312 



SOIL MICRO-ORGANISMS 



decomposing organisms is determined by adding i-cc. portions of several of the final dilutions 

 to flasks of urea medium; cellulose-decomposing organisms in media containing cellulose as 

 the only source of energy; nitrogen-fixing organisms in media containing no combined 

 organic or inorganic nitrogen, but containing a source of energy, such as glucose or mannitol; 

 protozoa on media favorable for the development of bacteria and protozoa, etc. If i cc. of a 

 dilution of i : 100,000, or i gm. of soil diluted with 100,000 cc. of water, gives no growth, 

 while a dilution of i : 10,000 gives growth, the number of specific organisms per i gm. of 

 soil is estimated to be between 10,000 and 100,000. On repeated determination, using nar- 

 rower dilutions, closer results can be obtained. This method is very convenient not only for 

 determining the abundance of specific organisms, but also for their isolation from soil, and 

 separation from other forms. By selecting carefully a series of dilutions and using two plates 

 with agar medium or two tubes or flasks with liquid medium for each dilution, incubating 

 for seven to twenty-eight days, then determining the number of positive and negative 

 growths, a more or less accurate picture of the abundance of specific protozoa in the soil is 

 obtained. 



TABLE I 



Numbers of Micro-organisms in Soil, by Direct Microscopic Method 



DISTRIBUTION OF MICRO-ORGANISMS IN SOIL 



The numbers of various micro-organisms found in a definite quantity of soil de- 

 pend upon the nature of the soil, depth of soil, season of year, state of cultivation, and 

 a number of other factors. By the use of the direct microscopic method of Wino- 

 gradsky, Richter found the numbers shown in Table I in i-gm. portions of different 

 soils. 



By the use of the plate method, the number of bacteria found in the soil is con- 

 siderably less, due to the various limitations of the method. The numbers vary from 

 two or three hundred thousand to a hundred million cells per gram of soil. The or- 

 ganisms developing on the plate consist of actinomyces (10-40 per cent of the col- 

 onies), non-spore-forming bacteria (50-80 per cent of the colonies), and spore-forming 

 bacteria (3-10 per cent of the colonies). 



The numbers of fungi found in the soil by the plate method range from 10,000 to 

 1,000,000 per gram. Acid soils as well as aerated soils rich in organic matter have the 

 highest number of fungi. The most important limitation of the plate method in this 

 connection is the lack of differentiation between the numbers of spores and vegetative 

 mycelium found in the soil. Yeasts are rare in soil, occurring somewhat more abun- 

 dantly in acid soils, in vineyards, and in orchards. 



