1/2 THE INTERCHANGE OF CROPS. 



desired purpose is to be accomplished. The forma- 

 tion of ammonia cannot be effected on cultivated 

 land, but humus may be artificially produced ; and 

 this must be considered as an important object in 

 the alternation of crops, and as the second reason 

 of its peculiar advantages. 



The sowing of a field with fallow plants, such as 

 clover, rye, buck- wheat, &c. and the incorporation 

 of the plants, when nearly at blossom, with the 

 soil, affect this supply of humus in so far, that 

 young plants subsequently growing in it find, at a 

 certain period of their growth, a maximum of nu- 

 triment, that is, matter in the process of decay. 



The same end is obtained, but with much greater 

 certainty, when the field is planted with esparsette 

 or lucern. These plants are remarkable on account 

 of the great ramification of their roots, and strong 

 development of their leaves, and for requiring only 

 a small quantity of inorganic matter. Until they 

 reach a certain period of their growth, they retain 

 all the carbonic acid and ammonia which may have 

 been conveyed to them by rain and the air, for that 

 which is not absorbed by the soil is appropriated 

 by the leaves : they also possess an extensive four or 

 six fold surface capable of assimilating these bodies, 

 and of preventing the volatilization of the ammonia 

 from the soil, by completely covering it in. 



An immediate consequence of the production of 

 the green principle of the leaves, and of their 

 remaining component parts, as well as of those of 



