When the season moves indoors things change dramatically. Now my plants are confined to a pot of growing medium that may or may not have much buffering capacity. And, like most of you, I use tap water. I may temper the water's temperature, but I don't often consider the effects of its quality on my plants. The fact is that, in Minneapolis the tap water can have a pH of over 8. Most plants prefer their growing medium pH to be in the neighborhood of 5.5 to 6.5. At higher levels, nutrients become unavailable and salts begin to build up. By the time the days have reached their shortest and the sun angle is the lowest, my windowsill plants are also beginning to feel the stress of the pH problem. The symptoms of their distress vary. For instance, spider plants, Chlorophytum, develop brown tips on their leaves. Citrus become chlorotic and English ivies (Hedera) fall prey to spider mites.
Beginning in late January, the problems begin to multiply. I used to believe that I just hadn't been paying attention, and that if I would concentrate on my plants more, the problems would go away. Instead, they got worse. Nothing seemed to help - not more careful monitoring of the water, not feeding with any kind of fertilizer, not spraying with any chemical. The insects multiplied, the leaves continued to discolor and fall. Soon I began to wonder why I gardened indoors at all.
At this low point in my gardening year, I learned that the alkalinity or ability of the water to buffer acid was extremely important to the pH of my potting medium. I don't call that medium soil anymore, because it isn't. It is an artificial environment without the living organisms that help buffer against these problems in the ground outside. I also learned that the alkalinity is related to water hardness, but not exactly the same thing. In a nutshell, when the amount of dissolved solids such as calcium is high and the pH of the water is neutral or above, salts begin to build up in the pots. When enough has accumulated and the pH has become high enough, nutrients become unavailable and the plants begin to suffer nutritional problems too. The more stressed a plant is, the more susceptible to insects and disease. So there it was. The problem was literally at the roots where I couldn't see it.
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