XX LIST OF ILLUSTRATIONS 



PAGE 



Surface soil and subsoil facing 92 



Relative quantities of potash, lime, phosphoric acid and 



nitrogen in a soil ........ 94 



Equipment for making the litmus paper test .... 123 



Apparatus for making the zinc sulfide test .... 124 



Relative sizes of bacteria and soil particles . . . . 128 



Appearance of some soil bacteria (after Lohnis) . . . 131 



Diagrammatic representation of the nitrogen cycle . . 139 

 Apparatus for estimating the relative rate of air movement 



through soils 153 



Apparatus to demonstrate the presence of carbon dioxide in 



soil air .......... 153 



Apparatus to demonstrate the formation of carbon dioxide in 



soil 154 



Effect of certain fertilizer constituents on plant growth facing 156 



Extent to which fertilizers are used in the several states . 197 



Tag representative of the kind often used on bags of fertilizer 201 



Plan for fertilizer experiments 212 



Field plat experiments facing 212 



Influence of soil moisture on the effectiveness of fertilizers 



facing 218 



Composition of farm manure 223 



Storage of farm manure facing 226 



Movements of plant-food materials between soil, air and plant 237 



Cover crops which are also green manures . . facing 238 



