68 FARM DEVELOPMENT 



which evaporation or the action of roots has depleted 

 the supply of water. The interstices are not filled with 

 water, unless the soil is flooded or not properly drained. 

 Soils rarely have even all their interspaces of a capillary 

 size filled with water ; in other words, the capillary power 

 of soils is usually only partly satisfied. The larger in- 

 terspaces are filled with air. Should the land be flooded 

 with water, however, all these openings are filled, and 

 the air, being lighter than water, is driven out. 



The soil needs air to supply oxygen to the growing 

 roots and to the germinating seeds; to assist in 

 fermenting manures, dead roots and other fertiliz- 

 ing materials; to furnish oxygen for bacteria and 

 other low organisms, part of which assist in pre- 

 paring the soil for higher plants. Air needs to cir- 

 culate in the soil to remove carbon dioxide and probably 

 other deleterious gases, and to oxidize substances which 

 may be unwholesome to plants. Roots of plants and 

 germinating seeds will suffocate if not supplied with 

 oxygen. 



Earth worms, insects and small animals perforate the 

 ground with holes and thus allow the air to circulate 

 more freely; and they also help to mix the subsoil with 

 the surface soil. Clover, alfalfa, corn and other plants with 

 long roots which die and decay, leave air passages in the 

 soil. After rains in summer, when the soil is moist and 

 at a temperature suited to the growth of bacteria and 

 fungi, cultivation lets the air in, presumably aiding many 

 of these minute agencies in their work. When saline 

 substances have been brought to the surface by capillary 

 water, they are left there as a cement after the water 

 has evaporated, and bind together the particles of sur- 

 face soil into a crust. Here cultivation is necessary to 

 open the soil to a freer circulation of the air. The soil 

 is not such an inert mass as is generally believed. The 

 particles are moved by the air, water, animals, plants, 



