SOILS CLASSED BY pH
Years ago, Dr. Edgar T. Wherry devised a classification of soils by degrees of acidity; it is still useful but should be qualified by the fact that many plants spill over into two or more classifications while some are relatively sensitive to pH. Under his system, soils are classified as follows:
Superacid: Bogs, largely of sphagnum origin, with a pH range of 3.0 to 4.0. Only a few plants thrive under these superacid conditions. Because bacteria and fungi cannot function at this low reading, organic matter breaks down slowly or not at all. (It is interesting to see that two plants which do well in superacid soils-pitcher plants and sundews-do not rely upon soil for nitrogen, but are carnivorous.)
Mediacid: Bogs of sedge and sphagnum where no run-off from lime-bearing soils drains in. The pH is from 4.0 to 5.0. Broad-leaved evergreens thrive on moist mediacid soils, while hemlock, spruce and oaks grow on somewhat drier areas.
Subacid: Older gardens and fields from which lime has been all but exhausted, resulting in a pH of 5.0 to 6.0. Also includes old upland woods and some swamps.
Minimacid: Gardens and fields which are limed from time to time; woods on soils over limestone; old untilled grasslands or soils under oaks. The pH ranges from 6.0 to 7.0.
Minimalkaline [including Neutral]: Marshes and lowlands into which water drains from lime-rich soils. Contain debris from limestone ledges and cliffs, and leaf mold from hardwood forests except, under most instances, from oaks. The pH is from 7.0 to 8.0.
To the above classification we might add a group for gardeners who live in the Great Plains area where rainfall is too light to leach out alkalizing chemicals, resulting in alkali- and salt-sick soils typical of such regions with a pH of from 8.0 to 9.0.
TWO IN ONE
Dr. Wherry linked minimacid and minimalkaline soils into a broader class he called Circumneutral. This may be a somewhat bookish word, but it does convey the impression of a wide range of plants that will thrive in a wide range of soil reactions, from 6.0 to 8.0. It has been my experience, however, that even these tolerant plants-including most annuals, perennials and so on-respond better to a narrower range of pH, say from 6.0 to 7.3 or from 6.0 to 6.9. Under these less alkaline conditions the plants' leaf color is better, even though there is no other sign that soil pH has affected growth.
My gardening preference is for plants classed as circumneutral at a pH range of 6.0 to 6.9. Within this range all the food elements they need are available in highest concentration except perhaps for iron, zinc and copper, which are, however, present in large enough amounts for normal growth.