48 THE SOIL POPULATION AND ITS DISTRIBUTION 



another gram taken a few feet or even a few inches distant from the 

 first. Results obtained from studies of the influences of environ- 

 mental factors on the abundance of microorganisms in field soils 

 should therefore be interpreted with an appreciation of the natural 

 variation in the normal distribution of these organisms in the soil. 



Methods for Determining Numbers of Microorganisms 

 IN THE Soil. — There is no one method available at the present 

 time which can measure the total abundance and activities of soil 

 microorganisms. The various methods commonly employed are 

 adapted to the enumeration of only a certain few groups of organ- 

 isms commonly found in the soil or to the measurement of one 

 product of the development of some microbes. The organisms 

 which are counted make up only a part, frequently a very small 

 one, of the total soil population, and the product measured is 

 but one of many. The methods used for counting soil organisms 

 are based either upon the observation of microorganisms in stained 

 preparations of soil, or upon their development in solid or liquid 

 culture media which arc specially prepared for the cultivation of 

 these organisms. In either case only a part of the total soil popu- 

 lation is measured. In the case of stained preparations, some of 

 the organisms are either destroyed in the process of preparing the 

 specimen, as in the case of the protozoa, or are not visible at all, or 

 are found in clumps, so that the individual cells cannot be counted. 

 In the case of those methods which are based upon the develop- 

 ment of microbes in culture media, the greatest difficulty is experi- 

 enced in finding any single medium which will permit the growth 

 of a large fraction of the soil inhabitants. 



Each medium is more or less selective as far as the growth of 

 certain organisms is concerned, but some media permit the devel- 

 opment of fewer forms than others. Some media are purposely 

 made favorable for the development of only those microbes that 

 are capable of bringing about a specific transformation. Such 

 media can be used for the determination of the abundance of 

 specific physiological groups of organisms which play known roles 

 in certain important soil processes. 



Methods for Studying Activities of Soil Microbes. — 

 For the determination of the activities of soil organisms, the 

 methods commonly employed are based upon a knowledge of those 

 soil processes which are considered to be essential to soil fertility. 

 These may deal with the transformation of a single element of 



