102 THE SOIL SOLUTION 



alkaline or neutral rather than acid. It is also promoted by the 

 addition of various minerals salts, notably by nitrates, phosphates, 

 or lime salts. Potassium salts promote the oxidation but slightly, 

 and in some experiments have even produced a slight decrease. 

 The corresponding sodium and ammonium salts are more favor- 

 able than those of potassium. 1 It appears altogether probable, 

 therefore, that the mineral salts in commercial fertilizers may 

 have some importance in this connection. 



Whatever may be the role of mineral fertilizers towards 

 organic substances toxic to growing plants, it is certain that they 

 have an importance and one that is probably specific, as indi- 

 cated by some recent investigations. 2 Culture solutions contain- 

 ing the constituents potassium, nitric acid and phosphoric acid 

 were prepared in such manner that they covered the range of all 

 possible ratios of these constituents in intervals of ten per cent, 

 in each. Into one set of these solutions was introduced 

 dihydroxystearic acid, into another set cumarin, and into a third 

 set, vanillin, and into a fourth set, quinone. The growth of 

 wheat seedlings in these several sets showed indubitably that 

 these several organic substances which are all deterrent to the 

 growth of wheat, were modified in their influence by the presence 

 of the mineral salts ; but that nitrates were more efficient than the 

 other minerals in the case of the solutions containing dihydroxy- 

 stearic acid or vanillin; phosphates were most efficient in the 

 case of the solutions containing cumarin, and potassium most 

 efficient in solutions containing quinone. As the organic sub- 

 stances used in these experiments, either in themselves or as 

 typifying classes of compounds, may be anticipated in soils under 

 natural conditions, it is again apparent that mineral fertilizers 

 have a function in addition to the traditional one of increasing 

 the supply of mineral nutrients. 



The fact that the oxidizing power of roots is more marked 

 when grown in aqueous extracts of soils in good tilth than in 

 extracts made from soils in poor tilth, shows that cultural 



1 Action of fertilizing salts on plant enzymes, by M. X. Sullivan, Jour, 

 biol. chem., 6 (1909), proceed. XLIV. 



"Private communication by Dr. Oswald Schreiner and Mr. J. J. 

 Skinner. 



