?hyv.ic'<;:fj 



In this situation, the gas diffusing throu'^h an exceedingly small porfi 

 space would not follow the initial assumptions of free diffusion that I 

 made a moment ago, in that molecules act independently from one another. 

 VJhat factors in the soil then will affect this diffusion rate? One is 

 called compaction. The effect of compaction is essentially to increase 

 the bulk density of the soil. Again assuming that there is no change in 

 the percentage composition of that soil, as the bulk density increases, 

 the value of percentage pore space will decrease, other things being 

 equal. Therefore, the rate of gaseous diffusion will decrease. Some 

 data in the literature indicate that one can decrease the rate of dif- 

 fusion as much as UO^ by merely compacting the soil and thereby reducing 

 the effective pore space through which diffusion occurs. 



Naturally, surface compaction will only affect diffusion rates through 

 that surface, whereas the presence of a plow sole or some other impeding 

 structure deeper in the soil profiles could affect diffusion at that 

 point. These compacted zones, perhaps, are the rate limiting factors in 

 the diffusion process, as is well known. Very often some constriction 

 in a pipe, as an example, serves as the rate limiting factor and affects 

 the flow in that pipe. Likewise, the amount of pore space present in the 

 impeding structure would be the rate limiting factor that would affect 

 the total diffusion rates through the entire system. 



The second factor which will affect porosity and the diffusion rate is 

 the amount of moisture present in the system. Dr. Bartholomew showed in 

 one of his charts that imder various moisture tensions various size pores 

 in the system will drain. If you started with dry soil and added mois- 

 ture, first the smaller pores would fill, and as more and more moisture 

 was added to the system, larger and larger pores would fill with moisture 

 until the soil became saturated. If, on the other hand, you started with 

 the saturated soil and added increments of tension to the soil system, 

 first the large pores would drain, and as more and more tension was ap- 

 plied, smaller pores would drain under the influence of this greater ten- 

 sion. The total porosity of the soil system will, therefore, be a func- 

 tion of the tension with which the moisture in that soil is held. Start- 

 ing with a saturated soil, the gas in that system would be zero. Then, 

 as more and more tension was applied to that system, pores would drain, 

 films of water surrounding the soil particles would decrease, and more of 

 the soil volume would be available for gaseous diffusion. Thus, at high 

 moisture tensions or low percentages of water, there would be more volume 

 for diffusion. As the water content increased, the rate of diffusion 

 would decrease. 



Another point of Dr. Bartholomew's that T think should receive some eiapha- 

 sis is with respect to size of pores and its affect on diffusion. The 

 size of porfis will affect the rate of mass flow, and it is a limiting 

 factor in water movement, because water generally moves by some sort of 

 mass flow arrangement or where there is actually some pressure differen- 

 tial. If gases were moving under some sort of a head, then the gas move- 

 ment, as well as the water movement, would be regulated by Darcy's or 

 Poiseuille's law, rather than by kinetic diffusion or kinetic pucrrj. In 



