TB026-1929.Pdf

Magnesium and Calcium in Zeolitic Soils

Item Type text; Book

Authors Breazeale, J. F.

Publisher College of Agriculture, University of Arizona (Tucson, AZ)

Download date 27/09/2021 10:36:46

Link to Item http://hdl.handle.net/10150/190616

CONTENTS Page Introduction '—37 Solubilities of Magnesium and Calcium Carbonates 38 Base Replacement 40 Preparation of Soil Samples 41 Effect of Common Ions in Solution 42 Effect of Calcium Carbonate upon Base Replacement 48 Experiment with Calcareous, Magnesium-Zeolite Soil 50 Equilibrium Between Magnesium and Calcium in a Non-Calcareous Soil 51 Effect of Basic Magnesium Carbonate upon Base Replacement 53 Magnesium Carbonate in Black-Alkali Soils 55 Effect of Magnesite upon Base Replacement, When no Carbon Dioxide is Present 57 Effect of Dolomite upon Base Replacement When no Carbon Dioxide is Present 58 Effect of Carbon Dioxide upon Base Replacement 58 Base Replacement Property of Dolomite in a Saturated Solution of Carbon Dioxide 60 Reaction between Magnesium Zeolite and Calcium Carbonate 60 Reaction between Barium Zeolite and Calcium Carbonate 62 Reaction between Calcium Zeolite and Calcium Carbonate 62 Summary 64 Bibliography 65 TABLES Table I.—Base Replacement in a Sodium-Zeolite Soil as Affected by a Common Ion in Solution 43 Table II.—Base Replacement in a Potassium-Zeolite Soil as Affected by a Common Ion in Solution 43 Table III.—Base Replacement in a Mixture of Sodium-Zeolite and Calcium-Zeolite Soils as Affected by Common Ions in Solution 44 Table IV.—Base Replacement in a Mixture of Potassium-Zeolite and Calcium-Zeolite Soils as Affected by Common Ions in Solution 45 Table V.—Base Replacement in a Mixture of Barium-Zeolite and Calcium-Zeolite Soils as Affected by Common Ions in Solution -.47 Table VI.—Base Replacement in a Mixture of Magnesium-Zeolite and Calcium-Zeolite Soils as Affected by Common Ions in Solution 48 Table VII.--Replacement oi Sodium by Magnesium in a Calcareous, Black-Alkali Soil in the Presence of Sodium Chloride 48 TABLES—Continued Table VIII.—Equilibrium between Magnesium and Calcium in a Calcareous, Calcium-Zeolite Soil 49 Table IX.—Equilibrium between Magnesium and Calcium in a Calcareous Magnesium-Zeolite Soil .• 51 Table X.—Equilibrium between Magnesium and Calcium in a Non- Calcareous, Calcium-Zeolite Soil 52 Table XI.—Equilibrium between Magnesium and Calcium in a Non- Calcareous, Magnesium-Calcium-Zeolite Soil 53 Table XII.—Effect of Basic Magnesium Carbonate on Base Replacement Reactions in a Calcareous, Calcium-Zeolite Soil 54 Table XIII.—Effect of Basic Magnesium Carbonate in Base Replacement Reactions in a Calcareous, Magnesium-Zeolite Soil 55 Table XIV.—Effect of Basic Magnesium Carbonate upon Base Replace- ment in a Sodium-Zeolite Soil in the Presence of Sodium Chloride ...... 56 Table XV.—Effect of Magnesite upon Base Replacement Reactions in a Calcareous, Calcium-Zeolite Soil , 57 Table XVI.—Reaction between Magnesium Zeolite and Calcium Bicarbonate 59 Table XVII.—Reaction between Calcium Zeolite and Magnesium Bicarbonate in a Non-Calcareous Soil 60 Table XVIII.—Equilibrium between Magnesium Zeolite and Calcium Carbonate, and "Build-up" of Soil Zeolites 61 Table XIX.—Equilibrium between Barium Zeolite, Calcium Carbonate and "Build-up" of Soil Zeolites - , 62 Table XX.—Equilibrium between Calcium Zeolite, Calcium Carbonate, and "Build-up" of Soil Zeolites 63 ILLUSTRATIONS Fig. I.—Illustration of apparatus used in percolating soils 41 MAGNESIUM AND CALCIUM IN ZEOLITIC SOILS By

J. F. BREAZEALE

INTRODUCTION The development of irrigation agriculture in the United States during the past few decades, has brought up many problems, among which some of the most important are those asso- ciated with the presence and accumulation of soluble salts, or alkali, in the soil. For a long time the agriculturists in the irrigated regions were interested in alkali chiefly because of its toxicity to grow- ing plants, and their efforts at reclamation and many of their irrigation practices were inspired by the desire either to reduce the concentration of the alkali already in the soil, or to prevent its accumulation in amounts that might be toxic to crop plants. However, in recent years, it has been demonstrated that there are other factors connected with reclamation that are of more importance economically than the mere removal or reduction of the concentration of the undesirable salts. Upon reclamation by the ordinary method of leaching with irrigation water, with- out the use of remedial agents, such as gypsum, practically all alkali soils, except those that contain an excess of a soluble calcium salt, tend to become deflocculated and impermeable to water. In Arizona, there are few soils that contain appreciable amounts of calcium sulphate or gypsum, and largely on this account reclamation has been attended with many difficulties. In many cases the soils, after being leached free from alkali, are of less agricultural value than they were before the reclamation was begun. Therefore, the farmer has become more interested in the penetration of water and its subsequent removal by drainage, than he is in the toxicity of the alkali salts. In other words, the indirect effect of alkali upon the soil is of more agricultural'importance than its direct toxic effect upon the plant. The deflocculation of soils during reclamation, or under continued cultivation, is brought about usually by a change in the colloidal complexes, or zeolites, in the soil. An intelligent inter- pretation of certain soil phenomena is obviously necessary in order to plan and execute practical methods of reclamation. The (37) 38 TECHNICAL BULLETIN No. 26 demand for specific information on the part of the farmer, has caused an extended investigation into the nature and function of soil zeolites, and many methods have been proposed recently for measuring the quantity of these zeolites in the soil, and for determining the relation of one zeolite to another. When a single zeolite, or a complex possessing the property of base replacement, occurs in a soil it is a relatively easy matter to treat the soil with a solution that carries another replaceable base, and to measure the degree of base replacement by an analysis of the percolate. However, there are few soils that can be thus treated. Calcium carbonate occurs in nearly all irrigated soils, and this salt, while only slightly soluble in pure water, is much more soluble in the aqueous solutions of all salts that are suitable for base replacement. The presence of calcium carbonate is responsible for many of the errors that come into base replacement determinations. Many of our western soils contain magnesium carbonate also, and the presence of this salt adds; to the difficulties of base replacement determinations. The presence of relatively large amounts of magnesium zeolite in many soils has been a source of surprise to some investigators. The presence of either calcium or magnesium zeolite in the soil is of great importance. These two zeolites do not hydrolyze readily as do the zeolites of sodium and potassium. They tend to keep the soil flocculated and permeable to water, while sodium and potassium zeolites hydrolyze readily and deflocculate soils. The retention of potassium and other basic plant foods in the soil is often, indirectly, a function of the quantity of the zeolites of calcium and magnesium (1). This bulletin deals with the relation of calcium and magnesium to each other, and with their relation to other replaceable bases in the soil, and it is hoped that these data will add some- thing to our knowledge of the reactions that take place in the soil, and that it will stimulate the development of practical methods of handling certain soil problems.

SOLUBILITIES OF MAGNESIUM AND CALCIUM CARBONATES The source of calcium carbonate, and the fact of its presence in such large amounts in arid soils, has been the subject of much investigation. In the irrigated sections of Arizona it usually runs from a fraction of 1 percent to 6 percent or more, upon MAGNESIUM AND CALCIUM IN ZEOLITIC SOILS 39 the basis of dry soil. Synthetic calcium carbonate is soluble to the extent of only about 10 parts per million in pure water. While requiring a longer time to come to equilibrium, the crystalline calcium carbonate in the soil has about the same solubility as the precipitated salt. In the presence of carbon dioxide, calcium carbonate is converted into the bicarbonate, and is soluble to the extent of 1,000, or more, parts per million of that salt. The presence of white alkali, sodium chloride or sodium sulphate, increases the solubility of calcium carbonate in the soil. Many salts of magnesium, and also other salts of calcium, exert a solvent action upon calcium carbonate. Most calcium salts are less soluble than the corresponding salt of magnesium, the hydroxides and crystalline carbonates, however, are exceptions. Calcium hydroxide is soluble about 1,600 parts per million, while magnesium hydroxide is soluble only about 8 or 10 parts per million. Magnesium carbonate in arid soils is derived chiefly from the minerals magnesite and dolomite. In either of these crystalline forms it is very insoluble. Like calcium carbonate, its solubility ia| increased greatly by the presence of carbon dioxide in solution. It is also soluble in many other salts of magnesium, and in salts of calcium.* Under certain conditions, that are not well understood, calcium and magneisum carbonates combine and crystallize out as a double salt, known as dolomite. This salt is very difficult to prepare synthetically and, like magnesite, it is very insoluble. With the same conditions under which it was formed the solubility of the mineral dolomite could not have exceeded the solubility of either of its components, hence it is more insoluble than either calcium or magnesium carbonate. The solubility of artificially prepared magnesium carbonate is given by some investigators as high as 225, or more, parts per million. Pure magnesium carbonate, MgCO3, is practically insoluble in water, and it is represented by the mineral magnesite, and not by the ordinary synthetic magnesium carbonate. In all probability the artificially prepared salts, upon which many solubility determinations have been made, have been mixtures of the normal carbonate and the basic or hydrated salts, which have a much higher solubility than pure magnesium carbonate. It is true that, when a solution of a magnesium salt is added to an

* For reviews of work upon the solubilities of calcium and magnesium, see Cameron and Bell, U. S. D. A. Bui. 49, 1907. Seidell, Solubilities of inorganic and organic substances. Comey, Dictionary of solubilities. 40 TECHNICAL BULLETIN No. 26 alkaline carbonate solution, a precipitate is obtained that has a relatively high solubility. However, it has been exceedingly interesting to note repeatedly during this investigation that, when a solution of a soluble magnesium salt is passed through a calcareous soil, a precipitation of magnesium carbonate takes place in the soil. This precipitate is insoluble, and apparently has the properties of magnesite or dolomite, and not those of ordinary artificially prepared magnesium carbonate. This fact has an important bearing upon zeolitic reactions. Under such conditions the precipitation of magnesium carbonate takes place at near neutrality, usually at a pH between 7 and 8, and there is not the hydroxyl ion concentration present, as when a precipitation of magnesium carbonate is made from an alkaline carbonate solution. As the solubilities of magnesium carbonate and magnesium hydroxide are so nearly equal, a precipitation in the presence of a high hydroxyl ion concentration brings down both magnesium carbonate and magnesium hydroxide.

BASE REPLACEMENT It is well known that, when salt solutions containing replaceable bases are added to, or percolated through, a zeolitic soil, an exchange of bases takes place between the zeolite and the percolating solution. The extent of this reaction is largely a function of the energy of replacement of the bases employed, the concentration and pH of the percolating solution, and the concentration of common ions in solution. Nearly all of the methods for measuring the base replacement power of soils consist in bringing a solution that contains a replaceable base in contact with the soil and determining, by chemical means, the amount of zeolitic base that this solution replaces. In all of the experiments to be reported, the percolation method was used. An arrangement of apparatus as shown in figure 1 was employed. One-hundred-gram samples of dry, pulverized soil were placed in glass tubes, and 250 cc. of the replacing solution was added to the soil columns, by inverting graduate flasks into the mouths of the tubes. The percolates were caught in the bottles below. With the soil used, it was found that 250 cc. of solution would yield a percolate of, about 210 cc, while about 40 cc. was held by the soil. Two hundred-ten cc. was, therefore, adopted as a convenient volume, and this amount of solution was percolated through the soil in nearly all of the experiments. If, when percolating a soil with barium chloride, for example, 210 MAGNESIUM AND CALCIUM IN ZEOLITIC SOILS 41 cc. of percolate was found, upon analysis, to contain 60 milli- grams of calcium, it was assumed that this represented the amount of replaceable calcium in 100 grams of soil. In some cases the percolation was not continued until complete equilibrium was established. This would have required a much longer time, and was not considered necessary in obtaining the data desired.

Fig. 1.—Apparatus used in percolating soils.

PREPARATION OF SOIL SAMPLES The soil from the old farm of the University of Arizona is a typical black-alkali soil, and is representative of large areas in the State of Arizona. It contains about 0.27 percent of replaceable sodium, about 0.04 percent of replaceable calcium, together with a little soluble sodium salts, and about 5 percent of calcium carbonate. It is highly dispersed, and does not take water readily. If this soil is leached with pure water, there is a gradual replacement of the sodium in the zeolite by the calcium that occurs as calcium carbonate, until at a point where perme- ability practically ceases, and, where percolation cannot be carried further, the zeolites exist largely as calcium zeolite with but a trace of sodium zeolite. This condition is represented in most of our impermeable soils, in many "slick spots," and in many soils where reclamation by leaching has been attempted (2). Large stock samples of soils that contained as nearly as possible one zeolite only, were prepared by percolating Univer- sity Farm soil with 5-percent solutions of barium, calcium, 42 TECHNICAL BULLETIN No. 26 magnesium, potassium, and sodium chlorides, respectively, and washing the* soils free from excess salt. The soils thus treated will be referred to as barium-zeolite, calcium-zeolite, magnesium- zeolite, potassium-zeolite, and sodium-zeolite soils. An equal mixture of two of these soils, calcium-zeolite and magnesium- zeolite for example, will be referred to as a calcium-magnesium- zeolite soil. With such bases as calcium, it is not difficult to prepare a soil with practically no other zeolite, as calcium zeolite does not hydrolyze readily, and the treated soil may be washed easily. But with sodium and potassium in calcareous soils, it is difficult to produce only one zeolite, as these salts hydrolyze readily upon washing, some sodium or potassium is removed, and calcium zeolite is formed from the calcium carbonate. The preparation of magnesium zeolite soil is also attended with difficulties, as will be explained later. It must be remembered that black-alkali soils thus treated, often contain small amounts of sodium zeolite. This often causes a small error in replacement work. There is also a certain amount of "break-down" or "build-up" in many replacement processes, and this must be borne in mind when balancing the amount of base, that disappears from solution, with the base that it is supposed to liberate from this zeolitic complex (3). The standard strength of replacing solutions used in this work was based upon the concentration of calcium in a saturated solution of gypsum. Gypsum is the only mineral remedial agent that can be applied economically to black-alkali soils, so its saturation concentration was adopted as a standard. A saturated solution of gypsum at ordinary temperatures, contains 0.618 grams of calcium per liter. Two hundred-ten cc. of such a solution would contain .1298 gram of calcium, or 6.49 milli-equivalents. For convenience 6.50 milli-equivalents were used. With the exception of the first two tables, all results are expressed in termsi of milli-equivalents.

EFFECT OF COMMON IONS IN SOLUTION It has been shown in a former bulletin, (2) that the presence of a common ion in solution will retard the ionization of any zeolite, and thus interfere with base replacement. This phenomenon was made use of in many of the experiments that follow. In Tables I and II are shown the effects of the ions Na and K upon the replacement of sodium and potassium by calcium. One-hundred-gram samples of untreated, calcareous, Uni- versity Farm soil, a soil that contained sodium zeolite with a MAGNESIUM AND CALCIUM IN ZEOLITIC SOILS 43 very small amount of calcium zeolite, were percolated with standard solution of calcium chloride corresponding to a saturated solution of gypsum, to which was added increasing amounts of sodium chloride. The percolations were continued until the percolates amounted to 210 cc. each. Calcium was then determined in the percolates.

TABLE I.—BASE REPLACEMENT IN A SODIUM-ZEOLITE SOIL AS AFFECTED BY A COMMON ION IN SOLUTION.

Percent of Na present Grams calcium No. as NaCl in percolat- Added to 210 cc. Found in 210 cc. ing solution percolating solution percolate 1 0.0 0.1298 0.0040 2 0.4 0.1298 0.0395 3 0.8 0.1298 0.0630 4 1.2 0.1298 0.0802 5 1.6 0.1298 0.0940 6 2.4 0.12,98 0.1102 7 3.2 0.1298 0.1228 8 4.0 0.1298 0.1260

It is apparent that, as the common ion, Na, was added to the percolating solution, the ionization of the sodium zeolite was gradually forced back until, at a concentration of about 4 percent of sodium as sodium chloride, base replacement practically ceased. This phenomenon is shown also in the case of a calcareous, potassium-zeolite soil, Table II.

TABLE II.—BASE REPLACEMENT IN A POTASSIUM-ZEOLITE SOIL AS AFFECTED BY A COMMON ION IN SOLUTION.

Ppyppn-f- of TC Drpspnt Grams calcium No. as KC1 in percolating Added to 210 cc. Found in 210 cc. solution percolating solution percolate 1 0.00 0.1298 0.0028 2 0.13 0.1298 0.0525 3 0.26 0.1298 0.0789 4 0.52 0.1298 0.0976 5 1.04 0.1298 0.1165 6 1.56 0.1298 0.1228 7 2.08 0.1298 0.1298 8 3.12 0.1298 0.1298 9 4.16 0.1298 0.1298 10 5.20 0.1298 0.1298

It will be seen that, at a concentration of 2 percent of potassium ion in solution, the ionization of the potassium zeolite 44 TECHNICAL BULLETIN No. 26 in the soil was forced back so that no base replacement was possible. Apparently 2 percent of potassium accomplished results similar to 4 percent of sodium. Ionization is necessary in all base replacement, and the fact that a common ion in solution will prevent replacement, fur- nishes a suggestion of a method for determining the energy of replacement of the different bases. This work is being developed further in this laboratory. Under the above described conditions, it required over 4 percent, or about 174 milli-equivalents of sodium in each 100 cc. of percolating solution, to prevent hydrolysis of the sodium zeolite, while it required only 2 percent, or about 51 milli-equivalents of potassium, to prevent the hydrolysis of potassium zeolite. However, this proportion is not a direct index to the energy of replacement of these two ions. Sodium and potassium zeolites have widely different hydrolysis constants, they differ in other respects also, and, therefore, require different amounts of common ions to stop reactions.

TABLE III.—BASE REPLACEMENT IN A MIXTURE OF SODIUM- ZEOLITE AND CALCIUM-ZEOLITE SOILS AS AFFECTED BY COMMON IONS IN SOLUTION.

Na as NaCl Ca as CaCl2 Ca found in Na Ca No. added to added to 210 cc. replaced replaced 210 cc. 210 cc. percolate M.E. M.E. M.E. M.E. M.E. Control 0.00 0.00 0.25 0.00 0.00 1 6.50 0.00 1.22 0.00 1.22 2 6.50 1.62 1.34 0.28 0.00 3 6.50 3.25- 1.45 1.80 0.00 4 6.50 6.50 3.46 3.04 0.00 5 6.50 13.00 9.29 3.71 0.00 6 6.50 26.00 21.16 4.84 0.00

7 0.00 6.50 2.52 3.98 0.00 8 1.62 6.50 2.84 3.66 0.00 9 3.25 6.50 3.06 3.44 0.00 10 6.50 6.50 3.47 3.03 0.00 11 13.00 6.50 4.09 2.41 0.00 12 26.00 6.50 4.96 1.54 0.00 13 39.00 6.50 5.67 0.83 0.00

In Table III is shown the effect of a common ion upon replacement, under a different set of conditions. Calcium-zeolite and sodium-zeolite soils were mixed in equal amounts, and percolated with solutions of calcium chloride and sodium chloride that were mixed in the proportions shown in the table. The table MAGNESIUM AND CALCIUM IN ZEOLITIC SOILS 45 is divided into two sections, in numbers 1 to 6 the concentration of sodium was kept constant, while the concentration of calcium was increased. In numbers 7 to 13, the concentration of calcium was kept constant, while the concentration of sodium was increased. A control was percolated with distilled water. Both calcium and sodium zeolite were present in each sample of soil, and either calcium or sodium could be replaced by the other unless other factors intervened. These results are expressed in milli-equivalents. Under the conditions of this experiment, 6.50 M. E. of sodium in No. 1 replaced some calcium, but all replacement of calcium by sodium ceased as soon as 1.62 M. E. of calcium was introduced into the percolating solution No. 2. As more and more calcium was introduced into the standard sodium solution, more and more sodium was replaced by the calcium. It required over 39 milli-equivalents of sodium to stop the replacement of sodium by calcium in this soil mixture. This experiment was repeated, using a mixture of calcium- zeolite and potassium-zeolite soils, and the results are shown in Table IV.

TABLE IV.—BASE REPLACEMENT IN A MIXTURE OF POTASSIUM- ZEOLITE AND CALCIUM-ZEOLITE SOILS AS AFFECTED BY COMMON IONS IN SOLUTION.

K as KC1 Ca as CaCl2 Ca found in K Ca No. added to added to 210 cc. 210 cc. 210 cc. percolate replaced replaced M.E. M.E. M.E. M.E. M.E. Control 0.00 0.00' 0.29 0.00 0.00 1 6.50 0.00! 1.58 0.00 1.58 2 6.50 1.62 2.21 0.00 0.59 3 6.50 3.25 3.39 0.00 0.14 4 6.50 6.50 5.91 0.59 0.00 5 6.50 13.00 10.50 2.50 0.00 6 0.00 6.50 2.29 4.21 0.00 7 1.62 6.50 3.47 3.03 0.00 8 3.25 6.50 4.64 1.86 0.00 9 6.50 6.50 5.91 0.59 10 13.00 6.50 7.24 0.00 o.oo 0.74 Upon the addition of calcium to the standard solution of potassium, all replacement of calcium by potassium ceased be- forq a concentration of 6.50 M. E. of calcium was reached. As in the case of sodium, the replacement of potassium by calcium was decreased by the addition of a common potassium ion to the percolating solution, until a concentration above 6-50 M. E. 46 TECHNICAL BULLETIN No. 26 in 210 cc. was reached, No. 9. Above this concentration the potassium replaced some calcium, No. 10. In the two experiments described under Tables I and II, the common ion effect was the chief factor. The replacement reaction involved calcium chloride and sodium zeolite in Table I, and calcium chloride and potassium zeolite in Table II. In each case equilibrium was established between a single zeolite and a calcium-chloride solution. In Tables III and IV the conditions were more complex. In these tables, equilibrium was established between two zeolites and two salt solutions. In Table III, the sodium in the sodium chloride replaced some calcium in the calcium zeolite, in addition to acting as a common ion. The calcium of the calcium-chloride solution replaced some sodium in the zeolite and acted as a common ion also. Another ion, OH, appeared in some of the solutions due to hydrolysis of the zeolite. There were also smaller amounts of sodium and potassium zeolites present in Tables III and IV than in I and II, and this materially reduced the solubility of the two zeolites. These facts explain why equilibrium was different in Tables I and III, and in II and IV. In both of the last two experiments the solubility of calcium carbonate in solutions of sodium and potassium chloride, brought a small error into the results. This error will be discussed later. In the last two experiments calcium was placed in competition with the monovalent ions, Na and K, which ions have a smaller energy of base replacement than does calcium. In the next two experiments, the ion Ca was placed in competition with the ions Ba and Mg, and the effect of the common ion of these bases was noted. It has been found very difficult to add a definite amount of barium as a soluble salt to a zeolitic soil, and to re- cover the entire amount that is fixed by subsequently displacing the barium with some other base. This is due to the fact that certain amounts of barium are precipitated out in the soil as a sulphate, or as some other insoluble salt. This fact is shown in the data of Table V. It will be seen, that, as the concentration of the calcium ion was increased in the standard barium solution, less and less calcium was replaced in the soil mixture. On the other hand, as the concentration of the barium ion was increased in the standard solution of calcium, more and more calcium appeared in the percolate, and less and less barium was fixed in the soil, until at a concentration slightly less than 6.50 M. E. of barium, and MAGNESIUM AND CALCIUM IN ZEOLITIC SOILS 47 6.50 M. E. of calcium, all replacement of both barium and calcium was stopped.

TABLE V.—BASE REPLACEMENT IN A MIXTURE OF BARIUM-ZEO- LITE AND CALCIUM-ZEOLITE SOILS AS AFFECTED BY COMMON IONS IN SOLUTION.

Ba as BaCl2 Ca as CaCls Ba found in Ca found in Ba Ca No. added to added to 210 cc. 210 cc. replaced replaced 210 cc. 210 cc. percolate percolate Control M. E. M. E. M. E. M. E. M. E. M. E. 0.00 0.00 0.00 0.12 0.00 0.00 1 6.50 0.00 1.91 3.31 0.00 3.31 2 6.50 1.62 2.49 4.25 0.00 2.63 3 6.50 3.25 2.95 5.52 0.00 2.27 4 6.50 6.50 4.59 6.61 0.00 0.11 5 0.00 6.50 1.91 3.31 1.91 0.00 6 1.62 6.50 2.4*9 4.25 0.87 0.00 7 3.25 6.50 2.89 5.52 0.00 0.00 8 6.50 6.50 4.62 6.61 0.00 0.11

It is interesting to note that the concentration of the percolate with respect to both barium and calcium was practically the same in Nos. 1 and 5, in Nos. 2 and 6 and on. Evidently it makes little difference with a soil containing both barium and calcium zeolite, whether the soil is percolated with a barium- or a calcium-salt solution, provided these salts are used in equivalent amounts. It also makes very little difference in what concentration these bases occur in a mixture, provided the mixture remains a definite molecular concentration. This phenomenon is shown in an equally striking way when replacement is carried on in a mixture of magnesium-zeolite and calcium-zeolite soils. The magnesium chloride in the solution dissolved some calcium carbonate, which effect will be discussed later. There were other factors also coming into the reaction in addition to the common ion effect, but, eliminating other factors, it will be seen that, as calcium chloride was added in increasing amounts to the standard magnesium chloride solution, less and less calcium was replaced in the soil. The same was true when magnesium chloride was added to the standard solution of calcium chloride. As with barium, it will be noted that the concentration of the percolates with respect to calcium and magnesium was practically the same for solutions Nos. 1 and 5, 2 and 6, and so on. It apparently makes no difference whether such a soil column is 48 TECHNICAL BULLETIN No. 26 percolated with calcium or magnesium, provided these bases are added in equivalent amounts. Furthermore, it makes no difference in what proportion these bases are mixed, provided the mixtures are of the same molar concentration with respect to the sum of calcium and magnesium.

TABLE VL—BASE REPLACEMENT IN A MIXTURE OF MAGNESIUM- ZEOLITE AND CALCIUM-ZEOLITE SOILS, AS AFFECT- ED BY COMMON IONS IN SOLUTION.

Mg as Ca as CaCl2 Mg found in Ca found in No. MgfClo added to 210 cc. 210 cc. Mg Ca added to 210 cc. percolate percolate replaced 210 cc. replaced

M.E. M.E. M.E. M.E. M.E. M.E. Control 0.00 0.00 0.15 0.08 0.00 0.00 1 6.50 0.00 1.26 3.78 0.00 3.78 2 6.50 1.62 1.82 5.18 0.00 3.56 3 6.50 3.25 2.80 6.30 0.00 3.05 4 6.50 6.50 4.90 7.56 0.00 1.06 5 0.00 6.50 1.20 4.02 1.20 0.00 6 1.62 6.50 1.96 5.51 0.34 0.00 7 3.25 6.50 . 2.80 6.46 0.00 0.00 8 6.50 6.50 4.76 7.71 0.00 1.21

TABLE VII.—REPLACEMENT OF SODIUM BY MAGNESIUM IN A CALCAREOUS, BLACK ALKALI SOIL, IN THE PRES- ENCE OF SODIUM CHLORIDE.

Na as NaCl Mg as MgCl2 Mg found in Ca found in No. added to added to 210 cc. 210 cc. 210 cc. 210 cc. percolate percolate M.E. M.E. M.E. M.E. 1 0.0 6.50 Trace 0.60 2 36.5 6.50 1.54 0.58 3 73.0 6.50 3.05 1.26 4 109.5 6.50 3.79 1.68 5 146.0 6.50 4.16 1.93 6 219.0 6.50 4.77 2.23 7 292.0 6.50 4.97 2.35 8 365.0 6.50 5.31 2.89

EFFECT OF CALCIUM CARBONATE UPON BASE REPLACEMENT As has been stated before, most black-alkali soils are calcareous, and the University Farm soil is no exception. MAGNESIUM AND CALCIUM IN ZEOLITIC SOILS 49

Samples of untreated University Farm soil were percolated with standard solutions of magnesium chloride, to which were added increasing amounts of sodium chloride. The analyses of the percolates are shown in Table VII. It will be seen, that, as the concentration of sodium chloride was increased, there was a gradual decrease in the replacement of sodium by magnesium, that is, the concentration of magnesium increased in the percolate. At the same time the concentration of calcium in the percolate increased also, although no calcium was added to the soil column, and very little replaceable calcium existed in the soil. This increase in calcium was due to the reactions between calcium carbonate and magnesium and sodium chlorides. It will be noted also that some magnesium disappeared from solution even in the higher concentrations of sodium chloride. In these concentrations the reaction between the magnesium chloride and sodium zeolite amounted to practically nothing, because of the common ion, Na. The disappearance of magnesium from solution was due, therefore, to its reaction with calcium carbonate, with the formation of insoluble magnesium carbonate and soluble calcium chloride. This phenomenon is illustrated more fully in the next experiment in which a calcium-zeolite soil was used.

TABLE VIIL—EQUILIBRIUM BETWEEN MAGNESIUM AND CALCIUM IN A CALCAREOUS, CALCIUM-ZEOLITE SOIL.

Mg as Ca as Mg Ca Mag- No. MgCl2 CaCl2 found in found in Calcium nesium added to added to 210 cc. 210 cc. displaced fixed 210 cc. 210 cc. percolate percolate M.E. M.E. M.E. M.E. M.E. M.E. Control 0.00 0.00 0.00 0.00 0.00 0.00 1 6.50 0.00 0.20 5.04 5.04 6.30 2 6.50 1.62 0.40 6.30 4.68 6.10 3 6.50 3.25 0.83 7.48 4.23 5.67 4 6.50 6.50 0.94 9.76 3.26 5.56 5 6.50 13.00 3.07 14.07 1.07 3.43 6 0.00 6.50 0.20 5.04 0.00 0.00 7 1.62 6.50 0.25 6.46 0.00 1.37 8 3.25 6.50 0.40 7.56 1.06 2.85 9 6.50 6.50 1.30 9.45 2.95 5.20 10 13.00 6.50 3.85 14.57 8.07 9.15

In most of the percolates, the magnesium occurred in such small amounts that, in the presence of so much calcium, it could not be measured accurately. Evidently the magnesium in the 50 TECHNICAL BULLETIN No. 26

percolating solution was reacting with the calcium carbonate of the soil, as well as with the calcium zeolite, and was being precipitated both as magnesium zeolite and as magnesium carbonate. As mentioned before, there was a striking similarity in the character of the percolate with respect to the concentration of calcium and magnesium. No. 1, that was percolated with 6.50 M. E. of magnesium alone, was exactly like No. 6 that was percolated with 6.50 M. E. of calcium alone. No. 2 was like No. 7, and so on through the series. It apparently makes no difference in the character of the percolate from such a soil, whether it is percolated with a calcium or with a magnesium salt of equivalent concentration, and mixtures of these salts, in all proportions, yield the same kind of percolate, provided equivalent total salt concentrations are maintained. In No. 5, when magnesium chloride was added in relatively large amounts, and when, due to the common calcium ion, practically all replacement with the calcium zeolite was checked, nearly one^half of the magnesium disappeared from solution and remained fixed in the soil. This was probably due to its precipitation as insoluble magnesium carbonate.

EXPERIMENT WITH CALCAREOUS, MAGNESIUM- ZEOLITE SOIL A sample of calcareous, black-alkali soil was percolated with a strong solution of magnesium chloride until base replacement of the zeolite was assumed to be over. Iti was then washed free from chlorides. The soil after treatment still contained over 4 percent of calcium carbonate. Samples of the soil were then percolated with the solutions described in Table IX. In this, as in the two other experiments just described, the concentration of the percolates with respect to calcium and magnesium correlated with each other, No. 1 was like No. 6, No. 2 like No. 7, and so on. Although this soil had been thoroughly leached with a large volume of a solution of magnesium chloride of a concentration of about 5 percent, or probably 50 times as much as that required to replace all zeolitic bases, when treated with 6.50 M. E. of magnesium, as in No. 1, the percolate showed a relatively large amount of calcium. Evidently a part of the magnesium was reacting with the calcium carbonate, precipitating out as magnesium carbonate and liberating an equivalent amount of calcium, which appeared in the percolate as calcium chloride. MAGNESIUM AND CALCIUM IN ZEOLITIC SOILS 51

TABLE IX.—EQUILIBRIUM BETWEEN MAGNESIUM AND CALCIUM IN A CALCAREOUS, MAGNESIUM-ZEOLITE SOIL.

Mg as Ca as Mg Ca Mag- No. Mg:Cl2 CaCl2 found in found in nesium Calcium added to added to 210 cc. 210 cc. displaced displaced 210 cc. 210 cc. percolate percolate M.E. M. E. M.E. M.E. M.E. M.E. Control 0.00 0.00 0.08 0.21 0.08 0.21 1 6.50 0.00 2.87 2.52 0.00 2.52 2 6.50 1.62 4.00 3.27 0.00 1.65 3 6.50 3.25 4.60 4.16 0.00 0.91 4 6.50 6.50 6.05 6.04 0.00 0.00 5 6.50 13.00 7.40 11.17 0.90 0.00 6 0.00 6.50 2.73 2.68 2.73 0.00 7 1.62 6.50 3.67 3.65 2.05 0.00 8 3.25 6.50 4.38 4.41 1.13 0.00 9 6.50 6.50 6.44 5.97 0.00 0.00 10 13.00 6.50 9.59 9.03 0.00 2.53

These results indicate that when calcium carbonate occurs in a zeolitic soil, although only slightly soluble, the calcium will react with a soluble magnesium salt and enter into base replacement reactions.

EQUILIBRIUM BETWEEN MAGNESIUM AND CALCIUM" IN A NON-CALCAREOUS SOIL The calcareous black-alkali soil from the University Farm that was used in the preceding experiment, was percolated with dilute hydrochloric acid until all the calcium carbonate was dissolved. It was washed free from acid and soluble salts, and then percolated with a tenth-normal solution of neutral calcium chloride until as much as possible of the hydrogen zeolite had been converted into calcium zeolite, and the "build-up" of zeolites had reached its maximum under these conditions. It was then washed free from calcium chloride. This sample constituted a* calcium-zeolite soil that was free from calcium carbonate. One-hundred-gram samples each were placed in tubes, and these percolated with the same solutions that were used in the previous experiment. When solutions of any concentration are passed through a large column of soil, there is a certain error brought into the results, due to occlusion and to other factors, so that it is practically impossible to balance exactly the base that is displaced with the base that is fixed by the soil. This error is evident in 52 TECHNICAL BULLETIN No. 26

TABLE X.—EQUILIBRIUM BETWEEN MAGNESIUM AND CALCIUM IN A NON-CALCAREOUS CALCIUM-ZEOLITE SOIL,

Mg as Ca as Mg Ca Mag- No. MgCl2 CaCl2 found in found in nesium Calcium added to added to 210 cc. 210 cc. fixed displaced 210 cc. 210 cc. percolate percolate

M.E. M.E. M.E. M.E. M.E. M.E. Control 0.00 0.00 0.00 0.12 0.00 0.12 1 6.50 0.00 1.10 5.04 5.40 5.04 2 6.50 1.62 2.02 5.71 4.48 4.09 3 6.50 3.25 2.82 6.59 3.68 3.34 4 6.50 6.50 3.79 9.03 2.71 2.53 5 6.50 13.00 4.82 14.15 1.68 1.15 6 0.00 6.50 0.00 6.30 0.00 0.00 7 1.62 6.50 0.58 7.05 1.04 0.55 8 3.25 6.50 1.41 7.83 1.84 1.33 9 6.50 6.50 4.14 8.82 2.36 2.32 10 13.00 6.50 9.23 9.87 3.77 3.37

Table X. However, it will be seen that there is a close correlation between the calcium that was displaced and the magnesium that was fixed by the soil. It will be noted also that, as the concentration of calcium in the percolating solutions was increased, Nos. 1 to 5, the common ion, Ca, prevented base replacement in proportion to the amounts added. In the same way, when the common ion Mg was added to the standard calcium chloride solutions, Nos. 6 to 10, the mass action of this ion increased the replacement of calcium in the zeolite, in proportion to the amount added. However, in this experiment when a non- calcareous soil was used, the concentration of the percolate with respect to calcium and magnesium in the two series 1 to 5, and 6 to 10, did not correlate with each other as they did in the experiment just described, Table IX, when a calcareous soil was used. When 13 M. E. of magnesium and 6.50 M. E. of calcium, as in No. 10, Table VIII, were added to the calcareous, calcium- zeolite soil, 9.15 M. E. of magnesium were fixed by the soil. When the same amounts of magnesium and calcium were added to the non-calcareous soil that was used in Table X, only 3.77 M. E. of magnesium were fixed by the soil. These two samples possessed almost the same base replacement capacity, so the difference in the fixation of magnesium can be accounted for only by the presence of calcium carbonate in the soil. In the next experiment, magnesium-zeolite and calcium- zeolite soils, that contained no calcium carbonate, were mixed in equal proportions and percolated with the following solutions: MAGNESIUM AND CALCIUM IN ZEOLITIC SOILS 53

TABLE XI.—EQUILIBRIUM BETWEEN MAGNESIUM AND CALCIUM IN A NON-CALCAREOUS, MAGNESIUM-CALCIUM-ZEO- LITE SOIL.

Mg as Ca as Mg Ca No. MgCl2 CaCl2 found in found in Mg Ca added to added to 210 cc. 210 cc. displaced displaced 210 cc. 210 cc. percolate percolate M. E. M. E. M.E. M.E. M.E. M.E. Control 0.00 0.00 Trace Trace 0.00 0.00 1 6.50 0.00 3.71 2.81 0.00 2.81 2 6.50 f.62 4.73 3.53 0.00 1.91 3 6.50 3.25 5.53 4.20 0.00 0.95 4 6.50 6.50 6.36 6.55 0.00 0.05 5 0.00 6.50 2,87 3.94 2.87 0.00 6 1.62 6.50 3.51 5.25 1.89 0.00 7 3.25 6.50 4.38 5.67 1.03 0.00 8 6.50 6.50 6.61 6.50 0.11 0.00

These results with the magnesium-calcium-zeolite soil, were very similar to those with calcium-zeolite soil alone. Attention is called to the ratio of the concentration of magnesium to calcium in percolates Nos. 1 and 5, 2 and 6, etc. In No. 1 when standard magnesium chloride was used as the percolating solution, 3.71 M. E. of magnesium were balanced against 2.81 of calcium. These figures were almost exactly re- versed in percolate 5, when calcium chloride was used as a percolating solution. It is interesting to note that in both percolates 4 and 8, when calcium and magnesium chlorides were added in equivalent amounts to a soil containing both magnesium and calcium-zeolites, no base replacement took place. The tendency of the calcium to replace the magnesium was counterbalanced by the tendency of the magnesium to replace the calcium. This and other phenomena are strongly indicative that the energy of replacement of magnesium is the same as that of calcium, provided all other factors such as common ions, etc., are eliminated.

EFFECT OF BASIC MAGNESIUM CARBONATE UPON BASE REPLACEMENT As many soils of the West contain both calcium and magnesium carbonates, the question arose whether or not magnesium carbonate would enter base replacement reactions, as does calcium carbonate. As mentioned before, the magnesium carbonate that is ordinarily used in the laboratory, is not a true carbonate 54 TECHNICAL BULLETIN No. 26 but a mixture of the carbonate and the hydrated oxide. The solubilities of such mixtures may vary widely. The sample that was used in these experiments, on being shaken with ordinary distilled water and allowed to stand at room temperature for 50 days, had a solubility of 40 parts per million of magnesium. The solubility was reduced to 30 parts per million, when boiled distilled water was used. A sample of calcareous, calcium-zeolite soil was mixed with the basic magnesium carbonate just described, so that the soil contained 2 percent magnesium carbonate in addition to 5 percent of calcium carbonate that it contained originally. One- hundred-gram samples of this mixture were then treated as shown in Table XII.

TABLE XII.—EFFECT OF BASIC MAGNESIUM CARBONATE ON BASE REPLACEMENT REACTIONS IN A CALCAREOUS, CAL- CIUM-ZEOLITE SOIL.

Mg as MgCl2 Ca as CaCl2 Mg found in Ca found in No. added to added to 210 cc. 210 cc. 210 cc. 210 cc. percolate percolate

M. E. M.E. M.E. M.E. Control 0.00 0.00 1.60 0.12 1 6.50 0.00 7.49 0.84 2 6.50 1.62 9.03 1.43 3 6.50 3.25 9.57 2.05 4 6.50 6.50 12.04 2.27 5 6.50 13.00 16.03 4.41 6 0.00 6.50 7.53 1.26 7 1.62 6.50 8.87 1.63 8 3.25 6.50 9.97 2.10 9 6.50 6.50 12.05 2.85 10 13.00 6.50 15.64 4.86

It was evident that the basic magnesium carbonate was entering into the reaction. In No. 5, only 6.50 M. E. of magnesium were added to the soil in the percolating solution, yet 16.03 appeared in the percolate as magnesium chloride. In No. 6, where 6.50 M. E. of calcium chloride were added, while no replaceable magnesium, as a zeolite, existed in the soil, 7.53 M. E. of magnesium appeared in the percolate. At the same time 5.24 M. E. of calcium were fixed by the soil. This was caused by the reaction between the calcium chloride in the percolating solution, and the basic magnesium carbonate in the soil. Under such conditions, magnesium carbonate, which has a solubility of only 40 MAGNESIUM AND CALCIUM IN ZEOLITIC SOILS 55 parts per million of Mg in the distilled water that was used, may, in a soil, act like a strong solution of a soluble magnesium salt. This same phenomenon is shown with a calcareous magnesium-zeolite soil to which 2 percent of basic magnesium carbonate had been added.

TABLE XIII.—EFFECT OF BASIC MAGNESIUM CARBONATE IN BASE REPLACEMENT REACTIONS IN A CALCARE- OUS, MAGNESIUM-ZEOLITE SOIL.

Mg as MgCl2 Ca as CaCl2 Mg found in Ca found in No. added to added to 210 cc. 210 cc. 210 cc. 210 cc. percolate percolate

M. E. M.E. M.E. M.E. Control 0.00 0.00 1.58 0.02 1 6.50 0.00 6.71 0.63 2 6.50 1.62 8.42 0.71 3 6.50 3.25 9.55 0.88 4 6.50 6.50 12.04 1.09 5 6.50 13.00 18.93 1.47 6 0.00 6.50 6.71 0.63 7 1.62 6.50 8.52 0.59 8 3.25 6.50 9.51 0.92 9 6.50 6.50 11.92 1.22 10 13.00 6.50 18.92 1.47

The reactions noted in the calcium-zeolite soil, Table XII, were very similar to those of the magnesium-zeolite soil, Table XIII. The characters of the two sets of percolates were similar also. In both experiments the basic magnesium carbonate en- tered into the reaction.

MAGNESIUM CARBONATE IN BLACK-ALKALI SOILS Black-alkali soils usually contain some soluble salts of sodium in addition to the sodium zeolite. A black-alkali soil that contained 4.7 percent of calcium carbonate, was mixed with 2 percent of basic magnesium carbonate, and 100-gram samples of the mixture were percolated with the solutions of magnesium and calcium chlorides as shown in Table XIV. Sodium chloride in increasing amounts was added also to the percolating solutions. The most striking feature of this table is that, in the second series, Nos. 7 to 12, when no soluble salt of magnesium was added in the percolating solutions, and only the slightly soluble basic 56 TECHNICAL BULLETIN No. 26 magnesium carbonate existed in the soil, the concentration of magnesium in the percolates was almost exactly like those in the first series, 1 to 6, when a fairly strong solution of magnesium chloride was used. That is, the concentration of magnesium in percolate 1 was almost exactly like that of percolate 7, No. 2 was like 8, and so on through the series. In other words, the basic carbonate of magnesium was behaving like a soluble salt.

TABLE XIV.—EFFECT OF BASIC MAGNESIUM CARBONATE UPON BASE REPLACEMENT IN A SODIUM-ZEOLITE SOIL IN THE PRESENCE OF SODIUM CHLORIDE.

Na as NaCl Mg as MgCl2 Mg found in Ca found in No. added to added to 210 cc. 210 cc. 210 cc. 210 cc. percolate percolate M.E. M.E. M.E. M.E. Control 0.00 0.00 1.17 0.09 1 0.00 6.50 3.30 0.18 2 1.62 6.50 3.58 0.18 3 3.25 6.50 3.58 0.18 4 6.50 6.50 3.90 0.21 5 13.00 6.50 4.30 0.29 6 19.50 6.50 4.95 0.33 Ca as CaCl2 added to 210 cc. 7 0.00 6.50 3.27 0.09 8 1.62 6.50 3.42 0.09 9 3.25 6.50 3-62 0.09 10 6.50 6.50 3.94 0.17 11 13.00 6.50 4.39 0.21 12 19.50 6.50 4.89 0.42

In the first series, the basic carbonate probably remained inert, and did not enter the reactions on account of the soluble magnesium in solution, but in the second series, when no magnesium ions were added in the percolating solution, the magnesium carbonate brought about the same equilibrium. In the first series, some magnesium was removed from each solution by the soil. This magnesium went either to replace the sodium in the zeolite, or to react with calcium carbonate and precipitate out as magnesium carbonate. In the second series, the basic magnesium carbonate went into solution, replaced part of the sodium in the zeolite, and reacted with the calcium choride in the percolating solution and precipitated the calcium as calcium carbonate. MAGNESIUM AND CALCIUM IN ZEOLITIC SOILS 57 This phenomenon was shown to be true also with a potassium-zeolite soil that was treated with the same concentration of magnesium and calcium chloride, with increasing amounts of potassium chloride added. Under the conditions of these experiments, very little carbon dioxide was present in the soil during the reactions. In some experiments that follow, the role of carbon dioxide in such reactions will be discussed.

EFFECT OF MAGNESITE UPON BASE REPLACEMENT, WHEN NO CARBON DIOXIDE IS PRESENT Experiments similar to those just described, were carried on with pulverized crystalline magnesite. This mineral was ground in a ball mill into a state of extreme subdivision. After being shaken and allowed to stand in contact with carbon-dioxide- free water, it was found to be soluble to the extent of only 2 or 3 parts per million of magnesium. A calcium-zeolite soil, that contained about 5 percent of calcium carbonate, was mixed with 2 percent of magnesite. This soil was then percolated with the following solutions:

TABLE XV.—EFFECT OF MAGNESITE UPON BASE REPLACEMENT REACTIONS IN A CALCAREOUS, CALCIUM-ZEOLITE SOIL.

Mg as MgCl2 Ca as CaCl2 Mg found in Ca found in No. added to added to 210 cc. 210 cc. 210 cc. 210 cc. percolate percolate

M. E. M. E. M. E. M. E. Control 0.00 0.00 Trace 0.31 1 6.50 0.00 Trace 5.46 2 6.50 1.62 0.20 6.38 3 6.50 3.25 0.91 7.45 4 6.50 6.50 2.28 9.24 5 0.00 6.50 0.21 5.75 6 1.62 6.50 0.62 6.38 7 3.25 6.50 1.09 7.40 8 6.50 6.50 2.39 9.45

In all probability, when magnesium chloride appeared in the percolating solutions, in concentration far above the solubility of magnesite, the magnesite did not enter into the reaction. When no magnesium chloride was added, as in No. 5, there was a limited reaction between the magnesite and the calcium zeolite. 58 TECHNICAL BULLETIN No. 26

EFFECT OF DOLOMITE UPON BASE REPLACEMENT WHEN NO CARBON DIOXIDE IS PRESENT An experiment similar to that described under Table XV, was carried on, using pulverized crystalline dolomite instead of magnesite, and results almost identical with those when magnesite was used, were obtained. Eliminating the phenomenon of "build-up," and of the reactions that take place in such systems when hydrogen zeolite is present, it appears probable that neither magnesite nor dolomite, when crystalline and unweath- ered, enters readily into base replacement reactions in a calcareous soil, when no carbon dioxide or other solvent is present. This is due, probably, to the low solubility of these minerals.

EFFECT OF CARBON DIOXIDE UPON BASE REPLACEMENT Under the slow process of rock weathering, and in the presence of carbon dioxide or other solvents, a different equilibrium from that just described is produced in a soil, and both magnesite and dolomite weather into amorphous compounds of magnesium and calcium, and enter readily into base replacement reactions. Assuming that a certain soil contains both calcium zeolite and magnesite, if, under any system of weathering, or in the presence of any solvent, the solubility of the magnesium in the weathered product of magnesite should equal or exceed the solubility of magnesium in magnesium zeolite, the magnesium will partially replace some of the calcium of thei zeolite. The same may be said of dolomite. Calcium carbonate in the dolomite also will enter into the reactions, and the presence of so many factors makes a determination of the final equilibrium almost impossible, even under a definite set of conditions. As pointed out in a former bulletin, (2), carbon dioxide cannot exist, in appreciable amounts, in a moist, black-alkali soil. The solutions of such soils contain hydroxyl ions, and it is obviously impossible for carbon dioxide and water, or carbonic acid, and the hydroxyl ion to exist in the same solution. In soils where carbon dioxide does occur, however, its effect upon base replacement is very pronounced. Both magnesium and calcium carbonates are soluble to a limited extent, in solutions of carbon dioxide. At zero partial pressure of carbon dioxide, and at 15 degrees, one liter of a saturated solution will contain 0.641 gram of magnesium carbonate, plus 1.954 grams of magnesium bicarbonate. Under the same conditions, one liter of a saturated solution should contain 0.385 gram of calcium bicarbonate. With MAGNESIUM AND CALCIUM IN ZEOLITIC SOILS 59 such concentrations, base replacement reactions involving magnesium and calcium carbonate, may be rapid and practically complete. A hydrogen-zeolite soil, that contained no calcium carbonate, was percolated with a tenth-normal solution of magnesium chloride, and washed free from soluble magnesium and chlorine. The soil now contained magnesium zeolite as well' as some hydrogen zeolite. The soil was dried, and three 100-gram samples weighed out. One of these samples was percolated with distilled water that was saturated with carbon dioxide, one was percolated with a saturated solution of calcium bicarbonate, and the third was first mixed with 5 percent of calcium carbonate, and then percolated with distilled water that was saturated with carbon dioxide. The results are given in Table XVI.

TABLE XVI.—REACTION BETWEEN MAGNESIUM ZEOLITE AND CALCIUM BICARBONATE. Magnesium found Magnesium-Zeolite Soil in 400 cc. percolate M.E. 1. Percolated with 400 cc. water saturated with carbon dioxide 0.16 2. Percolated with 400 cc. saturated solution

of Ca(HCO3J72 2.75

3. 5% CaCo3 added, percolated with 400 cc. water saturated with carbon dioxide 3.27

The percolating solutions were cold at the beginning of the percolations, and the saturated solution of calcium bicarbonate, drawn from a bottle, beneath an atmosphere of carbon dioxide, contained 0.330 Ca per liter. The percolations were carried on at room temperatures, and there was a loss of carbon dioxide from the solutions, which lowered the concentration of calcium. Some difficulty was experienced also in getting the carbon dioxide in columns 1 and 3 into the soil. These data are, therefore, not presented as a quantitative method for base replacement determinations, but merely as a qualitative example, to illustrate how carbon dioxide may affect the equilibrium between magnesium and calcium, in a soil that contains both carbonates and zeolites. While there was a low solubility of magnesium zeolite in the control solution of carbon dioxide, No. 1, the evidence is con- vincing that the calcium in calcium carbonate, when dissolved in water that contains carbon dioxide, will replace magnesium from zeolitic combination. 60 TECHNICAL BULLETIN No. 26

The same phenomenon is true, when calcium zeolite, in a non-calcareous soil, is treated with a solution of magnesium bicarbonate, as shown in Table XVII. TABLE XVII.—REACTION BETWEEN CALCIUM ZEOLITE AND MAG- NESIUM BICARBONATE IN A NON-CALCAREOUS SOIL. Calcium found Calcium-Zeolite Soil in 400 cc. percolate M.E. 1. Percolated with 400 cc. water saturated with carbon dioxide 0.27 2. Percolated with 400 cc. saturated solution

of Mg(HCO3'J2 4.84 3. 5% of magnesium carbonate added and percolated with 400 cc. water saturated with carbon dioxide ... 1.77

The magnesium bicarbonate solution used in No. 2, contained 0.800 gram per liter of magnesium, and the base replacement with this solution was very pronounced. The same difficulties presented themselves in this experiment, as they did in the case of calcium bicarbonate, and therefore, magnesium bicarbonate is not advocated as a suitable solution for base exchange work. However, it is evident that magnesium carbonate, when dissolved in water that contains carbon dioxide, has the property of exchanging its base with that of any zeolite in the soil. BASE REPLACEMENT PROPERTY OF DOLOMITE IN A SATURATED SOLUTION OF CARBON DIOXIDE A finely ground sample of dolomite was placed in a large bottle of cold, distilled water, and carbon dioxide passed through the mixture. One liter of this solution when filtered contained 4.9 M. E. of magnesium and 4.1 M. E. of calcium. A non-calcareous, barium-zeolite soil was percolated with this solution, and the amount of replaced barium determined. Eight hundred cubic centimeters of the dolomite solution replaced 3.81 M. E. of barium from zeolitic combination. It is evident that both magnesite and dolomite enter into base replacement reactions in the soil, provided carbon dioxide or any other solvent is present in solution. REACTION BETWEEN MAGNESIUM ZEOLITE AND CALCIUM CARBONATE It has been demonstrated repeatedly by the writer, that synthetic magnesium zeolite will react with calcium carbonate, MAGNESIUM AND CALCIUM IN ZEOLITIC SOILS 61 with the formation of a certain amount of calcium zeolite. The reverse of this reaction is true, the reaction between synthetic calcium zeolite and magnesium carbonate, will produce magnesium zeolite and calcium carbonate. The soils of the humid areas are usually saturated, either with hydrogen or calcium zeolites. When magnesium carbonate is added to such soils, it will disappear, wholly or in part. This disappearance of magnesium from such soils may be explained by its reaction with the soil zeolites of calcium or hydrogen. A sample of University Farm soil was percolated with dilute hydrochloric acid until all carbonates were decomposed. It was then washed free from calcium and chlorine. There was a certain amount of hydrogen zeolite formed in the soil, and some "break-down" of the zeolites. This "break-down" probably left what might be termed "zeolitic residue," consisting of alumina and silica, capable of being reformed into a zeolite when a base was introduced and a suitable pH established (3). The soil column was then percolated with tenth-normal magnesium chloride, until base replacement was over. As the percolate did not get above a pH value of 6.00, there was little "build-up" of the zeolites. The column was washed until free from magnesium and chlorine. This soil now contained 5.27 milli-equivalents of replaceable magnesium, as a zeolite, together with a small amount of hydrogen zeolite, and the zeolite residue. The soil was then dried, and 5 percent of calcium carbonate mixed with it. It was placed again in percolating tubes, and percolated with 600 cc. of distilled water. The analysis of the percolate is shown in Table XVIII.

TABLE XVIIL—EQUILIBRIUM BETWEEN MAGNESIUM ZEOLITE AND CALCIUM CARBONATE, AND "BUILD-UP" OF SOIL ZEOLITES. M.E. pH of percolate 7.45 Total Mg in 600 cc 1.86 Total Ca in 600 cc 0.40 Total HCO in 600> cc 2.37

Whatever hydrogen zeolite that remained in the soil after the treatment with magnesium chloride, was converted largely into calcium zeolite by the calcium carbonate, and this reaction produced a certain amount of carbon dioxide. There was a reaction also between the calcium carbonate and the zeolitic residue, with the formation of calcium zeolite and more carbon dioxide. The magnesium zeolite reacted with the calcium car- 62 TECHNICAL BULLETIN No. 26 bonate with the formation of some magnesium carbonate. The carbon dioxide that appeared in solution, dissolved both calcium and magnesium carbonates, which appeared in the percolate as bicarbonates. The bicarbonate ion in the percolate was, therefore, an index of the amount of replacement of hydrogen zeolite, and the "build-up" of zeolite in the soil. The bicarbonate ion reduced the pH value to 7.45, for, had not the bicarbonate appeared, the pH of the solution would have approached that of a solution of calcium carbonate, or above 8. Attention is called to the fact that the amounts of bases, magnesium, and calcium, in the percolate balanced the amount of acid, HCO3. REACTION BETWEEN BARIUM ZEOLITE AND CALCIUM CARBONATE A hydrogen-zeolite soil that contained no calcium carbonate, was percolated with a tenth-normal solution of barium chloride, and washed free from barium and chlorine. This soil now contained 4.41 milli-equivalents of barium zeolite that had been formed from the hydrogen zeolite, some hydrogen zeolite, and the zeolitic residue, or "break-down," from the hydrochloric acid treatment. The soil was dried, and mixed with 5 percent of calcium carbonate. A 100-gram sample was then percolated with 600 cc. of water, and the analysis of the percolate is shown in Table XIX.

TABLE XIX.—EQUILIBRIUM BETWEEN BARIUM ZEOLITE, CALCI- UM CARBONATE AND "BUILD-UP" OF SOIL ZEO- LITES. M.E. pH of percolate 7.50 Total Ba in 600 cc - —0.85 Total Ca in 600 cc 1.29 Total HCO in 600 cc 2.07

Either a replacement of hydrogen in the hydrogen zeolite, or a "build-up" of zeolite from the zeolitic residue took place in the experiment, as it did when a magnesium zeolite soil was used, and the bicarbonate ion in the percolate was an index to the amount of base replacement of hydrogen zeolite, and the "build- up" of zeolites in the soil. REACTION BETWEEN CALCIUM ZEOLITE AND CALCIUM CARBONATE The same phenomenon is true when a non-calcareous, calcium-zeolite soil is used intead of a magnesium-zeolite, or a MAGNESIUM AND CALCIUM IN ZEOLITIC SOILS 63 barium-zeolite soil. The percolate of 100 grams of a calcium- zeolite soil that had been mixed with 5 percent calcium carbonate and treated with 600 cc. of water, gave the following results:

TABLE XX.—EQUILIBRIUM BETWEEN CALCIUM ZEOLITE, CAL- CIUM CARBONATE AND "BUILD-UP" OF SOIL ZEO- LITES. : M.E. Total Ca in 600 cc. percolate 1.69 Total HCO8 in 600 cc. percolate 1.76

The amount of base in the percolate, balanced the amount of acid. The reactions just described took place very slowly. Several American and European investigators have ob- served the phenomenon, which is referred to in this laboratory as "build-up" (4, 5). Most of the observers attribute this reaction to "hydrolytic acidity," or to the fact that certain hydrogen ions require higher pH values for their displacement from the zeolitic molecule, than other. Prianischnikov and Golubev cite the unpublished work of Lambin, who determined the base exchange capacity of a certain soil, with barium chloride solutions of various reactions. With solutions of pH 5, he obtained a base replacement, which he called 100 percent. With pH 6, he obtained 108.1 percent, with pH 7, 112.1 percent, with pH 7.5, 116-2 percent and with pH 8.2, 126.8 percent. A maximum per- centage was usually obtained at a pH of about 11. None of these investigators appear to have considered the possibility of a "build-up" of zeolites from their ingredients, or from\ the zeolitic residue. It has been shown in a former bulletin from this Station (6), that a zeolite may be built up from finely ground orthoclase. Zeolites have been formed also from silica and alumina. There are patented processes in use for making permutite from alumina and siliciousi materials (7). There is no doubt but that certain hydrogen ions may require higher pH values for their replacement than others, but thef facts seem equally clear that in many replacement reactions, a "build-up" of zeolites actually takes place. The European investigators just quoted evidently did not give due weight to the time factor that is involved in the build- ing-up process. The replacement of basic ions usually is very rapid, but in "build-up," the reactions may be very slow. The "build-up" that is indicated in Tables XVIII, XIX and XX took place when 100 grams of soil were percolated with 600 cc. of 64 TECHNICAL BULLETIN No. 26 water, which percolation required about 48 hours. This, however, does not represent the total "build-up" that is possible at this pH. Additional percolations produced more "build-up," and this "build-up" probably will continue until all of the zeolite residue that can react at this pH will be converted into a zeolite.

SUMMARY 1. A common ion in solution retards the ionization of a zeolite, and thus interferes with base replacement. 2. It makes no difference in the final equilibrium of the percolate, whether a calcareous, calcium-zeolite soil is percolated with a magnesium or a calcium solution in equivalent concentrations. It also makes no difference in what percentages these bases occur in a percolating mixture, provided the mixture is maintained at a definite molecular concentration. 3. A calcareous, magnesium-zeolite soil acts in much the same way in reference to base replacement by magnesium and calcium salts, as does a calcareous, calcium-zeolite soil. 4. Practically the same relation exists between barium and calcium, and between barium and magnesium, as between magnesium and calcium. 5. When calcium carbonate exists in a zeolitic soil, although only slightly soluble, the calcium enters into base replacement reactions, as readily as do the more soluble calcium salts. 6. Magnesium carbonate when present as magnesite or dolomite, which have a very low solubility when no carbon dioxide or other solvent is present, apparently does not enter rapidly into base replacement reactions. 7. When carbon dioxide and water occur in a soil, both magnesite and dolomite enter readily into base replacement reactions. 8. When soils are treated with dilute hydrochloric acid, there is probably a "break-down" of zeolitic material in addition to the replacement of the zeolitic base by the hydrogen. When such soils are treated with a solution containing a replaceable base, there is apparently a "build-up" of a zeolite, the extent of the "build-up" depending largely upon the pH of the percolating solution. MAGNESIUM AND CALCIUM IN ZEOLITIC SOILS 65 BIBLIOGRAPHY 1. Magistad, 0. C. 1928. The hydrolysis of sodium and potassium zeolites, with particular reference to potassium in the soil solution. Ariz. Agr. Exp. Sta. Tech. Bui. 22. 2. Breazeale, J. F., and McGeorge, W. T., 1926. Sodium hydroxide rather than sodium carbonate, the source of alkalini- ty in black alkali soils. Ariz. Agr. Exp. Sta. Tech. Bui. 13. 3. Magistad, O- C, 1928. The action of aluminum, ferrous and ferric iron, and manganese in base replacement reactions. Ariz. Agr. Exp. Sta. Tech. Bui. 18. 4. Prianischnikov, D. N-, and Askinasi, D. L. Uber die Aziditat und Adsorptionskapazitat der Boden. Proceedings and Papers of the first Inter. Cong, of Soil Sci. Washington, D. C. June 13-22, 1927. 5. Prianischnikov, D. N., and Golubev, B. A. Die Bodena- ziditat und ihre Bedeutung bei der Kalkung und bei der Phos- phoritdungung. Proceedings and Papers First Inter. Cong, of Soil Sci. Washington, D. C. June 13-22, 1927. 6. Breazeale, J. F., and Magistad, 0. C, 1928. Base exchange in orthoclase. Ariz. Agr. Exp. Sta. Tech. Bui. 24. 7. Gans, R. Jahr. Konig. Preuss. Geol. Landeanst. Vol. 26: 179-211. 1905.