Soil Formation 77 



as well as a few species of birds spend at least a part of their lives 

 burrowing in the soil. 



Of smaller animal forms, the numbers present in the soil are much 

 greater (Chapman, 1931, Ch. 18). Earthworms have been estimated 

 at hundreds of thousands per hectare, and their burrows may extend to 

 depths greater than 2 m. Insects, especially in the larval stages, are 

 very numerous in the upper centimeters of the soil (Salt et al., 1948). 

 In some regions population densities of several million soil insects 

 per hectare have been found. Spiders, tardigrades, millipedes, and 

 isopods are also abundant inhabitants of the land substratum. An 

 extensive study in Illinois showed that the invertebrates of the soil 

 reached an average summer maximum of 3300 per sq m— or roughly 

 one animal under every 3 sq cm of surface. Since many of these 

 forms are short lived and populations succeed one another in the soil, 

 the study indicated that at least one or two invertebrate animals had 

 existed during the year under every square centimeter of soil surface. 

 In mineral soils of Jutland nematodes have been found to range in 

 number from 175,000 to 20,000,000 per square meter ( Nielsen, 1949 ) . 



The numbers of microorganisms in the soil are, of course, much 

 greater and produce a profound effect on the substratum. Protozoa 

 may exist in concentrations of hundreds of thousands per gram of soil. 

 The mycelia of fungi penetrate the soil wherever suitable conditions 

 are found, and bacteria are extremely abundant almost everywhere. 

 In raw humus as many as 20,000 bacteria per gram are a common 

 occurrence. In rich loam the bacteria population may rise to 50 or 

 even 100 million cells per gram of soil (Waksman, 1932). All these 

 denizens of the soil from sizable burrowing animals to the smallest 

 microorganisms add their influence to that of the underground parts 

 of plants in modifying the substratum. 



Soil Formation. Soil is formed by the combined action of several 

 agents. First may be mentioned the process of fragmentation, the 

 mechanical breakdown of rock material into smaller pieces. The 

 process is carried forward partly by geological agents, including 

 especially the freezing and thawing of moisture in the ground. The 

 action of roots in splitting rocks is also important and represents the 

 biological part of the process. No matter how small the rock par- 

 ticles may be, however, plants cannot obtain nutriment from the ma- 

 terial until the minerals are rendered soluble. The second step in 

 soil formation, and one that goes on simultaneously, is termed cor- 

 rosion and includes the chemical processes of oxidation, reduction, 

 hydration, hydrolysis, carbonation, and others. These processes go 

 forward and soil materials go into solution under the influence of rain 



