The necessity for the steady supply of phosphorus and lime to the body is the cause of the popularity of Mapes's superphosphate of lime as a manure. The farmers who buy it, perhaps, do not know that their bones and other parts are made of it, and that this is the reason they must furnish it to their land; for between the land and the farmer's bones are two or three other factories that require the same material. All the farmer knows is, that his grass and his corn grow better for the superphosphate. But what he has not thought of we will tell you,—that man finds his phosphate of lime in the milk and meat of the cow, and she finds her supply in the grass and corn, which look to the farmer to see that their stock of this useful mineral compound does not fall short. Thus in milk and meat and corn, which constitute so large a part of our diet, we have always our phosphate of lime. There are many other sources whence we can derive it, but these will do for the present. And thus, when an animal dies and has no further use for his phosphate of lime, it is washed into the soil around, after decomposition of the body has set it free, and goes to make new grass and corn. Bone-earth (pounded bones) is a common top-dressing for grass-lands.
A small proportion of sulphur is found in flesh and blood. We prove its presence in the egg by common experience. An egg—from which it escapes more easily than from flesh—discovers its presence by blackening silver, as every housekeeper knows, whose social position is too high for bone egg-spoons or too low for gold ones. This passion which sulphur entertains for silver is very strong, as every one knows who has ever been under that wholesome discipline which had its weekly recurrence at the delightful institution of Dotheboy's Hall; and what Anglo-Saxon ever grew up, innocent of that delectable vernal medicine to which we refer? Has he not found all the silver change in his pocket grow black, suggesting very unpleasant suspicions of bogus coin? The sulphur, being more than is wanted in the economy of the system, has made its escape through every pore in his skin, and, of course, fraternizes with the silver on its way. But it was of the sulphur which is natural to the body and always found there that we were speaking. When the animal dies, and the vital forces give way to chemical affinities, when the phosphorus and the rest take their departure, the sulphur, too, finds itself occupation in new fields of duty.
Chlorine and sodium, two more of the elements of animal structures, produce, in combination, common salt,—without which our food would be so insipid, that we have the best evidence of its being a necessary article of diet. The body has many uses for salt. It is found in the tears, as we are informed by poets, who talk of "briny drops" and "saut, saut tears"; though why there, unless to keep the lachrymal fluid from spoiling, in those persons who bottle up their tears for a long time, we cannot divine.
Perhaps we had better take the rest into consideration together,—the magnesia and iron, and whatever other elements are found in the body. Though some of them are there in minute quantities, the structure cannot exist without them,—and for their constant and sufficient supply our food must provide.
To see what becomes of all these materials after we have done with them, we must extend our inquiries among the articles of ordinary diet and ascertain from what sources we derive the several elements.
It has been sometimes believed that none but animal food contains all the elements required for the support of life. Thanks to Liebig, we have discovered that vegetable substances also, fruits, grains, and roots, contain them all, and, in most cases, in very nearly the same proportions as they are found in animals. We are not lecturing on dietetics; therefore we will not pause to explain why, although either bread or meat alone contains the various materials for flesh and bone, it is better to combine them than to endeavor to subsist on one only.
Whither, then, go these elements when man has done with them? The answer is,—All Nature wants them. Every plant is ready to drink them up, as soon as they have taken forms which bring them within its reach. As gases, they are inhaled by the leaves, or, dissolved in water, they are drunk up by the roots. All plants have not the same appetites, and therefore they can make an amicable division of the supply. Grasses and grains want a large proportion of phosphate of lime, which they convert into husks. Peas and beans have little use for nitrogen, and resign it to others. Cabbages, cauliflowers, turnips, and celery appropriate a large share of the sulphur.
The food of plants and that of animals have this great difference: plants take their nourishment in inorganic form only; animals require to have their food in organic form. That is, all the various minerals, singly or combined, which compose the tissues of plants and animals,—carbon, hydrogen, phosphorus, and the rest, which we have already named,—are taken up by plants in mineral form alone. The food of animals, on the other hand, consists always of organized forms. There is no artificial process by which oxygen, carbon, and hydrogen can be brought into a form suitable for the nourishment of animals. As oxygen, carbon, and hydrogen, they are not food, will not sustain our life, and human art cannot imitate their nutritious combinations. Artificial fibrine and gluten (organic principles) transcend our power of contrivance as far as the philosopher's stone eluded the grasp of the alchemists. We know exactly how many equivalents of oxygen, hydrogen, carbon, and nitrogen enter into the composition of each of the animal elements; but we can no more imitate an organic element than we can form a leaf. What we cannot do the vegetable world does for us. Thus we see why it was necessary that the earth should be clothed with vegetation before animals could be introduced. A field-mouse dies and decays, and its elements are appropriated by the roots around its grave; and we can easily imagine the next generations of mice, the children and grandchildren of the deceased rodent, feasting off the tender bark which was made out of the remains of their parent. The soil of our gardens and the atmosphere above it are full of potential tomatoes, beans, corn, potatoes, and cabbages,—even of peaches of the finest flavor, and grapes whose aroma is transporting.
Plants, as well as animals, have their peculiar tastes. Cut off the supply of phosphate of lime from a field of corn, and it will not grow. You can easily do this by planting the same land with corn for three or four successive years, and your crop will dwindle away to nothing, unless you supply the ground every year with as much of the mineral as the corn takes away from it. All plants have the power of selecting from the soil the materials necessary to their growth; and if they do not find them in the soil, they will not grow. It is now a familiar fact, that, when an old forest of deciduous trees has been felled, evergreens will spring up in their places. The old oaks, hickories, and beeches, as any observer would discover, pass their last years in repose, simply putting out their leaves and bearing a little fruit every year, but making hardly any new wood. An oak may attain to nearly its full size, in spread of branches, in its first two hundred years, and live for five or six hundred years longer in a state of comparative rest. It seems to grow no more, simply because it has exhausted too much of the material for its nourishment from the ground around its roots. At least, we know, that, when we have cut it down, not oaks, but pines, will germinate in the same soil,—pines, which, having other necessities and taking somewhat different food, find a supply in the ground, untouched by their predecessor. Hence the rotation of crops, so much talked of by agriculturists. Before the subject was so well understood, the ground was allowed to lie fallow for a year or two, when the crops began to grow small, that it might recover from the air the elements it had lost. We now adopt the principle of rotation, and plant beans this year where last year we put corn.
It is not merely that plants deprive themselves of their future support by exhausting the neighboring earth of the elements they require. Some of them put into the ground substances which are poisonous to themselves or other plants. Thus, beans and peas pour out from their roots a very notable amount of a certain gum which is not at all suited to their own nourishment,—so that, if we plant beans in the same spot several successive seasons, they thrive very poorly. But this gum appears to be exactly the food for corn; if, therefore, we raise crops of beans and corn alternately, they assist each other. Liebig gives the results of a series of experiments illustrating the reciprocal actions of different species of plants. Various seeds were sprouted in water, in order to observe the nature of the excretions from their roots. It was found "that the water in which plants of the family of the Leguminosae (beans and peas) grew acquired a brown color, from the substance which exuded from their roots. Plants of the same species, placed in water impregnated with these excrements, were impeded in their growth, and faded prematurely; whilst, on the contrary, corn-plants grew vigorously in it, and the color of the water diminished sensibly, so that it appeared as if a certain quantity of the excrements of the Leguminosae had really been absorbed by the corn-plants." The oak, which is the great laboratory of tannin, not only lays up stores of it in its bark and leaves, but its roots discharge into the ground enough of it to tan the rootlets of all plants that venture to put down their suction-hose into the same region, and their spongioles are so effectually closed by this process, that they can no longer perform their office, and the plant that bears them dies. Plants whose roots ramify among the roots of poppies become unwilling opium-eaters, from the exudation of this narcotic principle into the ground, and are stunted, like the children of Gin Lane.