The Elements of Agriculture
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1. The action of frost. When water lodges in the crevices of rocks, and freezes, it expands, and bursts the rock, on the same principle as causes it to break a pitcher in winter. This power is very great, and by its assistance, large cannon may be burst. Of course the action of frost is the same on a small scale as when applied to large masses of matter, and, therefore, we find that when water freezes in the pores13 of rocks or stones, it separates their particles and causes them to crumble. The same rule holds true with regard to stiff clay soils. If they are ridged in autumn, and left with a rough surface exposed to the frosts of winter, they will become much lighter, and can afterwards be worked with less difficulty.
What is the effect of water on certain rocks?
How are some rocks affected by exposure to the atmosphere? Give an instance of this.
2. The action of water. Many kinds of rock become so soft on being soaked with water, that they readily crumble.
3. The chemical changes of the constituents of the rock. Many kinds of rock are affected by exposure to the atmosphere, in such a manner, that changes take place in their chemical character, and cause them to fall to pieces. The red kellis of New Jersey (a species of sandstone), is, when first quarried, a very hard stone, but on exposure to the influences of the atmosphere, it becomes so soft that it may be easily crushed between the thumb and finger.
What is the similarity between the composition of soils and the rocks from which they were formed?
What does feldspar rock yield? Talcose slate? Marls?
Does a soil formed entirely from rock contain organic matter?
How is it affected by the growth of plants?
Other actions, of a less simple kind, exert an influence on the stubbornness of rocks, and cause them to be resolved into soils.14 Of course, the composition of the soil must be similar to that of the rock from which it was formed; and, consequently, if we know the chemical character of the rock, we can tell whether the soil formed from it can be brought under profitable cultivation. Thus feldspar, on being pulverized, yields potash; talcose slate yields magnesia; marls yield lime, etc.
The soil formed entirely from rock, contains, of course, no organic matter.15 Still it is capable of bearing plants of a certain class, and when these die, they are deposited in the soil, and thus form its organic portions, rendering it capable of supporting those plants which furnish food for animals. Thousands of years must have been occupied in preparing the earth for habitation by man.
As the inorganic or mineral part of the soil is usually the largest, we will consider it first.
As we have stated that this portion is formed from rocks, we will examine their character, with a view to showing the different qualities of soils.
What is the general rule concerning the composition of rocks?
Do these distinctions affect the fertility of soils formed from them?
What do we mean by the mechanical character of the soil?
Is its fertility indicated by its mechanical character?
As a general rule, it may be stated that all rocks are either sandstones, limestones, or clays; or a mixture of two or more of these ingredients. Hence we find that all mineral soils are either sandy, calcareous, (limey), or clayey; or consist of a mixture of these, in which one or another usually predominates. Thus, we speak of a sandy soil, a clay soil, etc. These distinctions (sandy, clayey, loamy, etc.) are important in considering the mechanical character of the soil, but have little reference to its fertility.
By mechanical character, we mean those qualities which affect the ease of cultivation—excess or deficiency of water, ability to withstand drought, etc. For instance, a heavy clay soil is difficult to plow—retains water after rains, and bakes quite hard during drought; while a light sandy soil is plowed with ease, often allows water to pass through immediately after rains, and becomes dry and powdery during drought. Notwithstanding those differences in their mechanical character, both soils may be very fertile, or one more so than the other, without reference to the clay and sand which they contain, and which, to our observation, form their leading characteristics. The same facts exist with regard to a loam, a calcareous (or limey) soil, or a vegetable mould. Their mechanical texture is not essentially an index to their fertility, nor to the manures required to enable them to furnish food to plants. It is true, that each kind of soil appears to have some general quality of fertility or barrenness which is well known to practical men, yet this is not founded on the fact that the clay or the sand, or the vegetable matter, enter more largely into the constitution of plants than they do when they are not present in so great quantities, but on certain other facts which will be hereafter explained.
What is a sandy soil? A clay soil? A loamy soil? A marl? A calcareous soil? A peaty soil?
As the following names are used to denote the character of soils, in ordinary agricultural description, we will briefly explain their application:
A Sandy soil is, of course, one in which sand largely predominates.
Clay soil, one where clay forms a large proportion of the soil.
Loamy soil, where sand and clay are about equally mixed.
Marl contains from five to twenty per cent. of carbonate of lime.
Calcareous soil more than twenty per cent.
Peaty soils, of course, contain large quantities of organic matter.16
How large a part of the soil may be used as food by plants?
What do we learn from the analyses of barren and fertile soils?
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 constitutes 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 analyses 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 assistance of the following table:
What can you say of the soils represented in the table of analyses?
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 quantities of lime, magnesia, and the other constituents of the ashes of plants (except ox. of manganese). It is not necessary that they should be present in the soil in the exact quantity 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 assistance 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 mass 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.
GEOLOGYWhat 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 passed. The river Nile, in Egypt, yearly overflows its banks, and deposits large quantities 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 washings 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 grass 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.
CHAPTER II
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 quantities. 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 quantity 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 constitutes nearly the whole of the dry weight of the clover, aside from the constituents of water; and, when we calculate the immense quantity 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 assumes 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 generate17 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 quantities of the fertilizing gases of the atmosphere.
3. It gives it greater power to absorb moisture.
4. It renders it warmer.
1. Carbon (or charcoal) makes the soil retentive of manures, because it has in itself a strong power to absorb, and retain18 fertilizing matters. There is a simple experiment by which this power can be shown.
Ex.—Take two barrels of pure beach sand, and mix with the sand in one barrel a few handfuls of charcoal dust, leaving that in the other pure. Pour the brown liquor of the barn-yard through the pure sand, and it will pass out at the bottom unaltered. Pour the same liquor through the barrel, containing the charcoal, and pure water will be obtained as a result. The reason for this is that the charcoal retains all of the impurities of the liquor, and allows only the water to pass through. Charcoal is often employed to purify water for drinking, or for manufacturing purposes.
Will charcoal purify water?
If a piece of tainted meat, or a fishy duck be buried in a rich garden soil, what takes place?
What is the reason of this?
How does charcoal overcome offensive odors?
How can you prove that charcoal absorbs the mineral impurities of water?
A rich garden-soil contains large quantities of carbonaceous matter; and, if we bury in such a soil a piece of tainted meat or a fishy duck, it will, in a short time, be deprived of its odor, because the charcoal in the soil will entirely absorb it.
Carbon absorbs gases as well as the impurities of water; and, if a little charcoal be sprinkled over manure, or any other substance, emitting offensive odors, the gases escaping will be taken up by the charcoal, and the odor will cease.
It has also the power of absorbing mineral matters, which are contained in water. If a quantity of salt water be filtered through charcoal, the salt will be retained, and the water will pass through pure.
We are now able to see how carbon renders the soil retentive of manures.
1st. Manures, which resemble the brown liquor of barn-yards, have their fertilizing matters taken out, and retained by it.
How does charcoal in the soil affect the manures applied?
Why does charcoal in the soil cause it to appropriate the gases of the atmosphere?
What fertilizing gases exist in the atmosphere?
How are they carried to the soil?
Does the carbon retain them after they reach the soil?
What can you say of the air circulating through the soil?
How does carbon give the soil power to absorb moisture?
2d. The gases arising from the decomposition (rotting) of manure are absorbed by it.
3d. The soluble mineral portions of manure, which might in some soils leach down with water, are arrested and retained at a point at which they can be made use of by the roots of plants.
2. Charcoal in the soil causes it to appropriate larger quantities of the fertilizing gases of the atmosphere, on account of its power, as just named, to absorb gases.
The atmosphere contains results, which have been produced by the breathing of animals and by the decomposition of various kinds of organic matter, which are exposed to atmospheric influences. These gases are chiefly ammonia and carbonic acid, both of which are largely absorbed by water, and consequently are contained in rain, snow, etc., which, as they enter the soil, give up these gases to the charcoal, and they there remain until required by plants. Even the air itself, in circulating through the soil, gives up fertilizing gases to the carbon, which it may contain.
3. Charcoal gives to the soil power to absorb moisture, because it is itself one of the best absorbents in nature; and it has been proved by accurate experiment that peaty soils absorb moisture with greater rapidity, and part with it more slowly than any other kind.
How does it render it warmer?
Is the heat produced by the decomposition of organic matter perceptible to our senses?
Is it so to the growing plant?
What is another important part of the organic matter in the soil?
4. Carbon in the soil renders it warmer, because it darkens its color. Black surfaces absorb more heat than light ones, and a black coat, when worn in the sun, is warmer than one of a lighter color. By mixing carbon with the soil, we darken its color, and render it capable of absorbing a greater amount of heat from the sun's rays.
It will be recollected that, when vegetable matter decomposes in the soil, it produces certain gases (carbonic acid, etc.), which either escape into the atmosphere, or are retained in the soil for the use of plants. The production of these gases is always accompanied by heat, which, though scarcely perceptible to our senses, is perfectly so to the growing plant, and is of much practical importance. This will be examined more fully in speaking of manures.
How is it obtained by the soil?
What offices does the organic matter in the soil perform?
Another important part of the organic matter in the soil is that which contains nitrogen. This forms but a very small portion of the soil, but it is of the greatest importance to vegetables. As the nitrogen in food is of absolute necessity to the growth of animals, so the nitrogen in the soil is indispensable to the growth of cultivated plants. It is obtained by the soil in the form of ammonia (or nitric acid), from the atmosphere, or by the application of animal matter. In some cases, manures called nitrates19 are used; and, in this manner, nitrogen is given to the soil.
We have now learned that the organic matter in the soil performs the following offices:—
Organic matter thoroughly decomposed is carbon, and has the various effects ascribed to this substance on p. 79.
Organic matter in process of decay produces carbonic acid, and sometimes ammonia in the soil; also its decay causes heat.
Organic matter containing nitrogen, such as animal substances, etc., furnish ammonia, and other nitrogenous substances to the roots of plants.
CHAPTER III
What effect has clay besides the one already named?
How does it compare with charcoal for this purpose?
The offices performed by the inorganic constituents of the soil are many and important.
These, as well as the different conditions in which the bodies exist, are necessary to be thoroughly studied.
Those parts which constitute the larger proportion of the soil, namely the clay, sand, and limy portions, are useful for purposes which have been named in the first part of this section, while the clay has an additional effect in the absorption of ammonia.
For this purpose, it is as effectual as charcoal, the gases escaping from manures, as well as those existing in the atmosphere, and in rain-water, being arrested by clay as well as charcoal.20
What particular condition of inorganic matter is requisite for fertility?
What is the fixed rule with regard to this?
What is the condition of the alkalies in most of their combinations? Of the acids?
What is said of phosphate of lime?
The more minute ingredients of the soil—those which enter into the construction of plants—exist in conditions which are more or less favorable or injurious to vegetable growth. The principal condition necessary to fertility is capacity to be dissolved, it being (so far as we have been able to ascertain) a fixed rule, as was stated in the first section, that no mineral substance can enter into the roots of a plant except it be dissolved in water.
The alkalies potash, soda, lime, and magnesia, are in nearly all of their combinations in the soil sufficiently soluble for the purposes of growth.
The acids are, as will be recollected, sulphuric and phosphoric. These exist in the soil in combination with the alkalies, as sulphates and phosphates, which are more or less soluble under natural circumstances. Phosphoric acid in combination with lime as phosphate of lime is but slightly soluble; but, when it exists in the compound known as super-phosphate of lime, it is much more soluble, and consequently enters into the composition of plants with much greater facility. This matter will be more fully explained in the section on manures.
How may silica be rendered soluble?
What is the condition of chlorine in the soil?
Do peroxide and protoxide of iron affect plants in the same way?
How would you treat a soil containing protoxide of iron?
On what does the usefulness of all these matters in the soil depend?
The neutrals, silica, chlorine, oxide of iron, and oxide of manganese, deserve a careful examination. Silica exists in the soil usually in the form of sand, in which it is, as is well known, perfectly insoluble; and, before it can be used by plants, which often require it in large quantities, it must be made soluble, which is done by combining it with an alkali.
For instance, if the silica in the soil is insoluble, we must make an application of an alkali, such as potash, which will unite with the silica, and form the silicate of potash, which is in the exact condition to be dissolved and carried into the roots of plants.