4 BULLETIN 355, U. S. DEPAKTMEISTT OF AGEICXJLTUBE. 



agencies which have most affected the formation of soils are tem- 

 perature changes, or heat and cold, water, ice, and wind. 



A chemical change, or reaction, is one which separates or rear- 

 ranges the elements of a substance or compound. Chemically, an 

 element is a single substance which can not be separated into two 

 or more different substances; a compound is a union of two or more 

 elements in certain definite proportions. Gold, silver, quicksilver, 

 oxygen, and nitrogen are examples of elements. There are about 

 80 known elements. Common salt is a compound of the elements 

 sodium and chlorin; water is a compound of the elements hydrogen 

 and oxygen; carbon dioxid, present in the air, is a compound of the 

 elements carbon and oxygen. The formation of carbon dioxid in 

 the decomposition of vegetable matter and the uniting of this gas 

 with other substances to form carbonate compounds, are common 

 examples of chemical changes in the soil. 



A biological change is one resulting from plant or animal life within 

 the soil and may affect soil substance physically or chemically. Insect 

 life in the soil is a matter of common knowledge. When plant or 

 animal organisms are so small that they can be identified and studied 

 only by the use of the microscope, they are called microorganisms, 

 and a study of those commonly occurring in the soil is called soil 

 microbiology or soil bacteriology. Nitrification, or formation of 

 nitrates, is a typical example of microbiological (bacterial) changes 

 in soils. The work of nodule-forming bacteria upon the roots of red 

 clover, alfalfa, and other leguminous plants, is another example of 

 such changes affecting the productiveness of soils. The biological 

 changes produced in the soil are very extensive and important. See 

 Lesson VI. 



The physical, chemical, and biological factors which have been 

 potent agencies in the formation of soil for past ages are constantly 

 producing soil changes. Their action may be advantageously con- 

 trolled to some extent by the farmer, as will be shown in other les- 

 sons. 



Residual soils (Ref . No. 3, pp. 31-35). — Soils formed from the rocks 

 immediately underlying them are called residual soils. On examin- 

 ing a stone quarry, it is usually found that the upper portion of the 

 quarry rock is more or less broken up and pieces of the rock are 

 embedded in the lower layer of the soil. In fact, the finer pebbles 

 and cobbles of stone often extend all the way to the surface of the 

 soil. A careful study will show that the soil itself has really been 

 formed from the rock. This has resulted from the action of several 

 agencies. Among them the expansion and contraction of the rock due 

 to alternate heating and cooling are very important. The expansion 

 of water as it freezes has much the same effect. During the long 

 period of transition from solid rock to thoroughly disintegrated rock, 



