« 上一頁繼續 »
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 80 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, adding a little more earth and haulm if necessary. All the urine, night-soil, soap-suds, soot, and all the domestic refuse, should be carefully added to the heap; the coal-ashes, unless sifted to a small powder, being kept by themselves. Sea-weed, when to be had, is an excellent ingredient in such composts; and all brush-wood, furze, &c., chopped small.
All animal remains, refuse of fish, hair, wool, rags, horn, bruised bones, and all refuse of the kind, should be carefully collected : animal remains are the richest in nitrogen. To a mixture of the above substances, half the bulk of manure when coarse, and about I or when more concentrated, should, when rotted together, be equal in value to the same bulk of rotted manure; preventing the loss in the way manure is generally managed, and increasing the quantity in an immense degree. When the heap is near the field less dung may suffice to mix, but when it is to be carted far it should be rich, to save expense; the heap ferments more perfectly with a good proportion of hot manure. For such as root-weeds and seeds of weeds, there must be a hot fermentation to destroy them. When there is not much stable manure to mix with the compost, a mixture of wood-ashes, lime, and other substances yielding alkalies, is very beneficial, in causing the production of humic acid, forming humates with the alkalies in place of carbonic acid. Where much ammonia is in the compost, which will be the case wherever animal remains and excrements abound, lime is apt to cause the escape monia, by decomposing its carbonates and humates; and only so much should be used as will saturate the surplus of humic acid not taken up with ammonia. Sprengel recommends about To for some composts; too much should, above all, be avoided. When sulphate of lime, sulphate of soda, and muriate of soda (salt), are cheap, they will be useful to add. The refuse of glue manufactories is rich in nitrogen. The refuse of woollen factories is rich in soap-suds, urine, &c. The refuse of tanners and skinners, the hair, skin, wool, and hoofs, is rich in nitrogen. The refuse of gas-works is well known as beneficial.
As regards the way in which carbon, forming the greatest proportion of any of the elements in plants, is obtained, there is still very considerable difference of opinion. As we noticed in former essays, Professor Liebig is of opinion that most, if not all, is got by the leaves from the air; while Professor Schlieden, one of the most eminent physiologists of the day, in a criticism of the Chemistry and Physiology applied to Agriculture, of Liebig, (translated into the Gardener's Chronicle,) seems to be of a completely opposite opinion. The action of leaves on a growing branch confined in a vessel filled with air, in which it has been said the carbonic acid has, after a time, been found diminished and the oxygen accumulated, has always
been brought forward as proof of the fact that plants get most of their carbon from the air. These experiments, however, are difficult to manage so as to prevent error; and Schlieden asserts that, in the average of recorded experiments, it has been found that the enclosed air of the vessel has neither been altered in its quantitive nor qualitive relations. Dumas expresses himself hesitatingly on the point, and, though he leans to the supposition of the carbon being got principally from the air, and undoubtedly, he thinks, from carbonic acid, yet notices the great quantity of carbonic acid found by Boucherie to issue from the trunks of divided trees when felled in full sap, evidently derived, he says, from the roots. Professor Johnson seems to lean to the supposition that the greater part is from the air. Professor Sprengel and Dr. Madden seem to be of opinion that the greater part of the carbon is got in the humic acid absorbed with the ammonia : the latter allows that very little of the carbon is got in the state of carbonic acid by the roots, while many are of opinion that what is got by the roots is principally in that form. I shall have an opportunity of entering more at length into this subject in the article «Vegetable Physiology." While so much difference of opinion, however, prevails among learned men on the subject, practical men will do well to preserve and deposit in the soil the carbon, as well as the other portions of the manure. Fortunately, in the mixed manures generally applied, there are carbon, nitrogen, and inorganic matter; and in most soils, though not all, as asserted by some, there is generally a reserve of humus to assist when neglected. I doubt it would be found very difficult to grow plants luxuriantly in washed sand, with either ammonia or saline matter, or both, in an ordinary atmosphere. Such an experiment, however, might throw more light on the subject than most of those tried. Solutions of nitrates, sulphates, and phosphates of ammonia, potash, soda, lime, and magnesia, with silicates of potash and soda, and a little common salt, would furnish the nitrogen and inorganic substances wanted; but, I fear, would not produce luxuriance of growth without carbon.
On the subject of inorganic manures, the experiments recorded this season are manifold, and, as might have been expected, much at variance. Some applications, in certain circumstances, appearing to have had little effect; others to have done much good, and some to have done harm. The tables furnished of the quantities of these substances found in the different plants under cultivation, and the quantities furnished by the different kinds of manure, will, when properly regulated and corrected, show what are the wants of plants as to constituents, and how far the food deposited is capable of supplying these wants, a proper allowance being given for the portion washed away from soils; but there is still more to be done before correct data can be furnished. It is evident that saline substances are wanted for other purposes besides forming constituents. The quantity found in the young branches and leaves of trees is great, in comparison with what is found in the trunks; the quantity found in trees is comparatively much less than that found in annual crops; and the quantity in these is also much greater in the young succulent growing portions, than in the ripened tissue.
These facts all show that a liberal comparative supply is needed for the young growth, teaching the necessity of applying these and ammonia early to young seedling plants; and also teaching that these substances are necessary to assist in the transformations going on where life is most active, to fit the circulating juice for the purposes of the organs of assimilation, and that, where extra vigour is wished, a liberal supply of these substances must be furnished. The effects of this supply may be observed, wherever the burnt ashes of young unsaleable trees, or the clippings of hedges, have been applied to vegetable crops ; I have seen the effect often such as to defeat the end intended, by an over-luxuriance of stem and leaves to root crops.
I have seen very powerful effects, this season, follow the mixing of composts for pots with ashes of small branches burned and bruised to small pieces. At Roselle, the geranium leaves were like those of tussilago for size, of a deep green, and the vigour of growth so great as to injure the flowering. The excellent preparation of branches, straw, cabbage leaves, and other haulm, set fire to and kept at a smothered heat till charred, as pointed out in the November Number of the Magazine by Mr. Barnes of Bicton Gardens, will be very powerful. The greater the variety in the small branches, leaves, roots, &c., charred, the more likely are the ashes to answer general purposes. They contain the great variety of saline substances found in the most vital portions of the plants burned, which accounts for much of their action. Leaves will not grow, nor vital activity become active, till all the essentials of vital chemistry are provided; though the compost they are furnished with contains sufficient of carbon and ammonia. The ashes, also, act mechanically, as keeping the soil open by their elasticity, which is one of the principal benefits of farm-yard manure. They also absorb oxygen and ammonia from the soil and air; or, if the oxygen is from water, perhaps form ammonia from the nascent hydrogen, and absorb it, as most oxidising substances likely do. These absorbed gases will be given off to the roots. Roots are always exceedingly fond of running round such porous substances, and are found to increase