gain, all the Brassicas, whether Kales, Cauliflower, or whatever
else, agree nearly with the Cabbage in the prominent presence of lime
and sulphur; ingredients which fully account for the offensive odour of
these vegetables when in a state of decay. Fruits as a rule are highly
charged with alkalies, and are rarely deficient in phosphates; moreover,
stone-fruits require lime, for they have to make bone as well as flesh
when they produce a crop. As regards the alkalies, plants appear capable
of substituting soda for potash under some circumstances, but it would
not be prudent for the cultivator to assume that the cheaper alkali
might take the place of the more costly one as a mineral agent, for
Nature is stern and constant in her ways, and it can hardly be supposed
that a plant in which potash normally predominates can attain to
perfection in a soil deficient in potash, however well supplied it may
be with soda. The cheaper alkali in combination as salt (chloride of
sodium) may, however, be usually employed in aid of quick-growing green
crops; and more or less with tap-roots and Brassicas. Salt, too, is very
useful in a dry season by reason of its power of attracting and
retaining moisture. As regards Potatoes, it is worthy of observation
that they contain but a trace of silica, and yet they generally thrive
on sand, and in many instances crops grown on sand are free from disease
and of high quality, although the weight may not be great. The
mechanical texture of the soil has much to do with this; and when that
is aided by a supply of potash and phosphates, whether from farmyard
manure or artificials, sandy soils become highly productive of Potatoes
of the very finest quality. On the other hand, Potatoes also grow well
on limestone and chalk, and yet there is but little lime in them. Here,
again, mechanical texture explains the case in part, and it is further
explained by the sufficiency of potash and phosphates, as also of
magnesia, which enters in a special manner into the mineral constitution
of this root.

Thus far we have not even mentioned nitrogen, or its common form of
salts of ammonia; nor have we mentioned carbon, or its very familiar
form of carbonic acid. These are important elements of plant growth; and
they account for the efficacy of manures derived directly from the
animal kingdom, as, for example, the droppings of animals, including
guano, which consisted originally of the droppings of sea-birds. Some of