Richard Smith, Joji Muramoto, Tim Hartz and Michael Cahn

UCCE Emeritus Farm Advisor, Extension Specialist, Emeritus, Extension Specialist and Irrigation and Water Resources Farm Advisor.

The winter of 2023 had the highest rainfall years in the last 25 years. The high rainfall resulted in flooding onto farmland along the main branch of the Salinas River in both January and March. The flood waters disrupted planting schedules as well as inundated established plantings resulting in a disruption to the beginning of the vegetable production season.

The river also deposited a layer of sediments in flooded fields (Photo 1). The sediments came from several sources: river sediments from as far away as San Luis Obispo County; sediments from side channels; and soil sediments scoured from upstream farms. Several growers and industry personnel have asked what is the composition of these sediments? In April after the flooding had subsided, we collected samples at river crossings from San Lucas to Salinas. The layer of sediment left by the flood waters tended to curled up as it dried out and were easy to collect. Any field soil was brushed from the bottom of the sediments and they were sent to the UC Davis Analytical Laboratory for analysis.

Tables 1 and 2 have analysis of the sediments collected. The data in the table is arranged with sites from south to north; the two side channels, Arroyo Seco and Monroe Canyon are listed separately. Monroe Canyon is the drainage that comes from the west side of Hwy 101 just south of the intersection of Hwy 101 and Central Avenue north of King City; it cuts through a large section of the Monterey shale formation that contains elevated levels of cadmium.

The San Lucas, Arroyo Seco and Monroe Canyon samples are coarser indicating that they were transported by rapid water movement, while the rest of the samples are dominated by silts and clays, indicating that they were transported by slower moving water. In general, there is a good correlation between the clay content of the sediments and nutrient and organic matter content. Higher nutrients in the silt and clay sediments include total nitrogen, calcium, magnesium, sulfate, zinc and iron. The sediments are generally fertile which may indicate that they are at least partially composed of soil eroded from farmed fields farther upstream. Sediments that are low in phosphorus likely originated from non-farmed or vineyard areas.

The elevated cadmium levels measured in sediments from the Arroyo Seco and Monroe Canyon indicate that these side channels carried sediments from the Monterey shale formation which has naturally high levels of cadmium into the Salinas River. Presumably these sediments originating in the Monterey shale formation are transported to areas further downstream by flood waters. 

Photo 1. Sediments deposited in a field along the Salinas River

Table 1. Analysis of river sediment samples from locations from San Lucas to Salinas and two side channel locations.

Table 2. Analysis of river sediment samples from locations from San Lucas to Salinas and two side channel locations.

CropManage Hands-on Workshop 
Bringing Irrigation and Nutrient Management Decision-Support to the Field
Date: Wednesday, March 29th, 2023

9:30 am – 2:30 pm
Location: Animal Services Center

Address: 12425 Monterey Road, San Martin, CA 95046

• Learn how to use CropManage to support irrigation and nutrient management decisions and record-keeping for your crops
• Learn about the latest updates to CropManage
• Learn how CropManage can assist with reporting requirements for Ag Order 4.0

CropManageis a free online decision-support tool for water and nutrient management of vegetables, berry, agronomic, and tree crops. Based on in-depth research and field studies conducted by the University of California Cooperative Extension, CropManage provides real-time recommendations for efficient and timely irrigation and fertilization applications while maintaining or improving overall yield. 

At this free workshop, we will provide hands-on training so that you can learn to use the newest version of CropManage. Crops currently supported include many vegetables (carrots, cabbage, celery, broccoli, lettuce, tomato, spinach, etc.), berry crops (raspberry and strawberry), tree crops (almond, walnut, pistachio, prunes, and pear), and agronomic crops (alfalfa and corn). CropManage is also available in Spanish. 

 
Who should participate? Growers, farm managers, other farm staff, crop advisors, consultants, and technical service providers are welcome. The workshop is for both new and current CropManage users. Spanish translation will be available.  Lunch will be provided.

What to bring? This is a participatory workshop. Please bring a tablet or laptop computer so that you can follow along and participate in the exercises. Each participant will need a user account for CropManage. Please set up a free user account at https://cropmanage.ucanr.edu/ before the workshop. Please arrive early to set up your laptop or tablet computer on the wifi and get logged on to CropManage.

Registration is free: Please register here

by March 28th, 2023. Seats are limited to the first 40 registrants.

Agenda 

Tuesday, February 21;

7:55 a.m. to 12:15 p.m.

1432 Abbott St, Salinas CA

Habrá traducción al Español

Free Workshop + Pizza!

7:55         Introduction

8:00         Tuning up your drip irrigation system: pressure regulation, system design, and scheduling.

Michael Cahn, UCCE Irrigation Advisor, Monterey County

8:30         Basics of pumps, pump tests, and variable frequency drives      

Bill Green, Education Specialist, Center for Irrigation Technology, CSU Fresno

9:05         Practices for improving soil health and its broader impacts        

Richard Smith, UCCE Weed and Vegetable Advisor, Monterey County

9:35         How to Maximize Cover Crop Benefits and Credits in Ag Order 4.0 and Beyond.

Eric Brennan, Research Horticulturist, USDA ARS

10:05       Break

10:20       Strategies for factoring in nitrate in irrigation water in nutrient management plans

Michael Cahn, UCCE Irrigation Advisor, Monterey County

10:40       Status of the Third Party Program for Ag Order 4.0

                Sarah Lopez, Executive Director, Central Coast Water Quality Preservation Inc.

11:00       Getting organized for AgOrder 4.0 

Jillian Flavin and Caroline Webster, Environmental Scientists, Central Coast Regional Water Quality Control Board

11:30       On Farm Experiences: Improving irrigation and Nutrient Management (Grower Round Table Panel)  Karen Lowell USDA-NRCS Moderator (Mark Mason, Huntington Farms, Sergio Casillas, D'Arrigo Bro., Salvador Montes Christensen and Giannini, Eric Morgan, Braga Fresh)

12:15        Adjourn (free Pizza Lunch)

CCA continuing education credits have been requested.  Pre-registration encouraged but not required.

For more information, contact Michael Cahn @ 831-759-7377, email: mdcahn@ucdavis.edu 

The University of California prohibits discrimination or harassment of any person in any of its programs or activities. (Complete nondiscrimination policy statement can be found at  http://ucanr.org/sites/anrstaff/files/107734.doc). Inquiries regarding the University's equal employment opportunity policies may be directed to Affirmative Action Contact and Title IX Officer, University of California, Agriculture and Natural Resources, 2801 2nd Street, Davis, CA 95618, (530) 750-1397; titleixdiscrimination@ucanr.edu.

Richard Smith¹, Eric Brennan², JP Dundore Arias³, Daniel Geisseler⁴, Peter Henry², Danyal Kasapligil⁵, Nicholas LeBlanc², Karen Lowell⁶, Jeff Mitchell⁴, Joji Muramoto⁷, Radomir Schmidt⁴, Kate Scow⁴ and Yu-Chen Wang¹

1 – UCCE Monterey; 2 – USDA ARS, Salinas; 3 – CSU, Monterey Bay; 4 – UC Davis; 5 – De La Valley Labs, Fresno; 6 – NRCS, Salinas; 7 – UC Santa Cruz

Lettuce production in the Salinas Valley has suffered unprecedented losses in the last three years due to infection with Impatiens Necrotic Spot Virus (INSV) and co-infection with soilborne pathogens. Pythium wilt (Pythium uncinulatum) has been the primary soilborne disease associated with INSV, but Fusarium wilt and other diseases have also been observed. Given that soilborne pathogens play a significant role in the observed losses and the lack of effective control measures, growers have been asking if there are practices they can employ to improve soil health that may reduce the frequency and intensity of outbreaks of soilborne diseases in their fields. However, it is not clear if the traditional practices used to manage the soil's chemical and physical characteristics that are known to improve soil health could also impact soilborne disease pressure. This article will briefly explore general strategies to manage soilborne diseases (with specific references to Pythium wilt of lettuce), as well as soil health, and their potential interrelationships.

Resistant varieties

For a plant disease to occur, three factors must be present at the same time: a susceptible host, a virulent pathogen, and a conducive environment. Soilborne diseases, just like any plant disease, need those three factors to occur, and therefore,  all disease management strategies are aimed at disrupting or weakening the disease triangle. One of the most effective management strategies to control soilborne diseases is the use of tolerant/resistant cultivars. Selection of resistant varieties is a promising strategy for addressing Pythium wilt (https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=55733 ) as well as Fusarium and Verticillium wilts in head and romaine lettuce types (https://calgreens.org/wp-content/uploads/2022/04/monitoring-the-population-of-the-lettuce-fusarium-wilt-pathogen-in-california_2022.pdf and https://calgreens.org/wp-content/uploads/2022/04/verticillium-biology-epidemiology_2022.pdf . There may be limits to the effective use of resistant varieties due to the lack of resistance in desirable lettuce types and to the development of new races of the disease that have overcome current varietal resistance (e.g. Fusarium).  As a result, other strategies are also much needed to fight pathogens; here we focus on ways to boost potential benefits of crop rotations and soil biodiversity in fighting soilborne diseases.

Rotations to reduce soilborne disease

Soilborne pathogens use different strategies to persist in the soil such as by 1) Forming chitin-based survival structures ; 2) by saprophytically colonizing plant debris as cellulose-based survival structures; and 3) by colonizing the root tissue of non-susceptible plants .

Crop rotations address the specific survival strategies of a pathogen and may provide specific or general suppression of soilborne diseases. A local example of specific suppression is broccoli's ability to suppress Verticillium wilt in subsequent lettuce crops when grown in rotations. Recent research suggests that broccoli rotations can be enhanced by the addition of crab meal amendment that is rich in chitin compounds to create a "substrate-mediated” microbial community shift which increased fungal antagonists and effectively reduced Verticillium dahlia microsclerotia in the soil by 50 – 78%, thereby reducing crop loss. However, there are only limited examples of rotations in vegetable production that provide specific suppression of soilborne diseases and none that specifically suppress Pythium wilt of lettuce.

Pathogens may be generally suppressed by the rotations with non-susceptible crops because their abundance in the soil usually declines with time. Types of rotational crops used, how frequently they are included in the rotation, and the length of non-host intervals, all influence how much disease suppression can be realized by rotations.  Some soilborne pathogens persist in the soil for a very long time and are challenging to effectively reduce with rotations.  One such example is the white rot of onions and garlic (Sclerotium cepivorum), whose sclerotia can persist for 20-30 years in the soil. In this case, even in the absence of a plant host, the organism will remain, and future plantings of susceptible crops may be infected. Other pathogens like Fusarium and Pythium, may persist in the soil as saprophytes infecting non-living plant tissue and only express themselves as diseases when a susceptible crop is planted. However, recent research on strawberries has shown that inclusion of rotations with weak hosts can result in a reduction in the amount of Fusarium oxysporum f. sp. fragariae. However, at present, this type of effect has not been shown for F.o. lactucae, which infects lettuce.

As the above examples illustrate, rotations can reduce Verticillium and possibly Fusarium, but other important pathogens, such as Pythium wilt of lettuce, have no field-based observations showing impacts of rotations to date.

What is soil health?

While the phrase ‘soil health' has become commonly used to discuss soil management and stewardship it is important to highlight that the phrase “is a metaphor, not a literal scientific construct” (Janzen et al. 2021).  Metaphors like ‘soil health' are used in science to help us understand abstract concepts, because they can make us think about and relate to more familiar things such as our own health.  Janzen et al. define soil health as “‘the vitality of a soil in sustaining the socioecological functions of its enfolding land.' And they add that “soil health reflects not the composition of soil per se, rather its capacity to promote the pertinent functions of the land in which it is embedded. This means that the term has little meaning for a soil divorced from its ecosystem, and that properties conferring such health depend on place and time.”  So for example, an undisturbed sandy soil in the foot hills of the Salinas valley may be considered healthy if one is considering its ability to support beautiful native flowers, but may not be able to support vegetable production unless it is regularly amended with organic matter and fertilizers. In other words, ‘soil health' depends on who is looking. 

Improving Soil Health

Practices to improve soil health for agricultural production mainly revolve around improving carbon management in the soil such as increasing soil organic matter. Sources of carbon to the soil in vegetable production on the Central Coast includes the incorporation of crop residues. For instance, broccoli residue commonly returns 3-4 tons/A of dry biomass to the soil of which 40-42% is carbon. However, given the low C:N ratio of the biomass, mineralization of a large portion of the residue occurs quickly. Inputs of soil carbon can be augmented using cover crops and compost. Cover crops with high C:N ratios are particularly helpful in this regard (https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=55520). In organic production, dry organic fertilizers often contain 30% carbon and, depending on the crops being grown, provide substantial quantities of carbon to the soil.  Soil carbon can also be increased by reducing the loss of carbon from the soil by reducing tillage.

Additions of carbon have beneficial impacts on the soil by increasing the diversity, abundance, and activity of soil organisms by providing their carbon food source and facilitating the building of good soil structure that in turn provides good soil organism habitat. These stimulated beneficial organisms may, in turn, outcompete or predate upon pathogens and reduce impacts on crops.  In addition, the same soil health practices can reduce abiotic plant stress by improving nutrient availability, reducing compaction, and enhancing drainage. These abiotic stresses can predispose plants to disease, as the defenses of weaker plants are more easily overcome. Reducing abiotic stress can lead to improved crop productivity even with pathogens present.

Growers recognize the benefits of increasing additions of carbon to the soil and have used these practices for many years. However, widespread use of over-wintered cover crop (Oct/Nov to Feb/March) has declined due to planting schedule conflicts; use of compost applications has also declined due to food safety concerns regarding the potential for composts to introduce pathogens of human health  impacts. Given the risk of missing spring planting slots with over-wintered cover crops due to wet soils in the spring, some growers are looking at the use of fall-grown cover crops (Aug/Sept to Oct/Nov). This earlier planting window provides an opportunity for growers to grow the cover crops for 50 to 60 days in the fall and incorporate them into the soil before the onset of the rainy season. Fall-grown cover crops can produce 3 - 4 tons biomass/A in this short window which provides a substantial input of carbon to the soil (https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=54986 ).  This is just one creative example of how growers can overcome constraints to the use of practices that improve soil health.

Measures of Soil Health

Numerous measures of soil health have been proposed. Examples include evaluating water infiltration rates, measurements of soil enzymes, evaluating the soil microbial biomass and many, many others. Soil health measures may be chemical, biological or physical. The USDA Natural Resource Conservation Service considers several factors when evaluating soil health. Two of the most critical are – is there enough carbon readily available to support abundant and diverse soil organisms and is the soil well aggregated and free of compaction so that the physical environment of the soil favors soil biological activity. Aside from management factors related to contaminants (e.g. excess salts) or disturbance (e.g. tillage), soil health is largely driven by biological activity in the soil.

Soil organic matter is a standard measure in most soil tests and offers insight into whether management is building or depleting soil carbon. But not all carbon is equally available to soil organisms. Carbon may be in chemical structures that are hard for microbes to break down and access as a food source, for example lignin in woody materials, or in forms with simple chemical bonds easily broken, for example sugars. Carbon exudates from living plant roots offer readily available carbon and is a primary driver for maintaining abundant microbial diversity and activity in the rhizosphere. Thus, an important practice in soil health management is to try to avoid long fallow periods when there are no living roots in the soil.  

Measurement of Soil Carbon

The most straightforward and practical measure of soil health is measuring total soil carbon. The preferred method to measure soil organic carbon is dry combustion. This method, when corrected for soil carbonates, offers the most reliable and repeatable measure of organic carbon (5%). This test is available from many soil testing labs. Other methods for measuring soil carbon such as Loss on Ignition (LOI) and Walkley-Black Wet Digestion measure soil organic matter and then make an estimate of soil carbon based on this measurement. These methods are less accurate (20%) than the dry combustion method. Good reliable measurements of total soil carbon is the cornerstone method for evaluating soil health.

A portion of the total carbon is “active carbon” that is stored in more simple chemical structures that is available to feed soil biological activity. This form of carbon can be measured by the peroxide oxidizable carbon (POX-C) test. It may offer a more robust insight into how well management provides readily available carbon for soil microbes. For example, a long-term organic systems study with vegetable production in Salinas valley found that POX-C increased with frequent cover cropping and compost inputs (White et al. , 2020).  However, this test and many others proposed by the research community are not currently offered by commercial soil labs. This is an indication that the surge of grower interest in soil health is in front of the ability of commercial labs to offer these services. However, commercial labs are responding to demand for soil health measures by growers and certification agencies and this situation may change in the future.

Some soil health assays can be done by the grower: measurements of CO₂ respiration in a 24-hour incubation can be done with purchased test kits (e.g. Solvita). This test gives an indirect measure of the activity of soil microbes by measuring how much CO₂ is evolved from soil in a 24 hour incubation. In addition, measurements of water stable aggregates which is also an indirect measure of soil health can also be conducted by growers using the SLAKES app on their cell phones. The test measures the stability of soil aggregates against falling apart when wetted with water. For more information go to: ( https://doi.org/10.1002/saj2.20012, and https://www.ars.usda.gov/ARSUserFiles/30501000/SoilAggStabKit.pdf ) .

At this point there are no locally calibrated baseline standards for  California growers to compare their soil health test results against. The best approach for a grower is to carry out these tests and watch the trends in the results over time to get a sense if the practices are moving the soil health indicators in a positive direction and whether those changes correlate with positive impacts on crop health and productivity. In almost all cases, it is most helpful to compare results across time and management change in the same field. Inherent soil properties, for example texture, drainage, parent material, etc. may all influence some properties and thus comparisons across fields are less clear cut than those in a single field over time. Given all the research being conducted on soil health, there will undoubtedly be refinements to recommended soil evaluations to assess soil health.

If you choose to send soil samples to a lab out of state, it is important to keep in mind that soil tests are often designed with the local conditions in mind and soils from California my not measure up well when compared with soils from other areas (e.g. lower organic matter levels, higher salts, etc.).

Summary

Frameworks for soil health assessment have been proposed that can help growers identify practical indices that can guide them in assessing the health of their soils. Fundamental to all soil health practices, increasing carbon inputs, is critical to increasing soil microbial activity which may have an impact on suppressing soilborne diseases. Therefore, reliable measurements of soil carbon with the dry combustion test can provide a basic understanding of the impact of management practices on soil health. The impact of soil health practices on soilborne diseases still needs to be examined on a disease-by-disease basis, as undoubtedly each disease will respond differently to beneficial practices. Pythium wilt of lettuce is proving to be an elusive and difficult to understand pathogen; much more work remains to be done to understand biological processes that can effectively suppress it.

The soil health field is an area of active research by scientists collaborating with growers to test the concepts discussed above and to find ways to optimize suppression of soilborne diseases. These crucial collaborations will help us better understand the directions we need to go in to get a better handle on these issues. 

For additional reading:

Soil Borne Diseases : Soil Health (ucdavis.edu)

Janzen H.H., D.W. Janzen, E.G. Gregorich. 2021. The ‘soil health' metaphor: Illuminating or illusory? Soil Biology and Biochemistry 159:108167.