POTASH.

[How may we obtain potash from ashes?

What are some of its agricultural uses?]

When we pour water over wood ashes it dissolves the _potash_ which they contain, and carries it through in solution. This solution is called _ley_, and if it be boiled to dryness it leaves a solid substance from which pure potash may be made. Potash left exposed to the air absorbs carbonic acid and becomes carbonate of potash, or _pearlash_; if another atom of carbonic acid be added, it becomes super-carbonate of potash, or _salaeratus_. Potash has many uses in agriculture.

1. It forms a const.i.tuent of nearly all plants.

2. It unites with silica (a neutral), and forms a compound which water can dissolve and carry into the roots of plants; thus supplying them with an ingredient which gives them much of their strength.[D]

3. It is a strong agent in the decomposition of vegetable matter, and is thus of much importance in preparing manures.

4. It roughens the smooth round particles of sandy soils, and prevents their compacting, as they are often liable to do.

5. It is also of use in killing certain kinds of insects, and, when artificially applied, in smoothing the bark of fruit trees.

The source from which this and the other inorganic matters required are to be obtained, will be fully considered in the section on manures.

SODA.

[Where is soda found most largely?

What is Glauber's salts?

What is washing soda?

What are some of the uses of lime?]

_Soda_, one of the alkalies contained in the ashes of plants, is very much the same as potash in its agricultural character. Its uses are the same as those of potash--before enumerated. Soda exists very largely in nature, as it forms an important part of common salt, whether in the ocean or in those inland deposits known as rock salt. When combined with sulphuric acid it forms sulphate of soda or _Glauber's salts_. In combination with carbonic acid, as carbonate of soda, it forms the common washing soda of the shops. It is often necessary to render soils fertile.

LIME.

_Lime_ is in many ways important in agriculture:

1. It is a const.i.tuent of plants and animals.

2. It a.s.sists in the decomposition of vegetable matter in the soil.

3. It corrects the acidity[E] of sour soils.

4. As chloride or sulphate of lime it is a good absorbent of fertilizing gases.

[How is caustic lime made?

How much carbonic acid is thus liberated?

How does man resemble Sinbad the sailor?]

In nature it usually exists in the form of carbonate of lime: that is, as marble, limestone, and chalk--these all being of the same composition. In manufacturing caustic (or quick) lime, it is customary to burn the carbonate of lime in a kiln; by this means the carbonic acid is thrown off into the atmosphere and the lime remains in a pure or caustic state. A French chemist states that every cubic yard of limestone that is burned, throws off _ten thousand_ cubic yards of carbonic acid, which may be used by plants. This reminds us of the story of Sinbad the sailor, where we read of the immense _genie_ who came out of a very small box by the sea-sh.o.r.e, much to the surprise of Sinbad, who could not believe his eyes, until the _genie_ changed himself into a cloud of smoke and went into the box again. Sinbad fastened the lid, and the _genie_ must have remained there until the box was destroyed.

Now man is very much like Sinbad, he lets the carbonic acid out from the limestone (when it expands and becomes a gas); and then he raises a crop, the leaves of which drink it in and pack the carbon away in a very small compa.s.s as vegetable matter. Here it must remain until the plant is destroyed, when it becomes carbonic acid again, and occupies just as much s.p.a.ce as ever.

The burning of limestone is a very prolific source of carbonic acid.

MAGNESIA.

[What do you know about magnesia?

What is phosphoric acid composed of?

With what substance does it form its most important compound?]

_Magnesia_ is the remaining alkali of vegetable ashes. It is well known as a medicine, both in the form of calcined magnesia, and, when mixed with sulphuric acid, as epsom salts.

Magnesia is necessary to nearly all plants, but too much of it is poisonous, and it should be used with much care, as many soils already contain a sufficient quant.i.ty. It is often found in limestone rocks (that cla.s.s called _dolomites_), and the injurious effects of some kinds of lime, as well as the barrenness of soils made from dolomites, may be attributed entirely to the fact that they contain too much magnesia.

ACIDS.

PHOSPHORIC ACID.

_Phosphoric acid._--This subject is one of the greatest interest to the farmer. Phosphoric acid is composed of phosphorus and oxygen. The end of a loco-foco match contains phosphorus, and when it is lighted it unites with the oxygen of the atmosphere and forms phosphoric acid; this const.i.tutes the white smoke which is seen for a moment before the sulphur commences burning. Being an acid, this substance has the power of combining with any of the alkalies. Its most important compound is with lime.

[Will soils, deficient in phosphate of lime, produce good crops?

From what source do plants obtain their phosphorus?]

_Phosphate of lime_ forms about 65 per cent. of the dry weight of the bones of all animals, and it is all derived from the soil through the medium of plants. As plants are intended as food for animals, nature has provided that they shall not attain their perfection without taking up a supply of phosphate of lime as well as of the other earthy matters; consequently, there are many soils which will not produce good crops, simply because they are deficient in phosphate of lime. It is one of the most important ingredients of manures, and its value is dependent on certain conditions which will be hereafter explained.

Another use of phosphoric acid in the plant is to supply it with a small amount of _phosphorus_, which seems to be required in the formation of the seed.

SULPHURIC ACID.

[What is sulphuric acid composed of?

What is plaster?

What is silica?

Why is it necessary to the growth of plants?

What compounds does it form with alkalies?]

_Sulphuric acid_ is important to vegetation and is often needed to render soils fertile. It is composed of sulphur and oxygen, and is made for manufacturing purposes, by burning sulphur. With lime it forms _sulphate of lime_, which is gypsum or 'plaster.' In this form it is often found in nature, and is generally used in agriculture. Other important methods for supplying sulphuric acid will be described hereafter. It gives _to_ the plant a small portion of _sulphur_, which is necessary to the formation of some of its parts.