450 
THE DICTIONARY oF GARDENING, 
Soil—continued. 
varies much in Soils, according to their composition and 
the size of the particles of which they are made up. 
Sandy Soils rapidly absorb the water; but they allow it 
to drait away almost as quickly; so that plants growing 
in sand are liable to suffer from drought. Fine sand 
is about twice as retentive of water as coarse. Clay re- 
tains twice or thrice as much as sand. Vegetable earth, 
or humus, absorbs and retains about twice or thrice as 
much as clay, and becomes about two and a-half or three 
times as heavy when soaked in water as when dry. The 
more retentive Soils lose less by evaporation than do the 
others. Almost all Soils are more or less full of water, 
in the liquid form, at a few feet below the surface, the 
depth varying with the nature of the Soil and Subsoil 
and with the season of the year and the climate. This 
subterranean water is brought within reach of the roots 
of plants by ‘what is known as capillary attraction, the 
water rising in the fine crevices between and in the 
particles of the Soil. If the subterranean water is 
stagnant near the surface, substances are apt to be 
formed in the Soil that are injurious to plants, and that 
thus diminish its fertility. Drainage is usually necessary 
in clays and peat Soils, to prevent water from accumu- 
lating and doing harm in this way. 
Certain Soils also have the property of absorbing 
a good deal of vapour from the atmosphere, and con- 
_ densing it in their particles, the amount increasing in 
‘proportion to the moistness of the air. Experiments 
have shown that humus can absorb about half its own 
weight of water from air saturated with vapour; and 
clays absorb one-tenth to one-fifth of their own weight; 
but quartz sand absorbs little, if any, moisture in that | 
. 
way. source of moisture becomes less productive 
as the temperature rises. During the night, and in cold 
weather, it is probable that Soils absorb a good deal of 
moisture from the atmosphere; but when the air is 
warmer and drier than the soil, the latter loses water 
by evaporation, and what is lost in this way is replaced 
from below by capillary attraction. Evaporation cools 
the surface from which it is going on; hence, wet land 
is colder than that which is well-drained. Humus gives 
up least by evaporation, and quartz sand most, of all 
Soils under similar conditions. 
The Power of Absorbing and Retaining Chemical Com- 
pounds is one possessed to a greater or less extent by 
all Soils, and is of the utmost importance in the nutri- 
tion of plants. If solutions of various kinds (e.g., of 
Potassie Nitrate or Ammonium Nitrate, or of Phos- 
phates of Potassium, Calcium, &c.) are allowed to trickle 
th a moderately thick layer of Soil, it is found 
_ that the water flowing off contains little of these sub- 
stances for a time; but at last the Soil becomes satu- 
rated with them, and allows the solution to pass through 
unchanged. By this property of retaining certain com- 
- _ pounds, Soils are enabled to store up soluble ma: 
_ as well as Ammonia and Nitrates from the S. 
and various substances formed in the changes due t 
weathering of the rocks and Soils; and eae this os 
plants can draw, as they need these substances in their 
food. Owing to this process, filtering impure water 
through, or over, a sufficient extent of earth is a very 
pataras pot of removing impurities, and is frequently 
i t or pu: if . sew A te x qi } . 
it into streams. y ME 
The Capacity for Absorbing and Retaining Heat vari 
Z at 
with colour and texture, with the amount of epy i 
and of evaporation from, the Soil, and with the angle of 
exposure to the rays of the sun, direct or reflected. Of 
course, the actual temperature depends also upon the 
amount of protection afforded by buildings or other ob- 
ee in the neighbourhood 
tom-heat is of great use in stimulating the actio; 
of roots, but in Great Britain it can Me pea paee i 
- 
Soil—continued. 
hotbeds or hothouses. Exposure to the sun’s rays is, 
therefore, the only source of warmth that need be dis- 
cussed here. The more directly the rays fall, the greater 
is their effect. Evaporation keeps down the temperature, 
and it has been found that wet Soils are usually from 
10deg. to 15deg. Fahr. colder than dry ones of the .same 
composition. Hence, draining wet Soils renders them 
warmer, and hastens the ripening of the crops on them. 
Dark grey Soils absorb most heat; next come black Soils, 
then brown and dark red. Pale sands, marls, and clays 
absorb least. The temperature of the Soil exercises a 
marked influence on the growth of plants, since they 
suffer if the buds and leaves are stimulated by a warm 
atmosphere while the soil is too cold to permit of the 
roots supplying the necessary sap to the other organs. 
The Density and Power of Cohesion of the particles of 
Soils, and the Mode of Shrinkage, are of interest and 
importance chiefly on account of their influence on the 
capacities of Soils for moisture, chemical substances, and 
heat. Pure sands show little cohesion, and change 
little in bulk or form during dry weather. Clays are 
very coherent, and may lose as much as one-fifth of 
their bulk by shrinkage during droughts; and peaty 
Soils shrink even more than clays. Cracks form in 
these Soils to a considerable depth, and allow evapora- 
tion to continue; and roots are broken across, and 
exposed to the air in the cracks, or are crushed by the 
shrinkage. 
Soils can frequently be much improved by a judicious 
mixture with others: e.g., sand should be added to clay; 
clay or peat to sand; lime, clay, and sand to peat. 
Oriein. All Soils are formed in one or other of two 
ways, either from the weathering and breaking down 
of rocks, or from the decay of plants or animals, and 
most of them are produced more or less in both ways; 
though the former has been the source of by far the 
greater bulk of all Soils except peat. In some localities, 
it is not difficult to recognise that the Soils are of the 
same composition as the rocks on which they lie, and 
that they are formed by the action of the weather in 
breaking up the rocks and reducing them to fragments. 
In course of time, also, the decay of successive genera- 
tions of plants gives origin to an admixture of humus 
in almost all Soils. But, in most parts of Great Britain, 
the Soils are different in composition from the rocks 
on which they lie, and must have been brought into 
their present situations from a greater or less distance. 
That this should be the case in valleys, is easily under- 
stood, for the Soil on the slopes is constantly being 
carried down by showers and streams, and spread 
over the lower ground. This also occurs on fields 
or meadows along fivers in the lowlands liable to 
be overflowed, as the finer particles of mud in the 
water are deposited on the flooded ground, where free 
from the action of currents. Frost splits pieces of stone 
from exposed rocks and cliffs, and causes them to roll 
down the slopes, and it is also a powerful agent in re- 
ducing rocks and stones into the finer particles of which 
Soils are composed. But a layer of a few inches of 
earth will protect the rocks very greatly, if not 
entirely, from the action of the air, rain, and frost; 
and these agents are not sufficient to account for the 
amount and depth of soil, or for the relations of the soil 
to the subjacent rocks, now prevalent over the country, 
especially in Scotland and in the northern half of England. 
A far more powerful force than any now existing in 
Britain was, however, at work during a comparatively 
recent geological period. This was ice, which, for a long 
time, covered the land (much as Greenland is cove 
now) with a continuous sheet. Formed and renewed oD 
the higher tracts of country, it spread down the valleys, 
and over all but the highest peaks, extended over the 
lowlands of Scotland and Northern England, and met ice 
