SOIL 



137 



should be plowed under to form humus, and thus keep 

 the soil in good condition. 



Clay soil. Most soils contain both sand and clay, 

 and the class to which a soil belongs is determined by 

 the relative amounts 

 of these of which it 

 is composed. A clay 

 soil is composed of 

 very fine particles, 

 which, when dry, 

 seem to contain no 

 grit or sand. Clay 

 soils retain water a **~"V*^ 

 long time, and be- 

 come very hard in 

 drying. 



Sandy soil. Sand 

 varies greatly in fine- 

 ness. Sand particles 

 are very hard, and 

 have sharp edges, 

 giving a gritty feel- 

 ing in handling. Wa- 

 ter passes through 

 sand very freely. 

 Sandy soils are 

 quickly warmed by the sun in the spring. They wash 

 badly and are not durable under tillage. Both sand 

 and clay may contain organic matter; that is, decaying 

 plant or animal life. 



Courtesy of D. W. Working 

 FIG. 43. THE WEATHERING OF ROCK 



Under the influence of the weather 

 heating and cooling, wetting and dry- 

 ing the solid rock is then broken into 

 fragments. 



