Gardening Gardening tools Light Needs of Plants Soil analysis

The 3-in-1 Soil Tester: A Virtually Useless Tool

Two models of 3-in-1 soil testers, square and oval. They also come in exciting designer colors! Photo: amazon.ca

You’ve probably seen a device like the one above in a garden center. If not, they’re easy enough to locate on the Internet: you’ll find them everywhere! It may be called a “3-in-1 soil tester”, or maybe a soil analyzer, a multi-purpose soil meter, a soil test kit or something similar. (Manufacturers can’t seem to agree on what to call it.) Possibly the name that covers it all, but which requires a good intake of breath to say out loud, would be “3-in-1 Multi-Purpose Moisture, Light and pH Meter.” Whew!

Usually made in China, the source of so many cheap, unreliable tools, this soil meter is indeed inexpensive to purchase (probably costing less than $20) and run (most models don’t even require a battery) and against such a surprisingly modest cost, appears to offer a wide range of potentially valuable services to the amateur gardener. 

Here’s how one of those manufacturers on amazon.com describes it:

  • ✔ Three different soil test meters in one device; Measures moisture, pH/acidity and light
  • ✔ 100% Accuracy; Easy to read moisture, pH and light levels; Perfect monitor for growing healthy plants
  • ✔ Takes the guesswork out of gardening; Know exactly when to water, amend your soil or adjust lighting
  • ✔ Compact & easy to use; No battery needed, just plug* and use; Compact soil meter works indoor/outdoors
  • ✔ Saves you money; Moisture meter tells you when to water, so you can save money by not overwatering

*I think the manufacturer means “insert into the soil” rather than “plug.”

It sounds wonderful, doesn’t it? Yet, I consider it probably the least useful gardening tools I’ve ever encountered … and it’s so easy to accidentally take the measurement in the wrong mode and thus really get the results wrong. Although the idea of running around stabbing gardens and flower pots with a pointy tool and watching a little needle swing back and forth does, I have to confess, somewhat please my inner child, I don’t really see how this kind of device could help my gardening in the slightest.

And there is also a 4-in-1 model … but let’s take a look at the 3-in-1 device first.

The Three Most Common Functions

Close up of the reader of a 3 in 1 soil tester.
Close up of the reader of a 3-in-1 soil tester. Photo: french.alibaba.com

The three different functions are usually activated by a switch on the face of the device. In the model shown here, we see the indications “Moist” (on others, the word “Moisture” is used), “Light” and “pH.”

So, if you want to measure humidity, set the switch to Moist/Moisture; if you want to know the light level, move it to Light and to test the pH, slide it to pH. Be forewarned that if you put the switch in the wrong position when you any test—an easy mistake to make! —, the results displayed will be totally off.

soil meter with 2 metal probes, green top.
Typically, a 3-in-1 soil meter has 2 metal probes, although some have only one. Photo: fenz.si

Next, you simply sink the metal probes (there are usually two) into the soil, whether it’s in the garden outdoors or the soil of houseplants or other container plants. In a moment, you’ll have a reading. 

Actually, according to the instructions that come with the device, you’re supposed to take soil samples separately or do other complicated calculations in order to interpret the device’s results. What you see is most certainly not what you get! But most people, though, just stab it into soil and read what the device says. This is well known to manufacturers and is the use I’m commenting on here.

So, at purchase time, the soil test meter sounds as if it ought to be very useful. Where the problem?

Let’s look at the results one test at a time and see.

Important: This article is about the low-end range of soil meters, those that are sold at bargain basement prices to home gardeners. There are also professional devices that can be very precise, but they cost a lot more. None, I think, are of the “3-in-1 meter” type. Rather, each is dedicated for a single type of analysis.

Soil Moisture

Pink 3-in-1 soil tester, a red arrow points to the light function.
In this model, the middle reading is the moisture meter. Photo: walmart.ca

Let’s start with “moisture”: soil humidity. In this function, the device works like a moisture meter and thus claims to be able to tell you how moist the soil is. That is, how much water it holds. And since most plants prefer soil that is always at least slightly moist, this would seem useful. And how simple! If, when you push the probes into the soil, the screen indicates moist or too wet, just don’t water; when the display indicates dry, logically, you should water. Easy enough.

But why pay for a tool if you want to know how humid any given soil is? You already have an even more sophisticated moisture probe attached to your body: your index finger! If you insert your finger into the soil all the way to the second joint, you’ll be perfectly able to tell if the soil is dry, moist or wet. So, you end up paying for a device that does the same thing … only less well! Do you honestly think your index finger is lying to you?

The moisture meter function is made all the more unnecessary in that the probe inevitably ends up giving false results.

Sometimes it does so from the get-go. It’s not adapted for properly analyzing very light, very dense or very alkaline or mineral-rich soils. For example, it generally doesn’t work correctly on orchid potting mixes, which are highly aerated. Plus, with use, the probe eventually starts to corrode and give false results. If the device claims the soil is dry, and yet you can clearly see the soil is soaking wet, that probably means it’s already been giving false results for months. Your index finger, on the other hand, will efficiently measure soil humidity, not only for a few months, but for your entire gardening life!

Soil meter checking the humidity level of a spiny cactus. There are several cacti in the photo.
A moisture meter may be useful for people who grow spiny plants. Ill.: womansweekly.com & sevenpros.com

One exception: if you grow a lot of very spiny plants, like cacti, I will grant you that the moisture meter function could be of use. (For a cactus, though, do let the needle move far into the dry side before watering.) After all, who wants to risk getting their finger stabbed every time they want to know whether or not their plant needs watering? 

And remember too to replace the device regularly, certainly at least annually, before it starts to give false results.

Light Level

The light mode is on top on this device, indicating either light or dark. The scale below shows the light measurement, usually in foot candles. Photo; amazon.com

When the light function is turned on, the 3-in-1 soil meter also serves as a light intensity meter, showing on its dial either “Light” (sun) or “Dark” (shade). How handy! Except, that’s a detail that you can easily see … just by opening your eyes! I mean, you can see the plant: is it in sun or shade? If your vision is so weak that you can’t distinguish between sun and shade, you probably won’t be able to read the meter anyway. That’s all you really learn directly from the device: whether the plant is in sun or shade.

Rather, what you really want to know is whether the plant has received the equivalent of direct sunlight, of shade, or of something in between (partial shade) on average over the day. The equivalent of full sun, for example, would be 6 hours of sun shining directly on the plant.

Knowing the sunshine at the precise moment of the analysis, though, is actually quite useless! Say the plant tested is in a sunny spot when you trialled it. Well, it could easily be in deep shade only 30 minutes later. If you calculate a plant rate as getting excellent sunshine just because the device shows the needle is on the “light” side of the screen, you’d often be wrong.

To truly analyze the light that a plant receives in such a location, you’d have to repeat the measurements several times a day, at least morning, noon and early evening, and take an average. A lot of effort when, if you only look at the plant three times a day, you can already clearly see that: you don’t need a light meter to tell you.

Even then, you’d also need to repeat your tests in different seasons, especially for plants that grow all year, like houseplants, because the light a plant receives changes enormously through the year, especially between summer and winter.

Now, there is also a scale showing a series of numbers (often they are supposed to represent foot candles, but that depends on the model used), but if you test your device against an actual dedicated light meter, it turns out that the results of the 3-in-1 model are rather vague: they don’t really correspond to the scale printed on it.

What it would really take to know a plant’s light situation would be a meter that measures brightness throughout the day and calculates the average light the plant receives, giving you a truly accurate analysis. This device does not.

pH

Green 3-in-1 soil tester. An arrow points to pH (acidity/alkalinity)
The pH level is shown at the bottom of this device’s screen… it really isn’t very precise, is it? Photo: amazon.ca

💡 FYI: The pH of a soil indicates the degree of its acidity or alkalinity, an important factor in growing plants. Most plants like a slightly acidic or neutral pH (a pH of around 5.5 to 7.0).

Finally, the pH of the soil, a very useful detail for the gardener, because we know that plants have preferences in this regard! But … this gadget is a bit vague about the details. You need to see the decimals for it to be truly useful. I mean, a pH of 5.0 could be disastrous for many plants, but 5.6 would be fine. The meter doesn’t show that.

Plus, it seems to give false results as often as the verifiable ones! Try stabbing it into the soil and you’ll see. It may tell you that the pH is 4 one day and 9 a week later, especially as the soil moisture level changes from very wet to dry or if you apply fertilizer or other treatments. You could get a result much closer to reality just by using a piece of litmus paper as is used in testing the pH of swimming pool water.

By the way, if you want to see a soil scientist roll their eyes, show them a little pH gadget like this one and ask what they think of it!

Again, there are many very sophisticated pH meters that can correctly analyze the pH of a soil, but, in addition to their much greater expense, they need meticulous maintenance and regular calibration. A pH meter of the type discussed here is essentially designed to be a disposable commodity; you cannot calibrate it, nor will it operate with any accuracy other than in the short term.

Soil Fertility: The Fourth Musketeer

Electronic 4-in-1 soil test device. Tells temperature, light, pH and moisture.
This model also claims to measure soil fertility … whatever that means! Photo: amazon.ca

The soil meter I’ve introduced you so far checks for moisture, light, and pH, but other models, in a similar price range, claim to analyze soil fertility rather than light.

4-in-1 meter with three probes.
Usually, models that give a soil fertility reading come with three probes instead of the usual two. Photo: Home Science Tools

Also, you will find all 4 functions on the same device in the case of a 4-in-1 soil tester in the same low-end category.

What to think of this last analysis?

The device claims to reveal the “combined level of nitrogen, phosphorus and potassium” in the soil and usually indicates, when you slide the switch to Fertility mode, either Too Little, Ideal and Too Much.

However, it should be understood that the “combined level” doesn’t really tell you much. You would really need to know the levels of the three minerals independently, as in parts per million of nitrogen (ideally 50 to 200 ppm), of phosphorous (4 to 14 ppm) and potassium (50 to 200 ppm). Logically so, because what you want to know is whether the soil contains enough of the minerals plant requires for healthy growth. 

In fact, though, the device could easily indicate “ideal” even when the soil has serious shortcomings. Theoretically, it could have an excessive or even toxic level of one element (say, phosphorus) and yet be completely lacking in nitrogen and potassium, a situation few plants could tolerate.

Sometimes Even a Fifth Musketeer!

Device with 3 probes.
This model indicates temperature of the soil as well as the usual light, pH and humidity. Photo: ebay.com.

There are even 3-in-1 or 4-in-1 soil test devices that drop one of the other measurements (usually the basically useless soil fertility factor) and check the soil temperature instead. Most of these are electronic models requiring a battery.

You rarely have to measure soil temperature when gardening, but if so (maybe to know when the soil is warm enough to transplant fragile seedlings in the spring), maybe you already own a compost thermometer you use for your backyard composter? If such is the case, you don’t need a second thermometer.

Meat thermometer testing soil temperature in the garden.
Soil temperature can be easily measured with a meat thermometer. Photo: butterflygardening.wordpress.com

The rare times I feel the need to check the temperature of the soil outdoors, I simply place my hand on the soil and feel whether it is cold or warm. That’s all I need to know. If you want more precision, a meat thermometer borrowed from the kitchen would also give an accurate result. That’s what thermometers do! It doesn’t matter what adjective comes with it!


Yes, I know some gardeners do appreciate gadgets like these, especially as a watering guide, but in my opinion, they’re really quite useless, a device designed more for your amusement than anything else. This is the kind of tool that most people will use two or three times, then put aside and never touch again. You can have a great green thumb without ever using a “3-to-1 soil tester”.

Larry Hodgson is one of Canada’s best-known garden communicators. He has notably been editor-in-chief of HousePlant Magazine, Fleurs, Plantes et Jardins, À Fleur de Pot and Houseplant Forum magazines and is currently the garden correspondent for Le Soleil and radio garden commentator for CKIA-FM Radio. He has written for many garden publications in both the United States and Canada, including Canadian Gardening, Harrowsmith, Horticulture, Fine Gardening and Organic Gardening. He also speaks frequently to horticultural groups throughout Canada and the U.S. His book credits include The Garden Lover’s Guide to Canada, Complete Guide to Houseplants, Making the Most of Shade, Perennials for Every Purpose, Annuals for Every Purpose, and Houseplants for Dummies, as well as nearly 60 other titles in English and French. He is a past president of the Garden Writers Association (now Garden Communicators International) and the winner of the prestigious 2006 Garden Media Promoter Award offered by the Perennial Plant Association. He resides in Quebec City, Quebec, Canada.

3 comments on “The 3-in-1 Soil Tester: A Virtually Useless Tool

  1. EXACTLY!
    A ‘weather rock’ is more useful.

  2. I spent $20 on one off of Amazon. It got 5 stars, and was complete crap, and fell apart after 3 uses. Amazon wouldn’t stand good for it. A finger in the soil is just as good.

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