Figuring out EC readings

Just got a new meter the Apera EC 20. I calibrated it, but im a little confused about readings and what what they mean.
Tap =650
Filtered fridge =550
distilled water= 47

Does this seem right? what do these numbers mean to mean as far as feeding go? Im running organic fer now and switching to Jacks next run. Please tell me what i need to know., as this isnt as straight forward as PH apparently, thanks.

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Dilstilled water readings are not valid. Any water at less than 100 PPM won’t give a reliable reading.

PPM is a measure of dissolved solids in a liquid. Cooler temps will cause some of the molecules in the water to solidify, thus reducing the amount of dissolved solids in the water.

Maintain a runoff PPM of ~1,000 when using Jacks. If you tap is 650 PPM I would target a higher level since 650 of the total PPM can be attributed to minerals.

650 PPM tap is really high. I would consider getting an RO system.


You have to add the TDS of the water to your total nutrient load: if you are shooting for 900 ppm then 650 of that is your water. If you add 900 ppm of nutes on top of your base water you will burn plants. This is why folks use an R/O system in high mineral content water. Especially in hydro. With pure water you know what is in your nutrient solution.


Does this mean i dont need to add cal mag to my current organic fert grow ( in coco) ???

If you are reading conductivity. (Umho) Your PPM value will be about half.
Tap at 650 (umho) = 325 ppm

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Thank you !

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Idk about organic growing in coco, but you don’t have to add calmag when using Jack’s. As long as you buffer the coco correctly Jack’s has everything you need. You can adjust part b or epsom salt to deal with calcium or magnesium issues. Every other salt nutrient line I can think of requires adding calmag.


I recently cut out the epsom and switched to 4g part A and 2g part B. Part A has calcium and magnesium. No issues at all so far to report


Has anyone paid the $40 to send in a water sample to Jacks in PA. for a personal Jacks formula recommendation?

Good to know it’s working out for you. I’ve been thinking about trying that during flowering. Adding .21 grams of part a and dropping about 1.5 grams of B and epsom salt should free up some EC for bloom boosters. I’m planning on using MKP, big bud powder and liquid kool bloom at different times during flowering this grow.


Hmmm, is 325ppm tap start too high, or im good to go?

Use microsiemens as your units of measurement for EC. There are two different scales to measure ppm and nobody says which one they are referencing. It can get confusing. microsiemens Is also the most precise

I get only 1 value on my meter and im supposed to double it a for salt PPM reading. kellydans said im in the 325 range i agree. is this too high?

You don’t really want to use anything higher than 300 µS/cm which is microsiemens. Otherwise the solution will have too high of an EC (electrical conductivity) after your nutrients are added to the water and your plant will not be able to absorb water through osmosis.

Hmmm is see, My options seem to be either run a water test via Jacks which is a service they offer for $44, i guess they, test, turn around and tell you exactly what to buy and put in im guessing?

It would be nice if some that has done this would chime in.

Or i guess I go with the osmosis system. i have no idea about this investment.

I kick in the FF Boosters at flower.

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If your tap water is really 650 I would buy a reverse osmosis filter system or set up a rainwater collection system if possible.
There is a lot of good information about understanding EC and fertigation on the coco for cannabis website

its 325 ppm

I am not concerned with Whatever it is in PPM you stated above the EC was 650 microsiemens and that is too high to use for fertigation.
Do you get reliable rainfall where you are? You could set up a collection system and use that

rainfall collection is totally an option, ty for showing me that. i have sent Jacks a sample of my water for them to analyze and give me suggestions. I really didn’t want to go the OS route, could you please give me an idea what id have to pay for the install and a decent Reverse Os unit?