608 



TRANSACTIONS OP SUB-SECTION K. 



w;is thought that sufficient care had not been taken that the whole of the carbo- 

 hydrate had been used up in the culture solution. 



To meet this criticism the experiments have been repeated with duplicate 

 flasks, containing glucose as the carbohydrate, and the determinations of nitro- 

 gen were not made until every trace of carbohydrate had disappeared from the 

 solutions. Cultures were made with different races of bacteria, one set of experi- 

 ments having £ grm. glucose per 100 c.c., the other set having 1 grm. Thirty- 

 eight separate nitrogen determinations were made, and in every series the 

 mixed cultures gave a greater increase of nitrogen fixation per unit of carbo- 

 hydrate than when the organisms were grown separately. It was also found in 

 every series the mixed cultures used up the carbohydrate more rapidly than 

 the pure organisms. The following are the averages of the nitrogen deter- 

 minations :— 



£ grm. glucose. 

 038 



Control . . . , 



Pseudomonas 



Azotobacter 



Pseudomonas -j- Azotobacter 



1-51 (6 days) 

 2-32 (6 days) 

 3-31 (5 days) 



1 grm. glucose. 



0-47 



248 (10 days) 



3-21 (10 days) 



4-76 (8£ days) 



7. Some Effects of Bacteriotoxins on Soil Organisms. 

 By Professor W. B. Bottomley, M.A. 



If well-rotted manure or a fertile soil be treated with a 0-9 per cent, salt 

 solution (100 grm. of manure or soil to 500 c.c. of solution), the liquid extracted 

 obtained by means of a Pukall filter has a marked effect on the growth of pure 

 cultures of certain soil organisms. The denitrifying bacteria thrive well in such 

 an extract, whilst the growth of the nitrogen fixers Azotobacter and Pseudomonas 

 is inhibited. Boiling the extract for an hour destroys the toxic effect of the 

 extract. 



Erlenmeyer flasks of 300 c.c. capacity filled with Giltay's solution, diluted 

 with (a) 100 c.c. distilled water, (b) 100 c.c. manure extract, (<■) 100 c.c. boiled 

 manure extract, each inoculated with 5 c.c. of a pure culture Bacillus denitrificans 

 showed a displacement of liquid after two days' incubation at 30° C. of — 

 (a) 80 c.c, {b) 100 c.c, (c) 50 c.c. With soil extract the figures were 25, 42, and 

 30 respectively. In agar tube preparations the proportions of colonies in a 

 square inch were, manure («) 20, (b) 65, (c) 16; soil (a) 24, (b) 76, (c) 26. 



The inhibiting, effect of the extract on nitrogen fixing organisms was seen by 

 rrowing them on nutrient agar plates containing 50 per cent, of the extract. 



These effects of manure and soil extracts on soil organisms may have some 

 significance as regards the beneficial effect of heating soils. 



