PhysicG;10 



from one place to another. 



Q. Can you tell us how much you think we may have been wrong in tlio 



past fumigating in the manner in which we have? lA/hat I mean by th:it 

 is, most growers do not work the soil when it is too wet or too dry. !\ 

 lot of times we use the system of picking up soil, clenching it lightl v 

 in our hand; if it barely hangs together, we say it is about the moistur 

 level we want for a good f-umigation. Very few of us, I think, would go 

 out and fumigate a very wet or a dry soil. Perhaps the moisture range 

 judged in this manner is fairly critical. In other words, I wonder if 

 our data have not been reasonably comparable in spite of the simple way 

 we have judged the soil moisture. V/ould you care to comment on this? 



A. I know so little about fumigation problems that it would be rather 



hard for me to answer that question, I am sorry to say. I think 

 that these emperical methods of determining field capacity or less than 

 field capacity are perhaps fairly good. I cannot answer the question, 

 because I really do not know how critical moisture is in the fumigation 

 problem. 



Q. I think probably what we are getting at is that we must fumigate 



somewhere near the point where the soil moisture characteristic is 

 such that the soil has lost its excess water and before it is down to 

 very fine water films. I would like to make this comment. For a long 

 time we were puzzled about what to do in regards to moisture content, 

 but, as you know, out in the field you cannot be too particular. You 

 take the problems the way they are encountered. On our sandy soils we 

 have gradually come to the point where we have fimigated enough of them 

 when the soi] ^as been at or above field capacity that we have decided 

 that this actually, in a sandy type soil, is the best time to fumigate. 

 We get our best control \jnder these conditions. As the soil dries, gas 

 losses through the surface reach the extent that they exceed penetration 

 into the soil spaces. I think there are some data to support this pub- 

 lished by the Dow Company concerning diffusion of methyl bromide in sandy 

 soils. 



Comment: I would like to make a comment about Dr. Cairns' earlier ques- 

 tion about compaction. Actually, under field circumstances you shoiild 

 not be really concerned with compaction, because one of the elements of 

 fumigation is that you work the soil up and get it into good seedbed con- 

 dition. So what we are concerned with may be a layer down deeper that is 

 packed. My experience is that growers usually take care to be rid of 

 that packed zone, because they know they have to. They break it up sonp 

 way, if possible in their cultural operation. Any measurement of the 

 soil compaction would have to be made after all the cultivatin;-: had taken 

 place and the soil was in seedbed condition, which it should be for Av li- 

 gation. 



',:. '/e have to differentiate br5twenn oconomic control lovols nn(^ no'iic 



kinds of theoretical or exact f:xpor imf nl,al uorl;. TC W'^ nrr "oli-^; to 



