42 PHYSICAL CONSTITUTION OF SOIL 



TABLE III 



SPECIFIC GRAVITY OF SOIL CONSTITUENTS 



Of all the solid constituents of soil, hunius is by much the 

 lightest. Clay is a little lighter than quartz sand, and 

 crystallized calcium carbonate (calcite) a little heavier. These 

 are the most common constituents of soil. Dolomite (calcium 

 and magnesium carbonate) is distinctly heavier than calcite. 

 Of the common siliceous minerals, felspar and talc have 

 specific gravities quite similar to quartz and calcite ; mica 

 is distinctly heavier. Hornblende and augite may contain 

 a good deal of iron, and then show a decided increase 

 in specific gravity. Limonite is a natural hydrated ferric 

 oxide; hematite is the same oxide in its anhydrous state. 

 These oxides of iron are the constituents of soil which possess 

 the highest specific gravity. It is clear, from what has been 

 stated, that humus soils will be the lightest, and soils rich 

 in iron the heaviest, if we have regard to their true specific 

 gravity. The true specific gravity of ordinary arable soil 

 is usually about 2-5. 



We have already seen that a soil is composed of particles 

 which touch each other only in certain points, spaces filled 

 with water or air lying between them ; a cubic foot of dry 

 soil has not therefore the weight which we might assume 



