Calibrating Soil Moisture Sensors for Irrigation

Capacitive soil moistures sensors are inexpensive and fairly easy to program for determining "wet" and "dry" soil conditions. However, arbitrary sensor thresholds are not adequate for determining irrigation start times or permanent wilting points in response to soils losing water to evaporation over time.
This video summarizes an experiment to see if capacitive soil moisture sensor response may be fine tuned by carefully measuring volumetric water content of a silty clay over time and comparing results to published recommendations.
The spreadsheet referenced in this video is posted here : docs.google.com/spreadsheets/... . This contains additional references and worksheets that might of interest to tinkerers - feel free to double check my calculations or use it to run your soil experiment ;)
Here's the paper that I studied in detail-- see Table 1 sourced to Ratliff et al. (1983); Hanson et al. (2000) used for deriving that multicolored chart for predicting VWC thresholds:
1. "Understanding Soil Water Content and Thresholds for Irrigation Management" extension.okstate.edu/fact-sh...
These are the papers for Table 1 referenced in my primary source:
2. "Field Limits of Soil Water Availability as Related to Laboratory Measured Properties"
www.ars.usda.gov/ARSUserFiles...
(My sincere apologies to ARS and Ratliff for the mispronunciation in the video.)
3. "Monitoring soil moisture helps refine irrigation management"
calag.ucanr.edu/Archive/?arti...
Here is another excellent reference from the UA Cooperative Extension
4. "Methods of Measuring for Irrigation Scheduling"
itc.tamu.edu/files/2018/05/az...
5. The multicolored chart in the video is sourced to a commercial irrigation sensor website, but I lost the link for proper attribution :( It sources its data to Ratliff which matches the numbers in Table 1 in my primary reference, so the attribution should really go to Ratliff and Hanson.
Note that it's been about 25 years since I took a soil science class. To be clear, this video is meant to share my exploration and lab notes on a topic of interest to me personally. Any corrections or suggestions for improvement are most welcome via the comments section.

[@sebakiller9736]

Nice Video and a lot of informations. Thanks for your work

[@ModestMaker]

Glad you liked it!

[@RonnieTheSim]

Excellent! In the "maker" world we sometimes call a build successful if it even survives long enough to log some data. Suppose it does -- now, about that data?
I clicked on this video looking for 'pro tips" about a specific device and its application. But in teaching that, @Modest Maker is also demonstrating the thoughtful approach of a skilled integrator. Prototypers of all kinds on whatever platform, watch: this is how one coaxes a component to reveal capabilities beyond what its pricetag or (non-)documentation might suggest.
Yes please, OP @Modest Maker, I will enjoy getting your test data when you upload it!

[@ModestMaker]

Of course - updating description now with link to spreadsheet. I removed some confusing notes from preliminary tests that weren't relevant, but you'll find other calculations and references in the spreadsheet that might be useful: docs.google.com/spreadsheets/d/1Jl9fN1wNGhqj5SkljHGKHVYtNJAVK36e/edit?usp=share_link&ouid=100450587141122348038&rtpof=true&sd=true

[@Aragorn450]

@@ModestMaker Thanks for the spreadsheet, although, it's a link that requests access, so you may get a few emails asking for access to the document... Like from me! 😄

[@RonnieTheSim]

@@ModestMaker It's just lovely. thank you!

[@ModestMaker]

@@Aragorn450 Thanks Charlie - fixed!

[@ModestMaker]

@@RonnieTheSim Appreciate that - thank you!

[@ConstantinosFrantzis]

Great video! I've got a couple of these sensors myself (for irrigation monitoring for my home garden) and this is good to know but unfortunate. I'm wondering if there's an issue of adhesion and compressed soil.
1) Water sticks onto the coating of the sensors, so it may be that if there's any water present, it'll be on the
2) Clayey/silty soils are pretty compressible. Inserting the sensor into soil will create a dense region around the sensor which may reduce water permeability away from the sensor itself.

[@ModestMaker]

Possibly. This is why I qualified my conclusions for “silty clay soils”, although I have witnessed the same behavior in premium potting soil (no measurements taken though). Thanks for those observations and suggestions 👍.

[@theirrigationguy700]

Interesting experiment. Was the soil loose or compacted?

[@ModestMaker]

The soil was initially broken up but settled when I saturated the columns. The full playlist with subsequent experiments is here : th-cam.com/play/PLqJ5k4cakypy1E4J_tvEiD0kpFK1tGY3D.html&si=CV6QYCToKY1i2yJY

[@J0stik]

trying to use capacitive soil moisture sensors for irrigation of tomato plants on my balcony. Re soldered timer chips for TLC 555 (because NE 555 chips). using Arduino NANO 33 IoT with set // Set ADC to use 12 bits, analogReadResolution(12);, that is giving me resolution from DryValue1 = 3480; to about WetValue1 = 1910; (dry soil to almost dripping wet saturation). i probably need to look more at soil type (bulk density), field density, irrigation point and Willting Point.
overal great video, I have to learn a bit more. It is not as easy as to set "two point calibration" and hope for the best :( :D

[@ModestMaker]

I agree on the two point calibration challenge. My concern is the sensor response is non linear as evidenced by relative insensitivity to water loss. I don’t think increasing resolution will help since (at least for my soil type) the signal itself is insensitive, but please share if you come up with any interesting results or further mods. Thanks for sharing!

[@J0stik]

@@ModestMaker yop, i have resolution from about 1000 (fully saturated soil) to 3550 (air), using simple kalman filter to at least somehow smoother spikes, mapped and constrained values to percentage (0 to 100 percent). but i will need to do full calibration for each sensor (for "medium" that i am using) determining field capacity and wilting point for coco coir + perlite. oh why have i gone through this rabbit hole ? :(

[@ModestMaker]

I know- right? I am working on a radiation shield for temp but as soon as I think I have it figured out, something else creeps up 🙄. The reward will be to finally get it all right but it’s a long and winding road though.

[@J0stik]

@@ModestMaker yop, i have set up second monitor for one particular plant. Two sensors in one pot (15cm apart) protected from water (resoldered timinig chip, glue heat shrink on electronic part pcb and epoxy on sides of pcb not to soak moisture from sides) just to determine wilting point by observation (to look at plant with eyes, when it will start to suffer from not enough of water) roots are in fairly constant temperature (20-21 °C) plant have not been watered for about 4 days and moisture percentage is slowly but steadily raising on one sensor (dont know why) plant is still pretty much thriving ...

[@ModestMaker]

@@J0stikEyes are the best!

[@mr.blacktown197]

is it possible that your sensor in this setup have a faulty reading because of the bad sensor? I have come across this video: th-cam.com/video/IGP38bz-K48/w-d-xo.htmlsi=Af19SZJ_XlXwAuC0 , Did your sensor use ne555, doesn't have voltage regulator or missing output resistor?

[@ModestMaker]

I checked all that here: th-cam.com/video/lb4HdcpYWCQ/w-d-xo.htmlsi=_xzaAGwenIYx4eBV . All sensors went through thorough quality control before selection.

[@Lisas4us]

Exactly, all this hobby soil moisture sensors are crap. We tried a lot of them. The capacitive once are just so bad. Soil type is a factor, but changing the sensor a bit, a mouse or worm will give you different readings.

[@ModestMaker]

Yes- in addition, two of the three I have installed in my formal setup just started reporting weird data after 6 weeks installed in the field. Temperature shows some promise- I will post some new results shortly, suffice it to say I haven’t had much luck with capacitance.

[@iddddd900]

coding ?

[@ModestMaker]

Buying, Programming, and Testing Capacitance Soil Moisture Sensors
th-cam.com/video/lb4HdcpYWCQ/w-d-xo.html