portions and forms almost
poisonous to plants. But there are profound resources in most clays, so
that if it is difficult to tame them, it is also difficult to exhaust
them. Hence a clay that has been well cultivated through several
generations will generally produce a fair return for whatever crop may
be put upon it. Limestone soils are usually very porous and deficient of
clay, and therefore have no sustaining power. Many of our great tracts
of mountain limestone are mere sheep-walks, and would be comparatively
worthless except for the lime that may be obtained by burning. On the
other hand, chalk, which is a more recent form of carbonate of lime, is
often highly productive, more especially where, through long
cultivation, it has been much broken up, and has become loamy through
accumulation of humus. Between the oldest limestone and the latest chalk
there are many intermediate kinds of calcareous soils, and they are
mostly good, owing to their richness in phosphates, the products of the
marine organisms of which these rocks in great part, and in some cases
wholly, consist. For the growth of cereals these calcareous soils need a
certain proportion of silica, and where they have this we see some of
the finest crops of Wheat, Trifolium, Peas and Beans in these islands.
If we could mix some of our obdurate clays with our barren limestones,
the two comparatively worthless staples would probably prove remarkably
fertile. Although this is impossible, a consideration of the chemistry
of the imaginary mixture may be useful, more especially to the gardener,
who can in a small way accomplish many things that are impracticable on
a great scale. Sandy soils are characterised by excess of silica, and
deficiency of alumina, phosphates and potash. Here the mechanical
texture is as serious a matter as it is in the case of clay. The sand is
too loose as the clay is too pasty, and it may be that we have to
prevent the estate from being blown away. It is especially worthy of
observation, however, that sandy soils are the most readily amenable of
any to the operation of tillage. If we cannot take much out of them, we
can put any amount into them, and it is always necessary to calculate
where the process of enrichment is to stop. It is not less worthy of
observation that sandy soils can be rendered capable of producing almost
every kind of crop, save cereals and pulse, and even these can be
secured where there is some basis of peat or loam