Complementary Ion Effects in Soils as Measured by Cation Exchange Between Electrodialyzed Hydrogen Clay and Soils

One of the most complex problems challenging the progress of soil fertility lies in the quantitative measurement of the suite of ions which the soil colloid presents to the plant root. Attempts to measure this suite of ions have been beset by many difficulties, the most important of which has been the development of suitable techniques for measuring the dynamic equilibria and exchange reactions between the ionic environment of the soil colloid and that of the plant root. The desirability of following the changes in this suite of ions becomes evident in view of the fact that the response of plants to fertilizer applications represents the gross effects of all ions added to the soil and that these may modify nutrient uptake and plant composition.^8,10,11,12 Jarusov⁷ and Wiklander¹³ have presented data to show the influence of one cation upon the replaceability of another and have related the differences in exchangeability of a given ion to its bonding energy for the exchange material. Jenny and his coworkers^4,5,6 as well as Marshall⁹ have shown the significance of the complementary ion principle in cation exchange reactions as they affect nutrient uptake by plants. They indicate that the percentage exchange of a particular cation will be affected least by the nature of the complementary ion for infinitely large or small percentages of exchange. Thus, it is for the percentage of exchange which lies in between these two extremes that the complementary ion will exert its greatest influence upon the suite of cations readily exchangeable from the soil colloid. Marshall⁹ has presented an excellent discussion of the relationship between the chemical environment of the plant root and the composition of the plant and indicates that the complementary ion effect may in some cases alter plant composition.

There is a need for additional techniques for measuring the complementary ion effects in soils which are not encumbered with the numerous variables associated with electrodialysis or leaching methods. Attempts to remove a definite portion of the total exchangeable cations of the soil by these methods involve many obvious variables which make quantitative and reproducible complementary ion data, difficult to obtain. An exchange of only the most active portion of the exchangeable cations of the soil is desirable for evaluating the complementary ion effects. Based upon this premise, the most logical technique for measuring the most active suite of ions of the soil lies in the use of a cation exchange material.

The purpose of this investigation was to measure quantitatively the effect of increasing levels of sodium and calcium upon the most readily exchangeable cations of the soil, by using electrodialyzed hydrogen clay as the cation exchanger. This implies similarities between the exchange properties of the plant root and those of the hydrogen clay colloid, insofar as (a) both exhibit a negatively charged surface, (b) both present a source of ionizing hydrogen ions, and (c) both possess colloidal exchange properties. The suite of ions exchanged from the soil to the hydrogen clay, therefore, becomes analogous to that which the plant root with its ionizing hydrogen ions would exchange from the clay colloid into its own environment during the initial step of nutrient uptake, i.e., the removal of the cations from the cationic environment of the soil colloid to that of the plant root.

Plan and Procedure

Complementary ion effects due to increasing levels of sodium and calcium were measured by utilizing the potential for cation exchange established between electrodialyzed hydrogen clay and soil when these two systems were separated by a collodion membrane. Twenty-five soil treatments were used, including five levels of sodium saturation, 0, 5, 10, 15, and 20% of the exchange capacity; these were repeated at five levels of calcium ranging from 65% calcium saturation with no free calcium carbonate to include the additions of 0.5, 3.0, 6.0, and 9.0% free calcium carbonate.

The following procedure was used for quantitative measurements of the suite of the readily exchangeable cations of the soil. Ten milliliters of distilled water was placed in a freshly prepared collodion tube (20 X 150 mm) and 20 grams of soil added by means of a large stemmed funnel. Sufficient water was then added from a wash bottle to just saturate the soil. The resulting soil:water ratio was approximately 1:1. The tube of soil was then immersed in the required aliquot of freshly electrodialyzed hydrogen clay suspension. The exchange of hydrogen ions of the clay for the cations of the soil was allowed to proceed. The stage of the exchange reaction was followed by measuring the pH of the hydrogen clay suspension at regular time intervals as it increased from an initial pH of 3.2 to 7.0. The soil was removed from the suspension when the pH of the suspension became constant. The cations exchanged to the hydrogen clay were removed by alternate washing and centrifuging with neutral normal ammonium acetate, and quantitative determinations for calcium, magnesium, potassium, and sodium were made by standard analytical methods. Sodium and potassium were determined by use of the Elmer Perkins flame photometer. Calcium was precipitated as calcium oxalate and subsequently titrated with potassium permanganate, and magnesium was determined by the Lundegardh flame photometer.7 Calculations from these data included (a) the fraction of each cation’s total in the soil exchanged, and (b) the suite of most readily exchangeable cations as expressed by the fractional distribution of each cation in the total cations removed from the soil.

The pH of the hydrogen clay suspension was obtained by use of a pH machine equipped with large glass electrodes which were conveniently immersed into the clay suspension without pouring it from the glass tube container. Electrodialyzed hydrogen clay was prepared from the colloidal clay fraction (<0.2 FA) of Putnam subsoil clay (beidellite). After electrodialysis, its pH value was 3.2 and its exchange capacity 68.10 M.E./100 grams of clay. It was prepared for use by adjustment to 8% concentration and the desired aliquots of clay (3.54 M.E. exchangeable hydrogen) obtained by weighing. 

Preparation of synthetic soil.–Since this investigation was designed for quantitative measurements of the effects of increasing levels of sodium and calcium upon the suite of cations exchanged from the soil, it was necessary to prepare a synthetic soil. This soil was prepared from electrodialyzed Gila clay (montmorillonite) which had an exchange capacity of 32 M.E. / 100 grams of soil.¹ Preparations of separate quantities of sodium, potassium, magnesium, and calcium soils were carried out as homoionic systems. These cations were added as the carbonates, except for magnesium which was added as the oxide, to give 100% saturation of the exchange complex with the respective cation. Each homoionic system was allowed to come to equilibrium with its respective cation and was then air dried and gently pulverized. In preparing the soil, the required portions of each of the homoionic soils were mixed together to give the following percentages of cation saturation in the final soil: calcium 65, magnesium 10, potassium 5, and sodium was varied from 0 to 20% saturation of the complex. Methylene blue saturated soil was added in quantities such that it varied from 0 to 20% reciprocal to the sodium saturation. The methylene blue saturated soil was used in order to avoid the complementary ion effect of the hydrogen ion which would occur if used reciprocally to the sodium ion. Horner3 recently employed this technique to avoid the complementary ion effect of the hydrogen ion in plant nutrition studies. 

Preparation of collodion membranes.–The membranes used in this investigation were cast on the inside of pyrex glass tubes 20 by 150 mm. A stock solution of collodion (24% alcohol) adjusted to a concentration of 5.0% with a 1:1 solution of absolute alcohol and ether was used. The tube was filled one third with collodion; it was then tilted down to a 45° angle, and the collodion allowed to drain while the tube was rotated rapidly for from 20 to 30 seconds to insure a uniform layering of collodion. The tube was then placed on a horizontal rotating axle and rotated at a speed of 24 rpm and allowed to dry for 3 minutes. The ether and alcohol vapors were drawn from the bottom of the test tube by applying a vacuum for about 15 seconds just prior to removing the tube. After drying, the tubes were placed in cooled distilled water for several hours. They were then removed from the glass tubes, washed thoroughly, replaced in the tubes, and kept in distilled water until used. The cation exchange properties of the membrane were reproducible under moderately rigid conditions of temperature and relative humidity, as evidenced by actual exchange experiments in which no significant variations in cation exchange were evident. 

Results and Discussion

Total cations exchanged.–The hydrogen clay removed sufficient cations from those soils which contained no free calcium carbonate and a low saturation by sodium to increase the pH of the clay suspension from an initial pH of 3.2 to 5.0 over a period of 98 hours. Those soils containing free calcium carbonate and a high percentage of sodium saturation exchanged sufficient cations to increase the pH of the clay suspension to a final pH of 6.8. The exchange of hydrogen ions for cations of the soil was sufficient in all cases for quantitative determinations of calcium, magnesium, potassium, and sodium. The total cations exchanged from the soil increased greatly with additional increments of calcium and sodium in the soil. The total cations exchanged increased from 1.0 milliequivalent for 0% sodium and 65% of calcium saturation to 3.8 milliequivalents for 20% sodium saturation and 9% free calcium carbonate. With increasing percentages of sodium and calcium carbonate the amount of sodium and calcium exchanged increased proportionally.

The influence of the degree of sodium saturation upon the exchange of the complementary cations of calcium, magnesium, and potassium was quite evident. Increasing the sodium saturation from 0 to 20% at the 65% calcium level repressed the exchange of calcium from 0.70 to 0.45 milliequivalents. Likewise, the exchange of magnesium from the soil was decreased from 0.26 to 0.05 milliequivalents. Sodium had only a slight effect upon potassium, increasing it from 0.07 to 0.08 milliequivalents at the highest sodium level. The repression effects of sodium were still evident for the 0.5 to 3.0% calcium carbonate additions but showed only moderate effects on calcium and no effect on potassium at the 6.0 and 9.0% calcium levels. Sodium continued to depress the exchange of magnesium at the 6.0 and 9.0% calcium carbonate levels.

The influence of calcium upon the exchange of its complementary ions was also quite significant. In general, the exchange of magnesium increased up to the 3.0% calcium carbonate addition, but was depressed by 6.0 and 9.0% of calcium carbonate. Increasing levels of calcium increased the exchange of sodium at all levels, increasing from 0.16 to 0.28 milliequivalents with 5% sodium and increasing from 0.43 to 0.76 milliequivalents with 20% sodium saturation. Potassium exchange was increased only slightly by increasing levels of calcium. The sluggish reaction of potassium to the complementary ion effects of sodium and calcium may be attributed to some potassium fixation brought about in the preparation of the soil.

The fraction of each cation exchanged.–The effect of sodium and calcium upon the fraction exchanged of each cation of its total in the soil was quite significant. Sodium decreased the fraction exchanged of the total calcium in the soil at the 65% saturation and 0.5% calcium carbonate level, but only slightly affected it in the presence of 6.0 and 9.0% calcium carbonate. With increasing percentages of calcium carbonate the fraction of calcium exchanged increased initially for the 0.5 and 3.0% calcium carbonate additions, but decreased markedly with the 3.0, 6.0, and 9.0% levels of calcium carbonate. Sodium decreased the fraction exchanged of the total magnesium in the soil at all levels of sodium saturation, the relationship being linear in the case of 65% calcium saturation and 0.5% calcium carbonate. The fraction exchanged decreased from 0.37 to 0.07 with an increase in the sodium saturation from 0 to 20% with 65% of calcium saturation. The fraction of magnesium exchanged was increased with successive additions of 13 5 calcium carbonate under all levels of sodium saturation. The increase was from 0.37 at the 65% calcium level to 0.66 at the 6.0% calcium level under 0% sodium saturation. Likewise at 20% sodium the increase in the fraction of magnesium exchanged was from 0.07 to 0.51, with the depressing effects of sodium being apparent in all cases. The fraction exchanged of the total potassium in the soil increased from 0.20 with 0% sodium and 65% of calcium saturation to 0.26 with 10% sodium saturation. Fifteen and twenty per cent sodium saturation depressed the fraction of potassium exchanged. The addition of 0.5% calcium carbonate increased the fraction of potassium exchanged from 0.20 to 0.31 with 15% sodium, but was depressed slightly at 20% sodium level. The effect of sodium was not as marked at the higher calcium levels. Increasing levels of calcium increased the fraction exchanged of the total potassium in the soil at all levels of sodium saturation, being particularly significant at the 0% sodium level where it increased from 0.20 to 0.32 as a result of the addition of 0.5% calcium carbonate. As might be expected, the fraction of the total sodium in the soil exchanged decreased with an increase in sodium saturation. Increasing levels of calcium increased significantly the fraction of sodium exchanged, increasing from 0.46 with 5% sodium saturation and 65% calcium saturation to 0.80 at the 9% calcium carbonate level.

The suite of cations exchanged.–Of greater significance than the total quantity of cations exchanged, or even the respective fractions of the total cations exchanged, is the suite of cations exchanged, i.e., the percentage distribution of each cation in the total cations exchanged from the soil. This represents a measure of the suite of cations most likely presented to the plant root by the soil. The fraction of calcium, magnesium, sodium, and potassium comprising the suite of cations exchanged is shown graphically in Fig. 1.

Calcium.–Increasing the sodium saturation decreased the fraction of calcium in the suite of cations exchanged regardless of the level of calcium carbonate (Fig. 1). The decrease was most significant in the case of 65% calcium saturation where the fraction of calcium in the exchanged suite of cations was reduced from 0.68 to 0.45 by increasing the percentage of sodium from 0 to 20%. Increasing the calcium level increased its percentage in the suite of cations exchanged; however, the effect of sodium saturation was still evident at the highest calcium levels.

Magnesium.–The complementary ion effect of sodium upon magnesium is evident in Fig. 1. Increasing the sodium at the 65% calcium level decreased the fraction of magnesium in the suite of cations from 0.25 to 0.05. The effect of sodium was most pronounced at the 65% calcium and 0.5% calcium carbonate levels, and only slightly evident at the 3.0, 6.0, and 9.0% calcium carbonate level. The complementary ion effect of calcium was quite evident in the case of 0% sodium saturation where 0.5% calcium carbonate decreased the fraction of magnesium from 0.25 to 0.17.

Potassium.–Increasing the percentage of sodium saturation at the 65% calcium level increased the fraction of potassium in the suite of exchanged cations from 0.07 to 0.08. The effect of sodium was also evident at the 0.5% calcium carbonate level where potassium was increased from 0.04 with 0% sodium to 0.5 with increases of 10, 15, and 20% sodium saturation. With 3.0, 6.0, and 9.0% calcium carbonate additions, the effect of sodium became relatively uniform, decreasing the fraction of potassium from 0.04 to 0.03. Increasing levels of calcium carbonate reduced the fraction of potassium in the suite of exchanged cations from 0.07 to 0.04 through the addition of 0.5% calcium carbonate. This effect is also evident at the 3.0, 6.0, and 9.0% calcium carbonate level. It is to be emphasized that some potassium fixation probably occurred in the preparation of this soil, and indicates that potassium can be expected to give greater responses to complementary ion effects under field conditions.

  

FIG. 1.–Complementary ion effects of sodium and calcium upon the suite of cations exchanged from Gila soil by means of electrodialyzed hydrogen clay.

 

Sodium.–Increasing the sodium saturation at the 65 % calcium level increased the fraction of sodium in the exchanged suite of cations in a linear relationship, increasing from 0.14 to 0.43. This relationship was significantly modified in the presence of 0.5% of calcium carbonate, reducing the fraction of sodium from 0.43 to 0.28 at the 20% sodium saturation. Additions of 3.0, 6.0, and 9.0% calcium carbonate gave further decreases in the fraction of sodium constituting the suite of cations exchanged.

Some emphasis should be given to the differences in the complementary ion effects of sodium and calcium as they are manifested in (a) the fraction of each cation exchanged of its total in the soil, and (b) the fraction of each cation in the suite of cations exchanged. In considering the fraction of each cation exchanged of its total in the soil, all levels of calcium increased the fraction exchanged of the total sodium, magnesium, and potassium in the soil. This effect was not the same in regard to the suite of ions exchanged. Increasing the level of calcium carbonate increased the fraction of calcium in the suite of exchanged ions, but decreased the sodium, magnesium, and potassium markedly.

The complementary ion effects of sodium upon the fraction of each cation exchanged of its total in the soil was evidenced by a decrease in the fraction exchanged of the total calcium and magnesium in the soil, and an increase in the fraction exchanged of the total potassium in the soil. However, in considering the effect of sodium upon the suite of cations exchanged, there was a marked decrease in the fraction of calcium and magnesium in the suite of exchanged cations. Increasing levels of sodium increased the fraction of sodium in the suite of ions exchanged. There is a need to shift our emphasis from the more conventional values of the per cent saturation of the colloid by a given cation and the fraction of the total cations exchanged, and to give more consideration to the suite of cations most likely exchanged to the plant root. These data indicate the magnitude of the complementary ion effects of sodium and calcium and the extent to which they may modify the suite of most readily exchangeable cations of soils. They also indicate that the use of a cation exchanger such as electrodialyzed clay may afford a useful technique for studying the suite of most readily exchangeable cations in calcareous soils as well as noncalcareous soils.

Summary

Complementary ion effects in soils were measured through the utilization of the potential for cation exchange established between electrodialyzed hydrogen clay and soil when these two systems were separated by a collodion membrane. Quantitative data were presented to show the effect of increasing levels of sodium saturation and percentage of calcium carbonate in the soil upon the suite of cations exchanged. Increasing the percentage saturation by sodium decreased the fraction of calcium and magnesium in the suite of cations exchanged under all levels of calcium carbonate. The fraction of potassium was both increased and decreased. The fraction of sodium in the suite of cations exchanged increased proportionally to the degree of sodium saturation at the 65% calcium level. The effects of sodium were greatest at the lowest levels of calcium carbonate, the effects being modified by additions of 3.0, 6.0, and 9.0% free calcium carbonate. The advantages of using a cation exchanger such as electrodialyzed hydrogen clay as a means of measuring the suite of the most readily exchangeable cations in the soil lie in (a) the use of the hydrogen ion as the principal exchange ion is significant since this ion is probably the dominant exchange ion of plant roots, (b) the exchange reaction between the hydrogen clay in the soil can be stopped and a quantitative determination of the exchanged cations be made at any desired stage, (c) the soil remains near its natural condition both physically and chemically throughout the exchange period, (d) the cations exchanged from the soil represent the most active suite of cations and can be expected to measure the influence of the complementary ion principle, and (e) a minimum disturbance occurs between the exchangeable and water soluble salts present in alkaline and calcareous soils, thus making the suite of ions exchanged in this manner more representative of that which occurs under field conditions.

 

References Cited:

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