How often should a soil moisture sensor actually measure?
Measuring more often always sounds better. With soil moisture it is the other way around: measure smarter instead of more often and you get more reliable sensors, batteries that last far longer and a wireless network that does not trip over itself. This article explains why, without assuming any technical background.
Soil is slow, and that is good news
Put a bucket of water next to a plant and watch it: nothing happens. Pour it out and it takes minutes for the water to soak in, and hours before the moisture level at root depth changes noticeably. Soil moisture moves on a scale of hours, sometimes days. A heatwave does not dry out a field between two cups of coffee, and a drip system does not wet the ground in thirty seconds.
That slowness is the key to the question of how often a sensor should measure. A reading every minute gives you sixty data points an hour that nearly all say the same thing. You do not get a better picture of your soil, you get the same picture sixty times over.
What a measurement really costs
Why should you care? Because every measurement your sensor sends costs three things you would rather save.
Battery
A wireless soil sensor is almost always what is called a sleeper: it spends nearly the whole day in deep sleep and only wakes up to measure and send its numbers. It is that waking moment, with the radio switched on, that eats power. A sensor that wakes every two minutes uses roughly three times as much as one that does so every five minutes. That makes a real difference to how often you have to change batteries.
Airtime
Every wireless device in and around your business shares the same air: your sensors, your wifi, the neighbours, the baby monitor. Every measurement sent takes up a slice of that shared space. A few sensors transmitting unnecessarily often go unnoticed; a growing network in which everything transmits unnecessarily often starts waiting on itself, and then it is precisely the weakest sensors, the ones hanging furthest away, that fall silent first.
Attention
A sensor that reports the same thing a thousand times a day teaches you nothing extra, but it does make it harder to spot the alert that actually matters.
Measured in our own test network
Cutting back needless message traffic made the difference between sensors that regularly “disappeared” and a network that simply stays up. Not because the sensors got better, but because they got out of each other’s way.
Every five minutes is plenty
For growing decisions, meaning when you irrigate, how much, and whether your irrigation is doing its job, a measurement every five minutes is more than enough.
Do the maths: say you have set an alert on a dryness threshold. On a five minute rhythm that alert arrives, at the very worst, five minutes “late”. On a process that plays out over hours, five minutes is nothing. No root will notice the difference.
What you get in return is substantial. Compared with a rhythm of every one to two minutes it saves roughly sixty to seventy percent of the message traffic, and with it a proportional share of battery use and crowding in the air. The same information, at a fraction of the cost.
When faster does make sense
If you grow on very light sandy soil, in pots or gutters, where moisture does drain quickly, you may want a faster rhythm for a while. And while commissioning a new drip system it is useful to be able to watch minute by minute.
But keep it to those moments
Those are moments, not settings for the whole season. Put the rhythm back to a calm one after commissioning, and your batteries and your network benefit for the rest of the year.
How FairGrow handles this
In FairGrow the measuring rhythm is adjustable per garden or greenhouse, without anyone having to walk out to your sensors with a laptop: it is a setting, not a rebuild. The default is deliberately set to a calm rhythm, and you as the grower decide whether your crop needs something different.
Every sensor judged by its own yardstick
Some sensors report dutifully every minute, others come from the factory with an erratic rhythm and long silences that are perfectly normal for them. A monitoring system that holds everyone to the same yardstick either raises a false alarm every day for such devices or, worse, misses the moment a dependable sensor really does fail. FairGrow learns what is normal for each sensor and only warns when a device deviates from its own pattern.
Change the rhythm and the expectation moves with it
A sensor going from two to five minutes will of course deliver fewer measurements per day. That is not a fault but the new norm, and the platform accounts for it automatically. So you do not get a stream of alerts because you made a deliberate choice yourself.
A small gift for your network: the smart plug
Wireless sensor networks such as Zigbee work like a relay race: battery sensors sleep as much as they can, and mains powered devices, which are always awake, pass messages on for them. Every mains powered Zigbee device does this automatically, even if you never intended it to: a smart plug, a curtain motor, a hard wired switch.
What goes wrong with only one relay point
If you have only one, part of your sensors depends on it, and if that single device falters they fall silent along with it while there is nothing wrong with them.
We have seen this in practice: a jammed curtain motor took a perfectly healthy motion sensor into silence with it for hours.
The fix costs around fifteen euros
Plug a certified Zigbee smart plug into a socket somewhere halfway. It does two things at once: it gives your network a second route, so if one relay point drops out the other takes over, and it extends the range towards your most distant sensors.
For the sensor in the far corner of the greenhouse that always sits right at the edge of its range, such a plug is the difference between struggling and simply delivering.
And soon: measuring at the rhythm of your crop
What we are working towards is a measuring rhythm that adapts by itself to what is happening. Looking a little faster during an active irrigation window, slower during the quiet night hours, and the frugal default rhythm the rest of the day. The technology for it is there; it will become a feature that wins you even more battery life without giving up any insight at all.
For soil moisture that means: every five minutes is plenty, your batteries last longer, your network stays calm and reliable, and you miss nothing at all. Soil is slow. Use that to your advantage.
Measuring rhythm, frequently asked questions
Do I miss anything if my sensor only measures every five minutes?
No. Soil moisture changes on a scale of hours. An alert on a dryness threshold arrives five minutes later at worst, and that is negligible for a process that takes hours.
Why does measuring more often cost so much more battery?
Because waking up and transmitting is what costs power, not sleeping. A sensor that wakes every two minutes uses roughly three times as much as one that does so every five minutes.
Can I change the measuring rhythm myself?
Yes. The measuring rhythm is adjustable per garden or greenhouse in FairGrow. Nobody has to walk out to your sensors with a laptop: it is a setting, not a rebuild.
When is a faster rhythm sensible after all?
On very light sandy soil, growing in pots or gutters, and while commissioning a new drip system. Put it back to a calm rhythm afterwards, because as a fixed setting for the whole season it costs needless battery and airtime.
What does a smart plug have to do with my sensors?
Every mains powered Zigbee device automatically passes messages on for sleeping battery sensors. A second relay point gives your network an extra route and more range. Read on about what Zigbee actually is and why we use standard Zigbee sensors.
Curious which measuring rhythm suits your crop?
Tell us about your location and your crop, and we will think along about the sensors, the measuring rhythm and the network that go with it. We are happy to set up a no obligation pilot for you.