4 MICROORGANISMS IN SOILS AND WATER BASINS 



preaching the freezing point of water j others, known as thermophilic 

 forms, can withstand very high temperatures, some being active even at 

 60° to 70° C. 



The reaction of the soil is also a factor influencing the nature of the 

 population. Many microorganisms are active within a very limited 

 range of /)H values j others, notably many of the fungi, are adapted to 

 much wider ranges of reaction. In acid soils, larger numbers of fungi 

 are present, because of the fact that they tolerate more readily the 

 more acid reactions, which limit bacterial competition. On the other 

 hand, actinomycetes comprise a large percentage of the microbial popu- 

 lation of dry and alkaline soils. 



CHEMICAL COMPOSITION OF THE SOIL 



The solid part of the upper or surface layer (20 to 30 cm.) of the soil 

 commonly is made up of i to 10 per cent organic matter and 90 to 99 

 per cent inorganic or mineral matter. The concentration of organic mat- 

 ter may be even less than i per cent, as in desert and poor sandy soils, or 

 more than 10 per cent, as in certain virgin prairie soils and, especially, 

 peat lands which consist of 50 to 99 per cent organic matter, on a dry 

 basis. 



The organic matter of the soil is markedly different in chemical na- 

 ture from that of plant and animal materials. It contains much less cellu- 

 lose and hemicelluloses than the majority of plants and is higher in 

 lignins and proteins. It is characterized by a narrow ratio of the two 

 important elements carbon and nitrogen, usually about 10: i j it is much 

 more resistant to microbial decomposition than are plant and animal 

 residues. It is black, is soluble to a considerable extent in alkalies, and is 

 partly reprecipitated by acids. These alkali-soluble constituents have 

 often been designated as "humic acids" or "humic bodies," thus impart- 

 ing the idea that soil organic matter is made up largely of these "acids" 



(94^). 



The inorganic constituents of f he soil comprise largely sand, silt, clay, 

 and, to a more limited extent, a number of soluble and insoluble salts, 

 notably phosphates, sulfates, and silicates of calcium, magnesium, potas- 

 sium, iron, aluminum, manganese, zinc, copper, and others. Some of 



