396 



Journal of Agricultural Research 



Vol. VI, No. II 



gm. of lead arsenate and the moisture content made up to i8 per 

 cent and then weighed. One-half of the samples thus prepared were 

 sterilized in the autoclave and all of them placed in an incubator at a 

 temperature of from 28° to 30° C. The moisture was made up weekly to 

 its initial content. Beginning at the end of 20 days, six samples, three 

 autoclaved and three not autoclaved, were used for the making of dupli- 

 cate total-nitrogen determinations. The average excess of nitrogen in 

 the unsterilized soil over that in the sterilized is given in Table IV. 



Table IV. — Quantity of nitrogen (in inilligrams) fixed in lOO gm. of soil containing 



0.0728 gm. of lead arse?taie 



Time incubated 



The greatest quantity of nitrogen was obtained at the end of 20 days. 

 During the next 10 days, however, 24.72 mgm. of combined nitrogen 

 disappeared. During the next 14 days there was a loss of only 4 mgm. 

 From this time on there was a gradual increase in the amount of com- 

 bined nitrogen found within the soil up to the end of the experiment, but 

 even after 172 days' incubation there was less nitrogen in the soil than 

 there was at the end of 20 days. 



The great loss of nitrogen can not be entirely charged up to the arsenic 

 added, for other workers (Ashby, 1907; Hoffmann and Hammer, 1910, 

 p. 164) have noted, when working with impure cultures, a loss of nitrogen 

 on prolonged incubation in the absence of arsenic. The loss is probably 

 due to the soil's becoming compact, vnth the production of anaerobic 

 conditions. This, assisted by the protozoa (Miller, 1914, p. 217), which 

 appropriate too large a share of the limited supply of oxygen in the soil, 

 prevents entirely the activity of the aerobic nitrogen-fixing organisms 

 and greatly stimulates the activity of the denitrifying organisms of the 

 soil. This can, however, only partly account for the phenomena; other- 

 wise there would be a continual decrease in the nitrogen as the soil 

 became more compact. 



The fact that aeration plays a considerable part in the reaction is 

 brought out by the following experiment, which differs from the pre- 

 ceding only in that the soil was thoroughly stirred, thus aerating it each 

 time before making up the moisture content. The results of this experi- 

 ment are given in Table V. 



