May 28, 1917 
Influence of Crop , Season , Heater on Soil Bacteria 305 
of chlorids in the soil solution was not sufficient to affect the sensitiveness 
of the method (55). 
The number of organisms was determined by growing them on a 
modified synthetic agar having the following composition: 
1,000 c. c. of distilled water. 
10 gm. of dextrose. 
0.5 gm. of dipotassium phosphate (K 2 HP 0 4 ). 
0.2 gm. of magnesium sulphate (MgSOJ. 
2 gm. of powdered agar per 100 c. c. of media. 
After the samples of soil had been carefully mixed by shaking, 100 gm. 
were weighed on a sterile watch glass, using a small sterile spatula. 
This soil was transferred to 200 c. c. of sterile water and shaken for one 
minute, 1 c. c. of this suspension transferred to 99 c. c. of sterile water, 
and the dilution continued with 9 c. c. of sterile water. The plates were 
made so as to give a dilution of 1 to 2o;ooo and 1 to 200,000. They 
were incubated at 28° C. for four days and then counted. No attempt 
was made to differentiate between bacteria and molds, but all were listed 
together as total numbers of colonies. 
The ammonifying power of the soil was determined by weighing 100- 
gm. portions of the soil and 2 gm. of dried blood into sterile tumblers and 
covering them with petri dishes. The dried blood was thoroughly mixed 
with the soil by means of a sterile spatula and the water content made up 
to 18 per cent with sterile water. The samples were incubated at. 28° to 
30° C. for four days and the ammonia determined by transferring to 
Kjeldahl flasks with 250 c. c. of distilled water, adding 2 gm. of mag¬ 
nesium oxid and distilling into N/10 sulphuric acid. The determinations 
were all made in duplicates and compared with sterile blanks. 
The nitrifying power of the soils was determined in tumblers like the 
ammonifying power, except that they were incubated for 21 days. The 
moisture content was made up weekly to the initial 18 per cent. 
At the end of the incubation period each soil was transferred with 250 
c. c. of distilled water to a i-pint Mason fruit jar. Two gm. of powdered 
lime were added and the jar placed in the shaking machine for 10 minutes, 
after which it stood in the closed jar until clear. This never required 
over two hours. At the end of this time an aliquot part (100 c. c.) was 
measured into a flask and the nitrates determined by the aluminum 
reduction method (4). 
The soil on which the experiments were conducted is extremely fertile, 
as is shown by the fact that the soil has been cropped for 48 years without 
the addition of barnyard manure or commercial fertilizers, yet during 
the last 4 years it has yielded fair crops. This is shown in Table IV, 
which gives the average yearly yield in pounds per acre. From these 
yields and the average percentage of nitrogen in the crops under similar 
irrigated conditions (64) the average amount of nitrogen removed per 
year has been calculated. 
82983°—17 - 3 
