196 SOIL CONDITIONS AND PLANT GROWTH 



compounds could absorb free nitrogen under the influence of 

 silent electric discharges, and at first attributed the natural 

 recuperation to this cause. He also examined the possibility 

 of bacterial action, as micro-organisms at that time were play- 

 ing a large part in French science under Pasteur's influence. 

 Accordingly he exposed sterilised and unsterilised sands and 

 clays poor in nitrogen ('Oi per cent, or less) to air in large 

 closed flasks for five months, and found distinct gains in 

 nitrogen in the unsterilised, 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 Hellreigel and Wilfarth the key to the clover 

 problem (p. 25), and led Winogradsky (312) to search for the 

 actual organism. 



No investigator of our subject has shown greatering enuity 

 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 assimi- 

 lating gaseous nitrogen could therefore develop, 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 in- 

 vestigation, and as unfavourable 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, o*i per cent. KgHPO^, 0*05 of 

 MgSO^ and traces of NaCl, FeSO^, and MnSO^, together with 

 a little soil. Under aerobic conditions nitrogen was assimi- 

 lated and the sugar was decomposed with evolution of carbon 

 dioxide and hydrogen and formation of n-butyric and acetic 

 acids in the proportion of three or four molecules of the 

 former to one molecule of the latter, the two acids together 



