CHEMICAL ANALYSIS OF SOILS 327 



of each other. Even if an analysis should show the nutrients, 

 which would be available over a term of years, it would still be 

 inadequate, since available nutrients are only one of a great 

 number of factors which govern productivity. 1 This produc- 

 tivity equation may be indicated as follows : 



Productivity = Texture X structure X organic matter X 

 moisture X available nutrients X soil reaction X weather X 

 plant disease X care of farmer, etc., etc. 



The factors of this equation are variables, their importance 

 in determining productivity depending on many things. An 

 accurate knowledge of the available soil nutrients, even if 

 procurable, would aid but little in solving such an equation. 



The solution of individual or community fertility problems 

 is best accomplished by the aid of experienced and technically 

 trained men, who understand the scientific principles under- 

 lying the common field procedures and who also are in touch 

 with the experiences of farmers over wide and diverse areas. 

 Such men may advise not only in regard to the crops that 

 should be grown but also as to their rotation, management, 

 and fertilization from seeding until harvest. These men may 

 also institute such cooperative experiments and tests as will 

 best throw light on fertility problems untouched by practical 

 experience. 



1 The samples sent to a chemical laboratory by farmers are gen- 

 erally improperly taken and consequently are not representative. It 

 would be unwise to analyze such soils even if the methods were capable 

 of showing all that could be wished for. 



