Ask a Question forum→How trustworthy are test instruments you see at store's garden section ?

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Name: Hank Alvarez
Moscow Mills, Missouri (Zone 6a)
MoscowMillsHank
May 19, 2020 8:56 AM CST
Last winter I purchase a number of instruments. I grow mostly vegetables in containers. One instrument measures light intensity, soil pH and moisture. Another measure fertility and pH. Two others measure only pH. They all seem to read the same so I'm assuming that they're either all wrong or all right.

I have a number of tomato and pepper varieties in Eco pots and I noticed that the ones producing the most fruit are taking in more water so when they get into the lower level of the moisture scale I add water before seeing any thirst stress in the plants.

When the fertility reads in the lower end of the scale I add fertilizer.

When the pH level rises above 7 I'll add pH Down to the water corrected to around 6 to 6.5 to bring it back down to 7 which they seem to like.

Am I on the right track trusting these instruments?
Name: Will Creed
NYC
Prof. plant consultant & educator
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WillC
May 19, 2020 9:06 AM CST
The moisture meters do not measure water directly. They operate on the principle that water is a good electrical conductor so the more water the better the conduction and the higher the meter reading. That is true, but it overlooks the fact that conductivity is also affected by the mineral salts in the soil and the water you use as well as how compact the potting soil is. The bottom line is that they work well in some soil conditions and poorly in other potting mixes and are therefore not reliable.

The light meters measure light intensity but do not take into account the portion of the spectrum that meets the needs of plants. They can give you some sense of relative light intensity levels but they are not very accurate in specific situations. Expensive light meters are color-corrected and more reliable.
Will Creed
Horticultural Help, NYC
www.HorticulturalHelp.com
Contact me directly at [email protected]
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Name: Baja
Baja California (Zone 11b)
Cactus and Succulents Seed Starter Foliage Fan Xeriscape Container Gardener Hummingbirder
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Baja_Costero
May 19, 2020 10:41 AM CST
Cheap moisture meters are perfectly reliable if you learn how to use them right. You have to calibrate the reading to some independent measure of moisture (like your finger inserted a couple of inches into the soil) and then you can rely on the reading to tell you the same thing each time. People who tell you they are unreliable either had a defective unit or failed to learn how to use it.

Cheap light meters are usually very helpful and should be completely adequate for your needs. They will tell you a lot about the different areas where you could grow plants. There's no need to concern yourself with the spectrum if you're just checking natural light (which has pretty much the same spectrum everywhere you might look in your house or patio). There is definitely no need to spend lots of money on a light meter for color correction unless you are measuring artificial light, as they are no more reliable than a functioning inexpensive model. The technology for an informative light meter is pretty basic and does not require rocket science.

I recommend walking around your house and patio with a light meter and putting it in different locations at different times and seeing how they vary. A quick survey like this may change your mind about where you choose to put some plants. What's important is not so much the absolute number but the relative amount of light, which varies on a log scale and may be 100 times brighter in one location compared to another.

I have both types of meter and have gotten a lot of valuable use out of them.

Most cheap meters that purport to tell you fertility are not accurate and should not be relied upon to decide when to use nutrients. Stick to a schedule, and measure the nutrients, and stay on the low side. If the soil is fresh then nutrients are overrated for most plants anyway.

Cheap meters that purport to tell you pH are also not accurate and should not be relied upon. Use a reliable test kit (like the cheap drop test kits sold in your local aquarium store for $5) to test the pH of your water accurately. If you are going to adjust the pH of your water (which is pretty much unnecessary for most plants if your water is near neutral) it's important to be consistent about the results. I actually adjust the pH of our very alkaline well water (pH>9 out of the tap) to near 6.0 for my plants (mostly succulents) every time, and would recommend doing this but only if the tap water is extremely alkaline like ours. Also be aware that the product pH Down contains phosphoric acid and will boost the phosphate in your soil significantly if you use enough of it. I prefer a product called Acid Buffer (Seachem brand) which is bisulfate and does not flood the zone with phosphate.

Any time you adjust the pH or add nutrients, it's important to flush the soil with extra water each time you water, so that the added salts in the water do not accumulate in the soil over time.
[Last edited by Baja_Costero - May 19, 2020 11:12 AM (+)]
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Name: Luis
Hurst, TX, U.S.A. (Zone 8a)
Dog Lover Region: Texas Salvias Roses Hibiscus Plumerias
luis_pr
May 19, 2020 12:44 PM CST
My moisture meter wad disappointing so I quit using it. My color based pH meter is not accurate but still useful, as it can still tell me when things are very alkaline, which is all I need to know.
Name: one-eye-luke US.Vet.
Texas (Zone 8a)
Quitter's never Win
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oneeyeluke
May 19, 2020 1:21 PM CST
My pH meters are accurate as well as my TDS meter. Before I fertilize a plant I check the discharge of my drain water and find out how many ppm the soil has. I never burn plants using a EC or TDS meter and I never let a plant get a nutrient deficiency. When I run into problems with a plant, I test the drain water and get a pH and a EC reading. Or when I get a new soil, I test both the pH and the TDS for the best plant care. I've tested soil I have sent to the soil labs and have always matched the soil with the soil lab readings when it comes to pH. These are the best investments for anyone in the Horticulture business or are serious about plants. Super accurate!
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NOT A EXPERT! Just a grow worm! I never met a plant I didn’t love.✌
Name: one-eye-luke US.Vet.
Texas (Zone 8a)
Quitter's never Win
Hummingbirder Birds Organic Gardener Dog Lover Cat Lover
oneeyeluke
May 19, 2020 1:39 PM CST
These meters are not accurate for getting moisture or pH and will cause problems when used. Warning J__K
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NOT A EXPERT! Just a grow worm! I never met a plant I didn’t love.✌

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