Methods of Stiidving the Soil Population 43 



of the organisms in a manner not comparable with their existence 

 in the natural soil itself. An especially distorted picture is thus ob- 

 tained of the actinomycetes in the soil. 



Contact Slide Method. Rossi and Cholodny proposed a method, 

 designated as "contact slide," "soil plate," or "surface growth plate." 

 This method consists in making a slit in the soil with a sharp knife 

 and inserting into the slit a clean cover slide. The soil is then pressed 

 gently to bring it in contact with the slide, which is left in position 

 for 1-3 weeks. The slide gradually becomes covered with some of 

 the soil solution and with soil particles. When the slide is removed 

 from the soil, it is cleaned on one side with a cloth to remove the 

 soil particles and allowed to dry in the air. The preparation is fixed 

 by passing the slide over a flame; it is then washed gently in tap 

 water to remove the coarse soil particles, followed by distilled water, 

 and stained with phenol erythrosin for 30 minutes, at room tempera- 

 ture. The slide is finally washed, dried, and examined under the 

 microscope. 



The microscopic population observed on the contact slide may not 

 be exactly the one that may be found in the soil at a given moment, 

 since it results from the development of specific organisms on the 

 slide in contact with the soil. In experiments on decomposition of 

 organic matter in soil, Jensen found that the results obtained by the 

 direct method agreed with those secured by the plate method; hence 

 the two methods were believed to be able to compensate each other. 



In their physical relation to the soil, the microorganisms are found 

 chiefly upon the solid soil particles; only a small number of organ- 

 isms are found in the liquid phase, chiefly because of the adsorption 

 of the organisms by the soil particles. Thus the mechanical compo- 

 sition of the soil, its chemical nature, especially its reaction, and the 

 nature of the base in the adsorbing complex are all significant in 

 determining the degree of adsorption of soil organisms. 



Conn found that an increase in the moisture content of the soil 

 results in a rapid change from a natural flora of fungi and actinomy- 

 cetes to one in which bacteria predominate. In a natural soil and 

 under normal conditions of wetting and drying, filamentous organ- 

 isms are active, especially in acid soils high in colloidal material. 

 With an increase in the moisture content of such a soil, due to ex- 

 cessive rainfall, bacteria became more active; such increased activity 

 is even greater than that following addition of lime. 



The contact slide method was found to offer great possibilities 

 not only for the quantitative evaluation of the soil microscopic popu- 



