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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 ammonia; 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 i 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 Jolinson in his Elements, would cause a greater quantity of nitrogen to be suspected in the rotation than that of Dr. Madden's state
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 2174 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.
nitrogen is got wholly from the soil, crops should impoverish the soil according to the nitrogen they extract. Many plants which are found to abound in fungi at the roots must excrete a great deal of nitrogen by the process of exosmose, fungi being well known to be voracious of nitrogen for their food. Horse-dung 'contains about the double of the nitrogen that cow-dung does, yet most
crops and most soils are more benefited by the latter than the former. Some crops, as wheat, are found to have gluten deposited in greater abundance in the ear, when manures have been furnished abounding in nitrogen; the proportion of gluten to the starch being much greater. But when manures containing much nitrogen are applied to oats and barley, the gluten is not increased there as in the wheat. All these circumstances seem to point out that there is not always a regular ratio between the nitrogen deposited in the manures, and that carried off in the crops; and that nitrogen is got in some way not yet understood. It is probable, though not hitherto admitted, that, as plants contain a system of air vessels by which the air containing the greater part of its volume in nitrogen is constantly brought into contact with the circulating juices of the plant, ammonia may be formed from the newly liberated or nascent hydrogen, developed in the transformations of the circulating sap, coming in contact with the nitrogen of the air, perhaps also recently deprived of its oxygen by absorption, which is well known to take place. The carburetted hydrogen of the air, stated by Dumas as equal in quantity to the carbonic acid, and the sulphuretted hydrogen lately found so beneficial by Mr. Solly, may, perhaps, also furnish
hydrogen to the plant to assist in the formation of ammonia. The nascent nitrogen from the air, deprived of its oxygen, and confined in the vessels of the plant, if it come in contact with nascent hydrogen there, should be as capable of forming ammonia in that situation as in the manure heap. That plants do get nitrogen, under a form capable of assimilation, in this or some such manner, is evident from their producing it in greater quantity than the ammonia from manure, or that from the air in rain-water to the roots, could be capable of furnishing. It has been generally said by our most scientific writers, that the ammonia is wholly got by the roots : the experiment of Mr. Milne, however, lately narrated in the Gardener's Chronicle, in which, having hung up tin cans containing ammoniacal liquor, and sprinkling it on the floor of a vinery, he found, in 48 hours, the leaves to assume a dark green appearance, and the after-growth to be exceedingly luxuriant, is, I should think, sufficient proof that leaves absorb ammonia from the air when they fall in with it. That nitrogen is not wholly from that provided in the form of ammonia to the roots is evident; that some plants get it from the air, and that probably all have partly that power, seem also evident: the exact tabular data of the quantities in crops and manures may not, therefore, be a perfect guide, and we may not always reap benefit in proportion to the quantity furnished; yet, generally speaking, manures containing much nitrogen are found to have a powerful effect; the exceptions will be best learned by practice.
As urine, and other liquid drainings of the farm-yard, and those furnished domestically, which are so rich in nitrogen, are too frequently allowed to run to waste, the necessity of collecting these substances and depositing them in the compost heap, with plenty of humus earth, which in many places abounds to so great an extent, cannot be too much inculcated.
Peat earth is plentiful in many situations, and has long been known as capable of fermenting and becoming as good as manure, if mixed with it. Sawdust of deciduous plants (hard wood) rots also, and is good for composts; that of fir wood is found to decay very slowly, from the insolubility of the resinous substances, which might, perhaps, be helped to dissolve by mixing with the alkalies of wood-ashes, in which they are soluble. Where leaves can be collected, and clippings of hedges, they are excellent for composts; also the stems, leaves, and roots of garden plants and weeds, in fact all the haulm and refuse of the garden. The straw of grain crops, the stems of potatoes, turf, and scourings of ditches (avoiding those places where there is much deposit of iron from the water), are all capable of yielding humus to the manure heap. Even the roots of couch grass, and other root-weeds, if well fermented, are capable of adding to the bulk and value of the manure heap; and almost all vegetable and animal refuse, which cannot be more beneficially employed. They should all be collected, a layer of these substances and earth put below, and alternate layers of hot fermenting dung and these put together, watered if needed, and too much wetness thrown off by coverings open at the ends and sides. There should be most of the humus earth in the bottom and around the outsides, the fermenting substance will not then escape so much. A proper state as to moisture, neither too wet nor too dry, is very necessary to be attended to, substances being found to putrefy incomparably more quickly when moist, than either wet or dry. Heat should be encouraged by loose strawy matter, to allow the admission of air, and when too great the heap should be turned over to allow it to cool. When too loose and dry it burns and gets white, and a proper degree of consolidation and moisture is necessary. The drainings of the dunghill should not be encouraged by too much wetness being allowed to fall on the heap, but provision should be made for collecting all that comes away, and throwing it on again,