THE NITROGEN CYCLE 457 



an arable soil is kept saturated with water to the exclu- 

 sion of air, nitrates added to the soil are decomposed, 

 with the evolution of nitrogen gas. As lack of drainage 

 is usually most pronounced in early spring, when the soil 

 is likely to be depleted of nitrates, it is not likely that 

 much loss arises in this way unless a nitrate fertilizer has 

 been added. Among the many difficulties arising from 

 poor drainage, denitrification of an expensive fertilizer 

 may be a very considerable item. 



The addition of a nitrate fertilizer to a well-drained soil 

 receiving stable manure is not likely to result in a loss of ni- 

 trates unless the dressings of manure have been extremely 

 heavy. Hall * states that at Rothamsted, where large quan- 

 tities of nitrate of soda are used every year in connection 

 with annual dressings of farm manure, the nitrate produces 

 nearly as large an increase when added to the manured 

 as when added to the unmanured plat. In other words, 

 there appears to be no loss of nitrate by denitrification. 



It is possible to reach a point in manuring at which 

 denitrification may take place. Market-gardeners some- 

 times reach this point, when fifty tons or more of farm 

 manure, in addition to a nitrate fertilizer, are added to 

 the soil. Plowing under heavy crops of green manure 

 may produce the same result. In either case, the best 

 way to overcome the difficulty is to allow the organic 

 matter to partly decompose before adding the fertilizer. 

 The removal of the easily decomposable carbohydrates 

 needed by the denitrifying organisms decreases or pre- 

 cludes their activity. 



377. Nitrogen fixation through symbiosis with higher 

 plants. — It has long been recognized by farmers that 



1 Hall, A. D. The Book of the Rothamsted Experiments, 

 pp. 114-115. New York, 1905. 



