ORIGIN AND FORMATION OF SOILS. 137 
the circumstances of their origin. In the depths of the 
soil similar matters are formed by the decay of roots and 
other subterranean parts of plants, or by the inversion of 
sod and stubble, as well as by manuring. 
Decay of Vegetation.—When a plant or any part of a 
plant dies, and remains exposed to air and m»isture at the 
common temperatures, it undergoes a series of cliemical 
and physical changes, which are largely due to an oxida- 
tion of portions of its carbon and hydrogen, and the 
formation of new organic compounds. Vegetable matter 
is considerably variable in composition, but in all cases 
chiefly consists of cellulose and starch, or bodies of simi- 
lar character, mixed with asmall proportion of albuminous 
and mineral substances. By decay, the white or light- 
colored and tough tissues of plants become converted into 
brown or black friable substances, in which less or none 
of the organized structure of the fresh plant can be 
traced. The bulk and weight of the decaying matter 
constantly decreases as the process continues. With full 
access of air and at suitable temperatures, the decay, 
which, from the first, is characterized by the production 
and escape of carbonic acid and water, proceeds without 
interruption, though more and more slowly, until nearly 
all the carbon and hydrogen of the vegetable matters are 
oxidized to the above-named products, and little more 
than the ashes of the plant reinains. With limited access 
of air the process rapidly runs through a first stage of 
oxidation, when it becomes checked by the formation of 
substances which are themselves able, to a good degree, 
to resist further oxidation, especially under the circum: 
stances of their formation, and hence they accumulate in 
considerable quantities. This happens in the lower layers 
of fallen leaves in a dense forest, in compost and manure 
heaps, in the sod of a meadow or pasture, and especially 
in swamps and peat-bogs. 
The more delicate, porous and watery the vegetable 
