CARBON AND NITROGEN CYCLES IN THE SOIL 93 



nitrogen, an observation that has been confirmed by Bouillac (43 and 

 44). Sand has been found to gain nitrogen where the growth of algae 

 was possible and the proper bacteria were present.^ 



How far azotobacter is active in the soil in natural conditions has 

 not been definitely ascertained, partly because of the analytical difficulties 

 of measuring sniall gains of nitrogen, and partly because of the losses 

 of nitrogen that, as we have seen, go on in presence of organic matter. 

 The mere occurrence of azotobacter in the soil is no proof that it is 

 actually fixing nitrogen, the only satisfactory evidence would be a 

 demonstrated gain ir\ nitrogen effected by azotobacter, all other possi- 

 bilities being ruled out by the experimental conditions. The usual 

 method of investigation has been to add sugar, or other carbohydrates, 

 to the soil and measure the change in nitrogen content after various 

 intervals of time. Generally, there is a gain of nitrogen ; losses are, 

 however, often recorded (248, 151, etc.), whilst a certain loss of 

 nitrate invariably sets in (p. 99). A. Koch (151) added successive 

 small doses of dextrose to 500 grams of loam, mixed with sand and 

 spread on plates to secure copious aeration, kept uniformly moist and 

 at 20° C. Nitrogen fixation began very soon and reached its maxi- 

 mum after eighteen weeks, when losses set in ; the results are given in 

 Table XXXVIII. 



Table XXXVIII. — Nitrogen Fixed in Soil by Bacterial Action in Presence 

 OF Dextrose. Koch (151). 



For each gram of dextrose supplied in the small doses about 8 

 milligrams of nitrogen were fixed during the first eight weeks ; but only 

 4 or 5 milligrams later on. In larger doses the sugar was less effective, 

 only 5 to 6 milligrams of nitrogen being fixed per gram of sugar at 

 first and 3 milligrams later. 



Pot experiments showed that the nitrogen thus added to the soil 

 became available for plant food. Dextrose and sucrose first depressed 

 ' A. Koch has collected instances in Lafar, Tech. MykologU, Bd. iii., p. 15, 



