BIOLOGY OF THE SOIL. 81 



this discovery attracted considerable attention, and pure cul- 

 tures of the nodule bacteria were put on the market for appli- 

 cation to soils which might be deficient in them. These 

 preparations were known as nitragin and were not so suc- 

 cessful in actual practice as had been hoped. They are, I 

 believe, no longer commercially obtainable. 



Nevertheless, Hellriegel's discovery is very important and 

 affords a satisfactory explanation of many hitherto puzzling 

 facts in reference to the nitrogen question. Some investi- 

 gators have obtained results which show that plants other than 

 the leguminous ones assimilate free nitrogen, but to a much 

 less extent." With the exception of lupins, the author just 

 quoted found that all the plants he tried were developed best 

 when combined nitrogen was also supplied. In many cases 

 the amount of free nitrogen assimilated was increased if com- 

 bined nitrogen was also supplied. 



Other experimenters, e.g., Lotsy,| do not confirm the fixa- 

 tion of free nitrogen except in the case of leguminous crops. 



According to later investigations the bacteria in the nodules 

 of leguminous plants secrete an enzyme (i.e., a so-called un- 

 organised or soluble ferment) and the assimilation of the 

 atmospheric nitrogen really occurs in the leaves of the plant 

 under the influence of the enzyme. J That the fixation takes 

 place in the leaves is denied by Nobbe and Hiltner, who 

 found that if the nodules on the roots were kept under water, 

 fixation of nitrogen ceased. 



A summary of the natural , sources of the nitrogenous food of 

 plants may usefully be given here : 



(!) The ammonium compounds and nitric acid brought down 

 in the rain (see Chap. III.). 



(2) Gaseous ammonia absorbed from the atmosphere by the 



soil constituents. This is probably a comparatively 

 unimportant source. 



(3) Compounds of nitrogen (chiefly nitric acid, probably) 



formed by micro-organisms existing in the soil itself, 

 air furnishing the necessary nitrogen and oxygen. 



* Frank, J.C.S. 1892, abst, 370. t U 8 Dept. of Agric., Bulletin 18, 1894. 



+ Stoklasa, Jour. Chein. Soc. 1900, abst. ii., 610. Jour. 1 Chem. Soc. 19CO, abst ii., 234. 



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