P?iy3ics t'6 



to tills increase in molecular size. The process of diffusion, which is 

 perhaps the itt,ain means by which gases move in soils, is one in which the 

 mixing of gases will be due to the kinetic energy that the molecules 

 possess where there is no external force entering into the reaction. 



Diffusion always takes place ffom a zone of greater concentration into a 

 zone of lesser concentration, in other words, according to the concentra- 

 tion gradient. 



As I say, most gaseous movement in soil is a result of the diffusion 

 process. There are several factors that affect the rate of diffusion. 

 First, one might mention temperature. As I mentioned a moment ago, the 

 kinetic energy that molecules possess is a direct function of the abso- 

 lute temperature. As we increase the temperature of the system, if 

 everything else remains constant, the quantity of kinetic energy in that 

 system will increase. If the mass of the molecules remains constant, 

 then the velocity must increase accordingly, and the process of diffusion 

 will increase also. Certain studies concerning gaseous movement have 

 shown that in some instances the rate of diffusion will increase by a 

 factor of about S0% for each 10° C. rise in temperature. 



A second factor that will affect diffusion is the difference in pressure. 

 If there is a higher pressure of a gas in one volume than there is in 

 another volume, the rate of diffusion will be greater than if the pres- 

 sures are essentially the same. This is not only true of the total pres- 

 sure on the two sides of some membrane, but it is also related to the 

 partial pressures of the gases that make up the total pressure. 



You will perhaps recall Dalton's Law of Partial Pressures. In the gase- 

 ous system, the total pressure is equal to the sum of the partial pres- 

 sures, and the partial pressures are proportional to the percentage com- 

 position of the various components in the gas. In other words, if you 

 introduce into a vacuum various quantities of gases to make a gaseous 

 mixture, each gas would fill entirely the volume into which it was intro- 

 duced and would yield a partial pressure. The suranation of these partial 

 pressures would equal the total pressure in that volume. 



Diffusion not only occurs according to total pressure differentials but 

 also according to partial pressure differentials. If the soil's gas 

 system and the atmosphere above ground each consist of around QOfo nitro- 

 gen-and 20/o oxygen, then we can say that in these systems the partial 

 pressure of nitrogen would be 0.8, and the partial pressure of oxygen 

 would be 0.2. If, in the soil through respiration processes, the oxygen 

 was consumed to a certain extent and CO2 replaced it, the partial pres- 

 sure of oxygen would be less than 0.2, and the partial pressure of the 

 CO2 would be greater than .03^, and diffusion would take place from the 

 atmosphere into the soil according to the partial pressures of the vari- 

 ous gases. So the relative partial pressures of gases will affect dif- 

 fusion in to and out of the sail. 



The next factor, of course, is the size of the various molecules of the 

 system, as I mentioned awhile ago. As the mass increases, the velocity 



