SOIL 99 



earth. There was no soil and there was no vegetation. 

 But this condition could not remain long. There are 

 forces at work in nature which cause even the hardest 

 rock to crumble at the surface, and thus the formation of 

 soil begins. Soil has been aptly called " rock meal " or 

 " rock flour," for it is principally composed of substances 

 which once were hard rock. Another important part of 

 soil is the material in it which has come from the decay 

 of vegetation. This material is called humus or mold. 

 It is dark colored. Rich surface soil, especially in shady 

 forests, is largely composed of humus. 



It is evident that the ingredients of soil may be classed 

 as organic and inorganic. The organic ingredients result 

 from the decay of plants and animals. The inorganic in- 

 gredients result from the decay of rock. 



The inorganic ingredients of soil may be classed as to 

 their physical condition, and upon this basis the two 

 principal classes are clay and sand. Clay, when dry, 

 is a fine powder. When wet, it forms a solid, plastic mass. 

 It is derived from those parts of rock which, principally 

 by the action of water, become reduced to fine particles. 

 Sand, whose particles are much larger than those of clay, 

 comes from those parts of rock which are insoluble, but 

 which may, chiefly by grinding, become reduced to fine 

 grains. Such grinding occurs on rocky beaches where the 

 waves rub the rocks together. The sand thus produced 

 may be carried away by waves and currents and form 

 beaches which are composed of little else than sand. The 

 sand bars in rivers show that river water may likewise 

 produce sand. 



We have referred to the " parts of rock " which form clay 

 and the " parts of rock " which form sand. This indicates 



