Soil Classification

Soil classification is important. Understanding your soil type is the first step to turning your unproductive garden soil into a glorious gardeners loam. Once you understand where you are, you can begin to figure how to get where you want to go.

Here is a soil classification according to particle size:

Classification Description
Stony loams Soils containing more than 50 per cent stones over 1 inch in diameter. If remainder is sufficiently fertile, this soil type may have gardening value, although it will be hard to work.
Gravels Soil with over 50 per cent gravel and much sand. Practically no garden value.
Sands Soils with more than 75 per cent sand. Low garden value.
Fine sandy loams Soils with 50 to 75 per cent fine sand mixed with much silt and some clay. Fairly good garden soils.
Sandy loams Soils with 50 to 75 per cent sand and much silt, some clay. Among the better light garden soils.
Loams Soils with 35 to 50 per cent sand mixed with much silt and some clay. Most of the better garden soils fall in this class.
Silt loams Soils with more than 50 per cent silt and less than 15 per cent clay. Are too "tight" to be good soils without some modification.
Clay loams Soils with 15 to 25 per cent clay, much silt and little sand. Usually are good garden soils if not worked when wet.
Clays Soils with more than 25 per cent clay, usually with much silt. Can be good if handled properly.
Mucks Soils with 15 to 25 per cent partially decomposed organic matter with much clay and silt. Good for certain crops, but modification is usually needed for general garden use.
Peaty loams Soils with 15 to 35 per cent organic matter mixed with much sand and some silt and clay. If acid, are good for broadleaved evergreens.
Peats Soils with 35 per cent or more organic matter, mixed with some sand, silt and clay. Need more mineral matter to be suitable for garden use.

Soil Classification by pH

Years ago, Dr. Edgar T. Wherry devised a soil classification 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, soil classification is 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 soil 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.