1058 ; A. F. Vass 
represented the bottom as well as the top of the soil, the increase could no 
have been due to the drawing-up of the organisms from the lower depths, 
as suggested by Harder. 
EFFECT OF TIME, TEMPERATURE, AND RATE OF THAW ON 
BACTERIA COUNT 
IN CLAY SOILS . 
The effect of the time of freezing and the rate of thaw on the number 
of bacteria in frozen soil, and on the bacteria in the dilutions from the 
soil, is shown in table 4. In the test to determine the latter, samples were 
taken from the flasks containing the last dilution—the one that was 
plated out — and were frozen for different lengths of time. 
TABLE 4. Errsct or Time, TEMPERATURE, AND Rate ¢ or THAW ON THE 
BAcTERIA Count IN CLAY SOILS 
Bacteria, 
Sample Treatment per gram of 
dry soil 
Mel Ser cae Bie ake evar Normal soil, water blank at 30° C............2...: 7,500 ,000 
Pais Ne Mee eu) SUE AES Normalisoil) water-blankiat® Om Ci. .3.3 eee 6,800 ,000 
ee teens exe teeneh Unrate Frozen 2 hours at —15° C., water added at O° C....; 5,600,000 
AMR Urs ticena peer ey Frozen 2 hours at —15° C., water added at 15° C....| 7,100,000 
Ey ache ec hae me eR eters ge Frozen 2 hours at —15° C., water added at 25° C....| 8,200,000 
(Gyaeteio MLS al aeao Dae EY Frozen 2 hours at —15° C., water added at 40° C....} 5,800,000 
gate ieee mage trea Dilution of sample 1, frozen 1 hour....,........... 6,400 , 000 
SARE en Ate Cerne Nena Dilution of sample 1, frozen 12 hours.............. 4,300,000 
ORGY r gare BEY ny Dilution of sample 1, stood 12 hours............... 5,700,000 
OR ee ane emp are ten hy Frozen in liquid air at -190° C. 1 minute.......... 8,000,000 
Teiipeecsse Nenu RS aaa Aisi Frozen in liquid air at —190° C. 30 minutes......... 7,100,000 
DES OC et RMs AN Frozen in liquid air at -190° C. 6 hours........... 3,400,000 
The temperature of the water used in making the dilution seemed to 
have a marked effect on the bacteria in the frozen soil and a slight effect 
on the bacteria in the unfrozen soil. These results seem to indicate that 
the present method of making plate counts should be standardized, and 
that the temperature of the water blanks used is one of the factors that 
must be controlled if the results are to be comparable. 
When the soil was frozen in liquid air at a temperature of —190° C. 
for one minute, there was a noticeable increase in the bacteria count 
which could not be accounted for by actual growth and multiplication. 
