The -Soil and Soil Water 



LOAHY 

 SOIL 



CLAYEY 

 5U660IL 



FIG. 15. 



ROCK 

 ROCK 



U. S. D. A. 



The soil and the material 

 below it. 



the forces of Nature 

 that slowly but steadily 

 worked through the ages 

 to produce the soil and 

 to distribute it as we 

 find it today (Fig. 16). 



Probably the most 

 DECOOPOSED jmportant of the soil- 

 forming processes is 

 chemical action. We 

 are all familiar with the 

 rusting of iron. This is 

 chemical action. If ex- 

 posed to the weather, the brightly polished metal sur- 

 face soon loses its luster and becomes coated with rust. 

 Rust is not iron ; it is a compound of iron and oxygen, 

 as we saw in Chapter Two. And if nothing is done 

 to protect iron from the weather, the process of 

 rusting will go on until the iron is entirely con- 

 sumed. In a similar way stones that are on or near 

 the surface of the ground are acted upon by the 

 weather so that they gradually crumble and change 

 into new substances. (Exp. 3.) An example of this 

 is to be had in the slow change of the rock feldspar 

 into clay. 



The effect of uneven expansion is illustrated when 

 we break a piece of glassware by pouring hot water into 

 it. The surface in contact with the hot water expands 

 more than the rest of the surface, and the strain 

 breaks the glass. Stones are often made up of several 

 kinds of minerals, and as the temperature changes from 



Chemical 

 action 



Soil 



formation 

 and the 

 rusting of 

 iron 

 compared 



Changes of 

 temperature 



