20 



The Atmosphere <>J the Soil 



At least two cases occur in which the oxyj^ten decreases to a greater 

 extent than the COg increases : 



(1) At times of active nitrification. 



(2) After heavy rainfall. 



In the first case the falling oft' of oxygen is partly at any rate the 

 result of oxidations such as the production of nitrate which do not 

 yield a volume of COg equal to that of the oxygen absorbed. Table V 

 gives the results obtained for all the periods of rapid nitrate accumula- 

 tion : in all except two the fall in oxygen is greater than the rise of COg. 



The second case is seen in wet weather particularly in February, 

 1013 and 1914. but it reaches its ma.xiniuni development on Geescroft 

 during the period when the soil lies waterlogged ; the o.xygen then falls 

 as low as 2'6 per cent, but the COg does not rise above 9-1 per cent. 

 There is no evidence of rapid biochemical change; it appears more 

 probable that the COg is being dissolved in the soil water. 



There are still other instances where the fall in oxygen precedes the 

 rise in COj : these are readily seen by inspecting the curves. 



A third case presents more difficulty and has not yet been satis- 

 factorily explained. Reference to the figures shows that several periods 

 occur when the oxygen and COg rise simultaneously: such are Ma}'- 

 June 1913 and April 1914 on Broadbalk unmauured plot (Fig. 1), 

 February, April and November 1913 on Broadbalk dunged plot 

 (Fig. 2), March, April and October 1913 on Broadbalk wilderness 

 (Fig. 3), etc. The phenomena suggest an evolution of COg from the 

 water or colloids in the soil. 



In general the o.xygen falls below that present in atmospheric air 

 (20-97 per cent.) but in a few cases it exceeds this amount^. The 

 occurrence is so rare that we have been unable to make a satisfactory 

 investigation, but we incline to the view that the additional oxygen 

 comes dissolved in the rain (p. 23). The following are instances: 



See also .^piKmdix, Tal)lo XI, Hoo>i field fallow. 



