148 THE SOIL: ORGANIC MATTER 



pounds other than these few classes mentioned. Notwith- 

 standing this great complexity, the black material has, or 

 perhaps more correctly, these numerous compounds colored 

 black have certain general properties which afford sufficient 

 excuse for the term humus, and to consider them as a single 

 kind of material in the soil. 



Using the term " humus," then, in this general and popular 

 sense, it can be said that it is composed of the same elements 

 as are plants, except that there is more carbon and nitrogen, 

 and less oxygen, hydrogen, and ash, or inorganic material. 

 Table VIII gives the percentage composition of cellulose, 

 grass, oak wood, decayed oak wood, and humus, showing the 

 changes in composition from fresh material to humus. 



Table VIII. — Composition of Humus and Humus-forming 



Carbon .... 



Hydrogen 



Oxygen 



Nitrogen 



Results on the ash of humus are not of sufficient number 

 for anv definite statement to be made, but it can be said that 

 whereas plants on the whole contain 6.5 per cent, ash (Section 

 14), humus probably does not contain more than 2 per cent, 

 on the average. On the whole humus is insoluble in water 

 and organic solvents. 



Humus can be divided into two kinds in the soil: Acid 

 humus, and neutral humus, both with the same physical 

 properties. 



124. Acid Humus. — Acid humus is formed in soils lacking- 

 sufficient neutralizing materials. It is insoluble in water, 

 acids, and organic solvents. It combines with bases to form 

 salts, particularly those of the alkaline earths, which are 

 insoluble in water, and of the alkalies, which are soluble in 

 water. By treating a soil containing acid humus — and for 

 practical purposes this means an acid soil — with ammonium 

 hvdroxide the acid humus unites with the ammonium, the re- 



