348 H. W. Turpin 
2. Millet produced about the same increase in carbon dioxide as did 
oats, but the production of carbon dioxide by each millet plant was 
approximately half as much as the production by each oat plant. The 
most marked rise in the carbon-dioxide content of the soil occurred at 
a later period of growth in the case of the millet than in the case of the 
oats. 
3. The cropped soil, after the crop was harvested, maintained a higher 
carbon-dioxide content than was found in the bare soil. This was due 
probably to the decomposition of plant roots left in the soil. 
4. It would seem that increased plant activity (growth) is accompanied 
by increased carbon-dioxide production. This theory is supported by 
the fact that a relationship was shown between the carbon dioxide pro- 
duced presumably by the crop, and the water transpired. 
5. Fluctuations in the content of carbon dioxide in the bare soil were 
accompanied by similar fluctuations in the cropped soil only after the 
removal of the crop and before the crop had made much growth. 
6. There appeared to be little relationship between the temperature of 
the soil at the time of sampling, and the carbon dioxide in the cropped 
soil or that assumed to be produced by the crop (determined by sub- 
tracting the carbon dioxide in the bare soil from that in the cropped soil). 
7. In the bare soil the carbon dioxide was usually high durmg warm 
weather and low when the temperature decreased. 
8. Very low atmospheric pressures were usually accompanied by an 
increase in the content of carbon dioxide in the bare soil. 
9. The carbon dioxide produced presumably by the plant was about 
the same in soils having a high initial carbon-dioxide content as in those 
low in carbon dioxide, indicating the probability that plants and soil 
organisms act independently in producing carbon dioxide. 
10. It is concluded from this work that the plant itself, and soil 
organisms, produce most of the carbon dioxide in the soil; that the plant 
often produces at the period of its most active growth many times as 
much carbon dioxide as is produced by soil organisms; and that the 
excess carbon dioxide in the soil growing a crop is due to respiratory 
activity of the plants rather than to the decay of root particles from 
the crop growing on the soil at the time of analysis. 
