THE PRINCIPLES OF FERTILIZER PRACTICE 477 



growth. That such a condition is more or less ideal and is 

 seldom realized is obvious, from the fact that the various fer- 

 tilizer carriers undergo more or less radical changes after 

 being applied to the soil. The composition of the soil itself 

 is also a disturbing factor. Nevertheless, the nearer an ap- 

 proach can be made to such conditions, the greater will be the 

 economy in fertilizer practice. 



Numerous persons have investigated the question as to what 

 effect an increase of an element in the minimum may have on 

 crop yield, and various ideas have been advanced to explain 

 the effect. The idea of a definite law governing the increase 

 of plant growth according as the element in the minimum is 

 increased, was first suggested by Liebig. Wagner x later 

 stated definitely that up to a certain point the increase yield 

 was proportional to the increase in the application. This, 

 however, evidently cannot apply except over a very limited 

 field, since it is a matter of common observation that increased 

 crop yield becomes lower as the lacking element is continu- 

 ously supplied. 



Mitscherlich 2 has formulated a law which is a logarithmic, 

 rather than a direct, function of the increase in the element 

 occupying the position of the minimum. Mitscherlich 's law 

 may be stated concisely as follows : the increased growth pro- 

 duced by a unit increase of the element in the minimum is 

 proportional to the decrement from the maximum. In other 

 words, the increase is proportional to the difference between 

 the actual yield and the possible yield at which the element 

 ceases to be a limiting factor. 



Mitscherlich has proposed a definite formula for such a 



1 Wagner, H., Beitrage zur Dungerlehre; Landw. Jahr., Band 12, 

 Seite 691 ff., 1883. 



2 Mitscherlich, E. A., Das Gesetz des Minimums und das Gesetz des 

 Abnehmen den Bodenertrages; Landw. Jahr., Band 38, Seite 537-552, 

 1909. 



Also, Ein Beitrage zur Erforschung der Ausnutzung des im Minimum 

 Vorhandenen Nahrstoffes durch die Pflanze; Landw. Jahr., Band 39, 

 Seite 133-156, 1910. 



