O, the organic horizon. This is the surface of the soil and contains dead and decaying plant materials. Typically you will find deserts to have the absence of O horizons due to the lack of vegetation. Forests and grasslands will have varying thicknesses of O horizons, but usually two inches or so deep at the most. This horizon contains almost exclusively organic material and little to no minerals such as sand, silt, or clay. Digging through an O horizon can be quite interesting, or gross depending on your prospective. Of course, I personally find them fascinating. This is where plant litter falls and decays so various bugs and fungi are quite abundant here as they eat through the dead materials, turning them into rich compost. Fungi can be identified by tiny white, moldy like threads that penetrate through dead materials. With some careful digging you can also identify many different species of bugs in a very small area. Even more careful digging you might be able to identify how long it takes for plant materials to decay. You may find whole leaves from the previous year, and mostly decomposed leaves from two years ago. O horizons function like mulch in the garden, holding in soil moisture, create biologically active soils below, and serve to fertilize or enrich soil below. Gardeners should imitate or create their own O horizon by mulching.
|A diagram of the soil profile containing the major soil horizons. Taken from Wikipedia.|
B horizon. Given how original some scientists are the B horizon is named thus, simply because B comes after A. The B horizon is always below the A horizon and typically is thicker then the A horizon. This horizon is usually less of a concern to the gardener than the A horizon being most plant roots are in the A horizon. Deserts have a different story though, but more on that some other time. Most of the time the B horizon is lighter in color and has been enriched with clay. Clay particles simply are leached, or washed downwards, by water moving through the soil. The clay particles are deposited lower in the soil and form a B horizon. For the gardener if you have a very small A horizon you might want to add sand and/or organic matter to the B horizon, making it into an A horizon. B horizons are not completely useless however. Being the B horizon often has higher clay content, and that clay holds water very well, a gardener can utilize a B horizon to keep plants well watered even in times of drought. Some roots will penetrate the B horizon and collect water even if the A horizon is completely dry. The key to this is simply soaking the soil. The A horizon will dry out first but the B horizon will hold water much longer. A lot of people complain about clay soils but the reality is they are easier to engineer then sandy soils. Sandy soils, and especially sandy B horizons, are quite frustrating. You can water them like crazy and most of the water simply drains right through. Fortunately most soils have a higher clay content.
C horizon. This particular horizon is simply the parent material that the above horizons originally developed from. You typically have to dig at least three feet deep, if not eight feet deep or more, to find this horizon. Save yourself some time and energy and don't worry about this horizon much. The gardener simply needs to be concerned with the O, A, and B horizons. Every garden or yard should have a healthy O, A, and B horizon. Concern yourself with these and you can forget the C horizon...
These are the most important basics, it obviously can get a lot more complicated. So if you are ready to be bedazzled with more information check out Wikipedia Soil Horizons.