Physics ;8 



you have to relate it to some energy concepts of water; you cannot re- 

 late it to percentage water in the system, for percentage water means 

 absolutely nothing with respect to the forces by which water is held. 

 This is related to the thickness of the film or the size of the pores 

 that will drain under a given tension. They would be related to each 

 other, if you held the texture of the material constant. I would like 

 to draw a graph on the board which may help relate various terms we have 

 been discussing. This is similar to the information given by Dr. Bar- 

 tholomew. The graph shows more or less typical moisture characteristics 

 for three different soil types: 



CLAY 



AP«c(tV) 



% (MOISTURE 



The values of A, B, and C are the so-called available water. So you 

 see the percentage available water will be much greater for a clay 

 soil than for sandy soil. Also, the amount of water present at wilt- 

 ing will be much greater in the clay soils than for sandy soils. Do 

 you see how, as Dr. Jones has said, the percentage moisture itself has 

 to be related back to some physical factors in order to be really mean- 

 ingful. You can get these water tensions by putting the soil sample on 

 a so-called tension table, or you may want to go over to more elaborate 

 pressure membrane type of apparatus. It is not too difficult to set up 

 a tension table to get an accurate measure of large pores. Some of you 

 may want to explore that possibility. To get up to liigher tension 

 ranges, specialized and expensive equipment would be involved. 



Comment: Acknowledging the necessity for precise work in certain cir- 

 cumstances, I would like to point out that in practice there is a cer- 

 tain range of conditions in a particular soil type in which certain 

 fumigants can be expected to be effective. I have had quite a little 

 experience in chemical control in the western states. 



I know the soil 



