10(3 Infiuence of Plant Residues on Nitrogen Fixation 



pots, and in their absence the breakdown of the nitrogen comijounds 

 stored up in the Azotobacter cells woidd therefore proceed but slowly. 



As a result of inoculation with Azotobacter the nitrogen in the two 

 crops was increased from 34 mgrm. (without inoculation) to 94 mgrm. 

 with Azotobacter: the total nitrogen in *sand and crop showed still 

 greater differences, being 460 mgrm. in the control pots as against 534 

 and 893 mgrm. in the treated pots. (Table III.) 



Further experiments consisted in the application of sugar to soil in 

 pots similar to those used in the above experiments. Since Azotobacter 

 is normally present in these soils it was not necessary to add this 

 organism nor was it considered desirable to introduce other factors by 

 subjecting the soil to heat prior to the application of the sugar, as was 

 done in the case of the sand cultures. One set of pots was retained as 

 control, and of the two sets treated with sugar, one was stored at a 

 temperature of 5-10° C. whilst the other was stored at between 15-30° C. 



The influence of temperature on nitrogen fixation has been noted in 

 various laboratory experiments and the general result was obtained that 

 when the temperature falls to the region of 5° fixation comes to a stand- 

 still, whilst it is most active between 18-31°. This is well borne out by 

 these pot experiments, the plants of both the first and second crops grown 

 in soils receiving sugar and incubated at 30° being not only heavier but 

 also richer in nitrogen than those in the other two sets. The general 

 results suffice to show the importance of supplying the source of energy 

 when the soil temperature is favourable lor the growth of Azotobacter 

 and thus confirm those obtained in the field experiments discussed in 

 the first part of the paper. (Table IV.) 



Table IY. Utilisation of Sugar for Nitrogen Fixation (Soil Series). 



The rapiditv with which the assimilated nitrogen becomes available 

 for plant growth appears to differ with the medium employed. In the 

 case of sand treated with sugar (or hay dust) little effect was apparent in 

 the first crop but was marked in the second crop; with soil, the first crop 

 was benefited most and the second crop was shghtly below the control. 



