THEORY OF SOIL FERTILITY 189 



an illustration, the soil could be likened to a 

 trust fund, of which only the interest was 

 available. If the beneficiary lived faster than 

 his means, he must either put himself in debt, 

 or sit down and wait for more interest to 

 accrue. 



Liebig announced that his researches ex- 

 plained the tonic effects on the soil of crop- 

 rotation, a practice which had been established 

 for more than a thousand years before his day. 

 An examination of the ash of plants showed, 

 he said, that certain families of plants require 

 more phosphorus than others, while other 

 families may require a greater percentage of 

 potash or nitrogen. 



Thus single cropping might reduce the avail- 

 able nitrogen, potash or phosphorus content of 

 that soil so low as to render the soil infertile 

 for a particular plant. The remedy was ob- 

 vious. Seed the land to another crop, requir- 

 ing less of one, and more of the other ele- 

 ments. Thus by a judicious selection, founded 

 both on the facts gleaned from chemical 

 analysis of the ash of plants and on experience, 

 a farmer could arrive at a system of crop- 

 rotation that would render his soil continuously 

 productive, at a certain rate. It was merely 



