33 THE CHEMISTRY OF THE FARM 



ing from the soil and subsoil dissolved or easily soluble 

 matter, storing these in its tissues, and at its death 

 leaving them upon the surface soil. When natural 

 vegetation has continued this action for ages, as in an 

 undisturbed prairie or forest, a surface soil is produced 

 rich in vegetable matter, and containing an accumula- 

 tion of plant food in a very available form. 



6. Organic Constituents, — In all sedimentary rocks 

 there is some organic matter, containing both carbon 

 and nitrogen, a residue of ancient vegetable or animal 

 life ; the nitrogenous organic matter in our deeper 

 subsoils is mostly of this ancient character. The much' 

 larger quantity of organic matter present in a surface 

 soil is, on the other hand, a residue of recent hfe, or 

 of applications of organic manure, and has a different 

 composition (p. 40). 



The brown or black organic matter of surface soils, 

 to which we give the name of humus, is a product of 

 processes of fermentation (as in a peat bog), or of par- 

 tial oxidation (as in an arable soil) ; and when exposed 

 to air in a surface soil is continually undergoing 

 further change. Humus is a mixture of many ill- 

 defined bodies ; it may be roughly divided into humic 

 acids and humin. If a soil is treated with cold dilute 

 hydrochloric acid, and washed, the bases with which 

 the humic acids were combined are removed ; if now 

 ammonia be added, the humic acids come into solution 

 as ammonium salts, while the insoluble humin is left. 

 The basic alkali humates are readily soluble, but 

 the acid humates of potassium and sodium are very 

 sparingly soluble. The combinations of humic acid 

 with calcium and iron are insoluble. Humic acid 



