BACTERIA OF OLD SOILS 



107 



deeper. The other soils free from excessive alkah salts, Nos. 

 6, 7, 8, and 9, possess a flora of twice the number present in 

 surface adobe soil No. 3. These figures suggest either fewer 

 numbers originally in the clay soils, or that a flora of less resisting 

 power existed in them, or that the loam soils offered a more 

 suitable medium when dried. As the original number of organ- 

 isms had not been determined and since so few soils were used, 

 no definite conclusions as to the determinant factors involved can 

 be stated. 



As indicated by Rahn's work, the greatest decrease in numbers 

 occurs during the earliest periods of the drying, and subsequent 

 drying has little effect, so it is very possible that further drying 

 of the soils used in this experiment would have caused little or 

 no decrease in the numbers found. In short, the number of 

 organisms in a soil is quickly reduced to a constant minimum by 

 drying. Though little of a decisive nature can be deduced from 

 the results, yet it is a striking observation in soil biology that 

 even after 30 years of drying the average count of seven normal 

 soils is 358,000 organisms per gram. 



AMMONIFICATION 



The ammonifying power of a soil, though not dependent on 

 the number of organisms present, performs, as is well known, 

 an important function in the supplying of available nitrogen to 

 plants, and accordingly it was deemed essential for a complete 

 knowledge of the existing flora that the ammonifying power of 

 the soils be determined. Owing to the limited amount of soil 

 at my disposal, the determination was carried on as follows: 

 50 cc. of a 1 % tap water solution of peptone, distributed in 

 Ehrlenmeyer flasks and sterilized, was inoculated with 5 grams 

 of soil. The cultures were incubated three days and the ammonia 

 then formed was distilled off with magnesium oxide and received 

 in ^ HCl. The excess HCl acid was then titrated with ^ NHiOH. 

 DupUcates and sterile controls were run for each soil. The 

 results appear in table 2. 



" Science 35: no. 893, p. 227. 1912. 



