Seeking inspiration: Soil moisture sensind that isnt 💩 - Page 1
Forum section: electronics / projects

author: Amper
author_rank: Frequent Contributor
timestamp: May 08, 2025, 11:35:12 am
content: Hi!Im looking for a way to masure soil moisture level, as with all the automated growing projects people did this seems to be a problem that isnt properly solved. Almost all sensors i can find (starting with chinese arduino stuff made from leaded PCB to expensive industrial) work by conductivity and they all either corrode away, are horribly inaccurate or contaminate the soil. I really dislike conductivity and capacitive is a bit better but also seems like its not the best solution.Is there measuring principles that are in reach for hobby use (no neutron flux sensors) and work "indefinitely" without loosing calibration or dying from corrosion?One idea i had was putting an inverted cup into the soil with a "dome" above the soils surface and put an air humidity sensor inside of that. I assume the air moisture level will change depending on the moisture of the soil and since its in a bubble above ground its water protected even when watering begins and the soil gets flooded.The downsideis that calibration wont be easy i suppose and the dome has to be at the same temperature as the ground even in sunlight.Open for suggestions

author: mtwieg
author_rank: Frequent Contributor
timestamp: May 08, 2025, 12:43:14 pm
content: I've never worked with soil moisture sensors before, but I'm curious... I see there are different types of sensors (Tensiometers, resistance, capacitance, TDR, etc), is there even an expectation that their measurements will be comparable with each other? Can any of them actually be verified against some sort of standard? Can one even expect comparable results between sensors of the same type (for example, can two resistance-based sensors be expected to agree if they use different electrode shape, or one uses AC while the other uses DC)?Quote from: Amper on May 08, 2025, 11:35:12 amThis might work. You will only be measuring the soil at that air-soil interface, but that might be good enough. Disturbing the soil might cause the air pocket to collapse though.If this is for isolated potted plants, you could just put the entire thing on a good mass balance and track weight over time. Of course the plant's weight will also contribute, so depending on the pot/plant this might not be accurate enough. I recall my dad used to grow five-foot tall cacti from gallon-size pots, definitely wouldn't work there.

author: Marco
author_rank: Super Contributor
timestamp: May 08, 2025, 02:14:03 pm
content: That's probably how soil moisture sensors should work. No direct soil contact, because it introduces massive amounts of confusing variables. Just a vertical interdigitated capacitor covered by PTFE (for antifouling when it gets wet) covered by a fine mesh box so the soil never reaches it.

author: MariuszD
author_rank: Regular Contributor
timestamp: May 08, 2025, 02:45:01 pm
content: Resistance measurement is bad idea, but is cheap, and easy to measure for Arduino noob. Capacitive sensor can be made watertight, electrodes should be insulated. But it requires high frequency AC voltage to measure the capacitance.

author: Amper
author_rank: Frequent Contributor
timestamp: May 10, 2025, 01:55:21 pm
content: I just had an idea that i may try:Making a sensor housing with a goretex membrane. They are available as housing vents for enclosures, water tight but moisture and air permeable. This way it should be possible to put the sensor under the soils surface and not have it die from water overflowing when watering.If i get around to it ill keep you up to date.

author: floobydust
author_rank: Super Contributor
timestamp: May 10, 2025, 02:51:33 pm
content: I've used Model200SSWatermark Irrometer Soil moisture Sensor."In use since 1978, the patented WATERMARK sensor is a solid-state electrical resistance sensing device that is used to measure soil water tension. As the tension changes with water content the resistance changes as well. That resistance can be measured using the WATERMARK Sensor. ""The sensor consists of a pair of highly corrosion resistant electrodes that are imbedded within a granular matrix. A current is applied to the WATERMARK to obtain a resistance value. The WATERMARK Meter or Monitor correlates the resistance to centibars (cb) or kilopascals (kPa) of soil water tension. ""The WATERMARK is designed to be a permanent sensor, placed in the soil to be monitored and “read” as often as necessary with a portable or stationary device. Internally installed gypsum provides some buffering for the effect of salinity levels normally found in irrigated agricultural crops and landscapes."https://www.emesystems.com/sensors.htmlCan use a 555 to convert R to F, as their SMX does.

author: Amper
author_rank: Frequent Contributor
timestamp: May 10, 2025, 07:45:57 pm
content: Arent those supposed to be only turned on a few times for measurement? To me the description sounds like its not for continuous supply.

author: DaJMasta
author_rank: Super Contributor
timestamp: May 11, 2025, 04:20:38 pm
content: You measure at specific moments, right? So if you want to measure every minute or hour of the day but your measurement time is a few milliseconds, most of the time is spent off, right? Not sure what benefit having an always-energized sensor would be, and it basically guarantees more issues with corrosion.I'm not going to say it's the ideal solution, but what's the specific aversion to the in-ground sensors? What sort of replacement interval is unacceptable?A buried air humidity sensor (most of those are capacitive, but air is more predictable a medium than soil) will probably do the job just fine, but keeping water, mineral deposits, and roots out of it in the long term is going to be the design challenge, and making it physically large would be one of the easier ways to mitigate it, which sounds like not a good option in the case of potted plants.If they are in pots indoors, you may be able to infer fairly well just from monitoring the ambient humidity, and a relative measurement from an inverted cup on the soil surface to ambient air would probably do a fine job when calibrated.