DISCUSSION OF PLANT INDUSTRIES 449 



410. The nitrogen supply. The nitrogen supply of plants 

 has already received attention in Sect. 343, Chapter XXI, 

 which should be reviewed in the present connection. Long 

 before the action of the soil bacteria was known, agricultur- 

 ists knew of the value of clover and other leguminous plants 

 as a means of helping to maintain or regain the fertility of 

 the soil. It is now known that when soils are poor in nitrogen 

 compounds it is possible to replenish the nitrates from the 

 atmospheric nitrogen by the use of clover and its relatives, 

 upon the roots of which grow tubercles containing the nitro- 

 gen-fixing bacteria. Sometimes it is difficult to get clover to 

 grow in old and much-worn soils. This may be due to the 

 fact that there are no nitrogen-fixing bacteria in the soil to 

 start the tubercles. In such cases they must be introduced 

 from a soil in which they are growing, or from artificial cul- 

 tures. The best way of introducing them consists in scatter- 

 ing over the impoverished field some soil from fields in which 

 tubercle-bearing plants have grown. Successful clover fields 

 and waste places in which the common sweet clover (Melilo- 

 tus~) grows, furnish good soil for infecting worn-out lands. 



Some much-used soils have become quite acid, and this 

 acidity seems to interfere with growth of the tubercle-forming 

 bacteria. It has been found necessary in many cases to coun- 

 teract this acidity with limestone before the tubercle bacte- 

 ria can flourish upon the clover roots and thus produce the 

 nitrates that are needed for nutrition of the clover and the 

 enrichment of the soil. 



411. Is fertility permanent? It is a fact of common obser- 

 vation that when a given crop of plants is cultivated upon the 

 same soil for a long period, of years the yield of the crop 

 diminishes. Agriculturists learned a very long time ago that 

 by growing different crops in rotation better yields were 

 secured. But even with this rotation of crops and with careful 

 cultivation, the annual yield decreases unless the soil is replen- 

 ished in some way. The oldest experiments of which there 

 are complete records are still in progress at the Rothamsted 



