EXTENSION COURSE IN SOILS. 3 
are believed to have been formed mainly through the agency of 
water. Examples of one class of these rocks are the deposits of 
gypsum and phosphate beds. The most important classes of the 
aqueous rocks, however, are those of sedimentary origin. They are 
composed of the materials resulting from disintegration of older 
rocks and from the mineral remains of animal and plant life. These 
rocks are largely distributed over the earth "s surface and include 
the limestones, the sandstones, and the shales. 
Organic matter as a soil factor (Ref. No. 7, pp. 120-125). — The 
organic matter of the soil has many important relations to the soil's 
fertility. Vegetable matter, commonly in the form of leaves, and 
of stems and roots of plants which have died, undergoes a process 
of decomposition in which it breaks down into simpler substances. 
When moisture and the air have ready access to it, vegetable 
matter slowly decomposes into the substances which were taken by 
the plant, in growth, from the soil and those which were absorbed 
from the atmosphere. The process is much the same as though the 
vegetable matter were slowly burned, and, like burning, it pro- 
duces volatile gases and mineral ash, which again serve as plant- 
food materials. However, when the air does not have ready access 
to the decomposing vegetable matter, it undergoes much slower 
and often different changes, yielding residues known as humus, 
muck, and peat. 
Humus may be defined for present purposes as vegetable matter 
in such an advanced stage of decomposition as to have lost its 
original physical identity. The degree of fertility of soils is very 
closely related to the amount of humus which they contain, and one 
of the most important problems of a farmer is to manage his soil 
so as to retain a high humus content. The quantity of vegetation 
returned, the drainage, the temperature, and the character of the 
soil are conditions affecting humus content. Peat and muck are 
terms applied to vegetable matter which has undergone changes 
under water, largely without air, and which may be in various stages 
of decomposition. Marsh soils are largely composed of muck and 
peat. 
FORMATION AND COMPOSITION OF SOILS. 
Agencies oj soil formations. — The principal agencies which have 
formed soils from rocks and organic matter may be classified as 
physical, chemical, and biological. (Ref. No. 9, pp. 1-6.) 
A physical change in matter is one which does not produce a 
substance or substances of different composition. For example, 
the changes of water to ice or to steam are physical. The form of 
the matter is changed, but not the composition. Likewise, the dis- 
solving of salt in water produces a physical change. The physical 
