332 SOILS AND PLANT LIFE 



phosphorus and twenty-five pounds, of potassium from the 

 land. 



In a very true sense, then, the farmer who sells his crop 

 to others is really selling the fertiUty of his soil ; or we 

 may say that, little by little, he is selling the land itself. 

 To continue such a practice year after year must inevitably 

 result in the gradual exhaustion of the soil. It is chiefly 

 due to this fact that much of the land of the older sections 

 of the country has become, as we say, " worn out," by 

 which we mean that it has reached the stage at which it 

 will not produce crops that will yield a profit above the 

 actual cost of care and cultivation. 



239. How the Three Important Elements of Fertility 

 may be restored to the Soil. — 



Nitrogen. — The store of nitrogen in the soil may be in- 

 creased in three ways : 



(1) By growing legumes. 



(2) By the apphcation of barnyard manure. 



(3) By the apphcation of commercial fertilizers. 



How the bacteria which Uve on the roots of legumes take 

 nitrogen from the air and convert it into a form that 

 plants can use was explained in Section 180. The actual 

 amount of nitrogen that is received from the air in this 

 way by the legumes varies with the different plants. A 

 crop of red clover, sweet clover or crimson clover, yielding 

 one and one half tons of hay per acre, receives from the 

 air as a rule between sixty and seventy pounds of nitrogen ; 

 a cutting of alfalfa or cowpeas, yielding the same amount 

 of hay per acre, contains usually from seventy to eighty 

 pounds of nitrogen which has been taken from the air; 

 and it follows that if a crop of any one of these plants which 

 would yield a ton and a half of hay per acre were plowed 

 under, nitrogen would be added to each acre of the soil 



