PLANT FOOD IN THE SOIL 79 



legume is repeatedly grown on the land, the bacteria may even- 

 tually cause the nodules to form on its roots. It has been found 

 that the type of bacteria which grows well on clover roots, for 

 instance, does not grow well on other legume roots. In other 

 words, different legumes have different types of these bacteria 

 especially adapted to their roots. Where a new legume is intro- 

 duced, it is usually necessary to supply the bacteria suited to it, 

 if the crop is to succeed. Supplying the proper bacteria is known 

 as inoculation, and usually it is accomplished by working into 

 the soil three or four hundred pounds per acre of soil taken 

 from land where the legume in question has been grown. Re- 

 cently, there has come into use a method of inoculation of the 

 soil with prepared cultures of the desired bacteria. These are 

 proving fairly satisfactory. In most parts of the United States 

 soils rarely need inoculation for clover, but for other legumes, 

 especially if they have not been grown before, inoculation is 

 often necessary for a satisfactory crop. 



87. How a chemical analysis of the soil helps. A chemical 

 analysis shows how much of each of the elements of plant food 

 a soil contains. It is important that the farmer have this knowl- 

 edge, in order that he may give special attention to maintaining 

 or increasing the amounts of those elements that are present in 

 the smallest quantities. But a chemical analysis fails to show 

 how much of each of these elements is immediately available 

 for the crops. It is not possible, therefore, to determine by 

 means of a chemical analysis the crop to which a particular soil 

 is adapted, or what fertilizers will give the best result. A field 

 test is the only way in which these questions can be accurately 

 answered. 



88. What the plant food in our soil is worth. If asked to 

 name the most valuable minerals of the United States, most 

 persons would promptly suggest gold, silver, iron, lead, zinc, and 

 copper. Yet the American farmer mines more wealth from the 

 soil in a single year than has been taken from the gold mines 

 of the United States in all the years since Columbus discovered 

 America. It is literally true that our most important mineral 



