IV.—Notes on the Growth of Soil

Extract

Previous to writing “Suggestions on Denudation,” I had not seen Mr. Darwin's paper “On the Formation of Soil;” since then, having read this essay, it appears to me that instead of Darwin proving “the disappearance of stones, etc., needs no growth of soil,” he advocates in a great measure the statement put forward in “Suggestions on Denudation.” For Darwin clearly proves that any foreign sustance placed on grass land will be gradually covered up by a growth of soil over it. This soil, however, he seems to believe to be entirely due to the labour of earth-worms, who excavate in the ground under the foreign sustance and deposit over it. From this it would appear that this eminent observer considers that the total thickness of the soil is not increased upwards by mould formed from vegetable decay, but that all is taken from below the foreign sustance and placed above it, thereby adding to the thickness of the upper stratum of the mould and diminishing the thickness of the portion below the foreign substance. This, within certain limits, may be correct; but can anyone asset that the decay of the yearly growth of vegetable matter is nil, and that it cannot possibly and its mite, no matter how small, to the thickness of the soil? If the above is the case, how can we account for the growth of peat bogs in Ireland and other countries? On the site of all peat bogs (or the bogs in the low flat country) there originally was vegetable soil, in which trees and lesser plants grew;—on this, by the decay of the vegetable matter, a spongy soil was formed that retarded the drainage, and was well fitted for the growth of such mosses as the Sphagnum, which began to luxuriate.