320 H. W. Turpin 
asides its importance as a direct solvent in the soil, carbon dioxide 
has considerable significance as an indicator of certain soil activities. 
Hutchinson (1912) observed a relationship between the biological activi- 
ties and the amount of carbon dioxide in the soil. Russell (1915, a and 
b) noticed a close parallelism between the carbon-dioxide and the nitrate 
production in the soil, there being more of these constituents in spring 
and fall than in midsummer and winter. It was pointed cut later by 
Russell and Appleyard (1915) that the curves for the bacterial numbers, 
the nitrate production, and the carbon-dioxide content in the soil thru- 
out the season, show marked similarity, indicating that the carbon dioxide 
may serve to some extent as an indicator of other soil activities. Neller 
(1918), however, could find in his experiments no correlation between the 
ammonia production and the carbon dioxide formed, except in cases in 
which he used pure cultures of bacteria. The lack of correlation he 
attributed to the predominating influence of fungi in the soil. 
In addition to its importance as a direct solvent. in the soil and as an 
indicator of certain soil activities, carbonic acid may possibly be significant 
as an inhibitor of the activity of soil organisms and perhaps even of plant 
growth. Large quantities of carbon dioxide in the air have been found by 
numerous investigators to be detrimental to the growth of the higher 
plants. E. Wollny (1897) observed an increased production of carbon 
dioxide with an increase in the organic matter in the soil, but the increase 
to the unit of organic matter was less with the larger application. This 
Wollny attributed to the inhibiting effect of carbon dioxide on the bacterial 
activities. The work of Plummer (1916), however, showed that exceed- 
ingly large amounts of carbon dioxide do not interfere with the activities 
of the ammonifying and nitrifying organisms, provided, in the latter case, 
that the oxygen supply is not reduced below a certain minimum. The 
same investigator showed that the maximum carbon-dioxide production 
in the soil corresponds with the point of maximum nitrification. In 
studies on the carbon dioxide produced in lysimeter tanks, Bizzell and 
Lyon (1918) noted a marked decrease in the production of this gas after 
the blooming period of an oat cropon Dunkirk clay loam. This decrease, 
they say, “was apparently due to the depressing effect of the crop on 
production by bacterial action.”” Such a dectease was not found to take 
place on a Volusia silt loam. 
