Methods of Stuching the Soil Popuhition 41 



lite in the soil, find their food there, and eventually leave their bodies 

 to become a part of the soil mass, just as the roots of the higher 

 plants penetiate and ramify throughout the whole of its surface area, 

 and leave their remains to decompose in the soil. In the many 

 changes that take place in the life cycle of the various microorgan- 

 isms in the soil, a definite equilibrium has become established be- 

 tween the various groups. This equilibrium is not stable, however, 

 but undergoes many changes as a result of the treatment that the 

 soil undergoes. 



The methods most commonly used for the enumeration of micro- 

 organisms found in the soil are commonly divided into several 

 groups: 



I. Microscopic methods. 



1. Staining of soil and diuxt niicroscoi)ic examination. 



2. Contact slide method. 



3. Direct examination of unstained .soil. 

 II. Culture methods. 



1. Plate culture methods. 



2. Electi\e cultiu-e methods. 



3. Soil enrichment methods. 



The numbers and types of organisms vary considerably, depending 

 upon the method. Each one of these methods has its advantages 

 and limitations. Some of the methods may have to be further modi- 

 fied in the study of a specific problem, or for the purpose of estab- 

 lishing the presence or abundance of a particular type of organism 

 under a particular set of conditions. 



Microscopic Examination of Stained Soil Preparations. This 

 method consists in the preparation of a suspension of soil in a dilute 

 fixative solution; one or two drops of the suspension is spread upon 

 a clean slide, which is then dried and stained with an acid dye, and 

 finally examined with a high-magnification microscope. The fixa- 

 tive solution is prepared by dissolving 0.15 gm of gelatin in 1 liter 

 of distilled water. The staining solution consists of 1 gm erythrosin 

 or rose-bengal dissolved in 100 ml of a 5 per cent aqueous solution of 

 phenol, containing sufficient CaClo (0.001-0.1 per cent) to give a 

 very faint precipitate of the calcium salt of the dye. 



The process of staining is carried out by placing a loopful of 

 the soil suspended in the fixative solution upon a glass slide and 

 spreading out with a needle until it covers a known area. The smear 

 is allowed to dry over a boiling water bath. A drop of the staining 

 solution is added to the smear and allowed to remain for 1 minute, 



