The unit of soil classification is the _soil type_, which is a soil having agricultural unity, as determined by texture, chemical character, topography, and climate. The types commonly named are clay, clay loam, silt loam, loam, fine sandy loam, sandy loam, fine sand, and sand. In general the soil materials are so heterogeneous and so remote from specific rock origin, that in such classification the geologic factor of origin is not taken into account. More broadly, soils may be classified into provinces on the basis of geography, similar physiographic conditions, and similarity of parent rocks; for instance, the soils of the Piedmont plateau province, of the arid southwest region, of the glacial and loessal province, etc. In such classification the geologic factors are more important. Soils within a province may be subdivided into "soil series" on the basis of common types of sub-soils, relief, drainage, and origin. USE OF GEOLOGY IN SOIL STUDY While the desirability of particular soils is related in a broad way to the character of the parent rocks, and while by geologic knowledge certain territories can be predicated in advance as being more favorable than others to the development of good soils, so many other factors enter into the question that the geologic factor may be a subordinate one. A soil expert finds a knowledge of geology useful as a basis for a broad study of his subject; but in following up its intricacies he gives attention mainly to other factors, such as the availability of common constituents for plant use, the existence and availability of minute quantities of materials not ordinarily regarded as important by the geologist, the climatic conditions, and the texture. As the geologic factors are many of them comparatively simple, much of the expert work on soils requires only elementary and empirical knowledge of geology. The geologist, although he may understand fully the origin of soils and may indicate certain broad features, must acquire a vast technique not closely related to geology before he becomes effective in soil survey work and diagnosis. For these reasons the mapping and classification of soils, while often started by geologists of state or federal surveys, have in their technical development and application now passed largely into the hands of soil experts in the special soil surveys affiliated with the U. S. Department of Agriculture and with agricultural colleges. FOOTNOTES: [13] A good summary of this subject may be found in _Engineering Geology_, by H. Ries and T. L. Watson, Wiley and Sons, 2d ed., 1915. [14] Marbut, Curtis F., Soils of the United States: _Bull. 96, Bureau of Soils_, 1913, p. 10. CHAPTER VII THE FERTILIZER GROUP OF MINERALS GENERAL COMMENTS