60 A MANUAL OF BACTEEIOLOGY 



a depth of four to six feet bacteria are rarely found except in. 

 places where local drainage currents carry them downward. 

 Various factors, such as temperature and moisture, influence the 

 number and activities of the soil organisms, but it is probable 

 that they are active during the greater part of the year. 



Again, the bacteria of the soil are of various species, and no 

 one species has uncontended sway in the field. Those which are 

 best suited to the environment thrive ; those which are not so 

 well suited are crowded out and diminish in numbers or die. 



In other words, the soil bacteria, like other organisms, have a 

 definite problem of existence which must be solved in the main 

 under the conditions which they find in arable soils. At times 

 one species of bacteria predominates in numbers and activity; 

 at other times, under changed conditions, other species predom- 

 inate. When the bacteria which set up certain activities pre- 

 dominate, the productiveness of the soil is enhanced ; when other 

 classes predominate, the reverse is true. 



An enumeration of the number of bacteria in soil does not 

 mean much in itself. However, if soil conditions are sufficiently 

 well controlled, a quantitative determination may shed light 

 on the effects of various factors upon microorganic life in soil. 

 Only comparative results can be obtained, even under the best 

 conditions, since many soil organisms do not grow upon the 

 media available for counting bacterial colonies. 



Exercise 87. Obtaining Soil Samples 



1. Sterilize in the dry oven several trowels or steel spatulas, 

 previously wrapped in Manila paper. Also sterilize three wide- 

 mouthed, glass-stoppered bottles. 



2. In the field scrape off the top layer of soil ; then, using a 

 fresh sterile spatula, loosen about 10 cm. of the soil, stir it thor- 

 oughly, and transfer 25-50 g. to a sterile bottle. The surface soil 

 varies widely in bacterial content, owing to seasonal extremes 

 of moisture, the mhibiting action of light, accidental contamina- 

 tion, and other causes. 



3. Carry the samples to the laboratory within an hour after 

 collecting them. 



