38 



SOILS AND FERTILIZERS 



neighbors. The calculated pore space in the first arrange- 

 ment is 47.64 percent. That in the second case is 25.95 

 percent. (See Fig. 5.) 



It is not actually the case, however, that soil particles 

 are of the same size in any natural soil. Consequently 

 small particles fit in between large ones, thus decreasing 

 greatly the actual pore space. These three cases, of which 

 only the last may occur in nature, illustrate pore space 



Fig. 5. — If all soil particles were spheres they could be arranged as 

 shown above, in which case the pore space would vary in volume as ex- 

 plained in the text. 



when the separate grain structure obtains, as in a dry sand 

 or a puddled loam or clay. 



The granular structure is the one most likely to be found 

 in nature, although all of the particles may not be in gran- 

 ules. The granules being of irregular form, with many 

 angles, there is likely to be a large amount of space between 

 them. It would be possible under this arrangement for a 

 soil to have a pore space of 72 percent. 



The weight of a given volume of soil, including the pore 

 space, as compared with an equal volume of water is termed 

 the apparent specific gravity. This it will be seen is not the 

 same as the absolute specific gravity because the amount of 

 pore space is the important factor in determining the ap- 

 parent specific gravity. Neither do the terms " light " soil 

 and " heavy " soil bear any definite relation to the apparent 

 specific gravity. A knowledge of the apparent specific 



