Plow horizon

The A horizons are slightly different from other horizons because they are plowed and planted. In many cases, a soil may have an Ap horizon, the p indicating that it is or has been plowed. The plowing need not have been done recently for the horizon to be described as Ap. Even a soil so newly developing that it has no horizons will have an Ap horizon designated if it is plowed. 

A-horizon (surface soil). This includes the mulch layer and plow layer. Living organisms are most abundant in this horizon, consisting of plant roots, bacteria, fungi, and small animals. 

Topsoil The surface zone, including the zone of accumulation of organic material (usually the A horizons). Topsoil can be modified by anthropogenic practices, such as road or foot traffic, plowing, and addition of fertilizers. 

The Rooting-Zone Soil Root-zone soil includes the A horizon below the surface layer. The roots of most plants are confined within the first meter of soil depth. In agricultural lands, the depth of plowing is 15-25 cm. In addition, the diffusion depth, which is the depth below which a contaminant is unlikely to escape by diffusion, is on the order of a meter or less for all but the most volatile contaminants. Soil-water content in the root zone is somewhat higher than that in surface soils. The presence of clay in this layer serves to retain water. Contaminants in root-zone soil are transported upward by diffusion, volatilization, root uptake, and capillary motion of water transported downward by diffusion and leaching and transformed chemically primarily by biodegradation or hydrolysis. 

AU fungi are heterotrophs and are predominantly aerobic, and hence are usually found at or near the surface of any organic matter upon which they may be growing. In the soils they are limited largely to the plow layer, or A-horizon, but this is because an abundant supply of energy sources is available only in this upper region [2]. 

Many of our soils now have no true A-horizon. Most often this is because extensive erosion of cultivated areas over a period of many years has removed most of the topsoil. It is also the result of various kinds of building operations, including the construction of houses, railroads, highways, and various pipe lines. In many of our cultivated soils the plowed layer now consists of a mixture of remnants of the A-horizon, and of the underlying layer. Since the farming of subsoils is often a far more difficult operation than growing crops on the A-horizon, it is well to consider some of the problems involved and. their possible solutions. 

Much emphasis has been placed on the plowed layer and its contribution to crop production, and comparatively little research has been done on the subsoil. In fact, in general discussions of the depletion of soil fertility the subsoil is often ignored, even though we know that it is often higher in essential nutrients, other than nitrogen, than is the A-horizon. These mineral nutrients may be less available to plants than those in the surface layer that is constantly subject to weathering and oxidation, but nevertheless subsoils do contribute in a major way to plant feeding. The plowed layer is only 6—12 inches deep whereas the subsoil tapped by the root system may have a depth of several feet. 

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