160 College of Forestry 
characteristic of this series of degradation products of plant 
tissues is the accumulation, of carbon at the expense of ogygen 
and hydrogen. 
The group of indefinite and complex organic substances 
formed in the decay of vegetable and animal matter are 
normal constituents of all soils and fulfill important fune- 
tions therein. The resulting compounds may be those that 
were in the living tissues and have resisted decay, those that 
result from a splitting or degradation of complex bodies in 
the living plant, or compounds’ arising through changes 
brought about by micro-organisms, and nearly all classes of 
organic compounds known may be represented. These prod- 
ucts have been classed under the general term ‘ humus”, 
and so long as the term is used in a collective sense it may 
be retained as a convenient term. 
The brown humus substances must be considered as a mix- 
ture of closely related bodies with and without nitrogen. 
These various substances have been divided into a number 
of groups by different writers, the subgrouping depending 
upon the solubilities of the substances. Schreiner and 
Shorey (1910) have grouped the various compounds they 
have isolated from the organic matter of soils as follows: 
Extract soil with alkali 
| 
| 
Alkaline solution, acidify Insoluble 
| 
Acid filtrate Precipitate, boil with alcohol 
CrENIC ACID Humic Actp, soluble Humic Acrp, insoluble HumIn. 
The brown humin substances are insoluble in water and 
alkahes, but are rendered soluble by fusing with caustic 
soda or potash, from the solution of which humic acid can 
*° In the light of recent investigations on the nature of organic matter 
of the soil it is necessary to revise this relict of the older terminology. 
The term “humus” is rather to be regarded as a loose generic term 
applicable to a group of organic compounds found in the soil. 
