10 SOILS OF THE TRUCKEE-CARSON IRRIGATION PROJECT. 
NITRIFICATION. 
Samples of soil were collected with the precautions previously 
described. In some cases 1-gram portions for counts of total num- 
bers of bacteria were removed from the bottle of soil and the remainder 
of the sample used for nitrification studies. 
Because of the great variation in the fertilty of different fields it 
was considered necessary to determine at what depths the nitrifying 
bacteria existed; therefore, instead of emptying the soil from the 
container and allowing it to dry, thus exposing it to some contamina- 
tion, one-half of the soil, approximately 50 grams, was removed with 
a sterile spatula and used for “‘original”’ determinations. Five cubic 
centimeters of 0.4 per cent ammonium sulphate was then added to the 
portion remaining in the botvle and the sample placed in the incu- 
bator at 28°C. With the original moisture of the soil this additional 
5 cubic centimeters frequently made the water content of the soil 
somewhat above optimum, but owing to the rapid evaporation in an 
arid climate this rapidly decreased and was adjusted as nearly as 
possible in subsequent waterings. All samples were weighed at 3-day 
intervals, and as any appeared to fall below optimum the required 
quantity of sterile distilled water was added to restore them. The 
incubation period was two weeks, the temperature being maintained 
at 28°C. 
The chemical work ord no little difficulty. The analytical 
determinations may be considered in two phases: (1) The prepara- 
tion of the aqueous extract of the soil both before and after incuba- 
tion with ammonium sulphate and (2) the determination of nitrites 
and nitrates in original and incubated samples. 
In the preparation of the aqueous extract considerable difficulty 
was experienced. All of the soils used contained variable and fre- 
quently quite large proportions of very fine clay, which would not 
settle out and leave a clear supernatant liquid, even on prolonged 
standing. It was thought advisable to determine the chlorids and 
sulphates in the original samples; therefore the common salts con- 
taining these radicals could not be used to flocculate the clay, 
although this method was sometimes used in the examination of the 
samples after incubation where only nitrites and nitrates were deter- 
mined. Pressure-pump facilities were inadequate for the large num- 
ber of samples used, the more so as the fine clay particles clogged the 
porcelain filter and caused filtration to be extremely slow with the 
low pressure available.t Heating the sample in the oven at different 
temperatures previous to adding the water seemed to have no effect, 
so the supernatant liquid was first drawn off turbid, evaporated to 
dryness, beleeL at 90° to 100° C., and then filtered. In all of the 
: pores 25 pounds to the square inch. 
