330 H. W. Turpin 
in the first week of June, at the time when the plants were starting to 
head. Thereafter the general tendency of the curve for the cropped scil 
was toward a decrease, altho it was not until the middle of July, two 
weeks previous to harvesting, that this decrease was very marked. It was 
pointed out by Russell and Appleyard (1917) that in their experiments 
a large increase in carbon dioxide was observed in the cropped soil at 
the time of ripening; but, as can be seen from figures 46 and 47, in neither 
1917 nor 1918 was any such increase noted in this work. If anything, 
the ripening was accompanied by a marked decrease in carbon dioxide, 
as is shown especially for the season of 1918 (fig. 47). Subsequent to 
the removal of the crop, the carbon dioxide in the cropped soil con- 
tinued to decrease, but never to a point below or equal to that in the 
uncropped soil. 
It is interesting to note that in 1917, fluctuations in the content of carbon 
dioxide in the uncropped soil were accompanied by similar variations in 
the cropped soil during the early part of the season and subsequent to 
harvesting. This was not true during the pericd of active growth of the 
plant, which would seem to indicate that at that time the life activity 
of the crop itself, rather than that of the soil organisms, is playing the 
dominant part in controlling the production of carbon dioxide. 
What has been said for the season of 1917 holds for 1918 also. During 
the latter season, however, there was a much more marked increase in - 
the carbon dioxide of the cropped soil. By the 11th of April, three months 
after seeding, more than 3 per cent of carbon dioxide was found, as com- 
pared with a little less than 0.2 per cent in the uncropped soil. This 
occurred four weeks previous to heading. Thereafter the content of 
carbon dioxide in the cropped soil increased to the maximum of 3.34 
per cent, which occurred a week before heading and coincident with 
the time of rapid elongation of the culms. Following the maximum there 
was a steady decline. The decrease was especially marked during early 
June, when the upper glumes were beginning to turn yellow and the 
plants were starting to mature. In figure 44 (page 325) the plants are 
shown a month before the period of maximum carbon-dioxide production. 
Since the maximum of 3.34 per cent of carbon dioxide found in the soil 
was about the same as that noted by Bizzell and Lyon (1918) in their 
studies with Dunkirk clay loam cropped to oats, it is evident that the 
decrease in the production of carbon dioxide in the cropped soil below 
