Water (more complicated than you'd think)

Growing cannabis is relatively easy once you understand some unusually complicated topics. This is one of those topics. Whenever you think of "water," you should think in the following terms.

Measuring water purity (TDS)

All water has Total Dissolved Solids (TDS) measured in parts per million (PPM). These "parts" are typically minerals, often called salts. Hard water has high mineral content (particularly magnesium and calcium). Soft water is low in those minerals (but "soft water" often refers to water from a "water softener," which is high in sodium. That's real salt, from the kitchen table, and bad for plants.).

TDS is a confusing topic because there are two types of meters (TDS & EC). TDS meters measure parts per million (PPM). Electrical conductivity (EC) meters measure microsiemens. Even more confounding: PPM meters can be in three different scales (relative to EC).[1]

I use an HM EZ-TDS (or TDS-3) meter meter which is the 500 (or .5) scale.[2]

Water purity

Distilled (deionized) water (produced by boiling and condensation) is the most pure (0-5ppm). Filtered drinking water (produced through Reverse Osmosis filtration) is almost as pure (10-40ppm. This water may be sold in jugs at the market, or dispensed through vending machines.).

However, pure water isn't optimal for your plant. Some trace minerals in the water are good. Therefore, you should start with water that's 150ppm (+/- 50ppm).

The problem: in some areas tap-water is extremely hard. In my area it can be as high as 800ppm. The reason that's a problem: When you add nutrients to water, it adds "salts." So, you might add 300-800ppm in nutrients to 800ppm water. The resulting nutrient mixture wold be too strong. The water may have an imbalance of minerals. Or, contain unusable sulfates and sodium which will interfere with uptake of nutrients.

I fill 5-gallon jugs of RO water from a vending machine, and mix that with tap water to start with approximately 150ppm (before adding nutrients).

Water quality

In addition to the purity of the water, the quality is important. What exactly is dissolved in the water?

The primary concern is the ratio of Calcium (Ca) to Magnesium (Mg), which should be 2:1 to 4:1. If you live in an unusual area which has high Ca or Mg, you may have to take steps to balance that.

In the US, all water providers are required to publish an Annual Water Quality report. Usually this can be found on your city's web site. Note: Tap water TDS can fluctuate through the year due to weather conditions, rainwater runoff, etc.

My water's Ca ranges between 44-100ppm (avg 58). Mg ranges 18-54 (avg 25). That's a reasonable ratio.

The average of my total TDS is 640ppm (it ranges between 320-1000). This means most of my tap water’s TDS is not useful (sodium & sulfates). Looking at it another way: the combined average of my Ca & Mg (83ppm) is just 13% of the average total TDS (640ppm). If I dilute my tap water down to 150ppm (using RO water), that also reduces the combined Ca & Mg to 20ppm.

The strength of the combined Ca and Mg isn't as important as the ratio. But, at that level it may be better to not use tap water (as the source of the starting 150ppm). Instead: add Ca & Mg to the RO water to make it 150ppm. (Or, add tap water to get 75ppm, and then add Ca & Mg to 150ppm.). This sounds excessively precise. I don’t do this with my water. But, the point is: you do have this control over your water.

NOTE: If your tap water is “softened,” it will have high sodium. Do not use that. You want to use your tap water before the softening process. This is typically available from an outdoor faucet. (For reference: my really-bad tap water averages 24% sodium. Or, 152ppm out of 650ppm total.).

Creating 150ppm water

For most people, their tap water's total PPM will be too high. You can dilute your tap water with filtered (RO) water to make 150ppm. I fill my bucket with 20-25% of my 650ppm tap water, then 75-80% RO water (which is about 20ppm).

It doesn't have to be exact. You want to be in the 100-200ppm range. I eyeball it.

In rare circumstances your tap water may have:

  • Too low TDS.
  • The Ca:Mg ratio may be imbalanced.
  • The combined Ca and Mg is much too low (or high) relative to the total TDS.

In those cases you can take additional steps to improve that condition. (It really has to be bad to worry about this because the soil and nutrients provide minerals too.).

To add Ca or Mg:

  • Magnesium can be increased with ordinary epsom salt sold at the pharmacy. This is pure magnesium sulfate. If you increase the water's PPMs with epsom salt, 10% of its weight is Mg, 13% is Sulfur (S).
    • Langbeinite (sul-po-mag) is another, more organic source of Mg. It’s useful if you want to add also potassium (cannabis likes potassium!). 11% of its weight is Mg (22% is S, and 22% is K).
  • Calcium can be increased using gypsum. 23% of its weight is Ca, 19% is sulfur. Gypsum is water soluble up to 2.0 to 2.5g/L. It takes some effort to dissolve. Shake it vigorously in a water bottle, for example.
    • Calcium acetate (crushed egg shells which have been dissolved in white/distilled vinegar for 3-4 weeks, then evaporated into a powder) contains 20% calcium by weight. This is calcium acetate, and may be useful if you didn’t want to add sulfur. I describe how to make this in the “how to treat Ca & Mg def” article.).


  • 1 gram epsom salt (per gallon) will add 26 PPM Mg (and 34 PPM S).
    • 1 gram/gal langbeinite will add 29 PPM Mg (and 58PPM each of S & K).
  • 1 gram/gal gypsum will add 61 PPM Ca (and 49 PPM S). Note: Gypsum which is commonly available for agriculture isn’t pure calcium sulfate dihydrate. It will typically have 10-20% less Ca & S.
    • 1 gram/gal calcium acetate (eggshells dissolved & dehydrated) will add 60 PPM Ca (and no S). Note: Crushed, dissolved & dehydrated eggshells is not pure calcium acetate. It may have phosphorous, etc. But, generally speaking, this should be a reasonable way to view it.

Note: both epsom salt & gypsum add sulfur. Cannabis likes sulfur. But, it can also acidify the soil. At these low amounts (to add 50-100ppm), there shouldn't be a problem to use both.

Other water topics

The water topic is also confusing because hydroponic (and soilless) growers face different challenges.

"Calmag" products

Whenever a new grower encounters Ca and/or Mg deficiencies (typical in flowering) the common advice is "use calmag." These products originated for hydroponics. I don't like to use “calmag” in soil because

  • It treats two problems when typically you only need to treat one.
    • For example, if there is a Ca:Mg ratio problem, it doesn't make sense to add to the other half of the ratio. That simply perpetuates the ratio's imbalance.
  • Contains Nitrogen which will alter your NPK ratio, perhaps causing the N toxicity. (If I want to add N, I can do that in other, more organic ways.).
  • Calcium nitrate is acidic. One of your bigger risks is soil acidity (due to peat as a medium, overfeeding and/or too little runoff, organic nutrients, etc.). Using Calcium nitrate will add to that challenge.
  • The acidity of calcium nitrate leads to the use of "Ph-Up". But, these products contain nutrients which will alter your NPK ratio too. (They aren't labeled showing their nutrient content because they aren't sold as "fertilizer."). Now you're adding more salts to your soil just to counteract the acidity of "calmag." More salts increases your risk of salt build up and acidic soil.
    • That last point implies you incurred the expense of a pH (pen) meter (and all the supplies to maintain it). You really shouldn’t need that to grow in soil. (I don't pH my nutrient solution.).

As you can see, it has a tendency to spiral into a vicious cycle.

Note: Some “calmag” products are based upon calcium (and magnesium) carbonates. These compounds are dissolved in acid. These may be more suitable for soil. However, the one product I looked at (General Organics CaMg+) seemed like vinegar. I didn't see the point of adding an acid to the soil when gypsum or epsom salt will do the job, for less expense.


When googling about how to grow, you’ll often read comments about tap water containing chlorine; "it's bad for the plant;” “you have to bubble it to evaporate the chlorine out."

  1. Tap water typically contains chloramine – which doesn’t aerate away. Hydro growers typically use RO water (which doesn't have chlorine). They bubble their water to prevent harmful (anaerobic) bacteria from growing, and to provide dissolved oxygen to the roots.
  2. Tap water has a very small amount of chlorine, only enough to keep it sanitary within a closed delivery system. I.e., it's not expected to come in contact with much unsanitary material, therefore it is kept to a minimal level.). That small amount of chlorine is exhausted as soon as it touches the soil. The unsanitary condition of the soil will overcome it very quickly.

To compensate for the small amount of sanitizer in tap water: I add a pinch of sugar to every gallon of water the night before mixing. This will encourage bacterial growth, depleting the chlorine. (Or, add it at the time you mix. It will compensate the soil microbes for whatever hit they take from the chlorinated water, encouraging them to rebound.). For example, if your soil has mycorrhizae, it would benefit from some carbs added to the water. It's a fungus which symbiotically assists the roots by supplying nutrients. Myco is fed by the roots exuding sugar when nutrients are needed. Supplementing sugar reduces that effort by the plant.

But, too much sugar is bad for a plant. It will also attract fungus gnats which live in the soil. Just a pinch (1/16 tsp, 0.3 ml) per gallon is sufficient. Too much can can create a hypertonic (osmotic pressure) condition which can pull water from the plant, causing shriveling and withering.

Some people use molasses instead of sugar because it has trace minerals. I don't think it's worth the expense. The soil, water and nutrients supply enough minerals. A pinch of ordinary sugar is fine. (I use kelp in late flower to supply minerals as well as growth hormones to harden the buds.).

Finally. to the extent you mix RO water with tap, the chlorine will be diluted even more.

pH (acidity to alkalinity)

This is a confusing topic because hydro (and soilless) growers must pay particular attention to the pH of their water. Because there is no soil to buffer/stabilize the pH, they must frequently adjust it using products called pH-up (and down).

Soil growers seem to obsess about pH too. But, it's really not necessary. The soil itself acts as a "buffer" (which will pull the nutrient solution's pH to the soil's pH. This is one reason to add dolomite lime to the soil, to help buffer its pH.).

Unlike hydro, soil pH will rise as it dries. Nutrients and minerals are more available at different pH. So, it's good for the soil to experience a range from wet to dry

As mentioned concerning “calmag” products, using pH-Up adds more nutrients (salts) to your solution, adding to salt buildup in the soil. Salt buildup will acidify the soil. So, trying to pH your nutrient solution can actually contribute to problems you are trying to avoid.

If you want to get out of pH'ing your nutrient solution: One of the best things to do is: 1) don’t overfeed, and 2) water enough volume for 10-20% runoff. Weaker nutrients have less "gravity" to pull the soil's pH. They also reduce the chance of salt buildup (which is your real concern as an acidification of the soil). Generous runoff also reduces salt build-up.

Another downside of testing your nutrient solution's pH is that you have to purchase a pH pen (meter). These are more delicate and shorter-lived than TDS meters. You have to calibrate them frequently (with calibration fluids you can't make yourself). You need a storage solution to keep the probe in good condition. Occasionally you need to use a special probe cleaning solution. It's an expense and headache which isn't worth it for soil.

The Control Wizard Accurate8 soil pH probe is useful when you develop more skills and wish to fine tune your starting soil pH. This is discussed in the soil article.

[1] The two TDS PPM scales are:

  • 500 scale – Called Hanna scale (however, Hanna makes 700 scale meters too.). This scale is sometimes called .5 scale, or 342 scale, or 1382ppm scale. Based upon measuring a solution’s NaCl (sodium chloride) content.
  • 640 scale – Called Eutech scale.
  • 700 scale – Called Truncheon scale. Sometimes called .7 scale, or 442 scale, or 1500ppm scale. Based upon measuring a solution’s KCl (potassium chloride) content.

500 scale appears to be the more common among inexpensive/hobbyist TDS meters.

EC can be converted to PPM by multiplying the EC value by 500, 640 or 700. (Convert PPM to EC by dividing PPM by its scale).

[2] I inexpensively calibrate my TDS meter using 1 gram of ordinary salt (from the kitchen table) dissolved in 1 liter distilled water. By definition this is 1000 PPM. I use a Horizon Pro-20B (20g capacity, 0.001 resolution) or Horizon HB-01 (100g capacity, 0.01 resolution). These are sold by eBay seller "anyvolume." Either scale would be accurate enough for this purpose.