30 



An Account of the Soil 



[PT. I 



heating some soil on a tin lid or in a crucible; the soil 

 blackens or chars, then little sparkles of fire can be seen, 

 and finally all the combustible part smoulders away 

 leaving only the mineral constituents. The organic 

 matter is so important that it must be dealt with in a 

 separate chapter by itseK. 



It has become customary to talk of the "nitrogen," 

 "pLosphoric acid," "potash," "lime," etc. in the soil, 

 but the student must at the outset reaHse that these do 

 not exist as such in the soil. The nitrogen meant is not 

 nitrogen as it occurs in the air, and which is better 

 spoken of as gaseous nitrogen: it is nitrogen combined 

 with other substances. "Phosphoric acid" does not 

 occur in the soil, but only its compounds, the phos- 

 phates; "potash" and "hme" do not occur, but only 

 potassium and calcium salts. These distinctions must 

 be clearly grasped: failure to understand them will re- 

 sult in considerable confusion later on. 



The mineral and organic constituents, however, do 

 not form the whole of the soil mass, but only one-half 

 to two-thirds of it; the remainder is filled with air and 

 water which are of vital importance to the roots of the 

 plants and to the soil organisms. The air resembles 

 ordinary atmospheric air in composition, but it contains 

 more carbon dioxide and more water vapour : 



