324 H. W. Turpin 
the most important part of all the factors concerned, in the production 
of carbon dioxide in the soil. Climatic factors, such as temperature, 
rainfall, and air supply, have a marked effect on the carbon-dioxide content 
of the soil. Crops increase the amount of carbon dioxide in the soil, 
either by direct excretions from the roots or thru the decay of root 
particles from the growing crop. Finally, the nature of the soil itself 
causes marked differences in the production of carbon dioxide. 
The results reported in this paper confirm some of the above con- 
clusions, but they also show that the influence of the crop has been under- 
emphasized. ; 
EXPERIMENTAL WORK 
In the author’s first experiment, a study was made for two seasons 
(1917 and 1918) in the greenhouse, with soil cropped to oats and with 
uncropped soil. The object was to try to establish some definite relation- 
ship between the carbon dioxide in a cropped soil and that in an uncropped 
soil, where the crop itself introduced the only variable. Such a relationship 
having been established, it was decided te determine in the second 
experiment whether or not it would hold for a different crop. The third 
experiment was designed to analyze the factors concerned in the production 
of carbon dioxide, and, if possible, to assign to each its respective part. 
EXPERIMENT 1 
The cylinders illustrated in figure 44 were used in the first experiment. 
These cylinders, eight in number, were made of galvanized iron, coated 
inside with a layer of paint to insure their being air-tight at the joints 
and to prevent rusting. They were 3 feet high by 1 foot in diameter, 
and each had a cone-shaped bottom leading to the cocks on the outside 
as indicated in figure 45. 
The cone-shaped bottom was filled with gravel, above which was placed 
a 12-inch layer of soil from the second foot of the field soil. Above this 
was placed a foot of surface soil. The soil used was Dunkirk clay loam. 
The moisture in the soil was maintained thruout the course of the 
experiment at 30 per cent on the oven-dry basis. The soil was covered 
with a half-inch layer of quartz sand in order to reduce the evaporation, 
the sand being added to the cropped soil immediately after seeding. The 
dry weight of the soil in each of the cans was 94.3 pounds. 
