d.

_Mode in which gypsum acts._

Despite the fact that gypsum is a most ancient manure, it is only of
late years that we have come to understand the true nature of its
action. For long it was believed that the reason of its striking effect
in promoting clover was due to the fact that, as clover was a
lime-loving plant, the action of gypsum was owing to the lime it
contained. That, however, the action of gypsum is not due to the fact
that it supplies lime to the plant, seems evident when it is stated that
were this so, any other form of lime would have the same beneficial
effect. It is well known, however, that this is not so. Besides, as we
have already pointed out, lime is not a constituent which most soils
lack, so far as the needs of the crop are concerned. There is a certain
amount of truth in the old belief that gypsum enriches the soil in
ammonia by fixing it from the air. The power that gypsum has as a fixer
of ammonia has already been referred to in the chapter on Farmyard
Manure; but in this case the gypsum is brought in contact with the
ammonia. The origin of this old belief was due to a misconception as to
the amount of ammonia in the atmosphere. No doubt gypsum greatly
increases the power of a soil to absorb ammonia from the air; but the
quantity of ammonia in the air is so very trifling, that its action in
this respect is hardly worth considering. The true explanation of the
action of gypsum is to be found in its effect on the double silicates,
which it decomposes, the potash being set free. Its action is similar to
that of other lime compounds, only more characteristic. As a manure,
therefore, its action is indirect, and its true function is to oust the
potash from its compounds. Its peculiarly favourable action on clover is
due to the fact that clover specially benefits by potash, and that
adding gypsum practically amounts to adding potash. Of course it should
be borne in mind that the soil must contain potash compounds if gypsum
is to have its full effect. Now, however, that potash salts suitable for
manuring purposes are abundant, it may well be doubted whether it is not
better to apply potash directly. Further, it must be borne in mind that
gypsum is applied to the soil whenever it receives a dressing of
superphosphate of lime, as gypsum is one of the products formed by
treating insoluble phosphate of lime with sulphuric acid.

It is possible that gypsum may act as an oxidising agent in the soil,