570 Cultivation of Plants. 
ur purity it is white, and receives the name of chalk. In this 
state it constitutes a poor soil, absorbs little heat from the 
sun’s rays, dissolves under the action of rain, blisters and flakes 
from frost, laying bare the roots of plants growing upon it. 
Earth containing from 40 to 60 per cent., the remainder being 
composed of equal parts of argillaceous and silicious matter, is 
termed calcareous, and is generally very favourable for cultiva- 
tion. It thus constitutes a light soil, easily worked; but a 
larger proportion of lime renders it very adhesive when moist. 
Silicious, or sandy soil is diametrically opposite in its 
physical qualities to argillaceous soils. It is rough or gritty 
to the touch, light, friable and loose, permitting the passage 
of water with great facility, drying with the slightest sunshine, 
and rapidly accumulating heat. Pure silicious sand, such as 
results from the disintegration of sand-rocks, is in its nature 
quite unproductive; but if mixed with a certain proportion of 
humus, and especially if it contain from 15 to 20 per cent. of 
argillaceous and calcareous matter, it is suitable for the support 
of many plants. A warm, poor, well-drained soil of this descrip- 
tion is eminently favourable for the growth of tender subjects. 
Fine silicious sandy soil, with sufficient humus or vegetable 
mould to appear brown or nearly black, plays an important 
part in horticulture under the name of peat-earth. On account 
of its lightness it is admirably adapted for raising small seeds, 
lut frequent waterings are necessary in consequence of the 
facility with which it dries up. It is, moreover, the only soil 
suitable for the culture of plants that grow naturally in peaty 
or boggy places, such as Heaths, Rhododendrons, Azaleas, and 
the Ericaces generally, and a few members of other families, 
collectively known ax American plants, though in point of fact 
many of them are not of American origin. 
Combinations of the three principal elements above de- 
seribed vary exceedingly in their relative proportions, and also 
by the addition of other matters which modify their physical 
properties. Hence the term mixed soils, as applied to the 
soils designated, accurding to their composition, argillo-arena- 
ceous, in which lime is almost wholly wanting; argillo-cal- 
careous, consisting mainly of clay and lime; caleareo-silicious, 
nearly destitute of clay. Ferruginous soil reccives its name 
from the presence of a large quantity of carbonate of iron, 
wiving it a reddish tinge. But these secondary distinctions 
are of little importance in practical gardening, as the soil, on 
