THE SOIL 45 



" sour," and can best be restored to fertility by the application 

 of free lime or of calcium carbonate. In soils containing an 

 abundance of calcium carbonate such sourness never occurs. 



Another important function performed by the calcium car- 

 bonate is that of acting as a basic material necessary for the 

 important process known as nitrification; this will be explained 

 hereafter. It is also important in the chemical changes which 

 are produced by the application of certain manures to soil — 

 e.g.^ in the case of ammonium sulphate, when the sulphuric 

 acid radical unites with calcium and runs off in the drainage 

 water, the ammonia being retained by the soil. 



Humus, the characteristic organic matter of soil, is of great 

 importance on account of its chemical and physical properties. 

 The chemical nature is little understood, despite many investi- 

 gations. Many analyses of humus extracted from soils have 

 been made, but no definite constitution has yet been assigned 

 to it. Thus four samples gave the following : 



Carbon 44-50 per cent. 



Hydrogen 3-6 „ 



Nitrogen 6-5-10 „ 



Oxygen 28-35 



Ash 4-12 



The ash was found to contain 7*5 per cent, of potash, 12-4 per 

 cent, of phosphorus pentoxide, together with silica, ferrit 

 oxide, alumina, soda, and other substances. According to 

 other investigations, humus contains several distinct and 

 complicated acids : humic, ulmic, crenic and apocrenic acids ; 

 but the nature of these bodies, which are said to be compounds 

 of carbon, hydrogen and oxygen, is not known, though several 

 formulae have been suggested for them. 



Humus seems to be of an acid nature, and the calcium 

 compound is insoluble in water. The ash constituents asso- 

 ciated with humus are thought to be of considerable import- 

 ance, because they are apparently easily available for plants. 

 The nitrogen is of the utmost value to plants, though in most 

 cases it canaot be directly utilised, but must first be inverted 



