METHODS FOR STUDYING ACTIVITIES OF SOIL MICROBES 49 



known importance to plant growth, such as the elements, nitrogen, 

 carbon, or sulfur, or with a single change in a group of transfor- 

 mations in which a certain element or compound is involved In 

 the decomposition of cellulose in the soil, for example, either 

 the disappearance of cellulose, the formation of intermediary 

 substances such as organic acids, or the formation of an end prod- 

 uct of the reaction, such as carbon dioxide, can be used as an index 

 of the microbial action. In studying the degradation of a protein 

 in the soil, the disappearance of the protein, the formation of 

 amino acids, the accumulation of ammonia, and frequently even 

 the transformation of ammonia into nitrate have served as meas- 

 ures of transformation. Many different groups of organisms are 

 concerned in such reactions, and the value of any single measure- 

 ment depends upon the object of the study. 



Cellulose and proteins are only two groups of chemical com- 

 plexes contained in the organic matter which is added to the soil 

 by the growing plants, either in the form of residues of the aerial 

 portions of the plant, such as leaves, needles, stems, and fruiting 

 parts, or in the form of the subterranean portions, namely, the 

 roots. The quantity of organic substances reaching the soil is 

 also increased through the introduction of stable manures, 

 green manures, various animal and plant residues, and such arti- 

 ficial organic preparations as urea and cyanamid. In the decom- 

 position of the organic complexes, various organisms are con- 

 cerned which in their development bring about numerous processes 

 of oxidation and reduction, decomposition and synthesis, which are 

 of extreme importance to soil fertility. 



To measure the activities of all these organisms, just as 

 to determine their total abundance, is beyond the scope of 

 any single method or group of methods. The nearest approach 

 to the ideal method is that which allows the determination 

 of an end product of the sum-total of the activities of a large 

 fraction of the soil microbes. One usually studies only those 

 transformations which appear to be of greatest economic impor- 

 tance to man, who utilizes these activities for the growth of his 

 crops and the nutrition of his animals. However, microorganisms 

 participate in a great many processes which take place in the soil 

 of which little or nothing is known. With the advance of our 

 knowledge of the soil microbes and their activities, the relation- 

 ships of soil microorganisms to the transformation of soil constitu- 



