141 



Carbon, as wo know, is taken by tho plants from the carbonic-acid 

 gas of the air, at least for the most part. Carbonic acid, like watvr, 

 exists everywhere, and if I remind you that we have succcetlcd in 

 transformino- it into some of the sufjars which exist so ijeneraiiy in 

 the vegetable tissues, you will agree with me in saying that the great 

 phenomenon of the assimilation of carbon by plants is at present 

 understood only in its smallest details. 



The mechanism of the assimilation of nitrogen is far from being 

 as well understood even as that of carbon. We as yet know nothing 

 of the chemical changes which cause this element to pass from a gas- 

 eous state to that of albuminous food: but its diU'erent modes of ])ene- 

 trating into the plant are welf known to us, and we can affirm to-day 

 that the atmosphere contributes as much as the soil to that portion of 

 vegetable nutrition. 



This fact, of which we shall shortly give the demonstration, was 

 almost evident, a priori. In fact the soil contains onlv very small 

 proportions of nitrogen. The store which it offers to" us (scarcely 

 10.000 kilograms per hectare) is insignificant in comparison with the 

 innnensity of time; but in comparison with it the atmosphere con- 

 tains an enormous quantity, about three-fourths of its entire volume; 

 hence the idea of a continual circulation of nitrogen betAveen its com- 

 ))ounds and the air — in other words, between the air, the earth, and 

 the living organisms — forced itself upon us, in the same way as the 

 circulation of water between the ocean and all points of the earth 

 obtrudes itself. 



It is therefore the more remarkable that this conception of the 

 subject has only quite recently been brought to light. Enunciated as 

 a principle more than thirty years ago, it has only been taken into 

 serious consideration in these latter years, after a series of researches 

 which we are now going to pass in review. 



But I should like first to establish, by experience alone, outside of 

 all speculative ideas, the fact that the intervention of atmospheric 

 nitrogen in the phenomena of vegetation is an absolute necessity. It 

 Avill suffice for that pur^Dose that I show a parallel, a sort of balance 

 between the sources of gain and the sources of loss to the soil in nitrog- 

 enous compounds; it is clear that if this comparison shows us a diff'er- 

 ence in favor of the enriching of the soil then we need have no fear of 

 seeing our soil become one day sterile ; if, on the contrary, the losses 

 are in excess of the gains from the exterior then we knoAv that it must 

 be receiving from the atmosphere the quantity of gaseous nitrogen 

 (>(jual to the difference. It is very easy to bring together the data for 

 this great problem. 



The most important cause of the decrease of nitrogen in the soil 

 is unquestionably the crop taken from it each year; .the amount of this 

 loss is, however, very variable; a crop of cereals — of wheat, for ex- 

 ample — takes from the soil about 50 kilograms of nitrogen per he(;- 

 tare; roots, beets, or others generally contain more; finally, certain 

 liinds of vegetation, such as clover or lucern grass, take as much as 

 100 to '200 kilograms, and even more nitrogen i)er hectare annually. 



Judging by these figures, we nuist conclude that by an average 

 rotation of crops, where root vegetables, leguminous plants, and 

 cereals are made to alternate one with the other, the earth loses every 

 year by the fact of cultivation alone a mininnnn of from GO to TO 

 kilograms of nitrogen in combination with other substances. 



