THEORY OF SOIL FERTILITY 187 



the total plant minerals in that cubic foot by 

 the amount of those minerals one crop re- 

 moves, then the question of the resources of 

 our soils is even more easily computed than the 

 resources of our mines. We have the resources 

 of our coal supply set before us in cubic yards, 

 pounds if you wish. 



It is not even necessary, in the light of this 

 hypothesis, to examine all the minerals en- 

 tering into plant growth. If any one of them 

 is lacking in a given soil, that soil is sterile. 



To get at the means that might be employed 

 to strike the balance of soil fertility, we will 

 review briefly the hypothesis of Liebig. He 

 incinerated plants and subjected the ashes to 

 chemical analysis. Plants take their food 

 from the earth, air and water. By burning the 

 plants, he eliminated the factors in plant 

 growth due to air and water, which seemed 

 to him infinite in resource. Thus, in his 

 crucible, he retained only the mineral in- 

 gredients. His analysis developed the fact 

 that among the principal elements required in 

 the growth of plants were potash and phos- 

 phorus. Among those elements derived from 

 air and water, he considered only nitrogen as 

 fugitive. As the result of his researches he 



