26 



Till Atiinixjihrn of till Soil 



The persistence of this iiioiease is its chief characteristic, and during 

 most of the year it does not assume very great dimensions nor does it 

 alter the shape of the curve relative to the iinmanured land. The 

 actual percentages of CO^ during the month before and the month after 

 ploughing in are as follows : 



September 22 

 October 

 ., 17 



Dunged plot 



before 

 ploughing in 



0-17 

 0-18 

 0-34 



Unmanured 

 plot 



Oil 

 010 

 010 



I Dunged plot 

 I after 

 ploughing in j 



November 10 



December 9 



12 



0-54 

 0-35 

 0-34 



Unmanured 

 plot 



O-Sf) 

 0-20 

 0-25 



Considerably larger diSerences however were observed during the 

 spring both in 00^ and oxygen in 1913 and in oxygen in 1914. 



C. The ejfecl of a growing crop. 



As already pointed out (p. 9) there has been considerable disagree- 

 ment as to the relative amounts of COg in the air of cropped and of 

 uncropped soils. Critical examination of the older work shows that 

 much of the discussion was irrelevant because the conditions in the 

 various experiments were not comparable. A cropped plot differs in 

 physical state, moisture content, temperature, etc. from uncropped land 

 and when the case is pushed to an extreme and a comparison is instituted 

 between grass land and arable land there arises a further complication 

 due to the difference in organic matter content of the two soils. 



The usual method has been to set up a comjjarison between cropped 

 and fallow portions of the same plot. We have done this in two fields. 

 Figs. 4 and 5 and Table VI give the detailed results and Fig. 11 a 

 simpler comparison for the Hoos wheat and fallow plots. These are 

 made to alternate each year : the land has been unmanured since 1851 

 and now yields a small crop averaging 16 bushels of wheat per acre. All 

 through the period of active growth (June to August) the cropped plot is 

 the richer in COg and it maintains its superiority even after the crop is 

 cut and right up to the time when the land is ploughed. Then the 

 COg sinks to a low level and remains low throughout the period of 

 fallow ; it rises again as soon as the land comes into crop. The physical 

 differences in the plots, iiowever, arc considerable. The fallow land is 

 left rough and is not harrowed, it is occasionally cultivated to kill weeds, 

 thus it readily allows of the escape of COo. The cropped iaiid has to 



