SOIL CONDITIONS AFFECTING PLANT GROWTH 33 



factor present and A is the maximum yield obtainable if the 

 factor were present in excess, this being calculated from the 

 equation. 



Mitscherlich's own experiments were made with oats grown 

 in sand cultures supplied with excess of all nutrients except- 

 ing phosphate. This constituted the variable x : the yields 

 actually attained when monocalcium phosphate was used and 

 those calculated from the equation shown in Table V. (Fig. 



2). 



Experiments were also made with di- and tri-calcic phos- 

 phates and constants were calculated corresponding to k. 

 The ratio of these constants 



2 (di-calcic phosphate) 



/^(mono-calcic phosphate) 



is a measure of the relative nutrient efficiency of the two salts ; 

 k is therefore called the efficiency value (Wirkungswert). 

 There are some very attractive possibilities about this method 

 of treatment, since it gives a constant independent of the yield 

 and having a definite mathematical meaning. 1 



TABLE V. YIELD OF OATS WITH DIFFERENT DRESSINGS OF PHOSPHATES. 



MlTSCHERLICH (2Ol6). 



The method has given rise to considerable controversy in 



1 For an interesting application see Zur Frage der Wurzelausscheidungen der 

 Pflanze (Landw. Versuchs-Stat., 1913, Ixxxi., 467-474), in which Mitscherlich 

 argues that the root excretions of clover cannot differ from those of oats. 



3 



