INTRODUCTION 



fertility problem. Some of their names may not 

 interest us. Six or seven of these elements are 

 in such abundance that we do not consider them. 

 A farmer may say that when a dairy cow has lux- 

 uriant blue-grass in June, and an abundance of 

 pure water, her wants are fully met. He omits 

 mention of the air because it is never lacking 

 in the field. In the same way the land-owner 

 may forget the necessity of any kind of plant- 

 food in the soil except nitrogen, phosphoric 

 acid, potash, and lime. Probably the lime is 

 very rarely deficient as a food for plants, and 

 will be considered later only as a means of making 

 soils friendly to plant life. 



Nitrogen, phosphoric acid, and potash are the 

 three substances that may not be in available 

 form in sufficient amount for a growing crop. 

 The lack may be in all three, or in any two, or 

 in any one, of these plant constituents. The nat- 

 ural strength of the soil includes the small per- 

 centage of these materials that may be available, 

 and the relatively large stores that nature has 

 placed in the land in inert form as a provision 

 against waste. 



The thin covering of the earth that is known 

 as the soil is disintegrated rock, combined with 



[3] 



