238 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 
caustic lime, generally in two grades, according to the quality of the 
stone so burnt and ground. Another form of lime on the market is a 
finely-powdered calcium hydrate or slaked lime, which, owing to its 
very fine state of subdivision, is very suitable for dry spraying. 
It will be seen from the foregoing review of the various forms of 
lime on the market that care should be exercised in their selection. 
Not only should the grower be sure that he gets the right form, but 
also that the stone from which it is made is suitable for agricultural 
purposes. The fat limes (from white chalk or mountain limestone) 
are preferable to the thin grey or stone limes, which are made from 
less pure and more argillaceous limestones ; fat limes slake better — 
thin limes are apt to set. Magnesian limestones should be avoided, 
as, although they make the best building limes, they contain from 
4 to 40 per cent, of magnesia, which is often considered harmful to 
plant life. It is therefore always advisable to buy lime by analysis, 
which should show at least 80 per cent, pure lime and not more than 
2 per cent, of magnesia. 
Action of Lime on the Soil. — We may now consider the action of 
lime on the soil, in order that we may be able to judge which form 
is the best to use for any particular purpose we have in mind. Liming 
is a very old practice, having been handed down to us from the Ancients ; 
our forefathers used it too heavily, our fathers too sparingly. 
Experience of these extremes is teaching the present generation to 
use it in the most effective and economical quantities. 
First we must mention that lime is in itself a plant food, calcium 
being one of the essentials to plant life ; it is, however, very seldom 
that a soil is encountered that does not contain a sufficient supply 
for the very small needs of most plants, and it is chiefly for what may 
be called its indirect action on plant nutrition that it is of value in 
horticulture. This indirect action may probably best be considered 
from three aspects, viz. (1) the mechanical, (2) the chemical, and 
(3) the biological. Let us first consider its mechanical action on 
the soil. Lime produces a different effect upon different soils. It is 
often said that lime makes heavy soils lighter and light soils heavier, 
an apparent contradiction which, as will be seen, is true at all events 
as far as it applies to heavy soils. On clay soils the adhesive property 
and tendency to puddle is lessened by the application of lime — such a 
soil is made more friable when dry, since lime, unlike clay, does not 
shrink in the process of drying, and therefore the mixture dries off 
in smaller particles instead of contracting into the hard lumps so 
familiar to the agriculturist. In order to understand clearly this 
action of lime on clay soils, it will be necessary to examine into the 
cause of the well-known adhesive and plastic properties of clay. 
The difference between a heavy and a light soil, as shown by 
mechanical analysis, is that the former contains a preponderance of 
very fine particles, some of them so fine as to assume an almost 
gelatinous nature when separated from the coarser particles, whereas 
