Figure 1. Limited microbial activity with reduced seed germination and vigor (left). Increased microbial activity (increased abundance and diversity) with increased seed germination and vigor (right).
Literally and figuratively, it is thelittle things that count when establishing crops. Yes, it is the microbes that we are talking about! When it comes to soil management, growers often have a plan to support their soil’s chemical and physical properties, but there is also a massive opportunity to see improvements in crop establishment by supporting soil biology. Bacteria, fungi and archaea live very close to plant roots or within the crop itself and have a symbiotic relationship to crops. The more active, abundant and diverse the microbiome, the better for crops. Around 80% to 90% of soil processes are impacted by the soil microbiome, which includes everything from soil health to quality and structure. In turn, that also means your soil microbiology can positively impact crop health, growth and overall performance throughout the growing season and after.
Some of the main benefits microbes provide an establishing crop include supporting germination, nutrient cycling, optimizing soil health, improved root development, pathogen defense and supporting stress tolerance.
Supports Seed Germination The spermosphere is a 2- to 12-mm area around your seed, and it’s the crop’s first interaction with the real world. This is where the symbiotic relationship between crops and microbes begins. Seeds naturally secrete enzymes and metabolites to provide microbes with a carbon source. In exchange, the beneficial microbes mineralize nutrients, reduce abiotic stressors and protect the seeds from pathogens.
Since this is such a brief yet vital stage, ensuring beneficial microbes are present and active at the time of planting can significantly impact early crop health by optimizing the habitat for germination, leading to more uniform stand and early root and shoot vigor (Fig. 1). There are interesting experiments that show sterilized seeds didn’t germinate as well compared to non-sterilized seeds because seeds’ germination enhances with the help of beneficial microbes. Also, native microbes help more than foreign microbes, according to the same research article.
Once the radical has emerged from the seed, the spermosphere no longer exists and has transferred into the rhizosphere, the next beneficial microbes crops encounter around the root zone.
Improves Root Development Microbes support an establishing crop with root development in two ways: First, some soil microbes produce phytohormones that promote plant growth and processes, and second, by improving soil structure and reducing compaction, soil microbes support root development.
Bacteria and fungi both assist with improving soil structure. Bacteria secrete extracellular polymeric substances that act as a glue and bind soil aggregates together. Fungi’s hyphae root network works as a net, holding soil in place, reducing erosion and acting as a nutrient superhighway. Once connected to the nutrient superhighway, the hyphae roots can determine which nutrients your crops need, seek out and find said nutrients and bring them back to the plant. Without these microbial networks, crops may struggle to access nutrients beyond their immediate root zone, limiting their ability to thrive, especially in harsh conditions.
When crops can grow healthier roots, they can also access more water and nutrients on their own, stimulating the plant’s overall growth.
Improves Nutrient Cycling Nutrient cycling has two key components: access and absorption. Once roots find nutrients, they also must be able to absorb them. Soil microbes can help an establishing crop both find and access nutrients. Recently dead and living plants release carbon as rhizodeposits in the soil. Beneficial microbes can use this carbon as food. In exchange, they provide the crop with other essential nutrients like NPK through atmospheric fixation or mineralization of organic matter (Fig. 2). The increase of organic matter decomposition provides an increase in NPK availability and absorption.
There are many microbes that aid in nutrient cycling. Two well-studied are rhizobium and mycorrhizal fungi. Rhizobium, for example, are nitrogen-fixing bacteria mainly associated with forming symbiotic relationships with plant roots. The bacteria form nodule structures on the root. There, colonies of bacteria convert atmospheric nitrogen into ammonia that can be used by the plant in exchange for carbohydrates. Plant arbuscular mycorrhizal fungi improve phosphorus availability by foraging with their hyphae root system.
Figure 2. Simplified plant and microbial nutrient cycling model
Optimizes Soil Moisture As previously mentioned, soil microbes can improve soil structure by binding soil aggregates together. Those microaggregates create pockets for roots to access water and air. Water plays a key role in enabling plants to access nutrients. For some nutrients, such as nitrates and sulfates, they depend on mass flow, or the movement of water, to carry nutrients to the plant. By optimizing water levels, nutrients become more mobile so they can move through well-structured soil to where they’re needed.
Drought-stressed plants uptake less nitrogen and restrict phosphorus uptake, stunting crop growth. Wet or poorly drained soil is susceptible to nutrient loss, which also negatively impacts crops and yields. A healthy microbiome leads to healthy soils that can both drain excess water and store water when needed.
Improves Stress Tolerance Young crops are especially susceptible to drought, cold, flooding and less-than-ideal salinity levels. To help crops manage stress, beneficial microbes secrete antioxidants, enzymes and osmoprotectants to reduce abiotic stress.
Beneficial microbes optimize water productivity. By improving water holding capacity, even during extremely dry conditions, crops have a better chance of establishing. Bacillus subtilis, for example, assists with stress tolerance by producing phytohormones, siderophores and enzymes to stimulate a crop’s natural defense mechanism against environmental stressors.
Impacts Positively to All Four Spheres A healthy and active soil microbiome doesn’t only support your crops at the beginning of the growing season. Yes, you want to create as hospitable of an environment as possible for seeds and newly germinating crops. By supporting microbes in the spermosphere and rhizosphere, you can improve growing conditions for your seeds and newly established crops. As crops continue to grow, some of the microbes in the soil will transfer to the phyllosphere. The phyllosphere is made up of the above-ground tissues of the plant where microbes live. An active phyllosphere supports your crops with managing abiotic and biotic stressors throughout the growing season. Come fall, the beneficial microbes in your soil help break down crop residue in the detritusphere, improving conditions and increasing available nutrients for the seeds you’ll plant in the spring.
Healthy microbes in any sphere positively impact the other three spheres (Fig. 3), which in turn positively impact your crops and soil each planting season.
Figure 3. All four spheres interact with each other and influence each other (courtesy PhycoTerra.)
Beneficial Microbes Are a Good Offense and Defense for an Establishing Crop Offensively, beneficial microbes help crops improve root development, nutrient cycling and optimize soil moisture. Defensively, beneficial microbes help crops manage biotic and abiotic stressors. The support of an active microbiome generally results in better growth and healthier plants overall, and higher yields, especially under less-than-ideal conditions.
Crops need an abundant, diverse and active microbiome to support them both offensively and defensively. Soil characteristics, organic matter levels, soil pH, soil depth, soil type, moisture content, soil structure, temperature, weather and agricultural practices can all impact the diversity and abundance of microbes in agronomic soil. Unfortunately, 75% of soil microbes on farms are dormant or too weak to support crops due to starvation.
The first step to leveraging beneficial microbes on your farm is to conduct a soil test to determine the biological health of your soil. Then, consider implementing farming practices that aid beneficial soil microbes so they can support your establishing crops and overall yields. These practices include crop rotation, no-till or minimal till, and providing the farm with optimal moisture, air, pH, food and nutrients for microbes. The great news here ismany practices that benefit soil physical and chemical health can also promote soil biological health, and vice versa. For example, a good soil structure can support better soil air and moisture levels, and thus soil microbial growth. In turn, this can further help crop establishment and growth. When it comes to establishing a crop, it really is the small things that count, the ones we can’t see but make a big impact on overall soil health and crop health. Hope you have a great year ahead with your crops and an abundant microbiome!
Resources
Why are your soil microbes dormant?: https://phycoterra.com/blog/why-are-your-soil-microbes-dormant/
Microbiome as a Key Player in Sustainable Agriculture and Human Health: https://www.frontiersin.org/journals/soil-science/articles/10.3389/fsoil.2022.821589/full
Management of Soil Microbes on Organic Farms: https://eorganic.org/node/34646#
Benefits to Plant Health and Productivity from Enhancing Plant Microbial Symbionts: https://pmc.ncbi.nlm.nih.gov/articles/PMC8072474/#
Study improves understanding of how bacteria benefit plant growth:
https://news.ucr.edu/articles/2023/07/24/study-improves-understanding-how-
bacteria-benefit-plant-growth#
Microbial controls on seed germination: https://www.sciencedirect.com/science/article/pii/S0038071724002657
Microbial co-operation in the rhizosphere: https://academic.oup.com/jxb/article/56/417/1761/484466
Early results suggest integrating natural habitat into pest management strategies could be a cost-effective way to enhance biological control (photo courtesy H. Cohen.)
Invasive pests can wreak havoc on orchards, and keeping them in check often means frequent pesticide applications. With increasing pesticide regulations and the rising costs of chemical applications, growers of all crop types need reliable alternatives for controlling key pests. Our research explores one such strategy: leveraging natural habitat to enhance pest control. By studying how natural vegetation, including both large protected areas and smaller on-farm hedgerows, impacts pest control, we’re asking how strategically incorporating native plants into an operation may reduce pest pressure and potentially cut down on pesticide use.
Small-scale plantings like hedgerows and floral strips have frequently been found to improve biological pest control and attract pollinators but there are challenges to implementing these approaches. First, some studies have shown small-scale plantings only deliver better pest control when they are within 100 to 1,000 meters of larger patches of natural vegetation (Chaplin-Kramer et al. 2011; Heath and Long 2019) and may thus be less effective in simple habitat-poor landscapes. Other studies also show substantial variation in the effects of natural vegetation on pests, natural enemies and crop yields (Karp et al. 2018). Sometimes adding natural vegetation works great but other times it seems to have no impact. This variation makes it difficult to develop broadly applicable management guidelines.
These inconsistent effects of noncrop vegetation on pest levels and crop yields might be attributed to the following explanations: 1. Researchers failing to monitor the entire biological control community and thus overlooking key interactions. 2. A poor understanding of how on-farm practices interact with landscape-scale processes. 3. Measuring pest levels and biological control in ways that are not relevant to the economic decisions made by growers (Karp et al. 2018, Chaplin-Kramer et al. 2019). For growers to determine whether natural vegetation is likely to deliver useful biological control and reduce expenditures on pesticides, we need studies that comprehensively survey the entire community of animals providing biological control, integrate experiments at the farm and landscape scale and measure biological control in a way that fits within the framework growers already use to make pest management decisions.
Figure 1. Bird species richness (a) and abundance (b) are significantly higher in natural habitats compared to orchards and significantly higher on orchard margins compared to interiors (Source: Cormier et al. in prep.)
Ventura County Study Targets Orchard Pest Control We have recently begun a study in lemon and avocado orchards in Ventura County, where we are assessing whether natural vegetation can suppress arthropod pest outbreaks below economic thresholds. We are compiling data on pest levels in orchards using both our own standardized surveys and data collected by PCAs. We are also comprehensively surveying the wildlife community that may contribute to pest control, including bats, wild mammals, birds and insect natural enemies. Critically, our study design allows us to assess how farm-level practices interact with landscape-scale variables. Half our study orchards are close to a large riparian corridor with extensive natural vegetation and half are more than 1 kilometer away. Additionally, half our orchard survey sites have bare margins and half have vegetated margins. This study design enables us to assess how both small on-farm plantings and large patches of natural vegetation interact to influence pest levels and wildlife abundance and diversity. We are also evaluating which specific plant species and plant traits in hedgerow plantings attract helpful insect predators, insectivorous birds, parasitoids and pollinators.
This study is in the early stages, but several key preliminary results have emerged. First, birds can be pest control allies. In orchards close to large patches of natural habitat, we found significantly more insect-eating birds like yellow-rumped warblers and bushtits. These species are known to consume pest insects like aphids, scale and ants. We found no evidence of bird damage to crop trees or fruit, but these effects will vary depending on crop type because avocado and lemon are not species typically targeted by birds. The diversity and abundance of beneficial bird species was significantly higher on orchard margins than interiors (Fig. 1). Additionally, bird diversity and abundance were significantly higher in orchards that had small patches of natural vegetation on their margins even if they were far from the large riparian zone. If you or your grower have natural vegetation nearby, you may already have a hidden pest control workforce, and growers might be able to attract more of these workers by planting the right vegetation on margins. We are currently analyzing exactly what vegetation characteristics attract beneficial insectivorous birds with the aim of providing precise guidance on how to put birds to work on farms.
We also find that hedgerows and floral strips attract the right bugs. Large patches of native vegetation and noncrop habitat around orchards bring in more beneficial insects, including wild bee pollinators and predatory parasitoid wasps. The closer orchards were to the large riparian zone, the stronger this effect. Our orchards close to the river had both fewer pests and more parasitoids (Fig. 2). We are curious if growers who have hedgerows experience increases in pest populations and pesticide costs. This might tell us if intentional hedgerow plantings harbor pests and will be a key future analysis.
A major concern for growers is whether adding native plants could inadvertently attract more pests. The good news? So far, we haven’t seen an increase in pest populations in orchards with hedgerows or floral strips. In fact, we see evidence for greater abundance of pests in orchards that are farther from the riparian zone (Fig. 2). We also anecdotally heard some growers needed fewer pesticide applications in blocks closer to natural vegetation.
Figure 2. Farms closer to riparian habitat have fewer insect pests than sites further away. Closer sites also have more insect natural enemies.
Actionable Strategies for Growers and Consultants Our study suggests some practical tips for incorporating native vegetation into farms. First, start small. Even a 10-by-10-meter hedgerow can make a difference over time when it comes to diversifying habitat for beneficial species. Choose native species that attract beneficial insects like buckwheat and salvias. Adding species with woody structure like laurel sumac, coyote bush and willows can attract birds that need cover and places to perch and nest. Second, think year-round by selecting plants that provide bloom year-round so that you can provide pollen and nectar for insects outside of your crop’s bloom period. Check out this UC site for more information on planting a hedgerow: ucanr.edu/sites/default/files/2018-12/295420.pdf. Finally, monitor and adapt. Keep an eye on pest levels and beneficial insect activity. Work with your PCA to track changes and adjust as needed.
Our study is ongoing, but early results suggest integrating natural habitat into pest management strategies could be a cost-effective way to enhance biological control. Healthier landscapes and support for beneficial insects go hand in hand.
Interested in learning more about our study? Reach out to us! We are a group of researchers from Cal Poly, Pomona, UCCE, CSU Long Beach and UC Santa Barbara with backgrounds in entomology, ecology, animal behavior and restoration. Any questions can be directed to Liz Scordato, Hamutahl Cohen and Erin Questad atescordato@cpp.edu, hcohen@ucanr.edu and ejquestad@cpp.edu, respectively.
References Chaplin‐Kramer, R., O’Rourke, M. E., Blitzer, E. J., & Kremen, C. (2011). A meta‐analysis of crop pest and natural enemy response to landscape complexity. Ecology Letters, 14(9), 922-932.
Heath, S. K., & Long, R. F. (2019). Multiscale habitat mediates pest reduction by birds in an intensive agricultural region. Ecosphere, 10(10), e02884.
Karp, D. S., Chaplin-Kramer, R., Meehan, T. D., Martin, E. A., DeClerck, F., Grab, H., … & Wickens, J. B. (2018). Crop pests and predators exhibit inconsistent responses to surrounding landscape composition. Proceedings of the National Academy of Sciences, 115(33), E7863-E7870.
Interactions in the rhizosphere. Plants influence their rhizosphere microbiome through exudation of compounds that stimulate (green arrows) or inhibit (red blocked arrows). Most microbes affect neither the plant nor the pathogen because they occupy different ecological niches (commensal microbes) but may affect every other organism to somewhat through a complex network of interactions.
“Soil health” and “healthy soils” have become popular topics in recent years as evidenced by the increased number of government programs and commercial products aimed at improving soil health. The desirable properties of healthy soils are efficiency and efficacy of nutrient cycling, capacity to hold and release plant-available water, an environment conducive to root growth, supportive of beneficial soil organisms and improved resilience of the vine to stress from pests, diseases, drought and/or heat.
Characteristics of a healthy soil are those that promote healthy plant growth:
• A living matrix of plant residues, plant roots, animal residue and microorganisms.
• Porous, with a range of pore sizes that allow a balance between water and air in the soil and space for a complex network of microorganisms (bacteria, fungi, etc.), microarthropods and roots to establish. • Chemically balanced to allow for nutrient cycling and conducive to the environmental needs of different types of soil organisms in the soil food web and vine roots. • High in organic matter, which adds nutrients and microbes to soil; those microbes support essential ecological functions of soil, including recycling of nutrients.
Different vineyards and different soil types support different soil ecosystems. What would be considered healthy for sandy soils may not be the same as what is considered healthy for clay soils. Assessing whether the functioning of the soil ecosystem is optimal for any given crop/soil combination is difficult as comparisons between combinations are not necessarily valid.
Roles of Microorganisms in Soil Health Healthy functioning of soil is promoted by complex networks of microorganisms and their grazers, such as beneficial microarthropods. The microbiome of a soil is composed of a host of organisms, including but not limited to bacteria, fungi, protists, nematodes, earthworms and microarthropods. Within these groups, some species can be beneficial, others pathogenic. This can be true even within a genus. For example, the bacteria Pseudomonas fluorescens is beneficial, while Pseudomonassyringae is a pathogen.
Soil microbes play an important role in nutrient cycling in the soil. Decomposers break down organic matter, making it available as an energy and nutrient source for other organisms. Macronutrients such as potassium and phosphorus, which are often immobile in soil, are made available to the vine by some soil microbes.
Soil microorganisms improve soil structure. Bacteria play an important role in aggregate structure and stability. They produce sugars that hold the mineral parts of the soil together. Fungal hyphae weave soils together as do plant roots. Collectively, soil minerals, roots, bacteria and fungi comprise soil aggregates.
Some microbes are biological control agents that antagonize or compete with deleterious microorganisms. For example, predatory nematodes are beneficial. As fungi and bacteria, respectively, Trichoderma spp. and Bacillus subtilis are other examples of well-known biocontrol agents.
Plant growth-promoting bacteria produce chemicals that stimulate vine growth, and amoeba protists stimulate lateral root formation by producing a plant hormone mimic. A vine might react to these compounds like a plant hormone. Other types of bacteria convert nutrients into forms more available to the vine.
Arbuscular mycorrhizal fungi (AMF) live in the soil and on vine roots in a symbiotic relationship with the plant. The plant delivers photosynthates to the fungi for energy, and the fungi provide additional water and nutrients such as phosphorus and nitrogen to the plant. AMF have structures called hyphae that extend great distances through the soil. Hyphae are essentially long tubes that can transport water to vines from areas beyond the root zone. This helps the vine cope with drought. Hyphae also play a role in soil structure.
Soil rich in organic matter supports a diverse microbial ecosystem that helps improve structure, nutrient cycling and plant resilience. Healthy vineyard soils often contain visible root systems and fungal hyphae interwoven through soil aggregates (photo courtesy Katie Bruce, Niner Wine Estates.)
Soil Microbial Consortia Soil microbial species do not function in isolation. The survival and success of any one type of soil microorganism is dependent on the presence and activity of many other collaborating microbes. One type of organism provides the resources another type of organism needs or changes the environment such as to favor a different type of organism. Collaborations of multiple species of bacteria and fungi are referred to as a soil microbial consortium. Applying compost to the field can be a method for delivering or manipulating these synergistic soil microbial consortia.
Like other food webs found in nature, soil food webs are composed of multiple trophic levels or positions in the food chain. Communities of organisms perform important ecological functions, such as contributing to plant productivity, decomposing dead and decaying matter, and returning energy and nutrients for use by plants. Numbers decrease as you move from bottom to top, but the biomass per individual increases from bottom to top. Soil food chains may be more complex than aboveground food chains, as they tend to exhibit a greater incidence of omnivory that are capable of foraging on multiple trophic groups.
The three basic pathways that energy is moved between and within trophic levels are roots, bacteria and fungi. Pathogenic fungi, bacteria and nematodes and their consumers comprise the root pathway. The bacterial pathway is made up of bacteria that feed on dead plant material (saprophytic), those that cause diseases in plants (pathogenic), plus the organisms that feed on them, such as protists and bacterial-feeding nematodes.
Fungi found in the fungal pathway include species that are saprophytic, pathogenic and/or mycorrhizal. This pathway also includes consumers of these fungi. Some mesofauna organisms occupy other trophic levels as secondary, tertiary and quaternary predators. Such organisms include protists, nematodes, mites, fly larvae, centipedes, spiders and beetles. The conversion and movement of energy and nutrients around the soil ecosystem is what allows the functions of decomposition, mineralization and soil aggregate formation to occur.
Soils with collaborative suites of microbial species are likely to be more resilient than single species, which are more vulnerable to disease or stress. Species within these communities turn “on” and “off” according to different environmental signals, such that when one classification of soil organisms declines, another one can fill that same role or function. An analogy is an orchestra that features different instruments at different times in a performance. Unfortunately, naming the species composing different soil consortia and their ecological functions in soil health is still in its infancy.
Monitoring Soil Microbiome and Soil Health Most methods for identifying and quantifying soil microbes are indirect. The methods include measures based on soil aggregation, biomass (estimated by a phospholipid fatty acid profile or counting cells under the microscope), biological activity such as production of extracellular enzymes, and identification by matching DNA fingerprints found in a soil sample to the known genomes of species of bacteria, fungi, protists or nematodes.
Aggregate stability can be a good measure of soil health because it reflects both physical structure and biology. The bulk density of soil is not a direct measure of soil aggregates but is related. A qualitative way of judging aggregate stability is to take a small sample of soil and drip water on it. If the soil crumbles and falls apart, that is an indication of poor aggregation. If the sample absorbs the water, that is a sign the soil has good structure and ability to hold water. Even if all the species of microorganisms in a soil are unknown, measuring aggregates comprised of bacteria and fungi is useful for monitoring changes through time.
Knowing the functional activity of fungi and bacteria provides a general description of the soil ecosystem and soil health. Functional activity can be measured as enzymes metabolizing specific substrates in soils containing cellulose, amino acids or phosphorus, for example.
Monitoring these and other variables can inform decisions about ground cover, cultivation and fertilization toward the goals of reducing compaction, improving soil aggregate stability, increased water infiltration and disease suppression. The limitation of this type of description is that it does not identify or differentiate what genera or species of these organisms are present. The diversity and complexity of the soil microbiome is crucial to the healthy function of the soil.
Techniques like aggregate stability tests and microbial enzyme analysis help monitor soil health and guide management practices (photo courtesy Katie Bruce, Niner Wine Estates.)
Biological Indicators Soil ecology is the study of the complex interactions between the environment and myriad soil biota. No single measure can capture all the variables that contribute to soil health, but choosing measurements that complement each other can help. Interpreting simple measurements of broad groups like fungi or bacteria is difficult because it does not distinguish pathogens from beneficials.
The biomass of bacteria and fungi can be estimated. Phospholipid fatty acid profiles or cell counts are two methods for estimating microbial biomass. Use of viability stains can distinguish active from dormant organisms. Measuring the ratios between fungi and bacteria can be useful as well because it reflects disturbance. A well-functioning vineyard soil will have a higher ratio of fungi to bacteria, which is promoted by reducing or eliminating tillage to keep vegetation with living roots in the system and avoiding the disruption of the physical characteristics of the microbial habitat.
Measuring respiration in the soil provides a picture of how much life there is in the soil, but it is hard to interpret because it combines respiration of roots, microorganisms and their consumers. Although these measures provide rough estimates of biomass, they do not reflect “who” is there.
Soil organic matter is composed of both living and decaying material. The active or living portion of total soil organic matter can be quantified using a technique based on changes in the color of a potassium permanganate solution mixed with soil. Measurements using this method correlate positively with soil biological activity and are sensitive to management practices.
Current research is being performed to identify sentinel species of microorganisms. If there are genetic markers for these organisms, then identifying specific soil microorganisms is possible. For example, DNA can be extracted from soil. Strands of DNA are replicated using polymerase chain reaction techniques. Those strands are compared to the known genomes of different organisms. The longer the strand of DNA replicated determines how specific identification can be. As the genomes of more soil microbes are mapped, identifying the composition of the microbial community in the soil will become more accurate and useful. This research is still in its infancy.
Encouraging and Conserving Soil Microbial Ecosystem Diversity of plants in the vineyard increases the diversity of the soil microbial community. This can be achieved with cover crops and grazing. Planting a blend of multiple species of grasses and legumes accomplishes this. Soil covered with vegetation is typically healthier than bare ground.
Applying compost is an excellent way of introducing more carbon into the soil. Compost can potentially inoculate soil with beneficial microbes, provide nitrogen in organic forms and increase soil organic matter overall. The carbon and nitrogen provided by compost feeds both vines and soil microorganisms.
Reducing tillage as much as possible is advisable. Excessive tillage disrupts the soil food web. The mechanical action of tilling severs earthworms and breaks up soil aggregates, which are habitat for beneficial soil bacteria. Hyphae of AMF are torn. Soil organic matter is lost to the atmosphere from tillage, reducing the food source and habitat of many soil microbes. Microorganisms are redistributed in space, separating them from their habitats and food sources such as predators from prey, decomposers from material that needs decomposing, and beneficial relationships between microbes and roots. Organisms surviving a tillage event will need to repopulate and recreate communities within the soil.
Conserving and encouraging the microbial community of the soil is crucial to improving and maintaining soil health. Differences between soil types and the necessities of vineyard management make comparisons difficult. Developing a soil health management program for any vineyard takes time, dedication and the willingness to experiment. Appreciating the role of the soil microbial ecosystems will contribute to the success of a grower’s efforts in improving and maintaining a healthy soil.
Plant-associated arbuscular mycorrhizal fungi (AMF) participate in soil carbon storage, improve soil aggregation and promote plant health and crop yield. Like other perennial crops, citrus trees create associations with AMF (Wu and Srivastava 2017; Xi et al. 2022) which have been shown to improve crop nutrition (Wu and Zou 2009), enhance tolerance against abiotic stressors like drought (Wu et al. 2019) and induce better root development (Wu et al. 2012). Due to multiple benefits of AMF to soil and plant health, AMF has gained much attention, leading to a rapidly expanding market in mycorrhizal biostimulants focused on improving crop yield and root development of horticultural crops (Igiehon and Babalola 2017; Chen et al. 2018).
In agricultural systems, abundance of AMF can be negatively impacted by intensive cultivation, leading to a decrease of AMF spores and infective mycelium; thus, native AMF are often promoted by cover cropping and reducing soil disturbance (Bowles et al. 2016b). AMF inoculation can be successful in soils with limited native AMF, poor soil health and low productivity (Verbruggen et al. 2012; Rog et al. 2025). Our study aimed to investigate the effects of inoculated and uninoculated triticale cover crops on soil health and carbon storage in the alley and tree rows of a commercial lemon orchard in the Californian Central Coast region.
Experimental Design and Soil Analysis
The study was conducted at a commercial citrus orchard located in San Luis Obispo County between fall 2019 and spring 2023. The testing site was a 6-acre block planted with Citrus limon (L.) Burm. f.‘Lisbon Lemon’. The experimental design was a randomized complete block design with three blocks and three treatments in each block. Treatments included a control (bare fallow with no herbicide application), a cereal cover crop (Triticale (Secale x Triticum L.) drill-seeded at 110 pounds per acre and a cereal cover crop inoculated with AMF (110 pounds per acre inoculated with commercial AMF inoculum Rhizophagus intraradices, 300 propagules per gram at 10 pounds per acre). Cover crops were seeded in alley rows every growing season from 2020 to 2022. The timing of cover crop seeding and AMF inoculation was selected in accordance with winter rain events. In winter 2022-23, early rain promoted germination of cover crop seed from previous years and no AMF inoculation was applied. Cover crops were completely rain-fed, with no supplemental irrigation during dry months, and were mowed in June each year.
In the first three years, soil samples were collected between trees from the tree rows and from the center of the alley row at 0 to 6 inches and 6 to 18 inches depth. In year four, soil sampling was adjusted to better understand the link between the position on the orchard floor, microbial community structure and soil carbon dynamics as affected by the cover crops and AMF inoculation. Composite soil samples were taken from each plot at four functional locations defined as follows: between two trees on a berm (Location 1), the transition section where the berm ends but no cover crop is grown (Location 2), the cover crop edge (weeds in control plots) (Location 3) and the center of the alley row (Location 4; Fig. 1). Fresh soil subsamples were sent to Ward Laboratories for soil microbial community structure analysis using phospholipid fatty acid (PLFA) and neutral lipid fatty acid (NLFA). Soil samples were sieved, air dried and analyzed for total soil C (%), permanganate oxidizable carbon (POXC) and mineralizable carbon (Min C). Min C, also referred to as soil respiration, is an indicator of microbial activity, while POXC, also referred to as active carbon, can be considered a microbial food source. Both Min C and POXC are commonly used as indicators of soil health (Moebius-Clune et al. 2016; Norris et al. 2020).
Figure 1. Modified sampling scheme across the orchard floor. Location 1 was between two trees on a berm, Location 2 was in the transition section where the berm ends but no cover crop is grown, Location 3 was in the cover crop edge (weeds in control plots) and Location 4 was in the center of the alley row.
The Overlooked Role of Weeds and Native AMF. After two years of treatment implementation, there was no effect of cover crops or inoculation with AMF on the total soil C, POXC and Min C in the tree or alley row (data is not shown). Likewise, there was no treatment effect on total soil C, POXC and Min C at any of the four locations across the orchard floor after three years of treatment implementation (Table 1). Our control treatment had weeds (predominantly Malva and some filarees during soil sampling) which added C sources to soil, likely causing the lack of a significant difference between cover crop treatments and control.
Our PLFA and NLFA microbial biomass data showed no significant persistent inoculation impact on the microbial community compared to the non-inoculated cover crop and the weedy control treatment plots after three years of treatment implementation. We did not observe any differences in microbial biomass between the cereal cover crop and the weedy control treatment, indicating weeds supported the microbial community similarly as the cereal cover crop (Fig. 2).
Figure 2. Mean abundance of AMF biomass NLFA (ng/g soil) and total bacteria biomass PLFA (ng/g soil) for the control, a cereal cover crop (CC) and a cereal cover crop inoculated with mycorrhizae (CC M) treatments in four functional locations: 1) on top of the berm next to the tree, 2) in the fallow area next to the berm, 3) on the fringe of the cover crop area and 4) in the middle of the cover crop area) in the 0- to 6-inch-depth increment. Error bars represent standard error (n=3). Different uppercase letters indicate significant differences between locations.
The lack of effect of the inoculated cover crop compared to the non-inoculated cover crop and control treatment on soil C metrics and microbial community structure in our study may be attributed to native AMF species forming associations with plants growing in the alley row and leading to similar results as those observed in the inoculated plots (Wilson et al. 2009; Bowles et al. 2016a,b; Agnihotri et al. 2021; Lin et al. 2023). These findings suggest native AMF can be successfully promoted in the alley rows by weeds or a cover crop and may provide a more effective strategy than inoculation in perennial citrus orchards. Therefore, AMF inoculations may not add value in soil with existing plant cover and low soil disturbance. Cover Crop Effects Extend Beyond Area Directly Underneath Plant Cover In contrast to treatment comparisons, soil C indicators showed greater values in the alley rows compared to the tree row (Table 1). In the topsoil, the cereal cover crop, weedy control and repeated additions of tree prunings in the alley row supported more microbial biomass, including AMF (Fig. 2), and had higher C storage potential compared to the tree row that had less plant matter input in the top soil and potentially experienced C loss due to pulses of C mineralization during dry-wet cycles associated with frequent irrigation (Lundquist et al. 1999; Denef et al. 2001; Lopez-Sangil et al. 2018). The location effects in this orchard trial suggest different managent of trees and alley rows leads to soil heterogeneity across the orchard floor.
Table 1. Average total soil carbon (%) concentrations, average permanganate oxidizable carbon (POXC, mg C kg soil-1), average mineralizable carbon concentrations (Min C, mg C kg soil-1 day-1) and the respective standard errors of the mean (n = 3) for the control, cereal cover crop (CC) and cereal cover crop inoculated with mycorrhizae (CC-M) treatments in four functional locations: 1) on top of the berm next to the tree, 2) in the fallow area next to the berm, 3) on the fringe of the cover crop area and 4) in the middle of the cover crop area) in the 0- to 6-inch-depth increment. Different uppercase letters indicate significant differences between locations within the same carbon measurement.
The management in the alley rows more positively impacted soil C cycling indicators and soil health with some carry-over from the cover cropped alley rows into adjacent areas. More specifically, total C and POXC concentrations declined from the center of the alley rows to the area next to the citrus trees (Table 1). In addition, total C and POXC were greater in location 2, which represents the plant-free area adjacent to the cover crop and impacted by wheel traffic, compared to location 1 immediately next to the citrus tree. An orchard floor gradient assessment from the center of the cover cropped row toward the tree row with bare soil showed a potential carryover effect from the cover crop row into the fallow area between the trees and the edge of the cover crop. Citrus trees may potentially receive more nutrients outside of the berm by extending lateral roots into alley rows and by making associations with AMF that have extensive hyphal networks. Therefore, lemon trees may benefit from soil improvements from cover crops grown in the alley row.
Mycorrhizae Affect Carbon Cycling through Recruitment of Bacteria We used correlation analysis to evaluate the relationships among microbial groups and C cycling indicators (Table 2). In our dataset, AMF and saprophytic fungal biomass were positively correlated with total bacterial biomass (Table 2), which supports previous findings on the cooccurrence of AMF or saprotrophic fungi with bacteria in soil niches (Yuan et al. 2021; Zhang et al. 2022). We also observed no correlation between NLFA biomass of AMF and saprotrophic fungi. These findings suggest both AMF and saprotrophic fungi are able to recruit bacteria but the two groups of fungi occupy different soil niches. Correlation assessment of linking AMF NLFA biomass with soil C indicators showed no direct significant impact of AMF on soil C storage. In contrast to AMF, bacterial biomass was significantly correlated with all soil C indicators supporting studies where bacterial community was a major contributor to soil organic C accumulation (Zhang et al. 2020; Guo et al. 2021; Hu et al. 2023). As such, our findings suggest AMF may have an indirect influence on soil C dynamics by promoting bacterial biomass. Therefore, management practices that promote AMF, such as cover crops and reduced soil disturbance, help build soil organic matter and store carbon.
Table 2. Pearson’s correlations between AMF biomass NLFA, saprophytic fungi NLFA, total bacteria biomass PLFA, Min C (mg C kg soil-1 day-1), POXC (mg C kg soil-1) and total soil carbon (%).
References
Agnihotri, R., A. Bharti, A. Ramesh, A. Prakash, and M.P. Sharma. 2021. Glomalin related protein and C16:1ω5 PLFA associated with AM fungi as potential signatures for assessing the soil C sequestration under contrasting soil management practices. European Journal of Soil Biology 103: 103286. doi: 10.1016/j.ejsobi.2021.103286.
Bowles, T.M., F.H. Barrios-Masias, E.A. Carlisle, T.R. Cavagnaro, and L.E. Jackson. 2016a. Effects of arbuscular mycorrhizae on tomato yield, nutrient uptake, water relations, and soil carbon dynamics under deficit irrigation in field conditions. Science of The Total Environment 566–567: 1223–1234. doi: 10.1016/j.scitotenv.2016.05.178.
Bowles, T.M., L.E. Jackson, M. Loeher, and T.R. Cavagnaro. 2016b. Ecological intensification and arbuscular mycorrhizas: a meta-analysis of tillage and cover crop effects. Journal of Applied Ecology 54(6): 1785–1793. doi: 10.1111/1365-2664.12815.
Chen, M., M. Arato, L. Borghi, E. Nouri, and D. Reinhardt. 2018. Beneficial services of arbuscular mycorrhizal fungi – From ecology to application. Frontiers in Plant Science 9. https://doi.org/10.3389/fpls.2018.01270.
Denef, K., J. Six, H. Bossuyt, S.D. Frey, E.T. Elliott, et al. 2001. Influence of dry–wet cycles on the interrelationship between aggregate, particulate organic matter, and microbial community dynamics. Soil Biology and Biochemistry 33(12): 1599–1611. doi: 10.1016/S0038-0717(01)00076-1.
Guo, Z., X. Zhang, J.A.J. Dungait, S.M. Green, X. Wen, et al. 2021. Contribution of soil microbial necromass to SOC stocks during vegetation recovery in a subtropical karst ecosystem. Science of The Total Environment 761: 143945. doi: 10.1016/j.scitotenv.2020.143945.
Hu, Q., T. Jiang, B.W. Thomas, J. Chen, J. Xie, et al. 2023. Legume cover crops enhance soil organic carbon via microbial necromass in orchard alleyways. Soil and Tillage Research 234: 105858. doi: 10.1016/j.still.2023.105858.
Igiehon, N.O., and O.O. Babalola. 2017. Biofertilizers and sustainable agriculture: exploring arbuscular mycorrhizal fungi. Appl Microbiol Biotechnol 101(12): 4871–4881. doi: 10.1007/s00253-017-8344-z.
Lin, J.S., M.V.M. Sarto, T.L. Carter, D.E. Peterson, C. Gura, et al. 2023. Soil organic carbon, aggregation and fungi community after 44 years of no-till and cropping systems in the Central Great Plains, USA. Arch Microbiol 205(3): 84. doi: 10.1007/s00203-023-03421-2.
Lopez-Sangil, L., I.P. Hartley, P. Rovira, P. Casals, and E.J. Sayer. 2018. Drying and rewetting conditions differentially affect the mineralization of fresh plant litter and extant soil organic matter. Soil Biology and Biochemistry 124: 81–89. doi: 10.1016/j.soilbio.2018.06.001.
Lundquist, E.J., L.E. Jackson, and K.M. Scow. 1999. Wet–dry cycles affect dissolved organic carbon in two California agricultural soils. Soil Biology and Biochemistry 31(7): 1031–1038. doi: 10.1016/S0038-0717(99)00017-6.
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Verbruggen, E., M.G.A. van der Heijden, M.C. Rillig, and E.T. Kiers. 2012. Mycorrhizal fungal establishment in agricultural soils: factors determining inoculation success. New Phytologist 197(4): 1104–1109. doi: 10.1111/j.1469-8137.2012.04348.x.
Wilson, G.W.T., C.W. Rice, M.C. Rillig, A. Springer, and D.C. Hartnett. 2009. Soil aggregation and carbon sequestration are tightly correlated with the abundance of arbuscular mycorrhizal fungi: results from long-term field experiments. Ecology Letters 12(5): 452–461. doi: 10.1111/j.1461-0248.2009.01303.x. Wu, Q.-S., J.-D. He, A.K. Srivastava, Y.-N. Zou, and K. Kuča. 2019. Mycorrhizas enhance drought tolerance of citrus by altering root fatty acid compositions and their saturation levels. Tree Physiology 39(7): 1149–1158. doi: 10.1093/treephys/tpz039.
Wu, Q.-S., X.-H. He, Y.-N. Zou, C.-Y. Liu, J. Xiao, et al. 2012. Arbuscular mycorrhizas alter root system architecture of Citrus tangerine through regulating metabolism of endogenous polyamines. Plant Growth Regul 68(1): 27–35. doi: 10.1007/s10725-012-9690-6.
Wu, Q.-S., and A. Srivastava. 2017. AMF diversity in citrus rhizosphere. Indian Journal of Agricultural Sciences 87: 653–659.
Wu, Q.-S., and Y.-N. Zou. 2009. Mycorrhizal influence on nutrient uptake of citrus exposed to drought stress. Philippine Agricultural Scientist 92: 33–38.
Xi, M., E. Deyett, N. Ginnan, V.E.T.M. Ashworth, T. Dang, et al. 2022. Arbuscular mycorrhizal fungal composition across US citrus orchards, management strategies, and disease severity spectrum. doi: 10.1101/2022.03.01.482593.
Yuan, M.M., A. Kakouridis, E. Starr, N. Nguyen, S. Shi, et al. 2021. Fungal-bacterial cooccurrence patterns differ between arbuscular mycorrhizal fungi and nonmycorrhizal fungi across soil niches. mBio 12(2): 10.1128/mbio.03509-20. doi: 10.1128/mbio.03509-20.
Zhang, X., G. Dai, T. Ma, N. Liu, H. Hu, et al. 2020. Links between microbial biomass and necromass components in the top- and subsoils of temperate grasslands along an aridity gradient. Geoderma 379: 114623. doi: 10.1016/j.geoderma.2020.114623.
Zhang, L., J. Zhou, T.S. George, E. Limpens, and G. Feng. 2022. Arbuscular mycorrhizal fungi conducting the hyphosphere bacterial orchestra. Trends in Plant Science 27(4): 402–411. doi: 10.1016/j.tplants.2021.10.008.
Barnyardgrass (Echinochloa crus-galli), one of the most competitive and destructive weeds in California rice production, has shown widespread insensitivity to multiple herbicides, posing significant challenges for growers managing resistance across the Sacramento Valley (photo by Luis Espino, UCCE Rice Advisor.)
California rice growers are wellacquainted with reduced herbicide efficacy, whether experienced on their own or witnessed on a neighbor’s acres. Weed populations tolerant or resistant to herbicides have been spreading throughout the California rice region since at least the early 1990s. The most competitive and destructive weeds in California rice production are undoubtedly the Echinochloa complex species, notably barnyardgrass (E. crus-galli), early watergrass (E. oryzoides) and late watergrass (E. oryzicola). Populations of each of these species found to be insensitive to available herbicides for grass management are found throughout the Sacramento Valley, where the majority of California rice is grown.
Fortunately, newer herbicide active ingredients are starting to hit the market, but due to cost and supply constraints, these new weed management tools will take some time to become widely adopted. In the meantime, it is important for all stakeholders to have up-to-date information about the current state of herbicide resistance in one of the most economically important crops in the state.
Greenhouse Herbicide Screenings UCCE researchers conducted a pair of greenhouse screenings of 63 samples of suspected resistant Echinochloa species over fall and winter 2021 at the Rice Experiment Station in Biggs, Calif. Seed samples of local weed populations suspected to be resistant to at least one herbicide mode of action (MOA) had been collected from grower fields in fall 2020 following UC ANR recommendations. Samples included barnyardgrass, late watergrass and coast cockspur (E. walteri) (Table 1). Coast cockspur is a newer weed species to California rice growers, although it is common in rice in the mid-South and is lately being found throughout the Sacramento Valley. Seedlings of each sampled population were subjected to a battery of common foliar (63 populations) and granular (62 populations) herbicide formulations (Table 2) in growing conditions simulating an early summer rice field.
Table 1. Echinochloa spp. samples collected from different rice counties of Northern California in 2020.Table 2. Herbicides and rates used for the 2021 watergrass screening. Rates are standard field rates for California rice growers with susceptible Echinochloa spp. biotypes.
Granular Herbicides Species response to granular herbicides varied between fall and winter applications. Almost every barnyardgrass, late watergrass and coast cockspur sample (Table 3) was insensitive to Bolero® (thiobencarb), Butte® (benzobicyclon + halosulfuron), Cerano® (clomazone) and Granite GR® (penoxsulam) when applied in fall. However, overall survival rates decreased in the winter application. Notably, seedlings of all species had greater than 70% survival from the fall Cerano (clomazone) application, yet no more than 20% survival in winter. Barnyardgrass survival rates in particular appeared to be affected the greatest by the different application times, especially from Cerano and Bolero. Cooler greenhouse conditions and slower plant growth during the winter trial may have allowed the herbicides’ active ingredients more time to enter and translocate through the plants. For instance, Butte efficacy is dependent on maintaining a constant and deep flood during the water-holding period (see product label for particulars). In addition, slower plant metabolism in the cooler winter would likely have reduced the rate of herbicide breakdown in the plants. This is an important consideration when dealing with suspected herbicide resistance since increased metabolic breakdown is one of the main mechanisms of herbicide tolerance in both weeds and crops.
Table 3. Proportion of Echinochloa spp. samples suspected resistant to granular formulated herbicides across rice-growing counties in California in comparison to a susceptible late watergrass (Echinochloa phyllopogon) population.Table 4. County-level proportions of Echinochloa spp. samples suspected resistant to granular and foliar herbicides in comparison to a susceptible late watergrass population.
Foliar Herbicides Species response to foliar herbicides was also variable between the fall and winter trials (Table 3). Barnyardgrass, late watergrass and coast cockspur samples were more tolerant to Clincher CA (cyhalofop) applied under the cooler winter greenhouse conditions, yet more sensitive to Regiment CA (bispyribac-sodium) during the same period. Survival rates of all species to both SuperWham® (propanil) applications were low to moderate except for late watergrass, which had an 80% survival rate in fall but only 30% in winter. Differences in greenhouse air and water temperatures between fall and winter were probably at play for the foliar herbicides. Regiment and SuperWham are both contact herbicides (although with different MOAs) and hotter temperatures can reduce contact herbicide uptake in the field through mechanisms such as rapid drying of the herbicidal solution, water-based in both cases or increased plant cuticle thickness. In addition, plant metabolic processes that can deactivate herbicides would be expected to act faster in warmer conditions. Nevertheless, SuperWham was relatively effective at killing most of the sample weed populations during both trials which ought to be reassuring to growers who still rely on SuperWham as their cleanup herbicide. On the other hand, Clincher CA is a solvent-based formulation that enters the plant and translocates rapidly, and higher temperatures may aid its activity as plants try to grow with rapidly disrupted cellular membranes. However, too-high application temperatures can result in Clincher CA volatilization. As always, consult the product label for application guidelines.
Multiple and Cross-Resistance Troublingly, most samples showed insensitivity to at least one herbicide MOA regardless of species or application time. Up to 80% of barnyardgrass and coast cockspur samples survived applications of all four granular herbicides and up to 80% of late watergrass samples were insensitive to all three foliar formulations. Given the relative differences in product efficacy between fall and winter applications, the fact that there’s potential for that many products failing in the field should be sobering. In addition, the observed multiple-MOA survival to granular herbicides was uniform across sampling counties (Table 5) indicating that the spread of those resistance mechanisms has already happened. The observed incidence of multiple resistance to foliar formulations was far more variable across sampling counties although Regiment CA was the least effective overall. The prevalence of ALS inhibitor resistance in weeds of California rice has long been established, so the high rates of observed insensitivity to both Granite GR and Regiment should not be surprising.
Table 5. Proportion of samples showing different resistance profile categories collected from California rice fields in 2020.
Implications for Advisors and Growers The theme of any report on herbicide resistance testing should begin and end with stewardship. This goes beyond simply rotating herbicides within a MOA or rotating MOAs; this should include the pesticide-use version of the 4Rs: the Right Product at the Right Rate with the Right Method at the Right Time. Herbicide efficacy is dependent on so much more than whether a weed has some biologic mechanism to block or detoxify the poison, and it is important to be aware of the interplay between formulation, mixing and application method, soil characteristics, temperature and humidity in affecting a given product’s ability to control weeds.
As a PCA, it is important to be proactive whenever possible in mitigating factors under your control that may inhibit herbicide effectiveness. Luckily, a lot of this information is already on the product labels; however, UCCE researchers are constantly adding to the available knowledge. Knowing that Cerano and Butte may be more effective in cooler temperatures, for example, may influence when to act during an application window. The results shown here indicate a high level of watergrass species resistance to most of the available grass herbicides in California rice. Although new herbicide MOAs are starting to hit the market, it is still important to avoid repeating the same errors of yesteryear. Growers rely on PCAs for timely and sage recommendations and therefore we must be the front line of stewardship for herbicides and other pesticides. Through research like the aforementioned study, we’ve learned to avoid overreliance on new products to the point that resistance also develops. Working together we can help ensure the continued efficacy of new and existing pesticides into the future.
Any questions about this study can be directed to Whitney Brim-DeForest, UCCE rice and wild rice advisor in Sutter-Yuba, Placer and Sacramento counties, at wbrimdeforest@ucanr.edu.
References
Vulchi R, Guan T, Clark T and Brim-DeForest W (2024) Echinochloa spp response to preemergence and postemergence herbicides in California rice (Oryza sativa L.). Front. Agron. 6:1349008. doi: 10.3389/fagro.2024.1349008
Generative artificial intelligence is becoming a valuable tool in agricultural consulting, helping consultants streamline research, analyze data and enhance decision-making.
There’s a powerful shift underway in agricultural consulting. The rise of generative artificial intelligence (AI) is rapidly changing how PCAs and CCAs conduct research, analyze data and support farm management. Russell Morgan, certified agricultural consultant with the American Society of Agricultural Consultants and owner of Morgan Ag Consulting Services, shared how AI is transforming his work and the wider industry.
“This is a burgeoning platform. It is ubiquitous, not just in ag production and consulting, but everywhere, engineering, all over,” Morgan said. “Because it’s such a fast-advancing platform. I saw that it will have applications both in my business and in production ag and the ag consulting industry, which is very diverse. Not just what I do, but agronomists, nutritionists and all types of ag tech folks.”
Morgan first began using AI for its ability to handle extensive research quickly. “My work was in research, and it can perform a tremendous amount of research that would take me days or weeks. It can do it in 15 minutes,” he said, noting Google’s Gemini AI as his preferred platform.
Morgan emphasized the importance of reviewing AI results critically. “I found you always have to watch for and use your, what I call, ‘critical discretion,’ which is above critical thinking. Look and say, ‘Wait a minute, that doesn’t look right.’”
He recalled a recent experience where he challenged Gemini’s results. “It was actually a debate with Gemini, and that was pretty cool. Because I hadn’t done that before. But finally, after some interaction back and forth, it recognized that it made a mistake and essentially apologized and corrected the mistake, which was really fascinating to me.”
Improving Efficiency, Not Replacing Experts
Morgan also noted how AI boosts efficiency in writing and reporting. “Old-school folks would say the worst thing that a writer can see is a blank piece of paper. The same thing with a blank screen or blank Word document. But if you can have Gemini or AI, ChatGPT or whatever, provide something, a base to start with, it really ramps up your efficiency. And that’s what I have found.”
He also sees AI’s value across ag consulting fields. “There are a number of areas where I can use it to leverage knowledge in my consultancy, but I also have read where, let’s just say, agronomists have found fascinating uses in agronomy. Of course, you never take the agronomist out of the picture, but some of the simpler things it can replicate the agronomist’s expertise. They can leverage their time and value tremendously by utilizing the tool.”
Three Pillars of AI Use
Morgan described his approach to AI through three key pillars:
Agricultural Intelligence: “All the data, the analytical data that an agronomist would use, can be built into and replicated by AI.”
Artificial Intelligence: “Machine learning. It learned from that interaction, so it will not make that same mistake again.”
Actionable Intelligence: “I take a lot of data, compile it, distill it and present it in an actionable way so the management team can look at it and make a decision.”
Ethics and Transparency
Morgan stressed AI should be used responsibly. “You don’t want to present somebody else’s work as your own.”
He referenced the American Society of Agricultural Consultants’ code of ethics. “If I created a report or business plan, let’s say, and I totally relied on Gemini or ChatGPT or whatever without modifying it and presented that as my work, that’s unethical, in my opinion, and also the opinion of the code of ethics for ASAC.”
In addition to the American Society of Agricultural Consultants’ code of ethics, PCAs and CCAs in California follow strict professional standards. PCAs must comply with ethical and legal requirements set by the state Department of Pesticide Regulation, while CCAs adhere to a national Code of Ethics established by the American Society of Agronomy. These guidelines emphasize accuracy, integrity, continued education and transparency to ensure responsible and trustworthy service to clients and the public.
He also cautioned about potential legal risks. “There may be some legal ramifications… Ag consultants, whatever area of consulting they’re working in, need to be considerate of those things and not violate the legal aspects and get themselves in a tremendous amount of trouble.”
As Morgan said, “You never take the agronomist out of the picture.” AI, he believes, is a tool for enhancing, not replacing, ag professionals. Used thoughtfully, it can improve efficiency, accuracy and the value consultants provide to their clients.
To hear more on this subject, check out this recent interview with Morgan on the MyAgLife Daily News Report. Additionally, Morgan led a roundtable discussion in late April titled, “Utilizing Generative Artificial Intelligence (Gen AI) In Your Agricultural Consulting Practice.” That recording can be found here.
Leaf death from sunburn. Photosynthesis in grapevines is generally optimal from 77 to 95 degrees F and is strongly reduced at temperatures above 105 degrees F (photo by Karen Block, UC Davis.)
The San Joaquin Valley (SJV) is already considered a hot growing region for winegrapes, and heat stress is expected to become more frequent and severe in this region over the next several decades (Livneh et al. 2015). Heat impacts many aspects of vine physiology, and the goal of this article is to provide consultants and growers with a broad overview of these impacts and the consequences for yield and berry quality.
Vegetative Physiology Heat strongly impacts grapevine carbon and water fluxes through effects on photosynthesis, respiration and transpiration. Photosynthesis in grapevines is generally optimal from 77 to 95 degrees F and is strongly reduced at temperatures above 105 degrees F (Greer 2018; Greer and Weston 2010). This reflects both direct effects from heat and indirect effects from water stress. Temperatures above 85 to 95 degrees F can directly impair photosynthesis by co-opting the leaf metabolism to generate toxins that damage the membranes where these reactions take place (Carvalho et al. 2015). Heat also increases evapotranspiration and vine water stress. Warmer air molecules spread apart, creating more room to hold water vapor and increasing the driving force for water to evaporate from the soil or vine (measured as a higher vapor pressure deficit, or VPD). Excessive dehydration damages vine tissues, so a higher VPD forces grapevines to restrict transpiration by closing the stomata, which in turn limits the CO2 entering the leaf and available for photosynthesis (Chaves et al. 2016). This process not only reduces the carbon available for growth and ripening but can also increase vine water stress and irrigation demand. Heat also accelerates respiration reactions, causing respiration rates to approximately double with every 18 degrees F increase in temperature (Palliotti et al. 2005). This combination of increased respiration and decreased photosynthesis can limit the carbon available for fruit set and ripening under hot conditions.
Vegetative growth can have complex responses to heat. Up to a point, warmer temperatures can increase vine transpiration and the transport of hormones (i.e., cytokinins) from the roots to the shoots, promoting lateral growth and increasing canopy size (Field et al. 2020). However, vegetative growth is one of the most sensitive physiological processes to water stress, so any positive effects on growth will rapidly reverse if heat is sufficient to produce water stress (i.e., pre-dawn water potentials < -0.3 MPa) (Deloire et al. 2020).
Fruit Physiology In general, warming has accelerated the rate of fruit development. Over the past 30 years, harvest has shifted 24 days earlier in Germany, mostly due to earlier bud break (10 days earlier) and faster sugar accumulation (i.e., the period from veraison to harvest becoming nine days shorter) (Koch and Oehl 2018). However, extreme heat can interfere with fruit development. The effects depend on temperature, duration and timing. At bloom, temperatures >95 degrees F can interfere with flower fertilization, preventing the pollen from forming the tunnels that allow it to reach the ovary, inducing shatter and berry thinning (Kliewer 1977). Heat generally has less impact during fruit set (bloom to veraison) (Greer and Weedon 2013; Greer and Weston 2010). Extreme heat (>100 degrees F) can limit cell division in the berries, but most impacts from heat during this period are indirect effects of water stress on cell expansion. At this stage, the berries receive most water (~80%) through the water transport tissue (xylem), and the rate and direction of xylem water flow is highly dependent on the water potential gradient between the fruit and canopy (Keller et al. 2015). Vegetative water stress at this stage (i.e., pre-dawn water potentials < -0.5 MPa) can decrease water flow to the berries, berry cell expansion and growth (Deloire et al. 2020).
At bloom, temperatures >95 degrees F can interfere with flower fertilization, preventing the pollen from forming the tunnels that allow it to reach the ovary, inducing shatter (pictured) and berry thinning (all photos by George Zhuang, UCCE.)
At veraison, berry water influx switches to the sugar transport tissue (phloem), which is less sensitive to canopy water potentials, and direct effects of temperature become more important. Heat especially impacts quality at this stage, and heat effects can be quite severe, since dark (red) berries can be ~30 degrees F warmer than the air (Venios et al. 2020). Berry temperatures will depend on multiple vineyard design and management factors, including factors affecting radiation exposure from the sky (e.g., trellising, shoot and leaf thinning decisions, shade netting, row orientation) and ground (e.g., cover cropping, fruit zone height) and transpirational cooling (e.g., misting, irrigation) (Keller 2010; Keller and Chang 2023). Heat can have complex effects on sugar accumulation. Warmer temperatures generally increase the rate of sugar accumulation through indirect effects of water stress on the phloem (Salmon et al. 2019). Leaves load sugar into the phloem to create a concentration gradient that pulls in water from the xylem, and this water influx pushes the sugar sap toward the fruit. When the canopy is water-stressed, and water potentials in the xylem are more negative, the phloem needs a higher sugar concentration to pull water away from the xylem, which delivers a more concentrated sap to the berries.
Insipient sunburn on grapes at Oakville Station during the hot 2022 season. Heat and light can interact to produce sunburn, which degrades the waxes in the berry cuticle, leads to severe berry dehydration and alters berry phenolics (photo by Karen Block, UC Davis.)
However, severe heat stress can also stall sugar accumulation. In Australia, a four-day heatwave at 105 degrees F downregulated photosynthesis and stopped sugar transport for two weeks, which could reflect persistent damage from heat or water stress (Greer and Weston 2010). Heat also directly impacts berry acidity and pigment (anthocyanin) levels. Heat accelerates berry respiration and the breakdown of malic acid, so that malate accumulation is optimal between 68 to 77 degrees F and significantly degraded above 105 degrees F (Coombe and McCarthy 2000; Venios et al. 2020). Heat also impairs anthocyanin synthesis and increases degradation above 95 degrees F (Cataldo et al. 2023). Heat and light can also interact to produce sunburn, which degrades the waxes in the berry cuticle, leads to severe berry dehydration and alters berry phenolics (Gambetta et al. 2021).
Heat has wide-ranging impacts on vegetative and fruit physiology. Many heat effects are strongly dependent on water stress or light exposure, making it difficult to predict changes in yield or quality metrics as a function of air temperature, though many processes begin to experience problems above 95 degrees F. We also lack important information on the interactions between duration and intensity in determining heat damage.
References
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Cataldo E., Eichmeier A., Mattii GB. 2023. Effects of Global Warming on Grapevine Berries Phenolic Compounds—A Review. Agronomy 13:2192.
Chaves MM., Costa JM., Zarrouk O., Pinheiro C., Lopes CM., Pereira JS. 2016. Controlling stomatal aperture in semi-arid regions—The dilemma of saving water or being cool? Plant Science 251:54–64.
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Deloire A., Pellegrino A., Rogiers S. 2020. A few words on grapevine leaf water potential. Technical Reviews.
Field SK., Smith JP., Morrison EN., Emery RJN., Holzapfel BP. 2020. Soil Temperature Prior to Veraison Alters Grapevine Carbon Partitioning, Xylem Sap Hormones, and Fruit Set. Am J Enol Vitic 71:52–61.
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Figure 1. Weed population fully recovered eight weeks post-application of glyphosate (A) and glufosinate (B) compared to the untreated area (C).
Developing a weed management program in conventional pear orchards is a challenge, varies from orchard to orchard and is influenced by weed species populations, weed pressure, management practices and local environmental condition. Integrated weed strategies ideally involve the use of multiple strategies, including mowing, chemically mowing, discing and cultivation in the row middles of trees, herbicide strip sprays and the adoption of selected cover crops.
Pear orchards usually require intensive irrigation and high moisture in the soil, especially in late May, June and early July, when the fruit is increasing in size. Optimum soil moisture and temperature in the orchard floor favors a high pressure of summer weed infestation, which requires multiple post-emergent herbicide applications to keep the pear orchard floor weed-free during the growing and harvesting season.
For California pear orchards, recommended herbicide programs may include a fall/winter (November to February) strip spray with preemergent (indaziflam, rimsulfuron, pendimethalin, or flumioxazin) in a tank mixture with post-emergent (glyphosate or saflufenacil). However, during the growing season, the strip spray herbicide programs are primarily performed using only post-emergent herbicides in April, late May and July.
Improve Herbicide Program Rotation Over the past few decades, glyphosate has been the most used herbicide registered for post-emergence non-selective weed management of annual and perennial weeds in conventional pear orchards. However, some weed populations, such as Italian ryegrass (Lolium multiflorum), junglerice (Echinochloa colona), annual bluegrass (Poa annua) horseweed (Conyza canadensis) and hairy fleabane (Conyza bonariensis), have developed resistance to glyphosate, and poor control of weeds with glyphosate application programs have been observed more frequently in California orchards.
Practitioners are seeking broad-spectrum herbicide alternatives to glyphosate. However, substitutes have scarcely been evaluated due to glyphosate’s history of being effective and affordable. Although this doesn’t seem imminent, if glyphosate were no longer available, glufosinate-ammonium appears to be the most efficient and economical option. Glufosinate is a contact non-selective herbicide that is very effective against annual broadleaf and grass weeds but is less effective against biennial and perennial weeds and may require sequential applications to achieve satisfactory control. Overall, the labels of glyphosate and glufosinate indicated outstanding performance in controlling a wide variety of weeds. For this reason, glufosinate has been ranked as the best alternative currently available.
Consider Adding Preemergent Herbicides in Late Spring Spray Programs Weed infestation in pear orchards is a year-round problem, especially during the growing season and preharvest with various annual and perennial species such as field bindweed (Convolvulus arvensis), nutsedge (Cyperus spp.) and summer grass such as Italian ryegrass (Lolium multiflorum), junglerice (Echinochloa colona), foxtails (Setaria spp.), crabgrass (Digitaria sanguinalis), common bermudagrass (Cynodon dactylon), etc. Due to high weed infestation during the growing season, post-emergent-only herbicide programs have short-lasting weed control (Fig. 1), requiring multiple sequential applications to keep weeds below an acceptable threshold. Beyond that, the use of many herbicide programs may require minimum intervals between the last herbicide application and harvest of up to 75 days for pear growers delivering the crop to specific markets. Therefore, it is difficult to select herbicide application programs that provide long-lasting control.
Generally, preemergent herbicide programs in pear orchards are typically applied in the fall/winter to early spring and have scarcely been evaluated for late spring application. This lack of information warrants more research regarding the effectiveness and crop safety of preemergent and post-emergent herbicide programs for late spring application. We believe preemergent herbicide programs added to the tank mixture may bring several benefits for late spring application, such as improved long-lasting control, prevention of herbicide-resistant weed evolution, reduction in total number of operations required for weed management and increase in preharvest minimum intervals.
Procedures In late spring 2024, we established two herbicide field trials in Lake and Mendocino counties to compare the effectiveness of glyphosate and glufosinate sprayed side by side and to evaluate the advantages and disadvantages of (preemergent and post-emergent) herbicide programs with different modes of action to improve weed management in pear orchards.
For these studies, we compared glyphosate (Roundup PowerMAX®) at 64 fl oz/ac and glufosinate (Rely® 280) at 56 fl oz/ac applied alone and in a tank mixture at two different rates with indaziflam (Alion®) at 3.5 and 4.5 fl oz/ac, rimsulfuron (Matrix®) at 3.0 and 4.0 fl oz/ac, or pendimethalin (Prowl H2O®) at 70.4 and 102.4 fl oz/ac, applied in a water carrier volume of 30 gallons per acre (GPA) with 11003 VS flat-fan spray nozzles. To reduce costs, growers may consider generic herbicides rather than the brand-name counterparts used in these trials. In many cases, generic herbicides may have the same effectiveness as brand-name counterparts at a lower cost (consult your local UCCE farm advisor or your PCA and always read the pesticide label.)
Efficacy The main weeds present at the trial sites were jungle rice, crabgrass, common bermudagrass, yellow nutsedge and field bindweed. Our results showed both glyphosate and glufosinate applied in late May provided excellent weed control greater than 90% for most weeds present at the trial sites. Overall, glufosinate proved to be a broad-spectrum herbicide with equivalent grass and broadleaf weed control to glyphosate (Fig. 2). However, both glyphosate and glufosinate have low residual activity, and most weeds began to germinate or regrow from regenerative underground propagules via roots, rhizomes or tubers, indicating the need for sequential application around four to six weeks after initial application for effective long-term weed control (Fig. 3).
Figure 2. Performance of glyphosate (A) and glufosinate (B) at two weeks post-application.Figure 3. Weed population germinating or regrowing at four weeks post-application of glyphosate (A) and glufosinate (B).
Overall, the results showed glyphosate or glufosinate applied in a tank mixture with the preemergent herbicides indaziflam, rimsulfuron or pendimethalin at the rates used in these studies provided excellent weed control for most weeds present in the orchard sites and improved long-lasting weed control when compared to glyphosate or glufosinate applied alone.
The tank mixture program with preemergent herbicides indaziflam or pendimethalin provided better long-term control of jungle rice and crabgrass than rimsulfuron. On the other hand, all preemergent herbicides showed similar control of broadleaf weeds present in the orchard sites.
Our results also indicated glyphosate or glufosinate applied in a tank mixture with pendimethalin provide inferior control of yellow nutsedge compared to indaziflam or rimsulfuron tank mix (Fig. 4).
Figure 4. Poor control of yellow nutsedge with tank mixture of glyphosate + pendimethalin (A) compared to glyphosate + indaziflam (B) or glyphosate + rimsulfuron (C) at eight weeks post-application.
In general, late spring application of glyphosate or glufosinate alone and in a tank mixture with indaziflam, rimsulfuron or pendimethalin at the rates used in these studies were safe for pear trees with no injury observed.
Our results indicated glufosinate may be a great alternative to glyphosate with similar efficacy for controlling a broad spectrum of weeds, and adding preemergent herbicide to the late spring herbicide application programs may improve long-term weed control.
The results of these studies and the literature review strongly suggest developing more efficient herbicide application programs or alternatives to glyphosate by using herbicides with different modes of action may help to reduce potentially resistant weeds. Also, adopting spray programs with preemergent and post-emergent herbicides promotes longer-lasting weed control, reducing the number of herbicide applications, increasing the time window between the last herbicide application and the harvest season, and reducing the risk of herbicide residue in the crop.
These studies may contribute to growers and PCAs developing a more complete integrated weed management program in conventional pear orchard systems and potentially result in a reduction of costs by not adopting preharvest strip sprays.
These studies will be repeated in 2025 to confirm preliminary data assessed in 2024. The mention of active ingredients or products in this article is not an endorsement or recommendation. Consult your local UCCE farm advisor or your PCA for a recommendation and always read the pesticide label; the label is the law.
The authors would like to thank California Pear Advisory Board and Pear Pest Management Research Fund for funding these studies. We thank Wilfredo Bello, UCCE agricultural technician in Lake County, for the technical support.
Figure 1. Organic market trend reports from the Organic Trade Association between 2013 and 2022 showing incremental growth in organic food sales (all photos courtesy C. Hight.)
The first concepts of organic agriculture as we now know it were developed in the early 1900s by Sir Albert Howard, Rudolf Steiner and F. H. King (Adamchak 2024). These individuals believed in the use of animal manures, composts, cover crops, crop rotation and a very early version of integrated pest management that, when combined, resulted in a better system approach to farming. After World War II and the invention of the Haber-Bosch process, there was an excess of nitrogen-containing compounds, and to relieve the excess, these products were applied to agricultural fields. While yields increased, there was an unforeseen detriment to natural populations of microbes and beneficial predators. Modern organic farming developed as a response to the environmental harm of synthetic pesticides and fertilizers used in conventional agricultural systems. Organic farming has been shown to lower pesticide usage, reduce soil erosion, increase cycling of nutrients (which decreases the likelihood of leaching to groundwater and surface water) and aid in recycling animal wastes. Although more ecologically friendly, organic farming tends to have a higher production cost and generally a lower yield. With an increasing concern for pesticide residues and consumer awareness of genetically modified organisms, organic food sales have steadily increased over the latter half of the 20th century and continue to show increases today (Fig. 1).
Organic Vegetable Production Practices Organic vegetable production systems rely on natural inputs, such as amino acids, proteins, composts and manures, to supply nutrients to the plants. These N-containing inputs must go through the process of mineralization from amino acids to ammonium (NH4+) and nitrate (NO3–) by microorganisms to become plant-available. The availability of nutrients supplied to the plants depends on many factors, such as carbon to nitrogen ratio and N% of the material as well as the moisture, temperature and texture of the soil. The release of N from these materials is variable but predictable in a laboratory setting, however in-field factors make release timing and quantity difficult to anticipate (Lazicki et al. 2020). This can lead to lower crop yields and difficulty controlling pests (Giampieri et al. 2022). An increased reliance on a whole systems approach is needed to effectively produce organic vegetables. There is evidence to suggest practices, such as legume cover crops and reducing tillage, can increase soil organic matter (SOM) and provide additional nutrients in both organic and conventional systems (Fig. 2). Growers who grow organically can expect a higher input cost but can typically also expect a higher price at the market. Consumers who purchase organic produce view this as a way to consume less synthetic pesticides and increased nutrient content (dos Santos et al. 2019). Organic inputs also provide a higher amount of C to the soil, increasing microbial biomass and activity, which are seen as positive soil health indicators. Published reports show under organic management, total and organic C, total N, available phosphorus and calcium, magnesium, manganese, zinc and copper were greater compared to conventional systems (Chausali and Saxena 2021). The dynamics of N mineralization (Nmin) may also be affected by long-term organic management compared to conventional management. Once again, a whole systems approach to organic farming is necessary to reap high yields with low pest pressure and a low environmental impact.
Figure 2. Example of a no-till or reduced-till field inter-seeded with clover, a nitrogen-fixing cover crop. There is evidence to suggest practices like legume cover crops can increase soil organic matter.
Conventional Vegetable Production Practices Conventional vegetable production systems rely on synthetic fertilizer and pesticides to provide nutrients to plants and protect them from disease and insects. While synthetic fertilizer inputs can target key growth points to maximize yield and reduce environmental pollution, synthetic fertilizers can be detrimental to natural soil microbial populations. Additionally, caution must be taken as overapplying N- and P-containing fertilizers can move with soil colloids and surface water to pollute rivers and streams. Consumers see synthetic pesticides with a negative connotation but may not understand the precision, regulation and care with which the synthetics are applied (Fig. 3). Improper use of insecticides, fungicides and other pesticides can cause insects and other pests to develop resistance to chemistry within said formulations. These same synthetics may reduce microbial populations, leading to decreased overall soil health. However, when managed correctly, these products can rescue crops from infestations of insects or other diseases. Conventional production provides growers more room for error as many products can provide nutrients immediately compared to organic systems that require mineralization of nutrients to become available to plants. Additionally, with a well-timed pesticide application, potential crop loss due to an insect swarm can be mitigated, often easier said than done in organic production. While all farming is difficult, conventional farming is more forgiving than organic farming on individuals learning the art of vegetable production.
Figure 3 . An example of an undisturbed 6-inch soil core fitted with parafilm and puncture holes to allow ventilation. Cores were then incubated at 25 degrees C and 60% water holding capacity to mimic ideal soil conditions.
Comparing the Two Paradigms On California’s Central Coast, a study is currently underway investigating Nmin dynamics of 20 pairs of organic and conventional fields with similar environmental conditions and soil types. After a vegetable crop is harvested, 6-inch undisturbed soil cores are taken alongside a composite 6-inch soil sample (Fig. 4). The soil sample represents the physical, chemical and biological characteristics of the soil pre-incubation. The undisturbed cores are then incubated for 10 weeks at 25 degrees C and 60% water holding capacity to determine how much N mineralizes or immobilizes within that period. The entirety of the samples will be analyzed as such, and analyses will be performed to determine the most significant characteristics driving N availability. We hypothesize the organic fields will have a lower starting inorganic N content but mineralize more N over a 10-week incubation, and characteristics that most impact the quantity mineralized will be water holding capacity, SOM content and N% in the soil.
Is a Combination of Practices Best? Conventional and organic management systems produce many of the same vegetables on the Central Coast, including broccoli, cauliflower, romaine and celery. Similar nutrient requirements are needed to produce adequate yields in both systems. Conventional systems provide inorganic nutrients that are immediately available for uptake bypassing the need for microbial mineralization. N-containing organic amendments require microbial decomposition to become available to plants. N dynamics in either system depend on a multitude of factors, including other cations and anions, SOM content, N% of the soil, moisture and temperature of the soil as well as amendments and crop residues added. While conventional practices allow for immediate applications of fertilizers and pesticides and organic fields require a whole systems approach and forward thinking, potentially bridging the gap between the two practices could be a practical approach. The added organic amendments with their high carbon content can contribute to soil health metrics and a robust microbial population, meanwhile the grower knows they have a failsafe in the back pocket in case something goes wrong. This lends to sustainable agriculture, which strives to provide resources necessary for our population to thrive while also conserving the planet’s natural ability to sustain future populations of plants, animals and humans. All this to say the preference for organic vs conventionally produced vegetables is for the consumer to decide. The practices that promote the best soil, air and water will be determined by a combination of growers and researchers interacting with and weighing practicality, sustainability and return on investment.
References
Adamchak, R. (Dec. 21st, 2024). Organic Farming. In Biritannica (Ed.), Britannica. https://www.britannica.com/topic/organic-farming.
Chausali, N., & Saxena, J. (2021). Chapter 15 – Conventional versus organic farming: Nutrient status. In V. S. Meena, S. K. Meena, A. Rakshit, J. Stanley, & C. Srinivasarao (Eds.), Advances in Organic Farming (pp. 241-254). Woodhead Publishing. https://doi.org/https://doi.org/10.1016/B978-0-12-822358-1.00003-1
dos Santos, A. M. P., Lima, J. S., dos Santos, I. F., Silva, E. F. R., de Santana, F. A., de Araujo, D. G. G. R., & dos Santos, L. O. (2019). Mineral and centesimal composition evaluation of conventional and organic cultivars sweet potato (Ipomoea batatas (L.) Lam) using chemometric tools. Food Chemistry, 273, 166-171. https://doi.org/https://doi.org/10.1016/j.foodchem.2017.12.063
Giampieri, F., Mazzoni, L., Cianciosi, D., Alvarez-Suarez, J. M., Regolo, L., Sánchez-González, C., Capocasa, F., Xiao, J., Mezzetti, B., & Battino, M. (2022). Organic vs conventional plant-based foods: A review. Food Chemistry, 383, 132352. https://doi.org/https://doi.org/10.1016/j.foodchem.2022.132352
Lazicki, P., Geisseler, D., & Lloyd, M. (2020). Nitrogen mineralization from organic amendments is variable but predictable. Journal of Environmental Quality, 49(2), 483-495. https://doi.org/10.1002/jeq2.20030
Figure 1. Aerial photo of the study vineyard blocks with north- and south-facing aspects and locations of the evapotranspiration (ET) measurement stations.
Many California specialty crop production areas often face significant water supply curtailments due to recurring droughts and stringent environmental regulations. In this context, the utilization of field-specific information is crucial to enhance irrigation management practices and pursue profitable and high-quality food production under more pronounced weather vagaries and increasingly variable fresh water supplies.
The rapid adoption of pressure-compensating microirrigation systems during the last 15 years has enabled California winegrape growers to establish vineyards in areas with marginal soils and sloping terrains that otherwise were unsuited to other irrigation methods. While some degree of slope can be beneficial in vineyards for improved drainage of excess water, better airflow through the vines and faster escape of cold air to reduce the risks of springtime frost damages, it can affect microclimatic conditions, radiation interception, vine water use and sometimes influence grapes ripening.
Several researchers documented winegrape quality ties with irrigation management and grapevine water status (Jackson and Lombard 1993; Kennedy et al. 2002; Downey et al. 2004). The amount of irrigation water required to grow quality winegrapes and the frequency of irrigation applications depend on several site-specific factors, such as vine growth stage, vine and row spacing, vine density, size of vine canopy (Williams, 2001), soil texture and terrain characteristics.
Little information is available to growers about water use of vineyards on sloping terrains with different aspects. Such information is necessary as growers seek more resource-efficient production practices and vine water stress monitoring techniques to manage grape yield and quality, and as future water supplies become increasingly variable, uncertain, limited and costly.
Recently, a team of UC researchers measured the actual grapevine evapotranspiration (ETa), its seasonal dynamics and vine water status in two winegrape vineyard blocks grown with microirrigation on sloping terrains with north- and south-facing aspects in El Dorado County during three consecutive seasons (2016, 2017, 2018).The goal of this field research study was documenting differences in grapevine water consumption (ET) due to slope and aspect for adapting irrigation management based on vineyard topography.
Study Site and Field Data Collection The UC team instrumented two adjacent north- and south-facing commercial vineyard blocks (Fig. 1) located near Pilot Hill in El Dorado County for collecting field data of biophysical parameters. El Dorado County is in the foothills of the Sierra Nevada mountains and is a relatively small but growing California winegrape production region falling within the California grape pricing district 10, where the top three varieties are Zinfandel, Cabernet Sauvignon and Syrah.
Both the vineyard blocks consisted of vines of Cabernet Sauvignon grafted onto 3309 Couderc rootstock, planted in 2000 at a density of 1,507 vines per acre and trained in a bilateral cordon vertical shoot positioned system with north-south vine row orientation. The vines were irrigated using single driplines with two pressure-compensating online button drippers per vine with nominal flowrate of 0.5 gph.
Both the north and south blocks had Auburn series very rocky loam soil with a typical depth of 2 feet, as mapped by the USDA-National Cooperative Soil Survey (California Soil Resource Lab 2019). The north-facing slope presented a different, shallower soil, with bedrock at 33 inches depth, but with less gravel content in the upper 20 inches and more finely textured clay retaining more moisture than in the south-slope soil. The terrain slopes in the north and south blocks were 24.5% and 25.5%, respectively.
ETa was determined with the residual of energy balance method from micrometeorological measurements of net radiation (Rn), ground heat flux and sensible heat flux obtained from one full-flux ET measurement station at each vineyard block (Fig. 1) consisting of a combination of eddy covariance and surface renewal equipment.
Actual crop coefficient (Ka) values were calculated dividing the measured ETa by atmospheric water demand (ETo) values obtained over the corresponding time-step from the automated weather station #195 (Auburn) of the California Irrigation Management Information System (CIMIS), according to the relation Ka = ETa/ETo.
The UC team assessed the vine water status during the three growing seasons with periodic measurements of the midday stem water potential on clear-sky days (between 11:00 a.m. and 2:00 p.m.) using a Scholander-type pressure chamber on six vines per vineyard block (one fully expanded and shaded leaf per vine), which were randomly selected within the footprint area of each ET station.
Table 1 reports the amounts of irrigation water applied in the north- and south-facing vineyard blocks, recorded with magnetic flowmeters during the three consecutive crop seasons and the monthly rainfall values recorded at the nearby CIMIS station. Differences in applied water were observed between the north and south blocks, which likely resulted from different application rates between the blocks but also from adjustments of irrigation frequency and duration based on visual assessment of vines’ and appearance. In fact, the vineyard manager applied irrigation water with varying frequencies and durations over the different months, the vineyard manager reported that irrigations for the two vineyards blocks were scheduled based on visual observations of the vines. The vineyard manager also considered the available soil moisture from periodic soil probing and existing water supply limitations.
Light interception by the vine canopies was measured during the 2018 growing season using the ‘Paso Panel’ canopy shade meter (Battany 2009), which consists of a solar collector panel, a voltage meter and power switch attached to a portable frame. Holding the Paso Panel underneath the grapevine canopy for a few seconds allowed for measuring the voltage current generated by sunlight passing through the foliage and striking the panel’s surface as illustrated in Figure 2.
Figure 2. Measurement of light interception by vine canopies in a commercial production vineyard on the California Central Coast (left) and the study vineyard in El Dorado County (right) (photos courtesy Mark Battany, UCCE, and D. Zaccaria.)
The current readings obtained placing the panel under vine canopies at multiple locations in the vineyards were then divided by current readings taken under full sun outside the vineyards to determine the shaded area by the vine canopy, which is a proxy of the vines’ fractional canopy cover. All the measurements in the north and south vineyard blocks were taken during clear-sky days at solar noon ± one hour and then calibrated against full-sun current readings. The values of shaded area by vines (%) were used to determine comparative differences in vines’ canopy growth and size between the north and south blocks.
Actual Grapevine Water Use for North- and South-Facing Blocks Figure 3 illustrates the ETa for the north and south vineyard blocks in 2016, 2017 and 2018 along with ETo. From the figure, it can be noticed that the north and south blocks had very similar season-long cumulative ETa values, but the time course of ETa differed between the two blocks during the three growing seasons. In the 2016 and 2018 growing seasons, ETa was slightly higher in the south block than the north block from April to early June, then ETa of the north and south blocks matched in late June. Afterward, the north block had slightly higher ETa from late June until late September to early October.
Figure 3. Season-long cumulative actual grapevine evapotranspiration (ETa) measured in the north- and south-facing vineyard blocks and reference grass evapotranspiration (ETo) obtained from the local CIMIS station (Station #195, Auburn, Calif.) for 2016-18 seasons.
The field dataset of 2017 shows very similar seasonal cumulative ETa values for the north and south blocks, but differences in ETa can only be noticed for the period from late June to early September, with the north block having slightly higher ETa than the south block, which is consistent with the pattern of 2016 and 2018. From mid-September to late October 2017, the south block had slightly higher ETa than the north block, which reveals a contrasting pattern to that of 2016 and 2018. The higher late season ETa in the north block in 2017 was probably due to larger water applications that occurred during irrigation events in late July and August in the area surrounding the ET measurement station of the north vineyard, which likely resulted from a dripline leak going unnoticed for more than a month as reported by the vineyard manager and as revealed by the flowmeter records of 2017 (Table 1).
Table 1. Applied irrigation water in the north (N) and south (S) facing study vineyards blocks obtained from flowmeter records and monthly cumulative rainfall during the 2016-18 crop seasons from CIMIS station #195.
Figure 4 shows the time course of weekly averaged ETa values measured in the north and south vineyard blocks for the three seasons. In this case, slightly higher ETa was observed in the south block early in the season from April to early June in all three years; afterwards, higher ETa occurred in the N block during the central part of the season from early to mid-June through early to mid-August in all three years, whereas slightly higher ETa was observed in the south block relative to the north block in the late part of the season in 2016 and 2017. On the contrary, slightly higher ETa was observed in the north block relative to the south block during the late part of the 2018 season.
Figure 4. Weekly average actual grapevine evapotranspiration (ETa) measured in north- and south-facing vineyard blocks during 2016-18 seasons.
The data in Figure 4 clearly show vines in the north block expressed higher water use during the central part of the growing season, which is possibly related to higher interception of solar radiation during the period around the summer solstice, when the sun reaches its most northerly excursion relative to the equator. Higher water use in that period may also be related to the north block having relatively larger canopy size or accessing larger soil moisture reserves, thus facing less water limitations during the hottest part of the growing season.
Figure 5 shows the weekly cumulative values of Rn measured in the north and south blocks during the study seasons. The Rn data show higher Rn values were measured in the south block during the early and late parts of the season in all three years, whereas Rn values were similar in both blocks in the central part of the growing season in 2016 and 2018, or slightly higher in the north than the south block in 2017.
Figure 5. Weekly cumulative values of the net radiation (Rn) measured in north- and south-facing vineyard blocks during 2016-18 seasons.
Rn is the main force driving crop evapotranspiration. In detail, when soil moisture is abundant and can support vine water consumption without restrictions, higher Rn leads to higher grapevine ETa, all other factors (vines’ canopy size, light interception, available soil moisture) being similar. In the Mediterranean climate of northern California, these conditions normally occur in the period between March and mid-June, when grapevine growth is supported by abundant soil moisture resulting from late winter and early spring rainfall, thus there is no need to irrigate. Later, Rn still drives ETa, which is however dynamically regulated by the available soil moisture from irrigation and by the amount of radiation intercepted by the vines’ canopy. As such, the ETa pattern may not necessarily follow that of Rn, especially when vines face water stress because of deficit irrigation or because of difference in vines’ canopy size or soil moisture available to roots. In other words, multiple factors regulate the actual vine ETa, including canopy size, row orientation and available soil moisture as well as the angle of incidence of solar radiation, which in turn depends on the position of the sun along the growing season relative to vineyard topography (i.e., slope and aspect).
A good relative indicator of vine water use is the actual crop coefficient (Ka), which reflects the actual ETa rate relative to ETo. Figure 6 shows the weekly averaged grapevine Ka values calculated for the north and south blocks over the course of the three consecutive seasons. Ka integrates the atmospheric water demand with the grapevine’s physiologic processes, regulating the actual vine evapotranspiration alongside the plant-available soil moisture, thus providing synthetic information on actual grapevine water use in the site-specific and plant-specific conditions of the vineyard study blocks.
Figure 6. Weekly averaged values of the actual crop coefficient (Ka) measured in north- and south-facing vineyard blocks during the 2016-18 seasons.
Figure 6 shows Ka was higher in the south block early in the season until mid-May 2016 and 2018, then the north block had higher Ka than the south block from early June to early August in all three seasons. Afterward, Ka was higher in the south block from mid-August to the end of the crop season in 2016 and 2017, whereas it was similar in the north and south blocks from mid-August to the end of the 2018 crop season. The figure also shows in 2016 and 2018 that Ka reached its peak values early in the season between mid-April to early May (whereas in 2017, peak Ka values were observed around late June) and progressively decreased during the course of the growing season, revealing increasing ETa reduction.
Following ETa or Ka could provide relevant information for tailoring irrigation management decisions (i.e., timing and amounts of water applications) based on actual grapevine water consumption, especially during periods of water supply restrictions. However, ET-based irrigation scheduling alone may not allow for targeting water stress levels that are conducive to reductions of grapevine vegetative growth and to specific fruit yield and quality targets.
Some additional considerations can be drawn observing Figure 7, which shows the values of midday stem water potential (ΨSTEM) measured in the north and south blocks over the course of the three crop seasons. In all three years, ΨSTEM values decreased progressively from values between -2 ÷ -4 bars early in the season to values between -12 ÷ -15 bars towards the final part of the season, revealing vines in both the north and south blocks were exposed to increasing water stress. Vines in the south blocks had relatively lower (more negative) ΨSTEM values from April to early or mid-August in 2016, 2017 and 2018. ΨSTEM values were lower in the north block from early August to the end of the season in 2016 and 2017, whereas vines in the north and south blocks had similar ΨSTEM values from mid-August to the end of the season in 2018.
Figure 7. Stem water potential (ΨSTEM) values measured in north- and south-facing vineyard blocks during the 2016-18 seasons.
As far as plant water status is concerned, the relatively lower ΨSTEM values of vines in the S block in the first half of the crop season for all three seasons could possibly be due to higher environmental water demand on those vines (i.e., higher Rn). Similarly, the lower ΨSTEM values of north vines during the central part of the season was possibly related to higher environmental water demand in the north block due to similar incidence of solar radiation between the two blocks but higher light interception by the vines (resulting from larger vines’ canopy) in the north block. Alongside, the flow meter records showed larger irrigation water applications in the south block in late July and August 2016 and 2017, which possibly relieved some water stress on the south vines.
Table 2 reports the values of light interception by the vines’ canopy measured during the 2018 season in the north and south blocks, which reveal slightly faster vegetative growth and larger vines’ canopy size in the north than the south during the 2018 crop season.
Table 2. Light interception by vines’ canopy measured in the north (N) and south (S) study vineyard blocks during the 2018 season.
According to Kurtural et al. (2007), faster canopy growth and larger canopy size in north-facing vineyards can be expected in Mediterranean climate as a result of relatively earlier bud-break and relatively lower impact of heat stress on vines relative to south facing slopes. All other factors being equal, in south facing slopes heat can increase during daytime above stress threshold levels, thus causing lower stomata conductance, less carbon assimilation and slower vegetation growth.
Controlling Water Use and Status at a Higher Level Many winegrapes production regions have hillside vineyards, where the actual water consumption is affected not only by grapevine age and health, vine density, canopy size, row orientation and irrigation management practices, but also by the terrain slope and aspect. Topography affects the amount of solar radiation the vines receive and intercept, which is a major driving force of grapevine evapotranspiration under abundant soil moisture.
Irrigation scheduling for winegrape vineyards must consider multiple factors that regulate actual grapevine water consumption in order to maintain vine water status at specific target levels for limiting vegetative growth while pursuing fruit yield and quality objectives. Among others, vines’ canopy size, row orientation and available soil moisture to vines are major factors. However, the field datasets collected in the UC research study show vineyard topography factors (i.e., slope and aspect) also play a significant role in regulating ETa in hillside vineyards. As such, following an evapotranspiration-based irrigation scheduling with generalized crop coefficients derived from other locations and vineyard conditions may not be appropriate. Instead, following ETa and Ka determined for the site-specific vineyard conditions provides relevant information for irrigation scheduling decisions, but may not enable growers to pursue vine water stress levels that are desirable in specific stages of the growing season for achieving grape yield and fruit composition objectives.
Integrating weather- and plant-based irrigation scheduling approaches allows for higher level of control on grapevine water status that is necessary for grape yield, composition and quality purposes. For example, following ETa and Ka while keeping track of ΨSTEM values can provide more integrative information on actual vines’ evapotranspiration and water status for more precise irrigation management decisions.
In the field, ΨSTEM values can help decide irrigation timings more precisely, while ETa and Ka enable to determine adequate irrigation amounts for maintaining the desired water deficit levels to balance vegetative growth with grape yield and composition goals.
References Battany, M.C., 2009. Estimating vineyard water use in the estrella-creston area of concern. Url http://cesanluisobispo.ucdavis.edu/files/71055.pdf
California Soil Resource Lab., 2019. Ssurgo (soil survey geographic), url https://casoilresource.lawr.ucdavis.edu/soilweb-apps
Cimis, 2018. California irrigation management information system, url https://cimis.water.ca.gov/
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Jackson, D.I., Lombard, P.B., 1993. Environmental and management practices affecting grape composition and wine quality-a review. American journal of enology and viticulture, 44(4), 409-430.
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