THE STUDY OF MICROORGANISMS IN THE SOIL 97 



Examine under high-dry and oil-immersion lenses. Make 

 drawings. 



METHODS FOR COUNTING NUMBERS OF MICROORGANISMS 



Directions for Drawing Soil Samples. — Samples from the sur- 

 face to 1 foot deep may be taken as follows: Remove the coarse 

 surface debris and sink a metal cylinder or soil sampler to the 

 desired depth. Samples of surface soil may be taken with a 

 sterile spatula. Draw several samples and empty into sterilized 

 paper bags or other vessels. Mix and pulverize the sample. This 

 may be done with a sterile spatula upon a large piece of sterile 

 paper. From the well-mixed sample remove a representative 

 portion for dilution, and at the same time make a moisture 

 determination. 



When it is necessary to secure accurate samples from various 

 depths, it is well to dig a ditch to the desired depth. By means 

 of a sterile trowel, representative samples may be drawn from 

 the sides of the ditch. In this way outside contamination is 

 largely prevented. 



In order to reduce the error common to determinations of this 

 character, it is well to use a large sample of soil. Balances 

 sensitive to 10 milligrams are satisfactory for this work. 



Exercise 3 



NUMBEK OF AlG^ IN SoiL ACCORDING TO DILUTION METHOD 



Add 50 grams of soil to 500 cubic centimeters of sterile 

 water and shake vigorously for 5 minutes. 



After the coarse particles have settled, dilute as follows: 



(a) Add 10 cubic centimeters of soil suspension to 90 cubic 

 centimeters of sterile water, giving a dilution of 1 : 100. 



(5) Add 10 cubic centimeters of dilution (a) to 90 cubic centi- 

 meters of sterile water, giving a dilution of 1:1,000. If the soil 

 is rich it is well to increase the dilutions. 



Inoculate one flask of algal medium (Medium 42 and 43) with 

 1 cubic centimeter of each of the above dilutions. 



The cultures should be kept near a window. Note when green 

 color appears, usually 30 to 90 days. Examine in wet mounts. 



Calculate the number of algae in 1 gram of soil. 



