There are, in addition to the foregoing, certain materials which in a
limited sense only can be called "manures," but the influences of which
are mostly seen in the mechanical and physical improvements which they
effect in soil. Such are salt, and also lime in its different forms.
_Salt._--The action of salt in liberating potash from the soil has
been explained. As a manure it is sometimes used along with nitrate of
soda as a top-dressing for corn crops, in the belief that it stiffens
the straw. For root crops also, and mangels in particular, it is
employed; also for cabbage and other vegetables.
_Lime._--The use of this is almost solely to be considered as a soil
improvement, and not as that of a manure. Sulphate of lime (gypsum)
is, however, occasionally used as a dressing for clover, and also for
hops. The fact that superphosphate itself contains a considerable
amount of sulphate of lime renders the special application of gypsum
unnecessary, as a rule.
As compared with "natural" manures, like farm-yard manure, artificial
manures have the disadvantage that they, unlike it, do not improve the
physical condition of the soil. Artificial manures have, however, the
advantage over farm-yard manure that they can supply in a small
compass, and even if used in small quantity, the needed nitrogen,
phosphoric acid and potash, &c., which crops require, and which
farm-yard manure has but in small proportion. They, further, present
the expensive fertilizing matters in a concentrated form, and by their
application save expense in labour. (J. A. V.*)