42 ELEMENTARY AGRICULTURAL CHEMISTRY 



micro-organisms capable "of assimilating free nitrogen from the 

 air and carbonaceous matter from carbon dioxide are to be 

 found on rock surfaces even near the summits of mountains. 

 Certain lichens and algse are also apparently able to grow 

 without combined nitrogen. Such growths, when they die, 

 furnish organic matter to the soil, and gradually fit it for 

 sustaining the life of higher plants. 



The Constituents of SoiL — A popular and convenient 

 classification of soil constituents is the following : 



1. Sandj — mainly silica, but containing small fragments of 

 felspar, mica, limestone, &c. 



2. Clay — mainly kaolin, but containing also finely divided 

 silica, felspar, &c. 



3. Ldmestone — finely divided calcium carbonate. 



4. Humus — the somewhat indefinite nitrogenous and car- 

 bonaceous material resulting from the decay of plants. 



These constituents have great influence upon both the 

 physical and chemical properties of soil. The physical pro- 

 perties of the constituents themselves can be gathered from 

 the following table : 



It may be desirable to explain the meaning of the terms in 

 the above table. 



Real specific gravity is the weight of any volume of the 

 solid material compared with that of an equal volume of water. 

 Apparent specific gravity is the weight of any volume of the 



