ORGANIC CONSTITUENTS OF THE SOIL SOLUTION IOI 



It has been found that, in general, a cultural medium which 

 has been rendered unfit for the continued growth of a crop, is 

 readily renovated by treatment with oxidizing agents, and is 

 sometimes rendered even better than ever by such treatment, 

 which would suggest that the oxidation products from plant 

 effluvia may be even beneficial to the plant. To this end the 

 growing plant seems itself to be an active agent, apparently 

 attempting automatically to protect itself against the products 

 of its own activities. It has been pointed out by Molisch 1 that 

 root secretions have an oxidizing power, apparently of an enzy- 

 motic character. Some doubt of the validity of Molisch's work 

 has been raised by Czapeck, Pieffer, and others; nevertheless it 

 is now accepted that while intercellular autoxidation or reduc- 

 tion processes may take place in living roots, the higher plants, 

 such as our common crop plants, also show a more or less well- 

 developed extracellular oxidizing power in the neighborhood 

 of the root tips and root hairs. 2 That this oxidizing power dis- 

 played by growing roots is enzymotic is indicated by the fact 

 that artificial culture media frequently display it also after plants 

 have been grown in them for a short while. 3 



It has been shown that the oxidizing action of growing roots 

 is generally promoted by having the cultural medium slightly 



1 t)ber Wurzelausscheidungen und deren Einwirkung auf organische 

 Substanzen, von Hans Molisch. Sitzungsber. Akad. Wiss. Wien, Math. nat. 

 Kl., 96, 84-109 (1888). 



'The role of oxidation in soil fertility, by Oswald Schreiner and 

 Howard S. Reed : Bull. No. 56, Bureau of Soils, U. S. Dept. Agriculture, 

 1909. 



3 From considerations as yet highly speculative, a different type of 

 oxidation by roots might be anticipated. It is recognized that in the 

 absorption of mineral nutrients by plants a certain amount of selection 

 enters. For example, a plant with its roots in a solution of potassium 

 chloride, absorbs more potassium than chlorine, relatively, and free hydro- 

 chloric acid is left in the solution. Obviously in the absorption, work 

 is done, and a possible explanation is that water is decomposed at the 

 absorbing surface of the root, with the liberation of oxygen. Theoret- 

 ically, it ought not to be difficult to investigate this by a study of the 

 energy changes during absorption, but growing plants do not lend them- 

 selves readily to such experimentation. 



