I live in the desert and actually have a few plants that aren't cactus. I have Mesquite and Palo Verde trees as well as a very few Chollas and Creosotes. I strictly forbid Desert Broom and Staghorn Cholla from almost all of the area because they are a constant mess and I get tired of the Cholla thorns sticking me in the leg. Over time, I got a little jealous of the fruit trees some of my neighbors have and decided to get a couple of my own. I got a Lemon and Lime tree because, well, you know Margaritas. And a Texas Ebony because it is a great looking tree with dark green leaves.
They immediately became a problem because they need to be watered more than the native trees, and I just can't seem to remember to do it when needed. That means stunted growth, burned leaves and no fruit. The Ebony did the best because it handles drought best, but the Lemon and Lime suffered.
I extended my drip watering system to include the new trees, and that didn't work well either. The problem is the calcium in the water left deposits in the drip emitters and plugged them up over time. I was constantly fighting the battle of cleaning the emitters to clear the calcium and restore flow to the little emitters. I finally, after a few dozen tries, just resorted to using that 1/4 inch tubing with a simple valve in the line to reduce the flow so it didn't wash away the dirt and a short piece of tubing after it to stick by the tree. It was just too much trouble to check the emitters all the time.
That sort of worked except for a problem that they never talk about in articles or ads for drip system emitters, heat. See, these things are plastic and the size of the orifice determines the amount of water that is passed through to water the plant. Heat expands the plastic and reduces the size of the orifice, reducing the amount of water let through. No, the water doesn't cool off the plastic and restore flow; the water is hot too. I live in the desert, and the ground temperature regularly exceeds 140F when the water is most needed.
I was fighting yet another battle against the desert elements, and I didn't have a solution at the time.
Now, this is the point where I'm going to point out that this isn't a well controlled scientific experiment. This is a guy trying to solve a problem with heavily advertised commercial products. What I found, and what I did with it. Sure, I could have set up tables of data and run careful evaluations of the devices I discuss, but who has the time or patience for that? If I was getting paid for it, maybe, but this is for my own use at my own house to solve a problem. I'm looking for results, not reputation.
Enter the idea of a plant moisture sensor. These battery-powered things measure the moisture in the soil and transmit it to a hub somewhere that you can easily check. Being able to see if the watering system was adequate easily seemed like the perfect solution. I could put it on a Home Assistant dashboard and check it every time I turned on a light. I could set up alarms based on the moisture to tell me to attend to it; I could even completely automate the watering so that I didn't even have to think about it until an alarm happened. What could be better?
Well, maybe something that actually worked like they say it does....
First I got the kind of device that looks like a pastel mushroom:
These are battery-operated Zigbee devices that measure the moisture based on a trickle of current through the soil. The three prongs are actually conductors that are inserted into the soil and hook to a microprocessor that interprets the soil moisture and uses a Zigbee transmitter to get the data into Home Assistant. When they arrived, I stuck them in a glass of water and observed what they read at different depths of water. The data was confusing.
When the probes were 1/4 covered with highly filtered water (reverse osmosis), I read about 25%; at half the probe, I read about 25%; the fully covered probe was the same as well. OK, that's because it isn't actually soil and pure water is not a good conductor. I tried a slurry of dirt and water and the reading actually followed better, the more probe submerged the higher the reading. So I just used dirt and added water to see what happened. The readings varied pretty nicely from dry, showing around 15% to soaking wet being around 75%. These things might actually work out.
I deployed them to different plants. The Lemon, Lime and Ebony trees as well as a Rubber Tree and a Sago palm that I had under shade in pots on my patio. The problems with the devices started about an hour after I put them in the dirt. The readings were erratic and varied a lot during the day. I'd see a 25% when the soil was soaking wet, and it actually went up as the water drained and dried. The devices would skip hours and hours without reporting at all. They lost contact with the network and had to be rejoined.
Maybe a different device would work better. I got a few of these that are capacitive rather than resistive sensors.
These were smaller, and the fat blade on the bottom contacted the soil better, but they gave much the same results when I stuck them in potted plants on the patio and in the house. Erratic readings and long periods of no communication.
There were things that made the sensor operate better. If I immediately watered the sensor after I put it in the soil so the material was in better contact with the sensor blade or rod, the readings followed more closely the actual condition of the soil. Both kinds seemed to have liked that a lot, and I thought that may be the secret, so for weeks I messed with them, trying to get the readings reliable. Net, the capacitive devices had some value, but the mushroom ones were pretty much useless. Both devices can take a long time between reports—I mean, as much as days, or in the case of the mushroom ones, a week or two. On the rare occasion when they were reporting, the capacitive ones actually helped. The mushroom ones just ate batteries and stayed silent most of the time.
My Lemon tree did not benefit from having one under it, and the Lime tree got a lot of its limbs eaten off by a rabbit, so I can't compare the two. The Ebony tree did OK, but it wasn't growing much. The potted plants did OK as well, but we actually felt the soil in them often enough to keep them watered.
So, the two devices that I tried were not the solution I thought they would be. There are other sensors, though. Was I willing to take on the expense of something like Sensoterra, which is true industrial-grade farming equipment with reams of research, or perhaps one of the EVO devices? Forums present personal experiences, and most of them are just opinions. The person that likes one kind is cancelled by the person that had bad results. Almost all of them blame the network they are hooked to for most of the problems.
Folks, if you're using Zigbee and have a reasonable network, the network is not the problem. If you're using wifi, it might be, but just set your phone next to the sensor and look at the wifi strength; that simple test will tell you what the signal level is. My Zigbee network is really robust; I've been improving it for years. Communication isn't the problem unless the measuring device is way underpowered. And that may actually be the problem with the two types of devices I tried. In the quest for maximum battery life, the trigger to transmit data may be too limited, or the radio transmitter may be throttled. I don't know if any of that is true, but it fits the performance I saw. Look at this chart for the Lemon tree that has one of the resistive sensors that looks like a mushroom:
I have another of the mushroom devices in the shade, monitoring a Sago palm. Take a look at this one:
The other style of moisture sensor is capacitive and behaves differently. Here is a month's worth of activity:
Exciting, isn't it? It just does my heart good to see that the plant stays flooded almost all the time with brief periods of slightly less flooding. When we walk by it and feel the soil, it's dry as a bone and gets immediately watered; don't you see the spikes where water was added?
What to do about it? I was floundering because devices change over time and manufacturers' changes constantly improve a project, but I didn't want to be the lab rat for their product development. That's their job! However, with some reservations, I decided to try Third Reality's latest offering for this kind of sensor, the "Smart Soil Moisture Sensor Gen2." The Gen 1 version got bad reviews due to short range, and there were other complaints as well, but this is Gen 2; this should be better, right?
I bought a three-pack of them and waited for arrival, and they came in the next day.
Aside --- I may sound like an advertisement for Amazon, but I'm not that guy. However, they have half the products in the world on their site, and they ship it fast. Yes, there are some problems at times, but overall, the reduced shipping and reliability are amazing. Plus, their return policy and customer service make this kind of experimentation possible.
Anyway, I grabbed one, a glass of water, a cup of coffee, and my laptop and started experimenting. Here's another chart:
The beginning line is right after joining it to my Zigbee network; it was immediately stuck in the glass of water and watched. It rose to around 80% immediately and stayed there. After watching it for a while, I pulled it out, dried it off with a towel, and emptied the water glass. With everything dry, I watched it again, and it held at 0%, so I joined the other two devices and went out to put them in the dirt. That's shown as the vertical rise up to just over 60%. Then I left them alone, went in the house, and watched them to see how they performed over a couple of days. Want to see another chart? Of course you do.
This horrible mess of complex intersecting lines that is almost impossible to read starts from the time I put them in the ground and spans days of activity and includes the ground temperature at the plants. Since my favorite right now is the Lime tree, let's just look at the charting for it:
Frankly, I'm amazed! After a couple of years of trying to make the well-advertised and discussed sensors actually do the job they were supposed to, I had some actual readings that made sense. Every bump, rise, and fall in that chart corresponds to an actual event. The accuracy of the readings is questionable, but I don't care! What I see in that chart is the watering system working, the intense heat evaporating the surface water, and the lowest reading still being high enough to encourage plant growth. All three sensors worked the same way. I had a little trouble joining one of them to the network, but on the second try, it joined and stayed there. Notice also that, so far, none of them have dropped out. Of course, the proof is in actual day-to-day use, and that won't happen as quickly as this early report.
They haven't been installed long enough to get a feel for battery life or survival in my climate, so I won't discuss something I don't have data for, but these things work.
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