SOIL MICRO-ORGANISMS 



155 



occurs well mixed with sand and clay. It adds to the soil various 

 organic substances, some useful and some harmful, enables the 

 soil to retain more moisture, makes the soil light, and makes the 

 soil a suitable place for micro-organisms to live. 



Soil Micro-organisms. — The soil is the home of innumerable 

 organisms, some plants and some animals, all of which are related 

 to soil fertility. They are of three kinds. Fungi, Bacteria, and 

 Protozoa. These organisms, influencing the soil fertility and 

 having their activities in turn influenced by the soil conditions, 

 add to the complexity of soils, which 

 are still far from being understood. 



Many Molds occur in the soil, where 

 their thread-like filaments, like those 

 of Bread Mold, aid in breaking up the 

 organic matter into soluble compounds. 

 Besides Molds, there are other kinds 

 of Fungi which act on the soil con- 

 stituents, as in case of Toadstools 

 which invade the soil with their root- 

 like filaments. Furthermore, some 

 Fungi are so intimately connected with 

 the roots of some plants as to replace 

 the root hairs. In this case the Fun- 

 gus weaves around the root a close 

 covering of filaments, thus forming 

 with the root the structure, known as 

 the mycorrkiza, in which the filaments 

 of the Fungus absorb water and solu- 

 ble substances, which afterward are 

 transmitted to the root. {Fig. 137.) Pines, Beeches, Oaks, Blue- 

 berries, and Orchids are some of the more familiar plants in 

 which the mycorhizas occur. Plants, like Blueberries, are so 

 dependent upon Mycorhizas that they can not be grown unless 

 the proper Fungus is present in the soil. 



The Bacteria, the smallest of all living organisms and well 

 known in connection with diseases of animals and plants, are ex- 

 ceedingly abundant in soils, often many millions being present 

 per cubic centimeter of soil. Like the Fungi, the Bacteria of the 

 soil are dependent in their development upon the presence of 

 some organic food material, such as decaying plant or animal 



Fig. 137. — A Mycorrhizaon 

 a rootlet of the Beech. The 

 felt-like mass of mycelial 

 threads closely enwraps the 

 root tip, extending back to 

 beyond the hair zone and 

 spreading into the soil like 

 root hairs. After Frank. 



