ORIGIN AND F<>li.\[ATi():>l OF SOILS. 1^7 



the circumstances of their oriL,Mn. Tii the depths of tlie 

 soil sitiiilar inattars 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 VcifCtation. — When a plant or any part of a 

 plant dies, and remains exposed to air and moisture at the 

 common temi'.eratures, it undergoes a series of chemical 

 and physical clianges, which are lai-gely due to an oxida- 

 tion of portions of it)6 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 a small proportion of albuminous 

 and mineral substances. By decay, the white or light- 

 colored and tough tissues of plants become conveited 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 aeid and water, proceeds without 

 interruption, though more and more slowly, until nearlj'' 

 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 remains. 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 



