SOIL CLASSIFICATION 35 



How Soils are Formed. — Many agents are active in the formation 

 of soils. Among these may be mentioned changes in temperature, the 

 mechanical action of wind and water, the solvent action of water, and 

 the action of bacteria, fungi and the higher forms of plants. 



The manner of formation gives rise to two general classes of soil 

 known as (1) residual soils and (2) transported soils. Residual soils are 

 those formed from rocks like those on which they rest, while transported 

 soils are those carried some distance either by the movement of glaciers, 

 or by moving water in the form of streams and tides, or by the action of 

 the wind. 



Weathering and Disintegration. — Rocks absorb more or less water. 

 Low temperatures cause a freezing of the water, which exerts a pressure 

 approximating one ton per square inch. This ruptures the rocks, and the 

 process repeated many times every year gradually reduces the portion 

 subjected to these changes in temperature to fragments. Little by little 

 rocks are thus reduced to soil. On the immediate surface the change in 

 temperature between night and day causes expansion and contraction 

 which also tends to sliver off particles of rock. The movement of soil 

 particles as the result of wind and rain also tends to wear down the surface 

 and break off minute particles that contribute to the process of weather- 

 ing and disintegration. 



In addition to this the vegetation which gradually secures a foothold 

 develops into larger plants, the roots of which penetrate the crevices, 

 exerting a pressure which still further moves and often ruptures the already 

 weakened rocks or fragments thereof. In this way, through generations, 

 the soils are gradually formed and become incorporated with the decom- 

 posed vegetation that gradually accumulates on and near the surface. 

 As a further aid to the process of weathering and disintegration we find 

 numerous worms and insects that burrow into the soil, living on the organic 

 matter and living plants. These not only move particles of soil from 

 place to place but carry the organic matter down into the soil. 



The rain which falls upon the soil is also a factor in soil formation. 

 When thoroughly wet the soils expand and when quite dry they contract 

 and little fissures open in the surface. A succeeding rain washes the fine 

 surface particles and organic matter into the fissures and causes a gradual 

 mixture of these two essential parts of the soil solids. 



Decomposition. — The processes of weathering and disintegration 

 result in a change in the physical properties of the soil without necessarily 

 changing the character of the compounds. Decomposition, on the other 

 hand, generally results in the formation of new compounds. The proc- 

 esses of decomposition are technical and we will not undertake to discuss 

 them. 



What is the Soil? — The soil consists of three principal parts, namely, 

 solids, a liquid and gases. The solids consist of the minerals and the 

 organic matter mingled with them. The liquid is the soil water in which 



