4 BULLETIN 355, U. S. DEPARTMENT OE AGRICULTURE. 
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, 
