136 



THE EOTHAMSTED EXPERIMENTS. 



Subsoil not 

 the main 

 source of 

 nitrogen 

 accumu- 

 luted by 

 leguminous 

 crops. 



Accordingly, the root-sap of many plants was examined, 

 and it was found to be more or less acid — that of the deep, 

 strong, fleshy root of the Medicago sativa being very strongly 

 so. The degree of acidity of the juice 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. These attempts were, however, unsuccessful. 



Experiments were next made to determine 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 Medicago sativa root-juice, or having a known relation 

 to it. These experiments and their results have been fully 

 detailed elsewhere. It is only necessary to say here that the 

 results did not justify any very definite conclusions as to the 

 probability that the action of roots in 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. 



Indeed, although significant indications have been obtained, 

 both as to the importance of nitric acid as a source of the 

 nitrogen of the Leguminosse, and as to the action of organic 

 acids in rendering soluble the otherwise insoluble nitrogenous 

 compounds of soils and subsoils, yet on neither of these 

 points is the evidence at present available adequate to 

 account satisfactorily for the facts of growth. 



Soil and 

 manure 

 main 

 sources of 

 nitrogen 

 for most 

 other crops. 



Lastly, in regard to the sources of already combined 

 nitrogen available to our crops, the evidence points to the 

 conclusion that, independently of the small amount of com- 

 bined nitrogen annually coming from the atmosphere in rain, 

 and the minor aqueous deposits, the source of the nitrogen, 

 at any rate of most of our crops, is the stores already existing 

 within the soil and subsoil, or those provided by manure. It 

 has further been seen that the combined nitrogen is largely 

 taken up as nitric acid, or rather as nitrates. But, it is 

 nevertheless obvious, that we have yet to seek for an ex- 

 planation of the source of the whole of the nitrogen of the 

 Leguminosse. 



We are brought to inquire, therefore, what is the evidence 

 relating to the question of the fixation of free nitrogen, by the 

 plant, by the soil, or otherwise ? 



