Pure water has a pH of 7, which is neutral. Higher numbers indicate alkaline water, the greater, the more alkaline. Lower numbers indicate acidity, the lower the pH, the more acid. Plants, each being adapted to some of the wide variety of conditions found on earth, have certain preferences in pH for rainfall and soil conditions.
When you measure the pH of your soil, you are actually measuring the pH of the moisture it holds, which is affected by the soil particles and microorganisms. Of course, the pH of the water you apply will affect this also. The pH does have an effect on plant growth.
Many of the elements most used by plants in their growth are less available when the pH is in the acid range, below 6, and many of the micronutrients, those used in small quantities, are less available when the water, or soil, is alkaline. Some plants prefer, even require, acid soils and are not bothered by the lack of nutrients. Other plants will tolerate, but do not require, pH levels above 7, and are adapted to the lack of micronutrients, such as iron and zinc.
One common effect of the application of water with a high pH, or of a high soil pH, is chlorosis, or the yellowing of the leaves in such a way that the leaf veins remain green. Common in gardenias and citrus, this indicates a lack of iron, usually caused by a pH that is too high for the plant. A lack of nitrogen is indicated by leaves that turn yellow evenly, without the green veins.
The tiny bacteria and other organisms that inhabit the soil, huge numbers of them in a teaspoon of soil, are most active at a pH of 6.3 to 6.8, and so the processes that break down plant waste, such as leaves, are most active at that level. Peat bogs, for example, having a very acid environment, act as preservatives for organic matter.
One of the more unusual effects of water pH is the change of color that can be seen in the blossoms of hydrangeas. Watering with an acid solution (as long as the soil isn't strongly alkaline) will give blue flowers. Applying alkaline water will cause the bush to bloom pink. Of course, varieties differ also, so if you want pink flowers, for example, it's best to get a pink variety and then keep the pH high also.
One group of plants that needs acid soil (below a pH of 6.0) is the heather family, which includes rhododendrons and azaleas as well as heathers. Some, such as cranberries and blueberries, need soil even more strongly acid, around 5.0.
Trying to grow these in an area where water from the faucet tests at a pH of 7.0 or higher will be difficult without amending the soil with a large amount of peat moss or watering with rainwater rather than tap water. If, however, your area has a problem with acid rain, you can try these in your garden.
Many common vegetables and ornamentals prefer soil with a pH of around 6.5 These include spinach, parsnips, dahlias, chrysanthemums, sweet peas and tulips. If your faucet water or soil is acid, or your area has acid rain, you'll likely notice that plants are stunted, without obvious reasons for their lack of growth. This is because the major nutrients, nitrogen, phosphorous and potassium, are simply unavailable to these plants. Apply lime to the soil--which increases pH--and plant growth should pick up.