Having already, -under the articles Arable Land ; Land ; Marsh; Moor; etc. stated the most approved methods of renovating or restoring exhausted soils, and of converting them from a state of nature ; we shall now communicate a few hints, by which their quality may be ascertained.
To effect this purpose, Bergman, Fordyce, Kirwan, and other eminent chemists, have analyzed the constituent parts of different soils, namely; carbon, lime, clay and siliceous sand; and, according to the respective portions of these ingredients, they conceive that the relative fertility of soils might be determined. But, as such analysis is very inaccurate and uncertain, Dr.Darwin proposes to dry a few pounds of different soils, in the same temperature : when their moisture is evaporated, they must be weighed, and exposed to a red heat. As carbon is a principal ingredient in calcareous earths, he conjectures, that the soil which loses the greatest portion of its weight, is the most fertile; because the carbonic matter, being the principal nutriment of plants, will be dissipated in the flame.
Another mode of examining the fertility of soils is, by calculating their specific gravity, when dried at equal distances from the fire, in bladders furnished with small apertures ; and, after immersing them in water, by accurately observing the difference between their respective weights, both in that fluid and in the air. But the most certain criterion, by which to judge of the value of land, is afforded by attending to the growth and colour of the vegetables spontaneously produced; and which in some measure indicate the nature of the soil beneath their roots. Thus, the Fox-glove, and Sandwort, abound in sandy situations; the Brook-lime, and some species of Cresses, in moist ground; the Corn Saw-wort, or Way-thistle, indicates a good, as the Dock shews an inferior, soil. Many plants might be added to this list; but, we shall conclude with remarking, that if an accurate Geographical Catalogue of such vegetables, as grow in particular situations, were published in every country, it would be of great service, in ascertaining the degree of fertility, as well as the nature of different soils.