28 BULLETIN 42, U. S. DEPARTMENT OF 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 
1 Bui. 73, Bureau of Soils, U. S. Dept. Agr. 
« 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 della Soc. degli Agr. Ital., 18, 974 (1908). 
