467 



square feet of land, Hessian measure, yield annually 2650 lbs. of dry 

 fir wood, which contain 5.6 lbs. Hessian of metallic o.xides. Now it is 

 ascertained in what proportion humic acid combines with the metallic 

 oxides, with lime for example. Having determined the metallic oxides 

 exising in such a product, he easily determines the amount of humic 

 acid thus introduced into the trees ; and, allowing humic acid to contain 

 58 per cent, of carbon, this would correspond only to the production of 

 91 lbs. Hessian of dry wood. But 2650 lbs. of fir wood are actually 

 produced. These calculations are well worth examining, and, if ac- 

 curate, it is difficult to deny the inference which follows from them, 

 that the humic acid existing in a soil, supposing all its carbon to be ta- 

 ken up and assimilated, will supply but a very small portion of that, 

 which exists in the crop grown upon the soil. 



The same remarks are applied to a crop of wheat. From the known 

 properties of metallic oxides existing in wheat straw (the sulphates and 

 chlorides also contained in the ashes of wheat straw not included), it 

 would be found, that the wheat growing on 40,000 square feet Hessian 

 of land would average 1780 lbs. Hessian of straw, independently of 

 the roots and grain, and the composition of this straw is the same as 

 that of woody fibre. Now, according to well-ascertained properties, 

 it could receive but 57i lbs. of humic acid, which would supply with 

 carbon only 85 lbs. Hessian of straw. 



Another calculation respects the amount of humic acid which plants 

 can receive through the agency of rain water. The amount of rain 

 falling in one of the most fertile districts of Germany, during the 

 months of April, May, June, and July, is estimated to be 17J lbs. Hes- 

 sian upon every square foot of surface, or upon 40,000 square feet 

 Hessian, 700,000 lbs. Hessian of rain water. Now this extent of land 

 averages a product of 2850 lbs. Hessian of corn (wheat) ; 390 lbs. of 

 humic acid calculated to be absorbed in this case, cannot account for 

 the quantity of carbon contained in the roots and leaves alone, even if 

 we suppose the whole of the rain water to be absorbed by the plants, 

 whereas a large portion of it must necessarily be lost or pass off" in some 

 other form than through the organs of the plants. If these calcula- 

 tions be correct, it is evident that a small portion only of the carbon 

 existing in plants can be derived from the humus of the soil. Another 

 idea is suggested, viz. that as humus results from the decay of plants, 

 none existed at the time of the creation to form the pabulum of the 

 primitive vegetation. This must have had other sources of supply. 



