224 HOW CROPS FEED. 
products of fermentation, a process that goes on in the 
juices of plants that have become a part of the soil or of 
a compost. 
These acids can scarcely exist in the soil, except tempo- 
rarily, as results of fermentation or decay, and then in but 
very minute quantity. They consist of carbon, hydrogen, 
and oxygen. Their salts are all freely soluble in water. 
Their relations to agricultural plants have not been studied. 
Humus (in part).—The general nature and origin of 
humus has been already considered. It is the débris of 
vegetation (or of animal matters) in certain stages of de- 
composition, Humus is considerably complex in its 
chemical character, and our knowledge of it is confessedly 
incomplete. In the paragraphs that immediately follow, 
we shall give from the best sources an account of its non- 
nitrogenous ingredients, so far they are understood, reserv- 
ing toa later chapter an account of its nitrogenized con- 
stituents. 
The Non-nitrogenous Components of Humus,—The 
appearance and composition of humus is different, accord- 
ing to the circumstances of its formation. It has already 
been mentioned that humus is brown or black in color. 
It appears that the first stage of decomposition yields the 
brown humus. It is seen in the dead leaves hanging to a 
tree in autumn, in the upper layers of fallen leaves, in the 
outer bark of trees, in the smut of wheat, and in the up- 
per, dryer portions of peat. ? 
‘When brown humus remains wet and with imperfect 
access of air, it decomposes further, and in time is convert- 
ed into black humus. Black humus is invariably found 
in the soil beyond a little depth especially if it be com- 
pact, in the deeper layers of peat, in the interior of com- 
post heaps, in the lower portions of the lcaf-mould of 
forests, and in the mud or muck of swamps and ponds, 
Uimic Acid and Ulmin.—The brown humus containg 
