xvi APPENDIX

The fertility ofa tract will,in some degree, depend upon the nature of the subsoil, sometimes the plant- bearing earth reposes upon pebbles of sand, which allow the water to escape too rapidly. In dry sea- 4, sons the crops of such fields will suffer from a lack of‘ moisture. A subsoil of clay will prevent the water from descending, as too much moisture may thereby be present, and artificial draining rendered necessary. Many of the sandy soils owe their fertility to their substratum. Mineral, vegetable, and animal matter, when exposed to the operation of the frost, heat, moisture, &c., undergo important chemical changes, and plants themselves change the character of the ground upon which they flourish. The trees of the forest and indigenous plants have arranged themselves in groups on the kinds of soils most favorable to their growth, and when these trees and plants are cut down they are succeeded by other varieties; rotation of crop in agriculture is thus clearly indicated by nature.

Soils are the basis on which plants fix their roots, and sustain upright positinns, and they supply vege- table food during their growth. This food is of a complex nature, and is not well understood. By some philosophers it has been called humus, humic acid, ulmin, ulmic acid, crenic acid, Ste. It is des— cribed as being a black substance, capable of being dissolved by the alkalies and ammonia, and when thus dissolved, it is supposed to enter plants and sup- ply them with carbon; but, from the experiments of Professor Playfair and others, it appears that the carbon ofplants is chiefly derived from the atmos- phere. Certain it is that dry sand, driven before the winds, contains neither vegetable matter nor car- bon, and will not produce plants. The Island that is raised from the sea by a volcano, and the crumbling surface ofthc naked rock,wherethere is no appreciable quantity ofcarbon, will produce vegetables, and cause seeds, deposited by birds ofpassage, to grow.

Soils consist chiefly of sand, lime and clay, with which are mixed certain saline and organic substan-