208 SOIL CONDITIONS AND PLANT GROWTH 



while azotobacter (except where it is associated with algae, a 

 case that requires further investigation) requires a supply of 

 organic matter in the soil, and therefore works only in fairly 

 rich soils where its effects are more difficult to measure. 



Few improvements in agriculture have produced more 

 marked effects than the extension of leguminous cropping. 

 Where a leguminous crop has not hitherto been commonly 

 grown it may be necessary to introduce the appropriate 

 organism, as has been successfully done in Canada by 

 Harrison and Barlow (126), in the United States by the 

 Bureau of Plant Industry, and on the North German moors 

 by Hiltner (135 ; see also 215 c and d]. In Great Britain 

 these inoculations have not proved useful, and they have never 

 come into farming practice : the high hopes sometimes enter- 

 tained that the whole problem of nitrogenous manuring the 

 most costly item in the farmer's fertiliser bill might reduce 

 itself solely to bacterial inoculation have never been realised. 



Other Nitrogen Fixing Organisms. Other organisms have 

 been described which have the power of fixing gaseous 

 nitrogen : among them Phoma, which in culture solutions 

 proved about half as effective as azotobacter 1 ; this organism 

 belongs to the mycorrhiza group and may play a part in 

 plant nutrition. 



Denitrification. 



If the air supply of the soil is cut off by water-logging, 

 or in the laboratory by means of an air-pump, the nitrates 

 rapidly disappear, whilst nitrites, ammonia, or gaseous 

 nitrogen are formed. The conditions can be so arranged 

 that the decomposition of nitrate-bouillon by soil shall give 

 rise to notable quantities of gaseous nitrogen, nitrous oxide 

 (18), or nitric oxide (168 and 277^). 



The reduction of nitrates to nitrite has long been known. 

 As early as 1867 Schonbein 2 stated that it could be brought 



1 B. M. Duggar and Davis, Annals Missouri Botanical Gardens, 1916, 3, 



4 I 3-437- 



2 C. F. Schonbein, Beitrage zur Physiologischen Chemie, Zeitsch.f. Biologic, 

 1867, 3, 325-340- 



