CHAPTER XIII 



PHYSICAL PROPERTIES OF SOILS 



1. Real or Absolute Specific Gravity. The real specific 

 gravity of soils varies with the kind and amount of minerals com- 

 posing them and the amount of organic matter present. The 

 specific gravity of some of the more important minerals in soils is 

 given in the following table: 



Specific Gravity of Soil-Forming Minerals 



Quartz I 2.65 



Albite ; 2.61 



Orthoclase 2.56 



Oligoclase 2.60 



Labradorite 2.68 



Anorthitc 2.72 



Dolomite 2.85 



Limonite ! 3.6 to 4.0 



Magnetite ' 5.0 to 5.1 



Zeolites... 2.25 



Kaolinite 2.50 



Amphibole 2.9 to 3.4 



Pyroxene 3.2 to 3.5 



Muscovite 2.7 to 3.0 



Biotite 27 to 3.1 



Calcite 2.70 



Gypsum 2.33 



Hematite 4.5 to 5.3 



Epidote 3.25 to 3.5 



Organic matter is the lightest soil constituent, its specific gravity 

 being 1.2 to 1..3. The specific gravity of the surface soil of brown 

 silt loam, the common prairie soil of the corn belt, is 2.(>'2, of gray, 

 yellow gray, or yellow silt loam 2.(>5, while of black clay loam it 

 'is 2.57. 



2. Apparent Specific Gravity. The real specific gravity is of 

 very little importance in comparison with the apparent specific 

 gravity, which is the ratio between the weight of a unit volume 

 of water-Tree soil and the same volume of water. The expression, 

 volume weight, is sometimes applied to this and represents the 

 weight of a unit volume of soil. The apparent specific gravity is 

 numerically smaller than the real specific gravity because, in the 

 latter, the pore space is eliminated. The apparent specific gravity 

 varies directly as the kind and amount of minerals and the com- 

 pactness, and inversely as the amount of organic matter present 

 and the porosity of the soil. Tt is obtained by dividing the weight 

 of a certain volume of soil by the weight of the same volume of 

 water, or. what amounts to the same thing, the weight of the soil 

 in grams by the volume of the soil in cubic centimeters. 



175 



