Phyaio" i'. 



PHYSICS OF VAPORS AND OAS'iS IN THE SOIL 

 Lloyd F. Seatz 



The nature of the soil - 



The soil is made up of a framework of solid matterj this solid matter 

 being minerals and organic material. This framework is separated by 

 voids that we call pore spaces. In an average soil, the soil frame- 

 work makes up roughly S0% of the total soil vol\ame, and the pore space 

 makes up the remaining ^0%. This pore space is filled with either 

 liquids or gases. ¥e call liquids the soil solution. The gas is the 

 soil atmosphere. Obviously, the amount of gas that will be present in 

 the soil will depend on the amount of moisture that is present. In 

 other words, the amount of pore space that is not filled with liquids, 

 naturally would be filled with gases. 



The size and distribution of this mineral and organic material is re- 

 ferred to as the texture of the soil. This material is usually in some 

 sort of arrangement that we refer to as soil structure, whether it be 

 single grained, crumb, or whatever types of aggregates that might be 

 present. 



It is the textural and s tructural relations in the soil that are most 

 closely related to the type of pore space and the size of pores found 

 in the soil. In particular, the size and continuity of this pore space 

 will determine the movement of liquids and gases in the soil. 



There may be many dead-end pores brought about by certain t^-pes of soil 

 arrangement. These dead-end pores are not effective in allowing move- 

 m.ent to occur. It is only those pores that are continuous that allow 

 for the movement of liquids and gases. So we see that the size, shape, 

 continuity, and distribution of the pores, as a reflection of the tex- 

 tural and structural arrangement in the soil, are the really important 

 things governing the air and water relations in soil. 



Another term I think we should review is this one called "bulk density." 

 This is defined as a mass per unit of bulk volume. In other words, we 

 have a certain volum.e, including both the solid material and the pore 

 space. Bulk density differs from the absolute specific gravity, which 

 refers to weight per unit mass of the solid material only. Other things 

 being equal, if we assume that we are using only mineral materials which 

 Dr. Bartholomew said had a specific gravity of about 2.6^, the bulk dens- 

 ity vri.ll be a reflection of the percentage pore space. If the bulk dens- 

 ity is low, then that means there is a higher per cent of pore space in 

 the soil than if the bulk is high, again assimiing that the same percentage 

 of minerals and organic material are present. If the per cent of organic 

 material, having absolute specific gravity of .9 varies, you see that the 

 bulk density would vary with the percentage minnral and organic materials, 



