Depending on where water originates, varying amounts of salts might be dissolved in it. Only distilled or deionized water have no dissolved salts. Water from wells, rivers, lakes, and other sources always has some portion of dissolved minerals.
The most common elements dissolved in water are calcium, magnesium, sulfur, chlorine, iron, and manganese. None of these elements are toxic to plants in the quantities normally found in drinking water, but contribute to other problems in the house.
Calcium and magnesium make “hard” water, which interferes with soap action and leaves deposits on pipes and faucets. The water softeners used to remove calcium and magnesium substitute sodium for them. Sodium is harmful to plants. However, calcium and magnesium are not. Softened water is the most common problem with water quality that gardeners encounter.
Alkalinity
Alkaline irrigation water will slowly make the soil more alkaline. This isn’t usually a serious problem outdoors because natural rainfall will leach the calcium and magnesium out of the soil before alkalinity becomes a problem.
However, in regions that don’t get summer rainfall, the water may make the soil less acid. Acid-loving plants, such as azaleas and rhododendrons, suffer iron deficiency if their soil isn’t acid enough. Although iron may be present in the soil, it is less soluble in alkaline soils. Use acid-reaction fertilizer on these plants to overcoming the alkalizing influence of the water.
If iron-deficiency symptoms occur (yellowing between the veins on the newest leaves), spray the plants and water their soil with a fertilizer containing chelated iron. This form of iron remains soluble and can be absorbed by plants readily.
Houseplants irrigated with alkaline water may suffer as the soil accumulates alkaline salts and becomes more alkaline. Leach the plants well with each watering to avoid mineral accumulation. Fertilizing with an acid-reaction fertilizer, such as Miracid Plant Food, counteracts the alkalizing effects of the water and keeps the soil acid.
The opposite problem — acidity — is seldom a problem with irrigation water.
Brackish Water
Some well water and some water drawn from sources near the sea contain sodium chloride (table salt). This water has the same effect on plants as softened water. If possible, irrigate your plants with water from another water district or a well that isn’t so high in sodium. If you don’t have too many plants, consider buying deionized water for use on your plants.
If you must use salty water, over-irrigate by at least 10 percent at each watering, and leach the soil frequently to keep the sodium from accumulating in the soil. Adding gypsum at regular intervals also helps to flush the sodium from the soil.
Other Chemicals
Other chemicals are sometimes found in drinking water in quantities that are toxic to plants.
Fluoride, which is put into drinking water to prevent dental cavities, can burn the tips of sensitive plants like dracaena, orchids, and spider plants. Boron, which is a necessary nutrient for plants, is sometimes present in toxic amounts in well water. You can’t do much about fluoride or boron except avoid using the water that contains them.
Chlorine gas, which is put into drinking water to sterilize it, is toxic to plants. Water districts are supposed to allow all the gas to escape from the water before use, but sometimes some is still dissolved in the water. If you let the water sit in an open container for a day or two, the rest of the chlorine should escape.