270 IMPROVEMENT OF THE SOIL. 



ing the fertility of his soil without the loss of a single crop ; 

 and even if his lands rest a year, and all their produce is 

 given back to them, they will more than return it in a few 

 years, by the larger quantity and better quality of their pro- 

 ductions. 



It will be seen that fallow crops and the turning in of 

 green crops, are somewhat similar in their influence upon the 

 soil. The object in both cases is to obtain alkalies or salts 

 and geine. Fallow crops yield mostly the former, green 

 crops principally the latter ; and by both processes taken to- 

 gether, a soil may be rendered very fertile, without the addi- 

 tion of manures; especially for crops not requiring much 

 nitrogen. 



Sect. 4. Rotation or Interchange of Crops. 



Rotation of crops is to cultivate, successively, on the same 

 field, crops of different kinds and of different habits, such as 

 common grains, roots and grasses. 



The necessity and utility of an interchange of crops has 

 been ascertained by experience. 



1. It was found that the growth of annual plants was ren- 

 dered imperfect, by cultivating them on the same soil in suc- 

 cessive years; and that a greater quantity of grain would be 

 obtained to let it rest for a season, during which time it 

 seemed to regain its original fertility. 



2. It was also observed that some plants, such as peas, 

 flax and clover, do not thrive well on the same soil until after 

 several years ; whilst others, such as tobacco, rye, oats and 

 Indian corn, may be cultivated in close succession. 



3. It was further ascertained by experience, that one class 

 of plants improve the soil, a second impoverish it, while a 

 third class exhaust it. 



4. To keep up the fertility, manure has always been em- 

 ployed. But however much a soil may be manured, it is 



