368 



SCIENCE 



[N. S. Vol. XXXII. No. 820 



vegetation, but the humus is the home of 

 the bacteria which have so much to do 

 with fertility. 



The discovery of nitrification was only 

 the first step in the elucidation of many 

 actions in the soil depending upon bac- 

 teria — for example, the fixation of nitro- 

 gen itself. A supply of combined nitrogen 

 in some form or other is absolutely indis- 

 pensable to plants and, in their turn, to 

 animals; yet, though we live in contact 

 with a vast reservoir of free nitrogen gas 

 in the shape of the atmosphere, until com- 

 paratively recently we knew of no natural 

 process except the lightning flash which 

 would bring such nitrogen into combina- 

 tion. Plants take combined nitrogen from 

 the soil, and either give it back again or 

 pass it on to animals. The process, how- 

 ever, is only a cyclic one, and neither 

 plants nor animals are able to bring in 

 fresh material into the account. As the 

 world must have started with all its nitro- 

 gen in the form of gas it was difficult to 

 see how the initial stock of combined nitro- 

 gen could have arisen; for that reason 

 many of the earlier investigators labored 

 to demonstrate that plants themselves were 

 capable of fixing and bringing into com- 

 bination the free gas in the atmosphere. 

 In this demonstration they failed, though 

 they brought to light a number of facts 

 which were impossible to explain and only 

 became cleared up when, in 1886, Hell- 

 reigel and Wilfarth showed that certain 

 bacteria, which exist upon the roots of 

 leguminous plants, like clover and beans, 

 are capable of drawing nitrogen from the 

 atmosphere. Thus they not only feed the 

 plant on which they live, but they actually 

 enrich the soil for future crops by the 

 nitrogen they leave behind in the roots and 

 stubble of the leguminous crop. Long be- 

 fore this discovery experience had taught 

 farmers the very special value of these 



leguminous crops; the Roman farmer was 

 well aware of their enriching action, which 

 is enshrined in the well-known words in 

 the Georgics beginning, "Aut ibi flava 

 seres," where Virgil says that the wheat 

 grows best where before the bean, the 

 slender vetch, or the bitter lupin had been 

 most luxuriant. Since the discovery of the 

 nitrogen-fixing organisms associated with 

 leguminous plants other species have been 

 found resident in the soil which are ca- 

 pable of gathering combined nitrogen 

 without the assistance of any host plant, 

 provided only they are supplied with car- 

 bonaceous material as a source of energy 

 whereby to efi'ect the combination of the 

 nitrogen. To one of these organisms we 

 may with some confidence attribute the ac- 

 cumulation of the vast stores of combined 

 nitrogen contained in the black virgin 

 soils of places like Manitoba and the Rus- 

 sian steppes. At Rothamsted we have 

 found that the plot on the permanent 

 wheat field which never receives any 

 manure has been losing nitrogen at a rate 

 which almost exactly represents the dif- 

 ferences between the annual removal of 

 the crop and the receipts of combined 

 nitrogen in the rain. We can further 

 postulate only a very small fixation of 

 nitrogen to balance the other comparatively 

 small losses in the drainage water or in the 

 weeds that are removed; but on a neigh- 

 boring plot which has been left waste for 

 the last quarter of a century, so that the 

 annual vegetation of grass and other herb- 

 age falls back to the soil, there has been an 

 accumulation of nitrogen representing the 

 annual fixation of nearly a hundred 

 pounds per acre. The fixation has been 

 possible by the azotobacter on this plot, 

 because there alone does the soil receive a 

 supply of carbohydrate, by the combus- 

 tion in which the azotobacter obtained the 

 energy necessary to bring the nitrogen 



