82 
OF GARDENING AS A SCIENCE. 
there was a known deficiency of such manure, they found proof conclusive of the 
fact, that humus was the nutrimental matter of vegetation. 44 Yet, this position, 
when submitted to a strict examination, is found to be untenable ; and it becomes 
evident, from most conclusive proofs, that liumus , in the form in which it exists in 
the soil, does not yield the smallest nourishment to plants.” Humus , dissolved in 
the water of the soil, is supposed to be conveyed through the roots ; but is humus 
soluble — can it be so dissolved ? We will first produce the opinion of Liebig, and 
then, at the first opportunity, add a few remarks, to a certain extent theoretical, but 
which will tend to reconcile conflicting opinions ; and being based upon the induction 
of facts, will, in a degree, interpret the phenomena of manure and exhaustion. 
First, then : — 44 Humic acid , when first precipitated, is a flocculent substance, is 
soluble in 2500 times its weight of w’ater, and combines with alkalies, lime, and 
magnesia, forming compounds of the same degree of solubility (Sprengel). Vege- 
table physiologists agree in the supposition that, by the aid of water, humus is 
rendered capable of being absorbed by the roots of plants. But according 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 temperature. (Sprengel.) 
44 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. 
44 The correctness of these observations is easily demonstrated by treating a 
portion of good mould with cold water. The fluid remains colourless, and is found 
to have dissolved less than 100,000th part of its weight of organic matter, and to 
contain merely the salts which are present in rain-water.” 
Physiologists have observed this insolubility, and, therefore, have assumed that 
lime, or some vegetable alkali, acts as the solvent : and here we request the parti- 
cular attention of the reader to the mode of reasoning;- by which our author over- 
throws this position. By a peculiar tact, he brings massive facts to bear upon a 
theory which he deems erroneous ; and herein consists much of the power of his 
work. He admits that 44 alkalies and earths do exist in the different kinds of soil 
in sufficient quantity to form such soluble compounds with humic acid. Now,” he 
adds, 44 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. — 40,000 square feet, Hessian measure, of wood-land, yield annually, 
according to Dr. Heyer, on an average, 2650 lbs. Hessian of dry fir-wood, which 
contain 5 T 6 ^ lbs. Hessian of metallic oxides.” 
Here, be it borne in mind, that dry wood is composed mainly of woody fibre, 
which, when reduced to charcoal by slow combustion, retains the precise figure, 
lines, and tracings of that fibre. Now, the basis of humus, or decaying vegetable 
