FOREWORD 



The soil is not a mass of dead debris, resulting simply from the 

 physical and chemical weathering of rocks and of plant and 

 animal remains through atmospheric agencies, but it is teeming 

 with life. Every small particle of soil contains numerous types 

 of living organisms belonging both to the plant and animal king- 

 doms, yet so small that they cannot be recognized with the naked 

 eye. These organisms are, therefore, called microbes, micro- 

 organisms or microscopic organisms. These microbes comprise 

 numerous types of bacteria, fungi, algae, protozoa, nematodes and 

 other invertebrates which vary considerably in their structure, 

 size, mode of living and relationship to soil processes. 



In the cycles of transformation of elements in nature, the 

 microbes play an important, if not a leading, role. Were it not 

 for them, the soil would soon become covered with a considerable 

 mass of undecomposed plant and animal residues; life would soon 

 cease, since the very Hmited supply of carbon and available 

 nitrogen, the most essential elements in the growth of hving 

 organisms, would become exhausted. It should be recalled that 

 carbon dioxide, the source of carbon for the growth of plants, 

 which in their turn supply the food for animals, is present in the 

 atmosphere only in a concentration of 0.03 per cent. This is 

 equivalent to 5.84 tons of carbon over each acre of land. A good 

 yield of sugar-cane will consume about 20 tons of carbon in a 

 single growing season; of course most of the surface of the earth 

 supports less vegetation than this, and diffusion tends to create 

 a uniform distribution of gases. It has actually been calculated 

 that the plant world consumes 64.8 millions of tons of carbon 

 annually, which amounts to 1,'35 of the total carbon content of 

 the atmosphere. The atmospheric supply of carbon dioxide is, 

 however, constantly replenished from the decomposition products 

 of the organic substances in the soil; only as a result of this does 

 plant growth not cease entirely through a deficiency of an available 

 supply of carbon. In the absence of microbes the available 

 nitrogen would also become very rapidly exhausted, as can be 



