THE CULTIVATOR. 
] 27 
It is chiefly from the carbonate that the lime used in agriculture 
is derived. By exposing the carbonate to strong heat, the carbonic 
acid is driven off, and that which remains is the caustic earth, to 
which we give the name of quicklime. This substance has a strong 
affinity for water, which it will absorb from the atmosphere. When 
the water is applied in quantity, it is absorbed by the lime, with a 
great evolution of heat; and this is the process of slaking so well 
known. The lime thus combined with water attracts carbonic acid, 
and again becomes carbonate of lime ; although, in this state ofcar- 
bonate, it presents external characters entirely different from those 
which it possessed in its original state of marble, limestone and chalk. 
But it is in external characters only, and in the lesser degree of co¬ 
hesion of its parts, that it differs, for otherwise the substances are 
the same. 
By the minute division of its parts by heat, we are enabled to ap¬ 
ply lime to the soil in the shape of a finely divided powder, and thus 
in the best form for improving the texture of the soil. It is from this 
cause doubtless, as well as those important purposes which it serves 
as a manure, that this earth is of such importance to the husband¬ 
man. Could we apply the earths silica and alumina to the soil in 
their pure state, or could we reduce them by mechanical or chemi¬ 
cal means to powder, we should be able to apply them in a form cal¬ 
culated to improve the texture of the soil. 
Lime exists in all soils formed by the decomposition of rocks ; but 
in soils formed wholly by the aggregation of vegetables, as peat, it 
does not necessarily exist. It improves the quality of all soils, whe¬ 
ther they are formed of silica, alumina or vegetable matter. 
Silica, and alumina, and lime, forming the principal part of soils, 
and, where any one of them prevails, giving its character to the soil, 
it is frequently convenient to distinguish soils, as being Silicious, 
Aluminous or Calcareous. Where silica prevails, as in the case of 
many sands, we may call the soil silicious ; where clay prevails, we 
may call the soil aluminous; and where lime exists in quantity, as 
in the case of chalk, we may call the soil calcareous. Thus, in ad¬ 
dition to the less artificial division of the farmer, derived from the 
texture and external characters of the soil, we may use those de¬ 
rived from its composition. 
Magnesia, in various states of combination, exists in nature in con¬ 
siderable quantity. It is generally found in combination with acids, 
as the carbonic. In mountain rocks and fossils, it exists along with 
silica, alumina, lime, iron, and other substances. The minerals of 
which it forms a part, generally feel soft and unctuous. It is the 
principal constituent of various mountain rocks, as serpentine and 
chlorite-slate ; and thus being an element in many rocks and fossils, 
it must form a considerable part of soils. Magnesia, however, is less 
generally diffused than lime, and may perhaps perform a less impor¬ 
tant function in the economy of vegetation. When it exists in such 
quantity as to give a character to the soil, we may term the soil 
Magnesian. 
The next substance that exists largely diffused in the mineral 
kingdom, is oxide of iron. 
Iron, as it is the most useful of the metals, so it is generally dif¬ 
fused on the earth. It is derived, for the purposes of the arts, from 
a series of minerals termed ores of iron. It is found extensively in 
mountain rocks, and many fossils; and it exists, accordingly, in more 
or less quantity, in almost every soil. Its precise effects, however, 
on the productive powers of soils, have not been well determined, 
some soils, where it exists, being extremely barren, while in some 
very fertile soils it exists in large quantity. Soils which contain 
much of iron may be termed Ferruginous. 
The alkalies, soda and potassa, are also found in soils, being ex¬ 
tensive products of the mineral kingdom. They are found in nature 
combined with various acids. Muriate of soda, or common salt, one 
of these combinations, is a widely diffused mineral in soils, a certain 
quantity of which is probably necessary to the existence of plants, 
while in excess.it is known to be injurious. 
Soils, then, consist chiefly of silica, alumina, lime, magnesia, ox¬ 
ide of iron, potassa and soda, together with a portion of organic 
matter. 
From various experiments, it is known, that plants consume, in 
growing, the decomposing animal and vegetable matter which the 
soil contains. It is rendered probable also by experiments, that a 
portion of the earthy matter of the soil—the silica, the alumina, the 
lime, as well as various saline substances contained in it—is absorb¬ 
ed by the plant, though in minute quantity, as compared to the ani¬ 
mal and vegetable matter absorbed. 
Further, the medium of supply of the matter of nutrition contained 
in the soil, may be regarded as water holding in solution the vegeta¬ 
ble, animal and other matters which pass into the roots of plants. 
The soil, then, may be chiefly regarded, 
1st. As the instrument for fixing the roots of plants in the ground ; 
and, 
2d. As a medium for conveying to them the water holding dissolv¬ 
ed the different substances which pass into the plant. 
The air may be considered as a vehicle for conveying water to the 
soil. It is continually charged with aqueous vapor ; which partly 
descends to the earth in rains, and is partly deposited in dews, in the 
cool of the night. In many countries, it never rains at certain sea¬ 
sons, and the whole moisture is supplied by the dew. In this case, 
in an especial degree, and in all cases in a certain degree, the pow¬ 
er of the earth to absorb moisture from the air, may be regarded as 
connected with the means of the soil to nourish plants. 
All our fertile soils, accordingly, have a power of thus supplying 
themselves with moisture, and of retaining it for the proper time; 
while infertile soils either have less of this absorbent power, or re¬ 
tain the fluid absorbed for a shorter time. 
Of the different matters which enter into the composition of soils, 
animal and vegetable substances possess the greatest power of ab¬ 
sorbing moisture ; and the addition of animal and vegetable substan¬ 
ces always increases the fertility of soils. 
Of the pure earths, the least absorbent is silica, and it is that also 
which parts the most readily with its moisture. A soil consisting of 
too great a proportion of silicious sand is always infertile. It im¬ 
bibes the aqueous vapor of the atmosphere with slowness, and parts 
with it quickly. A soil of silicious sand will scarcely be penetrated 
by the dew of night, and will part with it on the first action of the 
morning rays of the sun. 
While pure silica will imbibe scarce a fourth part of its weight of 
water, lime will absorb nearly its own weight, and alumina two and 
a half times its weight. But while the silica will absorb a smaller 
quantity than alumina or carbonate of lime, it will allow it to evapo¬ 
rate two times more quickly than carbonate of lime equally divided, 
and five times more quickly than alumina in the same state.* The 
addition of carbonate of lime or alumina to a soil containing too much 
silica, never fails to increase its powers of absorption and its ferti¬ 
lity. 
The order in which the principal substances that enter into the 
composition of soils, possess an absorbent power, is the following: 
1. Animal and vegetable substances. 
2. Alumina. 
3. Carbonate of lime. 
4. Silica.f 
It appears, too, that the more perfectly the parts of the soil are 
comminuted, decomposed and reduced, the greater is the power of 
absorption u liich they possess; and that the larger the proportion 
of the soil is which exists in this minutely divided state, the greater, 
cceteris paribus, is its fertility. 
But, although certain earths in their separate state have thus a 
greater power of absorption than others, it does not follow, that a 
soil consisting chiefly of that one earth would possess a greater 
power of absorption than a soil composed of a mixture of earths, 
even though these earths should in themselves be less absorbent. 
Thus, a soil consisting chiefly of aluminous earth, though alumina be 
itself the most absorbent of all earths, taking water up in the 
greatest quantity when poured upon it, as well as retaining it the 
longest, would not really be so absorbent as if it were more mixed 
with other earths. Hence, the stiffer clays are not the soils which 
absorb water readily from the atmosphere; for, when the weather 
is dry, such soils become indurated upon the surface, which presents 
an obstacle to absorption ; and thus we find, that in hot weather the 
vegetation of very stiff clays is almost as soon injured by drought 
as that of light soils, and much more quickly than that of good 
loams. 
A mixture of silicious sand, then, with a very aluminous soil, al¬ 
though the sand be the less absorbent snbstance of the two, seems 
to increase the general power of absorption from the atmosphere ; 
so also does a mixture of lime, and in an eminent degree, of animal 
and vegetable matter. 
It is not, therefore, the prevalence of any one earth that consti¬ 
tutes a soil well fitted to absorb humidity. A mixture of certain 
* Chimie appliquee al’Agriculture par M. le Compte Chaptul. f Chaptal. 
