elve years since it was
comparatively little known, but it has now come to be used in many cases
in which Peruvian guano was formerly employed. It produces a better
effect than that manure on light soils, although in general a mixture of
the two answers better than either separately. When Peruvian guano is
to be applied along with it, the farmer will naturally select a
superphosphate made from bone ash, and containing the largest obtainable
quantity of soluble phosphates; but when it is to be used alone, it is
advisable to take one made from bones, or at all events one containing a
considerable quantity of nitrogenous matter or ammonia. The kind to be
selected must, however, be greatly dependent on the particular soil, and
the situation in which it is to be used.

_Lime._--Lime is by far the most important of the mineral manures, and
an almost indispensable agent of agricultural improvement. It has been
used as chalk, marl, shell and coral sand, ground limestone, and as
quick and slaked lime, and its action varies according as it is applied
in any of its natural forms, or after being burnt. In all of its native
forms the lime is combined with carbonic acid in the proportion of
fifty-six parts of lime to forty-four of carbonic acid, and the
carbonate is generally mixed with variable quantities of earthy
ingredients, which in some instances are important additions to it, and
affect its utility as a manure.

_Chalk_ is a very pure form of carbonate of lime, and where it abounds
has been largely employed as an application on the soil. It is dug out
of pits and exposed to the action of the winter's frost, by which it is
thoroughly disintegrated, and in spring it is applied in quantities,
which, in many instances, are only limited by the question of cost.

_Marl_ is a name given to a mixture of finely-divided carbonate of lime,
with variable proportions of clay and siliceous matters, which is found
at the bottom of valleys and in hollow places in beds often of
considerable extent and thickness, where it is deposited from the waters
of lakes holding lime in solution, fed by streams passing over
limestone, or rocks rich in lime. The composition of marls differs
greatly in different districts, and they have been divided into true
marls, and clay marls, according as the carbonate of lime or clay is the
preponderating ingredient. The following table illustrates the
composition of different varieties:--

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