THEORY OF SOIL FERTILITY 209 



however, on the function of lime in the process 

 of plant growth. 



One more quotation from Doctor Hopkins 

 will conclude the presentation of the generally 

 accepted theory of the plant food requirements 

 of plants and its "availability": 



"Probably there has never been a greater 

 waste of time and effort in the name of science 

 than in the endeavor to determine the 'avail- 

 able' plant food in soils. The almost uni- 

 versal assumption has been that the plant food 

 in the soil exists in two distinct conditions, 

 'available' and 'unavailable' and that the 

 determination of the 'available' plant food 

 would reveal both the crop-producing power 

 of the soil and the fundamental fertilizer re- 

 quirements for the improvement of the soil 

 for crop production. 



"After ascertaining the total stock of plant 

 food in the plowed soil, the next important 

 question is not how much is 'available,' but 

 rather how much can be made available during 

 the crop season. In other words, we must 

 make plant food available by practical meth- 

 ods of liberation, by converting it from in- 

 soluble compounds into soluble and usable 

 forms; for plant food must be in solution be- 

 fore the plant can take it from the soil. For 

 the present, space is taken only to emphasize 

 the value of decaying organic manures in the 



