NATURE AND PROPERTIES OF SOILS 



The mineralogical complexity of rocks has an important 

 bearing on the question of soil formation and soil composi- 

 tion. The fragments of any soil are, for the most part, dis- 

 tinguishable as separate minerals rather than as mineral aggre- 

 gates. For example, a soil from a granite would be char- 

 acterized by separate grains of quartz, orthoclase, micro- 

 cline and perhaps mica rather than by fragments of the orig- 

 inal granite itself. Again, it is the composition of the easily 

 decomposable minerals rather than the composition of the 

 bulk rock that determines what simplifications shall occur, 

 what new substances shall arise in the soil and what elements 

 shall be liberated for plant use. 



3. Soil minerals. — Although hundreds of minerals have 

 been identified, comparatively few are common or important x 

 in rock formation. As a consequence, the list of im- 

 portant minerals found in soils will be correspondingly cur- 

 tailed, although enough are always present, especially in the 

 finer portions, to make the soil very complex mineralogically. 

 The minerals as to origin may be divided into two groups: 

 (1) those that persist from the original rock and (2) those 

 that are produced by the decomposition of the original min- 

 erals, during soil formation. For example, the quartz grains 



ir The following table indicates the approximate proportions of the 

 common minerals in the earth's crush to a depth of ten miles: 



Feldspars 57.8% Clay 1.0% 



Amphibole and Py- Carbonates 5 



roxene 16.0 Limonite 2 



Quartz 12.7 All others 8.2 



Mica 3.6 



Recalculated from Clarke, F. W., Data of Geochemistry ; U. S. Geol. 

 Survey, Bui. 695, pp. 32-33. 1920. 



