Taking a Soil Profile

A soil profile gives a gardener a cheap method to determining some soil problems.

If you have serious soil drainage, compaction or structural problems, it may pay you to take a soil profile. This is done simply by digging a pit into a "central" area of the garden to get an over-all idea of what lies underneath the topsoil. The pit ought to be at least two feet deep and wide enough so you can get down in it to "read" the profile.

At the top of the profile you will see the dark layer of topsoil (unless a building contractor got rid of clay from an excavation by spreading it over your lot, in which case a layer of hard, stiff, light-colored strata will overlay the darker topsoil). Sometimes, in semi-arid regions, topsoil may be lighter than layers underneath but this is rare in any soil where garden plants will grow.

The depth of the topsoil usually shows how deeply roots of grasses and shallow-rooted weeds have penetrated. In prairie soils where bluegrasses often penetrate as deep as two to three feet, a surface layer of that depth is often found. The darker color of the topsoil indicates where perishable plant tissue has decayed, leaving behind a residue of humus and carbon.

In the soil scientist's language, this upper layer is called the A horizon. In many soils, just below it is another darker layer, but separated from it, which is called the Ax horizon. In some soils, where organic matter is tightly held by the topsoil, no well-defined A horizon develops. In such cases the next lower layer is the B horizon or subsoil. The science of pedology makes a great deal of these horizons. From them, earth scientists can read the history of soil formation. However, our interest from a soil profile is in checking depth and condition of the topsoil and the subsoil.