56 LAND DRAINAGE 



broken into smaller blocks. As succeeding wettings and 

 dryings occur, the subdividing of the masses of soil con- 

 tinues until they become very small. 



79. Ventilation plays a part. As the cleavages in the 

 soil occur, air enters and comes in contact with the soil 

 masses ; the degree of ventilation, therefore, is determined 

 by the extent of the cleavaging. The contact of the 

 oxygen of the air with the soil particles in the walls of 

 the masses results in chemical reactions which mean 

 further soil and food preparation. The surface layer is 

 thus made ready to respond to the action of tools in its 

 preparation for the reception of the seed. 



80. Other agents. Crops may now be planted. Their 

 roots, developing, reach outward and downward. Root 

 progress is favored by the crumbling and mellowing al- 

 ready mentioned as having occurred. But these roots, 

 as they progress, wedge apart, disintegrate, and arch the 

 soil particles, with the result that, at the close of the sea- 

 son, the whole soil mass is more mellow and open because 

 of their presence. Then, as they later decay, the chemical 

 products resulting probably cause further soil ameliora- 

 tion. 



81. Animal-forms. The conditions have now become 

 propitious also for various forms of animal life to take up 

 their abode in the soil. Earthworms, ants, and other 

 creatures burrow in the soil, filling it with openings which 

 become passageways for both drainage and ventilation. 

 The materials removed from these passageways, chiefly 

 mineral matter, are deposited upon the surface to be sub- 

 jected to the direct action of sunshine and air, and to 

 cover the fragments of organic matter already lying upon 

 the surface. Again these creatures, for purposes which 

 may be attributed to them as economic, convey from the 



