- 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
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
10:15 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
CCA & DPR continuing education credits have been requested
- Author: Michael D Cahn
- Contributor: Thomas Lockhart
- Contributor: David Chambers
- Author: Gerry Spinelli
Pyrethroid pesticides strongly bind to suspended sediments carried in agricultural runoff and have been identified as a source of aquatic toxicity on the central coast by both the Central Coast Regional Water Quality Control Board and the California Department of Pesticide Regulation. Sprinkler irrigation of vegetable crops often results in significant volumes of runoff, especially during the germination and stand establishment phase. Unless growers can contain agricultural runoff on their ranches, mitigation practices are needed to minimize downstream water quality impacts.
Polyacrylamide has been shown to minimize erosion in furrow irrigated fields. Several field studies that we conducted more than 10 years ago also demonstrated that long chained anionic polyacrylamides can be used in pressurized sprinkler systems to minimize sediment losses from vegetable fields. These studies concluded that maintaining a concentration of 2.5 to 5 ppm PAM in the water during an irrigation can reduce sediment concentrations in run-off by more than 90%. These studies showed that the best method for injecting PAM into pressurized irrigation systems was to use a liquid formulation and a specialized metering pump developed for viscous liquids. Although the pump was relatively easy to use, it was expensive ($3500 per pump) and required that irrigators received training on how to operate the pump and correctly dose the irrigation water. In addition, the liquid formulation of PAM, which was emulsified with mineral oil, was more than twice the cost of dry granular formulations ($4/lb vs. $2/lb). The total cost for treating with PAM was estimated at $26 to $34/acre using the liquid formulation for four irrigations (Cahn 2006).
Another issue identified for liquid formulations of PAM was potential aquatic toxicity. Although research studies have shown that anionic PAM is not toxic to aquatic organisms, the mineral oil use to emulsify liquid PAM products was shown to have toxicity to Ceriodaphnia dubia and Hyalella azteca, both of which are test organisms used to evaluate aquatic toxicity. Although other liquid formulations of PAM using humectant ingredients were shown to be non-toxic to aquatic organisms and effective in controlling sediment (Cahn and Farrara 2009), they were more expensive and less widely available than the mineral oil-based formulations.
The solid tablet and granular forms of PAM are easier to handle than liquid formulations and cheaper but dissolving these forms of PAM at a rate sufficient to provide an efficacious concentration in a pressurized irrigation system is challenging. Rather than dissolving uniformly into solution tablet and granular forms of PAM tend to form clumps when added to water and require prolonged agitation to dissolve uniformly.
A new type of PAM applicator
During the past two years we designed, and field tested several prototype applicators that can add dry forms of PAM (tablet or granular) at a low but consistent rate in pressurized irrigation systems.After several modifications we settled on a design that maximized the efficiency of dissolving dry forms of PAM into flowing pressurized irrigation water. The current applicator consists of cartridges filled with PAM that insert into a series of six stainless steel cylindrical chambers (Fig. 1). A portion of the irrigation water can be diverted into the inlet of the applicator by partially closing a valve on the mainline or using a small pump on the inlet side. The diverted water passes through three pairs of chambers and then returns into the mainline. The dry PAM forms a viscous gel on the surface of the cartridge when exposed to water. The PAM dissolves into solution as the water streams through the gap between the cartridges and the outer wall of the chambers. Static mixers that fit around the cartridges force water to swirl around in the chamber, improving the dissolution of PAM (Fig. 2). The PAM applicator shown here was built by Ray Fab Inc. in Salinas, CA.
Field testing of the PAM applicator
Field testing of the final version of the PAM applicator was done in two commercial lettuce fields on the east-side of the Salinas Valley, north of Chualar CA. Testing was done during the germination phase of the crop using overhead sprinklers during April and May of 2020. The soil types were Chualar loam and Placentia sandy loam for trial 1 and Chualar loam for trial 2 fields. Lettuce crops were seeded in 6 rows on 80-inch wide beds on April 14 for trial 1 and May 7, 2020 for trial 2. The PAM used for testing was Soilfloc 100D, an anionic linear PAM developed for soil erosion control and manufactured by Hydrosorb Inc. The PAM applicator was positioned in the field next to the mainline (Fig. 1). Flowmeters were used to measure the flow rate in the mainline and at the inlet of the applicator. Another flowmeter monitored the volume of water applied to the adjacent untreated plot. The PAM treated plots measured 1.9 acres in trial 1 and 3.4 acres in trial 2. The flow rate of water applied to the PAM treated plot ranged from 235 to 260 gpm for trial 1 and 440 to 480 gpm for trial 2 (resulting in an average application rate of about 0.3 inches per hour). A portion of the irrigation water (160 to 190 gpm, or about 40% to 70% of the flow) was diverted from the mainline through the applicator and returned back to the mainline. Total water applied during the six irrigation events was approximately 4.7 and 6 inches for trials 1 and 2, respectively. Flumes were positioned 30 feet from the lower end of the field to measure run-off volume from 4 furrows in the PAM and untreated plots (Fig. 3). The flumes were equipped with a float mechanism calibrated to measure the height of water in the flume. A data logger recorded the height of the float mechanism which was transformed using the manufacturers' calibration curve to estimate the flow rate of the run-off. The dataloggers also automated sampling of the run-off into a container using a peristaltic pump. Composite samples of run-off were collected from the plots after each of six irrigation events and analyzed for turbidity, and concentration of suspended sediments, total N, nitrate-N, and orthophosphate.
Figure 1. Prototype PAM applicator in a sprinkler irrigated lettuce field.
Figure 2. PAM cartridge with static mixing element. Note that after exposure to water the dry PAM becomes a viscous gel that coats the surface of the cartridge.
Figure 3. Flume and peristaltic pumping system used to monitor run-off volume and automatically collect run-off samples from test plots in a commercial lettuce field.
Field testing of the PAM applicator demonstrated an average reduction in turbidity of 92% (Fig 4.) and an 86% reduction in total suspended solids (suspended sediments) in sprinkler run-off for the two trials (Table 1). The turbidity and concentration of suspended sediments increased with each irrigation in the untreated plot but remained low in the PAM treated plots. In addition to reducing turbidity and suspended sediment concentration, the PAM application reduced the volume of runoff. Total run-off volume from the PAM treated plot was 17% less than the untreated plot in trial 1 and 69% less in trial 2 (Table 2), indicating that a greater portion of the applied water infiltrated into the soil with the PAM treatment. The different effect of PAM on runoff volume measured for the two trials may be a result of several factors such as differences in soil texture, slope, field length, land preparation, irrigation frequency, etc. Nevertheless, the combined effect of PAM on runoff volume and suspended sediment concentration resulted in a large reduction in sediment loss from treated plots for both field trials. Total sediment loss after six consecutive irrigations was 66.3 lbs/acre in the untreated plot and 7.5 lbs/acre in the PAM treated plot (89% reduction) for trial 1 and 72.9 lbs/acre in the untreated plot compared with 2.7 lbs/acre in the PAM treated area for trial 2 (96% reduction). Concentration of nitrogen and phosphorus in run-off from the PAM treated plot were not reduced relative to the untreated plot (data not presented).
Figure 4. Sprinkler run-off from PAM treated (top) and untreated (bottom) plots in a commercial romaine lettuce field. Run-off is flowing through flumes positioned at the lower end of the plots.
Table 1. Average turbidity and total suspended solids in runoff from six irrigations collected from PAM and untreated plots.
Table 2. Cumulative runoff volume and sediment loss after six irrigations in PAM and untreated plots.
Because only a few specialized laboratories can measure PAM concentration we estimated the concentration in the treated irrigation water by evaluating the remaining quantity of PAM in the applicator cartridges after trial 1 was completed. We observed that about 90% of the initial volume of PAM remained, which would suggest that no more than 1 lb of PAM per acre was applied to the field during the six irrigation events. This amount would correspond to treating the irrigation water with 0.5 to 1 ppm of PAM.
Field testing of our prototype applicator demonstrated a simple and effective method to use polyacrylamide (PAM) to mitigate sediment losses from vegetable fields irrigated with overhead sprinklers. Our results showed both a reduction in sediment concentration in runoff as well as an improvement in water infiltration which would both conserve water and minimize the volume of runoff. The scope of this study did not include measurement of pesticide reductions but since pyrethroid pesticides, such as permethrin, strongly bind to sediment, minimizing suspended sediments in the runoff would presumably reduce pesticide loads and downstream water quality impacts. Although the applicator was set up in the field during these tests, the design could also be scaled up to accommodate greater volumes of water and be located at the well, much like a filter station. Further testing will continue during the upcoming season to evaluate the number of PAM cartridges required to optimize treatment at flow rates greater than 500 gpm, which would be more typical of wells in the Salinas Valley. If you have interest to test the PAM applicator in your field or learn more about how to use polyacrylamide (PAM) for mitigating sediment in runoff, please contact us and we may be able to arrange a demonstration or field test.
Past articles on polyacrylamide
Cahn M. (2005) Using polyacrylamide (PAM) for reducing sediment and nutrient losses. Conservation Currents. March 2005
Cahn M., Ajwa H., Smith R., Young A. (2004a) Evaluation of polyacrylamide for reducing sediment and nutrient concentration in tail water from central coast vegetable fields, Monterey County Crop Notes December 2004
Cahn M., Ajwa H., Smith R. (2004b) Evaluation of polyacrylamide (PAM) for reducing sediment and nutrient concentration of tailwater from central coast vegetable fields 12th Annual Fertilizer Research and Education Program Conference November 20, 2004 2004 Tulare, CA 17-22
Cahn, M. Qin, Z., Chambers, D. (2019) Mitigating pesticides and sediment in tail water using polyacrylamide (PAM). Progressive Crop Consultant. July/August 2019 p. 4-8.
Weston D., Lentz R., Cahn M., Ogle R., Rothert A., Lydy M. (2009) Toxicity of anionic polyacrylamide formulations when used for erosion control in agriculture. Journal of Environmental Quality 38
We thank the California Leafy Greens Research Board for financial support of this project and assistance of Mike Ray of Ray Fab Inc. for construction of the PAM applicator.
- Author: Michael D Cahn
The California Chapter of the American Society of Agronomy (CALASA) will host the 2021 Plant and Soil Conference as an online event February 1 - 3, 2021. The agenda can be found at the conference website: calasa.ucdavis.edu. Topics range from economic impacts of the pandemic on California agriculture, automation in agriculture, remote sensing, pest management, irrigation optimization, nutrient management, cover cropping, and soil health, and will include a student competition and present awards to honorees who have made significant contributions to California Agriculture during their careers.
Registration is currently $90 for the entire event and will increase by $25 after January 25th. There will be both CCA and DPR education units available.
The Plant and Soil Conference is annually organized by volunteers and supported by registration fees. If your company or organization would like to sponsor this event please visit our website.
- Author: Michael D Cahn
Currently, we are experiencing a prolonged heatwave on the central coast. Heatwaves have become a recurring phenomenon in recent years, especially in late summer. With thousands of acres of cool season vegetables in the ground, irrigation will be critical for keeping crops cool and for supplying enough moisture to meet their water needs.
Crops can be kept cool by maximizing evapotranspiration (ET). As liquid water vaporizes heat is lost from the surfaces of leaves and soil and from the surrounding air, which cools the temperature of the crop. Under water stress leaf stomates close during the hottest period of the day (11 am to 4 pm) and the temperature of the plant tissue can rise above the temperature of the surrounding air. If the temperature becomes too great leaves and other plant parts may become scorched (Figure 1).
Figure 1. Heat damage in broccoli
Since most ranches have a limited number of wells and personnel to irrigate, it is challenging to assure that each field has adequate soil moisture to prevent plants from overheating. A good strategy is to irrigate just enough to refill the soil profile to the rooting depth of the crop.
To prioritize which fields to irrigate one should consider the water holding capacity and existing level of moisture of the soil, as well as rooting depth and developmental stage of the crop. For example, a lettuce crop near maturity with a high ET demand, growing on a sandy textured soil that feels dry, should probably be irrigated soon. A young lettuce crop with a low ET demand, growing on silt loam soil that still feels moist, likely can be irrigated later without suffering heat damage.
Another consideration for prioritizing which fields to irrigate are recent field operations. A recently transplanted vegetable field may need to be irrigated first but may not need a long irrigation to re-saturate the soil around the roots. A crop that was recently cultivated may have pruned roots, and therefore may need water soon to prevent wilting under these hot conditions.
Table 1 estimates how much moisture is available to a vegetable crop between saturation and moderately dry or dry conditions for different soil textures. This table can be a guide for how much water should be applied to re-saturate the soil. For example, applying 0.42 inches per foot of rooting depth will bring a moderately dry silty clay soil back to saturation. Applying more than this amount of water will likely over-saturate the root zone.
Table 1. Estimated plant-available moisture for different textured soils.
Also, estimating the cumulative crop ET since the last irrigation can guide how long to irrigate. Reference ET values between south Salinas and Soledad during this hot spell have been as high as 0.27 inches per day. If the crop has a full canopy, 0.25 to 0.3 inches for each day since the last irrigation would be a good rule of thumb for how much water to apply as long as the total does not exceed the water holding capacity of the soil.
Lastly, one needs to convert the amount of water to apply to an irrigation run-time. To make this calculation one needs to know the application rate of the irrigation system. For impact sprinklers, the application rate can be estimated using Tables 2-4. Note that pressure and nozzle size have a significant effect on application rate. For drip, the irrigation time will depend on the tape discharge rate and pressure, as well as the spacing of drip lines. Assuming that the drip system is operated at the pressure recommended by the manufacturer (usually 8 to 10 psi) one can use Table 5 to approximate the application rate. For example, for one drip line of medium flow tape (0.45 gpm/100 ft) on 40- inch wide beds the application rate of the drip system is 0.13 inches per hour. If there are several drip lines per bed then multiply the application rate in the table by the number of drip lines.
The appropriate run-time can be estimated by dividing the amount of water to apply by the application rate of the irrigation system. For example, to apply 0.6 inches of water to a field with drip using medium flow tape the water would need to run for 4.6 hours:
Hours to operate the irrigation system = 0.6 inches of water/0.13 inches per hour = 4.6 hours
Irrigating the right amount of time to bring the soil back to saturation will maximize crop ET during these hot days, and hopefully prevent any heat damage to crops. Also, consider visiting the CropManage website (cropmanage.ucanr.edu) for further guidance on scheduling irrigations. This online tool can assist growers in quickly estimating how much water to apply to meet crop water needs.
Table 2. Sprinkler application rate for varying pressures and nozzle diameters for a solid set spacing of 30 × 30 feet (Rainbird 20JH).
Table 3. Sprinkler application rate for varying pressures and nozzle diameters for a solid set spacing of 30 × 33.3 feet (Rainbird 20JH).
Table 4. Sprinkler application rate for varying pressures and nozzle diameters for a solid set spacing of 30 × 40 feet (Rainbird 20JH).
Table 5. Drip application rates for varying bed widths and tape flow rates estimated for 1 drip line per bed. Multiply the rate in the table by the number of drip lines per bed to determine the actual application rate. (For 3 drip lines on an 80-inch bed multiply by 3)
- Author: Alejandro Del Pozo-Valdivia
This will be a ZOOM Webinar
When? Thursday August 6th
At what time? From 10:00 –11:15 am
If interested in participating, please register at:
We will send the ZOOM link 24 hours before the event/span>