Another measure, introduced by modern agriculturists, and the object of which, more especially, is to prevent the exhaustion of the soil, is, the rotation of crops. It is founded on the three following facts: First, some plants, such as wheat, rye, &c., as they have no leaves, and can therefore derive little nutriment from the air, are found to draw much more from the earth than the green crops, and those of the root kind, which do not mature their seeds; and therefore, if cultivated continuously, will in a short time entirely exhaust the soil. Hence the grains are succeeded by clover, beets, turnips, potatoes, &c. Secondly, each species of plants has its own proper food, so that a soil, containing (as most soils do) food for different species, may yield largely of one, after the food for others is exhausted. Thirdly, plants with bulbous roots, like the potato, turnip, beet, &c., serve to divide and loosen the soil, and at the same time send down their roots and radicles to its lower stratum, whereas the roots of grain, &c., spread near the surface.

MANURES are, mineral, vegetable, or animal.

1. Among the mineral manures,* are clay, sand, marl, and lime, in its various forms of quicklime, chalk, gypsum, &c. &c. Which of these is to be employed on a piece of land depends, of course, on the nature of the soil, and the plants to be reared. If the soil be deficient in the power of holding or circulating water, this deficiency is to be supplied by sand, if there is an excess of clay; or by clay, if there is too much sand; or by marl, according as it has predominance of clay or lime. If, again, the soil is wanting in some of those principles which constitute the food of plants, it must variety of roots and artificial grasses, the system of fallowing can no longer be supported by the shadow of a good reason."

*Strictly speaking, mineral manures are not so much food for plants as agents for preparing their food, and also for giving form, strength, and firmness, to their structure. Another use is to prepare vegetables to become food for animals. The bones of animals, the shell of the egg, &c., are formed from the lime and phosphorus taken in with the food and drink,

1

be manured by substances best calculated to afford these principles. We have already remarked, that lime and potass enter into the composition of plants, as may be proved by reducing them to ashes, which will always be found to contain more or less of these substances. It is obvious, that they are introduced through the roots into the plants, and that, if we would supply them, it must be by mingling them with the soil. Hence, one of the important uses, in manuring, of ashes, of gypsum, or the sulphate of lime, of quicklime, &c. In the preference of one of these manures to another, we must be determined by the nature, both of the soil and of the crop. If the soil be clay, it requires lime; and if the crop be Indian corn, clover, the grasses, &c., it should be gypsum, rather than quicklime.

2. Animal manures (among which are included the dung of various animals, urine, bone-dust,* horn-shavings, fish, woollen rags, &c.) are particularly valuable, first, because they decompose rapidly; and secondly, because they afford large quantities of azote, a principle necessary in all plants, but especially so in wheat, cabbage, broccoli, turnips, and radishes, of which azote is a principal constituent.

3. Vegetable manures are useful, on account of the carbonic acid and humic acid which they yield, on fermentation; and also, because, when applied before the straw is completely decomposed, they serve to loosen the soil, and render it lighter, while the process of fermentation, being continued, has the advantage of warming the soil and thus assisting germination. Much discussion has taken place in connexion with this subject,-Sir H. Davy maintaining that manures ought never to ferment

*The use of bone-dust has effected a vast and beneficial change in English husbandry. It is said to be adding annually sixteen millions of bushels of grain to the produce of her fields. Two bushels of bonedust, properly applied, will, on some soils, do as much good as a load of barn-yard manure. Eight hundred thousand dollars' worth are annually imported into Great Britain. Bone-mills have recently been established near Boston, Providence, New York, Albany, &c., but the manure, as yet, is hardly known in the United States.

at all, before they are used, while others, and especially practical farmers, have contended, that rotten dung, or that which has undergone at least a partial fermentation, is more valuable, inasmuch as it retains moisture longer, and has in it a large proportion of humic acid. The truth probably lies partly with both sides, entirely with neither.*

In regard to manures, we conclude with the following practical observations. "The most common manure consists in a mixture of animal, vegetable, and mineral, substances, such as farm-yard litter, night soil, mud from the streets, dust from the roads, or earth from the bottom of ponds and rivers, abounding with organic remains of fish, shells, and rotten plants. Before being laid upon land, it usually requires being well turned up, and exposed to the air for some time; but as soon as it is spread, it should be ploughed in, to prevent loss by evaporation. As to the depth, below the surface of the ground, to which it should be deposited, it may be remarked, that this should never be below the reach of the roots of the plants it is intended to nourish; for, in proportion as it is dissolved and liquefied, it will naturally descend. And it is better to manure lands in the Spring than in the Autumn, lest the Winter rains should dissolve it too much, and endanger its sinking below the roots of the crop. With regard to the quantity of manure, it is a commodity so scarce, that it is not likely to be employed in excess. This occurs,

however, sometimes in garden-culture, and it produces a strong and disagreeable flavor in the vegetables. But the stock of manure is generally so limited, that it has been the study of agriculturists to discover some means of compensating for deficiency, rather than to appre

* The truth is probably expressed by General Armstrong, in these words: "If we wish to obtain one great crop, the rotted dung is best; but when we look to more permanent improvement, the long dung is to be preferred." Experiments show that the first crop, after the manure is applied, is largest with short dung, but that in subsequent years it is not so great.

hend danger from excess. This compensation has been found in a judicious rotation of crops."*

CHAPTER II.

CHEMICAL AGRICULTURE CONTINUED.

THUS far, we have considered the food of plants. There are other important principles, which, if they do not serve as food, are yet as useful, by way of stimulants, as salt is to us; and which, as chemical agents for digesting the food of plants, are quite indispensable. These are, Light, Heat, and Electricity. How indispensable the first two of these are, to vegetation, is well known, by every practical cultivator. From light, plants derive their green color, their taste, smell, and nutritive qualities, as is apparent from the pale and sickly appearance, and the deficient flavor and odor, of such as are reared in the dark. Heat, as every one knows, is necessary, in order to have the seeds germinate, and to promote their rapid and healthy growth. It assists the fermentation and putrefaction, by which the necessary supply of carbonic acid gas and humic acid is produced; while, at the same time, it accelerates the flow of the sap through the vessels of the plant. Since, in Agriculture, the light and heat are derived from the sun, the proportion, in which the soil receives these principles, will depend upon its texture and upon the position of its

* Among other advantages of what is termed root culture, that is, the raising of beets, turnips, potatoes, &c., is the addition which it makes to the manure of a farm, by enabling the farmer to support a larger number of cattle. "It trebles," says Buel," the amount of cattle food, and doubles the quantity of manure.' See Farmer's Companion,' (forming the sixteenth volume of 'THE SCHOOL LIBRARY,') chapter xvi., on Root Culture, page 163.

+ Grapevines in the south of Europe are often furnished with electric conductors. Davy found that corn sprouted more rapidly in water positively electrified by the Voltaic battery, than in water negatively electrified."

surface. In our latitude, the rays of the sun fall so obliquely upon the earth, that a level surface will obviously receive less light, than if it were inclined towards the sun. When, therefore, early vegetation is required, and the soil is strong enough to bear much heat, it will be useful to give it an inclination in this direction. In light soils, however, where there is a tendency to parch, for want of moisture, this position would give the sun too great power, and is therefore to be avoided. The texture of the soil is broken up by the plough and harrow, and thus fitted to receive heat from the sun, while it carries off superfluous moisture, by evaporation. But it should be remarked, that such evaporation abstracts heat rapidly from the earth, and thus occasions, in some cases, the frosts, from which crops are apt to suffer. Similar frosts are occasioned, also, by what is termed radiation; that is, by the passing off of heat from the earth, at night, owing to which, it becomes very cold, and the vapor in the air is congealed. Hence the practice of covering plants, at night, and the use of snow, in protecting them. These serve to prevent the radiation, and thus keep the earth at a temperature friendly to vegetation, or at least not fatal to the life of the plant.

This brief sketch of the method, by which the productiveness of the soil may be maintained and increased, will be sufficient to prove the importance of knowledge, and especially of scientific knowledge. In many respects, the farmer may be regarded as a chemist, conducting experiments on an extensive scale. Plants are the substance with which he deals, and the natural properties of the soil, combined with manures, light, heat, &c., are the agents employed. If he would employ these agents successfully, he must surely understand their nature, and the mutual influence which they exert on each other. He must understand, also, something of the structure and functions of plants; the office performed by the roots, leaves, bark, &c. &c., a knowl