On the Sources of the Nitrogen of Vegetation, 111 



Although the evidence is c'ear that the nitrogen of raw clay sub- 

 soils, which constitutes an enormous store of already combined 

 nitrogen, is susceptible of nitrification, provided the organisms are 

 present and the supply of oxygen is sufficient, the data at command 

 do not indicate that these conditions could be adequately available in 

 such cases as those of the very large accumulations of nitrogen by 

 the Medicago sativa for a number of years in succession, or by the 

 red clover on the bean -exhausted land. 



The question arose — whether roots, by virtue of their acid sap, might 

 not, either directly take up, or at any rate attack and liberate for 

 further change, the otherwise insoluble organic nitrogen of the sub- 

 soil. Accordingly, in the autumn of 1885 specimens of the deep, 

 strong, fleshy root of the Medicago sativa were collected and 

 examined, when it was found that the sap was very strongly acid. 

 The degree of acidity was determined, and attempts were made so to 

 free the extract from nitrogenous bodies as to render it available for 

 determining whether or not it would attack and take up the nitrogen 

 of the raw clay subsoil. Hitherto, however, these attempts have been 

 unsuccessful. 



Also in the autumn of 1885, when this difficulty first arose, it was 

 decided, in the mean time, to examine the action on soils and subsoils 

 of various organic acids, in solutions of a degree of acidity either 

 approximately the same as that of the lucerne root-juice, or having a 

 known relation to it. The acids used were the malic, citric, tartaric, 

 oxalic, acetic, and formic. 



It was found that the weak organic acid solutions did take up some 

 nitrogen from the raw clay subsoil, and more from the poor lucerne 

 surface-soil. But when solutions of only approximately the acidity 

 of the root-sap were agitated with an amount of soil which it was 

 thought would be sufficient to yield so much nitrogen as to insure 

 accurate determination, it was found that the acid frequently became 

 neutralised by the bases of the soil, and that less nitrogen remained 

 dissolved after a contact of twenty-four hours, or more, than after 

 only one hour. The strength of the acid liquids was therefore in- 

 ci eased, and the relation of soil to acid diminished. More nitrogen 

 was then taken up, and more after the longer than the shorter period 

 of contact. Still, on adding fresh acid solution to the already once 

 extracted soil, a limit to the amount of nitrogen rendered soluble was 

 soon reached. 



Here again, the conditions of experiment in the laboratory are not 

 comparable with those of the action of living roots on the soil, and 

 the results obtained do not justify any very definite conclusions as to 

 whether the action of the roots on the soil by virtue of their acid sap 

 is quantitatively an important source of the nitrogen of plants having 

 an extended development of roots, of which the sap is strongly acid. 



