260 A TEXTBOOK OF BOTANY [Ch. V, 9 



Nurserymen are accustomed to transplant their trees and 

 shrubs two or three times before sale, in order to force root 

 formation near the stem. Thus more young roots are pre- 

 served when the plant is lifted for shipment, and it has better 

 chance for recovery when again planted out. This is a reason 

 why nursery-grown trees and shrubs usually survive transplan- 

 tation so much better than those brought from the woods. 



Much farming practice, ancient and modern, finds explana- 

 tion in root physiology, though there is not agreement on all 

 details. The addition of fertilizers, formerly interpreted as 

 a replacement of needful mineral salts removed with the 

 crop (page 242), may perhaps represent another method of 

 neutralizing unfavorable soil conditions introduced by root 

 excretion and decay. The ancient practice of letting a soil 

 lie fallow (or idle), for a period, may restore the fertility 

 either by giving time for the diffusion of more mineral salts 

 from lower levels, or through the removal by drainage or 

 oxidation of the injurious substances formed by the roots. 

 The rotation of crops may derive its value either from the 

 different demands made by different crops upon the mineral 

 supply of the soil, or from the fact that the organic materials 

 added by roots are usually not injurious to other kinds of 

 plants. There is, however, no question as to the reason for 

 the value of the old practice of green-manuring, that is, 

 plowing Clover, and other crops of the Pulse family, into the 

 ground. These plants, with nitrogen-fixing Bacteria in their 

 own roots, are very rich in nitrogen compounds which are 

 thus added to the soil. Plowing is primarily of benefit in 

 loosening a soil both for penetration of roots and admission 

 of air. Sub-soil plowing carries the air still deeper, while 

 likewise raising more mineral matters to the upper layers. 

 The value of fall plowing, leaving the soil exposed for the 

 winter, may possibly lie partly in destruction of the Protozoa 

 which are supposed to destroy the nitrifying Bacteria (page 

 246). Cultivating a soil, in the gardening sense, consists in 

 roughening the surface ; it breaks the homogeneity of the 



