42 Microbiological Population of the Soil 



while the slide rests on the bath. The stain is then washed off with 

 water and the preparation allowed to dry. 



Various modifications of the above method have been proposed. 

 In one such modification, a measured amount of soil is suspended in 

 a molten agar gel. Small drops of the agar are removed, placed 

 in a hemacytometer slide of known depth, and allowed to solidify. 

 The films are dried and stained in a solution of acetic-aniline blue, 

 followed by dehydration in alcohol and mounting in euparal. Differ- 

 ential counts of a measured area of the film will give a quantitative 



4 * 9 9 



'^^ ^ \: 



Fig. 20. Soil examination by direct microscopic method (from Winogradsky ) . 



estimation of the microorganisms present in a given amount of soil. 

 By adding a known suspension of bacteria to sterile soil, Jones and 

 Mollison were able to recover 95.7-98.4 per cent. According to this 

 method, the numbers of bacteria per gram of soil varied from 2,275 

 to 5,420 millions. The method will also detect large amounts of 

 mycelium and spores of fungi and actinomycetes. 



Thornton mixed a soil with a known amount of indigotin; by 

 establishing the relation between the numbers of erythrosin-stained 

 bacteria and indigotin particles, he was able to measure the abun- 

 dance of microorganisms in the soil. He found that 1 gm of soil 

 contained 1-4 billions of bacteria, whereas only 2 per cent of these 

 bacteria are measured by the plate method. Detailed studies of 

 stained soil preparations have established the fact that the numbers 

 of bacteria in the soil are far greater than those that have been ob- 

 tained by the plate culture methods. 



Cholodny believed, however, that the direct staining methods 

 cannot give a complete picture of the soil population in its natural 

 habitat, because the shaking of soil with water allows the distribution 



