Chapter XIX: Introduction (18)

Exhausted in the cold with very weak hydrochloric acid, 0.04 per cent; the residue treated in the cold with concentrated hydrochloric acid, the solution evaporated and fused with sodium carbonate and potassium nitrate, 0.07 per cent; again, the residue treated with aqua regia to oxidize the sulfur, the solution evaporated to dryness, fused with sodium carbonate and potassium nitrate, 0.14 per cent; in all 0.25 percent. A sample of the same soil treated directly with aqua regia, and then evaporated and fused as above, gave two-tenths per cent sulfuric acid. A sample of the same soil ignited in a crucible with sodium carbonate and potassium nitrate gave 0.16 per cent of sulfuric acid. The difference between 0.04 and 0.07 per cent can be attributed to the sulfur in the organic substance which was dissolved by the concentrated hydrochloric acid; the quantity, however, is too small to draw any safe conclusion. Possibly it might have been that the very dilute hydrochloric acid did not dissolve all of the sulfate. The quantity of sulfur combined in the organic substance in the above soil may be derived from the following equation; _viz._, (0.2–0.07)/(80) × 32 = 0.05 per cent of sulfur.

The estimation of the sulfur in a sample of soil from Deli was carried on with still greater exactness by three different methods.

The quantities of hydrochloric acid, nitric acid, and sodium carbonate employed were measured or weighed, and the minute content of sulfuric acid therein estimated and subtracted from the final results. The methods employed were as follows:

(A) Extraction with water and afterwards with very dilute hydrochloric acid.

(B) Extraction with cold hydrochloric acid, one part to three of water.

(C) Extraction with aqua regia.

(D) Ignition with sodium carbonate and potassium nitrate.

(F) Ignition in a combustion tube with sodium carbonate in a stream of oxygen.

The percentages of sulfuric acid obtained by the different methods were as follows:

(A) 0.058 per cent.
(B) 0.070 „ „
(C) 0.140 „ „
(D) 0.125 „ „
(F) 0.106 „ „

=387. Method of Wolff.=—In regard to the sulfuric acid Wolff calls attention to the fact that in soils which have been ignited, a larger quantity of this acid is found than in soils containing humus.[252] This, doubtless, arises from the oxidation of the organic sulfur. The following special method for determining the sulfuric acid is therefore proposed:

Fifty grams of fresh air-dried soil are placed in a platinum dish with a concentrated solution of pure sodium nitrate. After drying, the heat is raised gradually to redness. In this way the complete ignition of the humus present takes place. After cooling, the mass is diluted with hydrochloric acid, with the addition of a little nitric acid, and boiled. In the solution, the silicic acid is first separated and the sulfuric acid estimated in the usual way with barium sulfate.

=388. Method of the Italian Chemists.=—The determination of the sulfuric acid is conducted as follows by the Italian chemists:[253] The soil is completely extracted by diluted hydrochloric acid and the sulfuric acid precipitated in the solution with barium chlorid. If a soil is very rich in calcium sulfate it should first be treated with a warm solution of sodium carbonate to decompose the calcium sulfate, and the sulfuric acid determined in the solution after having added hydrochloric acid.

=389. Estimation of the Chlorin.=—The estimation of the chlorin is of great importance in certain cases. When this element is lacking in the soil, which, however, is rare, certain plants appear to suffer from its absence. The quality of the forage plants in particular is influenced by it; but when the chlorids are too abundant, which is a frequent case, they prevent or arrest completely the progress of vegetation. Salty soils are, in general, completely sterile. In the proportion of one pound in a thousand in the earth, sodium chlorid is to be regarded as injurious. It is necessary, therefore, in analysis to take account of two cases; _viz._, those of soils poor in chlorids and those of soils rich in chlorids.

For soils poor in chlorids the French method directs that[254] 200 grams of the earth are to be washed on a filter with boiling water. The liquor is evaporated to dryness and gently heated to a temperature inferior to redness in order to destroy the organic matter. The residue is taken up by small quantities of water and to the filtered liquor the volume of which should not exceed forty to fifty cubic centimeters are added ten cubic centimeters of pure nitric acid and a sufficient quantity of silver nitrate to produce a complete precipitation. The precipitate is vigorously shaken and allowed to stand for a few hours in a darkened locality. The precipitate is collected upon a double filter and the silver chlorid, after proper desiccation, is weighed.

When the soil is rich in chlorids it is washed as has just been described upon a filter. The wash-waters are made up to one liter and fifty cubic centimeters, equivalent to ten grams of the soil, are taken for analysis. This quantity is treated exactly as described above.

=390. Wolff’s Method of Estimating Chlorin in Soils.=[255]—Three hundred grams of the soil are treated with 900 cubic centimeters of pure water containing a little nitric acid for forty-eight hours with frequent shaking. Four hundred and fifty cubic centimeters are then filtered and the clear liquid evaporated to 200 cubic centimeters. The chlorin is then precipitated with silver nitrate. The quantity obtained, corresponds to that found in 150 grams of the air-dried soil.

A second method, Mohr’s, is as follows: Fifty grams of the soil are placed in a platinum dish and moistened with a concentrated solution of potassium nitrate, free from chlorin. The mass is evaporated to dryness and gradually heated to a red heat. After cooling it is moistened with water and washed into a beaker and the solid mass quickly separated. The clear liquid is poured off and the residue again washed with water. The clear liquid obtained is saturated with acetic acid, carefully evaporated to dryness and after solution in water, filtration and the addition of a little nitric acid, the chlorin therein is precipitated by a silver nitrate solution, and the precipitate collected and weighed as usual.

=391. Method of Petermann.=[256]—Chlorin in the soil is estimated at the Gembloux station by digesting 1,000 grams of the sample with two liters of distilled water with frequent shaking for thirty-six hours. After allowing to stand for twelve hours with the addition of one gram of powdered magnesium sulfate to facilitate the deposition of suspended matter one liter of liquid is siphoned and evaporated in a platinum dish with the addition of a few drops of a solution of potassium carbonate free from chlorin and nitric acid. The concentrated solution is filtered, washed, and made up to 250 cubic centimeters. Take 100 cubic centimeters of the solution add some nitric acid and precipitate the chlorin with silver nitrate. The rest of the solution is reserved for the estimation of nitrate.

=392. Estimation of Silicic Acid.=—_Direct Estimation._—The sample of soil in the method of Berthelot and André[257] is mixed with two or three times its weight of pure sodium carbonate and fused in a silver crucible until complete decomposition has taken place. The residue is dissolved in water and dilute hydrochloric acid. The silicates are decomposed by this treatment and the solution is evaporated to dryness on the water-bath, and when dry slightly heated. The silicic acid (silica) is by this treatment rendered insoluble. It is collected on a filter, washed, ignited, and weighed. The resulting compound should be mixed with ammonium fluorid and sulfuric acid, and after the disappearance of the silica the residue should be dried and weighed. The loss in weight represents the true silica. The loss in weight should be corrected by calculating the sulfates of the alkalies back to oxids. This correction can be neglected when the work has been carefully done, and the washing of the original silica has been well performed.

_Indirect Estimation._—The total silica may be estimated indirectly by subtracting from the total weight of the sample the sum of the weights of the other constituents resulting from the separate estimation of each of them after decomposing the sample with hydrofluoric acid.

=393. Simultaneous Estimation of Different Elements.=—The operations and processes for the estimation of each of the elements have been described, but it is often best to carry on an operation in such a way as to gain time by making a single decomposition upon a quantity of soil of some considerable magnitude, and using the results of the solution for the determination of the different substances. From the operations already described it will be easy to make a combination of methods by which all or nearly all the important constituents in a soil may be determined in a single sample. Of the various methods proposed, that of the commission of the French agricultural chemists may be taken as a type.[258] In the case of the estimation of lime, potash, magnesia, and sulfuric acid, in which the operation is carried on in a soil which is not incinerated, time may be saved by digesting a considerable quantity of the soil with concentrated nitric acid for a period of five hours. It is best to take 100 grams of the soil and increase proportionally the nitric acid. The filtrate, after washing, is made up to one liter and thoroughly shaken. From this amount of liquid, portions are taken corresponding to the weights of soil upon which the operation for the determination of each of the constituents would be conducted. For example, for the estimation of lime in the case of a very calcareous earth, ten cubic centimeters representing one gram of the original sample, in the case of a soil poor in carbonates 100 cubic centimeters representing ten grams, and for the estimation of potash, magnesia, and sulfuric acid 200 cubic centimeters, representing twenty grams of the soil, should be taken. This method avoids frequent weighings of the earth and separate treatments thereof by the acid.

On the other hand, in the same portion of the solution, the different elements can be estimated. For example, for the estimation of the potash as has been indicated, in the place of precipitating as a whole the sulfuric acid, lime, etc., and of afterwards separating the magnesia in the sole aim of eliminating these bodies, they can be collected separately and weighed, thus securing at a single operation several determinations.

At the end, some barium chlorid is added and if the barium sulfate is then collected and weighed, the estimation of the sulfuric acid is effected. To the filtrate there are afterwards added some ammonia and ammonium carbonate to precipitate, at once, the excess of barium and the iron and aluminum oxids, the lime and the phosphoric acid. This separation being effected the filtrate contains still the magnesia and the alkalies. The first can be separated by carbonation by means of oxalic acid, collected, and weighed. Finally the potash itself can be estimated in the state of perchlorate. It has thus been possible in the same suite of operations to estimate in a given quantity of the liquid, the sulfuric acid, the magnesia, the lime, and the potash.

=394. Estimation of Kaolin in Soils.=—True kaolin is a hydrated aluminum silicate, having the formula H₄Al₂Si₂O₉. This substance is, even in concentrated hydrochloric acid, almost completely insoluble. It contains, theoretically, 13.94 per cent of water of combination. The following methods, due to Sachsse and Becker,[259] can be used for its determination.

_Estimation of the Water of Combination._—Heat from one to two grams of kaolin, dried at 100°, for half an hour in a covered platinum crucible to a temperature which shows an incipient red heat when the crucible is partly protected from the daylight with the hand. This treatment does not quite give the whole of the water of combination but nearly all of it. A kaolin is changed by this treatment into a substance which is easily soluble in dilute hydrochloric acid.

_Estimation of the Kaolin in Impure Kaolins._—Mineral kaolin, or the kaolin obtained by silt analysis, is dried at 100° to constant weight. It is then heated with strong hydrochloric acid until all the matters which will pass into solution have been dissolved. The residual kaolin is then washed thoroughly with water and ignited for half an hour at a low red heat. The residual mass is a second time extracted with hydrochloric acid until silica no longer passes into solution. The soluble silica is then estimated in the usual way and calculated to kaolin. The result will give the pure kaolin in the sample examined.

The estimation may also be made as follows: Two samples of the impure kaolin are taken and dried to constant weight at 100°. One is extracted with hydrochloric acid in the manner described above and the amount of silica determined. The second is treated directly by ignition to low redness for half an hour, dissolved in hydrochloric acid and the amount of silica determined. The difference in the two percentages of silica corresponds to the silica equivalent to the pure kaolin.

_Statement of Results._—It is convenient to incorporate the data obtained by the above methods with the complete mass analysis of the silicate examined. In the sample given below the analysis was made on a clay silt obtained with a velocity of two-tenths millimeter per second.

The mass analysis gave the following data:

Loss on ignition 10.04
SiO₂ 51.52
Al₂O₃ 17.93
Fe₂O₃ 7.42
CaO 1.57
MgO 6.27
K₂O 4.1
Na₂O 1.61

The loss on ignition was made up of the combined water and a trace of humus. On gentle ignition only 7.52 per cent of water came off.

The examination of the non-ignited and the gently ignited silica by means of dilute hydrochloric acid, gave the following data:

Non-ignited. Gently ignited. Difference. Water 10.04 10.04 Insoluble residue 40. 34.54 –5.46 Al₂O₃ 9.04 10. +0.96 Fe₂O₃ 5.96 7.27 +1.31 SiO₂ 25.27 28. +2.73 Alkalies and alkaline earths 9.69 10.15 +0.46

By a comparison of these data with those obtained by the mass analysis, the following representation of the distribution of the various components in the clay is obtained:

23.52 per cent SiO₂ in the form of quartz and undecomposed silicates.

2.73 per cent SiO₂ in the form of kaolin.

25.27 per cent in the form of easily decomposable silicates and of the hydrates of SiO₂.

7.93 per cent Al₂O₃ in the form of undecomposed silicates.

0.96 per cent Al₂O₃ in the form of kaolin.

9.04 per cent Al₂O₃ in the form of easily decomposed silicates and of hydrates.

0.15 per cent Fe₂O₃ in the form of undecomposed silicates.

1.31 per cent Fe₂O₃ in the form of kaolin.

5.96 per cent Fe₂O₃ in the form of easily decomposable silicates and hydrates.

3.40 per cent of alkalies and alkaline earths in the form of undecomposed silicates.

9.69 per cent of alkalies and alkaline earths in the form of easily decomposable silicates.

10.04 per cent of water, including a trace of humus.

Collecting these results the following statement is obtained.

The clay analyzed contained:

10.04 per cent of water, a trace of humus.

35.15 per cent of undecomposed silicates and quartz.

5.00 per cent of kaolin.

50.27 per cent of easily decomposable silicates, hydrates of SiO₂ and hydroxids.

ESTIMATION OF NITROGEN IN SOILS.

=395. Introductory Considerations.=—The great economic and biologic value of nitrogen as a plant food renders its estimation in soils of especial importance. It is necessary, first of all, to remember that the nitrogen present in soils may be found in three forms; _viz._, first, in organic compounds, second, as ammonia, and third, as nitric or nitrous acid. Further than this each of these classes of nitrogen may be subdivided. The organic nitrogen may be in a form easily nitrified and rendered available for plant food, or it may be inert and resistant to nitrification, as in humus, or exist in an amid state. The ammoniacal nitrogen may exist in small quantities as gaseous ammonia, or be combined with mineral or organic acids. As nitric or nitrous acid the nitrogen will be found combined with bases, or perhaps in minute quantities as free acid, in passing under the influence of the nitrifying ferment from the organic to the inorganic state. To the latter state it must finally come before it is suited to absorption by plants.

In general, far the largest part of soil nitrogen, excluding the atmosphere diffused in the pores of the soil, is found in the organic state and is derived from the débris of animal and vegetable life and from added fertilizers. As ammonia, the nitrogen can only be regarded as in a transition state, arising from the processes of decay, or incomplete nitrification. As nitric acid, it is found as a completed product of nitrification, or as the result of electrical action. The processes of nitrification and the isolation and determination of the nitrifying organisms will be considered in a special chapter of this manual. By reason of the great solubility of the nitrates, and the inability of the soil to retain them, there can never be a great accumulation of nitric acid in the soil save in localities deficient in rain-fall or in specially protected spots, such as caves. The nitric acid, therefore, produced in the soil passes at once into growing vegetation, or is found eventually in the drainage waters.

The formation of ammonia in soil containing much vegetable matter is thought by Berthelot and André[260] to be due to the progressive decomposition of amid principles under the influence of dilute acids or alkalies, either in the cold or at an elevated temperature. Soils of the above description, of themselves, contain neither free ammonia nor ammoniacal salts, and the ammonia which is found in the analysis of these soils comes from the reaction above indicated. The ammonia which comes from these soils, in place of what is given off to the surrounding atmosphere, comes from the same class of decompositions, and these decompositions, in this case, are effected by the water itself, and by the alkaline carbonates of the soil. The amid principles which are thus decomposed belong either to the class of amids proper, derived by the displacement of hydrogen in ammonia by acids, or to the class of alkalamids derived from nitrogenous bases, both volatile and fixed. Among these alkalamids some are soluble in water and some insoluble, and the decomposition of these last by acids or by alkalies may furnish bodies which themselves are either soluble or insoluble in water.

To determine the nature of the nitrogenous principles in a soil rather rich in humus, Berthelot and André applied the following treatment:

A soil containing 19.1 grams of carbon and 1.67 grams of nitrogen per kilogram was first subjected to treatment, at room temperature, with a concentrated solution of potash. By this treatment 17.4 per cent of the nitrogen content was set free under the form of ammonia. One-quarter of this was obtained during the first three days; one-eighth during the next three days. Afterward the action became much more feeble and was continued during forty days longer, and the evolution of the gas was diminished almost proportionately to the time. It appears from the above observations that the amid principles of the soil, decomposable by potash, belong to two distinct groups, which are broken up with very unequal velocities. The soil, treated on the water-bath for three hours at 100° with strong potash, showed the following behavior in respect of its nitrogenous constituents: Nitrogen eliminated in the form of ammonia, sixteen per cent; nitrogen remaining in the part soluble in potash, ten per cent; nitrogen remaining in the part insoluble in potash, seventy-four per cent.

_Treatment with Acid._—The nitrogenous compounds of the soil are also decomposed by dilute acids, and often more rapidly than by the alkalies. The method of treatment is substantially the same as that set forth above. The decompositions effected either by alkalies or by acids tend in general to lower the molecular weights of the resulting products. The prolonged action of alkalies at the temperature of boiling water rendered soluble, after twenty-four hours of treatment, 93.6 per cent of the organic nitrogen found in the vegetable mould. By treating the earth successively with alkalies and acids 95.5 per cent of the total nitrogen were decomposed. These experiments show how the insoluble nitrogen in humic compounds can be gradually rendered assimilable. The action of vegetables is not assuredly identical with those which acids and alkalies exercise. However, both present certain degrees of comparison from the point of view of the mechanisms set in play by the earthy carbonates and carbon dioxid, as well as by the acids formed by vegetation. The reactions which take place naturally, while they are not so violent as those produced in the laboratory, make up by their duration what they lack in intensity.

For a more detailed study of the nature of the nitrogenous elements in soil the following method of treatment, due to Berthelot and André, is recommended:

_Treatment of the Soil with Alkalies._—1. Reaction with cold, dilute solution of potash. Take fifty grams of the sample, dried at 110°, and mix with a large excess of ten per cent potash solution and place under a bell-jar containing standard sulfuric acid. The mixture is left for a long time in order to secure as fully as possible the ammonia set free.

Example: Fifty grams of a soil contained 0.0814 gram of nitrogen. Treated as above it gave the following quantities of nitrogen as ammonia:

Nitrogen as Ammonia.
After 3 days 0.0034 gram.
„ 6 „ 0.0054 „
„ 11 „ 0.0065 „
„ 17 „ 0.0078 „
„ 25 „ 0.0093 „
„ 41 „ 0.0107 „
„ 46 „ 0.0141 „

It is seen that the action still continued after forty days. In the space of forty days 17.4 per cent of the total nitrogen contained in the soil had been converted into ammonia by dilute potash. According to the above observations the amid principles transformed into ammonia under the influence of dilute potash, exist in groups which are acted on with very unequal rapidity.

2. Reaction with hot dilute solution of potash. Take 200 grams of the soil sample, mix with one and one-half liters of dilute potash solution containing fifty grams of potash. Place in a flask and heat on boiling water-bath for six hours. The flask is furnished with a stopper and tubes, and a current of pure hydrogen is made to pass through the liquid, having the double object of preventing any oxidizing effect from the air and of carrying away the ammonia which may be formed. The escaping hydrogen and ammonia are passed into a bulb apparatus containing titrated sulfuric acid.

The sample of soil employed contained in 200 grams, 0.3256 gram of nitrogen. There was obtained at the end of six hours’ heating, 0.0366 gram of nitrogen. In other words, 11.24 per cent of the total nitrogen in the sample appeared as ammonia.

_Examination of Residue._—After the separation of the ammonia as above described, pour the residue in the flask on a filter, wash with hot water, and determine nitrogen in filtrate and in solid matter on the filter by combustion with soda-lime. The filtrate is, of course, first evaporated to dryness after being neutralized with sulfuric acid.

The insoluble part contained 0.041 gram of nitrogen, _i. e._, 12.84 per cent of the entire amount.

The soluble part contained 0.2411 gram of nitrogen, _i. e._, 74.05 per cent of the whole.

_Summary of Data._—In the sample analyzed the following data were obtained:

Of the whole.
Nitrogen as ammonia 11.24 per cent.
„ in insoluble part 12.84 „ „
„ „ soluble part 74.05 „ „
„ not determined 1.87 „ „
——————
Sum 100.00 „ „

The same experiment in which the heating on the water-bath was continued for thirteen hours gave the following data:

Of the whole.
Nitrogen as ammonia 16.03 per cent.
„ in insoluble part 9.98 „ „
„ „ soluble part 74.01 „ „
——————
Sum 100.00 „ „

_Further Treatment of Matter Insoluble in Hot Dilute Potash._—A portion of the insoluble portion from the last experiment was treated for thirteen hours longer under the same conditions with dilute hot potash. The soluble and insoluble portions were determined as already described. Of the nitrogen insoluble after thirteen hours, 64.21 per cent remained insoluble after the second thirteen hours. This fact shows that slow and progressive decomposition of the alkalamids in the soil occurs under the influence of hot dilute potash.

_Treatment of Matter Insoluble in Hot Dilute Potash with Hydrochloric Acid._—A part of the material insoluble in hot potash after thirteen hours is mixed with dilute hydrochloric acid, in such proportion as to have one-fifth the weight of pure hydrochloric acid to the dry solid matter. Heat in flask on a boiling water-bath for thirteen hours. Determine the nitrogen in the insoluble residue.

_Example_: In the case given it was found that 54.91 per cent of the nitrogen insoluble in dilute hot potash were dissolved by the hot hydrochloric acid.

This fact shows that insoluble nitrogen compounds contained in the soil are dissolved by dilute acids even more readily than by dilute alkalies at the temperature of boiling water.

Several reactions appear to take place simultaneously when potash is brought into contact with the nitrogenous principles of arable earth. Some of these principles, during the first period of the action become soluble and even form compounds which are not precipitable by acids. When, however, the action of the potash is prolonged, the dissolved bodies lose little by little a part of their nitrogen as ammonia or as soluble alkalamids. They become thus changed either to compounds no longer soluble in the potash, or to those insoluble in the solution when acidified. These compounds, it is true, contain nitrogen, but are poorer in this element and have a higher molecular weight, or, in other words, are condensation products. These last principles are not absolutely stable in the presence of potash, but are decomposed much more slowly than the original principles from which they were derived.

In general, it may be said that under the influence of alkalies on the nitrogenous principles of the soil there is a tendency to form two classes of bodies, the one more soluble with a lower molecular weight, the other less soluble with a higher molecular weight. The inverse relation between solubility and condensation is in agreement with what is observed in similar reactions with organic bodies in general. It certainly plays an important rôle in the transformations which an arable soil undergoes, either through the mild influences of the air and natural waters, or the more energetic action of vegetables themselves.

The methods of estimating nitric nitrogen will be made the theme of a special study in connection with the chapter on nitrification. There will be considered first, therefore, the methods of determining organic and ammoniacal nitrogen with only such incidental treatment of the methods for nitric nitrogen as the processes applicable to the other forms may contain.

=396. Provisional Methods of the Association of Official Agricultural Chemists.=[261]—The nitrogen compounds in the soil are usually placed in three classes.

1. The nitrogen combined with oxygen as nitrates, or nitrites, existing as soluble salts in the soil.

2. The nitrogen combined with hydrogen as ammonia, or organic nitrogen easily convertible into ammonia. The ammonia may exist as salts, or may be occluded by hydrated ferric or aluminum oxids and organic matter in the soil.

3. The inert nitrogen of the soil or the humus nitrogen.

_Active Soil Nitrogen._—The material proposed for reducing the nitrates to ammonia, and at the same time to bring ammonia salts and organic nitrogen into a condition for separation by distillation, is sodium amalgam. Liquid sodium amalgam may be readily prepared by placing 100 cubic centimeters of mercury in a flask of half a liter capacity, covering the warmed mercury with melted paraffin, and dropping into the flask at short intervals pieces of metallic sodium, the size of a large pea (taking care that the violence of the reaction does not project the contents from the flask), till 6.75 grams of sodium have combined with the mercury. The amalgam contains one-half per cent of sodium and may be preserved indefinitely under the covering of paraffin. To estimate the active soil nitrogen, weigh fifty grams of air-dried soil and place it in a clean mortar. Take 200 cubic centimeters of ammonia-free distilled water, rub up the soil with a part of the water to a smooth paste, transfer this to a flask of one liter capacity, washing the last traces of the soil into the flask with the rest of the water. Add twenty-five cubic centimeters of the liquid sodium amalgam and shake the flask so as to break the sodium amalgam into small globules distributed through the soil. Insert a stopper with a valve and set aside in a cool place for twenty-four hours. Pour into the flask fifty cubic centimeters of milk of lime, and distill, on a sand-bath, 100 cubic centimeters into a flask containing twenty cubic centimeters of decinormal sulfuric acid, and titrate with decinormal soda solution, using dimethyl-orange as indicator. Estimate the nitrogen of the ammonia found as active soil nitrogen.

If the ammonia produced is too small in amount to be readily estimated volumetrically, determine the ammonia by nesslerizing the distillate.

_Estimation of Nitrates in the Soil._—When it is desired to estimate separately the nitrates in the soil the following method may be used: Evaporate 100 cubic centimeters of the soil extract to dryness on the water-bath, dissolve the soluble portion, of the residue in 100 cubic centimeters of ammonia-free distilled water, filtering out any insoluble residue, place the solution in a flask and add ten cubic centimeters of liquid sodium amalgam, insert stopper with valve, set it aside to digest in a cool place for twenty-four hours, add fifty cubic centimeters of milk of lime, distill and titrate as above, and estimate the nitrogen as N₂O₅.

Nesslerizing may be substituted for titration when the amount of nitrates is small.

An approximate estimation of the amount of nitrates will be of value in determining which method of estimation to use. This may be done by evaporating a measured quantity of the soil extract, say five cubic centimeters, on a porcelain crucible cover on a steam-bath or radiator, having first dissolved a minute fragment of pure brucin sulfate in the soil extract. When dry pour over the residue concentrated sulfuric acid, free from nitrates, and observe the color reactions produced.

If the nitrate (reckoned as KNO₃) left upon evaporating the quantity of water taken does not exceed the two-thousandth part of a milligram, only a pink color will be developed by adding the sulfuric acid; with the three-thousandth part of a milligram, a pink with faint reddish lines; with the four-thousandth part, a reddish color; with the five-thousandth part, a red color.

By increasing or diminishing the amount of soil extract evaporated to secure a color reaction of a certain intensity, an approximate estimate may be made of the amount of nitrates present.

Blank experiments to test the acid and the brucin sulfate will be required before confidence can be placed in such estimations.

_Total Nitrogen of Soils._—The total nitrogen of soils may be determined by the usual combustion with soda-lime, but this process is often unsatisfactory because of the large amount of material required when the organic matter or humus is small in amount.

A modification of the kjeldahl method is more easy to carry out and gives results equally satisfactory. Place twenty grams of soil in a kjeldahl flask, and add twenty cubic centimeters of sulfuric acid (free from ammonia) holding in solution one gram of salicylic acid. If the soil contain much lime or magnesia in the form of carbonate, enough more sulfuric acid must be added to secure a strongly acid condition of the contents of the flask. Add gradually two grams of zinc dust, shaking the contents of the flask to secure intimate mixture. Place the flask in a sand-bath and heat till the acid boils, and maintain the boiling for ten minutes. Add one gram of mercury and continue the boiling for one hour, adding ten cubic centimeters of sulfuric acid if the contents of the flask are likely to become solid. Cool the flask and wash out the soluble materials with 200 cubic centimeters of pure water, leaving the heavy earthy materials. Rinse the residue with 100 cubic centimeters of water, and add this to the first washings. Place this soluble acid extract in a liter digestion flask, add thirty-five cubic centimeters of a solution of potassium sulfid, and shake the flask to secure intimate mixture of the contents. Introduce a few fragments of granulated zinc, pour in seventy-five cubic centimeters of a saturated solution of caustic soda, connect the flask with a condenser and distill 150 cubic centimeters into a flask containing twenty cubic centimeters of acid, using the same acid and alkali for titration used in the kjeldahl method under fertilizers.

Enter the nitrogen found in this operation as total soil nitrogen.

The difference between the total soil nitrogen and the active soil nitrogen will express the inert nitrogen of the soil.

=397. Hilgard’s Method.=[262]—The humus determination will, in the case of virgin soils, usually indicate approximately the store of nitrogen in the soil, which must be gradually made available by nitrification. Ordinarily (outside of the arid regions) the determination of ammonia and nitrates present in the soil is of little interest for general purposes, since these factors will vary with the season and from day to day. Kedzie proposes to estimate the active soil nitrogen (ammonia plus nitrates and nitrites) by treatment of the whole soil with sodium amalgam and distillation with lime. The objection to this process is that the formation of ammonia by the reaction of the alkali and lime upon the humus amids would greatly exaggerate the active nitrogen and lead to a serious overestimate of the soil’s immediate resources.

The usual content of nitrogen in black soil-humus is from six to eight per cent in the regions of summer rains. From late determinations it would seem that in the arid regions the usually small amount of humus (often less than two-tenths per cent) is materially compensated by a higher nitrogen percentage. It thus becomes necessary to determine the humus nitrogen directly; and this is easily done by substituting in the grandeau process of humus extraction potash or soda-lye for ammonia water, and determining the nitrogen by the kjeldahl method in the filtrate.

The lye used should have the strength of four per cent in the case of potassium hydroxid, three per cent in that of sodium hydroxid. The black humus filtrate is carefully neutralized with sulfuric acid, evaporated to a small bulk in a beaker or evaporating basin, and the reduced liquid finally evaporated to dryness in the kjeldahl flask itself by means of a current of air. The beaker or basin is washed either with some of the alkaline lye, or, after evaporation, with warm concentrated sulfuric acid, which is then used in the nitrogen determination in the usual way.

For the determination of nitrates in the soil it is, of course, usually necessary to use large amounts of material, say not less than 100 grams, and, according to circumstances, five or more times that amount. In the evaporated solution the nitric acid is best determined by the reduction method, as ammonia.

Usually the soil filtrate is clear and contains no appreciable amount of organic matter that would interfere with the determination; yet in the case of alkaline soils (impregnated with sodium carbonate) a very dark colored solution may be obtained. In that case the soil may advantageously be mixed with a few per cent of powdered gypsum before leaching; or the gypsum may be used in the filtrate to discolor it by the decomposition of sodium carbonate and the precipitation of calcium humate. The evaporated filtrate can then be used for the nitrate determination by either the kjeldahl, griess, or the nessler process, which will, of course, include such portions of the ammoniacal salts as may have been leached out.

For the separate determination of these and of the occluded ammonia, when desired, it is probably best to mix the wetted soil intimately with about ten per cent of magnesium oxid and distill into titrated hydrochloric acid. For general purposes, however, this determination is usually of little interest.

=398. Müller’s Modified Kjeldahl Method.=—Numerous difficulties, as stated by Müller,[263] have attended the attempts to apply the kjeldahl method for the estimation of nitrogen to samples of soil, and he has modified the method to some extent and made comparisons of the quantity of nitrogen by this modified method and by the soda-lime method.

The principal difficulty encountered by him has been in the regular heating of the mixture of fuming sulfuric acid and soil. The particles of soil are deposited at the bottom of the flask and the result is that the bottom layers become overheated, and, being poor conductors of heat, fail to transmit a sufficient quantity of heat to penetrate to the upper layers of the liquid to complete the reaction. In order to avoid this difficulty Müller heats his flask in a small stove formed with a straight vertical cylinder of iron or copper, the upper end of which is covered with a sheet of iron pierced with a hole which allows the neck of the flask to pass through, while the lower end is closed with a piece of sheet iron furnished on its upper surface with a layer of asbestos. This cylinder of metal is surrounded with a second one, concentric with the first through which passes a current of heated gases furnished by an ordinary bunsen. By heating the flask in this stove or furnace an even distribution of the heat is secured to all parts of the mixture, but the little drops of sulfuric acid, which are condensed on the cold part of the neck, sometimes lead to the fracture of the glass as they run down the sides of the flask to the hot portions. To prevent the reflux of this condensed acid, which only needs to be done near the end of the reaction, when it is necessary to heat to a very high temperature, the neck of the flask is bent at the point immediately above its emergence at the upper surface of the furnace, and carried into a flask of about seventy-five cubic centimeters capacity, which will receive the drops of sulfuric acid condensed during the operation. The furnace has the following dimensions; height, twelve centimeters; diameter of interior cylinder, five and one-half centimeters; diameter of exterior cylinder, seven and one-half centimeters.

It is supported on a triangle of large iron wire and is heated by an ordinary bunsen, or by a concentric bunsen, according to the temperature which it is necessary to obtain. The proportions which should be observed between the amount of earth employed and the sulfuric acid are about as follows:

Of the dry earth, fifteen grams; of the fuming sulfuric acid, thirty cubic centimeters. There should also be added to the mixture about three-tenths of a gram of pure stearic acid, or better, benzoic acid. When the soil to be analyzed does not contain carbonate, the sulfuric acid should be added in two portions. At first add about twenty cubic centimeters of the acid, and after shaking it, the other ten cubic centimeters, running it in from a burette or a pipette in such a manner as to wash thoroughly the neck and sides of the flask. If the earth contain carbonate, however, it is necessary to add the fuming acid in small portions of about five cubic centimeters at a time, waiting each time until the disengagement of the gas caused by the previous addition has ceased. A soil which contains from thirty to forty per cent of calcium carbonate should be carefully treated in a porcelain capsule with a slight excess of sulfuric acid, pure and dilute. The mixture is afterward to be evaporated to dryness upon a sand-bath and the residue heated in a drying oven to 110°. The mass is then pulverized, introduced into the flask, treated with three-tenths of a gram of benzoic acid and thirty cubic centimeters of fuming sulfuric acid, and treated as indicated above. In all cases it is necessary to continue the heating until the contents of the flask are colorless.

With soils containing considerable quantities of iron, however, a slight red color will probably be observed which will not interfere with the accuracy of the tests.

The heating should at first be gentle and the temperature afterward elevated little by little, and finally the heat should be sufficiently great to distill about one and one-half cubic centimeters of sulfuric acid. The operation lasts from twelve to thirteen hours. As the reaction is terminated the cooled mass is taken up with water absolutely free from ammonia. It is filtered into a flask, and washed upon the filter until the volume of the filtered liquid is about 350 cubic centimeters. Afterward an excess of soda-lye, at 50° baumé is added, then a few pieces of quartz to facilitate boiling. The flask is then connected with a condenser, the liquid distilled and received in a conical flask closed by a cork having two holes, of which one permits the entrance of the end of the condenser, and the other a glass tube which is connected with a small flask containing water, the neck of the receiving flask being inclined toward the condenser to avoid the entrainment of any of the alkaline liquid which may be distilled. The receiving flask rests upon two or three pieces of sheet iron and is heated with an ordinary burner, and ebullition is perfectly regular. From 170 to 180 cubic centimeters of the liquid are distilled in from three and one-half to four hours. The distilled liquid, treated with a few drops of litmus, is titrated by a solution of sulfuric or hydrochloric acid, of which one cubic centimeter corresponds to 0.001 cubic centimeter of nitrogen.

=399. Modification of the Kjeldahl Method by Arnold and Wedemeyer.=[264]—For the oxidizing liquid a mixture of three grams of benzoic acid with forty cubic centimeters of H₂SO₄ is employed. After placing in the digestion flask with the nitrogenous body the whole is gently shaken for a few minutes to prevent clotting. The temperature is then raised until acid vapors begin to come off, when one gram of copper sulfate and one gram of mercuric oxid are added; and after ten to fifteen minutes, to avoid foaming, ten to twenty grams of potassium sulfate. The sublimate noticed on the walls of the flask is benzoic acid and does not interfere with the accuracy of the determination.

This method has given good results with the alkaline nitrates, the nitrates of barium, mercury, silver, lead, and with strychnia, ammonia, pyridin, azobenzol, dinitrobenzol, and picric acid.

=400. Prevention of Bumping During Distillation.=—Daffert has employed the modified kjeldahl method, but found considerable difficulty in using the same owing to the violent bumping of the liquid in the distillation. This was especially the case where the sample contained a large proportion of sand. To overcome this annoyance and danger he devised the following process:[265]

Fit into the mouth of a large-mouthed distillation flask a stopper having two perforations. Through one of the perforations pass the usual distillation tube, through the other a similar tube connected with a supply of steam. Bring the contents to a brisk boil, after which a small current of steam is turned on, allowing the same to pass in a small stream throughout the distillation. By this means, not only is all danger from bumping avoided, but the time required for the distillation shortened. By the old method it usually requires from fifteen to twenty minutes, whereas the former requires from six to ten minutes.

It is advisable to filter all samples of soils having a large proportion of sand.

=401. Determination of Organic Nitrogen by the Soda-Lime Method.=—In the description of the method following, the directions of the French Commission of Agricultural Chemists have been taken as the basis of the analytical process.[266] This method is, in this country, almost superseded by the moist combustion process with sulfuric acid. By reason of its long use, however, and because it is still regarded as the best method by the agricultural chemists of France, Italy, and England, it merits a full description. It is recommended also by Berthelot and André,[267] by the International Congress of Chemists, held in Paris in 1889, by the Italian chemists, and by the official Belgian method,[268] in all cases where nitrates are not present in notable quantities.

The nitrogen which is found in soils in the organic state is transformed into ammonia when it is heated with soda-lime. This reaction is the base of the process of analysis which has so long been used for this class of bodies. The analytical process is conducted as follows:

A well-cleaned glass combustion tube, closed at one end, is used. The length of the tube is from thirty-five to forty centimeters. It is filled first to a depth of two centimeters with calcium oxalate; afterwards to a depth of five centimeters with soda-lime in small fragments; afterwards with the mixture to be analyzed; _viz._, of ten grams of the sample of soil, or twenty grams if poor in nitrogen and organic matters, with soda-lime reduced to a coarse powder. This mixture should occupy a length of about twenty centimeters in the tube. The soil and soda-lime are mixed in a mortar. Afterwards the mortar is rubbed with small quantities of soda-lime, and this, together with the copper boat which has been used in introducing the mixture, is thoroughly washed with the soda-lime, which is poured into the tube until it is filled to within four centimeters of its open extremity. The open end of the tube is then closed with a wad of asbestos packed sufficiently tight to prevent the carrying off of the soda-lime by the gas which may be generated during the combustion. The combustion should be commenced by heating the tube near the open extremity until it is red and carrying the heat progressively towards the part containing the soil mixed with the soda-lime. An ordinary gas combustion furnace should be used and the heat graduated in such a way that the bubbles of gas pass off regularly and not too rapidly. The gas is conducted into a bulb tube containing a decinormal standard sulfuric acid colored with litmus. The combustion is continued until the whole of the organic material is decomposed, care being taken not to raise the combustion tube above a low redness in order to avoid its softening. At the end, however, the temperature of the combustion tube should be raised to a bright red, and the part containing the calcium oxalate should be heated little by little for the purpose of evolving hydrogen, which is used to drive out the last traces of ammonia. After the combustion is completed, and the last traces of ammonia driven out, the standard acid which has received the evolved ammonia is removed, the tube leading to it washed, the wash-water collected with the rest of the liquid and titrated with a standard solution of lime-water, the strength of which has previously been determined against standard sulfuric acid.

=402. Preparation of the Standard Sulfuric Acid.=—The sulfuric acid to be used in making the standard solutions should be previously boiled for half an hour in a platinum dish and allowed to cool in a desiccator. It should contain 61.25 grams of sulfuric acid in one liter. It is recommended that the flask which holds the sulfuric acid should be one which has been used for a long time for holding concentrated sulfuric acid, in order to avoid any action of the alkali in the glass upon the acid after its strength has been determined. The solution before described is of such strength as to have each cubic centimeter equivalent to one milligram of nitrogen.

For the estimation of the nitrogen in the soil a tenth normal solution should be used, which is prepared by taking 100 cubic centimeters of the normal solution, described above, and diluting to one liter.

_Preparation of the Lime-Water._—From 200 to 300 grams of slaked lime are placed in a closed flask of about five liters capacity. This is filled with water and shaken frequently, and left to deposit the matter in suspension. The water which contains the saline particles which may have been present in the lime is then poured off. Fresh water is then poured on and the flask shaken from time to time. To use this lime-water the clear part of it is decanted into a flask, avoiding, as much as possible, access to the air. The flask is closed with a cork carrying two tubes drawn out and bent at a right angle. One of these serves for pouring off the water and the other serves for the entrance of the air. These two tubes are themselves closed by means of a rubber tube carrying a pinch-cock. The strength of the lime-water is fixed by titration with the decinormal standard sulfuric acid.

_Preparation of the Soda-Lime._—Six hundred grams of slaked lime in fine powder are saturated with 300 grams of caustic soda dissolved in 300 cubic centimeters of water. The whole is rubbed into a paste and introduced into a crucible which is heated to redness. The contents of the crucible, still hot, are poured out, and rapidly reduced to fragments in a copper mortar in such a manner as to have the pieces about the size of a pea, and without having too much finely powdered soda-lime mixed with it. While the matter is still hot it is placed in a flask and well-stoppered. In order that this reagent should contain no nitrogen it is indispensable to use in its preparation materials which contain no trace of nitrates.

_Preparation of the Calcium Oxalate._—In a small copper vessel place 100 grams of oxalic acid and add gradually, bringing it to boiling, enough water to dissolve it. Afterwards place in the solution small portions of slaked lime in a state of powder, constantly testing it until turmeric paper indicates that there is a little lime in excess. It is then evaporated, stirring vigorously on the open fire, and the evaporation is finally finished on a steam-bath. The dried material is placed in a flask and well-stoppered. The oxalic acid which is used in this preparation should be free from every trace of nitrogen.

_Preparation of the Litmus Solution._—Five grams of litmus are placed in a flask with a flat bottom. Afterwards a few cubic centimeters of ammonia are added, twenty five grams of crystallized sodium carbonate, and ten cubic centimeters of water. This mixture is left to digest for sometime, with frequent stirring, at a temperature of from 60°–80°. The digestion is finished in about four or five days, during which time, at intervals, a few drops of ammonia are added, sufficient to maintain always the ammoniacal odor. At the end of this time 200 cubic centimeters of water are added and the digestion allowed to continue several days more, still maintaining the solution alkaline with ammonia. A slight excess of hydrochloric acid is added, and the matter which is precipitated is received upon a filter where it is washed several times with cold water and allowed to dry at a low temperature.

For use, from one to two grams of this dry precipitate are dissolved in 100 cubic centimeters of alcohol, and there is thus obtained a litmus solution of extreme sensibility.

=403. Treatment of Soil Containing Nitrates.=—Nitrates exist in small quantities in all arable soils. When treated for nitrogen by the soda-lime method above described, a part of the nitric nitrogen is changed to the state of ammonia, while another part escapes estimation altogether, causing an error which it is important to point out. When the soils contain only small quantities of nitrates this error is insignificant and does not affect sensibly the results, but in the case of earths rich in nitrates it is necessary first to eliminate them before the determination of the nitrogen by the soda-lime method. The operation is carried on as follows:

Twenty grams of the soil are washed on a small funnel, furnished with a plug of asbestos, with small quantities of pure water, in such a way as to cause thirty to forty cubic centimeters of water to pass through. The whole of the nitrate is thus removed. The soil is now dried and submitted to analysis by the soda-lime method as just described. There are removed with the nitrate only small traces of organic nitrogen, too small to influence the results of the analysis. If, however, it is desired to remove altogether this slight cause of error, evaporate the wash-waters, above described, to two or three cubic centimeters; add a few drops of a concentrated solution of ferrous chlorid and as much hydrochloric acid, and boil some minutes in order to drive off, in the state of nitrogen dioxid, all the nitric acid. The residue is evaporated to dryness and contains the traces of organic nitrogen. This is added to the soil which is to be treated by the soda-lime method.

=404. Müller’s Method.=—The determination of nitrogen in the soil by soda-lime is carried on as follows by Müller:[269]