CULTIVATION. 101 



The depth to which ro-ots penetrate is largely de- 

 termined by the depth to which the soil is stirred in 

 cultivation. 



2. To admit air to the roots. 



Most agricultural plants of temperate climates will 

 live and thrive only in soil which is sufficiently porous to 

 admit air. 



Experiments show that the roots of such plants, as 

 well as the foliage, require the presence of free oxygen. 

 The plants either die, or produce a feeble, sickly growth, in 

 compact soil, or in soil continuously saturated with water. 



There are exceptions, as in case of the cranberry plant 

 and rice, which thrive best in soil completely covered 

 with water during a portion of the year. 



3. To hasten the decomposition of the soil, and the 

 formation of plant food. 



The chemical processes in the soil are hastened by 

 exposure to the air. They are partly dependent upon a 

 supply of oxygen, carbonic acid, etc., from the atmos- 

 phere. By loosening the soil, so as to admit air, and by 

 exposing different portions of it to the surface in culti- 

 vation, we increase its fertility by hastening the forma- 

 tion of plant food. 



The practice of " summer fallowing," which is some- 

 times followed, consists in preparing ground for a future 

 crop by plowing and harrowing repeatedly, so as to ex- 

 pose it as much as possible to the beneficial influences of 

 the atmosphere. 



4. To mix fertilizers with the soil. 



Plants thrive best, not where their roots find pure fer- 

 tilizer in some spots, and poor soil in others, but where 

 the two are so completely mixed as to render the whole 

 a uniform medium of rich soil. 



