Water quality is different in each location, from time to time and based on the season. Water supplies come from reservoirs that have collected surface water runoff, from several different wells, from rivers or lakes or some combination of these. In other words, water is not just water. It is water plus what it has been exposed to and what it has dissolved. Some of the water quality issues that affect plant growth and health are pH, chlorine and fluorine levels, temperature, microbiological levels, oxygen levels, and electrical conductivity.
In addition to the “natural” pH of the water, municipal water plants. have increased the pH of their water supply to meet the EPA lead rule. This is because higher pH levels don’t dissolve lead as well as lower pH water. This is the reason many water systems are running pH levels as high as 9 to 10 pH which is not good for plants that thrive at 5.6 to 6.5 pH levels.
The water supplier also may increase the level of chlorine from time to time based on higher bacteria levels in the supply or when repairs are made to water mains.
The temperature of the water is also a function of the weather, the source, and how the water is stored. Most plants do better with water that is between 65 and 75 °F.
One problem for the grower is the proliferation of bacteria. The water, its temperature, and the added nutrition are ideal for bacteria growth. As bacteria floats in water (planktonic bacteria), it comes in contact with tubing and piping walls and attaches (Stissel bacteria). The bacteria builds a slimy extracellular matrix called biofilm. This biofilm restricts the flow in piping and tubing and will plug water emitters and other devices. Periodically, the entire fertigation system will need to be disinfected.
Dissolved minerals (ions) are indicated by the conductivity of the water. Conductivity above 400 micro Siemens ( 0.4 milli Siemens) may indicate contaminants that might cause plant problems such as leaf burn, spotting, and staining of plants.
Starting with a pure water system ensures a consistent feed formula with repeatable results for every crop cycle regardless of the source of water. Commercial monocrops may get by with surface water, municipal water, or well water but for a high-value crop such as cannabis, every input will affect the final value of the product.