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- a agnilet af the Besa. “ihe 
gus weaves around the root a close feittike mass of mycelial 
covering of filaments, thus forming threads closely enwraps the 
with the root the structure, known as root tip, extending back to 
the mycorrhiza, in which the filaments beyond the hair zone and 
spreading into the soil like 
of the Fungus absorb water and solu- sob hain Abice Peale. 
ble substances, which afterward are 
transmitted to the root. (Fig. 187.) 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 
Fic.137.—A Mycorrhizaon 
