Growing crops on subsoils and mine spoils

Topsoil is a valuable material, as every gardener knows. It is so valuable that laws have been passed in European countries requiring that when excavations are made, the surface soil must be removed carefully and be replaced after the operation is complete. This shows indirectly how comparatively-less valuable is the subsoil for agricultural use. The various chapters of this book outline in considerable detail the many reasons for the high value placed on soil from the A-horizon. Most of these reasons center directly or indirectly on the presence of organic matter, much of it in colloidal form and in physical and chemical combination with the inorganic portion of it. 

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. 

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. 

Subsoils differ greatly in properties, and their suitability for use in cropping differs correspondingly. They may be deep or shallow, and uniform or non-uniform. As stated by Winters and Simonson (1951), subsoils range in texture from gravel to heavy clay, in 

Reference was made in Chapter 20 to some of the chief characteristics of the main types of soil horizons. For further information on the nature of the subsoil materials, reference should be made to the excellent discussion by Winters and Simonson (1951). They give much information on the physical properties, clay minerals and fertility levels of the B-horizons of podzols, latosols, chernozems, and desert soils. They also discuss pan layers. In many cases where these pan layers exist they play a major role in crop production and present problems in addition to those generally encountered in farming subsoils. 

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