SOILS AND FERTILIZERS 83 



sibility. To press the plowed soil down hard against 

 the subsoil so that it may get its moisture from the sub- 

 soil, many farmers have used the heavy roller success- 

 fully. The roller also crushes the clods on the surface, 



-PLOWED /SOIL 

 ; NOT HARROWED 



PLOWED SOIL 



SUBSOIL 



FIG. 25. A, Soil as left by the Harrow ; B the Same Soil packed by Rain. 



so that evaporation takes place rapidly and much soil 

 moisture is lost unless the surface be harrowed to pre- 



FIG/ 26. Harrow. 



vent it. If the surface is very cloddy, the heavy roller 

 may be a necessity. 



Another implement has been invented, called a sub- 



