Phyr.ics;9 



tjfpes for my area fairly well. The Shell Company has detemined the 

 moisture equivalents of these soil types for me. I have become able to 

 judge the soil when called to look at a field around planting time as 

 to whether the moisture is about 20°i or below its moisture equivalent. 

 If the soil is at 20$o or below^ 2$ gallons of D-D will give good control 

 of the sugar beet nematodes. If the moisture runs up around 25^, I know 

 well enough from past experience that I can tell the grower to go ahead 

 and rotate his crop that year, because he cannot afford to put on the 

 extra 10 gallons per acre necessary for effective control. I would like 

 to suggest that field workers try to get to the point of being able to 

 recognize the soil types and have someone in your locality determine the 

 moisture equivalents for you until you are able to make sound recommen- 

 dations to the growers. They cannot afford to fumigate if it will not 

 be effective due to unsuitable moisture conditions. 



Q. Why not report the condition of the field at the time we apply the 



fiomigant? Do you think that the soil type, compactness, temperature, 

 and some of these other things could be readily obtained for reporting 

 in fumigation experiments? 



A. If you are comparing soils with similar mineral and organic contents. 



then the bulk density value would give some measure of compactness. 

 You have to take bulk density with some precautions, as I pointed out, 

 because they are not universally comparable to each other. In addition 

 to that value, one should also, as Dr. Jones pointed out, have some meas- 

 ure of the tension with which the moisture is held in the soil at that 

 time. The only way to get at that is to run a complete pF versus percent 

 moisture curve on that soil and then determine the percentage moisture in 

 the soil at the time you did your fumigation. Relate that back to the pF 

 value of the soil on a predetermined curve and that would give you a more 

 or less absolute measure of the tensions or the physical forces that were 

 operating over moisture in the soil at that percentage moisture content. 



Q. Is there not an instruiaent of some t:fpe that could be put in the soil 

 to give us a reading? 



A. There have been several attenipts to r^et devices that will give the 



measures of soil moisture, and i)erhaps the most widely known one is 

 ':.he so-called Bouyoucos block. Variations of that have been devised. 

 The nylon cloth and gypsum blocks determine temperature and moisture at 

 the same time, but their readings have very limited value. For compara- 

 tive purposes for yourselves, I think that Bouyoucos blocks do have value. 

 However, they have to be closely calibrated for every particular soil 

 system. A good many people are for more accurate work, and I understand 

 they are getting away from the use of these moisture blocks and are going 

 back to the method of drying samples to obtain percentage moisture. I am 

 sure you understand percentage moisture in itself is quite meaningless 

 irnless it can be referred to something, and that is why one has to go to 

 the pF scale or atmospheres, tensions, or whatever you wish to get your 

 curve. This has to be determined, you see, eventually, long before you 

 put this percentage water on the scale. Then results can be compared 



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