Reverse osmosis and the argument among growers

It is not surprising to find reverse osmosis in use far more often in countries where the tap water is generally considered not potable. It makes sense that if you can’t drink it, your plants, that are dependant on you for their entire ecosystem, probably can’t drink it either.

If you live in a country where you can fearlessly drink the tap water without fear of any kind of infection or poisoning you may rest assured that your plants will be able to drink it as well.

Reverse osmosis is a filtration method that can remove up to 95-99% of dissolved salts in any water sample. It is the preferred method for desalinating water on an industrial scale for manufacturing and domestic water supply.

The resulting ultra filtered water is stripped of all minerals. You are now responsible for the entire mineral profile of your water before it goes to your precious plants.

Straight up reverse osmosis water will deplete your plants of calcium and magnesium so fast your head will spin. People that drink pure RO water are similarly at risk of osteo-type deficiencies and poor teeth because reverse osmosis water can pull calcium from the body.

Your nutrient product should be selected with the use of reverse osmosis water in mind. Remember you are now entirely responsible for the mineral content of your water. Organic soil growers will still need to modify their water. Soil is an excellent buffer against ph fluctuations and microbial attacks, but it will not save you from calcium deficiency. At all times, from spray bottle to the final flush stages, the reverse osmosis water will always need to be adjusted.

Do you use Reverse Osmosis and what is your Experience?


I haven’t ever used RO before, but it will be necessary in our new grow. Your article brought to light something I hadn’t thought of, mineral depletion. Would you be willing to tell me how to adjust the water for optimal growth? Camille


Hi Camille,
When using RO water it is very important to keep in mind that the water is stripped of all minerals and nutrients, which is nice when dialing in a precise feeding schedule but can be overlooked. One of the biggest losses going from tap water to RO is Calcium and Magnesium as well as other trace minerals. If you adjust accordingly for these losses your plants should be just fine.


Where tap water is used without RO, are the cal/mag ratios typically close to ideal for cannabis, or do you still need to test the levels and adjust with added cal/mag?

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It is always dependent on where you live and the water that you are using but there is typically a moderate level of calcium and other trace minerals in the public water supply. I wouldn’t say they are Ideal ratios and I do still typically end up adding some Cal/Mag just at lower ratios then in RO water. However, it is important to know the make up of the water that you are using for your grow so that you can adjust accordingly. This information is usually available on the municipal water supply website. The levels fluctuate from time to time and can be difficult to adjust for. Hence why people opt for Reverse Osmosis systems, You know whats going into your Grow :wink:


Exactly what kind of losses are we talking about? Any recommendations about products or amounts? The water I have used in the past has really needed very little treatment, the new grow site doesn’t have that option. Not much experience in adding minerals. Camille

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RO is the best way to any grow method. Rain water the has been UV or ozone treated is also very good. Make sure you use a rain diverter that take the first minute of the rain and diverts it to get most of the bird poop, pathogens, etc out of the rain water. A ball float in a pvc pipe is a cheap diy diverter.

If you drink RO water make sure you are getting enough minerals from your diet as pure water leaches mineral out of your tissues.

Depending on the RO system you get 1 to 4 times the amount of RO water produced in waste water. If you are on a well just put the waste water back into the ground. Otherwise use it to water your lawn.

The calcium found in well and municipal water is not in a form easily uptaken by plants. Also, many of the minerals found in well and tap water will bind with your nutrients and prevent them from being up taken.

Most growers only only monitor EC if your mixing up new nutrient daily that’s fine. If you have reservoirs or DWC that don’t get changed for two weeks or more, then have your nutrient water tested at least once a week and add specific minerals as needed. Most commerical greenhouse growers in europe have their water test daily.

For my small research growing I use Zero Water filters which like the name suggests gives you zero ppm. My local tap water has 125 ppm, my commercial RO system gets it down to 20 ppm.

People should not be drinking tap water s most has added flouride a posion. Why not feed your plants exactly what they need. To get the nasty flouride out of tap water you need to add an alumina filter.

Garbage in garbage out !