- Author: Michael D Cahn
Introduction
Traditional winter cereal cover crops planted in the Salinas valley have many potential benefits including, scavenging nitrate in the soil profile, increasing organic matter in the soil, and protecting the soil from erosion during storm events. However, when grown for 3 to 4 months during the late fall and winter, cereal rye, triticale, or barley can accumulate 5 to 6 tons of dry matter biomass that must be incorporated into the soil before planting a spring vegetable crop. Tilling in a high amount of cover crop biomass can be disruptive to spring planting schedules. Consequently, only a small fraction of the vegetable ground in the Salinas valley is cover cropped each year.
Previous studies demonstrated alternative strategies can limit the biomass growth of these cereal cover crop species so that they can more easily be tilled into the soil, and therefore less disruptive to spring planting schedules. After fall land preparation, the cereal cover crops are seeded into listed beds and/or in the furrow bottoms. After they become established they can reduce runoff and protect the soil from erosion during early winter storm events. Before the cover crops grow too big, they are terminated with an herbicide to limit the amount of above ground biomass that needs to be incorporated in the spring. For organic systems, planting a mustard cover crop on listed beds or furrows which can be terminated mechanically by mowing is another strategy to limit biomass. A good target for these low biomass cover crops is between 0.5 to 1 ton of dry matter per acre by the date of termination. Once terminated, the biomass begins to decompose. However, the residue on the surface continues to protect the soil from erosion and can significantly increase infiltration from rain events. This helps to leach accumulated salts in the soil as well as recharge groundwater aquifers. The remaining decomposed residue can easily be incorporated into the soil during bed preparation in the spring.
One risk of this low biomass approach is accessing fields during the winter to terminate the cover crop. If soil conditions are too wet or if there is not enough available labor, it may be difficult to fit in a spray application or to run a flail mower. This termination step also increases the cost of managing the cover crop. A possible solution is to use species that grow slowly during the winter when temperatures are cold. Sudangrass and sorghum-sudangrass hybrid are warm season adapted species that could be used in this low biomass approach to managing winter cover crops.
Field trial with warm season adapted cover crop species
A field trial was conducted with sudangrass and sorghum-sudangrass in the 2023-2024 winter to evaluate biomass growth, and the effect on storm water runoff and soil erosion compared to bare-fallow plots. The site was located on an Arroyo Seco gravelly loam soil with a slope of more than 5%. Plots measuring 1050 ft in length by four 40-inch wide peaked beds were planted with either sudangrass, sorghum sudangrass hybrid, or left bare fallow. Treatments were replicated 4 times. The cover crops were seeded at 60 to 80 lbs/acre on October 4th and were subsequently sprinkle-irrigated several times. Total water applied for establishment was 2.6 inches. One application of the herbicide Bromoxnil (Maestro) was applied about 45 days after planting to kill emerged broadleaf weeds. 
Results
Above ground biomass, N uptake, and carbon accumulation
Both cover crops had limited biomass growth, accumulating only 0.35 to 0.5 tons/acre of dry matter by early January and less than 1 ton/acre by mid March (Table 1). Growth was set back by cold conditions that occurred from mid November through early January, occasionally reaching freezing temperatures which caused damage to leaves (Fig. 1). However, the freezing temperatures lasted only a few hours and were not severe enough to kill the cover crops (Fig. 2). By March 13th the cover crops had taken up 45 to 55 lbs N/acre and had a carbon to nitrogen ratio of 15. The C:N ratio of 15 would suggest that after soil incorporation the residue would decompose rather quickly and release N for the following vegetable crop.
Runoff, rainfall infiltration, and control of soil erosion
Total rainfall measured at the trial site was 10.2 inches for the winter season. The most intense period of rainfall occurred in late January and early February which resulted in several runoff events (Fig. 3). During this period about 50% of the rainfall in the bare fallow plots was lost as runoff compared to 15% lost as runoff in the cover crop plots (Fig.4). Over the entire winter season, runoff was reduced by an average of 70% under the cover cropped plots compared to the bare fallow plots, and significantly more rainfall was infiltrated into the ground in the cover cropped plots. In addition, suspended sediment concentration was 90% and 77% less in the sudangrass and sorghum-sudangrass cover crop plots, respectively, compared to the bare plots. Turbidity, total P, and total N concentration in the runoff were also reduced under the cover crop plots compared to the bare fallow plots (Table 2).
Seasonal soil erosion losses could be calculated based on the volume of the runoff and sediment concentration in the runoff. The total loss of sediment averaged more than 3500 lbs per acre in the bare fallow plots during the winter, while erosion losses were reduced by 96% to 98% in the sorghum-sudangrass and sudangrass plots (Fig. 5). Total N losses were reduced by 83% to 86% in the cover crop plots compared to the fallow plots, and total P losses were reduce by 81% to 85% in the cover cropped plots compared to the bare fallow plots.
Conclusions
The use of warm season species such as sudangrass and sorghum-sudangrass hybrids as winter cover crops provides several advantages compared to planting cereal cover crops. The biomass growth through the winter is self-limiting due to the cold conditions that typically occur in the Salinas Valley. Because the final biomass would likely be less than 1 ton per acre, these species can be planted on listed beds in the fall rather than on flat ground. This means that in the spring, the remaining cover crop can be lillistoned into the peaked beds a few weeks before final bedshaping and planting. Cover crops planted on flat ground and have high amounts of biomass usually require many tillage passes to prepare ground for planting. Despite, having less biomass than traditional winter cereal species, sudangrass and sorghum-sudangrass hybrid cover crops provided excellent erosion control compared to leaving the ground bare, and increased infiltration of rainfall during storm events. Also these species may be able to scavenge significant amounts of nitrogen from the soil which can limit nitrate leaching during the winter months.
On the east-side of the Salinas Valley groundwater levels have been in the decline for several decades. Infiltrating as much rainfall as possible during the winter using strategies such as low biomass cover crops could potentially help recharge the aquifer in this region. We plan to conduct a second year of field trials with these warm season species to continue evaluating this approach to managing winter cover crops in vegetable systems.
Acknowledgments
This project was funded by the California Leafy Greens Research Board.
![1699385688184-b7678dfe-37ca-4187-b887-cec08c2a072c 1[1]](https://ucanr.edu/blogs/SalinasValleyAgriculture//blogfiles/103056_original.jpg "1699385688184-b7678dfe-37ca-4187-b887-cec08c2a072c 1[1]")
- Author: Michael D Cahn
- Contributor: David Chambers
- Contributor: Thomas Lockhart
- Contributor: Noe Cabrera
As the drought continues on the central coast, growers are trying to utilize water as efficiently as possible to produce their crops. Retaining and reusing sprinkler runoff, also referred to as tail water, can be an important strategy to increasing water conservation. Also, retaining runoff prevents suspended sediments, pesticides and nutrients from impairing rivers and estuaries downstream of agricultural fields.
Many ranches in the Salinas Valley have retention basins and infrastructure that can capture runoff and reuse tail water for irrigating crops. Most growers use this water during the pre-germination or the germination stages to avoid food safety risks from microbial pathogens. However, updates to the leafy green marketing agreement (LGMA) now require that water stored in open reservoirs and used for irrigating leafy greens maintain generic E. coli levels less than 10 MPN/100 ml. In most cases, tail water in open reservoirs will need to be treated with chlorine to achieve this low threshold for generic E. coli. Fine sediments suspended in the tail water can greatly reduce the effectiveness of chlorine to control bacterial growth.
Polyacrylamide (PAM), an inexpensive polymer molecule that has been used for controlling soil erosion in furrow irrigated fields since the early 1990s may be able to improve the efficacy of chlorine by reducing the suspended sediment concentration in sprinkler runoff. Additionally, if runoff is discharged from a ranch, treatment with PAM can greatly reduce the concentration of sediment-bound pesticides and nutrients that can degrade water quality downstream. Past field trials that we conducted have shown that adding PAM to irrigation water at a low concentration (< 5 ppm) is an effective way to minimize erosion in sprinkler irrigated fields and remove suspended sediments from tail water. However, for this strategy to be successful with sprinklers, we found that PAM must be injected continuously throughout each irrigation. In other words, a single application of PAM cannot control suspended sediments in runoff during subsequent irrigations.
New approaches to using PAM
Accurately injecting PAM into a pressurized irrigation system is not a simple process. Dry PAM powder becomes very gooey and viscous when moistened, and is almost impossible to uniformly dissolve into water. Emulsified oil formulations of PAM that mix up uniformly in water are available but are more costly than dry PAM products and require sophisticated pumps to meter it into a pressurized irrigation system, as well as trained staff to assure that the application rate is accurate. Another limitation of liquid PAM is that the mineral oil used to emulsify these products can be toxic to aquatic organisms. In contrast, dry PAM is less than half the cost of liquid PAM and has been shown to have no toxicity to aquatic test organisms such as Hyalella azteca and Ceriodaphnia dubia, even at concentrations 20 times greater than would be typically used for treating irrigation water. Hence, for these reasons, we have been developing and evaluating approaches of using dry PAM to control sediment in sprinkler runoff during the last several years.
Treating pressurized irrigation water with PAM
The first method that we describe in this article uses an applicator to dissolve dry PAM into pressurized irrigation water. The applicator consists of cartridges filled with PAM granules that insert into a series of cylindrical chambers (Fig. 1). A small pump can be used to divert a portion of the irrigation water from the mainline into the inlet of the applicator. PAM slowly releases from the cartridges (Fig. 2) as irrigation water streams through the space between the cartridges and the outer walls of the chambers. Vanes surrounding the cartridges increase turbulence of the flowing water to maximize the dissolution of PAM. The treated water then returns into the main line of the irrigation system where it distributes into the field through the sprinkler system.
Field-testing the dry PAM applicator
Field-testing of the prototype PAM applicator was conducted in commercial lettuce fields during 2020 and 2021. Each field test occurred during the germination phase of the crop (5 to 6 consecutive irrigations) using overhead sprinklers. The fields were divided into untreated, and PAM treated areas, where the PAM treated plots ranged from 1.9 to 4.2 acres. Soil textures at the sites varied from loam to sandy loam. A portion of the flow in the main lines was diverted through the PAM applicator. Flowmeters were used to measure the flow rate in the mainline and the inlet of the applicator. Another flowmeter monitored the volume of water applied in an adjacent untreated plot.
Flumes were installed 30 ft from the far end of the fields to measure run-off volume in the PAM treated and the untreated plots during the irrigations (Fig. 3). A stilling well and float mechanism were used to measure the height of the water in the flume. A datalogger recorded the height of the water in the flume and converted it to a flowrate using a calibration curve. The datalogger also automated sampling of run-off into collection containers using a peristaltic pump. Composite samples of run-off were collected from the plots during 5 to 6 irrigation events and analyzed for turbidity, pH, electrical conductivity, suspended sediments, total nitrogen (N), nitrate-N, total phosphate (P) and orthophosphate at the UC Davis analytical laboratory.
Results of field tests
The average concentration of suspended sediments in the untreated sprinkler runoff ranged from 466 to 1256 milligrams per liter (mg/L) during each trial (Table 2). Results of these field trials demonstrated that pretreating the irrigation water with the PAM applicator could reduce the concentration of suspended sediments carried in sprinkler runoff by 85% to 95%, depending on the soil type. The average reduction in suspended sediment concentration in the runoff was 90% across all trials. Turbidity of the runoff in the PAM treated plots was also reduced by an average of 95% across all sites (Fig. 4, Table 3). Runoff volume in the PAM treatment was reduced by an average of 26%, but the reduction in runoff volume varied from 8% to 67% depending on the site characteristics (Table 3).
Total and soluble phosphorus were reduced by an average of 65% and 14% respectively in the PAM plots compared to the untreated controls in the two trials conducted in 2021 (data not presented). Total nitrogen and nitrate nitrogen concentration in runoff from the PAM treated plots were not reduced compared to the untreated plots.
The combined effects of reduced runoff volume and suspended sediment concentration under the PAM treatment resulted in less loss of soil from these fields (Table 2). Soil erosion was reduced by an average of 93% compared to the untreated control, varying from 89% to 96% reduction in erosion among field sites. Cumulative losses of sediment during the germination phases of the crop were reduced from an average of 76 lbs per acre in the untreated plots to 5 lbs per acre in the PAM treated plots. The sediment lost in the untreated plots would equate to 15 tons for a 200-acre ranch that was planted with 2 crops of vegetables per season. This estimate is only for the germination phase of the crop and so total losses of sediment could be much higher for crops irrigated with sprinklers until harvest.
Potential soil health benefits
Improving soil structure and increasing soil organic matter may be an additional benefit of applying PAM to the field through the irrigation water and preventing the erosion of fine sediments. Much of the stable organic matter in soil is associated with the clay size particles, so preventing the erosion of these fine sediments would presumably help with retaining and building soil organic matter. Fine sediments and organic matter are also important in the development of better soil structure which can improve crop growth.
Summary
The dry PAM applicator that we field tested showed promise for greatly reducing soil erosion, as well as helping improve water quality. Presumably, by removing the sediment from the tail water, less chlorine would be required for controlling microbial pathogens. The six-unit PAM applicator tested in this study can treat up to 500 gpm. The applicator would need more mixing units to treat all the flow from a typical agricultural well in the Salinas Valley (flow rates of 1000 to 1500 gpm). To see a video showing runoff from PAM treated and untreated field plots during a sprinkler irrigation, follow this link. The second part of this article will discuss an additional method to use dry PAM to treat irrigation runoff. This second approach uses an applicator that directly treats runoff in drainage ditches.
Acknowledgments: We greatly appreciate assistance in fabricating the prototype PAM applicators from RayFab. This project was funded by the California Leafy Green Research Board.
- Author: Michael Cahn
- Author: Richard Smith
Tuesday, February 23;
7:55 a.m. to 12:00 p.m.
Habrá traducción al Español
Registration Cost: $10
Registration Link: https://ucanr.edu/survey/survey.cfm?surveynumber=32769
7:55 Introduction
8:00 Woodchip bioreactors for removal of nitrate and pesticides from tile drainage.
Pam Krone-Davis, California Marine Sanctuary Foundation
8:30 Nitrogen mineralization from organic fertilizers and composts
Daniel Geisseler, UC Davis
9:00 Improving the efficiency of drip germination of lettuce and weed control.
Michael Cahn and Richard Smith, UC Cooperative Extension
9:30 Using weather-based irrigation scheduling for optimizing red cabbage production.
Lee Johnson, NASA Ames Research Center-CSUMB
10:00 Break
10:15 Update on Ag Order 4.0.
Matt Keeling, Central Coast Regional Water Quality Control Board
10:45 Development of N removal coefficients for vegetables on the central coast
Richard Smith, UC Cooperative Extension
11:15 New approaches for using Polyacrylamide (PAM) for mitigating sediment and pesticides in irrigation runoff.
Michael Cahn, UC Cooperative extension
11:45 Using high-carbon compost for reducing nitrate leaching during the winter.
Richard Smith, UC Cooperative Extension
12:00 Adjourn
CCA & DPR continuing education credits have been requested
- Author: Cheryl Reynolds
This spring if you are looking for options to obtain your continuing education units (CEUs) and not sure where to get them, why not check out the online options that the UC Statewide IPM Program (UC IPM) has to offer. For license and certificate holders from the California Department of Pesticide Regulation (DPR) with last names beginning with the letters A through L, 2020 will be the year to renew.
UC IPM currently offers 16 online courses for DPR credit. Many of the courses are also accredited by the California Structural Pest Control Board (SPCB), Certified Crop Advisor (CCA), Western Chapter of the International Society of Arboriculture (WCISA), or Arizona Department of Agriculture.
If you are looking for CEUs in the Laws and Regulations category, check out these courses:
- Proper Pesticide Use to Avoid Illegal Residues (2.0 Pesticide Laws & Regs)
- Proper Selection, Use, and Removal of Personal Protective Equipment (1.5 Pesticide Laws & Regs)
- Providing IPM in Schools and Child Care Settings(1.0 Other and 0.5 Pesticide Laws & Regs)
Some of our courses do require a fee and are being offered at an early-bird price through October 31st. These courses can be purchased individually, or they can be purchased as a 4-course bundle for a special price of $85—a total discount of $20 versus purchasing each course separately.
In addition to offering online courses, UC IPM also hosts a monthly webinar series sponsored by
the Citrus Research Board. The UC Ag Experts webinar series is designed for growers and pest control advisers. It includes presentations on various pest management and horticultural topics, primarily for citrus and avocados. The next webinar will be held on April 8th from 3 PM until 4 PM with Dr. Elizabeth Grafton-Cardwell, UC Riverside Department of Entomology and Extension Specialist, speaking about citricola scale. This webinar has been approved for one hour of Other CEUs from DPR and 1 hour of IPM units from CCA. Registration is currently open. View past webinars on the YouTube UC Ag Expert Talk Playlist. CEUs are only available for attending the live webinar.
DPR always encourages license and certificate holders to avoid the last-minute rush and renew early to ensure your license will be renewed by January 1st. Take advantage of UC IPM's online courses and webinar series to get a jump start on your renewal today!
