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News and Best Practices

Using a Reverse Osmosis Filtration System

July 12, 2016

The terms “hard water” and “soft water” describe the amount of salt and alkalinity in the water supply. If there’s a buildup of potentially harmful salts or other contaminants, the plants in your professional grow may not receive sufficient nutrients. A best practice is to filter your water through a reverse osmosis (R/O) filtration system.

The purpose of an R/O system is to start with clean water and add only the nutrients your plants need. Whether your water comes from a well or a municipal source, you should filter it before adding nutrients. An R/O system removes salts and contaminants in your main water supply. It also removes any beneficial minerals such as calcium and magnesium, but you will add these back in your nutrient program. (R/O systems also produce wastewater, which must be disposed of properly.)

If you choose not to use R/O filtered water, your plants will most likely have salt buildup on the roots; this inhibits nutrient uptake and makes for a weaker plant with less bud growth. If water from the tap is 200 to 300 parts per million of contaminants, the salts will blend with your added nutrients and may produce “root lock.” You won’t know how existing salts and minerals in the water will affect your nutrients unless you clean the water first.

Starting with water that is nearly zero PPM puts you in total control, so you can add more beneficial nutrients. After you’ve added nutrients, PPM levels will be high, but you will control what makes them rise and fall. Your plants will love the clean water, at the same time reducing the salt buildup.

Which R/O system is right for you?

R/O filter systems are rated by number of stages, from small two-stage systems to massive six-stage systems. Choose a system based on how many gallons of water you need per day and how many contaminants you need to clean out. Medium-sized grows might use four-stage or home systems, which can be bought online or at a home supply store. For larger operations, you may need five- and six-stage systems.

The gallons per day are different for each system. You will know how much water you need a day after a few cycles. For example, 50 gallons of water a day might be needed for 40 plants.

Setting up an R/O station

To use R/O water efficiently, set up a water station:

  1. Place a 55-gallon or larger drum (200+ gallons for big operations) atop a platform. This is tank A, your main R/O water supply that is used in tasks such as cleaning and making sprays for foliage.
  2. Under the platform, place two drums of equal or smaller size. These are the nutrient drums (tanks B and C), tank B filled with vegetative nutrients and tank C with flowering nutrients.
  3. After filling tank A, run a hose from it to fill tanks B and C. It is recommended that you keep the three tanks filled. Each tank should have an air stone at the bottom to keep oxygen circulating.
  4. Place a sump pump, which is plugged into a wall or wired into a switch, at the bottom of each of the nutrient tanks for pumping to plants during watering times. You can attach a garden hose and spray gun for full control.
  5. Add nutrients to the bottom tanks for vegetative and flowering plants. You won’t be cross contaminating the tanks with different feeding mixes, and you will be able to water your vegetative room on the same day as your flowering room.
  6. After adding nutrients to the tanks, install a meter with probes that test PPM, pH level, electrical conductivity (or EC, the amount of total dissolved salts), and water temperature. This is a must-have to keep track of your water’s health before you add it to your soil. Always measure your water’s pH after the nutrients you’ve added have had time to mix well with the oxygen-rich water.
  7. Place the pH and EC probes in the middle of the nutrient tanks. It’s best to wait for an hour or more for each reading to stabilize. Be patient when waiting for an accurate pH reading after mixing a pH Up or a pH Down additive, since the additive can take a while to equalize.
  8. After achieving the pH level you are trying to reach, wait 24 hours. This can help the oxygen added to the water blend the nutrients.
  9. After you’re satisfied with the pH levels in the nutrient tanks, then feed your plants.

If the pH level has drifted after 24 hours, adjust it. Wait one hour while the pH meter tests it, until it is stable and at the level you want. The ideal pH level varies from cloning to vegetative to flowering stages. For clones, always keep pH at 5.5, during vegetative growth at the 5.7-6.3 range, and during flowering at the 6.3-6.7 range.

Finding the sweet spot

There are many options on the market for nutrients, and each of them will affect your water’s concentration levels of salt, nutrients and minerals. When adding nutrients, start with less than what’s recommended on the label. Then increase the amount until you find the sweet spot for your particular strains. Cannabis uses nutrients best when they are offered in a way they can be properly taken in through the roots. Starting with clean water also works with hydroponics and deep water culture systems, but the EC and the PPM levels will be different depending on your setup.


By Eric Stone
Cannabis Cultivation Today articles are for informational purposes only and should not be considered legal guidance or advice on grow practices. You should contact an attorney or a qualified cultivation consultant for specific compliance and cultivation advice.

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