Other formats

    Adobe Portable Document Format file (facsimile images)   TEI XML file   ePub eBook file  


    mail icontwitter iconBlogspot iconrss icon

The Pamphlet Collection of Sir Robert Stout: Volume 73


page 36


In the ash of plants is revealed the lime as well as the other constituents of their mineral food. This does not, of course, apply to the nitrogen of plants, which, though it must be reckoned as of mineral origin in the nitrates of the soil and manures, is expelled in the process of burning them.

The quantity of lime in 100lb. of the ash of various plants is as follows: Wheat grain, 10lb.; oats, 3½lb.; barley, 2½lb.; potatoes, 3lb.; turnips, roots 13lb., leaves 35lb.; mangels, 13lb.; clover hay, 20lb.; red clover, 22lb.; peas and beans, 6lb.; green grass, 15lb.

In 100lb. of the ash of milk there is 12lb. of lime. The ash of bones is nearly half lime.

In the soil, however, there are other important functions performed by lime besides supplying directly the lime food of plants. The quantity of lime in the ash, therefore, must not be taken as the only measure of its value as a manure. In the mineral world lime occurs in various states of combination: First, as carbonate of lime (CaCO3), 100lb. of which when pure contain 56lb. of quicklime (CaO) combined with 44lb. of carbon dioxide (CO2). In this state it constitutes marble, chalk, coral, calcspar (in which, being crystalline, it is quite pure), limestone of various degrees of purity containing from 20 to 98 per cent, of carbonate, stalactites (those pendent icicle-like masses of carbonate hanging from the roofs of many limestone caves), stalagmites (those accumulations of massive carbonate often forming the floor of these same caves), the shells of molluscs (shellfish), eggshells, marl (in which it is present in various proportions mixed with clay and other earthy matters), magnesian limestone or dolomite (in which it exists as carbonate of lime mixed or else combined with carbonate of magnesia).

Limestone, however, may be divided into two classes of different value as a fertiliser: (1.) Fossiliferous or sedimentary limestone, such as the Oamaru and Waihola limestone of Otago, the limestone of the Limestone Plains and Waiau in Southland, and the Forest Hill limestone of Canterbury. This kind of limestone, being of marine origin and fossiliferous, contains phosphate as well as carbonate of lime; and it is this, as well as the magnesia and potash salts which it contains, that gives it additional value. (2.) Lode or vein limestone and marble, stalactites, and stalagmites. These forms of the carbonate of lime were formed out of solutions by the drying up of the water that contained it. They are crystalline and not fossiliferous, and do not contain more than a trace of phosphate, and seldom even that. These are, however, purer or contain a higher percentage of carbonate of lime than do the others, and for this reason they make stronger lime when burnt, will slake more readily, and will act more energetically on the soil. A rich fossiliferous limestone, however, like the Oamaru and Waihola Gorge stone, is to be preferred for the phosphates, magnesia, and potash which it contains. Fossiliferous stone also often contains nitrogen in some state of combination, due, no doubt, to the organic animal and plant matter that was imbedded in it. Carbonate of lime is easily distinguished from other white minerals by the effervescence or boiling-up which is produced on it by the touch of any of the stronger acids (muriatic acid, nitric acid, or sulphuric acid), and by its ready solubility in any of these. The proportion of foreign or earthy matter (clay and sand) which a limestone contains may be roughly estimated by weighing out, say, 1oz. of it (powdered), and adding dilute muriatic acid (one of acid to six of water) till all effervescence ceases. This dissolves out page 37 all the carbonate of lime, leaving the clay and sand undissolved. The examination is finished by pouring the mixture on to a funnel lined with blotting-paper or filter-paper, letting the liquid run through, washing three or four times by pouring water on the residue in the filter-paper, and, finally, burning up the filter-paper and contents and weighing the ash. If an ounce of the limestone give, say, half an ounce of ash it must have contained about half its weight of earthy matter and sand. The difference between the weight of the ash thus got and that of the original limestone gives, of course, the quantity of carbonate of lime in the stone.

Secondly, lime occurs combined with sulphuric acid and water as sulphate of lime or gypsum (CaS04, 2H2O). One hundred pounds of gypsum contain 32½lb. of lime, 46½lb. of sulphuric acid, and 21lb. of water (not wet water, but water chemically combined with the other constituents of the stone).

There is a good deal of gypsum in the Oamaru district, probably of the same marine origin as the limestone itself; but, as it is easily soluble in water, it has mostly got washed out of the exposed part of the stone, and is now found crystallized into the beautiful glassy mineral "selenite," which has the same composition as gypsum itself.

Gypsum is easily identified by the following characters: Grind it to fine powder, and shake the powder up with water for some time; if gypsum is present this will dissolve a portion of it. Now pour off or filter the solution into two wineglasses, and to one of them add a few drops of ammonia: this will not effect any change; then add a solution of oxalate of ammonia, and there will be a fine white precipitate formed in the glass. To the other wineglass add a few drops of chloride of barium, and a white precipitate will form which will be found to be quite insoluble in dilute muriatic acid. If a mineral answer to these simple tests it must contain sulphate of lime or gypsum. Sulphate of lime, being soluble in water, gets into the creeks and rivers of the districts through which they flow, and its presence is the cause of the worst kind of hardness in such water, being a hardness which (unlike the hardness caused by carbonate of lime) is not removed by boiling the water.

The sulphate of lime in the English Thames is the chief cause of the permanent hardness of the London water-supply, and also accounts for the excellence of the London porter and ale, which require hard waters for their perfection.

The other lime-bearing minerals—scheelite, fluor-spar, &c.—would be out of place here, and the phosphates (apatite, coprolites, phosphorite, sombrerite, &c.) have already been described.

It is necessary to distinguish between five substances all containing lime (CaO) in different states of combination:—

1. Carbonate of lime, as it occurs in calcspar, marble, coral, limestone, marl, stalactites, stalagmites, &c. In all these the lime is present in combination with carbonic-acid gas (CO2), and hence the name carbonate of lime.

2. Quicklime, or burnt lime, or simply lime. In this state the lime is got by the process known as the burning of limestone in limekilns. From a chemical point of view it is absurd to talk of burning limestone. It is well enough known that limestone will not burn besides, in all cases of burning a union is formed between the thing that is being burned and the oxygen of the air; but in what is called the burning of limestone there is nothing burned but the fuel that heats the kiln and its contents; and, instead of the limestone forming a union with oxygen or anything else, it is a separation that takes place, as the limestone in the process parts with page 38 its carbonic-acid gas, which escapes away into the air, leaving the lime itself behind in the form of quicklime. The equation is—
  • CaCO3 = CaO + CO2; which means that 100 tons of carbonate of lime, when heated red-hot for several days, give off 44 tons of carbonic-acid gas, and leave a residue of 56 tons of quicklime.

3. Slaked lime, or slacked lime, is quicklime combined with water, which has been poured on it in the process of slaking. Quicklime has a strong attraction for water; in uniting with it the water is rapidly absorbed (appearing to an observer to be drying up); great heat is developed, due to the union of the two; clouds of steam are therefore produced, the mass swells up and bursts, flaking off and crumbling away as far as the water penetrates, till, if just enough water has been added, it has all fallen to a fine white bulky powder.

The equation for slaking lime is,—
  • CaO + H2O = Ca(HO)2 or CaO,H2O.

The molecular weight of quicklime (CaO) is 56, and that of water (H2O) 18. The equation given above therefore means that 56 tons of quicklime will absorb 18 tons of water, and in doing so will be changed into 74 tons of slaked time. If these proportions of quicklime and water be strictly observed the slaked lime will be perfectly dry—just as dry as the quicklime itself was before the water was put into it. The water, indeed, will have entirely disappeared. It is no longer wet water, but water chemically combined with the lime. If less than 18 tons of water be added to the 56 tons of quicklime, then the lime will not be all slaked; it will be a mixture of quicklime and slaked lime. If, on the other hand, more than 18 tons of water be added, then the lime will all be slaked, but it will be wet, for all the water in excess of the 18 tons will be there in its condition of liquid water. In "burning" limestone, either for agricultural use or for mortar, one or two points have to be attended to: (a.) The stone must contain a high percentage of carbonate of lime, to begin with; otherwise in the kiln the lime will be liable to unite with the clay and sand to form a kind of slag, which will not slake properly. This slag, too, will form a glassy mass on the outside of the lumps of stone in the kiln, and prevent the carbonic-acid gas from escaping from the interior, and thus the perfect "burning" of the stone will not be accomplished. (b.) The "burning" should be continued long enough and at a high enough temperature to insure the expulsion of all, or nearly all, the carbonic acid, for if this be not effected the parts left "unburnt" will not slake at all, but will remain as hard cores of raw stone, (c.) Again, too high a temperature in the kiln at an early stage of the burning will, if there is sand or clay in the stone, be apt to fuse or "run" the surface, and especially the sharp edges of the lumps of stone, into the glassy slag mentioned above, thus obstructing the free escape of the gas. The temperature should therefore be raised gradually to the full red pitch, and there maintained for a period depending on the size of the pieces of stone, the quantity of it in the kiln, and on its composition. If the stone be nearly pure carbonate (like Oamaru, or the best Waihola limestone), there is not much fear of overheating, as pure lime itself cannot be fused by the heat of the kiln, and there is little else in the best qualities of these limestones to "run" with the lime. Quicklime is tested to detect the presence of "unburnt" carbonate as follows: A pinch of the powder—as much as will lie on a threepenny bit—taken from the very centre of the largest pieces, is put into a wineglass; three times its bulk of water is added, and then muriatic acid (spirits of salt) till the powder is all dissolved; should there be page 39 any effervescence or bubbling or boiling-up during the addition of the acid, it is a sign that raw stone is present, and the briskness of the effervescence will be an indication of the quantity of such unburnt stuff.

A question might arise whether a given material is raw limestone or quicklime. This would be tested as follows: A portion of the questionable part of the stone is crushed to small pieces; these are shaken up in a bottle three-fourths full of water, allowed to stand for a day, and then shaken up again and filtered through blotting-paper, or filter-paper, in a funnel; or, instead of filtering, the liquid is poured off clear after the sediment has all settled. If it was "unburnt," or raw stone, the solution will not have any particular taste, as it will be just ordinary water: if, on the other band, any part of it was in the state of burnt lime, or quicklime, or slaked lime, it will have a faint caustic taste very perceptible, and if the breath be blown through it for two minutes (by inhaling through the nose and exhaling through a piece of hollow straw or other tube let down 3in. or 4in. into the clear water) a white, light, flaky chalky sediment will form in the water; whereas if it were raw stone such sediment would not form under these circumstances. Quicklime exposed to the air gradually slakes itself by drawing the necessary water from the atmosphere; but it draws carbonic-acid gas at the same time from the same source, and, instead of forming pure slaked lime, it will be, after long exposure, a mixture of slaked lime and carbonate of lime, the carbonate part of it being in the same state as it was before being "burnt." Such a mixture is then said to be "air-slaked." Air-slaked lime is not, of course, so caustic, nor so active in the soil, nor so soluble in water as water-slaked lime. It is, therefore, perhaps, safer in contact with tender young plants just immediately after germination than strong caustic lime would be; but its decomposing beneficial action on the soil would be weakened. When small heaps of quicklime are left out on the paddock for many weeks a good deal of it will have reverted or gone back to this raw condition, and much of its energy will be lost. If, on the other hand, it be at once spread on the ploughed land and harrowed in, it will then slake itself by combining slowly with the moisture of the soil, and the heat that is always produced when quicklime is being slaked will warm the soil itself, and thus provide for the seed a warm bed, which is very favourable to germination.

4. The fourth lime-substance to be considered is lime-water. This is water which has been shaken up with either quick-lime or slaked lime till the water cannot dissolve any more of it A gallon of cold water will, in this way, dissolve a little more than a fifth of an ounce of quicklime, or about a quarter of an ounce of slaked lime.

5. The fifth modification of lime is "milk of lime," which is nothing else than a mixture of lime-water and slaked lime It is made by shaking up with water a greater quantity of quicklime or slaked lime than the water can dissolve. The chemical symbols of these five kinds of "lime" are as follow:—
1.Carbonate of lime—CaCO3.
3.Slaked lime—CaO,H2O.
4.Lime-water—CaO,H2O + (H2O)x.
5.Milk of lime—CaOH2O + (H2O)y. (In numbers 4 and 5 the value of y is less than that of x.)

The action of quicklime or slaked lime in the soil may be described under six heads as follow:—

1. It supplies lime to plants directly, as a necessary part of their mineral food.

page 40

2. Quicklime decomposes and destroys undesirable accumulations of organic matters, such as the brown masses of vegetable débris (composed of roots, stems, leaves, &c., the remains of hundreds of generations of marsh plants) that prove so great an obstacle in the reclamation of swamps. Before applying the lime for this purpose the swamp should first be thoroughly dried by draining, and then the lime should be liberally applied, and at once mixed with the débris in the "quick" [unclear: o] newly-burnt state. It will now slake itself by taking water from the moisture of the organic matter, and, being a strong caustic alkali, will act chemically on the vegetable matter, disorganizing it, combining with part of it, burning up another part, and laying it open to the oxidizing action of those microbes whose life-work it is in the presence of lime or other alkali to consume organic matter, converting its carbon into carbonic acid and its nitrogen into nitric acid, both of which then combine with the lime, and thus furnish a very valuable contribution to the fertility of soil. Of course, by burning the dried vegetable masses of swamp growth a similar result is more speedily obtained, as the ashes will contain carbonate of potash, which serves in this case as a good basis for microbe-action, just as the lime did in the other case: but here the nitrogen, which is doubtless the most valuable constituent of dead organic matter, is lost by the burning. This burning action of quicklime on organic matter is utilised for the destruction of weeds in compost heaps. The weeds and other garden or field débris are collected and built up in alternate layers with lime (weeds and lime time about) into heaps, which, if dry, are then moistened with a sprinkling of water, and left to themselves for some weeks. A strong destructive burning action is soon set up, which rapidly reduces the mixed materials to a fine compost of sweet fertile vegetable mould. In this operation, however, some of the nitrogen will be lost unless the heaps be covered with a loose layer of 6in. or 8in. of damp clay to fix the ammonia that is always produced by such treatment.

3. Another very important chemical effect produced on clay lands by lime—either quick or slaked—is the liberation of potash, magnesia, and phosphoric acid, which are present in the clay in the insoluble, and therefore unserviceable, condition. In releasing these fertilising materials the whole character of the clay itself is changed for the better. If the service of lime is liberal, it loses in a great measure its plastic, stubborn, sticky, hard character, and becomes porous and friable, and thus more amenable to the ripening and pulverising action of the atmosphere, and more accessible to the tender rootlets and root-hairs of plants.

4. Quicklime has a very considerable warming and drying effect on land; the heat being produced, of course, by the spontaneous slaking of the lime by the moisture in the soil itself. In a late spring, after a cold, hard winter, this is a most desirable preparation for the seed-bed.

5. Quicklime or slaked lime also sweetens sour land by uniting with the low acids in the soil (humic, ulmic, and geic. acids, &c.) to which the sourness is due. It is also, for the same reason, in excellent addition in spring to low-lying land that has been for weeks under water.

6. It is claimed for lime, also, that it is very destructive to grubs, and the fungi that cause smut and rust in grain; and it has now been quite established by field experiments that it is the specific for finger-and-toe in turnips. It is not known whether the action of the lime in this last case is to neutralise the acids favourable to the finger-and-toe growth, or, by its caustic quality, to kill the fungus that causes the evil. In either case a dressing of slaked lime is strongly to be recommended for turnips on low-lying rich lands. Superphosphate is an acid manure—the very opposite of lime in that respect; and, if it is found that turnips manured with page 41 that fertiliser on dampish low lands are addicted to finger-and-toe, the lecturer would advise a trial with guano or bonedust, with or without lime, instead of the super. Besides all these benefits brought to the land by lime, it is justly claimed for it that it improves to a marked degree the quality of all the crops—grain, pasture, roots, and especially barley for malting purposes. It also hastens them on to maturity, and thus procures an early harvest. Indeed, it would be difficult to say too much in praise of lime as an improver of the soil. Even raw lime ground to powder, and scattered broadcast on the surface, would in the limeless districts of Otago and Southland have a very marked beneficial action on the soil. The whole district from Palmerston to the lower end of Waihola (with the exceptions of the belt of limestone that crosses from near the Camp on the Peninsula across the bay and the Green Island-Caversham belt), as well as all the country from Invercargill to Balclutha, is badly off for lime in the soil. Not, altogether, that there is a total absence of lime, but that what there is—and it is not much—is so combined with the silica in the clay as to be unavailable. With such splendid agricultural lime in such abundance along the railway-line at Waihola and in the Totara-Oamaru district, it is very provoking that the lime-starved lands between Edendale and Balclutha should not get the benefit of it. The lecturer suggested that, since the prosperity of the country is so much bound up with the success of its farming and grazing pursuits, the Government might, to the advantage of everybody, secure one or more blocks of good limestone country beside the railway, erect limekilns, and burn the limestone in quantities equal to the requirements of the country, or even get the stone ground to fine powder (without burning it at all), and distribute it to farmers at cost price along the line whenever there are trucks otherwise empty passing that way.

A farmer within this district could not, the lecturer believed, spend money more advantageously for the immediate and permanent improvement of his land than in procuring an abundant supply of lime—burnt if it can be had cheap enough, or unburnt and pulverised to the state of fine powder if the other is beyond his reach. Of course, this could not be done adequately without very great reduction in railway freight and the lowest possible first cost at the limeworks, to be arranged in some way by Government with the vendors, who, of course, have a perfect right to get the biggest price they can for their goods. It cannot be done, of course, without Government action in some form or other. In the meantime, travelling by rail from Oamaru to Invercargill, we have to look at our cultivated lands starving and hungering for lime, and to gaze vacantly and hopelessly at the rich interminable deposit of lime, lying ready and waiting for them, but unattainable, owing to the high price of the lime and the cost of conveying it where it is so badly wanted. A generous tariff in this matter of lime would, in increased products, repay to the country the cost of thus improving the land. Dr. Griffiths, in his "Treatise on Manures," 1892, gives the following rule for applying periodic dressings of lime to soils: "For heavy soils, use caustic lime; for light soils, use carbonate of lime; for soils rich in organic matter, use caustic lime; for soils poor in organic matter, use carbonate of lime."

Dr. Griffiths's "caustic lime" is what we call slaked lime, and his "carbonate of lime" is, of course, raw limestone. Gas-lime—the lime which has been used for purifying coal-gas at the gasworks—is another kind of lime-manure. Its value is very varied, according to the proportion of its constituents. Sometimes, indeed—when it contains much sulphite and sulphide of lime—it is positively injurious to plants. Free exposure to the air, with an occasional turning-over to bring up fresh surfaces. page 42 will gradually remove this poisonous quality, by oxidizing the injurious compounds named into sulphate of lime, which is to some degree beneficial, and (like the poisonous compounds) soluble in water. Gas-lime also contains a considerable proportion of slaked lime, ranging from 12 to 40 per cent.; and this portion of it does not differ in any way from ordinary slaked lime. After exposure to the air till all the sulphite and sulphide have been converted into sulphate, there will be from 15 to 40 per cent, of that salt present. The manure in this state should have a market value of, say, 1½. per unit for its slaked lime, and 2d. per unit for its sulphate, thus making up a value ranging from 5s. to 10s. per ton at a short distance from the farm. It is obvious, however, that this form of manure would not stand much expense in carriage.

In recounting the benefits of lime, either as carbonate or slaked, in small quantity, and well distributed in the soil in the latter case, we must not overlook the very important part that it plays in affording a nidus to the nitric-acid microbe and other acid-producing fermentation microbes. To the action of these microscopic organisms is due the production of the nitrates of the soil. Their raw material is the nitrogenous organic matter, chiefly plant débris. The nitrogen contained in this would lie dormant in the soil if it were not for the labours of these minute germs. It is their function to convert it through two, or perhaps three, stages into nitric acid; but they cannot do so unless the nitric acid at the moment of production gets some basic or alkaline substance with which to unite, so as to neutralise itself. Such a substance it gets in lime—nitrate of lime—a valuable fertiliser being the result of the union. In the absence of lime, or conjointly with it, a weak salt of potash, such as the carbonate of wood ashes, would serve the same purpose, nitrate of potash being formed in this case. In a former lecture the formation of nitrate of potash in the soil in some parts of India, Syria, Egypt, and other warm old populous countries was accounted for by this kind of microbe action on dead nitrogenous matter in the presence of these alkalies.