in cover crop treatments were on average +216 lbs./acre and +94 lbs./acre greater at the Merced and Kern county sites, respectively. Our Tehama county site is 2nd leaf as compared to the 16th leaf stage at the other two sites. As such, total manually-harvested yields are much lower. We measured a difference of +7 lbs./acre in cover crop plots compared to the control (resident vegetation) at this site. At the time of harvest, no remaining cover crop residues were observed in the orchard middles (Figure 6). As such, cover cropping did not interfere with harvest operations at any of the sites.
Figure 4. Seeders used in this study (a) Schmeiser Compact No-Till Cover Crop Drill, (b) John Deere 750 No-Till grain drill and (c) Great Plains No-Till Seeder NTS2511
Figure 5. Left: Pollinator mix (1 family/5 species), Right: Soil building mix (3 families/5 species) in March 2018
Figure 6. Orchard middle in a soil building mix plot at the time of harvest. No observable cover crop residues were found at harvest. As such, cover cropping did not interfere with harvest operations
Educational & Outreach Activities
All of our field sites are located on commercial orchards. As such, all of our cover crop operations were decided in close consultation to local UCCE extension crop advisers and integrated the valuable feedback and expertise of the orchards’ growers. In particular, proper soil preparation was discussed: light disking was compared on-site to mulching at 1.5 inches. We found that mulching provided better uniformity in orchard middles than disking, and was therefore better suited for soil preparation. Seeding time, width and rate, and additional irrigation were discussed as well as the compatible machinery for each site. Cover crop stand management, mowing and termination were also discussed with the growers and extension officers prior to each operation.
Two growers’ conferences were attended at which the research was communicated by both a poster and a talk. Our posters are available at the following link: https://almondcovercrop.faculty.ucdavis.edu/project-summary/research-activities/. The powerpoint to the talks are available here: https://almondcovercrop.faculty.ucdavis.edu/project-summary/presentations/. The study will also be presented at the California Climate and Agriculture Network Summit (CalCAN) in March 2019.
A website was created to provide recent updates and to compile the information and outreach material related to this project (https://almondcovercrop.faculty.ucdavis.edu/). News updates are included in a blog section. This platform is also used to add further visibility to the growers’ survey.
The study was reported in the Western Farm Press (https://www.farmprogress.com/tree-nuts/researchers-studying-cover-crops-near-almond-orchards), California speciality crops soil health website (https://cash-crop.org/portfolio-item/developing-cover-crop-systems-for-almond-orchards/) and in the Sacramento Valley Newsletter (https://cetehama.ucanr.edu/newsletter/Fruit – Nut Newsletters76786.pdf). The study is featured in the CalCAN recommendations to the incoming California Governor (p. 35 – http://calclimateag.org/wp-content/uploads/2018/08/AbundantSolutions.pdf).
A Growers’ Field Day is planned for early March 2019 with the support of CDFA Healthy Soil Demonstration Projects. Announcements are being distributed.