A range of these capacitance sensors were tested on the Flaura - Smart Plant Pot video. The youtube algorithm worked this time! This channel shows that many are made with flaws and he shows the flaws (82% of his test sample), how to choose when buying, and how to fix the flaws if you have one. It is definitely worth watching.

This is a re-upload of video#207 of June 17, 2018. Some other solutions proposed by the viewers are: - Connect an output pin of the MCU instead of VCC to the sensors and switch it to high only for measurement. This reduces the time the current is flowing - Use other probe materials like steel, nails, or even gold-plated material The "green stuff" is copper-oxide for sure not healthy, but it should not hurt the plants in minor concentrations. The same happens at the Statue of Liberty, BTW. Some viewers also used nail polish to protect the edges. And it seems that sensors with manufacturing errors are in the market. Search for the "Flaura - Smart Plant Pot" channel

I tried to measure moisture 45 years ago with two wires, and found out it worked for about a day. When I asked my teacher, he introduced me into the wonderful world of electrochemistry. Thank you, Jos Berenzten!

Hi Andreas! Just a comment about the home made PCB. It is very easy for copper clad FR4 to make 2 cuts through the copper down the center close together with a razor blade. Then lift a corner of the copper with the razor blade and grab it with a small pliers. It can usually be stripped off fairly easily. It should only take a minute or so to do.

Hi Andreas, Always a pleasure to view your videos. I have played with quite a few of the capacitive sensors. I found that with some of them, the 1M ohm resistor that is supposed to be connected between GND and SIG is there but is NOT connected to GND! . Seems to be a design defect as a whole batch I ordered from the famous Chinese supplier had the same problem. As a result, the reading was false. To correct it, I just soldered a 1M ohm resistor between the two pins on the connector. Easier than trying to correct it on the PCB with the surface mounted components.

What I can recollect about Soil moisture sensors is that you need to either operate them with an AC signal, or periodically run an AC signal through them to prevent the electrode from destroying itself, particularly sensors like the water mark that uses a gypsum (I think that's what's in there) in a small canister and if you don't there is a migration effect that will eventually destroy the sensor. Sorry, wish I had more details I hope that helps. The capacitive sensors are probably the best for you applications, but the cheap ones could potential work if you treat them right.

Very good tuto, as usual. You are our Master Theacher. Another good solution to protect the capacitive sensor pcb from moisture would be to cover the edges with neil varnish.

Wow, this video is incredibly fascinating! I'm really intrigued by the concept of building a multilevel soil sensor after watching this. However, I'm not quite sure where to begin or how to go about it. It would be amazing to see a video from you detailing the process and offering guidance on how to create one. Your content is always so informative and inspiring!

A very timely video. I'm working on a tool, a portable moisture sensor. Subscribed. Thanks!!

Well use missed the two most important principles for measuring soil moisture. 1. Time Domain Reflectometry (TDR). 2. Frequency Domain Reflectometry (FDR) / Capacitance (Frequency) Both are state of the art when measuring soil moisture within scientific context. When I saw your video I hoped that you have managed to build one. 😅

Thanks for pointing out the “if you let it sit for six months or so” issue - helped solve my mystery and fixed my shower. 👍

Your videos are always wonderful, instructional and your Swiss accent is adorable too

Thank you! This reminds me of the capacitive tank level sensors. Same principle.

Very nice view over the lake. Switzerland is beautiful. I have had the same problem with "open" sensors. I took bicycle spokes and solder it on the sensor. It works OK.

Excellent! I had to correct my shopping cart after watching this video. Capacitance types as mentioned in Flaura's video and yours will be the choice of sensor.

With your explaination it is very clear the one to use. Thank you for good work.

Great video. Thankyou for the upload!!!!

Nice re-up, Herr Spiess. :-) To further enhance this widely used humidity sensor, please have a look for the page titled "Hacking a Capacitive Soil Moisture Sensor (v1.2) for Frequency Output" from the Cave Pearl Project. There you will find 3.3V capability, how to use higher frequencies and some more tips and hacks around this sensor. In short: instead of the sensor with the NE555 chip you should favor the version with the CMOS TLC555 chip.

I have one project where I decided to solve this problem reading the sensors very quickly, and then turn-off the circuit where the sensors are connected. First the Arduino turn the circuit on, then I made 64 readings in less than a second, and calculate the average value of the readings. After that I turn off the circuit. As I have to make just one or two measures per day, I think this is a good workaround.

Great video, I tried explaining to a colleague in our makerspace why the moisture sensors they used for literally all the plants are bad for the soil and everyone was complaining what I was such a downer and that it should work fine because 'everyone does this' and 'why would they be so massively sold if they dont work'. Well, now we know why. Anyway, the moisture sensors all failed within a year and some plants died. Copper oxide is really bad for plant life. I'll be sure to share this video to prove my point, but on top, the best part of this video is that you offer a solid solution. I will get a bunch of these sealed sensors and seal the PCB edges and components with some additional epoxy coating. I might even design my own since you shared the schematics and it looks like this could also work with copper enameled wires since these some pre-insulated and might be easy to wrap around a customly designed 3D printed part (no worries, I use PolyCarbonate for the printed parts so it won't degrade like PLA).

I've never used them. I was expecting you to use Proximity sensor with it's sensitivity set extremely low. I have no idea if it would work but I'm my head it makes sense. Nice video.

Loving the SHT-35 for humidity, so far at least. They seem to be quite a bit more robust than the old DHT sensors. I run 2 of them 24/7 to add humidity to my plants via a relay-controlled humidifier.

Very insightful, A few years back someone shared this corrosion problem, i think the guy used graphite rods to solve it.

Good morning Herr Spiess, I hope your summer holiday is going well, we're missing your fresh content every Sunday morning :) WRT capacitive soil sensors - I coated their edges in 2 layers of nail polish but I was surprised to still find their _surface_ delaminated and corroded after little more than a year in use. Not going to buy again, they're kind of a lost hope.

Summer???? Spring is almost here in Australia. Great show, Andreas.

Thx for this topic... It's long time we develope this.. and have so many alternative.. But sometimes, its also depends on media that need to be measured..

I've used a similar circuit on the Farm-Data-Relay-System attached to a Watermark Soil Moisture Sensor. I've used the sensor for years and it works well. I also make Gypson Moisture Sensors that I have described on Printable. They provide similar readings but they tend to dissolve over a period of time. Watermark sensors are very expensive but are worth the money for monitoring my foundation in Texas.

Nice!, good research👍🏻👍🏻

Great video! I did some research a while back into these and came to a similar conclusion, that both of the main sorts are fatally flawed. Coating the capacitive sensor with a stronger layer (e.g. dipping in epoxy), or pulsing the conductive sensor for very short times were the results I came to. I’m not sure what the corrosion without any current is, but I figure it’s pretty low. A gold-plated sensor might last long even with a continuous current. As for the toxicity, considering the nonzero chance of that plating containing lead, I’d be very hesitant to use an off the shelf Chinese conductive sensor. Don’t want my broccoli to start lowering IQ points. Tin and nickel, and especially copper, can be toxic to plants too, but with the small concentrations after rain washing things it’s likely not a big deal. Actually, some people deliberately corrode copper wire in their soil to increase the amount of copper in the soil. Apparently copper deficiency is common enough in plants that it’s a cheap way of improving plant growth, though I haven’t looked into it myself.

Using stainless steel will help with the longevity of the sensor. This type of sensing is good for a small container where you don't care too much about the moisture content at different depths. One could use several of these at different depths in order to construct a map that describes the soil's behavior to watering. This information can be used to optimize water usage and similar concept is used in professional sensors.

Hi Andreas, I have tested the capacitive moisture sensors quite extensively in aquaponics. I use a ADS1115. I found considerable variation due to tempurature fluctuation.

NICE HAVE NOT SEEN THIS BEFORE THANKYOU. Have many old stuff from this time and have start growing my own food salads stock for added sustainability in a ever changing eco system, thinks I can grow indoor in window light controlled systems but as you know controlling these systems are harder than people think you for replay sir.

I remember getting the "chirp" sensors that ran on 3V coin cells back in - well, long ago. What I missed on these capacitive sensors was square wave output - I would assume that would be easier to transmit over longer distances. I've seen I^2C and RS485 versions, but they are pretty expensive.

There is the "tinovi I2C Non-Contact Capacitive Soil Moisture, Temperature sensor" which works very well (full waterproof). I wrote the ESPhome component (avalable in the last ESPhome dev)

these are some very interesting insights

Great video Andreas! I've been having problems with the capacitive sensor, making my house main differential circuit breaker go off as soon as the sensor contacts the soil. I'm assuming there's some kind of leakage? Will try the plastic bag to isolate the sensor and see if it helps

Hallo Andreas, der Beitrag kommt genau zur richtigen Zeit :-) bin gespannt, ich habe einen mit zwei V2A Stahlröhrchen "gebaut" :-) Den Kapazitiven Sensor hatte ich auch schon mehrfach verbaut, leider hat sich auch hier nach einiger Zeit die Farbe / Isolierung gelöst und die Kupferbahn aufgelöst, nicht gerade gesund für die Pflanzen

In production horticulture we use a neutron probe, but they are expensive. I have found the capacitive probes have a narrow range and are easily disturbed, somewhat difficult to calibrate IMHO, but definitely a cheaper option!

I testesd one of the capacitive V1.2 wrapped in film for resin UV resin detection and was surprised it worked.

Grutti Andreas, If you look at the schematic at 8:28 - the exposed "sensor-electrodes" still have a DC on them, albeit being used capacitively. It is a simple addition to place two large-value film-capacitors in series, one on each of the electrodes, to effectively"DC-isolate" these from any active electronics. The plasti-dip or whatever then has to do less work (esp. with pinholes etc. still being able to ruin the sensor). As the two series capacitors are much larger than the soil-dependent capacitance, they do not affect the frequency-modulation.

Interesting, just what I found using in my greenhouse. Thanks.

Could you propose a method to calibrate the output characteristics for the particular soil type, installation depth, and soil compaction? In other words can we compensate for these effects in order to improve repeatability? Maybe comment on use as tank fluid level sensing. Thank you.

I use a very good nail varnish to cover all the moisture sensor v1.2 including the chip and the pins and wires and works fine since 2-3 years now. But there is a missing connection between ground pin and the a ground trace. Simply solder a small diameter wire on both side of the pins header. Cheap China stuff but works.

It's super easy to build this kind of sensor yourself anyway. Just get copper rod, solder wires to two pieces, add the 500kOhm input resistor, to one and a ground wire to the other. Add a second wire to the one with the 500kOhm directly to an analog in of your microcontroller. That's it. You can have either the v+ or ground coming from a toggleable port so you can save a bit of life on your sensor.

Great info

Hello Andreas, Gardena (Husqvarna) uses a different approach for its irrigation/sprinkler system. Its moisture sensor contains a metal tip that is enclosed in felt. This metal tip heats up periodically (about every 30 minutes) and, based on its temperature decay (cooling time), the degree of soil moisture is calculated. When "wet", the connection between two pins is "closed," when dry the connection stays "open". The two pins are connected to the irrigation controller. Why periodically - to allow for battery operation. Greetings, Stefan.

you can use the cheaper ones without their electronics, and alternate the current thru them. the main problem with all these sensors is their resistance depends on the soil and the minerals present. takes allot of effort to calibrate them on specific soil now try it with changing soilminerals (rain water vs tap)

The first real Arduino project I built used these resistance sensors. I was so proud of myself, until the 3rd and 4th days when the sensors were ruined.

I have used that cheap sensor on my Christmas tree to warn about low water level for many years without a problem. I realised this design error and implemented very simple correction: I'm not powering it all the time. I use Arduino output pin to power sensor only for a couple of ms only when I do measuring once per each hour.

Thanks !

Arduino's can drive and measure capacitive changes between plates directly. There even an offical library for reading such sensors. It's also *easy* to create such plates oneself. If aluminum or titanium is used it's also possible to anodize the surface to insulate them. Far more even and resilient surface than varnish Nb the thickness of such an anodized layer adjusts the value/range of such a sensor

Did you check the RF-emissions of the capacitive sensor with a SA? I think there might be a lot of harmonics causing QRM in the HF bands. Maybe not relevant for the average houshold, but in fact, for example I've turned off my weather-station because of spurious emissions in the ISM-band, disturbing my satellite groundstation. So, maybe as a HAM-operator I'm a little bit over-sensitive to this . Anyway - nice video, thanks!

Andreas - good to see this topic again. I was a disappointed to learn that even capacitive sensors fail over time if they are just coated with resin, as the resin still allows water intrusion via osmosis. I wonder if that includes all forms of epoxy. Perhaps the probes and the base of the instrument body could be coated with "Plasti-Dip"? The plastic bag is a fine idea, I guess, but really the sensors should be fully sealed in plastic until the whole instrument is up out of the soil and well out of the way of splashes, etc. So rather than putting plastic "leggings" on the sensor probes, I'm talking about plastic "pants" that reach to the top of the sensing instrument. I am a bit sloppy when watering plants, yes. :^)

I bought the Xiaomi Miflora Sensor wich has also a lot of knockoffs wich are cheaper, Monitoring it for 7 Montths now and it works fine and reliable + has integration features Battery still shows 100% You pay a little extra for the enclosure ( water resistant ) and the ease of use ( not encrypted BLE ) and a nice byfeature is the EC measuring wich gives a rough ( not really useful ) estimate when its time to give a little more fertilizer or if there is too much salt build up Integrated in HomeAssistant with Automated Watering learning based of the dry / wet cycle and times it takes the plant to take up the water added I couldnt have or use more features and its working out of the box, no need to ask for more after fiddling around with diy solutions for years im happy that this project is now running and working

Good explanation as always.

Hi Andreas, did you ever try to put a Bosch BME sensor into a cavity underground like a small platic pot with the opening facing down? This would give you the relative humidity of the air in equilibrium with the soil.

I'm wondering if there are also moisture sensors that work on the principle that periodically a small resistor gets heated by electric current and then a circuit measures how long it takes for the resistor to cool down again, which should be dependent on ground moisture levels.

Another option to extend the lifespan of sensors with exposed copper conductors is to make measurements in a descrete manner. I.e. not constantly but once every 30 minutes for example.

Danke die für den super Beitrag! 73 von HB9TIA!

i would love to see you include a "Gypsum" soil moisture sensor in your comparison!

The capacitive approach is a simplified form of impedance spectroscopy. The real fun with that is when you sweep the frequency and take a spectrum instead.

there is a prefect soluiton from 70's: gypsum block. the response time is slow but extremly accurate.

Another technique can be to only power the sensor while the moisture content is being measured. No reason to read the sensor constantly, once an hour or so is more than enough. Cut the power to the sensor to only a few seconds per day will extend the life of the sensor.

You can use the first pcb very easy with an arduino and no electrolysis. Connect the pcb between to analog inputs. For example A1 and A2. In the first measurement make A1 output high, it will supply now 5V to the sensor, read the analog value from A2. For the second measurement you do the opposite, make A2 digitaal out high and read the analoge value from from A1. Repeat the above. With this way you measure a sort of with ac and the electrodes with not desolve. If the reading of the adc is to low, try setting the reference to 1.1V or other value.

What about sensing by weight instead of moisture level? A flower pot will have a certain consistent weight. As moisture evaporates, the weight will be reduced. At a certain point, you turn on the water. You might have to re-calibrate from time to time as the plant grows.

I have tested the capacitive soil moisture sensors extensively and they are not reliable whatsoever to measure anything accurately in soil, the only working solution is to make a resistive sensor that receive power only for a very short time with prongs in stainless steel or silver solder. That was my solution to manage my sprinkler system in Florida. Anyway, there is no need to measure the moisture in real time, 2 or 3 times a day is more than enough, humidity does not leave spontaneously the soil.

Hallo aus Österreich, Andreas! Ich mag deine Videos. And "the Swiss accent" :) Wegen des Kupfers... Wenn du, so wie du es gemacht hast, die Elektroden ins Leitungs-/Brunnenwasser steckst (nimmt man ja meist zum Gießen) und mit ein wenig Spannung und einigem Strom beaufschlagst, erzeugts du einfach Knallgas. Sauerstoff und Wasserstoff an je einer Elektrode, das entstehende Wasserstoffgas kann als Blasen an der Oberfläche der Kathode freigesetzt werden. Komplizierter ist es mit Folgendem: Im Leitungswasser sind üblicherweise verschiedene gelöste Salze enthalten, darunter Natrium- bzw. Kalziumionen und Hydrogencarbonationen (HCO₃⁻), die möglicherweise reagieren könnten, um Kupfersalze wie Natriumkuprat (Na₂CuO₂) oder Kupfercarbonat (CuCO₃) zu bilden. Verschiedenen Faktoren wie Wasserzusammensetzung, pH-Wert, Temperatur und Stromstärke beeinflussen die Reaktion. Wenn man Pflanzen mit einer solchen Flüssigkeit bewässert, bzw das im feuchten Erdreich auftritt, könnte dies potenziell schädlich sein, da Kupfer in höheren Konzentrationen toxisch für Pflanzen sein kann. Kupfer ist für Pflanzen notwendig, aber nur in geringen Mengen. Bei zu hoher Konzentration kann es jedoch die Aufnahme von anderen wichtigen Nährstoffen behindern und oxidativen Stress verursachen. Die Kalkablagerungen könnten auch den Boden pH-Wert erhöhen, was wiederum die Nährstoffverfügbarkeit für Pflanzen beeinflussen könnte. Ein erhöhter pH-Wert kann dazu führen, dass bestimmte Nährstoffe weniger leicht aufgenommen werden können. Es ist wichtig zu beachten, dass die Auswirkungen auf Pflanzen von verschiedenen Faktoren abhängen, einschließlich der Konzentrationen der beteiligten Ionen, der Dauer der Bewässerung und der Empfindlichkeit der Pflanzenart.

I remember reading about this problem in the 1980's (probably in Electronics Australia Magazine). If I recall correctly, there was an IC manufacturer who produced a chip specifically for this application. It applied a small AC signal to the probe for measurement to prevent electrolysis. I see lots of moisture probes advertised now and just assumed they would use the AC method. I forget how much junk is sold out there.

If price is not an issue, i have uses smt100 and smt50 from truebner on a research project. They currently have an uptime of 4 years (but they cost 100 and 50euros respectedly)

Danke!

Bedankt

Could you drive the original sensors with an AC voltage to avoid the electrolisys and then rectify the output signal with a diode?

The two-legged sensor with a gold plating most better than of cheaper lead. I've tested ENIG version about 2 month, if not scratch them, no corrosion will be even with fertilizers. And power on must be very short as possible for measuring.

I tried several capacitive sensors-cheap one, DF Robot one and two other Chinese brands. None can survive more than 3 to 6 months even if I coated it. Soil is salty due to fertilizer usage so it greatly reduce the life span of the sensors.

sonoff ms01 is looking pretty good, it sends data trough I2C, it is on my to do list to test one in esphome and if satisfied switch from all other ones .... but it is not on top of my list ....

The first sensors can benefit from probes made of stainless steel. But all have a major flaw in that they use a lot of power and they are not the best thing for IOT standalone sensors. Also resin coatings in pcb board suffer in the long run from water permeating through the pores because of osmosis. This causes blisters filled with acid under the coating. The latter I suppose one could accept it as a wear part in the system.

I have tried all the sensors shown as well. For the capacitive one, it consistently gave "wet" readings even when the soil had completely dried out. It seems like the moisture sticks on the sensor surface, it couldn't dry off quick enough as compared to the moisture in the soil. I had to remove the sensor to wipe it down for it to show the correct readings again, which makes it completely pointless.

I wonder if the resistance sensors would be better with graphite / carbon probes? Carbon is pretty inert in water electrolysis, unlike copper. Whilst they wouldn't last forever, they would significantly outlast copper traces. Also carbon is not toxic to plants whereas high concentrations of copper can be.

I have a commercially made sensor (Ecowitt WH51) that appears to have a continuous loop that goes into the soil. Perhaps under the coating it's not a continuous conductor though. It eems very reliable but now I want to see what is inside.

This was interesting, but might be a little out of context. This would probably be fine as a physical liquid level measure, but that's not how dirt and moisture work in most agricultural situations. If your plants roots are sopping wet, your plant is likely drowning. Usually there is much less liquid in soil, and that may make the readings much different. Could you repeat the experiment with the last sensor, using different types of soil, and adding water like a grower would? The best test scenario would be in a large planting container exposed to the outside air conditions and to the sun, but protected from direct rain. That way, there is a large amount of soil, there is air, heat, sunlight, wind, to dry the soil, and you can add water in small amounts, as if from a sprinkler or irrigation system. The moisture in the soil will need to equalize across the bulk of material, rather than just detect full saturation. Water would arrive on top, and then get wicked through the soil. Having a short or small container might give false readings from water pooling in the bottom, saturating the lower part of the sensor. The sensor needs to be affected by the correct amounts of moisture in the soil, not just "dry_ and" saturated" conditions. That would simulate regular growing medium better, and give a better idea as to the signals seen and the response time to detect actual soil conditions. _____ As an added thought, this looks like it would be a good wired system for a central garden plot. Consider a yard, though, or multiple garden plots, distributed areas, etc. Having wires running 10s or 100s of feet, in multiple directions, on opposite sides of structures, etc., and with foot traffic or vehicle traffic in the area. This would be hard to set up, calibrate, and maintain. A larger scale solution would call for wireless communications, sensor differentiation, logging, and probably power management. This is a job for SUPERMA-- Sorry, I got carried away. This is a job for: I don't know... It's got to be be cost, able to leap large distances, be identified, easy on the batteries, etc. LoRa? I don't know the costs or component size. 422mhz? ATtiny85? RaspiNano? BLE? I can think of a couple approaches. I can envision a mesh network for ease of adding sensors and extending the coverage of the network, but that seems complicated. I can see having a base station and just relying on low power rf transmitters, logging and sleep routines, and periodic polling fob the base station. I believe that BLE can operate as a mesh network. That's enough to think about now. I think I want more coffee.

Even the capacitive sensor isn't good, with exposed FR4 edges. I looked into this once and Steven's Hydraprobe looks good. It gives moisture, temp, salinity over RS-485 using 50MHz RF. The cost is $695 though (!).

So you made an unintentional electrolyzing cell. You are deplating the anode(+) side and plating the cathode(-) side. As well as liberating some hydrogen on one electrode and oxygen on the other. One way to make them last longer is to swap the polarity every other time you read it and only activate it when you want to read it. Nickel plating the electrodes will also help some. 😊 And for the capacitive version you might use some spray on conformal coating.

Cool DIY sensor ;-). But seriously, I think a capacitive DIY sensor can be a good option for makers and you can easily create a square wave signal with a microprocessor board.

I use hot-galvanized nails as probes. These are the gray, rough looking ones. Few years now, no corrosion. Avoid electro-galvanized (shiny) - they wont last.

Any electron flow in a damp environment will cause material to migrate either from your plate or to your plate. None contact is the only way to go, or treat the sensor like an LCD display where there it has to be treated the same, an AC signal with no DC component, or you destroy the segment.

Hello Andreas I use the last sensor (capacitive on), there is a but you forgot. The value measured is not stable since it is impacted by the temperature. It creates quite big differences. I ve not solved this yet.

Metal salt fertilizers that people use to fertilize the plants affect the readings and the life expectancy. Most powdered metal salt based plant fertilizers are extremely conductive Just like when you’re making a battery and you’re adding acid Like a nickel, and a penny the copper nickel with a piece of paper in between, and you add some vinegar As you’re going to summer in the soil gets dryer, faster, the salts from your fertilizer, build up and acidity or the salt content spikes . Throwing off your sensor readings .

Ah the classic ox redox reaction. You might even be creating metal complexes in the water (gets very complicated to know if its plain rust or a complex of Fe)

Great vid 👌

A super cheap workaround is running DC for 10ms every minute or so. You still get your readings and the sensor lasts much longer.

Durex is a brand to protect some sensors with a rubber layer

Why do you want to apply the dc voltage to the sensor all the time? Normally it is sufficient to check the moisture once a day and power the sensor for just one second or below? Then the resistance sensors are also quite durable..

Nearly 100% of this is Electroplating 101. Keeping DC measurements quick and short can prolong the electrode life as long as the time is less than the polarization time of the electrochemical cell - in this case the materials and the distance between electrodes can have an effect although my experience has been to keep the measurements lexx than a few milliseconds and, if possible, disconnect the probes from any cirtuitry which might provide a DC bias current to flow. Ground, any power connection, etc., a good double pole relay would work here. Also changing the polarity, as suggested elsewhere, can help. Another way is to transformer isolate the probes although this quickly gets difficult and requires AC measurements which are more difficult for the novice. Capacitive sensors are, by far, the best bet for this environment. All this the result of trying to measure pool water for various parameters.

Great video

I coated the edges of my capacitive sensors with clear finger nail polish. I set up an Arduino Nano with the sensor and an ESP-8266 (ESP-01) and can monitor the data via WiFi. If I didn't already have the Nano and ESP-01 I would probably use one of the Arduino's that have WiFi built in.

It has always bothered me that the bulk of Moisture Sensors being sold are nothing more than simple ohmmeters which measure conductivity. I guess most consumers and manufacturers never paid attention in Jr High science classes. Try measuring the moisture level of distilled water and they read dry. Water doesn't conduct electricity folks, only the minerals suspended in the water. Which of course changes when water is added to soil, the alkalinity of the source water, etc. If you want a cheap electrolysis instrument to stick in your soil to tear apart the water molecules, then by all means spend $2 for a moisture meter from Walmart or other box stores.

I wonder if an air humidity sensor placed under an upturned cup on the soil surface would be of any use. It could be an interesting experiment.

those two leg sensors basically suck, but you can prolong their life by measuring soil moisture only when needed, e.g. couple of times per day

Who would've thought that the verable 555 timer would get a new lease on life being used in a humidity sensor? That chip just keeps on giving.

Nice test, however, you cover only one aspect of moister sensing because you test the probe in water, which is the case maybe only in a swamp. With different types of soil, readings will differ and you may expect only trends not measure humidity with countable precision. I created and sold capacitance soil moister probes almost 10 years ago with limited success because of issues with soil (ph, soil compacting, type of soil and ingredients, temperature, and so on). Probe was tested in different climate ranges and soil types, even in the substrate in almost laboratory environment.