Tensiometers are useful for monitoring soil moisture in vegetable and row crops so that plants are not over-watered nor become water stressed.  As the name implies, tensiometers measure soil moisture (water) tension, otherwise referred to as matric water potential.   Soil water tension is essentially a measure of the force that plants need to exert to suck water from the soil pores into the roots.  A high tension means that the soil is dry and a plant needs more energy to extract water from the soil compared to when the soil is moist.  Tensiometers function without batteries and wires, although they can be interfaced to dataloggers by adding a pressure transducer.  Also, the readings are not affected by soil texture, temperature, and salinity unlike many electronic soil moisture sensors.

A previous article published in this blog described how to build a tensiometer from PVC parts and ceramic cups that can be purchased from a supplier in California.  The design results in a dependable and inexpensive tensiometer that can be built by anyone comfortable using a few hand tools. The most challenging step was gluing the cup to the end of the PVC shaft using epoxy cement which can sometimes result in a vacuum leak if the bond is weak.    Since publishing this article, we have built and used more than a hundred of these tensiometers and can suggest some modifications to the original design and a few other words of wisdom.  For example, we observed that the PVC “T” can crack if the gauge is threaded too tightly or during cold weather conditions which can cause the plastic to contract, so we suggest using a different type of “T” and not to over thread the gauge. We have also collaborated with a supplier of the ceramic cups, SoilMoisture Equipment Corporation, whose engineers have developed a cup that can be bonded to the PVC shaft using standard PVC cement and primer rather than a specialized epoxy (Fig. 1).  This improvement greatly reduces the time required to build the tensiometer which is why we call it the “ten-minute tensiometer.”  Once you have some experience, you will probably need less than 10 minutes to assemble one of these tensiometers from the parts shown in Fig 2.

Fig. 1 This newly designed ceramic cup (Part #Y2630C) can be bonded to the PVC shaft using standard PVC primer and cement.

Fig. 2. Components needed to build a tensiometer: a. ceramic cup, b. and f. ½ inch PVC pipe for shafts, c. PVC “T”, d. ½ inch male slip x ¼ inch female thread bushing, e. vacuum gauge, and g. rubber stopper.

Materials needed:

Ceramic cups

Vender: SoilMoisture Equipment Corporation, Santa Barbara CA (805-964-3525, www.soilmoisture.com)

Part Number Y2630C, Dimensions:  0.875 inch OD x 2.75 inch length

Cost: $25 ea.  Note that this is a new part that must be special ordered.

Vacuum gauge

Vender: Zoro.com/Grainger.com

Part Number 4FMK3, Description: ¼ inch MNPT 2 inch diameter test vacuum gauge

Weblink: https://www.zoro.com/zoro-select-vacuum-gauge-test-2-in-4fmk3/i/G0040792/?q=4FMK3  Cost: $18.99 ea.

#1 size rubber stopper

Vender: Grainger.com

Part Number 8DWU6, model RST1-S,  Description: 24 mm neck, bottom diam. = 14 mm.  Top diam. = 20 mm.

Weblink: https://www.grainger.com/product/GRAINGER-APPROVED-Stopper-8DWU6?internalSearchTerm=Stopper%2C+24mm%2C+Black%2C+PK52&suggestConfigId=8&searchBar=true&suggestConfigId=8&searchBar=true&suggestConfigId=8&searchBar=true&suggestConfigId=8&searchBar=true  

Cost:  $18.55 / 52 pieces

Schedule 40 PVC pipe (½ inch diameter)

Vender: irrigation supply or hardware store

Sand paper (60 or 80 grid)

Vender: hardware store

 ½-inch PVC “T” 

Vender: irrigation supply or hardware store

Description: ½ inch Female slip x ½ inch Female slip T.  Note that this T replaces part number 402-072 from Spears Inc., which we found to sometimes split during cold periods.  Also, the previous part was often not in stock.

PVC bushing

Vender: irrigation supply or hardware store. Description: ½-inch PVC Male slip x female ¼” thread bushing.

PVC cement (gray) and purple primer (Fig. 3)

Vender: irrigation supply or hardware store.  Note that the gray cement provides a stronger bond than the clear product.

Fig. 3 PVC cement and primer.

Gas pipe thread sealant (white or blue paste type)

Vender: irrigation supply or hardware store

Painters masking tape

Vender: hardware store

Petroleum Jelly (Vasoline)

Vender: pharmacy. A coating of petroleum jelly improves the seal between the rubber stopper and PVC tube.

Rubber gloves

Vender: hardware store, paint store, etc. Description: Nitrile disposable gloves to protect hands from PVC primer and glue.

Tools needed:

• PVC saw
• Miter box

Procedures

1. Cut PVC pipe sections in the following lengths

1 foot depth tensiometer:   top shaft = 4 inches*, bottom shaft = 17 inches

2 foot depth tensiometer:   top shaft = 4inches, bottom shaft = 30 inches

We advise first cutting the bottom shaft about 1-inch longer than indicated above using the miter box or an electric miter saw to assure that the cut at the lower end is at a 90-degree angle (Fig. 4).  The ceramic cup will fit crooked on the end of the shaft if the cut deviates from 90- degrees.  After assuring that the cup fits well, the top end of the shaft can be cut to the exact length.

*Note that the top shaft can cut shorter than 4-inches so that when the tensiometer is installed in the field it has a lower profile to the ground, thereby reducing chances of being hit by tractor implements.

Fig. 4 Use a miter box and PVC saw to cut the bottom shaft at a 90-degree angle.

2. Check that the ceramic cup fits into the bottom of the PVC shaft and is aligned straight. Sand the neck of the cup and/or the interior walls of the PVC shaft if the cup cannot be inserted at least half-way into the PVC.  Be careful not to over-sand the cup or the fit between the neck and the PVC shaft will be loose and not bond well when glued. The fit between the neck of the cup and the PVC should be very tight such that a lot of force is needed to insert the cup into the bottom of the PVC shaft.  If the cup is not in alignment with the PVC shaft, then recut the end with the miter box at a 90-degree angle and recheck the fit. 
3.  Wrap the bottom of the PVC shaft and the top of the ceramic cup with painter's tape to prevent cement from coating the outside of the ceramic cup (Fig. 5).
4. 

   Fig. 5 Painter’s tape will protect the outer surface of the ceramic cup from becoming coated with cement and primer.
5. In a well-ventilated location, apply PVC primer to both the interior of the PVC shaft and the outside of the PVC top of the ceramic cup. Then apply gray PVC glue to both surfaces, and push the parts together, and hold in place for about 30 seconds to 1 minute (Figs. 6-9).   Tip: slightly twist the parts by about 30-degrees immediately after gluing to assure a good bond before the cement begins to set.
   

   Fig. 6. Coat the PVC portion of the cup with primer and then PVC gray cement
   

   Fig. 7 Coat the inside of the bottom of the shaft with primer and PVC gray cement.
   

   Fig. 8 Insert the cup into the shaft.
   

   Fig. 9 After inserting the cup into the shaft immediately twist the cup about 30-degrees and hold in place for 1 minute while the cement sets. Wipe off any excess cement.
6. Next glue the top and bottom shafts and the bushing into the ½ inch, PVC slip “T” using the PVC primer and cement.  After inserting each of these parts into the T, slightly twist them and hold in place for about 30 seconds while the cement sets (Figs 10-12). Also cover the non-glued areas with painter's tape to prevent the outside from becoming covered with primer and cement.
7. 

   Fig. 10 Add primer to the interior of the “T” and the outside edges of the shafts and bushing. Then coat both surfaces with PVC cement.
   

   Fig. 11 Hold the parts in place for at least 30 seconds after gluing so that the cement can set.
   

   Fig. 12 The “T” shown with the bushing and upper and lower shafts connected.
8. Coat the ¼ inch male threads of the gauge with pipe thread sealant and hand screw on the vacuum gauge.  Tip: do not over tighten or the PVC “T” may crack.
9. Fill the tensiometer fully with degassed distilled water. The water can be degassed by boiling it and allowing it to cool.
10. Coat the lower end of the rubber stopper with a thin film of petroleum jelly and insert into the top end of the tensiometer with a light twist to firmly seat the stopper (A loose stopper is the main cause for vacuum leaks).

Fig. 13 Finished tensiometer is ready for testing.

Conclusion

So that is it—the tensiometer is ready for testing and installation (Fig. 13).  Hopefully, it did not take too long to build and with practice one should be able to assemble these tensiometers in 10 minutes or less.   Please visit our previous blog article on how to test and install tensiometers in the field.  Please let us know if you have any questions or feed back.  

The rain situation was beginning to look dire for our region before last week, with most major storms passing to the north of Monterey county.  However, the storms that occurred last week were generated by an atmospheric river that was focused on the southern part of Monterey County.  Cumulative depths recorded at CIMIS weather stations along the valley showed increasing amounts moving south in the valley with almost 9 inches recorded at the King City CIMIS weather station (station 113) (Fig. 1).  Also 8.3 inches were recorded at San Antonio and Nacimiento Reservoirs, where before these storms less than an inch of rain had fallen since October. 

Figure 1. Rainfall recorded at the CIMIS weather stations in the Salinas Valley between January 23 and February 2, 2021

This one weather event was able to significantly increase the water stored in Nacimiento reservoir and helped the situation in San Antonio (Fig. 2.)   Nacimiento water storage increased from 21% to 41% between January 23 and February 3, and San Antonio increased from 16% to 20% capacity.   In combination, water stored in the two reservoirs increased from 133,778 acre-ft to 216,858 acre-ft, representing 64% more water compared to before the storm events.   Total capacity of the two reservoirs is 712,900 acre-ft, so water stored in the reservoirs at this point in the season is still at 30% of maximum capacity.  

Figure 2. Reservoir storage before and after the storm events between January 23 and February 3. Storage is also expressed as percentage of total reservoir capacity above the bars

Our region usually receives a few atmospheric river events each winter, most of which usually pass too far north or south to greatly impact the Salinas Valley reservoirs.  This first major rain event of the season was a direct hit for the reservoirs.  Hopefully, more rain will be coming in the upcoming weeks.

The other benefit of this last storm was that by mostly passing over the southern part of the county, debris flows were minimized in the burned areas. 

If you want to keep track of the reservoir storage as we proceed through the winter visit the link at Monterey County Water Resource Agency website.

https://www.co.monterey.ca.us/government/government-links/water-resources-agency/home/daily-reservoir-dam-data

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

Background

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.

 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.

Conclusions

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. (2006) Tips on injecting polyacrylamide (PAM) into sprinkler systems to reduce sediment and nutrient losses, Monterey County Crop Notes May/June 2006 

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., Farrara B. (2009) Evaluation of polyacrylamide formulations for controlling suspended sediments and nutrients in sprinkler run-off, Monterey County Crop Notes November/December 2009  

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

Acknowledgments

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.