Do you think your tap water is fit for growing hydroponic crops? You might wonder what that question means. If your tap water is safe for humans to drink, it must be safe for plants…right? Mostly yes, but you may need to make some adjustments in your tap water to optimize your plant growth and crop production.
In this blog we’ll explore how water quality varies across water sources—even tap water drawn from different locations in your home. Before I set up any type of hydroponic system, I need to get a basic profile of my water.
I did not test for alkalinity levels, which are a good indicator of water “hardness.” General guidelines for classification of waters are: 0 to 60 mg/L (milligrams per liter) as calcium carbonate is classified as soft; 61 to 120 mg/L as moderately hard; 121 to 180 mg/L as hard; and more than 180 mg/L as very hard.[ii] I can find alkalinity levels for my tap water in my municipal water report.
To test my tap water, I’ll use three sensors to conduct a few basic chemical tests of the water. Since I have access to my local municipal water report data, I focused my home water testing on basic physical and chemical characteristics of the water. Specifically, I’m assessing the overall appearance, taste, smell, pH, Temperature (°Celsius), and Electrical Conductivity (EC).
Water quality guidelines suggest that good drinking water should be clear, have an EC between 0-800 μS/cm, and a neutral pH (near 7.0). I’ve chosen a TDS (total dissolved solids) meter and a BlueLab Guardian probe system that measures electrical conductivity (EC) and pH.
To have some comparison data, I tested the tap water form three different faucets—kitchen sink, 2nd level sink, and garage utility sink. I also included samples from rainwater and bottled water to see how these water sources differ from tap water.
What physical and chemical parameters are most suitable for plants?
Table 1 (shown below) provides a chemical profile of minimal requirements for water to be used in hydroponic systems. Compare human vs. plant pH requirements, and you’ll see how plant water preferences differ from those of humans. Most plants like more acidic water; humans prefer pH neutral water. Keep in mind pH requirements will vary across plant types and plant growth stages. Also note that it’s best to use water that has a low EC level, because as nutrients are added to the water, the EC will increase. In many cases plant growth will be negatively impacted by EC levels higher than 2 dS/m.[i]
Table 1 – Water Quality Guidelines for Hydroponic Systems
Figure 1 (below) shows how I set up my water quality testing of the six samples. I used two water monitoring probes. The Bluelab Guardian probe provides a digital screen readout of the EC, temperature, and pH to assess each of the six water samples. The Digital TDS Meter (DU-TDSW) was used as an alternative data collection device to collect EC, presented in the microsiemens (μS, uS) per centimeter = μS/cm format, which is 1000 times mS/cm.
What do the data tell us? Can this tap water be used for hydroponics?
pH: Five of the six the samples have pH levels above 7.0, which is slightly higher than what most plants prefer. Generally, most salad greens and vegetable plants grow best in pH levels of 5.4 to 7.0. Depending on what plant variety you’re growing, the higher pH may be acceptable or even ideal.
Electrical Conductivity: The EC levels are well within plant tolerances. However, keep in mind that EC levels will be higher once plant nutrients are added to the water. I’ve found that EC levels can double, or triple once plant nutrients are added. Because the EC calculation in μS/cm is a smaller unit of measurement, we can see more variability in the EC across the six samples, which are averaged out in the mS/cm EC data. While plants can tolerate up to 1 dS/m (equaling 500 μS/cm), EC levels above 300 μS/cm may cause leaf scorch on some salt sensitive plants.
Temperature: The 15-16°C temperature is acceptable for most plants but may be too high for many lettuce plants as well as varieties of salad greens commonly grown as cool weather crops that thrive in early spring and late fall temperatures.
Table 2 – Water Testing Data from Six Water Sources
In conclusion, can my tap water be used in a hydroponic system? Yes, but I will have to adjust the pH downward to provide optimal conditions for the salad greens I plan to grow. Later blog posts will offer more in-depth information about how to monitor, adjust, and optimize your water based on the plants you are growing and the type of hydroponic system you are using.
[i] Soil and water salinity is often measured by electrical conductivity (EC). The most used EC units are deciSiemens per meter (dS/m) and millimho per centimeter (mmho/cm), numerically: 1 dS/m = 1 mmho/cm.
[ii] Studies have generally found hard water to have positive effects on the health of its drinkers. Several studies have reported that calcium and magnesium in drinking water have a dose-dependent protective effect when it comes to cardiovascular disease. There is also some evidence that calcium and magnesium in drinking water may help protect against gastric, colon, rectal cancer, and pancreatic cancer, and that magnesium may help protect against esophageal and ovarian cancer. Hard water may also serve a protective role against atherosclerosis in children and teens. Read more at: https://www.mcgill.ca/oss/article/health-you-asked/you-asked-hard-water-dangerous-drink
[ii] Soil and water salinity is often measured by electrical conductivity (EC). The most used EC units are deciSiemens per meter (dS/m) and millimho per centimeter (mmho/cm), numerically: 1 dS/m = 1 mmho/cm.