228 



SOIL CONDITIONS AND PLANT GRO WTH 



a renewal of the air supply in the soil, but other factors, 

 diffusion, changes in pressure, air movements, etc., come in, 

 making the gaseous interchange still more complete. At soil 

 depths reached by plant roots — some 6 to 12 inches — the soil 

 air presents no abnormal features : there is some accumulation 

 of carbon dioxide, because this gas diffuses out more slowly 

 than water vapour, oxygen, or nitrogen, but the percentage 

 volumes of oxygen and nitrogen are nearly the same as those 

 of the atmosphere. Of course, if the air supply is cut off by 

 an accumulation of water on the surface, the oxygen may fall 

 considerably in volume, but this case is exceptional in England 

 on cultivated land. At still lower depths the volume of carbon 

 dioxide may rise above i per cent.^ 



As might be expected, the carbon dioxide is higher in 

 amount in summer than in winter, and higher in grass land 

 than in arable. It may rise considerably in grass land, or in 

 land recently dunged. 



Russell and Appleyard (241^) obtained evidence of a dis- 

 solved atmosphere in soil, composed mainly of COj with some 

 nitrogen. E. H. Richards has shown ^ that rain brings down 

 an appreciable amount of dissolved oxygen : — 



Dissolved Oxygen Brought Down in Rain. Richards. 



Leather has obtained much higher CO2 and lower oxygen 

 values in India under monsoon conditions when the soil is 

 virtually water-logged and at high temperature ; in land 



1 Pettenkofer's determinations at Munich at depths 1^-4 m. below the sur- 

 ace are published in iV. Rep. Pharm., 1873, xxi., 677-702, and abstracted in 

 Jomn. Chem. Soc, 1873, 361, and 1874, 36. 



'^Journ. Ag. Set., 1917, 8, 331-337- 



