The Gentleman's Magazine/Volume 253/July 1882/The Philosophy of Manuring

A READER writes for further information on this subject, treated in a Note in the March number, to which I must now refer in order to save repetition.

First. Should the weed refuse be dug in fresh, or in the decomposed condition produced by lying in a heap?

From a strictly economical point of view it is desirable to dig in the fresh weeds, seeing that their decomposition in a manure heap must be attended with some loss of carbon, of ammonia, and of heat, all of which are so useful in the soil itself. But there may be an objection to this; for if the weeds have grown far enough to bear seed, or if they consist largely of plants with an underground stem (like couch grass, otherwise named "squitch"), their burial is followed by a troublesome resurrection. Therefore, some judgment is required in suiting the proceedings to the particular case. Generally speaking, the late autumn crop of weeds should be heaped and afterwards buried in the following spring, but the early young weeds buried on the spot at once, by simply hoeing them in.

Second. Should the refuse of a certain crop be the manure for the same kind of crop?

This follows from the principle laid down of restoring what is removed from the soil. Of course, the whole cannot be restored thus when any is used, but in most cases the refuse far exceeds in bulk and manurial value the useful portion for which the crop is cultivated. This is strikingly the case in all kinds of sugar cultivation, pure sugar itself taking absolutely nothing from the soil, as explained in previous Notes.

Liebig tells the story of the poor vine-grower who could not afford to buy manure, and in despair carefully buried the trimmings of his vines close to their roots. He obtained thereby a good crop. In this case he returned a considerable proportion of the potash he had taken from the soil, this being the element which the vine so largely removes, but of course the soil lost all that was taken away by the grapes.

One of my pedestrian trips in Italy was during the most disastrous prevalence of the oidium. The manner in which the grape failed in its growth before the fungus was visible on its skin, led me to suppose that the vines succumbed to the parasite from weakness due-to exhaustion of the soil.

The general experience of vine-growers is very instructive. On one hand, we find records of partial and complete ruin from exhaustion due to neglect of mannring; and on the other, of mischief following as a direct consequence of ordinary manuring. This was the case with the celebrated Johannisberg vineyard while in the possession of General Killerman.

Thanks to the French Academy of Science, the prevailing error that attributed this to "over-manuring" has been dissipated. It was not the quantity but the quality that was in fault. To the unscientific farmer, dung and manure are synonymous terms; and if he has supplied the ground with customary muck, he supposes that he has done all.

This is sound for the grazier, who has to return what the animal takes away; but with such a crop as grapes, which removes so little ammonia and so much potash, the addition of nitrogenous manures is useless, and may even be poisonous. If all the trimmings of the vine and all the wine lees (grape skins and stones), plus as much potash as is contained in the precipitated argol (crude cream of tartar) and in the wine, are returned to the vineyard, its fertility will be uniform and perennial, provided its soluble material is not washed away by rains.

But there is an internal source of potash that must not be forgotten. Certain rock constituents, such as felspar, contain potash so combined that it can only dissolve out very slowly as the felspar decomposes. When the soil of a vineyard rests on a highly felspathic granite or similar rock, it may thus be fed from below with practically inexhaustible supplies, and its grape-producing fertility retained, even when the potash of the grape juice is carried quite away, provided this operation does not proceed more rapidly than the weathering of the rock restores it.

My theory of the appearance and prevalence of oidium for a few years in certain districts, followed by its apparently spontaneous disappearance, is that, in the first case, the potash supplies have been overtasked by abundant harvests; and in the second, they had been restored by the gradual dccomposition of the felspathic material of the soil or under-rock, during the compulsory fallow of the oidium period.

The above-named observations of the oidium were chiefly made on the Plains of Lombardy, the soil of which has been laid down by ancient glacier and torrent d├ębris from the granitic Alps. It therefore contains vast stores of felspathic detritus that is very slowly giving up its potash in soluble form. Hence its fitness for the vine. But even this may be overtasked.

The above-stated principles apply to domestic gardening. If all the weeds and all the stalks of peas, beans, &c., waste leaves of cabbages, rhubarb, and similar refuse be returned, the ground only loses what is actually eaten. This may be fully restored by giving to the soil the domestic refuse otherwise carted away by the dustman.