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Nitrogen Regulations in California and the Certified Crop Adviser’s Role

By
Mark Cady
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California Certified Crop Advisers (CCAs) are an integral part of the nitrogen management compliance picture (all photos by Vicky Boyd.)

In the last 10 or more years, water quality regulations that address nitrate in groundwater have expanded dramatically. Starting in 2012, the regulatory agencies charged with protecting California’s water quality have increased their scrutiny of and demands on agriculture. So, it is essential for crop consultants to understand the regulations and how regulations affect their customers.

 

Regulatory History

The challenges associated with nitrate in groundwater and its sources have been recognized for at least a generation. In 1987, the California State Legislature directed the State Water Resources Control Board to prepare a report on nitrate contamination in drinking water. The convened expert panel reported that agriculture was likely an important contributor to nitrate in groundwater.

In 2012, however, the regulatory landscape changed dramatically. First, a major study of nitrate in California drinking water was published by UC Davis. This sprawling effort, titled Addressing Nitrate in California’s Drinking Water, focused on the Tulare Lake Basin and the Salinas Valley. The multi-volume report was produced by the UC Davis Center for Watershed Sciences. It showed that nitrate problems would likely worsen for the next several decades and that most nitrate currently in drinking water wells was applied to the surface decades earlier. An important conclusion of the report was that agricultural fertilizers and animal wastes applied to cropland are by far the largest regional sources of nitrate in groundwater. Thus, in the last decade, the State and Regional Water Boards have been more assertive in regulating agricultural contributions to groundwater nitrate.

The Central Valley Irrigated Lands Regulatory Program (ILRP) started in the first part of this century with a focus on pesticides in surface water. That focus expanded in 2012 when the ‘Waste Discharge Requirements for the Eastern San Joaquin River Watershed (ESJ) General Order’ was first adopted by the Central Valley Regional Water Quality Control Board (CVRWQCB). This order required grower reporting of nitrogen fertilizer applied to cropland and estimates of the nitrogen removed with harvested crops, so the efficiency of nitrogen fertilizer use could be calculated. Growers record this information in their Nitrogen Management Plans (NMPs) as specified by the CVRWQCB. The reporting of NMP data was carried out through the ESJ Water Quality Coalition, a grower-led intermediary that anonymized the data and provided statistical analysis on a township basis. A component of the statistical analysis is identification of outlier values (i.e., parcels where the nitrogen efficiency is low relative to others in township.) These outlier growers are then targeted for outreach and increasing reporting requirements. Growers in all regions of the Central Valley are represented by coalitions. Nitrogen reporting requirements are now in place for all Central Valley regions and crops with the exception of rice.

In 2018, the State Water Board stepped into the picture and revised the ESJ Order to include new provisions to be precedential to all regional boards. The precedents adopted include reporting of nitrogen application (A) to and removal (R) from cropland, reporting of irrigation water used, testing on-farm domestic wells for nitrate and reporting nitrate exceedances to the well users. With these new requirements, the NMPs became the Irrigation and Nitrogen Management Plans (INMPs). The regional boards were directed to use both A/R and A-R (the nitrogen efficiency ratio and the total excess nitrogen, respectively) to evaluate compliance. All regional boards are required to adopt these precedents into orders by February 2023. The CVRWQCB updated its ILRP orders in 2019.

Another major nitrate-related regulatory effort in the Central Valley is the Central Valley Salinity Alternatives for Long-Term Sustainability (CV-SALTS). This is a multi-stakeholder effort that seeks to manage the long-term loading of salts in the Central Valley. Of interest here is the focus on nitrate. The CVRWQCB adopted the regulations proposed by the CV-SALTS stakeholders in 2018. The goals of CV-SALTS regulations are “1) to ensure a safe drinking water supply; 2) to achieve balanced salt and nitrate loadings; and 3) to implement long-term and managed aquifer restoration programs where reasonable, feasible and practicable.”

While the CV-SALTS process affects all discharges in the Central Valley, each of the above goals represent challenges to growers in terms of costs of compliance and improving nitrogen fertilizer management. Fortunately for growers, the administration of regulatory requirements is handled by the coalitions that were established for the ILRP.

Looking again at 2012, the Central Coast Regional Water Quality Control Board (CCRWQCB) adopted its first order to require reporting of nitrogen applications. This information was reported directly to the Regional Water Board. Just this year, the CCRWQCB adopted the updated “Ag Order 4.0,” incorporating the ESJ precedents and setting long-term limits on excess nitrogen applied to crops. Farming operations must now submit information on nitrogen applied to and removed from cropland. The order includes a schedule of targets for excess nitrogen fertilizer roughly defined as A-R. After 2026, specific excess nitrogen targets are in place for all crops. Those targets rachet down from 300 lbs/ac in 2026 to just 50 lbs/ac in 2050. For reference, the CCRWQCB estimates that currently only approximately 6% of the acres of high-value crops meet the 2050 benchmark.

The situation in the Central Valley is different than in the Central Coast. The Central Valley coalitions have developed a methodology to determine what those targets should be on a township basis. The methodology, a sophisticated modelling effort for the entire Central Valley, has been approved by the CVRWQCB Executive Officer and is scheduled to produce target excess nitrogen values in 2023.

Irrigated nitrogen management is a crucial area for CCAs.

 

The Crop Adviser’s Role

California Certified Crop Advisers (CCAs) are an integral part of the nitrogen management compliance picture. The CVRWQCB determined that CCAs who received special training in nitrogen management are qualified to certify growers’ INMPs. There are now nearly 900 CCAs eligible to certify INMPs.

For several years, CCAs became eligible to certify Central Valley growers’ NMPs through training received via a day-and-a-half in-person conference given once a year and presented by UC faculty. Funded by the CDFA Fertilizer Research and Education Program (FREP), this successfully certified nearly 1,000 CCAs. On October 1, 2020, the certification program changed to a Nitrogen Management Specialty category managed by the International Certified Crop Adviser organization. All CCAs who had the nitrogen management certification were “grandfathered” into the new Nitrogen Management Specialty category. CCAs who had not yet been certified now must take the Nitrogen Specialty category exam to be qualified to sign INMPs. UC staff developed online training modules to assist CCAs in passing the specialty exam. The training is available to any CCA and provides 16 continuing education units (CEUs) for a cost of $120. More information regarding the online training and the specialty exam can be found at certifiedcropadvisor.org/ca-nsp/. CCAs who have the California Nitrogen Management Specialty (CA-NSP) category must obtain eight CEUs in nutrient management and seven CEUs in soil and water management over two years but are still only required to obtain 40 total CEUs to maintain their certification. There is an additional fee required to maintain the CA-NSP.

CCAs may find that certifying INMPs, especially the irrigation portions of the forms, moves them out of the nutrient management comfort zones. Because we all recognize the importance of irrigation management in nitrogen management, we can also realize that it’s a crucial area for CCAs to step into. Information regarding anticipated crop evapotranspiration (ETc) and irrigation water to be applied is required in the INMPs.

Instructions provided with the plans suggest that the UC or coalition may provide the information to complete the ETc question, but the data are not readily available. To resolve the conflicts involving ETc determination, a statewide project was funded to create an accepted clearinghouse of coefficient values for the major crops in California. The project is nearing completion and should go a long way to helping CCAs provide accurate answers for the ETc questions in the INMP. Alternatively, this year, a project involving the National Aeronautics and Space Administration (NASA), the Environmental Defense Fund and a host of other partners will make ETc data available through interactive maps on the web. The project is called OpenET and is set to be released by the end of the year.

FREP is holding its annual conference this year in San Luis Obispo from October 26-28. There will be sessions on OpenET, ILRP water quality coalitions and other nutrient and irrigation management topics. For more information, see cdfa.ca.gov/is/ffldrs/frep/FREPConference.html.

Mark Cady is currently supervisor of the CDFA FREP. His understanding of water quality regulation comes from four years as an environmental scientist with the CVRWQCB. FREP’s role is to fund and facilitate research and education to advance the environmentally safe and agronomically sound use and handling of fertilizing materials. Please visit cdfa.ca.gov/is/ffldrs/frep for more information.

Protect Almond Tree Roots at Planting

By
Cecilia Parsons
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Irrigation systems for new orchards should be designed to ensure a wetting pattern that promotes good root distribution.

Protecting the roots of young almond trees at planting is a vital step toward long-term tree health.

At the 2021 San Joaquin Valley Almond Day, UCCE Farm Advisors Mae Culumber and Brent Holtz outlined care for new trees during orchard establishment.

If possible, avoid planting on hot, windy days, Culumber said. The small root hairs on dormant trees can dry out quickly when exposed to air. Since roots store carbohydrates needed to support new growth, the trees should be handled and planted carefully. Roots should not be pruned prior to planting unless they are damaged. Roots can also be treated with Galltrol before planting to prevent crown gall infections.

The holes for the new trees should be wide and deep enough so roots are not cramped and can spread. The largest, strongest roots should be oriented in the direction of the prevailing wind. The highest root should be slightly higher than the soil line, but covered with soil. Soil should be tamped down around the tree to eliminate air pockets.

After the trees are in place, Culumber said 1 to 3 gallons of water should be used to soak the soil around the tree.

Irrigation systems for new orchards should be designed to ensure a wetting pattern that promotes good root distribution. In the weeks and months ahead, over-irrigation should be avoided as saturated conditions kill small roots due to poor aeration. Over-irrigation also creates conditions favorable to phytophthora.

There is potential to lose irrigation efficiency when larger soil surfaces are wetted. With soil-applied nutrients, efficiency of the delivery system is dependent on water delivery.

Culumber explained one approach is to run tubing for button emitters. Using a riser with two outlets, one button is placed a half-foot from the center of the trunk at a rate of one gallon. Two more buttons are placed two feet from the first at two gallons per hour.

For second leaf trees, two more buttons are added at 8.5 gallons per hour. At third leaf, tubing is run for micro sprinklers with a pressure compensator in between each tree with a rate of up to 18 gallons per hour.

Holtz reported in his whole orchard recycling studies that young trees experience less water stress planted where wood chips are incorporated back into the soil due to increased moisture holding ability.

In WOR plantings, Holtz said the carbon to nitrogen ratio can become unbalanced and early N applications are important.

This drawing supplied by UCCE Farm Advisor Brent Holtz illustrates the correct procedure for planting almond trees on berms.

Water Stress Contributes to Smaller Vineyard Canopy

By
Cecilia Parsons
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Small canopy while clusters are at bloom indicates stunted early canopy growth due to a dry winter. Typically, the small canopy can recover after irrigation events; however, yield will be significantly reduced due to excessive cluster shatter (photo courtesy G. Zhuang.)

Smaller-than-normal canopies in wine grape vineyards are a common sight in the San Joaquin Valley this year due to lack of irrigation water.

George Zhuang, UCCE Fresno County viticulture advisor, said the dry winter and spring contributed to stunted growth and production issues growers are seeing in this year’s wine grape crop.

“In some varieties, we are seeing shatter in clusters and yield losses,” Zhuang said.

Postharvest irrigation is important every year to relieve water stress and help the vines store carbohydrates for next year’s production. This year, it is critical. The goal of postharvest irrigation is to avoid delayed, erratic bud break and ensure canopy growth the next growing season. It will also help with production and fruit quality, Zhuang said.

This year, securing enough water for postharvest irrigation may not be possible. Zhuang said growers and vineyard managers will have to decide if they want to apply water after harvest or save it for next spring. If the Valley experiences another dry winter, the water will help next year’s growth. Stressing the vines postharvest will have a carryover effect into the next year, he said.

If irrigation water is available postharvest, the goal is to maintain a photosynthetically functional canopy, but avoid overwatering to prevent vines from pushing new growth.

At least 10% of seasonal irrigation water should be applied postharvest. For early season wine grape varieties with a longer postharvest growth period, more water may be needed if hot temperatures persist. To ensure a more even bud break and an adequate carbohydrate reserve, irrigation, if possible, should continue until leaf senescence.

Adequate soil moisture postharvest and during dry winters will also hydrate vines, prepare them from cold hardiness and help with even budbreak the following season.

Fertilizer applications can be made in the fall to replace annual loss from harvest. It is important that the canopy is active to assimilate nitrogen applications. Early season varieties will have a longer period where this is possible.

Postharvest is a good time to apply soil amendments, including sulfur and gypsum, to adjust soil pH and improve infiltration.

If there is a positive to this year’s drought conditions, it is the lower pest and disease pressure in vineyards. This year, Zhuang said, there has been less fungal disease due to the smaller canopies. Vine mealybug has also declined in water stressed vineyards due to less vigor in the grapevines.

Keeping Pathogens Out of Produce Fields

By
Cecilia Parsons
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Vegetative barriers are narrow and parallel strips of stiff, dense vegetation planted on or close to the contour of slopes or across concentrated flow areas. These barriers can slow runoff water and have a filtering effect (photo by Ayanna Glaize, North Carolina State University.)

One of the pathways for foodborne bacterial pathogens such as E. coli, salmonella and campylobacter to contaminate fresh produce is proximity to livestock operations. Research has shown that vegetative barriers between livestock and produce fields could reduce foodborne pathogen transmission.

Most foodborne disease outbreaks can be traced back to contaminated fruits and vegetables, especially leafy greens. A research study conducted by North Carolina State University showed there was significant transmission from animal operations to fresh produce on farms where both livestock and crop systems were integrated.

Multiple pathways allow for transmission of foodborne pathogens. They include environmental sources, such as contaminated surface runoff water, insect and air transmission due to proximity of animals to fresh produce, manure contamination of irrigation systems and improper food handling.

Vegetative barriers have been proposed as a possible solution to transmission of foodborne pathogens.

Vegetative barriers are narrow and parallel strips of stiff, dense vegetation planted on or close to the contour of slopes or across concentrated flow areas. These barriers can slow runoff water and can have a filtering effect. They also loosen the soil, allowing for water penetration. The foodborne pathogens E. coli and salmonella can contaminate fresh produce through wind transmission and through surface runoff water, the study reported. Planting vegetation barriers can effectively reduce flow of runoff water and act as a wind barrier that traps spray droplets from animal operations, intercepting them and preventing them from reaching fields were produce is grown.

The study showed evidence of decreased rates of contamination of fresh produce by E. coli and salmonella from animal production when vegetative barriers were used. The study involved a five-layer vegetative barrier constructed between a dairy, a poultry operation and a produce farm. Fresh produce, manure and environment samples were collected over 15 months and tested for the level of E. coli and salmonella.

The results showed that only 18% of the total E. coli and salmonella samples isolated were present in the fresh produce after installation of vegetative barriers.

The study also noted that while the barriers are effective, other pathways, including manure applications on fields and improper food handling, can still lead to contamination.

Correctly Identify Mites in Strawberries

By
Cecilia Parsons
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Lewis mites on a strawberry leaf. Lewis mites have smaller spots that run along each side of their bodies. Identification of mite species is important for effective control (photos by Surendra Dara, UCCE.)

With mites in strawberries, it is important to correctly identify the species to achieve effective control.

At the 12th-annual Santa Maria Strawberry Field Day webinar, UCCE Santa Cruz Farm Advisor Mark Bolda reported on two strawberry pests, two-spotted mite and Lewis mite, and how to distinguish the two. This is important, Bolda said, because Lewis mite is not apparently responsive to the same materials and predators as two-spotted mite.

While the two-spotted mite generally has two larger dark spots on its back, Lewis mite has smaller dark spots that run along each side of the body. Lewis mite eggs are smaller than two-spotted mite eggs.

Lewis mite was found in Ventura-area strawberry production about ten years ago and it first appeared in the Salinas-Watsonville area in the last three years.

Feeding by Lewis mite kills leaves on strawberry plants and reduces yields. Both two-spotted mites and Lewis mites gather at the underside edge of leaves. Bolda said Lewis mite damage causes the leaves to have a purple tinge, and the damage tends to spread more slowly across a field. It is common to see both species on the same leaf at the same time.

Lewis mite has one larval and two nymphal stages prior to the adult stage. Eggs are laid on the leaf edges and larvae emerge after three days. Colonies are often found at leaf edges or veins.

A full life cycle of Lewis mite is about 14 days at 77 degrees F, according to UC IPM guidelines. The cycle can take fewer days at warmer temperatures. Two-spotted mites’ full lifecycle takes about five days at 75 degrees F.

Bolda noted that in mid-winter coastal strawberry growing areas, it is unusual for a large percentage of mites to become dormant. Instead, they continue to grow and lay eggs, although at a slower pace during the winter months.

In an efficacy study, Bolda said the most promising materials were tested first in a lab setting to get an idea of their control level.

The list included Oberon at 16 fl. oz. per acre; Vestis, a surfactant, at 13 fl. oz. per acre; Aza-Direct at 32 fl. oz. per acre; Agrimek at 16 fl. oz. per acre; and Nealta at 13.5 fl. oz. per acre. Ten leaves per treatment replicate were evaluated under a microscope. Adults and eggs were counted. Treatments were made January through mid-April. Numbers of Lewis mite were low until the end of March. Materials with the most efficacy were Vestis, Aza-Direct and Agrimek.

Complete information on this trial can be found on Bolda’s blog.

The Crop Consultant Conference Returns as In-Person Event Over Two Days

By
PCC
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Progressive Crop Consultant Magazine’s popular two-day Crop Consultant Conference will return this year as a live conference and trade show, featuring seminars worth 10 hours of CCA and 8 hours of DPR continuing education credits, a live trade show, and the presentation of Western Region CCA Association’s popular CCA of the Year Award and honorariums and scholarships. The Crop Consultant Conference will be held on Sept. 16 and 17 at the Visalia Convention Center.

The Crop Consultant Conference has become a premier event held in the San Joaquin Valley each September for Pest Control Advisors and Certified Crop Advisers. Co-hosted by JCS Marketing, the publisher of Progressive Crop Consultant Magazine, and Western Region Certified Crop Advisers Association, the event brings industry experts and suppliers, researchers and crop consultants together for two days of education, networking and entertainment.

“We are excited to be back to doing our events in person, and expect another sell-out event for crop consultants in the Western United States,” said JCS Marketing Publisher and CEO Jason Scott. “Agriculture is a relationship-driven business and there is no substitute for live events.”

Topics for the two days of seminars include: Various seminars on managing pests and diseases in high-value specialty crops, tank mix safety and regulations, fertilizer management, soil health, new technology, new varieties and rootstocks and their impact on tree nut pest management.

The conference will conclude with two one-hour panels offering hard-to-get CCA credits and moderated by Western Region CCA related to nitrogen monitoring, use, application and management as well as the various regulatory requirements around irrigated nitrogen management.

Registration fees for the two-day event are $150, or less than $15 per CE unit. Pre-registration is required and can be done at progressivecrop.com/conference.

Click Here to Register Now

Minimizing Drift from Orchard Spray Application by Spray Backstop System

By
Alireza Pourreza
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Cotton ribbon stretched around two rows of trees for continuous loop sampling.

The use of pesticides can be very effective in protecting trees from pests and diseases. However, many times this is also accompanied by negative impacts on humans and the environment. Off-target movement of chemical spray has always been a challenge for growers because it can contaminate the environment, reduce spray efficacy and impose liabilities.

California has stringent pesticide laws and regulations and orchard spray application is considered a high-risk operation. California law establishes a buffer between schools and any pesticide spraying location. Growers are required to notify the public when they spray pesticides. This includes schools, daycare facilities, and county agricultural commissions.

A prototype of the spray backstop system developed at the Digital Agriculture Lab at UC Davis.

Spray drift can be reduced by choosing the right type of nozzle, adjusting and calibrating sprayer settings, defining shelter zones and specifically modified practices in the downwind rows. Reducing the movement of spray droplets to sensitive areas might be accomplished by these methods, but they are in clear contrast with strategies that lead to a uniform on-target deposition. For instance, spraying with larger droplets can reduce the amount of drift, but it also decreases the effectiveness of the spray at higher parts of a tree. Likewise, reducing airflow results in reduced drift; however, it also diminishes the spray efficacy in zones farther from the sprayer, for example, on treetops. Drift can also be reduced when we use slower ground speed and higher liquid flow rate, but the impact is not significant.

Thermal view of spray cloud escaping the canopy from the top.

Spray Backstop

At the Digital Agriculture Lab at UC Davis, a sprayer attachment system called Spray Backstop was developed to minimize drift potential and possibly improve spray coverage on the treetops. The backstop system is a screen structure that can be raised above the trees using a foldable mast.

A test was conducted in a mature almond orchard to determine how much drifting could be reduced with the backstop system. A cotton ribbon loop was stretched around the trees to quantify the droplets scaping the tree canopy. The ribbon could capture all spray droplets that did not deposit on-target and were not blocked by the backstop system.

The orchard was sprayed with a mix of water and fluorescent dye. The ribbon was cut into sub-samples and analyzed with the fluorometry method. Comparing the ribbon samples from the test with and without the backstop showed that the Spray Backstop system could effectively block the spray droplets escaping the canopy from treetops or sides and reduce drift potential by 78%. Leaf samples were also collected from trees in both spray application conditions and analyzed by the fluorometry technique. Unlike the conventional drift control methods, using the spray backstop system does not change overall canopy deposition and could also help improve deposition on treetops.

A sprayer working in a young almond orchard in Northern California (all photos courtesy Digital Agriculture Lab.)

Adopting the spray backstop system into the orchard spray application practice will reduce the environmental degradation while protecting residential areas and schools from exposure to chemicals. On the other side, growers can adjust their sprayer for more air and finer droplets (that will improve spray coverage and efficacy in the upper canopy area) without being concerned about drifting because the backstop system can stop spray droplet movement above the trees. A uniformly applied treatment will significantly reduce the risk of crop failure due to pests and diseases.

The spray backstop system is simple and could be easily implemented without significant modifications to the grower’s spray rigs. This system can help growers to be compliant with pesticide regulations and maintain good environmental stewardship. You can find more information about the spray backstop project at the digital Agriculture lab website: digitalaglab.ucdavis.edu.

Spray backstop blocking the spray cloud from moving upwards.

Identifying the Potential and Impacts of On-Farm Groundwater Recharge

By
Jeanette Warnert
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On-farm recharge has the potential to clean up groundwater that has been contaminated with nitrogen and/or pesticides (photo by H. Dahlke.)

Aquifers have become depleted from decades of overuse. Drilling deeper is an option for farmers, but prohibitively expensive for low-income residents in disadvantaged communities in the San Joaquin Valley.

A UC scientist believes managed aquifer recharge on agricultural lands close to populations with parched wells is a hopeful solution.

Helen Dahlke, professor in integrated hydrologic sciences at UC Davis, has been evaluating scenarios for flooding agricultural land when excess water is available during the winter in order to recharge groundwater. If relatively clean mountain runoff is used, the water filtering down to the aquifer will address another major groundwater concern: nitrogen and pesticide contamination.

“The recharge has the potential to clean up groundwater,” she said.

Five years ago, UCCE Specialist Toby O’Geen developed an interactive map (casoilresource.lawr.ucdavis.edu/sagbi/) that identifies 3.6 million acres of California farmland with the best potential for replenishing the aquifer based on soil type, land use, topography and other factors. Dahlke and her colleagues analyzed the map and identified nearly 3,000 locations where flooding suitable ag land will recharge water for 288 rural communities, half of which rely mainly on groundwater for drinking water. The research was published by Advancing Earth and Space Science in February 2021.

“If we have the choice to pick a location where recharge could happen, choose those upstream from these communities,” Dahlke said. “Recharge will create a groundwater mound which is like a bubble of water floating in the subsurface. It takes time to reach the groundwater table. That bubble floating higher above the groundwater table might just be enough to provide for a community’s water needs.”

 

Filling Reservoirs Under the Ground

Many climate models for California suggest long-term precipitation amounts will not change; however, the winter rainy season will be shorter and more intense.

“That puts us in a difficult spot,” Dahlke said. “Our reservoirs are built to buffer some rain storms, but are mainly built to store the slowly melting snowpack in the spring. In the coming years, all the water will come down earlier, snowmelt likely in March and April and more water in winter from rainfall events.”

She is working with water districts and farmers to consider a change in managing water in reservoirs.

“We want to think about drawing reservoirs empty and putting the water underground during the fall and early winter. Then you have a lot of room to handle the enormous amounts of runoff we expect when we have a warm atmospheric river rain event on snow in the spring,” she said. “However, farmers are hesitant. They like to see water behind the dams.”

Interest in groundwater banking has been lifted with the implementation of the 2014 Sustainable Groundwater Management Act (SGMA). The law requires governments and water agencies to stop overdraft and bring groundwater basins into balanced levels of pumping and recharge by 2040. Before SGMA, there were no statewide laws governing groundwater pumping, and groundwater was used widely to irrigate farms when surface supplies were cut due to drought.

“For some of the drought years, overdraft was estimated to be as high as nine million acre-feet a year,” Dahlke said.

Dahlke believes wintertime flooding for groundwater recharge can help water districts meet SGMA rules. “We have to do anything we can to store any surplus water that becomes available to save it for drier times, and our aquifers provide a huge storage for that,” she said.

Helen Dahlke, professor in integrated hydrologic sciences at UC Davis, has been evaluating scenarios for flooding agricultural land when excess water is available during the winter in order to recharge groundwater (photo by Joe Proudman, UC Davis.)

 

Farming Impacts

The Dahlke Lab is collaborating with UC ANR farm advisors and specialists as well as scientists at other UC campuses to learn about agronomic impacts of flooding a variety of agricultural crops, including almonds, alfalfa and grapes.

In the San Joaquin Valley, UCCE Irrigation Specialist Khaled Bali led an intermittent groundwater recharge trial on alfalfa. The researchers applied up to 16 inches per week with no significant impact on alfalfa yield.

“You could do groundwater recharge in winter and then turn the water off completely and still get a cutting or two of alfalfa before summer,” he said.

This past winter, Dahlke was prepared to flood 1,000 acres of land with water from the Consumnes River. Even though winter 2020-21 was another drought year, the research will go on. Her team was able to flood a 400-acre vineyard and, in collaboration with scientists from UC Santa Cruz, deploy sensors in the field to measure infiltration rates to better understand whether sediment in the flood water could clog pores in the soil. Her team also collaborates with Ate Visser of Lawrence Livermore National Laboratory in using isotope and noble gas data to determine the groundwater age and flow.

The Dahlke Lab’s groundwater banking project has planned more studies in groundwater basins across the state to close knowledge gaps on suitable locations, technical implementation and long-term operation. They also plan to address operational, economic and legal feasibility of groundwater banking on agricultural land.

Mitigating Tree Nut Stress and Disease Requires a Multi-Pronged Irrigation Approach

By
Matt Comrey
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Flush lines regularly to prevent clogging, particularly after fertilizer injections (all photos courtesy Wilbur-Ellis Agribusiness.)

Nut growers are essentially paid in two ways: they can either produce higher yields or reduce the number of deductions from the processor. When nut trees aren’t receiving enough water, you’ll see an increased number of blanks, shriveled kernels and pinched kernels. It’s also common to see more disease and mite activity in the orchard. All of these issues can hurt final nut grade and yield, which ultimately affects grower payouts from the processor.

During this tight water year, it’s more important than ever to help growers implement a science-based approach to reduce the risk of disease and maximize orchard yields. In years of drought, salinity levels in the soil are going to rise, making it critical to keep orchards at proper field capacity so that trees do not suffer from physiological drought stress.

This is not as simple, however, as ensuring the right amount of water is applied to the orchard. Growers are going to be pressured to match water deliveries to crop stage and demands throughout the season, rather than honing in on evapotranspiration (ET) rates.

So the question for CCAs and PCAs is, “How do we help growers implement a strategic irrigation schedule?” My answer starts and ends with measurement. When you ask growers how much water they’re putting out, they often come back with a number of hours irrigated. But this number does not tell you how much water is being applied to an orchard; that can be drastically different based on application rates.

The first step in making the most of irrigation is working with your growers to understand how many inches of water per acre of soil the irrigation system puts out per hour. This piece of information is often overlooked but is essentially the backbone of a good irrigation schedule. Everything else should be calculated based on that application rate. When water is limited, it’s also helpful to know the exact depth of soil that needs to be wetted to avoid overirrigating.

An individualized irrigation plan and schedule takes into account a grower’s water quality and goals.

 

Measuring Orchard Demands

After the exact application rate is determined, growers can start designing an irrigation schedule that meets the needs of their orchards.

Irrigation scheduling maximizes the use of available water by applying the exact amount of water needed to replenish soil moisture. This practice offers growers numerous potential benefits:

  • Possibly reduces the grower’s cost of water through more efficiently timed irrigations.
  • Lowers fertilizer costs by limiting surface runoff.
  • Maximizes net returns by increasing crop yields and crop quality.
  • Aids in controlling root zone salinity accumulation through controlled leaching.
  • Decreases common disease pressures.

I once worked directly with a walnut grower who was able to increase edible yield by 4% just from having a better understanding of his irrigation output rates and adopting a more scientific irrigation approach. By measuring crop-water demand and optimizing water usage, the grower saw dramatic benefits in nut quality across the board, including both color and size advantages.

There are essentially three approaches to water management, and growers should use at least two of the following to design an irrigation schedule:

 

Soil-Based Approach

Soil-based methods are simply measuring how much water is being stored in the soil. If there is less water in the soil, and the water is held at a greater tension, it will be more difficult for the trees’ roots to take up that water, resulting in tree stress.

While irrigating based on the feel or appearance of the soil is a commonly used method, more precise measuring tools such as tensiometers, probes and electrical resistance blocks will provide more specific information that growers can utilize to produce healthier, more consistent nut crops.

Like the other approaches mentioned below, soil moisture readings can be used by themselves to schedule irrigations, but they are most beneficial when used in combination with other methods of irrigation scheduling.

Probes, for example, are very good at giving the user a snapshot of what the soil moisture looks like at varying depths. It will determine how deep in the soil profile water is infiltrating and ensure calculated water applications are not over- or under-irrigating trees. However, these probes do not tell you how often or how long to irrigate. Combining the data from the probes with ET measurements on a regular basis will provide enough information to make a strategic irrigation schedule.

 

Climate-Based Approach

This approach requires a deeper understanding and balancing of irrigation application rates and ET. This is the combination of transpiration, or water evaporation from plant leaves, and evaporation from the soil surface.

Many factors affect ET, including air temperature, humidity and wind speed; soil factors such as texture, structure and density; and plant factors such as plant type, root depth and canopy density, height and stage of growth.

Reference ET information is available from a variety of sources, but most well-known is the California Irrigation Management Information System (CIMIS) network of nearly 100 California weather stations that provide daily ET values.

Knowing how closely the amount of irrigation water plus rainfall matches estimates of real-time orchard ET can help make irrigation scheduling decisions, especially if this information is teamed with other measurement approaches.

 

Plant-Based Approach

Plants respond in different ways to keep their water supply and demand in balance, and most plant-based methods for irrigation management are based on the measurement of one or more of these responses.

The pressure chamber method for measuring the tension of water within the plant has been shown to be a reliable and commonly used measurement of stress in orchards.

One drawback of using a pressure chamber is that the tool doesn’t distinguish between types of stress. If you are utilizing a pressure chamber in a particularly weak or diseased portion of the orchard, it will not decipher between stress due to poor water supply, salinity and/or pests. This is why it’s important to measure crop-water demands with a climate- or soil-based approach in addition to the pressure chamber approach.

By using ET and a pressure chamber, we found that one almond grower was putting out the right amount of water in the orchard, but the watering interval was too wide. Once the water interval was shortened, trees experienced less stress and were able to produce a much better crop.

I always encourage growers to utilize at least two of the three water management approaches because in the orchard, data or feedback you get from each approach doesn’t always line up perfectly. There’s always some subjectivity. Carefully evaluating the information provided by multiple sources can help determine the best irrigation schedule for your growers’ orchards this year.

 

Maximizing Irrigation Scheduling ROI

There is not a one-size-fits-all irrigation approach or schedule, and choosing which tools to use depends on water quality available to the grower and operation goals.

Combining a plant-based approach with a climate-based approach has been the most effective in my experience, but there are certainly benefits to monitoring soil moisture status in combination with either of the other two methods .

Companies like Wilbur-Ellis Agribusiness are beginning to offer irrigation scheduling services (Figure 1), that include weekly plant stress readings, soil moisture conditions and watering interval recommendations based on replacement ET.

Figure 1: Soil monitoring reports can help growers and crop advisors understand what is happening with soil moisture in real time.

These programs have been shown to reduce drastic swings in irrigation protocols, which in turn reduces pest and disease pressures on orchards. These programs also allow growers to focus on other aspects of irrigation and orchard management rather than worrying about interpreting results.

Another critical piece of irrigation management to keep in mind is system maintenance, which includes in-season flushing. Keeping a clean system with good distribution uniformity is essential when growers are trying to get the most out of orchard irrigation scheduling (Figure 2).

Figure 2: System maintenance is key to more efficient water delivery.

 

Key In-Season Flushing Protocols

  • Regularly flush laterals (monthly, bi-weekly if needed).
  • Flush complete system after fertilizer injections.
  • Flush from larger to smaller lines; mains and submains, then laterals.
  • When flushing lateral lines, ensure proper velocity and volume to purge contaminants.
  • Never open more than five to eight laterals at a time, as additional open lines will reduce the velocity of flow, which reduces the effectiveness of the flush (dependent upon the total number of laterals per block).
  • Always note pressures and flows at the initial system start-up. Changes in these parameters often indicate in-season issues.

Cover Cropping to Achieve Management Goals

By
Shulamit Shroder and Jessie Kanter
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Honey bee on brassica flower in Shafter (all photos courtesy S. Shroder.)

Cover crops can provide many benefits to growers, like improving water infiltration and reducing nutrient loss. However, growers in California’s southern San Joaquin Valley worry that the lack of consistent winter rainfall and high cost of water in the area make cover cropping impractical (Mitchell et al. 2017).

In this article, we’ll discuss how well different cover crop mixes suppressed weeds, provided resources for beneficial insects and improved water infiltration. We’ll also delve into water requirements and effects on soil nitrate.

Irrigated (left) and non-irrigated (right) soil builder mix in Shafter on March 11, 2021.

Location of Trials & Cover Crop Species Selection

These trials took place at UC-managed research farms in Shafter (Kern County) and Parlier (Fresno County). The Parlier research farm is the Kearney Agricultural Research and Extension Center and will be referred to as “Kearney” for the remainder of this article.

Four common cover crop mixes were planted at Kearney and five mixes were planted at Shafter (Table 1). Three of the mixes were similar in both locations. We worked with Kamprath Seeds to select cover crop species based on mixes that had done well in a demonstration trial in Shafter the previous year. Two beginner-friendly simple mixes were also evaluated.

Based on the results of our trial, we have outlined some suggestions for trying out cover crops on your farm. Our suggestions depend on your goals for planting cover crops and your concerns about fitting them into your existing cropping system.

 

Goal: Weed Suppression

Based on the trial results, if your goal is weed suppression, you may consider the following.

Grasses appeared to be most effective at suppressing weeds, especially Merced rye which grew vigorously in both irrigated and non-irrigated plots at both locations. Brassicas also contributed to weed suppression, while legumes were least competitive with weeds, especially at Kearney.

If your goal is weed suppression, consider higher percentages of grasses and brassicas in your seed mix.

Irrigated (left) and non-irrigated (right) barley & common vetch in Shafter on March 11, 2021.

Higher seeding rates led to greater weed suppression. If weed suppression is your goal, consider seeding at rates higher than recommended.

If it is not realistic to purchase enough seed for higher seeding rates, pre-irrigating before the first rain to allow weeds to germinate, followed by cultivation prior to planting, can help knock back weeds that may compete with cover crops at germination.

If pre-irrigation is not possible, consider waiting for weed seeds to germinate after the first decent rain, then cultivating your field before planting your cover crop.

However, try to not wait too long, as colder temperatures may inhibit germination. This may have been another reason why cover crop establishment was not as vigorous at Kearney; we planted on December 18 when the high temperature was 53 degrees F and the low was 40 degrees F. Compare this to the planting date of November 5 in Shafter where the high was 20 degrees F warmer (high of 73 degrees F, low of 52 degrees F).

Irrigated (left) and non-irrigated (right) rye & peas in Shafter on March 11, 2021

With legume species, in particular, less initial germination correlated with a low percentage of legume species in the final stand. Grasses, however, germinated well in both locations and dominated the final stand.

Therefore, if you cannot plant until later in the winter, consider seeding grass seeds in higher proportions, especially if weed suppression is your goal.

Irrigating directly after seeding may help cover crops establish more vigorously and compete with weeds. If you are not able to irrigate your cover crop, planting right before a rain event can help your cover crops establish a better stand.

Do not assume that there is enough moisture in the soil to help your cover crops properly germinate if you are planting after a rain event.

Irrigated (left) and non-irrigated (right) soil health mix in Shafter on March 11, 2021.

 

Goal: Provide Resources for Beneficial Insects

The legumes in both locations did not perform well, so they did not offer much sustenance for pollinators.

The brassicas in Shafter started blooming at the beginning of February, while the radishes at Kearney started blooming in mid-March. This provided forage and diversity for the pollinating bees from the surrounding almond orchards. Butterflies were also observed on these flowers.

Ladybugs were seen on the grasses in both locations, which can serve to keep aphid populations low on surrounding or subsequent crops.

 

Goal: Improve Water Infiltration

At Kearney, water infiltration was measured using a mini disk infiltrometer before planting cover crops and right before termination of cover crops.

Infiltration rates were higher when cover crops were in the ground compared to the bare soil prior to planting cover crops (Figure 1).

Figure 1: Infiltration rates in both fields at Kearney before the cover crops were planted (blue) and while the cover crops were fully established (orange).

There were no significant differences in infiltration across seed mixes. Therefore, based on our results, there is not one particular mix we would recommend to improve water infiltration.

Having roots in the ground improves the ability of water to enter the soil surface. Roots create channels for water to enter so that more water can be stored in the soil.

With lower infiltration rates, water is more likely to run off the surface. This is why avoiding bare soil is important if your goal is to increase the amount of water that can be stored in your soil.

 

Concern: Will the Cover Crops Tie Up Soil Nitrogen?

In Shafter, we took soil samples zero to six inches deep on April 21 in the irrigated plots five weeks after termination. The fallow samples had an average soil nitrate level of 45.8 mg/kg, which was similar to the soil nitrate level that we measured before planting the cover crops.

Two of the mixes had soil nitrate levels that were very close to that of the fallow area: the brassica pollinator mix and the soil builder mix. Those mixes were mostly or entirely brassicas, so they decomposed quickly enough that the nitrate they had taken up had returned to the soil. The other three mixes (soil health, rye and peas, and barley and vetch) led to significantly lower soil nitrate levels.

Grasses have higher C:N ratios, which meant that soil microbes took longer to decompose their residue and needed to mine soil nitrogen to do so.

 

Concern: Can I Really Grow Cover Crops Without Irrigation?

Shafter
All of the non-irrigated plots contributed some amount of biomass. This means that even if you cannot irrigate your cover crops, you can still reap some soil health benefits.

For four of the mixes, the irrigated plots produced at least twice as much biomass as the non-irrigated plots. In contrast, the non-irrigated brassica plots produced almost as much biomass as the irrigated brassica plots (Figure 2).

Figure 2: Aboveground dry biomass was collected in Shafter on March 17, 2021.

Kearney
The differences in biomass between the irrigated and non-irrigated plots were not significant, except for the irrigated Merced rye and peas and the irrigated barley and vetch. These two irrigated mixes yielded nearly twice as much biomass as their non-irrigated counterparts (Figure 3). This biomass was mostly attributable to the grasses in both mixes.

Figure 3: Aboveground dry biomass was collected in Kearney on March 22, 2021.

The non-irrigated side produced an impressive amount of biomass given that it only received 3.71 inches of water.

The mixes that performed best without irrigation were the Merced rye and peas and the soil builder mix. The daikon radish performed well in the soil builder mix without supplemental irrigation, and the Merced rye performed well in the rye and peas mix without supplemental irrigation. Therefore, Merced rye and radish would be good options if you are unable to irrigate your cover crops.

 

Conclusion

Based on the results of our trial, cover crops can still produce biomass without irrigation in the San Joaquin Valley, even during a drier winter. While they may not produce huge amounts of biomass without irrigation, having roots in the ground is still beneficial for suppressing weeds, providing beneficial insect habitat, increasing water infiltration and feeding your soil biology.

Cover crops turn sunlight into carbon that feeds your soil microbes through their root system. Therefore, by simply maintaining living ground cover, you are feeding your soil. In addition, when you return cover crop residues to your soil, microbes will decompose them and slowly release available nutrients that can be used by your cash crop.

If you are growing cover crops in a limited water environment, consider planting Merced rye and brassica species, such as daikon radish or mustards. These are also good options if you plant your cover crops later in the winter.

Irrigated (left) and non-irrigated (right) brassica pollinator mix in Shafter on March 11, 2021.

Triticale also grows well with limited water and decomposes more quickly than Merced rye, which is something to consider if you are disking in your cover crop and planting an annual crop shortly after.

In both Shafter and Kearney, the legume species did not perform well because of the high soil nitrate levels and low water conditions. If your field is in a similar situation, it might be better to save money and not buy legume seeds.

You should also think about the cash crop that will follow the cover crop. For example, you do not want to plant a brassica cash crop after a brassica cover crop. This might increase pest pressure for your brassica cash crop.

Interested in trying out cover crops? The USDA and the California Department of Food and Agriculture have programs to help you pay for it. For more information about the USDA resources, reach out to your local USDA Natural Resource Conservation Service office. For more information about the CDFA Healthy Soils Program or getting started with cover crops, reach out to Shulamit Shroder at sashroder@ucanr.edu or Jessie Kanter at jakanter@ucanr.edu.

 

Resources

Mitchell, J. P., Shrestha, A., Mathesius, K., Scow, K. M., Southard, R. J., Haney, R. L., Schmidt, R., Munk, D.S. & Horwath, W. R. (2017). Cover cropping and no-tillage improve soil health in an arid irrigated cropping system in California’s San Joaquin Valley, USA. Soil and Tillage Research, 165, 325-335.

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