When Moisture Meters Fail

Typical moisture meter. Photo: Amazon.com

Question: I’ve been watering my houseplants using a moisture meter for over a year now and have found it very effective. When the meter reads “dry,” I water. It couldn’t be simpler. But I’ve started getting strange results: plants the meter says need watering, but whose soil seem moist to me. What’s going wrong?

Lisa T.

Answer: Simple moisture meters for houseplants are handy little devices and very inexpensive. Plus, they usually work quite well in preventing overwatering or underwatering … for a while. 

Usually, they show a scale of 1 to 10, with 1 being very dry and 10 being very wet. Most also have a color code: red means “dry,” green “moist,” blue “wet.” You just stick the probe into the potting mix and it gives instant results, telling you whether or not the plant needs watering. Then you check the next plant.

However, these devices rarely remain accurate very long. 

How They Work

It’s important to understand that these moisture meters don’t really measure moisture at all: they measure conductivity of electricity between two types of metal in the tip of the probe (or in the two different probes if your model has two prongs). Since water is a good conductor, if the soil is wet, the conductivity will be greater and the meter will indicate wet. If the soil is dry, there’ll be less conductivity and the meter will read dry. 

So far, so good. 

But these devices are designed for “normal conditions.” If the soil is extra light and airy (such as an orchid mix) or extra dense, the results will be off. If your water is very hard, containing mineral salts that increase conductivity, that too will lead to readings of moist even when the soil is clearly dry. So, under certain circumstances, the meters won’t work at all.

Under more normal conditions—and this likely the problem you’re having!—mineral salts, coming from fertilizer and tap water, tend to build up in potting soil over time. So, unless you repot annually, changing the old soil for fresh soil, readings will gradually become less and less accurate. If the soil is heavily contaminated with mineral salts, the meter will always read moist or wet, even when the soil is obviously dry.

Also, over time, often in as little as a year, mineral salts slowly penetrate the probe itself, or it may begin to rust and, again, both situations can lead to inaccurate results.

What to Do?

First, always back up your moisture meter results with an occasional “finger test.” (Personally, I always use the finger test rather than a meter of any kind: it’s always accurate!) Sink a finger into the soil and check whether what you feel corresponds to what the meter shows. If it says dry and your finger says moist or wet, trust your finger! That device may no longer be accurate.

Also, to keep your meter accurate longer, clean the probe with a soft, dry cloth after each use.

And when the meter starts regularly giving incorrect results (remember to check using a finger test), either buy a new one … or learn to use your finger!