268 Microbes and You 



ganic substances. A loose-textured soil will permit the entrance 

 of atmospheric gases which contain about 20% oxygen, but as active 

 metabolism continues in an organic-rich soil, carbon dioxide and 

 other gases produced during microbial metabolism tend to displace 

 the dissolved oxygen, sometimes reducing the latter to a sur- 

 prisingly low concentration. Organic matter helps the soil to re- 

 tain moisture, and it aids soil fertility in many direct ways. The 

 inter-dependence of these various chemical and physical factors 

 makes the soil far from a static medium; in fact, the biological 

 activities in the soil put a three ring circus to shame. Out of this 

 cauldron emerge the foods we depend upon for our survival. 



What is meant by the terms clay, silt, sand, and gravel? These 

 are arbitrary designations based upon the particle size of the mate- 

 rial, and they may be tabulated as follows : 



Clay 0. 005 mm. and smaller 



Silt 0.005 to 0.05 mm. 



Very fine sand . 05 to . 10 mm. 



Fine sand . 10 to . 25 mm. 



Medium sand 0.25 to 0.50 mm. 



Coarse sand . 50 to 1 . 00 mm. 



Fine gravel 1.00 to 2.00 mm. 



Surrounding these particles is a moisture layer, appearing as a 

 film, which transports in solution the minerals which are dissolved 

 from the inorganic soil constituents and the carbon dioxide and 

 other substances produced from the decomposition of organic 

 matter. 



Growing plants obtain their required nutrients by absorbing 

 them from this solution via the root hairs which penetrate between 

 the soil particles. Some of the soil elements are only slightly 

 soluble, and as the plants absorb these dissolved elements, more 

 of the chemical goes into solution and becomes available to higher 

 plants. 



SOIL ORGANISMS 



The microbial population of the soil is made up essentially of 

 bacteria, yeasts, molds, algae, and protozoa, and these organisms 

 are found principally in the upper lavers of the earth's crust grow- 



