Soil Moisture Meters – Do They Work – Should You Use Them?

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Robert Pavlis

I see soil moisture meters for sale all over the place and now there are numerous brands listed on Amazon. A lot of people must be buying them, but do they work? Are they accurate enough? Should you use soil moisture meters to determine when to water your plants?

Let’s have a close look at this garden device.

Soil Moisture Meters - Do They Work - Should You Use Them?
Soil Moisture Meters – Do They Work – Should You Use Them?

How Do Soil Moisture Meters Work?

Moisture meter probe showing two electrodes on each one, separated by white plastic
Moisture meter probe showing two electrodes on each one, separated by white plastic, credit: Kristine Lofgren

This might surprise you but they don’t measure moisture. Instead they measure conductance which is a measure of the amount of electrical current flowing between two probes. Many soil moisture meters only have one probe but if you look at the tip there are two parts. The tip is one electrode and the shaft is another and they are separated by a piece of plastic.

Air is a poor conductor of electricity and gives a low conductance reading. Dry soil is mostly air. Water is a good conductor and so wet soil gives a higher conductance reading. Moisture meters use this concept to indirectly measure moisture of the soil.

The problem with conductance meters is that other things also affect conductance as you will see below.

There are better quality moisture meters available but in this review I am focused on the relatively inexpensive garden meters.

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Readings From a Moisture Meter

Most of the current soil moisture meters for gardeners have a simple display with numbers from 1 to 10 and a needle that moves left and right to show you the value. These are not humidity numbers. They are just relative numbers that represent some degree of moisture.

The numbers might be color coded with red for dry, green for moist and blue for wet.

To take a measurement, stick the probe in the soil, wait a few seconds and take a reading between 1 and 10. Notice that the meter does not tell you if you should water the measured plant.

The next step is to look up your plant in the manual and see what its Moisture Number is. Let’s say your plant is a 5. If the reading is wetter than 5, don’t water. If the reading is a 5 or drier, it is time to water. There are two significant problems with this method.

Plants Not On the List

If your plant is not on the list, you are out of luck. The best you can do is try and find another plant on the list that has the same watering needs. A new gardener won’t know this and an experienced gardener doesn’t need a moisture meter.

What About a Plant Collection?

If you have a number of different plants, you will need to either memorize their moisture number, or mark it somewhere so you can refer to it each time you water. Good luck with that!

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Does the Soil Moisture Meter Work?

I wanted to see how well the meter works so I ran a number of tests. All of the following were done with a dual probe pH and moisture meter from Luster Leaf. The dual probe works just like a single probe meter except that one probe is for moisture and the other for pH.

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The soil tested was Promix potting media unless noted differently.

Repeatability

Test: The moisture meter was repeatedly inserted into the same pot, to a 3″ depth. Both a dry soil and wet soil were measured.

Results: Dry pot – 7,7,7,7,6.5. Wet pot – 4,4,4,4,4.

Conclusion: Repeatability was good.

Extreme Limits

Test: Measure moisture at the extremes. A dry pot of Promix with 15% added perlite had been sitting for over 4 weeks with no plant or watering. The wet pot was Promix that was watered well and allowed to sit in water overnight and then allowed to drain for 1 hour before measurements were taken.

Result: Dry pot gave a reading of 2.5, which is the same as when the meter is turned off or when it is measuring just air. It won’t go any lower than 2.5. The wet soil was a 9.

Conclusion: The 10 point range is actually a 6.5 point range on my unit, so it is less accurate than expected. It also means that plants with a moisture number of 1 or 2 will never get watered.

Soil Compaction

Test: Fill a pot with fresh moist Promix without packing it down. Measure moisture. Then pack it down hard and measure again.

Result: Un-packed soil gave a reading of 4 and packed was 5.5. The measurements were taken a few minutes apart to ensure no change in actual moisture amount.

Conclusion: The compacted soil has less air in it resulting in relatively more water, which increased the conductance (wetter reading). The moisture reading you get depends on how well you pack soil around the roots. This means a newly potted plant will measure drier than it will after a few months once the soil has settled, even if both pots have the same identical moisture level.

Soil Depth

Test: measure moisture at various depths in pots with plants.

Results: Some pots varied very little with depth, but other pots varied a lot – as much as 2 units from top to the bottom.

Conclusion: The probe does need to be inserted so that soil touches both electrodes which is about 2″ deep. Newly watered pots can have water sitting at the bottom of the pot so this should be avoided. The edge of the pot is drier then the center. It is important to measure at a consistent depth in all pots.

Effects of Fertilizer

Test: Fill two pots with fresh media and pack down normally. Measure both pots to make sure they give the same reading. Now water one with distilled water and the other with fertilized tap water. My water is fairly hard.

Results: There was a 1 unit difference between pots.

Conclusion: The salts in water increase conductivity. So both hard water and fertilizer should give a “wetter” reading. The Promix does have fertilizer in it so this may have kept the value for distilled water higher.

Moisture Meter vs Finger

Experienced gardeners use two methods to determine the need for water: finger or weight. The finger method is simple. Stick the finger in the soil and see if it is wet, moist or dry. Alternatively you can lift the pot. With a bit of practice this method, called hefting the pot, is quite accurate. Based on the weight you decide if it is time to water.

I went through my houseplant collection and did both a finger test and a meter test. Most of my plants are not on the plant list, so I don’t really know when to water using the meter. I do have some streptocarpus, which should be watered like an African violet which is a 3. Wet streptocarpus showed values in the 6 to 10 range. The one that was the wettest on the meter is almost ready for watering – maybe tomorrow. Others that showed a drier reading, are actually wetter. A couple that needed watering showed values of 5 so they are not ready for water according to the meter, but then my meter only shows a 3 in extremely dry conditions which would kill the plant.

My orchid media is very porous and the meter is useless in such media.

Problems with Moisture Meters

There are several problems with these meters.

Accuracy

There are too many variables involved: soil compaction, depth of reading, type of soil and limitations of the meter. There is no easy way to compensate for these.

Calibration

The units can’t be calibrated. In my case I am missing 2/3 of the range, making it even less accurate. A calibration feature might let you correct the readings for your type of soil.

Changes Over Time

Several things can affect the readings over time: corrosion of the probe, salt buildup in the media, fertilizer and soil compaction.

Soil Affects Conductivity

Conductivity is an indirect measurement of moisture and it’s affected by the type of soil. Clay and organic matter have a higher conductivity than silt and sand. That means that different gardens will give different readings even if they have the same amount of moisture. Different potting media will give different readings. A new plant from the nursery could give a very different reading than the same plant after you repot it because of a change in soil.

Do Soil Moisture Meters Work?

They do give you a relative moisture reading. They don’t tell you when to water although you can convert the displayed number into a watering recommendation.

Are they accurate enough to tell you went plants should be watered? They might work well enough is some soils but they don’t work in all cases. Lots of people report that they killed their plants because they did “what the meter said”.

The plant list provided with these devices only lists some plants and to be honest, I don’t agree with the list in some cases.

Even in commercial greenhouses, higher grade moisture meters are rarely used to control irrigation in part because of “unreliable measurements“. Should we be using cheap garden-grade devices when better quality units are not reliable?

YouTube video

Should You Use a Soil Moisture Meter?

Experienced gardeners don’t use them – that tells you a lot.

Inexperienced gardeners don’t know when to water and find the whole topic confusing so they like to use a meter. It will work, some of the time, for some plants. The problem with using it is that you will come to rely on it instead of learning to water properly. Or you will use it for a while, kill some plants because of it, and then throw it away. A moisture meter will prevent you from becoming an experienced gardener and that is probably the biggest reason for not using it.

How should you water? Watch the videos in this post.

Note: this post contains affiliate links to Amazon.

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

18 thoughts on “Soil Moisture Meters – Do They Work – Should You Use Them?”

  1. I have just bought a Reotemp meter, which it tells me I must calibrate, which is why I read this post. I bought it for my lawn. Which isn’t easy to stick your finger in to test, and is a reasonably large area for testing. How wet is too wet? or too dry? only a few inches under the lawn will tell me this. And without digging lots of holes all over my new turf, I was hoping that it may help. I’m still guessing but “guess” that the only way I’m going to calibrate it is by making a “dry”, “moist” and a “wet” sample to test, and adjusting the meter accordingly. But at least I know now how the thing actually works. Thank you for that.

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  2. I have a meter here that says that dracaena (it lumps them all) should be an 8, however on the internet it says between 2-3 which sounds more likely. Now I don’t trust any of the values on the insert that came with the meter.

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  3. My experience is that the electrodes don’t work unless you use something like a scouring pad or dry wire wool to get the oxide off the dissimilar metals, before each use. But, even this only gives you one go, because once you have got the junction moist in one pot, it will carry on reading the same in the following pots. So you have to dry it bone dry for every use, and buff off the oxide for every use. Hopeless. It would be better to get a proper conductivity meter with separate electrodes so there is no small gap to carry moisture over from one pot to the next. I suppose one could dip the probe in pure alcohol or acetone between readings, but even then it might take in moisture from the air while you were waving it dry. 🙁 My house plants have generally died of either waterlogging or drying out, because the dreadful black sphagnum that most supermarkets sell as compost, starts out impossible to wet, and then totally waterlogs and suffocates the plants. I’ve even put some in a perforated bucket and poured kettlefuls of boiling water on it, and none comes out of the bottom! On the other hand, good compost is incredibly expensive if you can’t get out to make or fetch it yourself. :/

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  4. Hi

    Very interesting Summary of Soil Moisture Meters. My interest is in UK Agriculture. For some e reason the RPA (in Uk) have decided to include purchasing a Soil Moisture Meter in a Grant funding package, which includes. Compaction & pH. These 2 are eminently helpful although digital pH meters can be misleading, but Soil Moisture simply doesn’t make sense to me for farmers for all the same reasons in your reviews. Like gardens Farm Soils vary from heavy Clay to Peat, from Sand to Silt, and farmers will know when they can travel on their fields without causing damage and the vast majority of Combinable Crops & Grass they grow, will not be irrigated if its very dry). The kit is simply not justified against the value of the crop. Additionally if they’re any good at managing their soils they will be encourage Soil Resilience by their good farming practices.

    Do you have any ADVICE about a sensibly priced Soil Moisture Meter (£100-£200 – USD x 1.2) that we could include in a sensibly priced package to meet the Grant Funding Programme?
    Thanks very much!
    Charles Goldingham

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  5. What I found is the expensive meters are probably good for professional growers who have seedlings, grow lights, in a climate-controlled environment. They have to know the moisture content, pH, etc., because they are adding fertilizer and monitoring the growth rates of baby plants. Those meters are very expensive and professionals know how to judge the readings because – as you mention in the article – moisture readings are not wholly accurate.

    In addition, when I looked reviews – supposedly written by knowledgeable authors – most of them rang hollow. If you read the reviews on Amazon that give 1 or 2 stars, you can easily see how well a cheap meter doesn’t work. The usual flaws are “the tip of meter broke off and got stuck in the pot”, “it quit working after 3 months”, etc. It makes you wonder who write the reviews because they rave about the same meter.

    I’ve always judged the moisture in the pot by sticking my finger down an inch or two (depending on the size of the pot). When it’s dry, I drench the pot, put it in the shower on an Ikea stainless steel rack (the thing with rubber feet that’s meant to sit near your stove when you want to move a hot pan away from the heat) and I let it drain over night. I do that for all my plants and so far, they all seem to be doing okay.

    It was a great relief to find your article because I haven’t been too keen on buying a cheap meter and paying hundred of euros for a quality professional meter makes having a jungle a bit pricey. Thanks for your good insights and also the video about how you water your plants.

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  6. What about the meters that you CAN calibrate? Such as the ReoTemp meter? I am NOT experienced, and I think one I can calibrate would help me to learn more because my finger doesn’t go as deep as the plant roots do?

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  7. I have a couple of things to point out about your essay: (1) In the “repeatability” discussion your dry-pot numbers are consistently around 7 and the wet-pot numbers cluster around 4. This is clearly wrong—did you accidentally reverse the results of your “reliability” test? (2) Later you assert that the resting value of the probe (in air) is 2.5; however, your first picture in the essay—of the meter not in use—clearly shows a resting value of about 1, not 2.5. Obviously, “1” is the lowest value on your meter, so 1 = very dry. Since a value of 3 is right at the border of dry and moist, I’d say 2.5 is probably just about right for many low-moisture loving plants.

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    • 1) typo – thanks for catching it.
      2) the first picture is not my meter – just one taken off the internet.

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  8. Actually, I have found mine quite helpful for deeper pots of house plants. I have been gardening for 40 plus years both outside and in, but I don’t know if that qualifies me as an experienced gardener. I don’t read the levels on the meter for each plant, I just check for moistness at the root level in deeper pots. The finger just does not go deep enough. (I confess to having short, fat fingers.) And since the pots are clay crocks and quite heavy to lift I am not going to go around lifting to measure that way. So I do feel they have their uses.

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  9. Excellent study of the Moisture Meters, Thank you. I agree with your findings and conclusion that experienced gardeners, (in most cases) have no need for the moisture meters. However, it all started with a question “how to tell if the soil is dry or wet” that question didn’t indicate Master Gardener expertise, that’s why I recommended a moisture meter. To be realistic, one wouldn’t expect $10 (give or take) instrument to do more than indicate Wet/Dry soil. A moisture meter will NOT prevent anyone from becoming an experienced gardener, the same as learning to drive a small car doesn’t prevent you from learning to drive a big truck. P.S. I do use moisture meter in a 30 + gal. containers which I find difficult to lift and my finger is not pointy enough to go deep. I enjoy reading your posts, please continue providing us with good, solid information, thank you. .

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    • If a gardener can’t use the finger and uses the meter instead – how do they every learn to use the finger? And until they do, they won’t be an experienced gardener.

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  10. Yeah, my husband bought me one of those as a gift years ago. I stuck it in the soil of a large schefflera pot to humor him, but pretty much ignored it after that. Didn’t find it useful at all.

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  11. Great review, and your results make sense. For some plants in greenhouse it was a little hard to do finger test so I used a meter, with mixed results. Best thing is to keep track of each pot and see how numbers change over time. essentially a calibration. but thats a hassle. I am wondering how those meters work for lawns. Its commonly suggested to have a meter connected to a watering controller to determine when sprinklers need to be turned on. I wonder how these compare with these cheap meters.

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