October 13, 1905.] 



SCIENCE. 



453 



which came next, three years after the 

 clover, showed a decided superiority on the 

 clover land. Thus a clover crop, itself 

 wholly removed from the land, exercised a 

 marked influence for good on at least the 

 three succeeding crops grown under the 

 ordinary conditions of farming. Next we 

 can make a comparison between red clover 

 and lucerne. On some of the Rothamsted 

 plots various leguminous plants have been 

 grown for many years, with indifferent 

 success indeed, because of the well-known 

 reluctance of the land to support such 

 crops except at intervals of four or more 

 years. Eventually the plots on which 

 these indifferent crops had been secured 

 were ploughed up and sown with wheat 

 without any manure. In five years the 

 wheat was thus growTi on the residues left 

 in the soil by the j^if^vious leguminous 

 crops, and from the table will be seen the 

 comparative value of these residues in the 

 ease of lucerne and red clover. 



As we have previously seen how great 

 the benefit of a single year's growth of 

 red clover may be on the succeeding crops, 

 an idea can be formed from the comparison 



in the latter table of how much more 

 lucerne may contribute towards building 

 up a fertile soil; a point which was very 

 markedly brought out in the experiments 

 of the late Mr. James Mason. 



The question of the fixation of atmos- 

 pheric nitrogen by bacterial agencies does 

 not, however, end with the organisms liv- 

 ing symbiotically on the leguminous plants, 

 for several other organisms have latterly 

 been discovered which possess the power 

 of fixing nitrogen independently, provided 

 they are supplied with the necessary nutri- 

 ment. Of late attention has been chiefly 

 directed to a conspicuous organism known 

 as Azotohacter chroococcum, which may be 

 readily identified in most cultivated soils. 

 The impure cultures (which may be 

 quickly obtained by introducing a trace of 

 soil into a medium containing no nitrogen, 

 but a little phosphate and other nutrient 

 salts, together with one or two per cent, of 

 mannite or other carbohydrate) fix nitro- 

 gen with considerable activity ; in one case, 

 for example, when working with a Rotham- 

 sted soil, as much as 19 mg. of nitrogen 

 were fixed for each gram of mannite 

 employed and partially oxidized. But 

 Beyerinck, the discoverer of the organism, 

 now attributes the nitrogen fixation to cer- 

 tain other organisms which live practically 

 in symbiosis with the AsotoTjacter, and 

 which are present in the impure cultures 

 just referred to. The exact source of the 

 nitrogen fixation may be left a little doubt- 

 ful; still the main fact remains that from 

 the bacteria present in many soils one or a 

 group may be found capable of effecting 



