29 



Liebig, that the organic principles of humus of every description suffer 

 entire decomposition under the action of fermentative germs before being 

 absorbed as plant nutriment. Recent investigations, however, tend to 

 show that in some instances the organic elements of the humus itself 

 may serve as food for plants without undergoing entire decomposition. 

 Whether or not the nitrogenous principles of the humus can thus be em- 

 ployed has not been determined, but that the humus itself, or some con- 

 stituents thereof, can be absorbed by the plant I have myself often no- 

 ticed, especially in the case of sugar cane grown upon a rich vegetable 

 mould. The juices expressed from such canes contain the organic mat- 

 ter of the humus to a certain extent uncbanged, and the sugar and mo- 

 lasses made therefrom are distinctly impregnated in the raw state with 

 this organic matter. 



These facts have a tendency to raise again the question concerning 

 the mineral character of plant food, which for many years was considered 

 as definitely settled. Recent progress in synthetic chemistry has shown 

 that there is no impassable barrier between organic and inorganic classes 

 of compounds. By the union, for instance, of lime and carbon under the 

 influence of the electric arc, a substance is obtained — calcium carbide — 

 which, when thrown upon water, evolves the gas, acetylene, which was 

 formerly supposed to be wholly of organic origin. In hundreds of other 

 instances the barriers between organic and inorganic substances have 

 been broken down in laboratory, and organic bodies as complicated in 

 their nature as sugars have been formed by pure synthesis. The chemistry 

 of the vegetable organism is admittedly superior to that of the chemical 

 laboratory, and while there is no doubt of the fact that the vast prepon- 

 derance of vegetable food is of a mineral nature, it would not be safe to 

 deny to the vegetable the ability to absorb to a certain extent organic 

 compounds. 



There is, however, at the present time but little evidence to show that 

 organic compounds of a nitrogenous nature are ever absorbed by plants, 

 and therefore, even in the case of humus, we must still contend, at least 

 for the present, that its nitrogenous constituents only become available 

 for plant food after having been fully oxidized by the action of the soil 

 ferments. 



DETERMINATION OF THE ACTIVITY OF SOIL FERMENTS. 



It is evident from the preceding pages that a study of the soil for agri- 

 cultural purposes is incomplete which does not include a determination of 

 the character and vigour of the ferments which it contains. This neces- 

 sarily introduces into the practice of soil analysis the processes of bacterio- 

 logical examination. It is not the purpose at the present time to describe 

 these processes, but to give only to the general reader as clear an idea as 

 possible of the principles which underlie the analysis of soils for the 

 purpose of determining the activity of their nitrifying ferments. 



