Does RO (Reverse Osmosis) water need special nutrients?

Updated Jan 28, 2021

by Nebula Haze

Splash of water

“RO” or “Reverse Osmosis” water has been treated to make sure you’re dealing with just water, without any other contaniments or minerals.

RO is purified using osmotic pressure with a membrane, as opposed to using a filter like many other water purification methods. This allows the water to be purifed of nearly all contaminents. In fact, the RO process is commonly used to purify sea water.

Many of the cannabis growers who choose to use RO water do so because, for one reason or another, their local tapwater is not suitable for growing cannabis.

Other growers use RO to “start fresh” as far as water is concerned. With RO water, you know exactly what you’re dealing with! There is nothing in RO water that you didn’t put there yourself.

Where to Get RO Water

RO water can be purchased at water stations (which are located in many cities), or growers can invest in an RO system to make reverse osmosis water at home.

Several cities offer water stations that sell Reverse Osmosis water in refillable jugs

5 gallon jugs are often used for refilling reverse osmosis from a water station

 Or many growers make RO water at home with a Reverse Osmosis Water Filter System

RO water system


Calcium & Magnesium Supplements

When using RO water for growing cannabis, it is important to use a Cal-Mag (Calcium and Magnesium) supplement alongide your regular cannabis nutrients. This is because you need to make up for some of the trace minerals that are normally found in regular water.

CaliMagic is a popular Cal-Mag supplement from General Hydroponics, but other Cal-Mag supplements (made for plants) will also work.

What nutrients work with RO water?

As long as you add extra Cal-Mag, any quality cannabis nutrients should work well with RO water.

What’s a good nutrient system that works for all hand-watered setups, including both soil and soilless growing mediums like coco coir?

I like the General Hydroponics Flora trio plus their Cal-Mag supplement Calimagic​.

General Hydroponics Flora trio - includes all your base nutrients!PlusGeneral Hydroponics CaliMagic Cal-Mag supplement

But there are also lots of other great nutrient options and Cal-Mag supplements that will deliver good results.

Whenever using liquid nutrients, and especially when growing in RO, you will need a way to test pH so your plant roots are able to easily absorb all the nutrients they need. I use a simple General Hydroponics pH test kit. Learn more about managing pH here:​

Generaly Hydroponics pH Test kit

Jump to…

Learn how to manage pH

What nutrients can I use?

General Hydroponics + CaliMagic nutrient schedule for RO water [PDF]

How do I fix growing problems?



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