402 MICROORGANISMS 



tumbler containing the soil, you will doubtless find some micro- 

 scopic animals similar to those you found in your hay culture. 

 The two kinds of organisms in the soil bear the same relations 

 to each other as they do in the hay culture. That is, the 

 bacteria of the soil feed on the decaying organic matter of 

 the soil, and the microscopic animals in turn feed on the 

 bacteria. 



457. The Nature of Soil. Before taking up the relation of 

 bacteria to the soil, it is necessary for us to know something 

 of the nature of soil. Geologists tell us that the outer crust 

 of the earth was originally solid rock, and that through the 

 influence of weathering agencies, such as the sun and wind, 

 freezing and thawing, and the flowing of water, much of this 

 crust of rock has been ground into exceedingly fine particles. 

 These fine particles now make up a deep layer of soft material 

 which we know as clay, sand, and gravel and which covers the 

 deeper solid rock of the earth's crust. For a few inches or 

 feet below the surface, this clay and sand and gravel is, in 

 most places, well mixed with decaying remnants of the bodies 

 of plants and animals. It is these few inches or feet on the 

 surface of the soft blanket of the earth that are well mixed 

 with organic matter that we call soil. It is in this thin layer 

 of soil, mainly, that the materials are contained which the 

 higher plants take in through their roots and use in building 

 up the more complex foods with which they nourish them- 

 selves and the rest of the living world. The organic matter in 

 the soil is collectively spoken of as HUMUS and it is this humus 

 which supplies food to the soil bacteria (see Art. 550). 



458. Soil Bacteria and Carbon. The soil bacteria complete 

 the work of destruction of the particles of plant and animal 

 matter which fall into the soil. All these materials contain 

 large quantities of carbon combined with other chemical ele- 

 ments and when the bacteria have digested, absorbed, assimi- 

 lated, and respired them, the carbon escapes to the air in the 

 form of carbon dioxide gas. This you will remember is the 

 form in which the green plants take up carbon from the air 



