ABSORPTION OF HUMUS. 35 



of being absorbed by the roots of plants. But ac- 

 cording to the observation of chemists, humic acid is 

 soluble only when newly precipitated, and becomes 

 completely insoluble when dried in the air, or when 

 exposed in the moist state to the freezing tempera- 

 ture. (Sprengel.) 



Both the cold of winter and the heat of summer 

 therefore are destructive of the solubility of humic 

 acid, and at the same time of its capability of being 

 assimilated by plants. So that, if it is absorbed by 

 plants, it must be in some altered form. 



The correctness of these observations is easily 

 demonstrated by treating a portion of good mould 

 with cold water. The fluid remains colorless, and is 

 found to have dissolved less than 100,000 part of its 

 weight of organic matters, and to contain merely the 

 salts which are present in rain-water. 



Decayed oak-wood, likewise, of which humic acid 

 is the principal constituent, was found by Berzelius 

 to yield to cold water only slight traces of soluble 

 materials ; and I have myself verified this observa- 

 tion on the decayed wood of beech and fir. 



These facts, which show that humic acid, in its 

 unaltered condition, cannot serve for the nourishment 

 of plants, have not escaped the notice of physiolo- 

 gists ; and hence they have assumed that the lime or 

 the different alkalies, found in the ashes of vegeta- 

 bles, render soluble the humic acid and fit it for the 

 process of assimilation. 



Alkalies and alkaline earths do exist in the differ- 

 ent kinds of soil in sufficient quantity to form such 

 soluble compounds with the humic acid. 



Now, let us suppose that humic acid is absorbed 

 by plants in the form of that salt which contains the 

 largest proportion of humic acid, namely, in the form 

 of humate of lime, and then, from the known quantity 

 of the alkaline bases contained in the ashes of plants, 

 let us calculate the amount of humic acid which 

 might be assimilated in this manner. Let us admit, 

 likewise, that potash, soda, and the oxides of iron 



