212 AIDS TO BACTERIOLOGY 



Success has not always followed the use of pure cultures 

 of the nodule bacteria. Bottomley's preparation of Azo- 

 tobacter is in the form of a powder. It remains virulent 

 for months, and has proved highly successful when 

 intelligently used. Bottomley has shown that by asso- 

 ciating together Azotobacter smdPseudomonas a very much 

 greater quantity of nitrogen is collected from the air, 

 and fixed in a form available as a plant food, than can be 

 fixed by them acting independently. He treated moss 

 litter with the nitrogen-fixing bacteria and other soil 

 organisms, and incubated for three weeks. Ammonia 

 was formed, which combined with the humic acid present, 

 making the whole alkaline and producing a large quantity 

 of soluble humates available for plant growth. The peat 

 was then sterilised by heat, and a pure culture of Bacillus 

 radicicola and the Azotobacter chroococcum added. After 

 a further period of incubation, the peat contained weight 

 for weight about fifteen to twenty times as much humates 

 as were present in two-year-old rotted farmyard manure. 

 With soil rich in nitrogen, bacterised peat is of little use, 

 but on poor soils experimental croppings after its use 

 have proved highly satisfactory. Professor Bottomley 

 describes substances which he calls auximones, which are 

 comparable to vitamines, in the treated peat. 



The small amounts of iron and silicates in humus have 

 been shown to assist Azotobacter chroococcum, which 

 explains the favourable action basic slag has on the 

 organism. Bottomley says a good medium for both 

 Azotobacter and Pseudomonas, or for a mixed culture of the 

 two, may be obtained by adding to distilled water 1 per 

 cent, of dextrin, 0-2 per cent, of dipotassium phosphate, 

 0-02 per cent, of magnesium sulphate, and 0-4 per cent, 

 of basic slag. 



A mild activation of the air by pitchblende has been 

 found to increase the amount of nitrogen fixed by Azoto- 

 bacter. Brilliantly successful results have sometimes 

 followed the addition of soil from a place where the 

 nitrogen bacteria were abundant to an area where develop- 

 ment of plants was slow. 



Nitrification. The conversion of nitrogenous substances, 

 as found in dejecta, cadavers, etc., into nitrates (the form 

 in which plants absorb nitrogen), takes place in three 

 stages, each being produced by bacteria, (a) Ammonisation: 



