HOW SOILS ARE FORMED 47 



illustration of oxidation is seen when iron is exposed to the 

 air; the red rust that forms is an oxide of iron a substance 

 very different from the iron itself. To test for ourselves 

 the action of air on iron we may take a piece of bright iron 

 or steel, as the blade of a knife. Cover one end with linseed 

 oil or wagon grease, and leave the other end exposed to the 

 action of rain and air for a few days. Notice the effect on 

 the exposed part. What lesson on the care of plows and 

 other farm tools may we learn from this? 



r - ' "~^ 



FIG. 26. Abroad valley of rich alluvial soil. This soil at some time was chiefly 

 part of the surrounding hills. (Plant Industry.) 



Water is the most powerful agent in decomposing or 

 breaking down rock (Fig. 25a). Its action, especially in dis- 

 solving rocks, is very important. Nearly all rocks are more 

 or less affected by this action, and some, as limestone, are 

 easily affected. 



W'ater breaks up rock by freezing. Most rocks contain 

 cracks or openings into which the water readily flows; when 

 the water freezes, it expands and forces the rock apart. 

 Solid rock is thus gradually broken to pieces by the freezing 



