90 SOIL CONDITIONS AND PLANT GROWTH 



The oxidising bacteria are usually credited with the change, but no 

 organism has yet been isolated capable of bringing it about. Nothing 

 whatever is known about the mechanism of the process. No experi- 

 ments appear to have been made with pure substances, or pure cultures 

 of organisms, but only with the highly complex mixtures present in soil 

 or dung. Further work is very desirable. 



The Fixation of Nitrogen. 



The first systematic search for a recuperative agency to make good 

 the losses of nitrogen from the soil was started thirty years ago by 

 Berthelot. He found that certain organic compounds could absorb 

 free nitrogen under the influence of silent electric discharges, and at 

 first attributed the natural recuperation to this cause. He also ex- 

 amined the possibility of bacterial action, as micro-organisms at that 

 time were playing a large part in French science under Pasteur's 

 influence. Accordingly he exposed sterilised and unsterilised sands 

 and clays poor in nitrogen (-01 per cent, or less) to air in large closed 

 flasks for five months, and found distinct gains in nitrogen in the un- 

 sterilised, but not in the sterilised soils. Fixation is, therefore, not due 

 to any external physical cause which would operate equally in both cases, 

 but to micro-organisms (26). This research was at once fruitful of results 

 because it gave Hellriegel and Wilfarth the key to the clover problem 

 (p. 1 6), and led Winogradsky (313) to search for the actual organism. 



No investigator of our subject has shown greater ingenuity than 

 Winogradsky in devising methods at once simple, direct and effective. 

 In looking for the nitrogen-fixing organisms he inoculated soil into a 

 medium containing every nutrient except nitrogen compounds : only 

 bacteria capable of assimilating gaseous nitrogen could therefore de- 

 velop, and these had a clear field. But he further recognised that the 

 process was endothermic and required some source of energy, hence 

 he added sugar to the solution. The method (known as the elective 

 method) thus consists in making the conditions as favourable as 

 possible for the group of organisms under investigation, and as un- 

 favourable as possible for all others ; it has proved extremely valuable 

 in the subsequent development of soil bacteriology. 



Winogradsky's solution contained 2 to 4 per cent, dextrose, a little 

 freshly washed chalk, cri per cent. K 2 HPO 4 , 0*05 of MgSO 4 and traces 

 of NaCl, FeSO 4 and MnSO 4 , together with a little soil. Under 

 aerobic conditions nitrogen was assimilated and the sugar was decom- 

 posed with evolution of carbon dioxide and hydrogen and formation of 



