| Potash and soda have been so invariably found 
| in all the numerous clayey soils which have been 
analyzed, that they may fairly be pronounced in- 
| gredients in absolutely every description of argil- 
| laceous land. Potash always exists as a distinct 
and separate substance in the leys of alum manu- 
factories; and it is formed from the ashes of the 
stone and brown coal, which contain much ar- 
gillaceous earth. The mixture of the clayey 
matter properly called loam with the quartz of 
the new red sandstone formation, or with the 
| lime in the different limestone formations, in so 
very trivial a proportion as one part to a thou- 
sand, affords as much potash to a soil only twenty 
inches deep as will supply a growing forest of 
pines with a sufficient quantity of alkali for a 
century; and a single cubic foot of felspar, a 
mineral which contains a large proportion of 
alumina, will afford as much as an oak wood, 
growing upon a surface of 26,910 square feet, will 
require during five years. 
The land around Vesuvius in the vicinity of 
| Naples, is one of the most fertile soils in the 
|| world; it possesses, in its different districts, a 
greater or less degree of fertility according to its 
proportion of clay or sand; and as it entirely con- 
sists of disintegrated lava, it owes its fertility, 
| not to the product of any vegetable decomposi- 
tion, but solely to the presence of mineral alkalies. 
When lava or volcanic ashes have been exposed 
for some time to the simple influence of air and 
moisture, they produce the most luxuriant crops 
of any sorts of plants which their occupants 
please to cultivate; and they obviously acquire 
all their power from the reduction of their al- 
kalies, their alkaline oxides, and their silica into 
such a condition as to be absorbable by the spon- 
gioles of the plants. The multitudes of soils 
which, in all parts of the world, have been formed 
by the disintegration of rocks, either through 
mere atmospheric influence upon the spot, or 
through the action of water in transmission to a 
distance, are generally rich in alkaline oxides, 
and yield them up to the purposes of vegetation 
only in a slow and gradual process co-ordinately 
with their own increasing pulverization. Thou- 
| sands of years have been necessary for the disin- 
| tegration of rocks into the condition of arable 
lands; and, in many instances, thousands of years 
more will be necessary to effect such a fine com- 
minution of these lands as to expose their minut- 
est particles to the dissolving power of air and 
moisture, and to exhaust them of their alkalies. 
Yet not only do virgin or uncultivated soils part 
|| slowly with their alkalies under the wasting 
| power of weather, but they continually regain a 
| compensating quantity of alkaline matter, and 
| 
| even add to its amount, by alkaline deposits from 
| their own vegetation, from the saline vapours of 
| the sea, and from the ammoniacal gases concen- 
| trated and brought down by rain. “ We see from 
} the composition of the water in rivers, stream- 
\ 
lets, and springs,” says Dr. Liebig, “ how little 
alkali the rain water is able to extract from a 
soil even after a term of years; this water is gen- 
erally soft, and the common salt which even the 
softest invariably contains, proves that the alka- 
line salts, which are carried to the sea by rivers 
and streams, are returned again to the land by 
wind and by rain. Let us suppose that a soil has 
been formed by the action of the weather on the 
component parts of granite, grauwacke, moun- 
tain limestone, or porphyry, and that the vege- 
tation upon it has remained the same for thou- 
sands of years; now this soil would become a 
magazine of alkalies in a condition favourable 
for their assimilation by the roots of plants. The 
interesting experiments of Streeve have proved 
that water impregnated with carbonic acid de- 
composes rocks containing alkalies, and then dis- 
solves a part of the alkaline carbonates. It is 
evident that plants also, by producing carbonic 
acid during their decay, and by means of the 
acids which exude from their roots in the living 
state, contribute no less powerfully to destroy the 
coherence of rocks. Next to the action of air, 
water, and change of temperature, plants them- 
selves are the most powerful agents in effecting 
the disintegration of rocks. Air, water, and 
change of temperature prepare the different 
species of rocks for yielding to plants their alka- 
lies. 
fluences which affect the disintegration of rocks, 
but from which the alkalies have not been re- 
moved, will be able to afford, during many years, 
the means of nourishment to [cultivated] vege- 
tables requiring alkalies for their growth.” 
But the processes of cultivation entirely alter 
the alkaline conditions of virgin soils, rapidly 
drawing off their native alkalies to exhaustion, 
and rendering requisite either a frequent arti- 
ficial supply of fresh alkalies in the form of 
manures, or the periodical abandonment of the 
land to a slow reacquirement of alkalies from 
saline vapours, rain water, and the chemical de- 
compositions of spontaneous vegetations. When 
any virgin soil, no matter how rich in native 
alkalies, is subjected to unmanured cropping, and 
worked onward to exhaustion, it may experience 
a series of recruitings merely by such periodical 
and prolonged exposures of it to the weather as 
will effect the further comminution of its parts, 
and lay open its more minute or ultimate alkaline 
constituents to solution; but, if continued to be 
worked without any other appliance, it will even- 
tually lose all its intrinsic alkalinity, or become 
intrinsically barren, and will be capable of reac- 
quiring fertility only by the attainment of fresh 
alkalies from manures or from the atmosphere. 
The first colonists of Virginia found a virgin soil 
remarkably rich in native alkalies; and they 
drew from it annual harvests of wheat and 
tobacco, without the aid of manure, for a cen- 
tury; but now their successors are obliged to 
treat whole districts of it as mere poor pasture- 
land, or occasionally to work portions of them 
| 
| 
ee eee 
A soil exposed for centuries to all the in- | 
