Emil Truog 29 



THE CONSTITUTION OF SOILS 



Turning now from the brief consideration of the origin, gross 

 morphology, and kinds of soils, there follows a discussion of the chemi- 

 cal, physical, and mineralogical constitution— that is, what might be 

 called the anatomy and histology of soils. Matter exists in three physi- 

 cally distinct conditions or states commonly referred to as the solid, 

 liquid, and gaseous phases. It is important to recognize that all three 

 phases of matter, and in addition a "living phase" consisting of soil 

 bacteria and other organisms, enter into the constitution of a soil when 

 it is in proper condition to function as a medium for plant growth. 



Thus, in the language of the physical chemist, soils are three- 

 phase systems: there is the solid phase consisting of innumerable 

 minerals and organic substances; the liquid phase consisting of the 

 soil moisture or water in which relatively small amounts of the solid 

 phase dissolve; and the gaseous phase, the soil air, which fills the 

 pore space not occupied by the soil moisture. The total volume of 

 the latter two in any soil must, of course, always equal the volume 

 of the soil's pore space, although the volume of each fluctuates with 

 the moisture content of the soil. 



But a soil serving as a medium for plant growth is more than a 

 three-phase system in the ordinary sense: it teems with microorgan- 

 isms, and has, in addition, what may be called "a living phase," which 

 must always be taken into account in any consideration of the soil 

 as a medium for crop production. The constitution of the three con- 

 ventional phases and also the "living phase" of the surface or plow 

 layer of a typical silt loam soil is given in outline form in Table I. 



The solid phase 



It will be noted that the solid portion or phase of a silt loam oc- 

 cupies only about one-half of the volume of the soil, the remainder 

 being, of course, pore space when the soil is free of moisture. The pro- 

 portion of solid-to-pore space varies greatly among the different types 

 of soils. In general, the heavier the soil— that is, the greater the content 

 of finer material, especially clay, and also the greater the content of 

 organic matter— the higher will be the percentage of pore space. Thus, 



