Phyoico:? 



the soil increases, the rate of diffusion of that gas throuf^h the soils 

 decreases as this moisture affects the porosity of the system. 



In concluding, let me mention again the matter of compaction. As we 

 compact the soil, for example, into a plow sole, or traffic sole, or if 

 there are some other naturally occurring barrier structures of one sort 

 or another in the soil, all these affect the rate at which diffusion 

 will occur and the depth to which gases might diffuse in the soil. If 

 you wish to have deep penetration, then sub-soiling or some other me- 

 chanical measure of this kind might be necessary to allow for deeper 

 penetration of the gas within a reasonable length of time in order to 

 prevent its complete dissapation without having done the job for which 

 it was used. 



Discussion 



Q. The efficiency of a fumigant seems to be tied directly to the mois- 

 ture equivalent of the soil. Up to about 20%, the average dosage of 

 D-D or EDB, or whatever it is we are using, is effective. Above that, 

 the rate has to be increased rapidly. Unfortunately, the data are always 

 published without any reference to the moisture equivalent of the soil. 

 I wonder if you would explain that term and something of the techniques 

 .hat are necessary for individuals to get such a measure. 



1. This goes back to a point that I overlooked. The amount of moisture 



in the soil, as I mentioned, will affect the rate of diffusion. There 

 seem to be some conflict in the literature concerning the moisture con- 

 tent at which diffusion will almost cease. Hagan, in the work he did with 

 carbon disulfide, said that the rate of diffusion became almost zero after 

 moisture equivalent was reached. Some later work by Taylor, who was at 

 Cornell at the time, indicates that there is a rather sharp break in the 

 curve between 20 and 30 cm. of water tension. This would be considerably 

 wetter than the moisture equivalent. 



To get to the question you asked as to how the moisture equivalent if- 

 termed, it is a defined term that has no theoretical implication. It is 

 defined as the percentage moisture in the soil remaining after the soil 

 to 1 cm. depth has been saturated and has been centrifuged at a force of 

 1,000 times gravity for 30 minutes. This moisture equivalent value is 

 theoretically close to the approximate field capacity which is the ten- 

 sion of a pF value of about 2.7. 



To actually state moisture equivalent, it has to be determined accord 

 ing to the technique of Briggs and McLane. According to their technique, 

 and I believe that the definition I gave is correct, it approxijnates fie3 

 capacity. Field capacity is approximated by about 500 cm. of water ten- 

 sion. 



Dr. Bartholomew showed a chart in which he rated five or six things to- 

 gether, such as relative humidity, pF, and centimeters of water tension. 

 dF and cm. of water tension are, of course, the same thing. pF is a loga- 

 rithm of the height of a column of water expressed in cm., but certainly 



