186

vegetable nourishment is long preserved in them,unless taken up by the organs of plants. Siliceoussands, on the contrary, deserve the term hungry,which is commonly applied to them ; for the vege-table and animal matters they contain not beingattracted by the earthy constituent parts of thesoil, are more liable to be decomposed by the actionof the atmosphere, or carried off from them bywater.

In most of the black and brown rich vegetablemoulds, the earths seem to be in combination witha peculiar extractive matter, afforded during thedecomposition of vegetables : this is slowly takenup, or attracted from the earths by water, andappears to constitute a prime cause of the fertility ofthe soil.

The standard of fertility of soils for differentplants must vary with the climate; and must beparticularly influenced by the quantity of rain.

The power of soils to absorb moisture ought tobe much greater in warm or dry counties, than incold and moist ones; and the quantity of clay, orvegetable or animal matter they contain greater.Soils also on declivities ought to be more absorbentthan in plains or in the bottom of vallies. Theirproductiveness likewise is influenced by the natureof the subsoil or the stratum on which they rest.

When soils .are immediately situated upon a bedof rock or stone, they are much sooner rendereddry by evaporation, than where the subsoil is of