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The Pamphlet Collection of Sir Robert Stout: Volume 73

Analysis of Soil

page 59

Analysis of Soil.

The process recommended by the lecturer for analysing samples of soil is as follows:—

A mere bald statement of the composition of a soil—the percentage of phosphoric acid, lime, potash, magnesia, oxide of iron, and nitrogen—would not be of much use to farmers. The lime, the magnesia, and the potash might be, partly or wholly, so combined with the silica which all soil contains that the growing crops could not take them up. Something more, therefore, must be stated in a useful analysis of a soil than the percentage of its constituents. To meet this difficulty the analysis should be so conducted as to show how much of the lime, magnesia, and potash are readily soluble in boiling hydrochloric acid of a particular strength (acid of specific gravity 1.15 is usually employed), and also how much of the same constituents are insoluble in such boiling acid. The former (the lime, magnesia, and potash soluble in the acid) may be regarded as immediately available for the use of the plants, because they will be also soluble in the carbonic-acid moisture of the soil; and the latter (the lime, magnesia, and potash insoluble in the acid) as a store of nutrition, held in reserve for future requirements, which, though not at once available, will slowly be rendered soluble by exposure to the air and frost and rain as the result of the proper working of the soil by ploughing, harrowing, draining where required, liming, &c. This division into "solubles" and "insolubles" will be sufficient for ordinary purposes; and it is the division the lecturer would take in the present course of lectures, although, as he informed the students, a further division of the "solubles" is sometimes made into "soluble in water" and "insoluble in water, but soluble in hot hydrochloric acid."

Before describing the analytical processes for soil the lecturer explained how to take a sample of soil from the paddock. A sample taken from the surface (3in. or 4in., say, on the top) would not fairly represent the general body of soil, nor would, of course, a sample taken exclusively from the subsoil. The former would contain an undue proportion of the residues of any manures that had ever been applied, and also an excess of vegetable roots and other débris of former crops; and the latter (the subsoil) would err on the other side. In order, therefore, to get a fair estimate of the quality of the soil it is necessary in all cases to take the sample in such a way that it will represent fairly both surface and subsoil to the depth for which the information is required. This depth is usually taken as 12in.; and the sample is taken as follows: A piece of galvanised iron, 12in. long by about 9in. wide, is bent round so as to form a tube or cylinder 12in. long, and open at both ends. The cylinder is pressed down into the soil till its upper end is level with the surface of the ground. Of course it will now be full of the soil through its whole length of 12in. The earth around it is now removed by a spade; the upper end is closed by a plug or piece of glazed stout paper tied on; and, on lifting it, the lower end is similarly closed. To prevent mistakes, each end should be marked "top" and "bottom" respectively. Not only, it is evident, will such cylinder now contain a fair sample of the soil to the depth of 12in., but it will show the quality unmixed all the way down from the surface to that depth, so that the analyst will be in a position to note any differences between the soil at different levels through that depth. Another method of taking a sample, quite simple and equally suitable, is thus given by the Royal Agricultural Society of England in their directions on this subject: "Have a wooden box made 6in. long, 6in. wide, and from 9in. to 12in. deep, according to the depth of the soil and subsoil of the field. Mark page 60 out in the field a space of about 12in. square; dig round in a slanting direction a trench, so as to leave undisturbed a block of soil with its subsoil from 9in. to 12in. deep; trim this block or plan of the field so as to make it fit into the wooden box; invert the open box over it, press down firmly, then pass a spade under the box and lift it up; gently turn over the box, and nail on the lid. The soil will then be received in the exact position in which it is found in the field. In the case of very light, sandy, and porous soils the wooden box may be at once inverted over the soil and forced down by pressure and then dug out."

On reaching the analyst's hands the first thing he does after the removal of the soil from the box or cylinder described above and careful inspection is to mix the whole quantity submitted and dry it by exposure to the air, spread out in a thin layer on a piece of glazed paper or our several sheets of foolscap.

A mechanical analysis is then made to determine the proportion of (a) stones; (b) gravel; (c) coarse sand; (d) fine sand; (e) clay. For this purpose, weigh out 3lb. or so of the air-dried soil, rub it up in a large dry basin, pick out the stones and free them, by rubbing in the hands, from the soil and dust that adhere to them (and which are returned to the basin), and weigh them, noting at the same time whether they are partly or wholly composed of barren useless quartz, or of limestone (known by its effervescence with a few drops of hydrochloric acid), or of basalt, or of mica-schist. A knowledge of the geology of the district will often throw light on this subject. Now rub up and gently pound with a wooden pestle in a mortar or clean dry basin ½lb. of the soil freed from the stones as above, and sift through a sieve (call it No. 1) of, say, sixteen holes to the square inch. The portion that will not pass through such sieve, after first rubbing off and finally washing off with water the particles of soil that adhere to it and drying it, may be weighed and set down as gravel. Now sift the soil that has passed No. 1 through a finer-meshed sieve containing, say, two hundred and fifty holes to the square inch, and treat the residue that will not pass through as before; weigh it, and call it "coarse sand." The part that has passed through No. 2 sieve now contains a mixture of the fine sand and clay with organic matter. To separate and determine the proportion of these, it must be boiled with water, with occasional stirring with a glass rod, for at least half an hour. This separates out the different qualities (clay, sand, &c.) from each other. It is now panned off by the gold-digger's system of panning, by means of a couple of basins. This floats off the light clay particles and the particles of organic matter from the heavier saud. The latter is then dried and weighed as "fine sand." Disregarding the organic matter (which may. however, be determined by drying each quality—stones, gravel, coarse sand, fine sand—at 100° C., and then roasting them, and noting the loss of weight in the roasting), the proportion of clay in the stoned soil (the soil after removal of the stones) will be calculated by adding together the quantities of gravel, coarse sand, and fine sand found as above, and subtracting the sum of these from the ½lb. we began with.

If the proportion of organic matter in the soil were also required we should have to dry each quality (gravel, coarse sand, fine sand, clay) at the temperature of boiling water (212° Fahr. or 100° C.) on a water-bath (in the steam that proceeds from boiling water), or in an air-bath, and then weigh and mark the weight. We should then have to roast each of these dried residues at a red heat, stirring it cautiously now and them with a glass rod to bring fresh portions to the service, in an open crucible, until it ceases to lose weight. On weighing again after roasting the loss observed would represent the organic matter in each case. This page 61 would be sufficient when there was found to be no limestone in the soil. But, if it were found that on adding to the soil some hydrochloric acid an effervescence was produced, it would mean that the soil contained limestone; and, in this case, before the last weighing mentioned above, we should moisten each quality with a solution of carbonate of ammonia, and then heat to 150° C. (about 302° Fahr.) in an air-bath till it ceased to lose weight, and then weigh and note the loss of weight as before.—[Alexander Burt and Co., of Dunedin, would supply these air-baths for about £1 5s. each; the necessary thermometer costs about 2s.; and a porcelain crucible suitable for the roasting would cost about 2s. 6d.]

The results of such a mechanical analysis would be stated somewhat as follows:—
Per cent.
Stones 12
Gravel 9
Coarse sand 18
Fine sand 10
Clay 46
Organic matter 5

If the proportion of moisture were required, it could be obtained by warming, say, an ounce of the soil, free from stones, in a saucer at 100° C. till it ceases to lose weight. The loss of weight would represent the moisture. In the case of a rich clay land or loam free from stones and gravel, the process would be much shorter, but otherwise performed as described above. With a statement of the character of the stones, gravel, and sand (whether limestone, shelly, quartz, basalt, schist, &c.), such an analysis as above, purely mechanical, would give a good idea of the general quality of the soil; and, without such an analysis, a mere chemical analysis showing the percentage of each constituent would not be of much value. In the chemical analysis the information required is the proportion of—(a) phosphoric acid, (b) nitrogen, (c) lime, (d) magnesia, (e) potash, (f) soda, (g) silica, (h) alumina, (i) oxide of iron, (k) sulphuric acid, (l) carbonic acid, (m) chlorine, (n) total organic matter, (o) moisture. Of these fourteen constituents the first five named above are the most important, and it may be said that on the proportion of these depends the fertility of the soil. We shall, therefore, in this lecture pay special attention to the determination of these five constituents—namely, phosphoric acid, nitrogen, lime, magnesia, and potash—drawing at the same time a distinction between the portions soluble in hydrochloric acid and those insoluble in that liquid.

1. Phosphoric Acid.—This is never free by itself in the soil, but always in combination with lime, or, perhaps, occasionally with oxide of iron or alumina. A fair sample of the soil is dried thoroughly in the air and ground to fine dust in a mortar. An ounce of this fine powder is accurately weighed out, heated cautiously in a porcelain dish with half its bulk of strong nitric acid, stirring it occasionally with a glass rod. The heating is continued till it is quite dry and has ceased to give off any acid fumes. More nitric acid (strong) is added, and the heating continued as before to dryness. This is repeated three or four times to insure the complete destruction of the organic matter, the conversion of all its phosphorus into phosphoric acid, and the solution of all the phosphoric acid present. Finally the residue is warmed with dilute nitric acid (one part of acid to five of water) and filtered, the filter being washed with boiling water till the washings pass through quite free from any acid reaction (or till they page 62 do not redden blue litmus-paper). The phosphoric acid will now be in the filtrate or solution that has passed through the filter-paper; and in this its amount has to be determined as follows: A solution of molybdate of ammonia in nitric acid is added to it, and the whole is heated to about 60° C. (140° Fahr.), and kept at that temperature for three or four hours, after which it is left to cool for another six or eight hours. This treatment throws down all the phosphoric acid combined with molybdic acid as a fine yellow powder. The mixture is then filtered, when the yellow powder remains on the filter-paper. This yellow powder has then to be carefully washed five or six times on the filter-paper with dilute nitric acid, containing a little nitrate and molybdate of ammonia. The washings and filtrate are thrown away (after being tested for more phosphoric acid by heating with more of the molybdate), and the yellow powder is then dissolved in dilute ammonia, from which it is finally precipitated by adding "magnesia mixture," stirring vigorously in the cold for fifteen minutes with an indiarubber-armed glass rod. It is then allowed to stand for two or three hours to allow the sediment to settle, when it is filtered, washed with water containing a little ammonia, and finally roasted and weighed as pyrophosphate of magnesia, 222 parts of which represent 310 parts of phosphate of lime, or 142 parts of pure phosphoric anhydride (phosphoric acid). The "magnesia mixture" mentioned above contains chloride of magnesia, chloride of ammonia, and ammonia; it is made by dissolving, separately, in water 11 parts by weight of chloride-of-magnesia crystals, 14 parts by weight of chloride-of-ammonia crystals, mixing these two solutions, adding 60 or 70 parts of strong ammonia solution, and then adding distilled water till the whole measures 130 cubic centimetres. Of this solution, 10 cubic centimetres will be enough to percipitate 3.7 grains of pure phosphoric acid, or the phosphoric acid of about 8 grains of phosphate of lime. The "molybdate of ammonia" solution used above is made by dissolving 10 parts by weight of the molybdate-of-ammonia crystals in about 30 parts of dilute ammonia (half strong ammonia, half water) and afterwards adding 200 parts of semi-dilute nitric acid of specific gravity 1.18.

2. Nitrogen.—This important element exists in the soil partly as nitrate and nitrite, partly as ammonia, and partly in the organic matter of the plant-roots and other vegetable débris. The nitrogen in the last two of these forms (ammonia and organic matter) may be estimated by the following process: 10 or 15 grammes of the soil is weighed, rubbed up with a few drops of hydrochloric acid, and then dried at a temperature no: higher than 50° C. It is then mixed with its own weight of cold, recently ignited soda-lime, and transferred into a hard glass tube, about ¾in. in diameter, which has been closed at one end in the blow-pipe flame, and which contains at the closed end about 10 grains of oxalic-acid crystals. The rest of the tube, which should be about 20in. long, is then filled to within 3in. of the open end with more soda-lime. A plug of recently-ignited asbestos is then inserted into the open end, and pushed down till it just touches the soda-lime. A well-fitting cork is then tightly worked into the open end so as to close it (the combustion-tube); and through the cork a narrow glass tube communicates with the inside of the combustion-tube, while the other end of such narrow tube leads, gas-tight, into the mouth of a bulbed U tube, containing a measured bulk of standard weak sulphuric acid of known strength. Heat is then applied to the combustion-tube by means of a series of gas-burners, or in any other way, heating first that portion of it near the corked end, and, while still main-taming that front end dull red-hot, working the source of heat gradually back towards the mixture of soil and soda-lime. Bubbles of air will, from page 63 the very commencement of the heating, issue from the combustion-tube, and pass by means of the connecting narrow tube into the standard sulphuric acid. Any ammonia in the soil will also pass by the same channel into the acid, and the nitrogen of the organic matter in the soil will, by the action of the red-hot soda-lime, be also converted into ammonia, which will then find its way into the measured standard acid. This will go on till all the ammonia and all the nitrogen of the organic matter of the soil will be received into this acid; the oxalic acid at the far end of the combustion-tube serving the purpose of forming gases which sweep out the last traces of ammonia from the combustion-tube. The bulb U tube, containing the standard sulphuric acid, and now also the ammonia, from the soil, is detached. Its contents are washed out into a clean glass beaker, and its ammonia determined by noting the volume of standardised solution of caustic soda required for neutralising the acid. From an hour to an hour and a half is sufficient time for the completion of the whole process. It does not, however, take any account of the nitrates and nitrites which the soil contains, but, as the process for the determination of these could not well be made intelligible in the course of one lecture, the lecturer would leave a description of it for some future occasion.

3. Lime; and 4. Magnesia.—These two constituents, as they keep together through the greater part of the analysis, will be best described together. They exist in the soil partly as carbonates, partly as sulphates, and partly as silicates.

That portion of them which is present as sulphates, being soluble in water, is easily determined. The soil is air-dried, ground to fine powder, weighed, and then shaken up with water for some time, filtered, and washed, the washings being added to the filtrate running through the filter-paper.

The solution thus got is then boiled with chloride of ammonia and ammonia, and again filtered. Oxalate of ammonia is now added; the mixture is again boiled for ten minutes, and allowed to cool. This throws down the lime as a white precipitate. The mixture is filtered, washed, dried, and ignited at a red heat over the blowpipe for ten minutes, when the lime is weighed. The last filtrate still contains the soluble magnesia, which is now to be thrown down by adding to it a solution of phosphate of soda and more ammonia. It is now to be stirred very vigorously with a rubber-covered glass rod, and allowed to stand for three or four hours. It is then filtered, washed, dried, roasted, and weighed as pyrophosphate of magnesia, 222 parts of which contain 80 parts of magnesia. The lime and magnesia present as carbonates can be determined by first removing the sulphates of these metals by washing well with water and then dissolving out the carbonates with very dilute cold hydrochloric acid, and then proceeding as described above for the aqueous solutions of the sulphates of these metals.

The soil that remains after dissolving out the carbonates with hydrochloric acid may now be analysed for silicates of lime and magnesia as follows:—

It is dried and pulverised to very fine powder. A small portion, say three grammes (a gramme is equal to 15.432 grains), is weighed and mixed with about four times its own weight of the double carbonate of potash and soda—"flux for silicates." The mixture is heated for one hour to a red heat in a closed crucible. When cold the contents are dissolved in hydrochloric acid and water. The solution is then to be evaporated to perfect dryness, with constant stirring towards the end, in a porcelain basin. The residue is then moistened and warmed for five minutes with page 64 half its bulk of strong hydrochloric acid. More hydrochloric acid is then added, and the mixture allowed to stand at least fifteen minutes. More of the same acid and water is then added, and the mixture is filtered and washed, dried, and roasted and weighed, the weight showing the quantity of silica the sample contained. The filtrate now contains the iron, alumina, lime, and magnesia, together with some, or all, of the phosphoric acid of the soil. To determine the lime and magnesia in it, it is first boiled with a slight excess of ammonia, which throws down the iron and alumina with, probably, some of the lime and magnesia as phosphates. Acetic acid is then added till the liquid smells distinctly of that acid after shaking up. This dissolves the lime and magnesia, and leaves the phosphoric acid in combination with the alumina and the oxide of iron in the sediment. The mixture is now filtered, and the filtrate tested for lime with oxalate of ammonia, and for magnesia with phosphate of soda, as described above.

5. Potash, lime, and magnesia, soluble in strong hydrochloric acid of specific gravity 1.15.

These constituents are determined as follows: A portion of the finely-ground air-dried soil (10 grammes or so) is accurately weighed, and then digested with strong hot hydrochloric acid, with occasional stirring, in a porcelain basin, for five or six hours. It is then allowed to stand for ten or twelve hours, when water is added, and the mixture filtered. The filtrate will contain the constituents named above, as well as oxide of iron, alumina, phosphoric acid, and probably some silica. It is then to be evaporated to dryness, with constant stirring towards the end. The residue is warmed with strong hydrochloric acid and then with water, and filtered for the silica, as already described. The filtrate is then divided into two equal parts, and in one of them the lime and magnesia are determined as described above, the other being treated as follows for the determination of the potash: Boil with chloride of barium, to precipitate any sulphuric acid that may be present; filter, and to the filtrate add ammonia, and filter again: this will now remove the phosphoric acid. To this last filtrate add carbonate and oxalate of ammonia, till all the barium is precipitated, or till a little more oxalate gives no further precipitate; boil for ten minutes, and filter; evaporate the filtrate to dryness, and roast the residue at a red heat in an open crucible till fumes cease to be evolved; now add a strong solution of oxalic acid, and shake it up; evaporate to dryness, and again ignite at a red heat: this makes the magnesia insoluble, leaving the potash and soda as soluble carbonates. Dissolve these alkaline carbonates in a little hot water, and filter; to this filtrate now add hydrochloric acid, which will cause effervescence; transfer the solution to a dish whose weight is known, and evaporate to dryness; weigh, and the weight will represent the potash and soda of the soil (as chlorides now). Stir up the white residue with a strong solution of chloride of platinum, and evaporate in a steam bath to just about dryness; stir up with strong alcohol; pour off the solution (which must be of a brownish-yellow colour if a sufficient quantity of the platinum salt had been added), and wash with alcohol by decantation so long as the washings show the slightest trace of a yellow colour. Now dry and weigh the yellow powder, which is the double chloride of platinum and potassium, 488 parts of which represent 94 parts of potash.

By Authority: Samuel Costall, Government Printer, Wellington.—1895.