1232. The soils improved by burning are all such as contain too much dead vegetable fibre, and which consequently lose from one third to one half their weight by incineration; and all such as contain their earthy constituents in an impalpable state of division, i.e. the stiff clays and marls, are improved by burning: but in coarse sands, or rich soils containing a just mixture of the earths, and in all cases in which the texture is sufficiently loose, or the organisable matter sufficiently soluble, the process of torrefaction cannot be useful.