THE ESSENTIALS OF AGRICULTURE 



subsoil. The surface soil varies in depth from a few inches to 

 several feet, and is usually coarser and more friable and much 

 more productive than is the subsoil. Other things being equal, 

 the deeper the layer of surface soil, the more productive it is. 



51. How soil is formed. Soil is formed by the breaking of 

 rocks through the action of the weather and water. No exposed 

 rock surface is free from the effects of weathering. Highly pol- 

 ished granite blocks lose their luster after considerable exposure. 

 The minerals which compose the rocks are transformed into a 

 powdered mass, and some of them may also undergo a more or 



less complete chemical 

 change. The action 

 of rain water dissolves 

 some of these materials, 

 while the finer particles 

 may be washed entirely 

 away or carried down 

 into lower layers. This 

 explains why the sub- 

 soil in humid regions 

 is usually of finer tex- 

 ture than the surface 

 soil. 



Various weathering agencies are concerned in the process of 

 breaking down rocks (Fig. 28). The change in temperature 

 causes rocks to expand and to contract, producing small cracks 

 in the rock surface, into which water enters. When the water 

 freezes it exerts a strong force which widens these cracks, and 

 thus hastens the breaking down of the rock. Certain of the 

 mineral substances in the soil are affected by oxygen as iron 

 oxidizes, or rusts, under the influence of air and moisture. Rain 

 water takes up carbonic-acid gas in its passage through the air, 

 and this makes it very much more effective in its action upon 

 the rock minerals than is water containing no carbonic acid. 

 This carbonated water not only aids in dissolving certain 

 rock substances but also brings about chemical action whereby 



FIG. 27. How vegetation helps in soil 

 building 



