In addition to adequate sunlight and fertile soil, a good water supply is the third essential component required for healthy plant growth. In some regions of the United States, groundwater contains higher levels of calcium and magnesium, which can contribute to salt buildup in soils. Municipal water supplies, most of which have added fluorine and chlorine, can also negatively affect plants which are sensitive to these elements.
A common problem of coastal and mountain regions, saline soils often result from the accumulation of minerals from weathered rock deposits in combination with low rainfall or high water evaporation rates. Failing to adequately water growing plants can lead to salt stress in regions where soils are naturally saline, but a more important problem is that plants frequently fail to germinate in soils of high salinity. Seedlings that do manage to germinate usually fail to thrive, leading to stunted, slowly maturing plants. High salt concentrations can reduce the volume of water plants are able to absorb, limit root growth and development, burn plant foliage, reduce flowering, and reduce fruit or vegetable yields.
Many municipal water supplies tend to be alkaline, which has effects on plant growth and soil fertility which many professional growers as well as amateur home gardeners little understand. Alkalinity is a measure of water's ability to neutralize acidity, or concentration of hydrogen ions present in either soil or water. Over time, irrigating with water of high alkalinity can lead to deficiencies of trace elements like magnesium and calcium, both of which are necessary for healthy plant growth.
Some plants, especially several species commonly kept as indoor ornamentals like the corn plant and spider plant, are sensitive to fluorine and chlorine. Plants typically display yellowish patches or a dry, burned appearance on leaf edges and tips. When irrigated over time with high-alkaline water, plants tend to become "chlorotic," or yellowed in appearance. This is due to either iron or manganese deficiencies, which are exacerbated by alkaline water. Conversely, plants irrigated with iron- or calcium-rich water can suffer as well, as deposits of iron and calcium appear as tiny brown or white crystalline accumulations on the leaves.
Soil and Water Testing
Sending samples to a soil laboratory for testing is the only reliable way to determine the chemical composition of your native soil. Sending samples of water for testing is also advisable for those living in areas where water quality may be suspect in plants' failure to thrive, especially if soil tests indicate local soils seem to be of adequate structure and fertility. Soil tests reveal the concentration of trace elements, presence or absence of salts and other inorganic compounds; water tests indicate the pH, or acidity, of the water supply, as well as the occurrence of other impurities. Most university agricultural extension offices offer soil and water testing for a small fee, or gardeners can send samples off to private labs for testing.
Periodic or habitual flooding of saline soils is a process known as leaching, which moves harmful salts out of the plant's root zone. Saline soils cannot be remedied by the addition of soil amendments such as compost, bark mulch or fertilizers. For chlorinated water used to irrigate indoor plants, allowing water to stand for several hours before watering allows evaporation of these additives and significantly reduces the effects of chlorine and fluorine on sensitive plants. Irrigation with high-alkaline water may need to be balanced out by fertilizing with highly acidic fertilizers, such as those containing ammonia; however, this should be done with care and research, as many types of vegetable plants are easily damaged by ammonia-based fertilizers.