104 Influence of Plant Residues on Nitrogen Fixation 



contradistinction to the first crop this difference was evident from the 

 earhest stages. The results of these experiments are briefly given in 

 Tables 11 and V, and the dry matter production is diagrammaticaily 

 represented in Figs. 2 and 3. 



As the result of treatment with putrefactive organisms alone, no 

 definite increase in the nitrogen of the sand and crop is evident, but where 

 Azotobacter was additionally present the nitrogen content of the sand was 

 distinctly increased. After deducting the amount of nitrogen supplied in 

 the hay (148 mgrm.) there still remains a surplus of about 120 mgrm. 



Similarly, the amount of nitrogen in the first and second crops was 

 increased from 49 mgrm. to 73 mgrm. and from 21 to 56 mgrm. re- 

 spectively. Taking all the gains into account one finds that by the 

 utilisation of the hay dust as a source of energy, a nett increase of 

 180 mgrm. occurred. When bacteria were not added but must have 

 been eventually introduced by air infection the crop nitrogen was 

 reduced from 49 to 26 mgrm. in the first crop, and only rose by 8 mgrm. 

 in the following rye crop. 



On tiie whole one must conclude either that the nitrogen supplied in 

 the hay dust was extremely badly utilised or that some degeneration 

 process resulting in the withdrawal or loss of nitrogen took place during 

 the initial period of growth. Since the pots with Azotobacter also passed 

 through this stage of depression it is probable that a more accurate gauge 

 of the intensity of assimilation would be obtained by a comparison 

 between the set with Azotobacter and that without any bacteria added. 



If the fact be taken into consideration that such gains occurred within 

 six to seven months from the time of treatment, the degree of utilisation 

 may be regarded as fairly satisfactory. 



Ulilisation of Sugar for Nitrogen Fixation. 



In order to allow of some comparison of the value of plaut residues 

 and that of some directly available source of energy, two further sets of 

 pot experiments were carried out in which sugar was applied to the sand 

 at the rate of 0-2 per cent. The general preliminary treatment was the 

 same as in the preceding sets — potassium phosphate and calcium car- 

 bonate being supplied. After being sterilised, the sand belonging to one 

 set of pots was maintained in a sterile condition, but the other portion 

 was inoculated with a pure culture of Azotobacter. Later treatment, 

 sowing, harvesting, etc. was the same as with the hay dust sets of pots. 



The initial depressing effect of the treatment with sugar on subse- 

 quent growth was even more pronounced than with hay dust, and by 



