DISCUSSION OF PLANT INDUSTEIES 437 



there is some air, though its amount is usually so small that 

 only water-enduring plants can grow therein. Observation of 

 any porous soil immediately after a heavy rainfall will enable 

 one to see bubbles of air emerging from the soil as the spaces 

 which they have occupied begin to be filled by the water. After 

 prolonged rains most of the air of the soil may have been ex- 

 pelled, and it is generally supposed that in such cases it is the 

 absence of air as much as the overabundance of water that 

 brings injurious results to suddenly submerged plants. Obvi- 

 ously the quantities of water and air in the soil are factors 

 that are constantly varying in amount. As a given soil becomes 

 dry it may also become compact, and it by no means fol- 

 lows that the total space occupied by water and air together 

 remains constant. 



403. Composition of the soil: living things within it. A 

 highly important factor of the soil consists of the many kinds 

 of living things that inhabit it. Microscopic plants and animals 

 of many kinds and in great numbers live upon one another, 

 upon plant roots, or upon dead organic matter, and as they 

 do so are constantly affecting the composition of the soil. 

 Then the roots of living plants, the molds, and the burrow- 

 ing animals such as the larvse of insects and the earthworm, 

 constantly take from, add to, or otherwise change the soil. 

 Earthworms eat their way through the soil, and as they do 

 so they make it more porous and excrete materials that add 

 to its available organic matter. Certain groups of soil bacteria 

 have already been discussed (Sect 343). The living things 

 of the soil may be said to constitute an extensive and intricate 

 society of plants and animals living close together and greatly 

 affecting the nature of the material in which they live. 



404. Drainage. The annual rainfall in different parts of the 

 United States varies from ten inches or less to more than sixty 

 inches per year (Fig. 381). In some parts of the country the 

 total annual rainfall occurs within a short period, while in 

 other regions it is usually fairly well distributed throughout 

 the year. In all regions shortage of water is often a source 



