56 THE SOIL POPULATION AND ITS DISTRIBUTION 



examined to determine whether or not growth of microbes has 

 taken place. Development will be readily detected by the disin- 

 tegration of the paper in the culture, especially in contact with the 

 surface of the liquid. If the flask that has been inoculated with 

 1 5,000 of a gram of soil shows evidences of growth, and those 

 flasks or tubes to which smaller amounts of soil were added show 

 no growth, one is justified in concluding that one gram of the 

 particular soil contains at least 5,000 cells of bacteria which are 

 capable of decomposing cellulose under aerobic conditions. Certain 

 formulae have been devised which can be used for accurate deter- 

 minations of the number of specific organisms in a given quantity 

 of soil from the number of cultures giving positive and negative 

 growth. 



It may be necessary to modify the method greatly so as to 

 encourage the growth of the various specific microbes found in the 

 soil. For the development of protozoa, sterile portions of nutrient 

 meat extract agar media are placed in sterile Petri dishes; these 

 are inoculated with 1 cc. portions of various dilutions of soil and 

 incubated for at least 28 days. The surface of the agar is kept 

 moist by frequent additions of sterile water. At various inter- 

 vals, a loopful of material is removed from the surface of the agar 

 and examined under the microscope to determine the presence of 

 protozoa in general and of specific forms in particular. 



For the development of nitrogen-fixing bacteria, media are 

 employed which are free from combined organic or inorganic nitro- 

 gen, but which contain the mineral elements required for growth 

 and a source of energy, such as simple sugars or higher alcohols. 

 Thus, by modifying the nature and composition of the culture 

 media and conditions of growth, the development of various soil 

 microbes can be favored. 



In spite of the fact that all of the methods mentioned previ- 

 ously are limited in their usefulness for determining the abundance 

 of the microscopic organisms in soils, they become quite useful 

 for comparative studies of different soils when interpreted in the 

 realization of their deficiencies. 



Abundance of Bacteria in Soil. — The numbers of bacteria 

 found in different soils depend entirely upon the method employed 

 for their enumeration. When the plate method is used, the total 

 number of bacteria is found to vary from 10,000 per gram, as in the 

 case of desert sands, sand dunes, and very poor sandy soils, to 



