Tap water filling a glass at a kitchen sink

Distilled, RO or Tap Water for Kratky?

By Kratky Nutrient Solution · · Water

Short answer: for most growers ordinary tap water is fine for Kratky, as long as it is not very hard or high in sodium. Distilled and reverse osmosis (RO) water are worth using when your tap water is hard, salty or has high alkalinity, because they start close to zero and let the nutrient supply everything. The trade-off is that pure water has no buffering, so pH moves more easily.

Start by knowing your water

Oklahoma State University Extension calls water analysis the very first step in hydroponics, testing for pH, EC and alkalinity. It lists acceptable levels for common salts in source water:

  • Sodium: under 50 ppm
  • Calcium: under 150 ppm
  • Magnesium bicarbonate: under 50 ppm
  • Chloride: under 140 ppm
  • Sulfate: under 100 ppm

Your water utility's annual quality report gives most of these numbers. An EC or TDS meter gives a quick overall reading of how much is already dissolved.

Tap water

Tap water brings minerals with it. Some, such as calcium and magnesium, are nutrients; others, such as sodium and chloride, are not. It also usually brings alkalinity (bicarbonate). Oklahoma State University Extension notes that water with alkalinity above about 75 ppm causes the pH of a nutrient solution to rise and needs checking more often.

Cornell University's hydroponic lettuce research shows how much the water itself contributes: the research trials used RO water and held the solution at an EC of 1150 to 1250 µS/cm, while the demonstration greenhouse used municipal water and maintained an EC of 1650 µS/cm.

Many utilities disinfect with chlorine or chloramine. The U.S. EPA notes that more than one in five Americans uses drinking water treated with chloramines, which are formed by adding ammonia to chlorine and last longer in the pipes. See do you need to dechlorinate tap water for Kratky for what that means in a container.

Distilled and RO water

Distilled water is boiled and condensed; RO water is pushed through a membrane. Oklahoma State University Extension mentions reverse osmosis as one way to remove alkalinity. Both start with an EC close to zero, so the reading after mixing is all nutrient.

  • Advantages: predictable strength, no unwanted sodium or chloride, and pH that does not creep up from bicarbonate.
  • Disadvantages: cost or equipment, no buffering, so a few drops of acid can move pH a long way, and no calcium or magnesium from the water. Some nutrients assume the water supplies a little of both, so read the label.

How I decide

  1. I read the utility report or test the water. If sodium, chloride and alkalinity are inside the limits above, I use tap water.
  2. If alkalinity is high but everything else is fine, I still use tap water and simply expect to check pH more often, as Oklahoma State University Extension advises.
  3. If sodium or chloride is high, or the plain water's EC is high enough to take up much of the crop's target range, I use distilled or RO water, or blend it with tap water.

Blending is underrated. Half tap and half RO halves the sodium and alkalinity while keeping some buffering, and it is cheaper than buying all pure water.

What changes when you switch to pure water

  • pH adjustments: use much smaller doses. A quarter of what you used with tap water is a sensible start.
  • EC: the reading after mixing will be lower than before, because the water no longer contributes anything. Mix to the label rate and read the EC, rather than chasing your old number.
  • Calcium and magnesium: check whether your nutrient is designed for soft water. If not, the label may suggest a supplement.

A quick way to compare waters

If you have an EC meter, measure each candidate water on its own before adding any nutrient. Oklahoma State University Extension explains that EC measures dissolved salts: a higher reading means more salt already in the water. Distilled and RO water should read close to zero. Tap water usually reads higher, and that difference is room the nutrient no longer has. I write the reading down for each source, so when a batch behaves oddly I can tell whether the water changed. Utilities can change sources or treatment during the year, so I recheck tap water now and then rather than assuming it stays the same.

Quick questions

Is bottled spring water a good choice?

It varies by brand. Spring water can contain as many minerals as tap water, so measure its EC before using it.

What about softened water?

Most water softeners swap calcium and magnesium for sodium, so softened water is usually worse for hydroponics, not better. I use the unsoftened tap if there is one.

Related guide: water for Kratky nutrient solution. Also see how hard water affects Kratky nutrient solution.

Sources

  1. Dunn, B. and Singh, H. Oklahoma State University Extension. Electrical Conductivity and pH Guide for Hydroponics (HLA-6722).
  2. Both, A.J. Ten Years of Hydroponic Lettuce Research (Cornell University Controlled Environment Agriculture program).
  3. U.S. Environmental Protection Agency. Chloramines in Drinking Water.