Is your tap water suitable for plants? Unless you know something about both hardness and alkalinity, you can’t tell if your tap water should be used.
Is RO water, distilled water, or rainwater suitable for plants? You probably think it is, but they also have problems that relate to hardness and alkalinity.
Let’s have a closer look at water and these two properties.
Key Takeaways
- Hardness and alkalinity are not the same thing.
- Tap water may not be suitable for plants.
- Pure water may also not be suitable
Hardness and Alkalinity
People commonly confuse the terms harness, alkalinity, calcium and magnesium. Let’s understand the differences.
Calcium and magnesium are two macronutrients that plants need.
Alkalinity is a measure of the total carbonates (CO₃), bicarbonates (HCO₃), and hydroxyl ions (OH) and is usually expressed as the equivalent of CaCO₃, e.g., 100 ppm CaCO₃. Although calcium commonly combines with carbonates, alkalinity is not a measure of calcium. I discuss alkalinity in more detail in What is Alkalinity – It May Not Be What You Think?
Hardness is the concentration of divalent cations, mainly calcium (Ca²⁺) and magnesium (Mg²⁺), in water.
I have had so-called “hard tap” water all my life. Everyone here uses an ion exchange water softener to lower the amount of calcium and magnesium in the water.
But what is hard water? How hard is it, and at what level is it a problem for plants?
What is Water Hardness?
Water hardness is generally expressed in milligrams per liter (mg/L) of CaCO₃ equivalent, or parts per million (ppm) of CaCO₃ equivalent. 1 mg/L = 1 ppm.
General classifications:
- Soft water: 0–60 ppm CaCO₃
- Moderately hard: 61–120 ppm CaCO₃
- Hard: 121–180 ppm CaCO₃
- Very hard: >180 ppm CaCO₃
What level of hardness is detrimental to plants?
- <100 ppm CaCO₃ (soft to moderately hard water): Usually fine, little risk for most potted plants.
- 100–200 ppm CaCO₃ (hard water): Still usable, but sensitive plants (like blueberries, azaleas, some orchids) may show stress from excess Ca/Mg or reduced uptake of iron and other micronutrients.
- 200–300 ppm CaCO₃ (very hard water): Often problematic in containers — can cause leaf chlorosis (iron deficiency), salt buildup in soil, and scaling on pots. Regular leaching and acidification may be needed.
- >300 ppm CaCO₃: Considered detrimental to most potted plants unless managed (acidifying agents, reverse osmosis, or blending with rainwater).
Ideally, the water should have a hardness of less than 200 ppm CaCO₃, and 100 is even better.
What level of calcium and magnesium is recommended for fertilizer solutions?
The recommended amounts vary depending on the source of the information; however, the following are the most popular.
- Calcium: 40-120 ppm, with 100 ppm a common suggestion.
- Magnesium: 20-40 ppm, with 25 ppm a common suggestion.
- The ratio of calcium to magnesium is also important and should be in the range of 3-5 ppm calcium per 1 ppm magnesium.
Note that there is a lower limit for both calcium and magnesium. The pure sources of water are well below this level and are therefore not suitable for plants unless you add these nutrients.
How does this compare with the recommended hardness values? If we convert 100 ppm Ca and 25 ppm Mg to hardness values, we get:
Calcium = 250 ppm CaCO₃ and Magnesium = 100 ppm CaCO₃, for a total hardness of 350 ppm CaCO₃.
That is significantly higher than the above recommended maximum value of 200 ppm CaCO₃?
There seems to be something wrong with the numbers? Fertilizer recommendations suggest calcium and magnesium values that result in a hardness that is far too high. To understand this discrepancy, we also have to look at alkalinity.
What is Water Alkalinity?
When commercial fertilizer solutions are made up, they tend to use pure forms of water, including RO, distilled, and rainwater. These have no carbonates, so the alkalinity is zero. Soft tap water also has very few carbonates.
However, hard tap water usually has higher levels of carbonates, and the carbonates are a bigger problem for plants than hardness (Ca + Mg). High alkalinity (i.e. a high amount of carbonates) causes salts to precipitate out as white deposits on soil, which can harm roots. It also raised the pH, which is harmful to plants.
When organizations develop hardness recommendations for plants, they take into account the fact that high hardness is commonly associated with high alkalinity. By limiting the hardness, they also limit the alkalinity, so they recommend lower, safer levels for hardness.
What are the safe limits for alkalinity? These guidelines are based on greenhouse studies.
- < 60 ppm CaCO₃ – Ideal.
- 60–120 ppm CaCO₃: Generally safe. Substrate pH may rise slowly; most houseplants tolerate this well.
- 120–180 ppm CaCO₃ – Borderline. Monitor substrate pH; acidifying fertilizer or occasional leaching may be needed.
- > 180 ppm CaCO₃ – Unsuitable for potted plants. Tends to raise pH too high, causing micronutrient deficiencies (especially iron, manganese, zinc).
What does all this mean?
If you are making fertilizer solutions with pure water, or if your tap water has a very low alkalinity, you can use higher levels of calcium and magnesium from the fertilizer. You can even make your own Cal-Mag mixture from eggshells and magnesium sulfate.
However, most tap water tends to have higher alkalinity and hardness, and you may need to reduce them before using the water. In that case, it is best to dilute the tap water with some pure water, which includes RO, distilled water, dehumidifier water, and rainwater.
How Much Do You Dilute Your Tap Water?
Let’s have a look at my tap water as an example. I checked with my city water department, and like most water departments, they publish annual reports that include water quality data. I obtained the following values from them.
- Calcium – 120 ppm
- Magnesium – 35 ppm
- Akalinity – 290 ppm CaCO₃
Some reports also give you the hardness, but if not, you can easily calculate that. To convert the first two numbers to a harness value, use this formula:
Hardness as ppm CaCO₃ = (calcium ppm / 0.4) + (magnesium ppm / 0.25).
My hardness is 440 ppm CaCO₃.
Based on the above recommendations, I should keep my hardness below 200 (100 is even better) and my alkalinity below 120.
A mixture of 1/3 tap water + 2/3 pure water would result in an approximate hardness of 150 and an alkalinity of 100.
That gives me water that has lots of calcium and magnesium, but not too much carbonate. For pure water, I use a combination of rainwater and dehumidifier water. Both are free, which is a good thing since I use a lot for my hundreds of streptocarpus and orchids.
To this water, I add synthetic fertilizer with an NPK ratio of 3-1-2 so that the nitrogen level is 100 ppm. This fertilizer does not contain calcium or magnesium, since my tap water already has enough.
Use my PPM Fertilizer Calculator to figure out how much fertilizer to use.
Hello Robert,
Interesting reading. How do you apply this to a garden environment? I live in California and rain water is not readily available during late spring, summer, into early fall. The rainy season starts about November ending in April. California stores water in reservoirs for use during the drier portion of the year. Also, the humidity is typically low & there is no need for dehumidifiers.
Second part of the article talks about using it on houseplants.
In the garden, your water quality is not so important because most people don’t use a lot compared to rain. It also depends more on your soil, than the water.