Subsoils root penetration

Soil often occurs as a series of horizontal layers called soil horizons, shown in Figure 15.7. The deepest horizon, which lies just above solid rock, is the substratum, which is rock just beginning to disintegrate into soil by the action of water that has seeped down to this level. No growing plant material is found in the substratum. Above the substratum is the subsoil, which consists mostly of clay. Only the deepest roots penetrate into the subsoil, which may be up to 1 meter thick. Above the subsoil is the topsoil, which lies on the surface and varies in thickness from a few centimeters to up to 2 meters. The topsoil usually contains sand, silt, and clay in about equal amounts. This is the horizon where the roots of plants absorb most of their nutrients. 

Two common features of soils in the study site are plinthite and fragipan. Both are cemented materials that tend to restrict water movement and root penetration. Plinthite forms in subsoils that have developed in the oldest areas of the landscape. 

Soils that have received heavy applications of fertilizers over the years, as in many sections in the southeast states, often have subsoils that are so acid, or even toxic due to high aluminum and manganese, that root penetration is limited (Foy and Brown, 1963,... 

Peat in mixture with soil, and adequately supplied with nutrients, serves as an excellent medium for root growth if it is not too acid. In very compact soils growth is often curtailed because of the inability of roots to penetrate them readily. Peat corrects this and in addition releases nutrients that favor root development. Root penetration into the subsoil is often curtailed because of either the presence of a hardpan, lack of calcium, or too much soluble aluminum. The addition of peat is not likely to correct these conditions unless it is mixed with the subsoil. Where this can be done, a comparatively small amount of peat, reinforced with the needed nutrients, may have a long-term effect in correcting the unfavorable condition. 

The B horizon is commonly referred to as subsoil , consists of mineral layers which may contain concentrations of clay or minerals such as iron or aluminium oxides. This layer is also known as the zone of accumulation . Plant roots penetrate through this layer, but it has very little humus. It is usually brownish or red because of the clay and iron oxides washed down from A horizon. The B horizon may be divided into B1, B2 and B3 types. B1 is a transitional horizon to an A3 containing some A-horizon characteristics. B2 horizons have a concentration of clay, minerals, or organic debris. B3 horizons are transitional between the overlying B layers and materials of whether C or D horizons beneath it. This layer is little affected by soil forming processes. The C horizon may contain lumps or more likely large shelves of unweathered rock. The C horizon also contains parent material. 

Better drainage is one benefit that may be realized from deep plowing and subsoiling under conditions where this operation breaks up an impervious layer. No appreciable improvement would be expected in more nearly normal soils. Unless a hardpan exists, the roots of crops, such as sweet clover and alfalfa, may be able to penetrate to such an extent that drainage is improved. Where subsoiling is practiced, the operation does not ordinarily result in permanent improvement, since the subsoil materials tend to become cemented together within a year or so. 

The term subsoil will not be used here in any very exact sense, but will refer to all of the layers below the A-horizon where plant roots commonly penetrate. The emphasis will be on the B-horizon, but not exclusively, for in actual agricultural practice it is seldom possible to study this horizon isolated from the others. A badly-eroded soil usually contains some A-horizon. Even where the B-horizon is exposed and crops are being grown on it, many of the plant roots may be in the C- and D-horizons and dependent upon these lower horizons for much of their nutrient supply and moisture. Furthermore, where extensive excavations or mining operations have been conducted the whole soil profile, including much parent material, is commonly mixed. The discussion here will, therefore, be concerned primarily with the growing of crops on soil or soil material that contains little or no A-horizon. 

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