112 SOILS: PROPERTIES AND MANAGEMENT 



particles of varying sizes, of a structure partly single- 

 grain and partly granular, to which has been added by 

 natural means sufficient organic matter in an advanced 

 stage of decomposition, we have the ideal soil conditions 

 for plant development. Yet the same laws govern here 

 in a general way as were found to function with homo- 

 geneous grains of spherical shape. 



81. The absolute specific gravity of the soil. — The 

 structural condition of any soil, be it single-grain, granular, 

 or a favorable combination of the two, has considerable 

 influence on certain other physical conditions. One of 

 those most affected is the weight. The weight of a soil 

 is determined by two factors : the weight of the individ- 

 ual particles, or the absolute specific gravity; and the 

 amount of actual space taken up by such particles in any 

 given volume. The latter is really a structural condition 

 and is independent to some extent of the size of particles. 

 The absolute specific gravity, or weight compared with 

 an equal volume of water, of some of the common minerals 

 is as follows : — 



Quartz .... 2.7 



Orthoclase . . 2.6 



Plagioclase . . 2.7 



Mica .... 3.0 



Olivine .... 3.4 



Calcite .... 2.7 



Dolomite ... 2.9 



Although a great range is observed in the absolute 

 specific gravities of these common soil-forming minerals, 

 it must be remembered that such minerals as quartz 

 and feldspar usually make up the bulk of a soil. As a 

 consequence it has been found that the absolute specific 



