100 Influence of Plant Residues on Nitrogen Fixation 



for nitrogen assimilation is quite simple, but in the case of plant residues 

 some of the effects of the treatment may be due to the nitrogenous com- 

 pounds contained in them apai-t from any nitrogen assimilated by the 

 action of soil organisms. 



Since it is impossible to make any accurate allowance for the nitrogen 

 originally contained in the residues and which becomes slowly available 

 for plant growth, the experiment resolves itself into a comparison be- 

 tween the effects of a mixed bacterial flora without nitrogen fixing 

 organisms, and such a flora with the latter organisms present. It 

 becomes necessary, therefore, to start with a sterile medium and for this 

 purpose sand, and not soil, must be chosen. 



The work was carried out in sixteen glazed earthenware pots, each 

 containing 3,500 grms. of clean sand to which 2-0 per cent, of calcium 

 carbonate, 0-1 grm. potassium phosphate, and 0-04 grm. magnesium 

 sulphate had been added. To each of twelve of the lots of sand an 

 addition of 5-0 grms. of finely ground hay was made, and after thorough 

 admixture each portion was transferred to a large narrow necked bottle 

 with a cotton wool plug. All the bottles were then heated to 95° for one 

 hour, and after the sand had cooled down the bottles were divided into 

 four lots and treated as follows: 



I. Sand alone. Heated and then reinoculated. 



II. Sand with hay du,st. Heated but not inoculated. 



III. Sand and hay dust. Heated and then inoculated with putre- 

 factive organisms. 



IV. Sand and hay dust. Heated and then inoculated with putre- 

 factive organisms and Azotobacter. 



Providing secondary effects did not come into play, the difference 

 between sets I and II might, be attributed to the nitrogen compounds 

 liberated from the hay by heat: that between sets II and III would be 

 due to the putrefactive organisms added; anv advantage derived from 

 the presence of Azotobacter would be expressed by the difference between 

 sets III and IV. 



The putrefactive organisms referred to above were obtained by trans- 

 ference of the colonies occurring on .six nutrient agar plates which had 

 been inoculated with a suspension of garden soil, and to this mixed 

 culture a few drops of a sub-culture of nitrifpng organisms in mineral 

 salt .solution were added. The term "putrefactive" is employed here in 

 a very broad sense, the main object being to secure a mixed and effective 

 bacterial flora not containing Azotobacter. One cubic centimetre of this 

 suspension was applied to the sand of sets I, III and IV, but set IV 



