Elements of Agricultural Chemistry

The ash contained—

More elaborate analyses of the same fluid have since been made by Dr. Voelcker, with the subjoined results per gallon:—

The differences are here very remarkable, especially in the quantity of ammonia, which is exceedingly large in the first sample. All of them are particularly rich in potash, and contain but a small proportion of phosphoric acid. The general inference to be deduced from them is, that liquid manure is a most important source of the alkalis and ammonia, and must be peculiarly valuable on soils in which these substances are deficient.

The system of liquid manuring, originally introduced by Mr. Kennedy of Myremill, Ayrshire, and which has since been adopted in some other places, differs from liquid manuring in its strict sense, for not only are the drainings of the manure-heap employed, but the whole solid excrements are mixed with water in a tank, and rape-dust and other substances occasionally added, and distributed through the pipes.

It has been abandoned on Mr. Kennedy's farm, but is in use at Tiptree Hall, and on the farm of Mr. Ralston, Lagg, where the fluid is distributed by gravitation.

The arrangements employed by Mr. Mechi are identical with those formerly in use at Myremill. The greater part of the stock is kept on boards, and the liquid and solid excrements are collected together in the tank, and largely diluted before distribution. The liquid from the tanks has been recently examined by Dr. Voelcker, who found it to contain per gallon—

The quantity of this liquid distributed per acre is about 50,000 gallons, at a cost of 2d. per gallon. As this quantity contains about 39 lbs. of ammonia, it must be nearly equivalent to 2 cwt. of Peruvian guano, which costs, with the expense of spreading, from 28s. to 30s. per acre, while the cost of distributing the liquid exceeds £1: 17s. per acre. On the other hand, the rapidity with which liquid manure produces its effect must be taken into account. It is on this that its chief value depends, and especially when applied to grass land in early spring, it produces an abundant crop just when turnips and other winter food are exhausted. Mr. Telfer, Cunning Park, who has used this system for a good many years, has come to the conclusion that it is only in this way that it can be made profitable; and though pipes are laid all over his farm, he has latterly restricted the use of the liquid manure entirely to Italian ryegrass. Its effect on the cereals is much less marked, and it can scarcely be considered as capable of advantageous application to the general operations of the farm. Neither can liquid manure be applied to all soils. It fails entirely on heavy clays, but is peculiarly adapted to light sandy soils; and even barren sand may by its repeated application, be made to yield luxuriant crops. It is not likely that the system of liquid manuring will extend, except in localities where it is possible to distribute it by gravitation; and even then, it will probably be found most economical to restrict its use to one portion of the farm; and for that purpose, the poorest and most sandy soil ought to be selected.

Sewage Manure.—The use of the sewage of towns as a manure is closely connected with that of the liquid manure produced on the farm. Its application must take place in a similar manner, and be governed by the same principles. Although numerous attempts have been made to convert it into a solid form, or to precipitate its valuable matter, none of them have succeeded; nor can it be expected that any plan can be devised for the purpose, because the most important manurial constituents are chiefly soluble, and cannot be converted into an insoluble state, or precipitated from their solution. In its liquid form, however, sewage manure has been employed with the best possible effect in the cultivation of meadows. The most important instance of its application is in the neighbourhood of Edinburgh, where 325 acres receive the sewage of nearly half the town, and have been converted from barren sand into land which yields from £20 to £30 per acre. The contents of the sewer, taken just before it flows into the first irrigated meadow, near Lochend, were found to contain per gallon—

It is interesting to notice that this sewage is superior in every respect to the liquid manure used at Tiptree Hall; and the good effects obtained from its application, in the large quantities in which it is used in the Craigentinny meadows, may be well imagined. It operates, not merely by the substances which it holds in solution, but also by depositing a large quantity of matters carried along in suspension, and is in reality warping with a substance greatly superior to river-mud. A deposit collected in a tank, where the sewage passes through a farm, is used as a manure, and contains—

And even, though containing more than half its weight of water and 20 per cent of sand, this substance has considerable value as a manure.

The growing evils of the existing system of sewage, and the enormous waste of a manurial matter, which the experience of the Craigentinny meadows has shewn to be productive of the most important effects, has recently directed much attention to the conversion of the contents of our sewers into a useful manure. Numerous plans for its precipitation and conversion into a solid manure have been proposed, but most of these have shewn an entire ignorance of the fundamental principles of chemistry, and the best only succeed in precipitating a very small proportion of its valuable matters, and leave almost the whole of the ammonia, as well as the greater part of the fixed alkalies, in solution. Nor is it to be expected that any process will be discovered by which these substances can be precipitated, because solubility is the special characteristic of their compounds, and no means is known by which it is possible to convert them into an insoluble form. If sewage is to be used at all, there seems little doubt that it must be by applying it entire, and in the liquid state. But here again, the expense of conveying it on to the land becomes an obstacle which it must frequently be impossible to overcome. When it can be conveyed by gravitation, as is the case in the neighbourhood of Edinburgh, it may undoubtedly be used with the utmost advantage, and with the very best economic results. But when it requires to be carried to a great distance through pipes, and raised to a high level by pumping, all these advantages disappear. If the cost of application amounts to 2d. a gallon, as in Mr. Mechi's case, or even to half that sum, it may be fairly concluded that it cannot be used with any great prospect of large economic results, and that, unless under very exceptional cases, it must be unprofitable.

The chances of success must also greatly depend upon the kind of soil on which it is used. Experience has shewn that its effects are most beneficial on light and deep sandy soils, but that on heavy retentive clays it is without effect, or even absolutely injurious. In clay soils it is important to use every means of getting rid of moisture, and any plan which adds 200 or 300 tons of water to them, only aggravates their natural defects to an extent which more than counterbalances the benefits derived from the manurial matter it contains. Whatever the ultimate result of the use of town sewage in the liquid form may be, it is unlikely that it will be employed in general agricultural practice. It is more probable that it will be found necessary to set apart a certain breadth of land to be treated by it exclusively. Many plans have been proposed for conveying it through considerable districts, and selling to the surrounding farmers the quantities which they require, but wherever large sewage-works are established, it will be impossible to depend on a precarious demand, and the promoters of such schemes will be compelled, as part of their speculation, to supply not only the manure, but the land on which it is to be used. Indeed, the difficulties attending the whole question are so formidable, that even those who are most anxious to see a stop put to the waste of manurial matter must admit that the prospect of a successful economic result is not encouraging. Nor is it likely that anything will be done until the whole system of managing town refuse is changed, and in place of deluging it with water, some plan can be contrived which, while fulfilling sanatory requirements, shall preserve it in a concentrated form, or convert it into a dry and inodorous substance.

CHAPTER IX.

COMPOSITION AND PROPERTIES OF VEGETABLE MANURES.

Many vegetable substances have been employed as manures, either alone or as auxiliaries to farm-yard manure. Like that substance, they are general manures, and contain all the constituents of ordinary crops; but, owing to the absence of animal matter, they in general undergo decomposition and fermentation much more slowly, although some of them contain a so largely preponderating proportion of nitrogen, that they may in some respects be compared to the strictly nitrogenous manures.

Rape-dust, Mustard, Cotton and Castor Cake.—Rape-dust has long been employed as a manure, and the success which has attended its use has led to the introduction of the refuse cake from some other oil seeds, such as those of mustard and castor-oil, which cannot be employed for feeding. Like the seeds of all plants, these substances are rich in nitrogen, and their ash, containing of course all the constituents of the plant, supplies the necessary inorganic elements. The following are analyses of these substances, which, in addition to the amount of nitrogen and phosphates, shew also that of water and oil, to which reference will be made in a future chapter, in relation to the feeding value of some of them. The detailed composition of their ash may be judged of from that of the seeds from which they are made, and which have been given under that head.

A general similarity may be observed in the composition of all these substances; they are rich in nitrogen, and contain as much of that element as is found in six or seven times their weight of farm-yard manure, and a somewhat similar proportion exists in the amount of phosphates, and probably of their other constituents. They have all been employed with success, but the most accurate observations have been made with rape-dust, which has been longer and more extensively used than any of the others. It has been employed alone for turnips, or mixed with farm-yard manure, and also as a top-dressing to cereals. But the most marked advantage is derived from it when applied in the latter way on land which has been much exhausted, and its effects are then very striking. An adequate supply of moisture is essential to the production of its full effects, and hence it often proves a failure in very dry seasons, and on dry soils. It must not be applied in too great abundance, experience having shewn that after a certain point has been reached, an increase in the quantity produces no benefit, and even sometimes positively diminishes the crop. The other substances of the same class, in all probability, act in the same way, but as their introduction is recent, and their use limited, less is known regarding their effects.

Malt-Dust, Bran, Chaff, etc.—The value of these substances as manures is chiefly dependent on the nitrogen they contain, though to some extent also on their inorganic constituents. Malt-dust contains about 4·5 per cent, and bran 3·2 per cent of nitrogen. But they are little used as manures, as they can generally be more advantageously employed for feeding. The value of chaff more nearly resembles that of straw.

Straw is occasionally employed as a manure, and sometimes even as a top-dressing for grass land. It is generally admitted, however, that its application in the dry state, and especially as a top-dressing, is a practice not to be recommended, as it decomposes too slowly in the soil; and it is always desirable to ferment it in the manure heap, so as to facilitate the production of ammonia from its nitrogen. Still circumstances may occur in which it becomes necessary to employ it in the dry state, and it will generally prove most valuable on heavy soils, which it serves to keep open, and so promotes the access of air, and enables it to act on the soil. On light sandy soils it generally proves less advantageous, as its tendency of course is to increase the openness of the soil, and render it less able to retain the essential constituents of the plant.

The quantity of nitrogen in straw does not exceed 0·2 per cent, and its value is mainly due to its inorganic constituents and to its mechanical effect on the soil.

Saw-dust has little value as a manure, as it undergoes decomposition with extreme slowness. It is a good mechanical addition to heavy soils, and diminishes their tenacity; and though its manurial effects are small, it sooner or later undergoes decomposition, and yields what valuable matters it contains. The saw-dust of hard wood is to be preferred, both because it contains more valuable matters than that of soft wood, and because the absence of resinous matters permits its more rapid decomposition. It is a useful absorbent of liquid manure, and may be advantageously added to the dung-heap for that purpose.

Manuring with Fresh Vegetable Matter—Green Manuring.—The term green manuring is applied to the system of sowing some rapidly growing plant, and ploughing it in when it has attained a certain size, and the success attending it, especially on soils poor in organic matters, is very marked. It is obvious that this mode of manuring can add nothing to the mineral matters contained in the soil, and its utility must therefore be due to the plant gathering organic matters from the air, which, by their decomposition, yield nitrogen and carbonic acid—the former to be directly made use of by subsequent crops, the latter, in all probability, acting also on the soil, and setting free its useful constituents. Hence those plants which obtain the largest quantity of their organic elements from the air ought to be most advantageous for green manuring. The plants used for this purpose act also as a means of bringing up from the lower parts of the soil the valuable matters which exist in it out of reach of ordinary crops, and mixing them again with the surface part. Many of the plants found most useful for green manuring send down their roots to a considerable depth; and when they are ploughed in, all the substances which they have brought up are of course deposited in the upper few inches of the soil. Vegetable matter when ploughed in in the fresh state, also decomposes rapidly, and is therefore able immediately to improve the subsequent crop; and as this decomposition takes place in the soil without the loss of ammonia and other valuable matters, which is liable to occur to a greater or less extent when they are fermented on the dung-heap, it will be obvious that in no other mode can equally good results be obtained by its use.

Many plants have been employed as green manure, and different opinions have been expressed as to their relative values. In the selection of any one for the purpose, that should of course be taken which grows most rapidly, and produces within a given time the largest quantity of valuable matters, but no general rule can be given for the selection, as the plant which fulfils those conditions best will differ in different soils and climates. The plants most commonly employed in this country are spurry, white mustard, and turnips. Rye, clover, buckwheat, white lupins, rape, borage, and some others, have been largely employed abroad. Some of these are obviously unfitted for the climate of the British Islands; and the others, although they have been tried occasionally, do not appear to have been very extensively employed. The turnip is sown broadcast at the end of harvest, and ploughed in after two months. White mustard and spurry are employed in the same way as a preparation for winter wheat, and with the best results. The latter is sometimes sown as a spring crop in March, ploughed in in May, and another crop sown which is ploughed in in June, and immediately followed by a third. The effect of this treatment is such that the worst sands may be made to bear a remunerative crop of rye.

It is not easy to estimate the addition made by green manuring to the valuable matters contained in the soil, but it is probably far from inconsiderable. A crop of turnips, cultivated on the ordinary agricultural system, after two months' growth, weighs between five and seven tons per acre, and contains nitrogen equivalent to about 48 lbs. of ammonia, and half a ton of organic matters; but nothing is known as to the quantity produced when it is sown broadcast, and is not thinned, although it must materially exceed this. Neither is it possible to determine the relative proportions derived from the soil and the air, although it is, in all probability, dependent on the resources of the soil itself,—plants grown on a rich soil obtaining their chief supplies from it, while, on poorer soils, a larger proportion is drawn from the atmosphere. Hence light and sandy soils are most benefited by green manuring, partly on this account, and partly also, no doubt, because the valuable inorganic matters, which are so liable to be washed out of these soils, are accumulated by the plants and retained in them in a state in which they are readily available for the subsequent crop.

Sea-Weed.—Sea-weeds have been employed from time immemorial as a manure on the coasts of Scotland and England, in quantities varying from 10 to 20 tons per acre. Their action is necessarily similar to that of green manure ploughed in, as they contain all the ordinary constituents of land plants.

The subjoined analyses of three of the most abundant species will sufficiently indicate their general composition.

The first four analyses give the composition of the weeds after they have been separated from all foreign substances; the last, that of the mixture taken from the heap just as it is used in Orkney; and its value is then enhanced by small shells and marine animals adhering to the plants, which increase the amount of phosphoric acid and nitrogen.

The ease with which all sea-weeds pass into a state of putrefaction, adapts them in a peculiar manner to the manurial requirements of a cold and damp climate. The rapidity of their decomposition is such, that when spread on the land they are seen to soften and disappear in a short time. They form therefore a rapid manure, and their effects are said to be confined to the crop to which they are applied; but this is probably due to the fact, that they are chiefly used in inferior sandy soils, in which any manure is rapidly exhausted. In good soils there is no reason why their effect should not be as lasting as that of farm-yard manure, which, in many particulars, they considerably resemble. The method of applying sea-weeds most generally in use, is to spread them on the soil, and plough them in after putrefaction has commenced, and it is on the whole the most advantageous. But they are sometimes composted with lime and earth, or mixed with farm-yard manure, and occasionally, also, they are used as a top-dressing to grass land.

On some parts of the western coast of Scotland and in the Hebrides, sea-weed is the chief manure. It gives excellent crops of potatoes, but they are said to be of inferior quality, unless marl or shell-sand is employed at the same time.

Leaves may be used as a manure, simply by ploughing them in, by composting them with lime, or by adding them to the manure heap.

Peat.—As a source of organic matter, peat may be used with advantage, especially on soils in which it is naturally deficient. Dry peat of good quality contains about one per cent of nitrogen, and a quantity of ash varying from five to twenty per cent. These substances, however, become available very slowly, owing to the tardy decay of peat in its natural state; and in order to make it useful, it is necessary to compost it with lime, or to mix it with farm-yard manure, or some readily putrescible substance, so that its decomposition may be accelerated. It may be most advantageously used as an absorbent of liquid manure, and on this account, forms a useful addition to the manure heap.

The observations which have been made regarding the use of these substances, lead directly to the inference that all vegetable matters possess a certain manurial value, and that they ought to be carefully collected and preserved. In fact, the careful farmer adds everything of the sort to his manure heap, where, by undergoing fermentation along with the manure, their nitrogen becomes immediately available to the plant; while the seeds of weeds are destroyed during the fermentation, and the risk of the land being rendered dirty by their springing up when the manure comes to be used is prevented.

CHAPTER X.

COMPOSITION AND PROPERTIES OF ANIMAL MANURES.

Manures of animal origin are generally characterized by the large quantity of nitrogen they contain, which causes them to undergo decomposition with great rapidity, and to yield the greater part of their valuable matters to the crop to which they are applied.

Guano.—By far the most important animal manure is guano, which is composed of the solid excrements of carnivorous birds in a more or less completely decomposed state, and is accumulated in immense quantities on the coasts of South America and other tropical countries. It has been used as a manure in Peru from time immemorial, but the accounts given by the older travellers of its marvellous effects were considered to be fabulous, until Humboldt, from personal observation, confirmed their statements. It was first imported into this country in 1840, in which year a few barrels of it were brought home; and from that time its importation rapidly increased. Soon after large deposits of it were found in Ichaboe; and it has since been brought from many other localities. The quantity of guanos of all kinds imported into this country and retained for home consumption now exceeds 240,000 tons a year.

The value of guano differs greatly according to the extent to which its decomposition has gone, and this is chiefly dependent on the climate of the locality from which it is obtained. When deposited in the rainless districts of Peru it still retains some of the uric acid and the greater part of the ammonia naturally existing in it, and the quantity which has escaped by decomposition is unimportant. But that obtained from other districts has suffered a more or less complete decomposition according to the humidity of the climate, which reduces the quantity of organic matters and ammonia, until, in some varieties, they are so small as to be of little importance. The following are minute analyses of three specimens of Peruvian guano, shewing all the different constituents it contains, and the amount of difference which may exist:—

These analyses illustrate two points—first, that in some samples the decomposition has advanced to a greater extent than in others; for we observe that the quantity of uric acid, or rather of urate of ammonia, is greatly less in the last analysis than in the other two, and much smaller than in the fresh dung, which contains from 50 to 70 per cent of uric acid; and secondly, that guano is rich in all the constituents of the plant, but especially in ammonia, the best form in which nitrogen can be supplied, in uric acid which by decomposition yields ammonia, and in phosphoric acid. But such analyses are too elaborate for ordinary purposes, and much less convenient for comparison and for estimating the value of the guano than the shorter analysis commonly in use, which gives the water, the loss by ignition (that is, the sum of the organic matters and ammoniacal salts), the phosphates, the alkaline salts, and the quantity of phosphoric acid contained in them, and existing there in a state similar to that in which it is found in the soluble phosphates of a superphosphate. In addition to these, the quantities of sand and other less valuable ingredients are also stated.

In the subjoined tables the composition of a great variety of different kinds of guano is given. Most of these are averages deduced from a considerable number of analyses of good samples. Those of some kinds of guano, such as Peruvian, which present a considerable amount of uniformity, afford a sufficiently accurate idea of the general composition of the variety, but in other cases they are of less value, because the imports of different seasons, and even of different cargoes, differ so greatly in composition that no proper average can be made. Several of these varieties are already exhausted, the importation of others has ceased, and new varieties are constantly being introduced.

Table showing the Average Composition of different varieties of Guano.