Plants and the Exchangeable Calcium of the Soil

The facts presented herewith are the result of the stimulating acquaintance of two great botanists, one in person, the other through publications; the one, Professor C. F. Hottes of the University of Illinois, the other, Professor R. H. True of the University of Pennsylvania. The invitation to appear here was accepted in no small measure as a means of visiting the headquarters of Professor True and of expressing indebtedness for the stimulating help received from one great principle which he established, when he said, “The presence of a certain minimal quantity of calcium ions was necessary for the normal absorption of ions of other elements,” and further, “The larger the variety of ions present the greater was the absorption of all electrolytes and the less marked the importance of the proportional concentration between ions (True, 1921).”

Calcium deficiency in the soil has come to be plant nutrient problem number one in agricultural production. That recognition of this fact should have been so long delayed must unfortunately be ascribed to fallacious reasoning, It has been the common belief that liming the soil is beneficial for plants through the reduction of the hydrogen-ion concentration which this carbonate treatment affects when, in fact, the benefit comes from the introduction of the nutrient, calcium, for plant use. Increasing soil acidity is disastrous to agricultural production, not because of the advent of the hydrogen into the soil, but because of the exit therefrom of the many plant nutrient cations replaced by the hydrogen. Among these, calcium is the most prominent. Soil acidity is therefore in reality a symptom and not the malady.

An understanding of the colloidal behavior of the clay fraction of the soil, with its adsorption of ions and their chemical exchange for others, has done much to provide a clearer concept of the mechanism of those soil and plant root interactions commonly spoken of as plant nutrition. There has always been a wide gap between the behavior of soil in the test tube, by which its stocks of plant nutrients are measured, and its behavior under test against the plant root as computed in terms of crop yield and crop composition. Reasoning from the chemical behavior of the soil in the laboratory to the crop behavior in the field has corresponded to a jump across a tremendous abyss into which most reasoners eventually have found themselves plunged. With the clearer concept of the chemical behavior of the colloidal clay fraction of the soil as it may give, or even take, nutrient cations and anions,¹ it is now possible to bring the clay composition and the plant growth together, and to observe their chemical interactions with laboratory accuracy. We are narrowing the abyss, not only by pushing the chemistry of the soil closer to the plant behavior, but also by using the plant’s metabolism as a biological reagent–possibly more delicate than chemical reagents–to give suggestions regarding the chemical nature and behavior of the colloidal clay fraction of the soil.

Colloidal Clay Replaces Aqueous Solutions as Growth Medium–The soil solution has been found an inadequate medium for the relatively large delivery of nutrients from the soil to the plant, and aqueous nutrient cultures may soon be discarded as media simulating plant behavior in the soil.⁷ Such solutions demand carefully controlled concentrations, osmotic relations and other physico-chemical conditions that are quickly upset with only partial removal of the ions by the plant. Only the more experienced plant physiologist with continually renewed dilute solutions seems successful with this research tool. On the contrary, the colloidal clay may serve as a nutrient medium which even an embryonic plant physiologist may manipulate successfully.⁶ The colloidal clay offers an ease of suspension but yet a low solubility. It gives a large supply, even to excess, of adsorbed nutrient ions but under nearly constant physico-chemical conditions, and it has the capacity to remove from solution those injurious substances bringing about what is commonly known by that cause-concealing term “toxicity.” It permits a wide range in kind and amount of nutrient offerings to plants under experiment while other conditions so disconcerting by their fluctuations in aqueous nutrient solutions remain almost constant. At a pH of 5.0 in an aqueous solution, for example, the presence of one hundredth milligram of hydrogen per liter is an approach to the danger point in acidity. In a 2 per cent colloidal clay suspension at the same pH of 5.0 there would be 650 times as much hydrogen with no great danger. When the chemistry of the behavior of anions on colloidal clay is understood as well as that of the cations, then the colloidal clay medium will permit laboratory research in plant nutrition to function in the interpretation of field results with a degree of satisfaction which aqueous nutrient cultures do not make possible. The colloidal clay medium was the research tool that brought the exchangeable calcium into its proper importance as a plant nutrient.

Simplified Concept of Nutrient Absorption by Plants–The ordinary equation of a chemical reaction at equilibrium may be helpful in formulating a concept of plant root and colloidal clay interactions. Suppose we consider the equation as a case of colloidal clay suspension with possibly some ions in solutions as the left side, and the plant cell protoplasm, also another colloid, plus its aqueous accompaniment as the right side of the equation. Then in place of, or rather along with, the arrows between and pointing in opposite directions, we interpose a membrane like the wall of the root hair. This may be represented as follows:

Imagine the removal of the water to the point of eliminating the solution phase on each side of the equation. Then we can write it as a case of soil colloid and plant colloid on opposite sides of the cell wall of the root hair.¹¹ This brings us to the concept of two different colloids in contact.¹³ We can believe them at equilibrium, or as exchanging ions in either or both directions as regular chemical laws dictate, except for the modifications dependent on the nature of the membrane and its changes in relation to the colloidal interactions or the plant’s metabolic effects.

Unfortunately, we know very little about the chemical properties of the plant colloid “in vivo.” Chemical behavior of the plant cell contents, like the goose that laid the golden eggs, does not submit to internal observation. Thus, the conditions prevailing on the right side are not well known. In addition, the time factor, as a kind of fourth dimension, must be introduced. Displacement of equilibrium by the plant is a matter of a growing season of approximately 100 days and not an instantaneous performance. Thus, we can measure the accumulated displacement result on the right side of the equation only after that interval of plant growth at which we choose to make analysis of the plants.

More fortunately, the colloidal clay and its properties, its behavior, and possible changes are known definitely enough to serve as the known side of the equation for solving the unknown or the plant side. The beidellite type of clay isolated from the claypan layer in the subsoil of the Putnam silt loam has been subjected to enough physical, chemical, mineralogical and other studies to establish its relative constancy in behavior as an anion. Its capacity for wide variation in kind of, and degrees of, saturation by nutrient cations and anions is also established. It is a large negative molecule of constant size and behavior. By electrodialysis it becomes an acid consisting of a multi-hydrogen alumino-silicate. For its numerous hydrogens, there can be substituted, by simple titration, different amounts and kinds of other cations whether nutrient or non-nutrient for plants.1 This fact offers in the clay medium, then, wide possibilities of variation in nutrient offering but all under accurate chemical control and yet not in solution.

The relative concentrations of the exchangeable ions are controlled by their respective degrees of saturation on the clay. Their total amounts are controlled by this character coupled with the amount of clay offered the plant. These conditions simulate, then, the degree of ionization and the concentration of the ions, respectively, in the ordinary solutions.

By means of this knowledge of the properties of the clay and its changes on the left side of the equation, we may observe or measure the plant growth behaviors, the incidence of plant disease, and the seed and plant compositions with their indications of the movements of both cations and anions from the colloidal clay into the plant or in the reverse direction. All these and other plant manifestations and clay changes can serve as helps to interpret what has happened chemically on the right side of the colloidal action equation known as plant nutrient feeding. From such we may learn whether plant nutrition may not finally conform to the more commonly accepted laws of chemical behavior.

Plate I.-A-F.-A. Calcium, rather than magnesium or potassium, is the first requisite among cations required by young plants. -B. Increasing clay content of sand to supply more calcium (left to right) even at pH 4.4 gives better growth of soybeans. -C. Calcium adsorbed on permutit was more effective than ionic calcium (acetate) or mineral crystal calcium (anorthite). (Increasing equivalent calcium left to right.) -D. Soybean growth according to different calcium levels through different degrees of acidity of colloidal clay. -E. Soybean growth as related to the degree of calcium saturation of the colloidal clay (left to right, 40, 60, 75, 87, 95 per cent with hydrogen [upper row], magnesium [middle row], and barium [lower row] as the balance of the saturation). -F. Degree of saturation of calcium is without effect when accompanied by methylene blue (lower row) in contrast to its accompaniment by hydrogen or potassium (upper two rows).

 

Calcium is the Most Important of the Adsorbed Nutrients–The experience of farmers with limestone use for legume crops pointed to the irregularities in the growth responses by these crops and cast doubt on the belief that the hydrogen-ion concentration of the soil, or its pH, is the causal factor in legume crop failure. Some acid soils, failing to grow clover, were given limestone and showed no change in pH after a year; yet they produced clover successfully. This beneficial effect of the added calcium, when there was no change in pH, pointed to calcium deficiency in the soil rather than to an injury by excessive hydrogen-ion concentration as the problem of so-called “acid” soils. Aqueous nutrient solutions served to demonstrate calcium as the first requisite⁵ for growth of soybeans (Plate I, A), a legume which did not require one nutrient, namely nitrogen, in the medium because of its introduction into the plant from the atmosphere. Growth occurred when potassium and magnesium were not supplied. Incidence of disease with low calcium and, conversely, healthy plants with high calcium, showed calcium to be requisite for healthy plants and plant growth⁵ (fig. 1). Clay on which only calcium was adsorbed produced growth (Plate I, B). It was superior to aqueous solutions (Plate I, C) in that it produced growth over a wider range of calcium offered the plant. These results suggested that calcium occupying so large a portion of the adsorbed and exchangeable store of cations on the clay is plant nutrient number one in importance, and this is true even for the soybean, a supposedly “acid tolerant” legume.

Fig. 1. Damping off as a plant disease was prevalent in the absence of calcium.

 

Separation of Effects of Calcium From Those of Hydrogen-Ion Concentration– Electrodialyzed hydrogen clay on which the exchangeable hydrogen was neutralized by calcium hydroxide to varying degrees to give soils of different pH figures, provided the means of separating the effects of the hydrogen-ion concentration from the direct influence of the calcium.^1,2 By use of properly prepared clays, controllable and variable amounts of calcium could be offered to the plant at any pH, merely by varying the amount of the clay that had been titrated to any desired pH. Plant growth on such a series of clays in sand bore some relation to the degree of acidity, or pH, but it was influenced far more by the amount of calcium offered to the plants (Plate I, D). Thus, in trying to relate plant growth to the pH of the soil, the facts indicate that it is related in reality to the approximate reciprocal of the hydrogen saturation and ionization, namely, the calcium saturation.

Some Nutrient Cations May Go in Reverse, or From the Plant to the Soil–Changes in the pH of the colloidal clay medium as the result of plant growth pointed to a displaced equilibrium, but a displacement toward both the right and the left in the suggested equation. Analyses for calcium of the seed and of the clay at the outset, and again of the plants grown, were the means of determining that the direction of movement of the calcium was to the right or into the plant.¹ Then by calculating the amount of calcium left in the clay and by determining from that the corresponding pH at the close of plant growth, it was discovered that the pH figures for the clays by actual measurement were higher than those by calculation (fig. 2). In other words, the clays were less acid than they should have been because of the calcium removal and its assumed substitution by hydrogen. The increase in calcium in the crop over that in the seed established movement of calcium from the clay soil to the seed and plant in every case. The growth was parallel to the amount of calcium delivered. The fact that the pH was not lowered as calculated for the calcium removal pointed to a return to the clay from the plant of some elements other than calcium, serving as bases or cations, to raise the pH of the clay. Whether anions of plant origin moved to the soil to add to the confusion may well be considered as a possibility.

Here was the first indication that exchange cations–possibly nutrients–may move from the plant to the soil as well as from the soil to the plant. Certainly, as shown by analysis, the element calcium did not go from seed back to the soil. Instead it moved into the plants. Its equilibrium was displaced by movement to the right. At the same time, some displacement toward the left occurred because of movement of other cations in that direction.

Changes in pH of Clay Caused by Plant Growth are Related to Calcium Supply–When the pH figures of the clays were below 5.5, the calculated reduction in pH by calcium removal and hydrogen substitution as shown by calcium increase in the plant over that in the seed corresponded to an average of approximately 0.12 pH. In clays with PH figures above 5.5, the lowering of pH through calcium removal was greater according as the pH figure was higher for the initial clay. These reductions were 0.45, 0.90, and 1.25 pH for clays at pH 5.5, 6.0, and 6.5, respectively. Equilibrium displacement through calcium removal from the clay by the plants was greater as the clay was more nearly saturated by calcium, or as its pH figure was higher. The plants removed a larger share of the exchangeable calcium as calcium occupied more of the exchange capacity of the clay.

Fig. 2. Changes in the pH of clay brought about by plant growth in contrast to those caused by calcium removal from clay into the plants. 

 

In spite of the calcium removal from the clay, which lowered the pH figure, the measurements of pH of the clays showed these to be higher by some rather consistent amounts than those obtained by calculations. These amounts of pH shift were not related to the pH level of the clay growing the crop. They were seemingly related inversely to the total calcium offered to the crop. With offerings of 0.05 M. E. of calcium per plant the clay became more alkaline by an average of 0.55 pH. For 0.10 M. E. of calcium offered, the induction of alkalinity was but little less, but where 0.20 M. E. of calcium were allotted, there was a shift toward alkalinity of only 0.25 pH. Even at the higher degrees of soil acidity, or with pH figures below 5.5 to 4.5, the plants gave less of their cations back to the soil as more total calcium was offered to them.

Not Only Ammonium Cations Go in Reverse Direction–Nitrogen determinations of the seed and crop showed losses of this element at the pH figures below 5.5 (fig. 3). Here may have been a cation that was going back to the clay in the form of ammonia to make it more alkaline. But since an increase in nitrogen in the system occurred at and above pH 5.5 for the offerings of 0.10 and 0.20 M. E. of calcium per plant, nitrogen fixation or use of atmospheric nitrogen was involved, with an increase of nitrogen in the system.² This raises the question as to whether nitrogen fixation may be going on at the same time that losses of it to the soil are occurring. Since the shifts in pH were so consistent for the calcium offerings at all six pH levels used, it seems doubtful whether the plant losses of the ammonium ion could be so consistent in affecting this change when coming from such widely varying sources as seed alone in some cases, and from seed plus atmospheric fixation in other cases. Doubtless the shift toward higher pH by cation movement from seed to soil must be ascribed to cations other than the ammonium of seed origin in this case, where a calcium hydrogen clay delivering only calcium to the plant is used.

Fig. 3. Nitrogen fixation (positive above and negative below horizontal line for seed content) as correlated with calcium levels rather than with degree of acidity. 

 

Calcium Delivery to Plants Related to the Degree of Calcium Saturation of the Clay–In order to test more accurately the significance of the degree of calcium saturation in the delivery of this element to the plant, clays were prepared with different degrees of calcium saturation, ranging from 40 per cent to complete saturation. The balance of the exchange capacity of the clay was taken individually by hydrogen giving variable acidity, by barium, magnesium, and potassium all giving complete neutrality in the form of readily exchangeable ions, and finally by methylene blue, a large, non-exchangeable ion of organic matter.⁹ Such quantities of clay were added to sand for the growth of soybeans as would supply equal amounts of calcium per plant.

Plant growth followed the degree of clay saturation by the calcium, whether it was accompanied by hydrogen or by the other inorganic cations (Plate I, E). In all of the trials in this test, as was true in the others, growth was insignificant unless the seed content of calcium was doubled within the growth period, which was five weeks. Increased degrees of saturation resulted in the delivery into the crop of an increased percentage of the exchangeable calcium on the clay, figures varying from six to twenty-five per cent of the constant but exchangeable supply³ (fig. 4). When a large complex organic ion like methylene blue accompanied the calcium, then the degree of saturation was without effect on growth (Plate I, F).

Fig. 4. Nitrogen fixation and efficiency of exchangeable calcium as related to the degree of calcium saturation.

 

These situations are not easily explained on chemical bases, though they certainly exclude any effect of the hydrogen-ion or the soil acidity. Seemingly, as more calcium is placed on the individual clay molecule, those calcium ions added at the more nearly complete saturation stage are more active in entrance into the plant, or they may be less forcibly held to the clay molecule. Much less of the same total exchangeable calcium moves into the plant when it is present at a low degree of saturation on many clay molecules, than when it is present at the higher degree of saturation on fewer clay molecules. Seemingly, chemical equilibrium pressure is changed; that is, it is increased when the clay surface for root contact offers the nutrient in the higher degree of saturation.

Such results suggest that if the calcium is to have the most pronounced effects, and if the application is to be used most efficiently, then we should place the calcium into limited soil areas for more complete clay saturation rather than place it throughout the root zone for only partial saturation. In agricultural practice this would suggest drilling the limestone in the manner used for fertilizers. It also gives credence to the theory of increased efficiency of fertilizers through their granulation.

Losses of Other Nutrients From Plant to Soil Under Calcium Deficiency–A more complete chemical inventory of seed and clay at the outset was undertaken in order to determine the behavior of other nutrient ions beside calcium and nitrogen. The latter had been found to move seemingly from plant to soil. The former had been shown to pass from soil to plant whenever growth occurred. Trials were undertaken to determine the behavior of phosphorus in connection with different amounts of calcium offered the plants. The element phosphorus, though a cation, should be classified with nitrogen in that it moves from plant to soil. Unless larger offerings of calcium were given the plant, it failed to contain all the phosphorus originally in the seed. The phosphorus seems, therefore, to have gone back to the soil¹⁰ (fig. 5).

Fig. 5. Nitrogen and phosphorus contents of the soybean crop in relation to calcium levels. (Seed nitrogen=385 mgms., seed phosphorus = 47 mgms.)

 

Phosphorus and nitrogen, both constituents of protein, are apparently moved into the plant from its seed only at high levels of calcium delivery by the soil to the plant. Whatever the nutritional role of the calcium in the plants may be, the question may certainly be raised as to whether it is not instrumental in metabolizing the nitrogen and phosphorus within the plant into insoluble protein to keep equilibrium displaced to the right. There is the further question as to whether it may not play some role in determining the nature and activity of the plant membrane interposed. The latter may be one of its functions, according to the work of the late Professor True;¹⁵ the former can scarcely be denied when calcium, nitrogen and phosphorus run so closely parallel in the plants growing near the lowest possible levels in these different trials. If calcium plays this role in membrane activity, its significance in the early life of a plant is greater than that of merely adding calcium to the original content of the seeds.

Fig. 6. Potassium contents of soybean crop with increasing calcium. (Seed potassium =171 mgms.)

 

In some other trials it was revealed that potassium moves from the plant back to the soil⁸ (fig. 6). Although this element appears in the seed in quantities larger than those of calcium or of phosphorus by fifteen and three times, by weight, respectively, it is interesting to note that, despite this fact, the potassium movement to the left in our type equation has been as high as 50 per cent of the seed content. It might be easy to imagine a “sour” soil serving as an acid extracting agent for taking potassium out of the plant. The potassium, however, moved back to the soils, even when they were neutral, moderately saturated with calcium and free of potassium. When one crop exhausted only part of the applied potassium the second or the following crop brought potassium returns to the soil. This brings our viewpoint nearer to the equilibrium concept again and to the belief that potassium must occur liberally on the clay, with calcium accompanying it, if the potassium content of the crop is to increase over that in the seed.

The Small Supply of Calcium in the Seed May Be Significant–The full significance of calcium in these cases where nitrogen, phosphorus and potassium have gone from the plant back to the soil, unless calcium was liberally supplied, is not yet determined. It may not be as important as at first indicated, but it is significant that in no case has growth been possible unless calcium moved into the plant in its early life. Growth has taken place, however, during the time when nitrogen, phosphorus, and potassium are being lost from the plant to the soil. The quantities of these different nutrients in the seeds may have some meaning when we note that calcium is present in soybeans in a lesser amount than any of the other nutrients. The calcium, magnesium, phosphorus, potassium and nitrogen occur there in the approximate ratio of 1:1:2:7:42 as molecular equivalents, respectively. Those present in the seed in larger quantities may be more readily lost from the plant to the soil, yet permit plant growth. The shortage of calcium in the seed may be related to the need for delivery of it by the soil in the early plant growth, or even in seed germination.

Magnesium and Manganese May Bear A Relation to Calcium–When magnesium is considered in relation to its influence on soybean growth and nitrogen fixation,⁸ its importance is seen to be indirect. Improvements in general plant performance, including nitrogen fixation, were not related to the amounts of magnesium taken in by the plant, but rather to the increased amounts of calcium from a constant source which went into the crop as the exchangeable magnesium on the colloidal clay was increased. Magnesium is apparently instrumental in bringing about greater effectiveness in calcium use by plants in much the same manner that calcium is important in making nitrogen and phosphate more effective in the plant.

Another interesting relation is that of the calcium to manganese. Recent studies of this so-called “minor” or “micro-nutrient” element in plant nutrition, which is present in such minute quantities that spectrographic technique is required to measure it, show that as more calcium carbonate is mixed throughout the soil, there is a reduction in the amount of manganese taken by such crops as bluegrass, redtop, lespedeza, and sweet clover. When, however, these same amounts of limestone are put into the surface part of the soil only, to “feed” larger amounts of calcium into the plants, then the plants take more manganese from the soil. Here calcium carbonate seems to inhibit manganese delivery to the crop by its neutralizing effect when it is evenly distributed throughout the soil, but to stimulate the same when it is provided to the plant as a nutrient in only a limited zone of the soil. Two distinct effects of calcium are here shown, if this visualization of its role is correct.

Calcium Significant in Ecological Array in Nature–If calcium is so significant in the plants, this may have come about by adjustment, during the course of evolution, to this one chief and widely fluctuating variable, which makes soils different according to climatic differences. The degree to which soil development has progressed is measured in terms of calcium depletion. Vegetation in its ecological array fits into the picture of the variable calcium⁴ as it gives plants of different chemical compositions. Soils liberally stocked with calcium support nitrogenous vegetation. Calcium depletion, and therefore potassium dominance, in the soils means a highly carbonaceous vegetation. Calcium fluctuation, then, may determine the different crop possibilities on our different soils. Alfalfa with its protein-rich herbage is commonly established without soil treatment in Kansas or Colorado because the low rainfall has not leached the calcium from the soil. Cellulosic cotton fiber, sugar cane, and other crops, rich in carbohydrates and deficient in proteins and minerals as animal feeds, dominate in regions of high rainfall and soils so highly developed that human and animal populations are disturbed by mineral deficiency diseases. Such relations of plant composition and of animal nutrition to the calcium levels in the soil will be no anomaly in the climatic scheme of plant arrangement when the full significance of calcium in the plants and the soils is understood.

Possible Chemical Linkage of Calcium to Phosphorus Suggested–The close linkage of phosphorus to calcium in behavior deserves particular attention in future studies in physiology in respect to their possible molecular as well as ionic activities. Plant behavior suggests that the two are used in combination, and the same thing is seen in the animal world among the vertebrates. This is suggested biochemically by bacterial behavior when colloidal clay with adsorbed calcium and phosphorus is used as a medium.¹² It is suggested in a purely chemical way when phosphorus shows a different chemical behavior when adsorbed on a calcium-saturated beidellite clay from its behavior on the same medium carrying no exchangeable calcium.¹⁴ Plant services credited commonly to phosphorus may in some measure be found to be services by calcium.

Summary

Calcium is important because of the roles it plays directly. In addition it is significant in relation to the behavior of other ions and to the entire physicochemical relationship of plants and soils.

To date, the behavior of all the plant nutrient ions adsorbed on the colloidal clay complex, or in the exchangeable form, can by no means be cataloged completely. However, a beginning has been made which has called attention first to calcium. Other cations also have come into the picture. Anions and their reactions to the colloidal clay and to the humus as an organic colloid are still unknown. One nutrient after another can doubtless be brought into the picture with respect to its relation to calcium, and then to the other nutrients, both cations and anions. If these relationships can be worked out, we may learn to understand the influence of these nutrients on the behavior of the root hair wall or membrane which intervenes between the plant and soil colloids.

The colloidal clay concept, and the clay behavior as it establishes an equilibrium with the plant colloids on the other side of a cell wall has suggested that the humus as a colloidal, organic matter fraction may be serving similarly as a nutrient ion carrier. This concept has opened fields of study in plant physiology and soil fertility that are bringing the soil and the plant closer together. These studies may have a much greater significance as their results are applied to animal and human physiology. Perhaps in time, even the mysteries of plant growth can be analyzed in terms of simple chemical behaviors.

References Cited:

1. Albrecht, Wm. A.: “Inoculation of legumes as related to soil acidity.” Jour. Amer. Soc. Agron., 25:512-522, 1933.
2. —”Physiology of root nodule bacteria in relation to fertility levels of the soil.” Soil Sci. Soc. Proc., 2: 315-327, 1937.
3. —”Some soil factors in nitrogen fixation by legumes.” Intern. Soc. Soil Sci. Third Com. Trans., U.S.A., Vol. A, 1939.
4. —”Calcium-potassium-phosphorus relation as a possible factor in ecological array of plants.” Jour. Amer. Soc. Agron., 32, 1940.
5. —and H. Jenny: “Available soil calcium in relation to ‘damping off’ of soybeans.” Bot. Gaz., 92:263-278, 1931.
6. —and T. M. McCalla: “The colloidal clay fraction of soil as a cultural medium.” Amer. Jour. Bot., 25: 403-407, 1938.
7. —and R. A. Schroeder: “Colloidal clay culture for refined control of nutritional experiments with vegetables.” Proc. Amer. Soc. for Hort. Sci., 37: 689-692, 1939.
8. Graham, Ellis R.: “Magnesium as a factor in nitrogen fixation by soybeans.” Missouri Res. Bull., 288, 1938.
9. Horner, Glenn M.: “Relation of the degree of base saturation of a colloidal clay by calcium to the growth, nodulation and composition of soybeans.” Missouri Res. Bull., 232, 1935.
10. Hutchings, Theron B.: “Relation of phosphorus to growth, nodulation and composition of soybeans.” Missouri Res. Bull., 243, 1936.
11. Jenny, Hans, and R. Overstreet: “Contact interchange between plant roots and soil colloids.” Soil Sci., 47: 257-272, 1939.
12. McCalla, Thomas M.: “Behavior of legume bacteria in relation to the exchangeable calcium and hydrogen ion concentration of the colloidal clay fraction of the soil.” Missouri Res. Bull., 256, 1937.
13. —”Physico-chemical behavior of soil bacteria in relation to the soil colloid.” Jour. of Bact., 40:33-43, 1930.
14. Ravikovitch, S.: “Anion exchange. I. Adsorption of the phosphoric acid by soil.” Soil Sci., 38: 219-239. “II. Liberation of the phosphoric acid adsorbed ions by soils.” Soil Sci., 38: 279-290, 1934.
15. True, R. H.: “The function of calcium in the nutrition of seedlings.” Jour. Amer. Soc. Agron., 13: 91-107, 1921.