HUMUS, OR VEGETABLE MOULD. 415 



soil, from the application of the dung of cattle, which is in the 

 main decomposed vegetable matter, and the extraordinarily 

 luxuriant vegetation always appearing upon dung-heaps left on 

 the field, or upon places where dung-heaps have been formed, 

 seemed to speak the same language. The supposition has been, 

 that this vegetable matter constituted, in fact, a part of the food 

 of plants, and went to assist, in forming their substance. 



The doctrine of Liebig denies directly the supposition that 

 this humus, or vegetable matter, is taken up as the food of plants, 

 because, where a forest grows, the vegetable matter in and upon 

 the soil actually increases, instead of diminishing ; but then, 

 although, in the case above referred to, of the volcanic soils near 

 Mount Vesuvius, one might be led to infer that he considered it 

 of no moment, yet this I think would be doing him an injustice. 

 He does consider the humus of the soil as furnishing, in its 

 decay, a necessary supply of carbonic acid to the plant in the 

 process of germination, though of no use after the plant gets 

 above ground; arid he supposes that the manures of animals 

 fed upon the product of the land return to the land those 

 mineral elements which they took from it, and which are indis 

 pensable to their perfect formation. This may be so ; and in 

 this view he does not deny the value of vegetable mould, or 

 humus. But certainly there was nothing improbable in the 

 supposition that plants might have found some portion of their 

 food in those decayed substances which once constituted a part 

 of the substance of their predecessors. Indeed, I see as yet no 

 sufficient grounds to conclude that their office in supplying 

 carbon to the growing plant ceases as soon as the plant is above 

 ground, and able, as he supposes, to gain its own supplies for 

 itself from the atmosphere. It is quite certain that the growth 

 of a forest would be checked, and the amount of humus in the 

 soil be diminished, if all the decayed leaves and limbs, which 

 fall from the trees, were constantly removed ; and it is as certain 

 that the continual cultivation of land, without supplies of manure, 

 exhausts its vegetable mould ; and that the application of vege 

 table manures to crops in a growing state is often as efficacious 

 as when applied, or ploughed in, with the seed. 



