GREEN-MANURES 



237 



Not all of the nitrogen contained in these crops is taken 

 from the air. On soils rich in nitrogen, a considerable pro- 

 portion may be obtained from the soil. On poor soils, the 

 proportion derived from the atmosphere is considerably 

 larger. Soils needing nitrogen most are those that benefit 

 most largely from its application. 



296. Transfer of plant-food materials. — There is a trans- 

 fer of plant nutrients in a double sense : (1) removal of these 



L055 LAR6ELY ORGANIC * 

 WITH SOME NITROGEN 

 AND PHOSPHORIC ACID 



ANIMAL 



TO MARKET 



LARGE L055 OF 0R6ANIC * 

 MATTER, NITROGEN, PHOS- 

 PHORIC ACID AND POTASH 



GREEN MANURE 



Fig. 34. — Movements of plant-food materials. After absorption by the 

 plant they may be returned in whole or in part to the soil. If grain and 

 straw or hay are sold nothing but the stubble and roots are returned. If 

 fed to animals, part may be returned in the manure. If plowed under as 

 green-manure, all are returned. 



substances from combination with other minerals and their 

 conversion into combinations with organic matter; (2) re- 

 moval from lower soil by absorption by roots and the deposi- 

 tion of this material in the upper layer of soil when the plant 

 dies and is plowed under. The first of these transfers results 

 in an improved condition of the plant nutrients, because in 

 the combinations with organic matter they are in general 

 more available to plants than when in combinations with 



