Phy3icr;:6 



this instance, the size of pores would be a factor, as vrell as the total 

 porosity of the system, but where free diffusion is occurring and mass 

 flow is not a factor, then it is total porosity that is governing the 

 amount and rate of diffusion and not necessarily the size of pores 

 through which the diffusion is occurring. 



I thinl<: I need to differentiate between these two types of movement and 

 how the effects of size of pore would be a controlling factor in the one, 

 whereas it is not a controlling factor in the other. Although Hagan, in 

 some work in California, did indicate that in some instances the size of 

 pore was a factor in carbon disulfide movement, his emperical equation 

 was more closely similar to x^oiseuille ' s law than it was to the diffusion 

 processes. The importance of moisture content and its effect on the 

 gaseous movement, of course, implies that a steady state has been reached. 

 By "steady state" we mean an eouilibrium has been established, which may 

 or may not be true in many soil systems. If a gas is introduced into the 

 system and it were to go into solution in the water present in the soil, 

 the state would not be reached and the diffusion rate would be increased, 

 because of the increased differential brought about by solution of the 

 gas. One of the components in the system may be adsorbed by soil parti- 

 cles. This, too, would prevent a steady state condition. If in some 

 instance (X)2 is being produced by biologic action in the system, this, 

 too, would upset a steady state arrangement, and diffusion rates would 

 be affected accordingly. 



I think there are certain interests in other factors as indicated in 

 other discussions by this group. One of these would be temperature 

 effects. If one were to add a fumigant to the soil, its partial pres- 

 sure in the system would be related to the temperature at which it vola- 

 tilized, assuming it volatilized immediately at ordinary temperature. 

 As the temperature raised, the partial pressure of that gas in the system 

 would increase and there would, of course, be effects on rate of diffu- 

 sion. This would be in addition to the ordinary kinetic effects that I 

 mentioned previously. 



The depth to which gas is placed would also affect the volume of soil 

 through which diffusion would occiu*. If you place the gas at the surface, 

 diffusion will occur in all directions, according to differential and 

 partial pressures of that gas. If the introduced gas is placed near the 

 surface, naturally, most of it would diffuse into the atmosphere rather 

 than against the steeper gradient in the soil. I think that this would 

 indicate in certain instances you would wish to place the fumigent deepei 

 into the soil to get reduced loss by diffusion into the atmosphere. 



Oftentimes some sort of plastic cover or something of that kind is placed 

 over the surface to prevent diffusion losses back into the atmosphere. 

 The amount of moisture in the soil will certainly affect the rate of dif- 

 fusion. If it is necessary for the introduced gas to go into solution, 

 then one is confronted with a dilemma. Is it more important to have the 

 gas go into solution, or is it more important to have rapid diffusion? 

 These things are reciDi'ocal to one another. An the moisture content of 



