SOILS AND FERTILIZERS 309 



water and be carried put into the sea, perhaps to be taken up even 

 there by plants or animals. 



Soils are made up of small particles of different kinds of min- 

 erals mixed with more or less organic matter. These small min- 

 eral particles were originally formed by the breaking down of rocks 

 through the agencies of weathering and decomposition. These in- 

 clude a variety of forces that do their work under different condi- 

 tions but all produce the same ultimate effect, that of changing 

 solid rock into fine material that may become the home of plants. 



One of the most common physical agents is that of tempera- 

 ture, which does its work in two ways: (1) by the freezing of 

 water contained in rocks and (2) by changes in temperature pro- 

 ducing an increase or decrease in volume. All rocks contain more 

 or less water, and when this water freezes it expands and exerts 

 enormous pressure about one ton to the square inch. This would 

 be sufficient to shatter most rocks if frozen when the pore space was 

 completely filled. As it is, the expanding water or ice breaks many 

 rocks into pieces, and weakens others so that the repeated freezing 

 is enabled to accomplish what a single freeze could not do. These 

 small pieces are acted upon in the same way until they are reduced 

 to material that will form soil. 



It is common knowledge that a substance expands when heated 

 and contracts when cooled. This change of temperature and con- 

 sequent increase or decrease in volume takes place on the surface 

 most readily, and the strain thus produced in rocks is sometimes 

 sufficient to cause pieces to scale off, and even when this does not 

 occur the rock is weakened so that other forces may do more effec- 

 tive work. When a sufficient amount of this fine material is formed 

 for the growth of plants, especially shrubs and trees, the roots pen- 

 etrate the crevices in rocks and by their growth pry off fragments, 

 which are broken down by other agencies. 



All rocks have more or less water passing through them, how- 

 ever slowly it may be. This water frequently dissolves out the 

 more soluble material, which is often the substance that cements 

 the crystals or particles of the rock together. This leaves it very 

 loosely cemented, and other agencies soon reduce it to sand or other 

 smaller particles. Rocks are made up of P. number of minerals 

 such as quartz, mica, feldspar, calcite, etc. Water containing oxy- 

 gen and some other substances may have the power of decomposing 

 or changing these minerals into other minerals. These may bo 

 more bulky than the first and tend to rupture the rock, or they may 

 be softer and will be acted upon more rapidly by atmospheric 

 agencies. 



In some or all of the>e different ways rocks are broken down 

 into particles of various sizes. These do not always remain where 

 they are formed, but may be transported by wind or water and in 

 their movement help to wear down solid rock or grind stones, gravel, 

 or sand into still finer material. The wind aids in soil forming by 

 driving the sand against hard surfaces, gradually wearing them 

 away. The wind is also a distributing agent for soil material, pick- 



