12 
NATURE AND OFFICES OF EARTHS AND SOI LSI 
7. Chemical Agency of Soils. Besides the mechanical uses of soil, there is, 
according- to Sir H. Davy, " another agency between soils and org-anisable matters 
which may be regarded as chemical. The earths^ and even the earthy carbonates, 
have a certain degree of chemical attraction for many of the principles of vegetable 
and animal substances. The extract from decomposing vegetable matter, when 
boiled with pipe-clay or chalk, forms a combination by which the vegetable matter 
is rendered more ditBcult of decomposition and solution. Pure silica and silicious 
sands have little action of this kind ; and the soils which contain the most alumina 
and carbonate of lime are those which act with the greatest chemical energy in 
preserving manure. Such soils merit the appellation which is commonly given to 
them of rich soils ; for the vegetable nourishment is long preserved in them, unless 
taken up by the organs of plants. Silicious sands, on the contrary, deserve the 
term hungry, which is commonly applied to them ; for the vegetable and animal 
matters which they contain, not being attracted by the earthy constituent parts of 
the soil, are more hable to be decomposed by the action of the atmosphere, or 
carried off from them by water. In most of the black and brown rich vegetable 
moulds, the earths seem to be in combination with a peculiar extractive matter, 
afforded during the decomposition of vegetables; this is slowly taken up and 
attracted from the earths by water, and appears to constitute a prime cause of the 
fertility of soil." 
8. Soils may he improved by pulverisation, or the minute divisions of the 
particles by mechanical labour ; and under this term are included the operations of 
ploughing, harrowing, digging, trenching, hoeing, and raking. It is of the most 
essential service to land, and induces fertility in a variety of ways. It opens the 
ground, and thus gives scope to the roots of vegetables ; increases its sponge-like 
properties, and thus promotes the regular diffusion of water. It tends to increase 
the quantity of vegetable food, by enabling the water holding nutritive matters in 
solution, to convey it more equally to the roots of plants. Pulverisation, by 
opening the soil, promotes and assists the free ingress of heated air, and thus 
regulates and improves the temperature of the soil ; it also introduces, and as it 
were buries, a portion of the atmospheric air, and thus furnishes another source of 
electro-chemical decompositions and combinations. 
" The depth of pulverisation,'' Sir H. Davy observes, " must depend upon the 
nature of the soil and subsoil. In rich clayey ground it can scarcely be too deep ; 
and even in sand, unless the subsoil contain principles noxious to vegetables, deep 
comminution should be practised. When the roots are deep, they are less liable to 
be injured either by excess of rain or drought, the radicles are shot forth into every 
part of the soil, and the space from which the nourishment is derived is more 
considerable than when the seed is superficially inserted in the soil." 
9. A free admission of air, and exposure to the iyiflueiice of heat and cold, tend 
to improve the ground. " If the soil be laid up in large lumps^ (or ridges,) it is 
evident that it will acquire more heat, by exposing a greater surface to the J^atmo- 
sphere ; and it will retain this heat longer, from the circumstance of the lumps (or 
ridges) reflecting back the heat radiated by each other. A clayey soil, in this way. 
