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Farm-yard manure, the average of several analyses:

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the phosphates in the analysis of bones having been partly contained in the easily dissolved matter.

He next compares them as to the quantities of the respective matters found in each, showing that the total organic matter and soluble matter are nearly alike: but that, as compared with farm-yard manure, there is contained in bones, of

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Thus chemical analysis shows, he says, that 1 ton of bone dust is equal to 30 tons of farm-yard manure. The same mode of analysis is also pursued with rape dust, 1 ton of which is made equal to 18 tons of farm-yard manure; and, as this nearly agrees with what is generally considered the ratio in practice, he considers that practice thus confirms scientific results.

In the above analysis, however, the phosphates are made to perform a double part, being compared, as to their action, both as phosphates and saline matter. The soluble matter in farmyard manure, also, should have been 43-565. If these had been properly attended to, the result would have been much less; probably nearer the true amount as regards practice, which he

states himself at 15 to 20. It may also be matter of doubt, that, though a certain quantity of phosphates and other saline matters is needed, we are therefore entitled to affirm that every addition to the quantity of these will produce a corresponding result. Certain quantities are needed as constituents; and, when they are abundant, the plenty of materials may produce more action. Another portion, not so easily estimated, may be needed to act chemically in the soil, in preparing the food, and in the transformations going on in the vital juices to fit them for the assimilating organs. It may be well to provide abundance; waste in these does not take place so readily as in nitrogen and carbon, but there will be a limit. If, however, we had many careful analyses of the different species of manures, conducted in the same way, they would come ultimately to be of great benefit, in enabling practical men to know, when there was a deficiency of ordinary manure, how much of the other substances should be added to make up the deficiency. When manure is required to be carried to great distances, a considerable saving of expense might be found to result from using the more concentrated manures. Peaty soils, or those abounding in brushwood, turf, leaves, branches, &c., producing carbon, are often situated at a distance from towns; and concentrated, nitrogenous, and saline manures might be carried there at much less expense. If tables were once furnished of the quantities of all the different substances drawn off by the different crops usually cultivated, and of the capability of the different manures to supply these, such as those now publishing by Professor Johnson in his Lectures; and if such tables, carefully made out, and corrected by separate analyses of plants grown in different soils and climates and in different seasons, and of manures in different stages of decomposition and in mixtures, were compared and digested; they would furnish data to enable the practical man to know, in all circumstances, how to proceed, how to supply deficiencies in manure, how to make up for exhaustion, and keep his ground properly supplied with food, so that his crops might be duly benefited by the blessings of Providence in sending good weather. To expect exact mathematical results, where so many contingencies are at work, might be Utopian; but something sufficiently near the truth may be hoped to be arrived at, so as to save a vast deal of expense, and greatly increase the value of produce. When substances are deposited as food, without a knowledge of their contents, or the requirements of the plants they are intended to feed, it is going blindly to work, and leaving to hazard what it may be in our power to provide against.

In endeavouring to elucidate the progress made in arriving at correct theoretical views on the action of the different indi

vidual substances forming the food of plants, or assisting in rendering that food available in increasing the produce of plants, the importance of nitrogen to both plants and animals, though undoubtedly sometimes overrated, entitles it to a prominent share of attention. It is the basis of fermentation, which cannot be carried on without nitrogen, whether we may reckon it the fermenting principle itself, or, as some, the food of the fungi which carry on fermentation. It appears that, as in the food of animals the necessary quantity of nitrogen is so mixed up with their ordinary aliment, that in attaining it the other substances, viz. the carbon, hydrogen, oxygen, and saline earthy matters, forming the structure of the body, follow of course; so in plants, when we deposit their food in the soil, the stomach of the plant, it is generally, as in farm-yard manure, a mixture of different substances containing all the elements requisite to build up the structure of the plant, and assist the vital energy of the system in the chemical changes necessary to enable the several organs to perform their functions. Nitrogen forming a constituent, less or more, in all plants and all the parts of plants, especially the youngest and most active parts, and being found in much greater quantity in animals, from the carbon given off by respiration, a mixture of these substances, especially when containing a due proportion of the latter, will always be found, along with the nitrogen, to convey the other substances wanted. It is probable, also, that, even in the function of absorption, the most essential elements are intimately united; the humate and carbonate of ammonia, apparently the greatest source of food to plants, furnishing the carbon and nitrogen, combined with water (or hydrogen and oxygen), the most essential elements of plants.

In the excellent papers lately published in the Gardener's Chronicle from Professor Sprengel, whose great eminence in his profession seems properly united to an intimate acquaintance with practical cultivation (a most essential requisite in bringing science to bear on practice), the benefits of humus have a very important station. In all the mixtures he recommends as necessary to prepare manures for becoming the food of plants, he gives humus a preference, as the most essential requisite in preventing the evaporation of ammonia, and retaining it in the compost in the state of humates and carbonates of ammonia. Even in the solution of bones, humus is the article he recommends, as both rendering the phosphates soluble by the humic and carbonic acid it furnishes, and at the same time absorbing the ammonia of the cartilage. It appears from his practice, that, where sufficient vegetable remains have been mixed with the animal substances usually employed as manures, the humus has been found sufficient to retain, not only the am

monia, but also the phosphuretted and sulphuretted hydrogens, of the composts. Dr. Madden, as we noticed in our former essay, considers ammonia as the greatest solvent of humus, and the way in which the principal part of the carbon and nitrogen of the plant is furnished. It is true that this is also doubted by such eminent men as Liebig, Johnson, and Schlieden, the great liability of the humates to decomposition seeming to be one of the principal objections; but if formed by the every-day action of the manures in the soil, and carried to the roots of plants, their liability to decomposition, if once absorbed, may be a benefit in place of the reverse, and may account for one of the principal objections of Liebig, that humates, or humic acid, are not found to descend to any great depth in the soil.

Nitrogen being so indispensable an article; being necessary, according to Dumas, in forming the fibrin of which all the vessels of plants are composed; being deposited in the form of diastase, gluten, and albumen, wherever food is stored up for the future use of the plant; and, by its action in the form of ammonia, which is largely formed in all young shoots wherever life is most active, probably assisting in the chemical changes necessary to prepare the food for the vital organs; it is of great consequence to know whether the food we administer contains this valuable substance in proper quantity. Being exceedingly volatile in the caustic state of ammonia, it is of great importance to prevent evaporation as much as possible; and, if deposited in composts containing humus or vegetable remains in a state of decay, it is reckoned by some sufficient to prevent escape, and will be united to another essential element of vegetation, thus simplifying and rendering more intelligible the feeding of plants. Such as do not believe in the power of composts to absorb and retain ammonia, and such as dwell in large towns where composts are not so easily to be had, use a variety of substances to fix the ammonia by uniting it to some more powerful acid, of which the best and most economical appears to be sulphuric acid or vitriol.

As to the comparative quantities of nitrogen in crops and manures, Dr. Madden, in his Prize Essay on Physiology and Chemistry applied to Agriculture, published in the Highland Society's Transactions for March last, reckons that in a fourcourse rotation of 30 tons of turnips, 42 bushels of wheat with 2000 lb. of straw, 200 stones of hay, and 48 bushels of oats with 2500 lb. of straw, which the four years would furnish from an acre of ground, there would be produced and carried off, in all, about 8183 lb. of carbon, 248 lb. of azote, and 1190 lb. of saline matter. The manure, at the rate of 30 tons of farm-yard manure, deposited in the soil as a preparation

for the turnips, he calculates at 12,730 lb. of carbon, 280 lb. of azote, and 6104 lb. of saline matter. We thus find that while the manure provides half as much again carbon as needed, and five times the quantity needed of inorganic matters, there is not above part of an overplus in the nitrogen, to allow for what may never reach the roots of plants (being carried off by evaporation or washed away), and for the ammonia which circulates in the vital juices, assisting in the transformations needed to prepare the food for assimilation, and stimulating the activity of the vital principle. Wherever nitrogen is furnished in abundance from the substances deposited in the soil as food, whether in the form of ammonia or nitrates, the plants are found to assume a dark green healthy appearance; the evidence, well known to practical men, of luxuriant vigour of growth, this colour being always assumed in the healthy condition of the plant; though perhaps the alkaline effect of the ammonia on the chromule of the leaf may only denote its presence, and the capability of action, other circumstances being favourable, as the colour has been sometimes found to appear, without the usual consequences of luxuriance in growth following. It might be more beneficial, when the manure is compounded of substances not known to abound in nitrogen, to make such as potatoes, containing little nitrogen, to precede wheat. The analysis of turnips, as given by Professor Johnson in his Elements, would cause a greater quantity of nitrogen to be suspected in the rotation than that of Dr. Madden's statement. He states the gluten and albumen in 25 tons of turnips at 1400 lb.; according to Dr. Prout's estimate of 15.55 per cent, about 217 lb. of nitrogen. Boussingault's estimate of 17 per cent of azote in the turnips would make only about 95 lb.

When the nitrogen of manures is so small in comparative amount, it seems to strengthen the opinion that part of this also is got from the air. Some crops, undoubtedly, derive a great portion of their nitrogen from the air. Boussingault has found it so in Jerusalem artichokes; a more familiar instance, however, is to be found in the bean, which, in an average crop, carries off a great deal more of nitrogen than wheat, and should proportionally exhaust the fertility of the ground in a greater degree; yet, while the wheat is one of the most exhausting crops we have, the bean is rather a fertiliser. The oat also is a very exhausting crop, and contains still less nitrogen than the wheat. The crops of oats which follow beans are more luxuriant than ordinary, as if the bean had been depositing rather than extracting nitrogen. If Professor Johnson's estimate of the quantity of nitrogen carried off by turnips be correct, it is another instance of ground being fertile after what should have been a scourging crop.

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