28 BULLETIN 42, U. S. DEPARTMENT OP AGRICULTURE. 



plots and the check plots increased their oxidizing power, the sodium 

 citrate especially. Addition of organic matter, such as darkened 

 pyridine, which is in a state of autoxidation, markedly increased 

 the oxidizing power of the soils in laboratory tests. 



As already stated, the oxidative power of a soil depends not so 

 much on the amount of manganese as on the form of the manganese 

 and the nature of the organic matter. Organic matter in a state 

 of autoxidation may oxidize aloin directly or act as an activator 

 of inorganic oxidizers such as manganese. Organic matter in a 

 state of autoxidation seems to be lacking in the plots. Addition 

 of manure to various plots on the Arlington farm has greatly in- 

 creased the oxidizing power and also the crop production, thus illus- 

 trating, as previously pointed out, 1 that the factors that favor oxida- 

 tion also favor, to some degree, soil fertility. In many soils, espe- 

 cially those carrying considerable organic matter and of neutral 

 reaction, the addition of manganese increases the oxidation in soil 

 and also the crop yield. The action of the manganese may be direct 

 or indirect; direct in that it stimulates the plant to an increased 

 growth, or indirect in that it increases the metabolism of microorgan- 

 isms which place at the service of the plant more material, organic 

 and inorganic, which is readily assimilated, or by furthering the 

 oxidation of material which is injurious to crop growth. Among 

 the injurious substances found in soils under study for low pro- 

 ductiveness are dihydroxystearic acid, 2 salicylic aldehyde, 3 and 

 vanillin, 4 all which are modified by oxidation. 



As regards manganese, Bertrand has shown it to be the most 

 active element in promoting oxidation changes. In soils we have 

 found it capable of promoting active oxidation, especially in the 

 presence of suitable organic matter. By its strong oxidizing power 

 manganese would render injurious material in the soil harmless or 

 even beneficial, and by the oxidation of inert or rather stable organic 

 matter might cause the nitrogen and other substances contained in 

 the organic matter to become more rapidly available to plants. 

 According to Giglioli, 5 the presence of compounds which accumulate 

 combined oxygen with a capacity of gradually giving it off, facilitates 

 the development of the roots in the deeper strata of the soil, which 

 otherwise they would not be able to penetrate. When a ditch was 

 dug in a region where manganese dioxide was acting as an oxidizer, 

 a deep system of roots was found. 



In various experiments abroad, as already shown, manganese salts 

 have been tested as fertilizers. Where the action of the manganese 



* Bui. 73, Bureau of Soils, U. S. Dept. Agr. 

 s Bui. 53, Bureau of Soils, U. S. Dept. Agr. 

 » Bui. 88, Bureau of Soils, U. S. Dept. Agr. 



* Jour. Agr. Research, vol. 1, p. 359 (1914). 



* Bol. quindicinale dellu Soc. degli Agr. Ital., 18, 974 (1908). 



