DEFINITION OF FERTILIZERS 199 



necessity of growing a legume in the rotation. And yet, 

 compared to the total amount of nitrogen removed in the 

 other crops of a rotation, this is inadequate to make up the 

 loss — 40 to 60 pounds returned, and 100 to 150 pounds 

 removed. 



There may be added to the soil of one field more or less 

 fine soil blown from an adjoining field or roadway. This, of 

 course, adds some plant food such as potash and phosphoric 

 acid, but it is at best only "robbing Peter to pay Paul," since 

 this plant food may in turn be lost to the next field in the same 

 way. There is, of course, some plant food brought to the 

 surface soil by the rise of capillary or film water, and the 

 decay of roots and stubble may add plant food which has 

 been brought up from the subsoil by deeply penetrating 

 roots. But analyses show very conclusively that a long 

 period of cropping reduces all the plant food elements. As 

 much as one-third to one-half of the amount contained in the 

 virgin soil has been found to disappear during 50 to 60 

 years of cropping with no return in the way of manure or 

 fertilizers. 



It is a well-recognized fact among farmers who have the 

 experience of centuries to guide them that if plant food is 

 removed it must be returned. F. H. King in his " Farmers of 

 Forty Centuries" states that to each acre of the 20,000 

 square miles of cultivated land in Japan there is added 

 annually 60 pounds of nitrogen, 32 pounds of phosphoric 

 acid, and 48 pounds of potash. These people have been 

 farming for centuries and are still maintaining the fertility 

 of their soil only by a most rigorous return to the soil of the 

 plant food elements removed. 



These facts all show that sooner or later plant food must 

 be added to the soil. 



147. Definition of Fertilizers. — The term fertilizer is ap- 

 plied not only to a compound which supplies a plant food 

 to the soil, but also to a compound which has other func- 

 tions in the soil, such as neutralizing acidity, making potash 

 available, etc. Hence, fertilizers may be defined as com- 

 pounds which are added to the soil to increase the yield of 

 crops — to increase the fertility of the soil. 



