Soil pH / Soil Acidity
Soil pH, or soil acidity, is one of the principal influences for good or bad in soil. This used to be the province of scientists and chemistry students, but over the years it has become part of the home gardener's everyday world. In many, many cases, soil pH is the key to proper plant growth, and a reading of soil acidity can tell you much about what is going on beneath the surface of your garden.
The acidity of soil can be a highly technical subject, but for us it need not be. Actually the pH scale is just as easy to work with as the thermometer scale. You don't have to know thermodynamics or heat transfer to understand what happens to your plants when thermometer readings drop to or below 32° Fahrenheit. Similarly, without knowing a thing about hydrogen and hydroxyl ions, logarithm exponents or other technical details of the pH theory, you can make practical use of soil pH elements that your plants (and the soil organisms) need.
Effects Of Soil pH
Effects of soil pH are both direct and indirect. Direct effects, while not numerous, can be critical. In the case of a soil that is too acid or too alkaline, there can be (1) toxic effects on the plants themselves, and (2) an unfavorable balance between acid and alkaline elements needed by plants.
Indirectly soil acidity can have an effect on one or more of the following:
(1) Availability of essential elements
(2) Activity of soil microorganisms
(3) Solubility and potency of toxic elements
(4) Prevalence of plant diseases
(5) Competitive ability of different plant species
(6) Physical condition of the soil (when lime is used to raise the pH)
[You can find out more here about soil classification by soil ph.]
The pH scale is a measure of balance between acidity and alkalinity of soil solutions. The scale is simplicity itself, being a series of numbers starting at 0.0, the most acid, and running in tenths up to 14.0, the most alkaline. The neutral soil reaction on the scale is 7.0, the mid-point where acid and alkaline elements are in balance. (Soil reaction refers to the degree of acidity.) Gardeners do not use the entire pH scale, since reactions from 4.0 to 9.0 are just about the limits for plant growth.
Each full step, or unit, up or down on the scale (say from 6.0 to 7.0 or 7.5 to 6.5) represents a tenfold increase or decrease in the degree of soil acidity. For example, a soil solution with a pH of 6.0 is ten times more acid than one with a pH of 7.0.