Читать книгу The Elements of Agriculture - George Edwin Waring - Страница 6

INORGANIC MATTER

What are ashes called?

How many kinds of matter are there in the ashes of plants?

Into what three classes may they be divided?

What takes place when alkalies and acids are brought together?

We will now examine the ashes left after burning vegetable substances. This we have called inorganic matter, and it is obtained from the soil. Organic matter, although forming so large a part of the plant, we have seen to consist of four different substances. The inorganic portion, on the contrary, although forming so small a part, consists of no less than nine or ten different kinds of matter.2 These we will consider in order. In their relations to agriculture they may be divided into three classes—alkalies, acids, and neutrals.3

Is the character of a compound the same as that of its constituents?

Give an instance of this.

Do neutrals combine with other substances?

Name the four alkalies found in the ashes of plants.

Alkalies and acids are of opposite properties, and when brought together they unite and neutralize each other, forming compounds which are neither alkaline nor acid in their character. Thus, carbonic acid (a gas,) unites with lime—a burning, caustic substance—and forms marble, which is a hard tasteless stone. Alkalies and acids are characterized by their desire to unite with each other, and the compounds thus formed have many and various properties, so that the characters of the constituents give no indication of the character of the compound. For instance, lime causes the gases of animal manure to escape, while sulphate of lime (a compound of sulphuric acid and lime) produces an opposite effect, and prevents their escape.

The substances coming under the signification of neutrals, are less affected by the laws of combination, still they often combine feebly with other substances, and some of the resultant compounds are of great importance to agriculture.

ALKALIES

The alkalies which are found in the ashes of plants are four in number; they are potash, soda, lime and magnesia.

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 salæratus. Potash has many uses in agriculture.

1. It forms a constituent 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.4

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 constituent of plants and animals.

2. It assists in the decomposition of vegetable matter in the soil.

3. It corrects the acidity5 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-shore, 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 compass as vegetable matter. Here it must remain until the plant is destroyed, when it becomes carbonic acid again, and occupies just as much space 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 quantity. It is often found in limestone rocks (that class 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 constitutes 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.

NEUTRALS

SILICA

How can you prove its existence in corn stalks?

What instance does Liebig give to show its existence in grass?

How do we supply silicates?

Why does grain lodge?

What is the most important compound of chlorine?

This is sand, the base of flint. It is necessary for the growth of all plants, as it gives them much of their strength. In connection with an alkali it constitutes the hard shining surface of corn stalks, straw, etc. Silica unites with the alkalies and forms compounds, such as silicate of potash, silicate of soda, etc., which are soluble in water, and therefore available to plants. If we roughen a corn stalk with sand-paper we may sharpen a knife upon it. This is owing to the hard particles of silica which it contains. Window glass is silicate of potash, rendered insoluble by additions of arsenic and litharge.

Liebig tells us that some persons discovered, between Manheim and Heidelberg in Germany, a mass of melted glass where a hay-stack had been struck by lightning. They supposed it to be a meteor, but chemical analysis showed that it was only the compound of silica and potash which served to strengthen the grass.

There is always enough silica in the soil, but it is often necessary to add an alkali to render it available. When grain, etc., lodge or fall down from their own weight, it is altogether probable that they are unable to obtain from the soil a sufficient supply of the soluble silicates, and some form of alkali should be added to the soil to unite with the sand and render it soluble.

CHLORINE

Of what use is chloride of lime?

What is oxide of iron?

What is the difference between the peroxide and the protoxide of iron?

Chlorine is an important ingredient of vegetable ashes, and is often required to restore the balance to the soil. It is not found alone in nature, but is always in combination with other substances. Its most important compound is with sodium, forming chloride of sodium (or common salt). Sodium is the base of soda, and common salt is usually the best source from which to obtain both soda and chlorine. Chlorine unites with lime and forms chloride of lime, which is much used to absorb the unpleasant odors of decaying matters, and in this character it is of use in the treatment of manures.

OXIDE OF IRON

Oxide of iron, one of the constituents of ashes, is common iron rust. Iron itself is naturally of a grayish color, but when exposed to the atmosphere, it readily absorbs oxygen and forms a reddish compound. It is in this form that it usually exists in nature, and many soils as well as the red sandstones are colored by it. It is seldom, if ever, necessary to apply this as a manure, there being usually enough of it in the soil.

This red oxide of iron, of which we have been speaking, is called by chemists the peroxide. There is another compound which contains less oxygen than this, and is called the protoxide of iron, which is poisonous to plants. When it exists in the soil it is necessary to use such means of cultivation as shall expose it to the atmosphere and allow it to take up more oxygen and become the peroxide. The black scales which fly from hot iron when struck by the blacksmith's hammer are protoxide of iron.

The peroxide of iron is a very good absorbent of ammonia, and consequently, as will be hereafter described, adds to the fertility of the soil.

What can you say of the oxide of manganese?

How do you classify the inorganic constituents?

Oxide of Manganese, though often found in small quantities in the ashes of cultivated plants, cannot be considered indispensable.

Having now examined all of the materials from which the ashes of plants are formed,6 we are enabled to classify them in a simple manner, so that they may be recollected. They are as follows:—

2

Bromine, iodine, etc., are sometimes detected in particular plants, but need not occupy the attention of the farmer.

3

This classification is not strictly scientific, but it is one which the learner will find it well to adopt. These bodies are called neutrals because they have no decided alkaline or acid character.

4

In some soils the fluorides undoubtedly supply plants with soluble silicates, as fluoric acid has the power of dissolving silica. Thus, in Derbyshire (England), where the soil is supplied with fluoric acid, grain is said never to lodge.

5

Sourness.

6

There is reason to suppose that alumina is an essential constituent of many plants.