EXTEHTSION 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 soUs 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 



