3i6 SOIL MICRO-ORGANISMS 



2. By determining the changes in the nitrogen content. When the organic materials are 

 rich in nitrogen, their decomposition will be accomplished by a rapid evolution of ammonia, 

 which is a waste product in the metabolism of micro-organisms. This ammonia will be rapidly 

 changed, in well-aerated soils, to nitrates. The determination of the formation and accumu- 

 lation of ammonia and nitrate nitrogen can serve as an index of decomposition of the organic 

 matter. However, with a low nitrogen content, especially when the organic matter contains 

 I per cent or less of nitrogen, the decomposition processes are always accompanied by the 

 assimilation of the nitrogen which has been made available from the decomposition of the 

 soil organic matter, or of the nitrogen which is present in the artificial medium. This nitrogen 

 is required by the micro-organisms for the synthesis of their cell substance. The lower the 

 nitrogen content of the decomposing organic matter, the greater will be the need for addi- 

 tional nitrogen to enable the organisms to decompose the organic matter added. There is a 

 very definite relation between the organic matter which is decomposed and the nitrogen (as 

 well as phosphorus) which is required by the organisms for the synthesis of their cell sub- 

 stance. 



3. The most accurate method of following the decomposition of organic matter by soil 

 micro-organisms is the complete analysis of this organic matter at the beginning and at the 

 end of the decomposition process. This method requires considerable time and a very careful 

 technique. It involves the determination of the various constituents of the organic matter, 

 especially the celluloses, pentosans, lignins, proteins, starches, sugars, fats, and waxes. 



The decomposition of natural organic matter added to the soil results in the for- 

 mation of soil organic matter or "humus." This so-called "humus" consists of certain 

 constituents of the natural organic matter added to the soil and which resist decom- 

 position, namely, the lignins, the cutins, the fats and waxes, the tannins; of certain 

 synthesized substances, namely, the living and dead cells of the soil micro-organisms; 

 and of a number of substances which are still undergoing decomposition. When one 

 hundred parts of organic matter are composted in the manure heap or are allowed to 

 decompose in soil, there will be left, after two to six months, depending on the acidity, 

 moisture, temperature, and nature of micro-organisms active in the process, nature 

 of organic matter, etc., forty parts of organic matter. The sixty parts that have de- 

 composed comprise practically all the pentosans, celluloses, and proteins. The forty 

 parts remaining comprise largely the lignins and other resistant constituents and syn- 

 thesized cell substance. The residual organic matter will tend to be uniform, as far 

 as the content of nitrogen and phosphorus is concerned. The organic matter of the 

 soil is more or less constant in composition, containing carbon and nitrogen in a 

 definite ratio which approaches 10: i. The exact nature of this organic matter is still a 

 matter of dispute. 



TRANSFORMATION OF NITROGEN IN SOIL 



When proteins are acted upon by micro-organisms, they are first broken down to 

 amino acids, and these are sooner or later decomposed with the liberation of a part of 

 the nitrogen as ammonia. A part of the nitrogen will be used by the organisms for 

 the synthesis of their own cell substance, giving rise again to proteins and other com- 

 plex nitrogen compounds. Since the carbonaceous substances are used by the or- 

 ganisms as sources of energy, and since the assimilation of nitrogen depends on the 

 amount of energy available, the liberation of nitrogen as ammonia will depend on the 

 ratio of carbon to nitrogen in the compound or substance which is decomposed by the 



