The Elements of Agriculture Part 10

We will now take under consideration that part of the soil on which depends its ability to supply food to the plant. This portion rarely const.i.tutes more than five or ten per cent. of the entire soil, sometimes less--and it has no reference to the sand, clay, and vegetable matters which they contain. From a.n.a.lyses of many fertile soils, and of others which are barren or of poorer quality, it has been ascertained that the presence of certain ingredients is necessary to fertility. This may be better explained by the a.s.sistance of the following table:

---------------------------+--------------+-------------+---------- In one hundred pounds. Soil fertile Good Barren.

without wheat soil. manure. ---------------------------+--------------+-------------+---------- Organic matter, 9.7 7.0 4.0 Silica (sand), 64.8 74.3 77.8 Alumina (clay), 5.7 5.5 9.1 Lime, 5.9 1.4 .4 Magnesia, .9 .7 .1 Oxide of iron, 6.1 4.7 8.1 Oxide of manganese, .1 .1 Potash, .2 1.7 Soda, .4 .7 Chlorine, .2 .1 Sulphuric acid, .2 .1 Phosphoric acid, .4 .1 Carbonic acid, 4.0 Loss during the a.n.a.lysis 1.4 3.6 .4 +--------------+-------------+---------- 100.0 100.0 100.0 ---------------------------+--------------+-------------+----------

[What can you say of the soils represented in the table of a.n.a.lyses?

What proportion of the fertilizing ingredients is required?

If the soil represented in the third column contained all the ingredients required except potash and soda, would it be fertile?

What would be necessary to make it so?

What is the reason for this?

What are the offices performed by the inorganic part of soils?]

The soil represented in the first column might still be fertile with less organic matter, or with a larger proportion of clay (alumina), and less sand (silica). These affect its _mechanical_ character; but, if we look down the column, we notice that there are small quant.i.ties of lime, magnesia, and the other const.i.tuents of the ashes of plants (except ox.

of manganese). It is not necessary that they should be present in the soil in the exact quant.i.ty named above, but _not one must be entirely absent, or greatly reduced in proportion_. By referring to the third column, we see that these ingredients are not all present, and the soil is barren. Even if it were supplied with all but one or two, potash and soda for instance, it could not support a crop without the a.s.sistance of manures containing these alkalies. The reason for this must be readily seen, as we have learned that no plant can arrive at maturity without the necessary supply of materials required in the formation of the ash, and these materials can be obtained only from the soil; consequently, when they do not exist there, it must be barren.

The inorganic part of soils has two distinct offices to perform. The clay and sand form a ma.s.s of material into which roots can penetrate, and thus plants are supported in their position. These parts also absorb heat, air and moisture to serve the purposes of growth, as we shall see in a future chapter. The minute portions of soil, which comprise the acids, alkalies, and neutrals, furnish plants with their ashes, and are the most necessary to the fertility of the soil.

GEOLOGY.

[What is geology?

Is the same kind of rock always of the same composition?

How do rocks differ?]

The relation between the inorganic part of soils and the rocks from which it was formed, is the foundation of Agricultural Geology. Geology may be briefly named the _science of rocks_. It would not be proper in an elementary work to introduce much of this study, and we will therefore simply state that the same kind of rock is of the same composition all over the world; consequently, if we find a soil in New England formed from any particular rock, and a soil from the same rock in Asia, their natural fertility will be the same in both localities.

Some rocks consist of a mixture of different kinds of minerals; and some, consisting chiefly of one ingredient, are of different degrees of _hardness_. Both of these changes must affect the character of the soil, but it may be laid down as rule that, _when the rocks of two locations are exactly alike, the soils formed from them will be of the same natural fertility, and in proportion as the character of rocks changes, in the same proportion will the soils differ_.

[What rule may be given in relation to soils formed from the same or different rocks?

Are all soils formed from the rocks on which they lie?

What instances can you give of this?]

In most districts the soil is formed from the rock on which it lies; but this is not always the case. Soils are often formed by deposits of matter brought by water from other localities. Thus the alluvial banks of rivers consist of matters brought from the country through which the rivers have pa.s.sed. The river Nile, in Egypt, yearly overflows its banks, and deposits large quant.i.ties of mud brought from the uninhabited upper countries. The prairies of the West owe a portion of their soil to deposits by water. Swamps often receive the was.h.i.+ngs of adjacent hills; and, in these cases, their soil is derived from a foreign source.

We might continue to enumerate instances of the relations between soils and the sources whence they originated, thus demonstrating more fully the importance of geology to the farmer; but it would be beyond the scope of this work, and should be investigated by scholars more advanced than those who are studying merely the _elements_ of agricultural science.

The mind, in its early application to any branch of study, should not be charged with intricate subjects. It should master well the _rudiments_, before investigating those matters which should _follow_ such understanding.

[In what light will plants and soils be regarded by those who understand them?]

By pursuing the proper course, it is easy to learn all that is necessary to form a good foundation for a thorough acquaintance with the subject.

If this foundation is laid thoroughly, the learner will regard plants and soils as old acquaintances, with whose formation and properties he is as familiar as with the construction of a building or simple machine.

A simple spear of gra.s.s will become an object of interest, forming itself into a perfect plant, with full development of roots, stem, leaves, and seeds, by processes with which he feels acquainted. The soil will cease to be mere dirt; it will be viewed as a compound substance, whose composition is a matter of interest, and whose care is productive of intellectual pleasure. The commencement of study in any science must necessarily be wearisome to the young mind, but its more advanced stages amply repay the trouble of early exertions.

FOOTNOTES:

[M] The s.p.a.ces between the particles.

[N] In very many instances the crevices and seams of rocks are permeated by roots, which, by decaying and thus inducing the growth of other roots, cause these crevices to become filled with organic matter. This, by the absorption of moisture, may expand with sufficient power to burst the rock.

[O] Some rocks contain sulphur, phosphorus, etc., and these may, perhaps, be considered as organic matter.

[P] These distinctions are not essential to be learned, but are often convenient.

CHAPTER II.

USES OF ORGANIC MATTER.

[What proportion of organic matter is required for fertility?

How does the soil obtain its organic matter?

How does the growth of clover, etc., affect the soil?]

It will be recollected that, in addition to its mineral portions, the soil contains organic matter in varied quant.i.ties. It may be fertile with but one and a half per cent. of organic matter, and some peaty soils contain more than fifty per cent. or more than one half of the whole.

The precise amount necessary cannot be fixed at any particular sum; perhaps five parts in a hundred would be as good a quant.i.ty as could be recommended.

The soil obtains its organic matter in two ways. First, by the decay of roots and dead plants, also of leaves, which have been brought to it by wind, etc. Second, by the application of organic manures.

[When organic matter decays in the soil, what becomes of it?

Is charcoal taken up by plants?

Are humus and humic acid of great practical importance?]

When a crop of clover is raised, it obtains its carbon from the atmosphere; and, if it be plowed under, and allowed to decay, a portion of this carbon is deposited in the soil. Carbon const.i.tutes nearly the whole of the dry weight of the clover, aside from the const.i.tuents of water; and, when we calculate the immense quant.i.ty of hay, and roots grown on an acre of soil in a single season, we shall find that the amount of carbon thus deposited is immense. If the clover had been removed, and the roots only left to decay, the amount of carbon deposited would still have been very great. The same is true in all cases where the crop is removed, and the roots remain to form the organic or vegetable part of the soil. While undergoing decomposition, a portion of this matter escapes in the form of gas, and the remainder chiefly a.s.sumes the form of carbon (or charcoal), in which form it will always remain, without loss, unless driven out by fire. If a bushel of charcoal be mixed with the soil now, it will be the same bushel of charcoal, neither more nor less, a thousand years hence, unless some influence is brought to bear on it aside from the growth of plants. It is true that, in the case of the decomposition of organic matter in the soil, certain compounds are formed, known under the general names of _humus_ and _humic acid_, which may, in a slight degree, affect the growth of plants, but their practical importance is of too doubtful a character to justify us in considering them. The application of manures, containing organic matter, such as peat, muck, animal manure, etc., supplies the soil with carbon on the same principle, and the decomposing matters also generate[Q] carbonic acid gas while being decomposed. The agricultural value of carbon in the soil depends (as we have stated), not on the fact that it enters into the composition of plants, but on certain other important offices which it performs, as follows:--

[On what does the agricultural value of the carbon in the soil depend?

Why does it make the soil more retentive of manure?

What is the experiment with the barrels of sand?]

1. It makes the soil more retentive of manures.

2. It causes it to appropriate larger quant.i.ties of the fertilizing gases of the atmosphere.