314 ANNUAL REPORT OP THE Off. Doc. 



©econdi 100 c. c. flask, and filled to the mark with sterile water, 

 shaken for one minute, and 1 c. c. of the dilution transferred to a tube 

 of melted gelatin (5 c. c. in each tube), the latter gently rocked, and 

 the contents poured into a Petri dish. The gelatin is made to solidify 

 rapidly on a cold plate, and then placed in a cold water incubator 

 for four days. At the end of this time the number of colonies on 

 tke plate are counted. 



The result of the analyses are expressed in number of bacteria per 

 gram of dry soil, hence it becomes necessary to know the percentage 

 of moisture in the sample. Fop this a given weight of soil from the 

 same tube is taken, and dried for three hours at 100° C, and from this 

 the percentage of dry matter in the sample is calculated. 



It is evident that in the mixing of the soil with water in the two 100 

 c. c. flasks there is in the 1 c, c. taken for the plate culture a one 

 ten-thousandth dilution of the original quantity, hence the number 

 of colonies on the gelatin plate must be multiplied by 10,000 to get 

 the true number in the quantity of moist soil taken. To calculate 

 the number per dry gram of soil the following formula is used: 



in which N equals the number of bacteria per gram of dry soil, N' 

 the number of colonies on the gelatin plate, P the percentage of dry 

 matter in the sample, D the dilution of the soil sample (in most cases 

 iOjOOOj, and W the weight of the moist soil taken for bacteriological 

 analysis 



V. THE NUMBER AND DISTRIBUTION OF SOIL BACTERIA. 



In the superficial portion of ordinary cultivated soil the number 

 of bacteria varies from several hundred thousand to several millions 

 per gram of dry soil. The following list will show the range of vari- 

 ation as observed by different authors: 



1. Park Montsouri, Paris (Miquel. 1879),2 700,000 



2. Sandy soil (Adametz, 1880),=^ 300,000 



3. Clay soil, 500,000 



4. Orchard, Potsdam (Fraenkel, 1887),* 31,000 to 218,000 



5. Soil of grain fields (Caron, 1895),^ 937,000 to 1,600,000 



6. Pear orchard, Del. Expt. Sta., ground under high 



state of cultivation (Chester, 1901),« 2,200,000 



7. Land in permanent grass for over 12 years, New- 



ark, Del. (Chester, 1901),^ 425,000 



8. Land in grass for four years, Newark, Del. (Ches- 



ter, 1901),« 425,000 



