What Every Grower Needs to Know About Water Quality

Water (H₂O) is a basic requirement for gardening, yet something that it is often given little thought after gardeners have learned the fundamentals.

As small children we learn that plants need water to grow, but we don’t think about it too much after that—we seem to just accept its presence as a given. For those interested enough to look a little deeper, here is some of the rest of the story.

Understanding the Chemistry of Water

Part of the magic of water stems from the fact that it occurs naturally as three different states of matter—solid, liquid, and gas—and that it possesses certain unusual surface tension properties. These properties have a variety of biological ramifications.

At sea level, water freezes at 32°F (0°C) and boils at 212°F (100°C). Freezing water expands and becomes less dense than its liquid form, which is why ice will float on water. Snow and ice are important agents for water storage, releasing trapped moisture in the summer months when plants need extra water the most.

Water vapor is caused by evaporation or as a result of boiling water. The amount of water vapor present in the air is recorded as humidity. High-humidity gardens are prone to molds, while low-humidity gardens are prone to spider mites and other insect infestations.

Under normal circumstances and air pressure levels, the temperature of liquid water will not exceed its boiling point. At sea level, water boils at 212°F—and it won’t get any hotter, since at that point it will turn into a gas and escape. At higher elevations, the boiling point of water is lower, due to the drop in pressure.

The higher the altitude, the lower the boiling point of water and the longer food must be boiled to heat to a given temperature. For example, water boiling at 212°F will cook eggs faster than water boiling at 190°F. Pressure cookers use the other side of the same principle—water under pressure boils at a higher temperature, so immersed food heats faster.

Pure or distilled water should be microwaved with care, because if it is heated in a clean smooth container it is possible for the water to become ‘superheated', which means that the temperature of the water in its liquid state is actually above the normal boiling temperature.

This can become dangerous when a contaminant is introduced to the water—like a spoon, for instance—as that can trigger a violent, almost instantaneous, boiling response. If the starting water is not pure (such as filtered or tap water) or if a contaminant is added (like coffee, tea, or sugar), then the water will boil normally.

While not technically a nutrient, (it’s actually a transport agent for nutrients) water is an important requirement for growing healthy plants, as it is required for photosynthesis. Light energy is absorbed by the plant, which divides the water molecules into hydrogen and oxygen. The hydrogen is combined with carbon dioxide to form glucose.

The quality of the end solution is in part determined by the quality of the water you’re using for the base liquid. If your local water is of poor quality, reverse osmosis filters or other filtering systems can be used to clean it.

Water is made up of a collection of molecules having two hydrogen atoms and one oxygen atom. It is particularly well-suited as a solvent for many materials. Hydrophilic (water-loving) materials—like many of the salts—dissolve easily in water, while hydrophobic (water-fearing) substances such as oils do not.

Because water molecules have a positive charge on the side with the two hydrogen (H) atoms and a negative charge on the oxygen (O) side, they are attracted and attach easily to many other molecules.

One reason that salts are so often used as nutrient sources is that they also have a positive and negative side, but are held together with a weaker ionic bond. Water molecules attach themselves to either side of the salt molecule and break them apart, dissolving the salt into the water—this is why many chemical hydroponic solutions make use of salts dissolved in water. Organic nutrients are often less miscible and should be shaken to create a suspension each time before use.

Water Quality and Your Plants

Pure water has no electrical conductivity, so an EC or PPM meter will display a zero conductivity reading. As the water becomes more contaminated with salts and other conductive materials from fertilizers, the nutrient solution will become more electrically conductive and the meter will show higher values.

The information gathered with a meter can be invaluable in maintaining a suitable nutrient solution, but keep in mind that EC and PPM meters only display the total amount of electrically conductive contamination present, not specifically what the contamination consists of.

These meters do not give specific N-P-K values—a solution with a high nitrogen content might give the same reading as a solution with a high phosphorus content.

Two common contaminants in tap water are calcium carbonate salt and magnesium, which are found in hard water. Since this can be a source of calcium to plants, nutrients developed for hard water applications generally take this into account and therefore contain little or no additional calcium.

Another common contaminant in city water is chlorine, which plants do not need in very high quantities. To vent off chlorine from tap water, fill your watering containers each time just after watering and the chlorine will have dissipated by the next time you water.

Checking the quality of the source water will assist in fine tuning your garden, but tap water clean enough to drink will often work well enough.

Best Watering Practices for Your Plants

Underwatering is less harmful than overwatering—if plants are slightly underwatered they will usually signal this by getting droopy and will respond favorably within hours when watered again.

Overwatering is more traumatic to plants and requires a longer recovery time. If you do happen to drown your plants, don't give up—just try to learn from the experience and try again. If your tap water is of a very poor quality, you might have to filter it before use, or use drinking water to hydrate your plants.

What Does pH Have to Do with Water Quality?

Growers use pH levels—assigned values between zero and 14—to denote how acidic or basic a liquid is. Pure water has a pH level of 7. Solutions with a pH value lower than seven are called acids and solutions with a pH value higher than seven are called alkaline or basic. Strong acids will corrode and dissolve many substances and strong bases such as lye can cause deposits and damage organic tissues.

The ideal range for nutrient solutions is around six to 6.2 (See: Perfecting pH). If you are within a half point or so, don't bother adjusting it. Proper pH is more critical in hydroponic systems than soil gardens, due to the additional buffering properties of soil.

Radically incorrect pH can cause physical damage to plants due to the caustic properties of acids and bases and less severe imbalances have an impact on the solubility of plant nutrients. Adjustments to pH are made by adding the opposite to the solution. (See: Understanding the Ups and Downs of pH)

Acids have positive hydrogen ions and bases have a matching negatively charged acceptor—when the two combine, they neutralize each other. If the solution has a pH value that is too low (acidic), add a base to raise the pH level. If the solution is too basic (high pH), an acid is added to lower it.

How Water Moves in the Garden

Water has a high surface tension and sticks to itself very well. Submerged molecules can attach on all sides to each other, but the molecules on the surface do not have water molecules above them to attach to, so they hold tighter to their neighboring water molecules.

This creates tension along the surface, allowing water to ‘bead’ into droplets and capillary action to force water to rise within a narrow tube. The water will stick to the walls (this action is called adhesion) and surface tension will pull to try to form a droplet (this is called cohesion), supplying an upward force.

This allows for the transport of water up the growing media to plant roots and up the roots and along the rest of the plant as part of respiration.

A key component to plant health—particularly in hydroponic systems—is a sufficient amount of available oxygen. While oxygen is a major component of water, the plant also requires oxygen in a more available form. The level of freely available oxygen in water is known as its dissolved oxygen (DO) content.

Air stones and fountains are both used to increase DO. Since the amount of air that water will hold is dependent on temperature, the warmer the water the less air it will hold and the more important proper aeration becomes. Cool winter water tends to have higher dissolved oxygen levels than warm summer water.

If the amount of dissolved oxygen in water drops too low, it creates conditions for anaerobic (air-fearing) bacteria to develop. Anaerobic bacteria are the rotting and spoilage bacteria responsible for the foul odors in stagnant water. Properly aerated water is conducive to aerobic (air-loving) bacteria, the beneficial bacteria that assist in composting and healthy plant development.

Water is a crucial part of gardening and an improved appreciation of its properties can help you to better understand how many gardening concepts fit together.

Read more about the quality of your water supply.