3. But it seldom happens that pure water is employed for the purposes of irrigation. The water of rivers, more generally, is diverted from its course, more or less loaded with mud and other finer particles of matter, which are either gradually filtered from it as it passes over and through the soil, or in the case of floods subside naturally when the waters come to rest. Or in less frequent cases, the drainings of towns, and the waters from common sewers, or from the little streams enriched by them, are turned with benefit upon the favoured fields. These are evidently cases of gradual and uniform manuring. And even where the water employed is clear and apparently undisturbed by mud, it always contains saline substances grateful to the plant in its search for food, and which it always contrives to extract more or less copiously as the water passes over its leaves or along its roots. Every fresh access of water affords the grass in reality another liquid manuring.
In the refreshment continually afforded to the plant by a plentiful supply of water, in the removal of noxious substances from the soil, or in the frequent additions of enriching food to the land—the efficiency of irrigation, therefore, seems entirely to consist.
SECTION VIII.—OF PARING AND BURNING,
AND OF BURNED CLAY.
A mode of improvement often resorted to is the paring and burning of poor land. The efficacy of burned clay, also, even in superseding manure on good lands, has been highly extolled by some practical men.
1. The effect of paring and burning is easily understood. The matted sods consist of a mixture of much vegetable with a comparatively small quantity of earthy matter. When these are burned the ash of the plants only is left, intimately mixed with the calcined earth. To strew this mixture over the soil is much the same as to dress it with peat or wood ashes, the beneficial effect of which upon vegetation is almost universally recognised. And the beneficial influence of the ash itself is chiefly due to the ready supply of inorganic food it yields to the seed, and to the effect which the potash and soda it contains exercise either in preparing organic food in the soil, or in assisting its digestion and assimilation in the interior of the plant.
Another part of this process is, that the roots of the weeds and poorer grasses are materially injured by the paring, and that the subsequent dressing of ashes is unfavourable to their further growth.
2. Much greater uncertainty hangs over the alleged virtues of burned clay. That benefits are supposed to have been derived from its use there can be no doubt, though in many cases the better tillage of the land generally prescribed along with the use of burned clay, may have had some share in producing the good results actually experienced during its use.
By the burning, in kilns or otherwise, any organic matter the clay may contain will be consumed, and the texture of the clay itself will be mechanically altered. It will crumble down like a burned brick into a hard friable powder, and will never again cohere into a paste as before the burning. It will, therefore, render clay soils more open, and may thus, when mixed in large quantity, produce a permanent amelioration in the mechanical texture of many stiff wheat soils. It cannot itself undergo any chemical change that is likely so to alter its constitution as to make it a more useful chemical constituent of the soil than before. Any saline matter we may suppose to be set free could be far more cheaply added in the form of a top-dressing to the soil.
Bricks, however, are generally more porous than the clay from which they are formed; burned clay is so also. And all porous substances suck in and condense much air and many vapours in large quantities into their pores. In consequence of this property, porous substances, like charcoal and burned clay, are supposed, when mixed with the soil, to be continually yielding air to decaying vegetable matter on the one hand, and as continually re-absorbing it from the atmosphere on the other, and by this means to be of singular service in supplying the wants of plants in the earlier seasons of their growth. The vapours of nitric acid and of ammonia, which float in the air, they are also supposed to imbibe, and by the beneficial action of the substances believed to be thus conveyed by burned clay into the soil, the fertilizing virtues ascribed to it are attempted to be explained.
It must be confessed, however, that on this point considerable obscurity still rests. It is in some measure doubtful what the true action of charcoal and of burned clay is, both in kind and in quantity. It is the part of science, therefore, to decline offering more than a mere conjecture till the facts to be explained are more fully and satisfactorily demonstrated.