Comparing Cheap Capacitative Soil Moisture Sensors With Commercial Sensors

When your residence has soil moisture sensors embedded that were dictated by your friendly neighborhood HoA, you may start asking questions about the system used. That’s what [Modest Maker] did and the resulting findings along with an attempt to beat the commercial system with some cheap capacitive sensors, are covered  in a recent video that’s also embedded below. Part of the motivation here was that the commercial system in the community was not clearly installed properly.

To make a long story short, the commercial system by Hunter (Soil-Clik) appears to be a tensiometer-based system that uses the pressure produced by moisture intrusion into the measurement column. This translates to how easy it is for plant roots to extract water, depending on the soil type. [Modest Maker] had to first dodge the broken-by-design capacitive sensors that are available everywhere, but after that was able to cobble together a measurement system that he hopes will allow him to validate the commercial system’s installation.


40 thoughts on “Comparing Cheap Capacitative Soil Moisture Sensors With Commercial Sensors

  1. These capacitive sensors are basically useless, since they operate in a low frequency where soil salinity and temperature has a great effect on the measurement outcome. Salinity or other dissolved electrolytes make the “capacitor” leaky, which throws the sensor off. This is a problem for irrigation since clean rain water washing the sensor and diluting the salts, and drier salty soil can appear the same.

    The more expensive capacitive sensors also include a soil conductivity meter to correct for salinity, and a temperature sensor to account for the difference in the dielectric constant of water over temperature.

    Plus, these simple sensors built directly on PCBs absorb water into the PCB matrix and don’t last very long because they start to de-laminate. The absorbed water also throws the sensor off.

    1. Can you elaborate a little on how salinity has an effect on the outcome? Is there some sort of double-layer capacitance effect going on with ions in the soil?

      If you can point me towards more info about this, that would be awesome. I’m interested in doing some high-accuracy soil moisture measurements.

      1. Imagine if you had a perfect conductor around the sensor – like immersing the sensor in liquid gallium metal – that would bridge the sensing capacitor plates to one another such that the only insulator in between would be the thin conformal coating on the PCB. The sensor then would be measuring only the dielectric constant and the thickness of the paint on the PCB.

        I remember there’s some frequency band high up in the MHz range where the effects of salinity and temperature converge to a point. I read a paper about it where they used measurements at 500 kHz and then correlated that to something like 15 MHz to deduce salinity and water content by the difference it makes to the reading.

        These cheap capacitive sensors built around the 555 chip usually operate at some tens of kHz which is way too low to exclude the effect of dissolved salt. It’s probably low enough to also cause other issues like dielectric absorption which shows up as hysteresis.

          1. Thanks so much for this detailed response!

            Why do the higher frequency measurement circuits mitigate salinity effects? Is it because of ion transport physics inside the soil?

          2. john – Pure capacitance sensors determine the relative permittivity of the soil they are impeded in to extrapolate the water content. (Water has a much higher permittivity than the background soil, so the higher the permittivity, the wetter the soil.)

            The application of an electric field will cause polarization within the soil + water medium. The complex permittivity is measured in response to the applied electric field, so the material’s polarization will always lag the applied field. When you increase the frequency of the applied field into the upper MHz ranges, the imaginary portion of the complex permittivity will plummet as the dipolar polarization can no longer follow the high frequency oscillations in the electric field.

            Below ~500MHz, the imaginary part of dielectric permittivity is predominantly dominated by the salinity and resultant electrical conductivity. Above those frequencies, dipolar polarization will be significantly reduced, and the overall dielectric permittivity will be predominantly determined by the water content.

            Most commercial sensors skirt around this issue by using a lower frequency, and applying a correction curve to account for soil properties and frequency.

    2. So that’s why these are all garbage. I spent so much time a few years ago trying everything to make those cheap sensors work. It seems they are only really good for once cycle, where you can see the moisture go down, then each time you water it goes to a different range of ADC readings for wet to dry. I tried all sorts of board coatings (conformal, silicone, nail polish, etc), thinking that the PCB itself was absorbing something from the soil to change its characteristics. I eventually gave up and declared every one of them to be be unusable.

      They work ok as a “am I being dipped in a glass of water” sensor, but I can already use my finger for that.

      1. Yep.

        Every time you pour water on or around the sensor, a number of things change. The salinity, the temperature, and the density of the soil around the sensor as well as the contact with the soil as it, or the soil, will probably move around a bit.

  2. Don’t get fooled by the HaD Headline: The guy in the video in NOT comparing the cheap sensors with commercial sensors. He is just using the cheapest ones and – surprise – they are bullshit. Still waiting for a HAD Article or YT Video about: “This is a cheap, but reliable moisture sensor that will work for 6+ years in the soil.” – I’m tired of these pcb sensors. They delaminate and also bleed out toxic chemaicals into your plants. The only decent ones I came across, were the ones with metal rods instead of the “so clever” PCB integrated sensors. But even they were not accurate enough.

  3. The commercial sensors come with a 5 year warranty. With meteorological style sensors a 5 year warranty usually translates to a 10 year average lifespan throughout there geographic installation range.

    I’ve heard the most of the capacitive sensors fail in much shorter time periods and can barely last a year in some soil types.

    1. For irrigation purposes, capacitive sensors are less useful because the volumetric water content doesn’t really tell you how well the plants are doing. They’re particularly bad when the soil is relatively dry, because the reading becomes increasingly inaccurate and you have to know the exact soil type to tell whether the plants are getting enough water.

      The tensiometric pressure sensor lets you skip all the difficult bit about the soil type etc. because it tells you directly how difficult it is for the plant roots to pull water. However, they operate on the osmotic pressure between the soil and a capsule of gypsum (or equivalent), which dissolves away over time and the sensor properties keep changing gradually. They are particularly bad at cases where the soil is frequently saturated with rain or flooding.

      1. Hello Dude
        Thanks for adding some really good points to the discussion.

        From reading through it all I understand that capacitive sensors might not be the best to use for accuracy and long term use.

        You suggest that tensiometers might be a better way to go but mention that they might also go bad over time due the capsule dissolving away over time.

        But would it not be possible to overcome that problem by using a ceramic tip that is probably more durable?

        Also I guess that tensiometers will need regular maintenance (e.g. refilling) which is another drawback compared to the capacitive sensors.

        But are there other concerns with this type of sensor?

        What would, in your opinion, be the best type of sensor providing enough acuracy to use in an DIY irrigation system for a greenhouse if expensive commercially available sensors are out of the question?

    2. The cheap capacitive sensors fail when you measure. Passing electricity through the sensor will cause electrolysis which is a bad thing. That causes degradation. If you don’t leave it to measure constantly, but go for, say, once a day, it will last for many years. If you constantly measure, it will degrade in a few months time.

    1. Any group of humans can be horrible when self-interest rises above common sense. Groups of humans can also do great things, of course. A HOA can protect you from awful neighbors, or they can make your life hell if you are perceived as the awful neighbor. Whether that’s good or bad depends upon that so-uncommon common sense.

      Anyway, for this video, the HOA is simply the owner of some soil monitoring equipment that no one appears to be keeping track of.

      1. The point of living in detached housing is that you don’t need to listen to your neighbors and live under the constant surveillance and harassment of demented old ladies with nothing better to do than complain about everything imaginable and unimaginable, and most of the time completely made up.

        It’s your house, your yard, your rule – except when the HOA steps in and starts dictating how short to cut your lawn and what color to paint your fence so “the property values don’t go down”. You see, it’s the same old demented ladies and people with nothing better to do sitting in the association and making shit up to harass people and feel powerful.

  4. Those sensors won’t last long. The electronics are exposed and will corrode quickly outdoors and die. All the electronics and the connecting cable should be potted and made water tight. Also, If you’re going to leave the microcontroller outdoors even, in a water-tight enclosure, I suggest putting a small desiccant packet in the enclosure to absorb any moisture in the trapped air to prevent condensation from corroding the microcontroller board.

  5. There are better moisture sensors that cost a little more but are reliable and rugged. I’ve been using the Vegetronix VH400 for a year without any problems. I have two buried at the root level in my grass to measure moisture. They come potted and waterproof. Another option, that I haven’t tried but also looks good, is the SoilWatch 10 on tindie. There are a whole bunch of sensors on tindie.

      1. It lists as a “dielectric” sensor, suggesting it’s a TDR type sensor that measures soil dielectric constant by the speed of signal reflection in a conductor surrounded by wet soil.

        Only problem is, the density of the soil affects the reading. You get a reading proportional to the dielectric constant of the stuff around the sensor, but you don’t know the contribution between the water and the dry matter unless you know how much of the volume is taken by each. Usually the amount of solid soil matter is more or less fixed, and the amount of water varies, but it becomes difficult to estimate because adding more water makes the soil expand and become loose.

        That won’t matter if you’re only interested in knowing whether the soil is “wet” or “dry”, once you learn the difference. Making finer distinctions requires that you calibrate the sensor to the soil type and local conditions.

  6. I have a Rainbird SMRT-Y in my backyard, — which I got on ebay in 2020 for $131 shipped, but seems to be almost twice that direct from rainbird. YMMV I guess.

    Anyway, it uses a “Digital TDT” ( sensor, over a clever protocol that rides on top of the usual 24VAC sprinker valve wires. I reverse engineered it and wrote a decoder that uses a RP2040 PIO. I probably could be convinced to write it up properly if there’s interest. It gives soil moisture as a percent and soil temperature and electrical conductivity.

  7. What ever happened to using galvanic soil testers. Just two metal spikes with an analogue meter between them. I’ve not seen one for a while. They used to measure light intensity and pH too, via a combination of different metals and a switch. Pretty much as simple and robust as it gets, bar a little corrosion on the ground spikes. No doubt it wouldn’t take much to add a simple ADC chip and a microcontroller, perhaps even a wireless mesh network.
    It would be interesting to see how long those capacitive sensors last in the ground before the traces corrode away.

    1. That extra price which going to sensor encasing results in less efforts in suppressing response to remove the setup after couple of years. And if there are more projects like this – it could make a place fell messy. But “feel” if a perception thing. And gradual improvements possible too.

      Great job to author.

  8. First, my sincerest thanks for all the interest realized by this video. Lot’s of good feedback here, much of which I agree with.

    Of greatest significance (and as Dude correctly noted), a rigorous approach requires having sensor response correlated to volumetric water content. Why? It’s because different soils can be considered “wet”, but will “hold on” to that water tightly as a function of soil texture. As such, a “wet” (high signal response) doesn’t necessarily mean that moisture is available to plants. Water availability depends on soil texture and composition.

    In addition to reliability, the challenge I see with these sensors is that even at very low volumetric water content, they can return a very high signal– at least for my tight soils here in Arizona. The lack of sensitivity to actual soil moisture (at least for my soils) means they are not particularly effective for rigorous scientific purposes. They MIGHT be useful if you can qualitatively correlate signal response to landscape conditions, and are willing to maintain them over time.

    Also, I agree that soil tensiometers are better for cutting to the chase of water availability. However, these are significantly more expensive and not perfect either. The issue there is that different plants are more or less effective at competing for water at a given soil-water tension. That important detail gets missed when we use there frequently-referenced standard of 15 bars for permanent wilting point. That number might be okay for commercial crops, but may not necessarily accurate for native desert landscapes.

    In summary, this is why I don’t think you can just put these sensors in the ground and walk away. I’ve learned you have to really know what you are doing to use them effectively. At a minimum, you have to at least calibrate signal response to landscape conditions as clearly noted in commercial sensor manuals.

    Finally, there may be another approach to monitoring soil moisture that doesn’t require soil tensiometers or capacitive soil moisture sensors exclusively, at least in the desert southwest where ambient temperature can be highly variable. Since water has a high specific heat, it cools soil down as the soil dries out (assuming all else equal). I’ve observed that you can actually see this in the temperature profile over time as soils dry out. As such, monitoring temperature in soils can supplement other monitoring data (sensors, feel, observation, etc). On that note, I’m putting a temperature experiment together right now using multiple DS18B20 temperature sensors coupled with capacitive soil moisture sensors and should have a video posted in the next month or so.

    If you’d like to learn more about what I’m getting at in using temperature as an analog for soil moisture, details are posted in this video:

    … and also here where temperature captures the impact of adding a thin layer of mulch to your landscape:

    I’ll post an update to this thread in a month of so– thanks to everyone for your feedback and interest!

    1. Hello Hans, thanks for write up and exploring this. Over the years I have had little success with the capacitive soil moisture sensors. Part of my career I worked in developing countries with low income farmers developing irrigation technologies. One tool we had to help with a water management plan for the famers was a low cost tensiometer. The tensiometer was a simple 12″ pvc pipe filled with water, the top has a silicone cork, the base is a clay spike, just below the cork is a color coded pressure gauge. As water is pulled out of the clay spike by the soil, pressure in pvc decreases and moves gauge, reverse when moisture is applied. It seemed to work great for our general purposes but we never did collect data. Replacing the gauge with a low cost digital barometer could make the devise into a low cost IoT soil moister sensor.

      I’m very much on the side of measuring soil water tension when developing a water management scheme for irrigating. While -15 bar is the standard permanent wilting point, you always aim to be above that value . It can be an in-depth analysis of crop production and the crop value in determining the ideal value that should be targeted.

      Feel free to reach out if you have any question on the described tensiometer or would like to see some photos. Here is a link I just found form UC Davis with a similar device,see%20Figure%201%2C%20below).&text=Why%20Use%20Tensiometers%3F,indirectly%20measure%20soil%20moisture%20tension.

      1. Ryan- I apologize for my delayed response and thank you for you for sharing your low cost alternative. I will certainly be reaching out to you for further details.

        Regarding these capacitive sensors, I have since learned quite a bit after attempting to calibrate the same for detecting plant wilting point. Details of that experiment are captured here.

        Calibrating Soil Moisture Sensors for Irrigation

        The bottom line is after a 60% reduction in soil volumetric water content, the sensor showed no variation in response. This means the sensor does not respond linearly to losses in soil water. As such, the rating system I developed really isn’t valid.

        This isn’t to say that the sensor doesn’t have some redeeming qualities. I think that if combined with soil temperature with data evaluated over time, it might serve some useful purpose. I am exploring that possibility right now with this setup:

        I am about to post some new results shortly and will be tracking all these experiments on this playlist : .

        I am familiar with the theory behind soil tensiometers but don’t have any practical experience. I appreciate you sharing your knowledge and will reach out to you shortly.


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