10 SOILS OF THE TKUCKEE-C ARSON lERIGATION 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 

 contamer 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 bot Je 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 presented 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.* Ih^ating the sample in the oven at different 

 temperatures ])revious to adding the water seemed to liave no effect, 

 so the supernatant liquid was first drawn off turbid, evaporated to 

 dryness, baked at 90° to 100° C, and tlien fiUered. In all of the 



' Approximately 25 pounds to the square inch. 

 211 



