Our own practice has proven that there is scarcely any manure more effective for one crop, particularly of spring sowing, than the clay of old mud walls of {325} houses, though applied in no larger quantity than is usually given of farm-yard manure, and though the clay appear quite free from vegetable matter. It is improbable that the resting of the clay from production could have any effect to occasion this fertility. We considered it to arise chiefly from a quantity of nitre having been formed in, or deposited about, the walls from their long proximity to animal effluvia and to atmospheric air. The fertilizing effect of the dike system of summer-fallow, and even of the present system, may also depend in part on the formation of nitre, well known to be a powerful manure or stimulant in this country. In dry seasons we have scraped together handfuls of salts, partly nitre, from the exposed surface of clay-banks. Should a considerable part of the fertilizing effect of fallowing arise from the formation of nitre, the application of lime and putrescent manures to fallows, in the early part of summer, will be advantageous, as the presence of both are favourable to the formation of nitre. Of course, the utility of encouraging the formation[63] of {326} nitre or other salts, combinations of potassa or of soda, will depend on the climate, whether much or little rain falls, and whether the rain water goes off by evaporation or by drainage. In the case of little rain, or the rain-water being nearly all evaporated, nitre and other salts will accumulate in the soil, so as, from their excess, to be injurious to vegetation; whereas, should much rain fall, or the rain-water be chiefly carried off by drainage, vegetation may languish from deficiency of these salts, there being less deposition of the salts, or the salts as they form being washed away. The same will apply to the graminivorous animals. Sea-salt, perhaps also nitre and other salts, will be serviceable in a moist country, or far from the sea, where the plants and water contain little saline matter, and probably pernicious in a dry climate, where the plants and water generally contain much saline matter.