Aug. 1909.] Soil Bacteriology. 225 



SANDY SOIL, PLOT A. 



Humus 3.25 per cent. 



Vplatiles 5.86 



Fine gravel 00 



Coarse sand 28 



Medium sand 38 



Fine sand 3.72 



Very fine sand 64.80 



Silt 17.18 



Clay 4.53 



Total 100.00 per cent. 



Number and Manner of Taking Samples. From each half 

 of the subplots which were plowed on March 18, and from each 

 half of the subplots which were plowed on March 18 then re- 

 plowed on June 20, six samples for bacterial determination 

 were taken every fourteenth day. Six samples with duplicates 

 for volumetric bacterial determination were taken from check 

 plot Al and the same number from check plot Bl. 



QUANTITATIVE BACTERIOLOGICAL ANALYSIS. 



One of the difficult problems in soil bacteriological technique 

 is that relating to the proper method of taking and measuring 

 samples for quantitative determination. It is the general cus- 

 tom of bacteriologists to compare the number of bacteria per 

 gram of dry soil. This is sometimes called the gravimetric or 

 gram method. 



In the present experimental work the volumetric method has 

 been used. One cubic centimeter of soil was placed in a sterile 

 test-tube to which was added 19 cc. of sterile distilled water. 

 This was shaken for five minutes, and from each tube, by means 

 of a sterile platinum loop, which was constructed and gradu- 

 ated to hold one cubic millimeter of water, approximately one 

 cubic millimeter of the soil suspension was transferred into 

 10 cc. of sterile liquefied agar. The inoculated medium was 

 then poured into a sterile Petri dish. This method is un- 

 doubtedly inaccurate, as it is difficult to measure such sub- 

 stances as soil in terms of cubic centimeters. However, for the 

 purpose of securing comparative results, as in the present 

 work, either method may be used. In other words, the com- 

 parative number of bacteria in soil which has been plowed two, 

 four, six, eight, ten and twelve inches deep may be found by 

 either the volumetric or gravimetric method and expressed as 

 so many bacteria per cubic centimeter or per gram. The final 

 results would appear to be correct so long as one or the other 



