FUNCTIONS OF ORGANIC MATTER 140 



suiting compound dissolving in the solution present. A dark 

 or black liquid results. For the sake of convenience in dis- 

 cussing humus we can call the acid humus, humic acid, 

 remembering, however, that it is not a single acid by any 

 means but a mixture which acts like an acid. The compound 

 with ammonium would then be called ammonium hum ate. 

 Sodium and potassium hydroxides react like ammonium 

 hydroxide. From a solution of alkaline humate a mineral 

 acid like hydrochloric precipitates humic acid which separates 

 out in black or brown flocks, drying to shiny scales. 



125. Neutral Humus. — Where the soil contains sufficient 

 calcium or other carbonate, humic acid is neutralized as 

 fast as it is formed and the humus may then be said to be 

 calcium (or other basic element) humate. This neutral 

 humus is insoluble in water and organic solvents, unchanged 

 by ammonium hydroxide, but partly decomposed by sodium 

 and potassium hydroxides, forming the humates of the 

 alkalies, soluble in water. When treated with a mineral 

 acid "like hydrochloric, the humus is decomposed, forming 

 humic acid insoluble in water, and calcium chloride soluble 

 in w T ater. On further treatment of the soil with ammonium 

 hydroxide, the humic acid forms ammonium humate soluble 

 in water. 



126. Functions of Organic Matter. — In considering the 

 functions of the organic matter in the soil it should be re- 

 membered (Section 118) that there are really two kinds of 

 organic matter: First, the active or decomposing organic 

 matter which is constantly, changing, with the production of 

 organic acids, carbon dioxide, water, and mineral salts, and 

 the release of nitrogen locked up in insoluble form; second, 

 the inactive organic matter or humus, which is a more or 

 less stable "compound" comparatively resistant to further 

 rapid decay, 



(a) The Active Organic Matter serves important pur- 

 poses in the production of chemical compounds active in the 

 decomposition of mineral particles; in the formation of 

 nitrates and soluble inorganic salts which serve as plant 

 foods; in increasing the moisture-holding capacity of the 

 soil; and in improving the structure of the soil. 



