10 OF THE ASSIMILATION OF CARBON. 



It has been taken for granted in these calculations, that the 

 basic metallic oxides which have served to introduce humic acid 

 into the plants do not return to the soil, since it is certain that 

 they remain fixed in the parts newly formed during the process 

 of growth. 



Let us now calculate the quantity of humic acid which plants 

 can receive under the most favorable circumstances, viz., through 

 the agency of rain-water. 



The quantity of rain which falls at Erfurt, one of the most 

 fertile districts of Germany, during the months of April, May, 

 June, and July, is stated by Schubler to be 17£ lbs. over every 

 Hessian square foot of surface (=0*672 square foot English) : 

 one Hessian acre, or 26,910 square feet, consequently receive, 

 in round numbers, 700,000 lbs. of rain-water. 



If, now, we suppose that the whole quantity of this rain is 

 taken up by the roots of a summer plant, which ripens foUr 

 months after it is planted, so that not a pound of water evaporates 

 except from the leaves of the plant ; and if we further assume 

 that the water thus absorbed is saturated with hum ate of lime 

 (the most generally diffused of the humates, and that which con- 

 tains the largest proportion of humic acid) ; then the plants thus 

 nourished Would not receive more than 350 lbs. of humic acid, 

 since one part of humate of lime requires 2000 parts of water 

 for solution. 



But the extent of land which we have mentioned produces 

 2580 lbs. of corn (in grain and straw, the roots not included), 

 or 20,000 lbs. of beet-root (without the leaves and small fibres 

 of the radicle). It is quite evident that the 350 lbs. of humic 

 acid, supposed to be absorbed, cannot account even for the 

 quantity of carbon contained in the fibres of the radicle and 

 leaves alone, even if the supposition were correct, that the whole 

 of the rain-water was absorbed by the plants. But since it is 

 known that only a small portion of V.he rain-water which falls 

 upon the surface of the earth is absorbed by plants and evapo- 

 rates through their leaves, the quantity of carbon which can be 

 conveyed into them in any conceivable manner, by means of 

 humic acid, must be almost inappreciable, in comDarison with 

 that actually produced in vegetation. 



