TRANSACTIONS OF SUB-SECTION B. 583 



The study of the behaviour of these organisms under controlled conditions 

 of artificial culture, approximating as far as possible to the natural conditions, 

 indicates a solution of the problem in the near future. 



In some work on the subject quantitative determinations of the nitrogen fixed 

 have been the measure of advance, while in others the formation of bacteroids, 

 or of slime, or even the power of infecting plants, has been taken to indicate ah 

 approach to natural conditions. 



A new method described and demonstrated for the first time indicates that 

 it is the reaction of the medium which plays an important role in nitrogen- 

 fixation. The cultures also disclose previously unobserved properties of the 

 nodule organism. 



Previous work is summarised, indicating that it is not only the acidity of the 

 root sap, but also the removal of the products of growth, the supply of carbo- 

 hydrate, and the slime production which must be regulated before artificial 

 cultures of the organism can be expected to fix nitrogen to an extent comparable 

 with that which takes place in the nodule. The ready adaptation of the organism 

 to its environment must also be borne in mind. 



MONDAY, SEPTEMBER 5. 



Joint Meeting with Section D. 

 The following Paper was read : — 



The Part flayed by Micro-organisms other than Bacteria in determining 

 Soil Fertility. By E. J. Kussell, D.Sc, and H. B. Hutchinson, Ph.D. 



Partial sterilisation of soil by heat or treatment with volatile antiseptics such 

 as toluene leads to a notable increase in productiveness. The authors find that 

 shortly after the treatment has ceased there is a great increase in the rate at 

 which; plant food is formed by bacteria and in the rate of multiplication of the 

 bacteria. This increased activity is not brought about by any heightened vigour 

 in the bacterial stock ; on the contrary, it is shown that these organisms are 

 actually weakened by the treatment. It follows, then, that the environment has 

 been improved. 



When some of the original untreated soil or an aqueous extract of the soil is 

 added to the partially sterilised soil there is at first a still greater increase in the 

 bacterial activity. This has been traced to the addition of the more vigorouo 

 organisms of the untreated soil. Later on there is evidence that a detrimental 

 effect is produced where soil was added, but not from the extract. It appears, 

 then, that the untreated soil contained some injurious factor not washed out by 

 water, which only slowly makes itself felt when introduced into a clean soil. This 

 factor is put out of action by any poisonous organic vapour by heating to 52° and 

 similar means. 



It is difficult to account for the experimental results in any other way except 

 by supposing that soil contains organisms capable of checking bacterial develop- 

 ment. Such organisms must be larger than bacteria and not readily detached from 

 the soil since they do not appear, or not to any extent, in an aqueous extract. 

 Search for large organisms has revealed the presence in every soil so far examined 

 of amoebae or amoeboid organisms, of colopoda and other protozoa. It is shown 

 that the mixed culture growing in hay infusion is capable of destroying bacteria. 

 It has not yet, however, been possible to ascertain definitely whether these protozoa 

 are active in the soil, although the conditions obtaining — an atmosphere saturated 

 with water vapour and a film of water round the particles of soil — would seem to 

 us favourable to their development. 



