INFLUENCE OF ORGANIC MATTER UPON SOIL MICROBES 205 



Frequently the decomposition does not proceed very far, but 

 stops at an intermediary stage, as in the case of the so-called raw- 

 humus in certain forest soils. The acid nature of the organic matter 

 (pH 4.0 and even lower), and the fact that this organic matter is 

 not appreciably mixed with the inorganic portion of the soil but 

 remains on its surface, are responsible for its accumulation. 

 These conditions favor an abundant development of fungi, many 

 of which belong to the Basidiomycetes (mushroom fungi). The 

 absence of extensive development of bacteria and actinomyces and 

 the relative absence of an animal population further favors the 

 persistence of the organic matter which is known as raw humus. 

 Under these conditions the essential plant nutrients either remain 

 in the undecomposed plant residues or become removed from 

 circulation by the fungi which assimilate these elements and build 

 them up into their own cell substance. The trees can obtain some 

 of these nutrients only through the agency of certain fungi (mycor- 

 rhiza) which grow in association with the roots and serve as root 

 hairs. Many of these mycorrhiza fungi are able to decompose the 

 resistant organic matter of the forest soil and make the nutrients 

 contained therein available to the growth of the forest vegetation. 

 This association is symbiotic in nature and appears to be of benefit 

 to both the tree and the fungus (see Chapter VIII). 



In certain other forest conditions an entirely different series of 

 changes takes place. When the fresh plant residues which are 

 first attacked by the fungus population are further acted upon by 

 the numerous soil bacteria and invertebrate animals, the so-called 

 mull type of soil develops. This is considered to be a richer soil, 

 since the nutrients are liberated from it much more readily by 

 processes of decomposition. This type of organic residue or humus 

 is formed only when the soil is well supphed with lime, and when 

 the organic substances are well mixed with the inorganic particles 

 of the soil. Under these conditions the organic matter has a 

 reaction of pH 5.0 to 7.0, and its thorough mixture with the soil is 

 brought about largely through the agency of invertebrate animals 

 which find these more nearly neutral soils a much more favorable 

 habitat for their development than the acid raw-humus soils. 

 In the latter, the nitrifying bacteria are absent and the nitrogen 

 does not become changed beyond the form of ammonia, but in the 

 mull type of soil, conditions are favorable to active nitrification 

 and the ammonia is rapidly changed to nitrate. One need hardly 



