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A large proportion of good deep arable soils, 
comprising the alluvial and the diluvial, were ori- 
ginally formed in the manner described in the 
articles Annuvium and Dituviem ; all moss-lands 
and peat soils were originally formed in the 
manner described in the article Bog; some ar- 
able lands were originally formed in a manner 
intermediate between shallow hill-soils and allu- 
viums, having acquired their mineral constitu- 
| ents partly from disintegrations of the surfaces 
immediately below them and partly from the 
washed-down debris of the neighbouring decli- 
vities ; and almost all cultivated soils, of what- 
| ever original formation, have undergone modifi- 
| cations first from the natural processes of spon- 
taneous vegetation, next from the artificial pro- 
cesses of tillage and culture, and, concurrently 
with both of these, from the chemical action of 
air, heat, light, moisture, and the weather. See 
the article ALKALIES. 
The Composition of Soils—The composition of 
some soils is very largely silicious,—of others, 
_ very largely aluminous,—of others, very largely 
| calcareous,—of others, very mixedly mineral,— 
| and of others, chiefly or almost solely humous ; 
| and this wide diversity, with all its intermediate 
_ gradations, together with differences of me- 
chanical condition, of subsoil, of situation, of 
climate, and of artificial or georgical character, 
gives rise to the classification of soils of which 
we must treat in the next section. But we 
speak here of the composition of soils in the 
aggregate ; and we may represent them as com- 
prising three great classes of bodies,—earths, 
salts, and organic matters,—and as, at the same 
time, involving a necessary and constant permea- 
tion by heat, moisture, and atmospheric gases. 
The earths, the salts, and the organic matters, 
are discussed separately and in a general way in 
respectively the articles Eartu, Sauts, and Hv- 
Mus; andallarealso discussed, collectively, in their 
relations to the soil, in the articles Manurny and 
Foop or Puants. The three principal earths are 
discussed separately in the articles Sinica, ALUMI- 
nA, and Lime; and the forms or combinations in 
which they most commonly exist are discussed also 
in the articles Sanp, Chay, CHALK, and Loam. A 
less common earth is noticed in the article Mag- 
NESIA; and another mineral constituent of soil, 
the rust or oxide of iron, not properly an earth, 
and comparatively very small in amount, but 
plentiful enough to give much of it a reddish 
colour, and sometimes existing under such con- 
ditions as to act like a poison on plants, is no- 
ticed in the article Iron. The most important 
salts, whether as to bulk, as to economy within 
the soil, or as to nutritional connexion with 
plants, are discussed in the articles Ammonra, 
Nirrates, Poospnares, SInicaTEs, CARBONATES, 
Sunpuates, and Ankaugs. And the principal 
organic matters, whether those which accumu- 
late by the decay of a long succession of different 
sorts of vegetables, or those which become soon 
‘ SOIL. 
abstracted from a virgin soil by constant culti- 
vation, and require to be periodically supplied 
in the form of manure in order to maintain fer- 
tility, are discussed in the articles Humus, Mounp, 
Excrements, Anima Manurss, and Farm-YAarp 
Manure. Connected views of the entire consti- 
tution of soils in their relations to vegetation 
and cropping, and of the slow mutations which 
they undergo during the processes of cultivation, 
are given also in the articles Manurz, AGRICUL- 
TURAL CHEMISTRY, and Roration oF CROPS. 
All the constituents of soil exist as com- 
pounds, and perform their offices as compounds, 
and are obtained in analyses as compounds. 
The earths, for the most part, are oxides, or 
combinations of metallic bases with oxygen; the 
salts, for the most part, are double compounds, 
or combinations of either alkalies or alkaline 
earths with acids ; and the organic matters are 
animal and vegetable principles in a decompos- 
ing state,—naturally very compound, and always 
passing by decay into other and less compound 
forms. The silica in soils commonly exists in 
mechanical combination, or intimate commix- 
ture, with alumina or oxide of iron, or with alu- 
mina, lime, magnesia, and oxide of iron, forming 
sand and gravel of different degrees of fineness. 
Most of the lime exists naturally in the state of 
carbonate,—most frequently in an impalpable 
form, but sometimes in the state of calcareous | 
sand; and even when added artificially in the 
state of mere earth or oxide, it exists as a hy- 
drate, and gradually passes into the state of a 
carbonate. The alumina commonly exists in an 
impalpable form, and in mechanical combination 
with minutely comminuted or very finely pul- 
verized silica, lime, and magnesia, constituting 
the various kinds of clays, argillaceous earths, 
and clayey loams. The magnesia, when not 
combined with unreduced portions of gravel and 
sand, occurs in a fine powder in the state of car- 
bonate. The salts occur variously in combina- 
tion with the mineral matters, in combination 
with the organic matters, and in evolution or 
progressive formation from the elements or prin- 
ciples let loose by the gradual decomposition of 
other bodies. The organic matters occur in dif- 
ferent states and stages of decomposition, from 
the slightest degree of appreciable decay to an 
ultimate resolution-of all their principles into 
powders and gases, and are therefore variously 
compact, fibrous, pulverulent, pulpy, and aeriform. 
All soils originally derived all their mineral 
constituents, directly or indirectly, from the dis- 
integration of rocks; and they owe their distinc- 
tive mineral characters,—the predominance of 
one earth, the alternations of two or more, or 
the prevailing intermixture of several,—to the 
character and distribution of the rocks which 
yielded them. The several rocks were dissevered 
into parts and ground into powder of different 
degrees of smallness by the action of air and 
water and weather; solutions of some of their 
