HOW SOILS ARE FORMED 



45 



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 hnseed 

 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? 



■fffrmm 



Fio. 26. — ^Abroad valley of rich alluvial soil. This soil at .'-ome time 

 part of the surrounding, hills. (Plant Industry.) 



chiefly 



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. 

 SoUd rock is thus gradually broken to pieces by the freezing 



