THE SOIL. 27 



to the soil. A loose, open soil allows air to enter 

 it more freely than does one of a close texture; 

 but most soils tend to pack and become less porous 

 after the crop is planted. Some means, then, must 

 be resorted to so that good soil-breathing may be 

 restored. This is usually done, with crops which 

 admit of it, by cultivation. In plowing the ground 

 between the rows of plants, the broken-up and 

 falling earth catches the air and drags it down into 

 the resulting spaces. If the roots of the crop go 

 deep into the earth, then the deeper the cultivation 

 the better for rapid growth. 



Sometimes a field of corn, or other crop, is not 

 cultivated because the weeds do not affect it; but 

 frequent cultivation should take place for air sup- 

 ply to the roots, and also for the conservation of 

 moisture, even when all weeds have been destroyed. 

 It is not only necessary to supply air to the roots 

 of plants during their early growing condition, but 

 at all times, from germination till fruiting. 



In a compact, water-filled soil, no air can circu- 

 late. Water always contains some air in solution 

 after falling as rain, but the amount is small, and 

 its free oxygen is soon exhausted by the roots of 

 the growing crop. If, however, there is a deep, 

 porous subsoil into which the rain may quickly 

 sink; or if the field is underdrained well with til- 

 ing, the water runs away quickly, leaving empty 

 spaces into which gravity pulls the air so that the 

 needed oxgyen and nitrogen are supplied. It is 



