No. 4.] NITROGEN AND FERTILITY. 167 



sure that to an audience of Massachu.setts farmers I need 

 make no apology if I say some things that lie outside of 

 their usual thinking in connection with soil fertility. 



The Chemical Theory of Soil Fertility. 



In its broad meaning soil fertilit}^ can be defined as the 

 power that a given soil has to produce crops of desirable 

 kinds of plants. Since Liebig's time, and until within a 

 very few years, soil fertility has been supposed to depend 

 chiefly upon the kinds and amounts of plant food that a 

 given soil may contain. This chemical explanation an- 

 nounced by Liebig was for many years closely followed 

 with the result that the chief inquiries into soil fertility 

 were made by chemists along chemical lines ; and , as it now 

 seems, undue importance was attached to the chemical com- 

 pounds contained and the purely chemical changes going on 

 in the soil. It was speedily found that plants as a rule 

 responded only to applications of materials containing nitro- 

 gen, phosphoric acid and potash, and to some extent lime; 

 hence it was assumed that plants were readily al)le to oljtain 

 all of the other elements which enter into their o-rowth from 

 the soil, water and air. A generation ago it was believed 

 that a chemical analysis of a soil, showing the amounts of 

 nitrogen, phosphoric acid and potash that it contained, 

 would furnish the needed data for proper manuring. Based 

 upon the chemical theory of soils and plant nutrition, such 

 very helpful books as Johnson's "How crops grow," and 

 "How plants feed " were written. As explained in these 

 and similar books, the whole story of plant growth reduced 

 to bald outline is something as follows : — 



A soil is formed from disintegrated rock, broken down by 

 physical and chemical processes called weathering. The soil 

 thus formed was regarded as dead, inert matter, containing 

 plant food material which by chemical action may become 

 available for plant growth. The simpler forms of plants 

 develop first. By their death they add organic matter to 

 the soil, thus fitting it for the higher forms of plant life. 

 This organic matter undergoes certain chemical changes, 

 and is thereby converted into hunnis. The coarser particles 



