71 



This relation of ease of aeration tu the 

 mean size of individual pore spaces applies, theoreti- 

 cally, only to ventilation by gas movement under pres- 

 sure. An expression representing gas exchange by diffu- 

 sion would contain the second power of the porosity, ■*• 

 but porosity refers to total relative pore space and 

 not at all to the mean size of the individual pore 

 spaces. On this theory the diffusional gas exchange 

 out of a clay soil would be just as great as out of a 

 sand; probably greater since the clay would usually 

 have a greater total pore space. Now, as a matter of 

 fact ail of the cultural, bacteriological and chemical 

 symptoms which are supposed to indicate bad soil aera- 

 tion are much more prone to appear on heavy clay soils 

 than on the lighter loams and sands. This may be due 

 to a failure of the diffusion formula to remain inde- 

 pendent of the individual pore-size when this size is 



1. See page <-*-£? above. 



