THE SOIL AND THE MICROBE 



CHAPTER I 

 THE SOIL AND THE PLANT 



The Nature of the Soil. — The upper layer of the earth's 

 surface, varying in thickness from 6 to 18 inches in the case of 

 some humid soils and up to 10 or 20 feet in the case of arid soils, 

 possesses certain characteristic properties which distinguish it 

 from the underlying rocks and rock ingredients. This very thin 

 surface layer of the earth's pellicle is spoken of as the soil. It is 

 distinguished from the lower layers by its mechanical, physical, 

 and chemical properties, but especially by the presence of hving 

 organisms including a variety of microbes, lower animals, and 

 roots of plants. Dead bodies of these organisms also occur in the 

 soil in all stages of decomposition. The science of the soil is 

 frequently spoken of as Pedology. 



The type of soil that has developed upon the underlying rock 

 is a result of climate and the organic life upon it or within it, 

 including the action of higher plants, animals, and microorgan- 

 isms. The soil is arranged in a series of characteristic layers or 

 horizons, which make up the soil profile, which is a direct result 

 of the conditions under which it has been developed. A soil 

 profile is obtained by making a vertical cut through the soil, 

 showing its various horizons (Fig. 1). The upper horizon is more 

 or less dark colored on account of the presence of organic matter 

 in different stages of decomposition. The color of the soil may 

 become darker or hghter with depth, depending on the accessi- 

 bility of air and movement of water through the profile, the 

 penetration of roots, and the activity of microorganisms. 



The soil is characterized morphologically by the texture, 

 structure, color, and chemical composition of the various horizons. 

 These horizons are designated by letters: A, usually at the sur- 



