NEUTRALS.

SILICA.

[How can you prove its existence in corn stalks?

What instance does Liebig give to show its existence in gra.s.s?

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 const.i.tutes 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 gla.s.s is silicate of potash, rendered insoluble by additions of a.r.s.enic and litharge.

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

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 _per_oxide and the _prot_oxide 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 const.i.tuents 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 cla.s.sify the inorganic const.i.tuents?]

OXIDE OF MANGANESE, though often found in small quant.i.ties 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,[F] we are enabled to cla.s.sify them in a simple manner, so that they may be recollected. They are as follows:--

ALKALIES. ACIDS. NEUTRALS.

Potash. Sulphuric acid. Silica.

Soda. Phosphoric " Chlorine.

Lime. Oxide of Iron.

Magnesia. " Manganese.

FOOTNOTES:

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

[C] This cla.s.sification 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.

[D] 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.

[E] Sourness.

[F] There is reason to suppose that _alumina_ is an essential const.i.tuent of many plants.

CHAPTER V.

GROWTH.

[Of what does a perfect young plant consist?

How must the food of plants be supplied?

Can carbon and earthy matter be taken up at separate stages of growth, or must they both be supplied at once?]

Having examined the materials of which plants are made, it becomes necessary to discover how they are put together in the process of growth. Let us therefore suppose a young wheat-plant for instance to be in condition to commence independent growth.

It consists of roots which are located in the soil; leaves which are spread in the air, and a stem which connects the roots and leaves. This stem contains sap vessels (or tubes) which extend from the ends of the roots to the surfaces of the leaves, thus affording a pa.s.sage for the sap, and consequently allowing the matters taken up to be distributed throughout the plant.

[What seems to be nature's law with regard to this?

What is the similarity between making a cart and raising a crop?

In the growth of a young plant, what operations take place about the same time?]

It is necessary that the materials of which plants are made should be supplied in certain proportions, and at the same time. For instance, carbon could not be taken up in large quant.i.ties by the leaves, unless the roots, at the same time, were receiving from the soil those mineral matters which are necessary to growth. On the other hand, no considerable amount of earthy matter could be appropriated by the roots unless the leaves were obtaining carbon from the air. This same rule holds true with regard to all of the const.i.tuents required; Nature seeming to have made it a law that if one of the important ingredients of the plant is absent, the others, though they may be present in sufficient quant.i.ties, cannot be used. Thus, if the soil is deficient in potash, and still has sufficient quant.i.ties of all of the other ingredients, the plant cannot take up these ingredients, because potash is necessary to its life.

If a farmer wishes to make a cart he prepares his wood and iron, gets them all in the proper condition, and then can very readily put them together. But if he has all of the _wood_ necessary and no _iron_, he cannot make his cart, because bolts, nails and screws are required, and their place cannot be supplied by boards. This serves to ill.u.s.trate the fact that in raising plants we must give them every thing that they require, or they will not grow at all.

In the case of our young plant the following operations are going on at about the same time.