696 PRINCIPLES OF SOIL MICROBIOLOGY 



The solubility of the organic matter and the ease with which it can decompose 

 decrease with an advance of this process. The further advanced the process 

 of peat formation, the more resistant the materials become to the attack of 

 microorganisms. When peat soils are brought under cultivation, they are at 

 first drained, and if lime is lacking (high-peat or low-lime soils) they are limed, 

 so as to produce favorable conditions for the development of the actinomyces 

 and certain bacteria which are able to decompose the "humus" complexes and 

 liberate the nitrogen in an available form. Since the constant washing of these 

 soils results in the removal of the soluble minerals, the decomposition processes 

 and especially the growth of higher plants are usually favored by the addition 

 of phosphates and potassium salts. Inoculation of a newly drained (and limed 

 if necessary) peat soil with a rich garden soil is essential, so as to introduce the 

 organisms active in the processes of decomposition of organic matter and nitrate 

 formation. 



Factors influencing the different processes of transformation of organic matter 

 in the soil are (1) temperature (low temperature favors peat formation; thorough 

 aeration and high soil temperature favor fungus development); (2) physical 

 and chemical soil conditions (excessive moisture favors peat formation, low 

 moisture and soil acidity with proper aeration and temperature favor fungus 

 development, high lime content favors the processes of decomposition by neu- 

 tralizing the acids formed by the fungi); (3) abundance of spores, and (4) the 

 nature of plant products (thick needles are decomposed less readily than loose 

 leaves, old needles are decomposed with greater difficulty than young ones). 



The various forms of "humus formation" are not distinct but may be 

 followed one by another, depending on the soil conditions. Forest 

 trees also thrive well in soils in which the process of decomposition is 

 incomplete, due to the interaction of mycorrhiza fungi, usually the ecto- 

 trophic types. 75a 



Chemistry and classification of humus compounds. While some in- 

 vestigators consider all the soil organic matter as "humus," others 

 designate by that name only that part of the soil organic matter which 

 is soluble in a solution of an alkali. Still others designate by "humus" 

 only that part of the organic matter which is precipitated from the alka- 

 line solution on acidification; this substance, insoluble in acids and solu- 

 ble in alkalies, was first regarded as an acid ("humic acid"). 



The decomposition of plant residues under aerobic or anaerobic conditions 

 was considered to yield different groups of substances, to which different names 

 and formulae were assigned: 



7Ba A detailed study of the nature of organic matter and nitrate formation in 

 forest soils has been recently made by Hesselman, H. Studien fiber die Humus- 

 decke des Nadelwaldes, ihre Eigenschaften und deren Abhlingigkeit vom Wald- 

 bau. Meddel. Statens Skagsfors., 22: No. 5. 1926. 



