MINERAL MANURES.
[How many kinds of action have inorganic manures?
What is the first of these? The second? Third? Fourth?
Do all mineral manures possess all of these qualities?]
The second class of manures named in the general division of the
subject, in the early part of this chapter, comprises those of a mineral
character, or _inorganic_ manures.
These manures have four kinds of action when applied to the soil.
1st. They furnish food for the inorganic part of plants.
2d. They prepare matters already in the soil, for assimilation by roots.
3d. They improve the mechanical condition of the soil.
4th. They absorb ammonia.
Some of the mineral manures produce in the soil only one of these
effects, and others are efficient in two or all of them.
The principles to be considered in the use of mineral manures are
essentially given in the first two sections of this book. It may be
well, however, to repeat them briefly in this connection, and to give
the _reasons_ why any of these manures are needed, from which we may
learn what rules are to be observed in their application.
[Relate what you know of the properties of vegetable ashes?
How does this relate to the fertility of the soil?
According to what two rules may we apply mineral manures?
What course would you pursue to raise potatoes on a soil containing a
very little phosphoric acid and no potash?]
1st. Those which are used as food by plants. It will be recollected that
the _ash_ left after burning plants, and which formed a part of their
structures, has a certain chemical composition; that is, it consists of
alkalies, acids, and neutrals. It was also stated that the ashes of
plants of the same kind are always of about the same composition, while
the ashes of different kinds of plants may vary materially. Different
parts of the same plant too, as we learned, are supplied with different
kinds of ash.
For instance, _clover_, on being burned, leaves an ash containing
_lime_, as one of its principal ingredients, while the ash of _potatoes_
contains more of _potash_ than of any thing else.
In the second section (on soils), we learned that some soils contain
every thing necessary to make the ashes of all plants, and in sufficient
quantity to supply what is required, while other soils are either
entirely deficient in one or more ingredients, or contain so little of
them that they are unfertile for certain plants.
[Would you manure it in the same way for wheat?
Why?]
From this, we see that we may pursue either one of two courses. After we
know the exact composition of the soil--which we can learn only from
correct analysis--we may manure it with a view either to making it
fertile for all kinds of plants or only for one particular plant. For
instance, we may find that a soil contains a very little phosphoric
acid, and no potash. If we wish to raise potatoes on such a soil, we
have only to apply potash (if the soil is good in other particulars),
which is largely required by this plant, though it needs but little
phosphoric acid; while, if we wish to make it fertile for wheat, and all
other plants, we must apply more phosphoric acid as well as potash. As a
universal rule, it may be stated that to render a soil fertile for any
particular plant, we must supply it (unless it already contains them)
with those matters which are necessary to _make_ the ash of that plant;
and, if we would render it capable of producing _all_ kinds of plants,
it must be furnished with the materials required in the formation of
_all kinds of vegetable ashes_.
It is not absolutely necessary to have the soil analyzed before it can
be cultivated with success, but it is the _cheapest_ way.
[How is the fertility of the soil to be maintained, if the
crops are _sold_?
What rule is given for general treatment?
Give an instance of matters in the soil that are to be rendered
available by mineral manures?]
We might proceed from an analysis of the plant required (which will be
found in Section V.), and apply to the soil in the form of manure every
thing that is necessary for the formation of the ash of that plant. This
would give a good crop on _any_ soil that was in the proper _mechanical_
condition, and contained enough organic matter; but a moment's
reflection will show that, if the soil contained a large amount of
potash, or of phosphate of lime, it would not be necessary to make an
application of more of these ingredients--at an expense of perhaps three
times the cost of an analysis. It is true that, if the crop is _sold_,
and it is desired to maintain the fertility of the soil, the full amount
of the ash must be applied, either before or after the crop is grown;
but, in the ordinary use of crops for feeding purposes, a large part of
the ash will exist in the excrements of the animals; so that the
judicious farmer will be able to manure his land with more economy than
if he had to apply to each crop the whole amount and variety required
for its ash. The best rule for practical manuring is probably to
_strengthen the soil in its weaker points, and prevent the stronger ones
from becoming weaker_. In this way, the soil may be raised to the
highest state of fertility, and be fully maintained in its productive
powers.
2d. Those manures which render available matter already contained in the
soil.
[How may silica be developed?
How does lime affect soils containing coarse particles?
How do mineral manures sometimes improve the mechanical texture of the
soil?]
Silica (or sand), it will be recollected, exists in all soils; but, in
its pure state, is not capable of being dissolved, and therefore cannot
be used by plants. The alkalies (as has been stated), have the power of
combining with this silica, making compounds, which are called
_silicates_. These are readily dissolved by water, and are available in
vegetable growth. Now, if a soil is deficient in these soluble
silicates, it is well known that grain, etc., grown on it, not being
able to obtain the material which gives them strength, will fall down or
_lodge_; but, if such measures be taken, as will render the sand
soluble, the straw will be strong and healthy. Alkalies used for this
purpose, come under the head of those manures which develope the natural
resources of the soil.
Again, much of the mineral matter in the soil is combined within
particles, and is therefore out of the reach of roots. Lime, among other
thing, has the effect of causing these particles to crumble and expose
their constituents to the demand of roots. Therefore, lime has for one
of its offices the development of the fertilizing ingredients of the
soil.
3d. Those manures which improve the mechanical condition of the soil.
The alkalies, in combining with sand, commence their action on the
surfaces of the particles, and roughen them--_rust_ them as it were.
This roughening of particles of the soil prevents them from moving among
each other as easily as they do when they are smooth, and thus keeps the
soil from being compacted by heavy rains, as it is liable to be in its
natural condition. In this way, the mechanical texture of the soil is
improved.
It has just been said that _lime_ causes the pulverization of the
particles of the soil; and thus, by making it finer, improves its
mechanical condition.
Some mineral manures, as plaster and salt, have the power of absorbing
moisture from the atmosphere; and this is a mechanical improvement to
dry soils.
[Name some mineral manures which absorb ammonia?]
4th. Those mineral manures which have the power of absorbing ammonia.
_Plaster_, _chloride of lime_, _alumina_ (_clay_), etc., are large
absorbents of ammonia, whether arising from the fermentation of animal
manures or washed down from the atmosphere by rains. The ammonia thus
absorbed is of course very important in the vegetation of crops.
Having now explained the reasons why mineral manures are necessary, and
the manner in which they produce their effects, we will proceed to
examine the various deficiencies of soils and the character of many
kinds of this class of fertilizers.