[vanderleck] bacteria OF FROZEN SOILS 19 



Methods. 



The methods employed were slightly different from the usual 

 ones. Most of the time the ground was frozen solidly so that an 

 auger could not be used. Therefore by means of pick and crowbar 

 holes of about 2 feet square were dug until the frost line was reached 

 and then samples of about 2 lbs. were taken from the walls of the 

 pit. The first sample was taken on a depth of 4 inches and subsequent 

 samples at a depth of 8'^ 12", 16" etc. The frozen soil was collected in 

 large chips or flakes, this being a guarantee that only soil of the 

 proper level was sampled. The chips were not thawed, but crushed 

 and the stones sifted out with a 2 mm. sieve. For water determination 

 3 samples of 50 gr. each were taken; for bacteriological determination 

 one gram of the frozen power was put in 100 c.c. sterile water and shaken 

 for 5 minutes. After this shaking further dilutions were made and 

 one cubic centimetre of the proper dilution was added to each plate. 

 All samples were plated in triplicate on beef peptone agar, beef peptone 

 gelatine, soil extract agar and soil extract gelatine. It was found 

 advantageous to use one dilution for each complete set of 12 plates. 

 Previous work enabled the writer to judge with sufficient accuracy 

 the numbers of bacteria present in the different soil samples. The 

 averaging of three counts of the same dilution was a very simple matter 

 and the results otbained were much more accurate than when three 

 different dilutions were used; the first one usually 5 times greater 

 than the second, and the second again twice the last dilution. 



The beef peptone agar and beef peptone gelatine were prepared 

 according to the formulae given by the A.P.H.A. The soil extract 

 media were prepared following the directions given by H. J. Conn. 

 The plates were incubated for seven days at a temperature of 15° C. 

 before counting. Besides, a number of biochemical tests were carried 

 out to determine the powers of ammonification, nitrification and 

 dentrification of the difïerent soil samples and also of the individual 

 species. 



Weather. 



The weather conditions throughout the course of the experi- 

 ments have been tabulated, and snow and rainfall have been recorded 

 on the temperature charts. The charts give the daily mean temper- 

 ature for the winters 1915-16 and 1916-17. During the former year 

 it thawed on four different occasions before the 1st of February, 

 accompanied by heavy rainfall, followed by a cold spell lasting until 

 the last week in March. On account of the rain the frost penetrated 

 very easily and the subsequent heavy snow covering prevented any 

 premature thawing, so that the soil was still frozen solidly on April 

 4th. 



