Chapter XV: Introduction (14)

The estimation of humus by combustion, in any form, of the total organic matter in the soil, gives results varying according to the season, and having no direct relation to the active humus of the soil. The same objection lies against extraction with strong caustic lye.

=312. Modification of Grandeau’s Method for Determining Humus in Soils.=—According to Huston and McBride[200] the function of the vegetable matter in the soil has long been a matter of contention among those interested in the science of agriculture. Two factors have contributed to the uncertainty existing in this matter: First, the very complex and varying nature of the compounds resulting from the decomposition of vegetable matter in the soils; and second, the lack of uniformity in the methods of determining either the total amount of organic matter present in a soil, or the amount that has been so far decomposed as to be of any immediate agricultural value. Prominent among these methods are the methods in which a combustion is resorted to, the substance being either burned in air or in a combustion tube with some agent supplying oxygen. The loss on ignition is no measure of the amount of organic matter present since it is practically impossible to remove all the water from the soil previous to ignition, and neither of the methods gives information regarding the extent of the decomposition of the organic matter. Pure cellulose and the black matter of a fertile soil are of very different agricultural value.

Determinations of carbon in soils by oxidation with chromic and sulfuric acid, and with alkaline permanganate have been used. The method with alkaline permanganate agrees fairly well with combustion with copper oxid or lead chromate, but the chromic sulfuric acid method gives only about eighty per cent of the carbon found by combustion processes. However valuable these processes may be for determining the total carbon in the soil, they furnish no information regarding the condition of the carbonaceous soil constituents, and as the determination is really one of carbon, the organic matter must be calculated by using an arbitrary factor. Generally the organic matter of the soil is considered to have fifty-eight per cent carbon; yet different values are given from forty to seventy-two per cent.

There is a general opinion that the black or dark brown material of the soil, resulting from the decay of vegetable matter, has a much higher agricultural value than the undecomposed vegetable matter. No very sharp dividing line can be drawn, for changes in the soil are continually going on, and material may be found in almost every stage between pure cellulose and carbon dioxid. The character of the intermediate products will vary according to the conditions of tillage and the supply of air and water.

For agricultural purposes some means of determining the amount of decomposed matter is very desirable. Several solvents have been tried for this purpose. The earlier attempts were made by treating the soil with successive quantities of boiling half-saturated solution of sodium carbonate until the soil appeared to yield no more coloring matter to the solvent. The solutions were then united, rendered acid with HCl, which precipitated the humic acid, which was then washed, dried, and weighed. This was considered the more soluble portion of the humic acid. The soil was afterward treated with caustic potash solution in the same manner, and the humus thus extracted was called insoluble humus. This last process was really more in the nature of manufacturing humus, for sawdust treated with caustic potash yields humic acid, and the inert organic matter in the soil was decomposed to some extent by the caustic alkali. Neither of the processes provided for the separation of the humic acid from the lime, magnesia, alumina, and iron with which it is usually combined in the soil.

In case results of different workers are to be compared, it is of the greatest importance that methods should be used that are of such a nature that errors resulting from difference of manipulation, and from difficulty of reproducing duplicate work can be reduced to a minimum.

Hence, a simple modification of the Grandeau method has been tried which has the advantage of keeping a definite amount of the soil in contact with a definite volume of ammonia for a fixed time, the strength of the ammonia remaining constant.

The process is as follows: The soil is washed with acid and water as usual. It is then washed into a 500 cubic centimeter cylinder with ammonia, the cylinder closed and well shaken and allowed to remain for a definite time, usually thirty-six hours. The material is shaken at regular intervals. The cylinder is left inclined as much as possible without having the fluid touch the glass stopper, thus allowing the soil to settle on the side of the cylinder and exposing a very large surface to the action of the ammonia. During the last twelve hours the cylinder is placed in a vertical position to allow the soil to settle well before taking out the aliquot part of the solution.

The process of washing the soil with hydrochloric acid, water and ammonia, is very tedious when performed in the usual way with the wash-bottle. A simple automatic washing apparatus was devised by which a fixed volume of the washing fluid can be delivered at regular intervals, giving ample time for the thorough draining between each addition of the fluid, and requiring no attention. By this apparatus work can be continued day and night. Instead of washing on the usual form of filter paper in funnels, it is preferable with this apparatus to hold the soils on a disk of filter paper resting on a perforated porcelain disk in the bottom of the funnel. This removes the necessity of washing out the filter papers, does not permit of the accumulation of humus on the edge of the filter paper when the Grandeau process is used, and insures that all the washing fluids pass through the soil and not around it. This form of apparatus reduces the labor to a minimum and permits many determinations to be carried on at once.

This form of apparatus was only lately devised and has only been used long enough to test it and to show its advantages. The reported results were obtained by the ordinary methods of washing.

In all the work reported, five grams were used, as the soils contained so much humus that this amount gave enough humus for good work in the final weighings. The results obtained so far appear in the following tables:

TABLE I.

COMPARISON OF METHOD OF GRANDEAU WITH HUSTON’S MODIFICATION AND OF
INFLUENCE OF STRENGTH OF AMMONIA SOLUTION. TIME OF DIGESTION IN MODIFIED
METHOD THIRTY-SIX HOURS.

Two per cent Four per cent
NH₃. NH₃.
Grandeau. Huston. Grandeau. Huston.
1. Peat soil, 16.40 20.06
Bogus „ 13.98 20.80
„ „ 17.43
————— —————
Mean 15.94 20.43

2. Peat subsoil, 13.98 19.38
Bogus „ 13.85 20.30
————— —————
Mean 13.92 19.84

3. Peat soil, 9.05 15.60 14.71 21.24
Good „ 10.27 15.88 15.34 20.20
————— ————— ————— —————
Mean 9.61 15.74 15.03 20.72

4. Peat subsoil, 16.75 24.34
Good „ 18.60 23.52
————— —————
Mean 17.68 23.93

5. Black soil, A 3.90 6.90 (1.86) 7.42
„ „ „ (1.67) 6.98
„ „ B 3.88 7.00 4.42
„ „ „ 4.20
————— ————— ————— —————
Mean 3.99 6.95 (3.05) 7.20
„ 4.31

6. Clay loam, 1.86 4.20 2.40 4.26
West side, A 4.28
„ „ B 1.76 4.36 2.48 (3.40)
„ „ „ (3.10)
————— ————— ————— —————
Mean 1.81 4.28 2.44 (3.76)
„ 4.27

7. Clay loam, A 1.90 4.12 (1.60) (4.59)
Lysimeter soil, B 1.61 4.22 (1.41) (4.58)
„ „ C 1.80 4.12
„ „ D 1.95 4.04
„ „ E 1.92 3.85
„ „ F 1.95 4.08
„ „ G 1.90 3.93
„ „ H 1.90 3.80
————— ————— ————— —————
Mean 1.76 4.17 (1.80) (4.12)
„ 1.90 3.97

Seven per cent Eight per cent
NH₃. NH₃.
Grandeau. Huston. Grandeau. Huston.
1. Peat soil,
Bogus „
„ „

Mean

2. Peat subsoil,
Bogus „

Mean

3. Peat soil, 19.77 21.70 16.05 21.42
Good „ 19.85 21.90 15.40 21.80
————— ————— ————— —————
Mean 19.81 21.80 15.73 21.61

4. Peat subsoil,
Good „

Mean

5. Black soil, A
„ „ „
„ „ B
„ „ „

Mean
„

6. Clay loam, 2.14 4.02 1.85 4.12
West side, A
„ „ B 2.13 4.48 1.90 4.40
„ „ „
————— ————— ————— —————
Mean 2.14 4.25 1.88 4.26
„

7. Clay loam, A
Lysimeter soil, B
„ „ C
„ „ D
„ „ E
„ „ F
„ „ G
„ „ H

Mean
„

NOTE.—Numbers in parentheses indicate results, generally the earliest
ones, which the authors do not consider strictly comparable with the
rest of the work. They are given solely for the purpose of exhibiting
all the work that has been done to date. When a mean is included in
parentheses it indicates that it is calculated from all the results
obtained, including those not considered strictly comparable. Bogus is
a name given to a peaty soil which is very sterile.

TABLE II.

INFLUENCE OF TIME OF DIGESTION. FOUR PER CENT OF AMMONIA USED
THROUGHOUT. HUSTON’S METHOD.

Thirty-six Forty-eight Sixty-eight Ninety-eight
hours. hours. hours. hours.
Peat Soil, 21.24 22.28 24.04
Good „ 20.20 21.70 23.94
————— ————— —————
Mean 20.72 21.99 23.99

Clay loam, 4.28 4.00 4.40 „ 4.26 4.01 4.85 West side (3.40) „ „ (3.05) —————— ————— ———— Mean 4.27 4.01 4.63

TABLE III.

INFLUENCE OF TIME OF EXTRACTION. TIME, TEN DAYS. GRANDEAU’S METHOD,
FOUR PER CENT AMMONIA. PEAT SOIL.

A. B. Mean. Remarks.
Per Per Per
cent. cent. cent.
1st extraction, 750 cc 16.90 18.96 17.93
2nd „ 250 „ 2.80 2.38 2.59
3rd „ 250 „ 1.77 1.10 1.44
4th „ 250 „ 1.34 1.30 1.32 Stood over night.
5th „ 250 „ 0.89 0.85 0.87
6th „ 250 „ 1.41 1.65 1.53 Stood overnight.
7th „ 250 „ 2.10 1.80 1.95 Washed again with HCl for Ca.
Trace found. HCl washed out,
but trace of chlorids found in
ash. Probably HCl absorbed
from air as humus showed small
quantity of a white volatile
solid on evaporation.
8th „ 250 „ 0.67 0.65 0.66
9th „ 250 „ 0.57 0.50 0.53
———— ————— ————— —————
Total 2750 „ 28.45 29.19 28.82

=313. Summary of Results.=—1. The modified method gives much higher results than the original method of Grandeau.

2. In the Grandeau method marked irregularities follow a change in the strength of the ammonia solution. These differences in results bear no relation to the strength of the solution used. They seem to be errors due to the difficulty of securing uniform and complete washing of the soil by the ammonia solution.

In the modified method the change in the strength of the ammonia solution makes practically no difference in the amount of humus extracted, except in the case of the peat soil where two per cent ammonia failed to extract all the humus. But the results show no considerable increase when the strength is increased to over four per cent.

3. The factor of time has not been fully investigated, but the results so far obtained indicate that the time exerts less influence in the modified than in the Grandeau method.

4. Table III shows that considerable quantities of the peat soil are still passing into solution in the Grandeau method at the end of ten days. With ordinary soils this is not true; but in the case of soil No. 5, a black soil, the solutions were colored at the end of a week. On the peat soil the modified method extracted from ten to fifty per cent more than the Grandeau, and on the ordinary soil from two to three times as much humus.

5. In comparing duplicate results by both methods it is found that with soil No. 3, peat soil, the following differences appear calculated to percentage of the total amount involved in the determination:

Per cent. Per cent. Per cent. Per cent.
Strength of ammonia 2. 4. 7. 8.
Modified 1.7 5. 1.0 1.8
Grandeau 13.0 4.3 0.5 3.4

Special attention was paid to this point in case of soil No. 7, an ordinary soil; taking all results into consideration the greatest difference in percentage of total amount involved was, by the modified method, nineteen per cent, and by the Grandeau, thirty per cent. In the set of six special determinations made by both methods to test this point and which are strictly comparable with each other, the maximum range was by the modified method 7.8 per cent and by the Grandeau 8.3 per cent of the total amount involved in the determination. From which it appears that the modified method is on the whole capable of yielding rather more concordant results than the Grandeau.

=314. Estimation of Free Humic Acids.=—This process, due to Müntz[201] is essentially that of Huston and McBride. Twenty grams of the soil are reduced to a fine powder and saturated with fifty cubic centimeters of concentrated ammonia and allowed to digest two or three days in a warm place. The volume is then made up to one liter with water, well shaken, and set aside for one day in order to permit the subsidence of the solid matter. At the end of this time 500 cubic centimeters of the supernatant liquor are taken and acidified with hydrochloric acid in order to precipitate the humic bodies. The humus is collected on a filter, dried and weighed. It is then ignited and the weight of ash deducted from the first weight thus giving the actual weight of the humus obtained, free from mineral matter. This process gives the free humic acids. By previous treatment of the sample with hydrochloric acid as in the process of Huston and McBride, the total humus is obtained. The estimation of the free humic acids is of importance in determining the quantity of lime or marl which should be added to acid lands.

=315. Humus Method of Von Bemmelén.=[202]—Von Bemmelén obtains the content of humus by the multiplication of the content of carbon in the soil by the factor of Wolff; _viz._, 1.724. The estimation of carbon, water, and of the loss on ignition is conducted in combustion tubes in a current of oxygen. The nitrogen estimation is carried on according to the method of Dumas.

In soils containing calcium carbonate the carbon content is derived from the carbon dioxid taken up by the potash bulbs during combustion (a); from other carbonates not decomposed on ignition and which are subsequently determined in the residue by treatment with hydrochloric acid in a carbon dioxid apparatus, (b) and the total carbon dioxid derived from the carbonates in the soil (c).

For each estimation from three to five grams of the soil are taken, because with smaller quantities the errors of analysis too strongly influence the results. The carbon is then calculated according to the formula:

Carbon = ³⁄₁₁ (a + b − c).

_The Carbon Dioxid of Carbonates._—It is necessary to expel the carbon dioxid at ordinary temperatures, because on heating to boiling, carbon dioxid would be formed from the humus. In a flask, as small as possible, the soil is treated at ordinary temperature, with dilute sulfuric or citric acid, the escaping gas dried over sulfuric acid and taken up with soda-lime. Behind the soda-lime is a small tube filled with pieces of glass and moistened with sulfuric acid, which retains any moisture taken out of the soda-lime. A stream of about one liter of air, free from carbon dioxid, is sufficient to drive out all of the carbon dioxid when the estimation is made at ordinary temperatures.

A volcanic earth from Deli, which contained five per cent of humus, gave, at a temperature plus or minus 15°, 0.01 per cent CO₂. At boiling temperature two analyses gave 0.54 and 0.56 CO₂. This soil contained no carbonate, and the carbon dioxid found at the boiling temperature, must have come from the humus substances under the influence of the dilute acids.

A heavy clay containing 6.9 per cent of humus gave, at plus or minus 15°, 3.60 per cent CO₂; at 100° without boiling, it gave an additional 0.53 per cent, and with boiling an additional 0.11 per cent, or a total of 4.24 per cent CO₂. A light clay containing 3.2 per cent of humus, gave, at 15°, 5.09 per cent CO₂; at a boiling temperature an additional 0.43 per cent, and by continued boiling an additional 0.27 per cent.

=316. Estimation of Humus by the German Method.=—The German experiment stations follow the method of Loges,[203] depending on the oxidation of the humic bodies with copper oxid after evaporation of the sample with phosphoric acid. The object of the preliminary evaporation is to set the humic acids free in order that they may be better and more easily oxidized than when burned in the combined state.

The sample of soil is placed in a Hoffmeister dish (Schälchen), moistened with dilute phosphoric acid and evaporated to complete dryness. The dish and its contents are rubbed up with pulverized copper oxid and placed in a combustion tube of sixty centimeters in length, open at both ends. There is then placed in the tube, and held in place by asbestos plugs, granular copper oxid to a length of twenty centimeters.

The combustion tube is placed in a proper furnace and one end connected with two washing-flasks, the first containing potash lye, and the other a solution of barium hydroxid. These flasks are to free the aspirated air from carbon dioxid. The other end of the combustion tube is connected with an appropriate apparatus for absorbing the carbon dioxid. Loges recommends the Pettenkofer absorption tube and a Fresenius drying cylinder.

Between the absorption apparatus and the aspirator, is also placed a washing-flask containing barium hydroxid solution, serving to detect any unabsorbed carbon dioxid. The layer of granular copper oxid is first heated, the air being slowly aspirated through the apparatus meanwhile, but not through the absorption bulbs. All the carbon dioxid is thus removed from the apparatus.

The absorption system being connected, the tube is heated slowly from the front, backwards, and after the tube is well heated a slow current of air is drawn through and continued until the combustion is complete, which is usually in about three-quarters of an hour.

After the tube is cool the powdered copper oxid and residue of combustion are removed, and for this reason the tube is stopped with a cork at both ends instead of being drawn out and sealed at one end. The tube can thus be refilled without disturbing the granular layer of copper oxid.

The drying cylinder used between the combustion tube and the absorption system has its upper part filled with cotton to avoid the deleterious effects of the nitric oxid produced in the combustion. With this arrangement the use of metallic copper in the combustion tube to reduce the nitric oxid can be dispensed with, the moist cotton holding back the acid fumes. The per cent of humus is obtained by multiplying the per cent of carbon found by 1.724.

=317. Method of Raulin for the Estimation of Humus.=[204]—The volumetric estimation of humus in soil by a solution of potassium permanganate would be convenient and practical if the combustion of the organic matter were complete, and if the browning of the liquor did not render the end of the reaction uncertain. The process of Schmidt, modified as below, has given satisfactory results.

In a small flask, with flat bottom, containing about 250 cubic centimeters, are introduced ten cubic centimeters of a solution of manganese sulfate containing sixteen grams of the anhydrous salt per liter, and ten cubic centimeters of a ten per cent solution of potassium permanganate. The solution is heated for a few minutes, the liquor is decolorized and manganese bronze is precipitated. One hundred cubic centimeters of water are added, and four cubic centimeters of sulfuric acid containing 150 cubic centimeters of monohydrated acid per liter. There is now added an exactly measured volume of the humic liquid properly prepared, so that in oxidizing completely it destroys at most only half of the manganese dioxid. The mixture is submitted to gentle ebullition for eight hours, the water being kept at a constant volume. The excess of manganese dioxid remaining is dissolved hot by a measured portion of decinormal oxalic acid in slight excess, and the excess of oxalic acid is removed by a solution of potassium permanganate containing one gram per liter. The volume of oxalic acid not destroyed by manganese dioxid is calculated from the amount of permanganate consumed. The volume of oxalic acid, which corresponds to the same quantity of dioxid as the introduced humus, is also calculated by taking the difference between the volume of oxalic acid necessary to destroy all the dioxid formed by ten cubic centimeters of the ten per cent permanganate solution, and the volume of the oxalic acid which has destroyed the dioxid remaining after the action of the humus. The first volume of oxalic acid, that is to say, that which destroys the dioxid formed by ten cubic centimeters of ten per cent permanganate is determined in a preliminary titration.

In regard to the humic liquor, it is prepared by treating ten grams of earth with soda solution in the usual manner. It will be easy to calculate the volume of the oxalic solution equivalent to the total volume of the humic solution, of which a determined fraction has been assayed, and consequently the volume of oxalic solution equivalent to the humus in ten grams of the dry earth. This number of cubic centimeters of the decinormal oxalic solution multiplied by 0.8 will express in milligrams the weight of oxygen necessary to burn the humus from ten grams of dry earth. Humus not being a definite compound, but a residue of complex organic matters partially oxidized, it will require as much more oxygen to complete the combustion as the previous oxidation has been less pronounced. This weight of oxygen necessary to burn the humus from ten grams of dry earth may serve to detect the total value as well as the weight of the humus itself. However, if we wish to have directly the weight of the humus, resource can be had to a table which, without being rigorous, can be regarded as sufficiently exact when the variability of the constitution of humus is taken into account.

Volume of decinormal oxalic acid Corresponding humus, directly
for ten grams of earth. determined.
Cubic centimeters. Milligrams.
50 80
100 150
200 280
300 400
400 510
500 610
600 705
700 790
800 885
900 975
1,000 1,060
1,200 1,225
1,400 1,390
1,600 1,560
1,800 1,720
2,000 1,890
2,500 2,315
3,000 2,735
3,500 3,170
4,000 3,605
4,500 4,035
5,000 4,460
5,500 4,890
6,000 5,310
6,500 5,745

=318. Pasturel’s Method.=—According to Pasturel[205] the process of Raulin does not furnish figures that are rigorously exact only with soil of which the humus contains forty-five per cent of carbon. When the richness in organic carbon is less, the results of the estimation are too high. Pasturel modifies the process as follows:

_Manganese Sulfate._—Dissolve sixteen grams of the pure anhydrous manganese sulfate in distilled water and make the solution up to one liter.

_Potassium Permanganate._—Make a solution of ten grams of potassium permanganate in one liter of water; 100 cubic centimeters of the liquor just mentioned are diluted to one liter and constitute the potassium permanganate solution one to ten.

_Oxalic and Sulfuric Acids._—A solution of oxalic acid is prepared containing 6.3 grams of the acid in one liter of water, and a dilute solution of sulfuric acid, by dissolving 150 grams of the monohydrated acid in one liter of water.

_Humus Solution._—The solution of humus is prepared by the following process: Ten grams of fine earth are freed from all their carbonates by dilute hydrochloric acid. After washing, the filter is broken and the dirt is washed into a small flask. Not more than twenty or thirty cubic centimeters of water should be employed for this purpose. Twenty cubic centimeters of a liquor containing two grams of caustic soda are added, and the flask is placed upon a sand-bath and maintained at a boiling temperature for six hours. It is then diluted with water, filtered and washed as long as the waters are colored. The liquor is treated with dilute sulfuric acid until almost the whole of the soda is saturated. It is indispensable, however, to maintain a slight alkalinity in order that the organic matter may rest totally dissolved. The precipitation of silica which is almost always produced is without inconvenience. Afterward the volume is completed to 500 cubic centimeters and the humus solution is then ready for use.

_Estimation of the Humus._—Ten cubic centimeters of the manganese sulfate are placed in a flask and ten cubic centimeters of the permanganate added, and the whole is then slightly heated, and afterward 100 cubic centimeters of water and four cubic centimeters of sulfuric acid are added. The humic liquor is now introduced in such proportion that the humus which it contains dissolves at the greatest, a half of the precipitated manganese and the rest of the process is continued as described by Raulin.

=319. Estimation of Carbonates in Arable Soil.=—The principle of the determination depends on the liberation of the carbon dioxid from its compounds in the soil by acting on them with strong acid, and the desiccation, absorption, and weighing of the evolved gas. Any of the ordinary forms of apparatus for estimating carbon dioxid may be used in this determination.

The apparatus of Knorr[206] has been used with satisfaction for many years in the laboratory of the Department of Agriculture.

FIGURE 65.

KNORR’S APPARATUS FOR THE DETERMINATION OF CARBON DIOXID.
]

The apparatus consists of a flask A, Fig. 65, in which the carbon dioxid in the soil is liberated. A condenser, D, fits by means of a ground-glass joint into the neck of the flask in which the liberated gas, together with any air or aqueous vapor which may be carried forward, is cooled. This prevents any excess of vapor of water from entering the absorbing bulbs, which could easily happen at the end of the experiments when the contents of A are raised to the boiling point. The bulb B contains the acid, usually hydrochloric, which is employed for decomposing the carbonates. It is provided with a guard bulb-tube, C, which serves to absorb any carbon dioxid which might enter the apparatus with the air during aspiration at the close of the determination. The carbon dioxid is dried in the bulb-tube, E, in oil of vitriol, and absorbed in the potash solution in F. It is advisable to aspirate a slow current of air through the apparatus by means of the tube G during the whole of the operation. The quantity of the sample to be taken depends on its richness in carbonates. Many soils are so poor in carbonates as to render any attempt at exact determination nugatory. On the other hand, a comparatively small sample of marls will be sufficient. A preliminary qualitative test will indicate, in a general way, the quantity of the sample to be taken. The sample of soil, five to fifty grams, having been transferred to A, which should be perfectly dry, is made into a batter with freshly boiled distilled water. When all the parts of the apparatus are properly connected gas-tight, the cock between B and A is slowly opened and the hydrochloric (nitric) acid in B allowed to flow into A at such a rate as will secure a moderate evolution of gas.

When the carbonate is entirely decomposed, a lamp is brought under A and its contents gradually raised to the boiling point. The aspiration of air, free from carbon dioxid, is meanwhile continued until all the liberated gas has been absorbed in F. Usually about fifteen minutes will be sufficient to accomplish this purpose.

=320. Bernard’s Calcimeter.=—For a rapid and approximately accurate method of determining the amount of carbonate in the soil, estimated as calcium carbonate, Bernard makes use of the well-known method of the volumetric estimation of carbon dioxid. The sample to be examined should not be powdered in any way. The sample in a natural state, but well air-dried, is gently broken up by the fingers and passed through a sieve having ten meshes to the centimeter. Of the fine earth thus obtained, one gram is taken, for the determination. If the percentage of carbonate in the soil exceeds fifty then only half a gram is taken.

FIGURE 66.

BERNARD’S CALCIMETER.
]

The apparatus employed is one well known. The small erlenmeyer C is fitted with a rubber stopper carrying an exit tube for the gas and a small thermometer. This flask is connected by means of a rubber tube and small glass tube to the measuring burette B. This burette is graduated from 0 to 100 cubic centimeters. Below, by means of a rubber tube, it is connected with the open bulb A, which, by means of a cord about its neck, can be suspended by the hook as shown in the figure. The measuring tube is filled with water through A until the level of the liquid in B is slightly above the zero mark. Meanwhile the one gram of earth has been placed in C, together with the tube D three-fourths filled with an equal mixture of water and strong hydrochloric acid. The greatest care must be taken that no part of the acid be spilled.

The rubber stopper is now forced into C until the level of the water in B is just at the zero mark. Grasping C in the right hand and A in the left, the operator inclines C until the contents of D are emptied. Meanwhile as the gas is evolved, A is lowered at such a rate as to always keep the level of the water in B and A on the same plane. In a few moments the evolution of gas is complete, and the volume given off is read at once without correction. This volume multiplied by 0.4 gives the percentage of carbonate in the sample examined. It is understood that the determination is made at ordinary temperatures; _viz._, 17° to 22°. Example:

One gram of a soil treated as above, gave of carbon dioxid
(uncorrected) 65 cubic centimeters. 65 × 0.4 = 26.00 = per cent
calcium carbonate in sample.

The above method is useful in the classification of soils and in determining approximately the quantity of calcium carbonate which they contain. The practical use of this method is of great value in determining the character of fertilizer to be applied. It is well to know the percentage of carbonate in selecting mineral fertilizers.

=321. Soils Deficient in Carbonates.=—When a soil contains but a small quantity of carbonates, Müller[207] has called attention to the fact that the carbon dioxid absorbed by the water in which the soil is rubbed up may vitiate the result. Instead of water a titrated solution of sodium carbonate is employed. The apparatus is composed of a flask containing the mixture of the sodium carbonate and the soil on which the hydrochloric acid is to act. The hydrochloric acid is contained in a small tube, as in Scheibler’s apparatus. The gas is received in a rubber tube 1.5 meters long and three to four millimeters interior diameter, and connected with a burette, the open mouth of which dips into the water of a cylinder of proper length. The volume of gas is read when the burette is raised or lowered in the cylinder until the liquid within and without stands at the same level.

During the action of the acid on the carbonates the flask is constantly shaken.

Several readings of the volume of gas are made, the evolution flask being vigorously shaken before each one. Finally, in order to allow for the variations in temperature and pressure of the exterior air which may take place between the beginning and the end of the reaction, a second flask containing air is placed by the side of the evolution flask and communicating with a narrow =ᥩ= tube half filled with water. Any variations in the volume of the air in the flask will be shown by variations in the height of the liquid in the two arms of the =ᥩ= tube, and the volume of the variation can be easily determined by having the =ᥩ= tube calibrated.

If now _a_ equals the volume per cent of carbon dioxid in the atmosphere of the evolution flask at the end of the reaction, _v_ the volume of gas disengaged, and V the volume of the atmosphere in the evolution flask, the per cent of carbon dioxid contained in a given length of the rubber tube will be equal to _a_/2. This arises from the fact that the first gas which passes into the rubber tube is composed solely of air, while the last contains a per cent of carbon dioxid. By reason of the shaking of the flask the mean richness of the contents of the tube in carbon dioxid, will be sensibly _a_/2.

From the above data the following equations are derived:

1. _v__a_/2 + V_a_ = _v_.

2. _a_ = _v_/(_v_/2 + V)

If the weight of the carbon dioxid dissolved in V′ cubic centimeters of the liquid in the evolution flask be represented by _q_, the coefficient of the solubility of pure carbon dioxid in this liquid will be, according to the law of the solubility of a gas, equal to _k_ = (_q_)/(V′_a_)

The volume of _k_ has been determined for various strengths of the sodium carbonate solution, using five cubic centimeters of hydrochloric acid containing 1.6 grams pure hydrochloric acid. For solutions disengaging from five to fifty milligrams of carbon dioxid, the mean value of _k_ was found to be 1.8 milligrams in the absence of calcium chlorid. When calcium chlorid was present in quantities varying from 0.03 to 0.07 gram per cubic centimeter of liquid in the evolution flask, the value of _k_ was 1.4 milligrams.

By adopting, according to circumstances, the one or the other of the above numbers and multiplying it by V_a_, as determined by experiment, results are obtained differing only 0.2 to 0.3 milligram from those secured by direct weighing of the evolved gas.

Dietrich[208] has called attention to the necessity of adding the volume of the dissolved gas to the measured volume in such determinations, and this volume or weight is easily determined by the above formulas.

=322. Belgian Method.=—The method pursued at the Gembloux Station[209] consists in taking from five to fifty grams of the sample of soil, according to its content in carbonate, rubbing it up in a porcelain dish with distilled water in order to make a thin paste. The mass is worked to drive out all the air, the whole washed into a flask of 300 cubic centimeters capacity, and the amount of carbon dioxid estimated by setting free with an acid, and collecting the carbon dioxid evolved in potash bulbs.

DIGESTION OF SOILS WITH SOLVENTS.

=323. General Considerations.=—There are two points in connection with the determination of mineral matters in the soil which must always be kept in view; _viz._, first, the estimation of the total quantities of material in the soil, and second, the study of those materials which are more easily brought into solution and thus made available for the food of plants.

It is well understood that the soil particles do not give up entirely to the plant the food materials which they contain. The practical value therefore of an analysis of a soil depends more upon the exact determination of the plant food available than upon its total quantity. From a mineral and geological point of view, on the other hand, an idea of the total composition of the soil is the object to be attained.

For the determination of the available plant food, various solvents have been proposed, none of which, perhaps, imitates very accurately the natural solvent action of organic life and moisture on the soil materials. A description of the standard methods of preparing soil extracts will be the subject of a few succeeding paragraphs.

=324. Estimation of the Quantity of Materials Soluble in Water.=[210]—Five hundred grams of the air-dried soil are treated in a flask with 1,500 cubic centimeters of water, less the quantity of water already contained in the air-dried soil, which is volatile at 125°. The mass is frequently shaken and, after seventy-two hours, 750 cubic centimeters of the liquid filtered. The filtrate is evaporated to dryness in a platinum dish, dried at 120° and weighed. This is then incinerated and, after treatment with ammonium carbonate and gentle ignition, is again weighed. The further examination of the residue for acids and bases is made by some of the methods hereafter described.

=325. Treatment with Water Saturated with Carbon Dioxid.=—Two thousand five hundred grams of the air-dried soil are treated with 8000 cubic centimeters of distilled, and afterwards with 2000 cubic centimeters of water, which have previously, at room temperature, been saturated with carbon dioxid. The mixture is left in a closed flask for seven days, frequently shaken, after which 7,500 cubic centimeters of the liquid are filtered. The clear filtrate, after treatment with a little hydrochloric acid and a few drops of nitric acid, is evaporated to dryness. After the separation of the silica the traces of iron, alumina, lime, sulfuric acid, magnesia, potash, and soda, are estimated in the liquid in the manner hereinafter to be described. Phosphoric acid is always present in such a case, in such small quantities as to make its estimation unnecessary.

=326. Treatment with Water Containing Ammonium Chlorid.=—In the flask containing the residue from the last experiment; _viz._, the soil with 2,500 cubic centimeters of liquid, are added 1,500 cubic centimeters of water saturated with carbon dioxid, and 8,000 cubic centimeters of pure water in which five grams of ammonium chlorid are dissolved. The mixture is then left for seven days, with frequent shaking, and 7,500 cubic centimeters of the liquid are then filtered, and the substances dissolved, determined in the filtrate. In addition to the usual quantities of lime and magnesia, from two to four times as much alkali is dissolved by this treatment as is found in the solution from the water containing carbon dioxid alone.

=327. Treatment with Water Containing Acetic Acid.=—The acetic acid should be of such a strength that after it has fully acted on the soil it should still contain twenty per cent of free acid. 1000 grams of the soil dried at 100° are taken and the acid added in proper proportions and treated in the manner to be described for determining the solvent action of hydrochloric acid.

=328. Treatment with Citric Acid Solution.=—In ascertaining the quantities of soil materials soluble in a solution of citric acid, Dyer[211] recommends the use of a carefully prepared citric acid solution. The digestion is carried on as follows: Place in a flask or bottle, holding about three liters, 200 grams of air-dried soil and two liters of distilled water, in which are dissolved twenty grams of pure citric acid. The soil is left, at room temperature, in contact with the one per cent acid for seven days, with thorough shaking several times a day. At the end of the digestion the solution is filtered and 500 cubic centimeters of the filtrate, corresponding to fifty grams of the soil, are taken for analysis for each ingredient to be determined.

The digestion in citric acid is especially recommended by Dyer because of its supposed near resemblance to the methods of solution of plant food practiced by the rootlets of plants. It is evident, however, that this process is in no sense an imitation of natural methods. The solution is to be used exclusively for the estimation of potash and phosphoric acid. Dyer concludes, from a comparison of the action of a solution of citric acid on soils of known fertility, that when as little as 0.01 per cent of phosphoric acid is dissolved from a soil by this treatment it is justifiable to assume that it stands in immediate need of phosphatic manure. The methods used by Dyer to determine the phosphoric acid and potash in the citric acid solution will be given in their appropriate place.

=329. Treatment with Hydrochloric Acid.=—The solutions of soils usually subjected to chemical analysis are those obtained by long treatment with hot mineral acids, among which the most common is hydrochloric.

It has long been assumed by soil analysts, perhaps not with justness, that such treatment removed from the soil, all those elements of plant food which could possibly be available for the needs of the growing crop. In this connection, however, the analyst must not forget that nature, in a series of years, with her own methods may easily accomplish what he in five days, even with the help of a hot mineral acid, may not be able to secure. Since, however, this method of solution has been so long practiced it is not the place here to throw doubt on its effectiveness without being able to suggest a better way. Of the mineral acids available no one possesses solvent powers for soils in a higher degree than hydrochloric. A somewhat detailed description will therefore be given of the methods of its use.

=330. Strength of Acid to be Employed.=—The fact that hydrochloric acid of nearly constant strength; _viz._, specific gravity 1.115, equivalent to 22.9 per cent hydrochloric acid, may be obtained by distillation, led Owen to use acid of this density in his classic work on soil analysis. Hilgard has lately reviewed the conditions of constant strength in the solvent with results confirming the statements of Owen.[212] He evaporated on a steam-bath, to one-half its bulk, fifty cubic centimeters of hydrochloric acid, specific gravity 1.116, obtained by using the distillate from a stronger acid after rejecting the first third. The same operation was conducted with similar acid diluted with ten per cent of water. The acid used contained 22.96 per cent hydrochloric acid. The residual acid contained 21.49 per cent hydrochloric acid. These results lead Hilgard to believe that the changes arising from evaporation in hydrochloric acid during soil digestion are insignificant, compared with those due to its action on the soluble matters, and that evaporation during digestion is effective in maintaining a definite strength in the solvent. For this reason it is contended that evaporation in a porcelain beaker covered by a watch-glass is more effective in constancy of conditions than digestion in a closed flask under pressure.

=331. Influence of Time of Digestion and Strength of Acid.=—Loughridge has made an interesting study of the influence of the strength of acid and time of digestion on the extraction of soils.[213] The method of preparing the soil for the determination of the above points is as follows:

The soil, having been passed through the appropriate number of sieves to obtain the fine earth is pulverized with a wooden pestle and thoroughly mixed. The hygroscopic moisture is determined, after exposing it in a place saturated with vapor, in a layer not exceeding one millimeter in thickness for twelve hours, and subsequently drying at 200° in a paraffin-bath. Of this dried substance, from two to three grams are used in the general analysis, the methods employed being in general those adopted by Peter.[214]

The quantities of materials dissolved by acids of different densities are shown below. The determinations were made by methods hereafter to be described.

Specific gravity of
acid.
Ingredients. 1.00 1.115 1.160
Insoluble residue 71.88 70.53 74.15
Soluble silica 11.38 12.30 9.42
Potash 0.60 0.63 0.48
Soda 0.13 0.09 0.35
Lime 0.27 0.27 0.23
Magnesia 0.45 0.45 0.45
Manganese oxid 0.06 0.06 0.06
Ferric oxid 5.15 5.11 5.04
Alumina 6.84 8.09 6.22
Sulfuric acid 0.02 0.02 0.02
Volatile matter 3.14 3.14 3.14
—————— —————— —————
Total 100.02 100.69 99.29

Amount of soluble matter 24.00 27.02 22.27
„ „ „ bases 13.50 14.70 12.83

From the above table it is seen that the strongest acid exerts the least soluble effect upon the substances present in the soil, while the greatest degree of solution was obtained by the acid of 1.115 specific gravity. This result indicates that while lime and magnesia are probably present chiefly as carbonates, potash as well as alumina, and to some extent lime, are present as silicates, and for that reason are not as fully extracted by acid of low strength as by that of medium concentration.

In regard to the influence of the time of digestion, the acid of specific gravity 1.115 being used, the data obtained are given in the following table:

Number of days digested.
Ingredients. 1. 3. 4. 5. 10.

Insoluble residue 76.97 72.66 71.86 70.53 71.79
Soluble silica 8.60 11.18 11.64 12.30 10.96
Potash 0.35 0.44 0.57 0.63 0.62
Soda 0.06 0.06 0.03 0.09 0.28
Lime 0.26 0.29 0.28 0.27 0.27
Magnesia 0.42 0.44 0.47 0.45 0.44
Manganese oxid 0.04 .06 0.06 0.06 0.06
Ferric oxid 4.77 5.01 5.43 5.11 4.85
Alumina 5.15 7.38 7.07 7.88 7.16
Phosphoric acid 0.21 0.21
Sulfuric acid 0.02 0.02 0.02 0.02 0.02
Volatile matter 3.14 3.14 3.14 3.14 3.14
————— —————— —————— —————— —————
Total 99.63 100.68 100.55 100.69 99.80

Amount of soluble matter 19.67 24.88 25.57 27.02 24.87
„ „ „ bases 11.05 13.68 13.91 14.49 13.68

From this table it appears that the amount of dissolved ingredients increases up to the fifth day, the increase becoming, however, very slow as that limit is approached. It is also found that the ingredients offering the greatest resistance to this action are the same as those whose amounts were sensibly affected by the strength of the acid; namely, silica, potash, and alumina.

In regard to lime and magnesia, one day’s digestion not being sufficient for full extraction, it is evident that they do not exist in the soil as carbonates or hydric oxids only, as has been supposed, but also as silicates. A comparison of the results of the five and ten days’ digestion shows that the solvent action of the acid has substantially ceased at the end of five days, there being no further increase of the amount of dissolved matter.

=332. Digestion Vessels.=—Hilgard prescribes that the digestion of the sample of soil with acid be conducted in a small porcelain beaker covered with a watch-glass.[215] Kedzie, however, prefers beakers of bohemian glass, and shows that hydrochloric acid attacks the porcelain with greater energy than the glass.[216] Platinum would be the ideal material for the digestive vessels, but its great cost would exclude its general use. In most cases it will be found that the error introduced into the analysis by the use of porcelain or bohemian glass beakers is quite small and not likely to affect the quantitative estimation of soluble soil ingredients to any extent.

In this laboratory some comparative tests made by Mr. W. D. Bigelow have shown that vessels of hard glass of special manufacture are less soluble in hot hydrochloric acid of 1.115 specific gravity than porcelain, thus confirming the observation of Kedzie. Following are the data showing the weights of material dissolved in fifty hours:

Berlin porcelain 2.8 milligrams
Bohemian glass 1.7 „
Kaehler and Martini glass 1.2 „

In each case twenty-five cubic centimeters of the acid were used. The vessels all had approximately a capacity of 200 cubic centimeters.

=333. Processes Employed—Hilgard’s Method.=—The sample of soil sifted through a 0.5 millimeter mesh sieve and thoroughly air-dried, is conveniently preserved in weighing tubes. The actual content of hygroscopic and combined moisture may be previously made on a separate sample of soil.

In determining the amount of material to be employed for the general analysis regard must be had to the nature of the soil. This is necessary because of the impracticability of handling successfully such large precipitates of alumina as would result from the employment of as much as five grams in the case of calcareous clay soils; while in the case of very sandy soils even that quantity might require to be doubled in order to obtain weighable amounts of certain ingredients. For soils in which the insoluble portion ranges from sixty to eighty per cent, two and a half to three grams are about the right measure for general analysis, while for the phosphoric acid determination not less than three grams should be employed in any case. It has been alleged that larger quantities must be taken for analysis in order to secure average results. It is difficult to see why this should be true for soils and not for ores, in which the results affect directly the money value, while in the case of soils the interpretation of results allows much wider limits in the percentages. Correct sampling must be presupposed to make any analysis useful; but with modern balances and methods it is difficult to see why five grams should be employed instead of half that amount, which in some cases is still too much for convenient manipulation of certain precipitates.

The weighed quantity, usually of two to two and a half grams, is brought into a small porcelain beaker, covered with a watch-glass, treated with eight to ten times its bulk of hydrochloric acid of 1.115 specific gravity, and two or three drops of nitric acid, and digested for five days over the laboratory steam-bath. At the end of this time it is evaporated to dryness, first on the water-bath and then on the sand-bath. By this treatment all the silica set free is rendered insoluble.

=334. Provisional Method of the Official Agricultural Chemists.=—Place ten grams of the air-dried soil in a round bottom 150 to 200 cubic centimeter bohemian flask, add 100 cubic centimeters of pure hydrochloric acid of specific gravity 1.115, insert the stopper, wire it securely, place in a steam-bath, and digest for thirty-six hours at the temperature of boiling water. Pour the contents of the flask into a small beaker, wash with distilled water, add the washings to the contents of the beaker and filter through a washed filter. The residue is the amount insoluble in hydrochloric acid. Add a few drops of nitric acid to the filtrate, and evaporate to dryness on the water-bath; take up with hot water and a few drops of hydrochloric acid, and again evaporate to complete dryness. Take up as before, and filter into a liter-flask, washing with hot water. Cool and make up to the mark. This is solution A. The residue represents the silica originally dissolved. In comparing the two preceding methods it is found that the former; _viz._, digestion in flasks covered only with a watch-glass gives a larger quantity of dissolved matter in five days than the digestion under pressure does in thirty-six hours. In comparative tests in this laboratory made by Mr. W. D. Bigelow the respective quantities of soluble and insoluble matter obtained by the two methods in two soils are as follows:

Soil No. 1. Soil No. 2.
Per cent. Per cent.
Method of Digestion. Insoluble. Soluble. Insoluble. Soluble.
Open flask 75.62 24.38 79.62 20.38
Closed flask 76.81 23.19 80.48 19.52

=335. The German Station Method.=—The method recommended by the German Stations[217] is greatly different from that described above, both in temperature and time of digestion. To one part of the soil are added two parts by volume of a twenty-five per cent hydrochloric acid solution, the quantity being increased to correspond to any excess of carbonates. The mixture is left for forty-eight hours with frequent shaking. As an alternate method, one part of soil is treated with two parts by volume of ten per cent hydrochloric acid, and heated on the water-bath, with frequent shaking, for three hours.

The soluble materials are determined in the filtrate by some of the methods usually employed.

=336. The Gembloux Method.=—The method of making the acid extract of the soil at the Gembloux Station does not differ greatly from some of those already described.

The quantity of air-dried material taken is such that it may weigh exactly 300 grams exclusive of the moisture which it contains. It is dried at 150° for at least six hours. The drying is necessary in order to obtain an extract in hydrochloric acid of exactly 1.18 specific gravity. The dry earth is placed in a flask of two or three liters capacity to which one liter of hydrochloric acid of 1.18 specific gravity is added, being careful to take precautions to prevent frothing if much carbonate be present. The acid is allowed to act for twenty-four hours, it being frequently shaken meanwhile. After settling it is decanted and filtered upon a double folded filter, the apex of which rests upon a small funnel covered with a plain filter of strong paper. Five hundred cubic centimeters of the filtrate are taken for the estimation, and in this filtrate are estimated the silica, phosphoric and sulfuric acids, potash, soda, iron, alumina, lime, and magnesia.

The filtrate is evaporated to dryness in a porcelain capsule, a few drops of nitric acid added and the liquid kept well stirred. The residue should be taken up with water, and if not perfectly bright a second and even a third evaporation with nitric acid should take place, until all the organic matter is destroyed, which will be indicated by the clear yellow or reddish-yellow color of the liquid, caused by the iron oxid. After the last evaporation the material is dried in a drying oven one hour at 110°.