Plant Nutrition and the Hydrogen: Ion I. Plant Nutrients Used Most Effectively in the Presence of a Significant Concentration of Hydrogen Ions

The increasing recognition of the fact that the entrance of the hydrogen ion into the soil is the approximate reciprocal of the exit therefrom of calcium and other nutrient cations, might lead us to believe that complete removal of the hydrogen ion by cation restoration should be good soil management. Almost legion are the determinations of the pH of the soil to leave the inference that a significant degree of soil acidity is associated with low productivity. Application of limestone for hydrogen-ion removal from the soils in the humid region has become so general as a farm practice that it is approaching a crusade against soil acidity.

In most studies designed to determine the detrimental effect of a low pH on the various soil processes and on the summation of these processes as production of crops, the relations have too often been qualitative rather than distinctly quantitative. More recently, numerous exceptions have crept into the generalization that soil acidity is detrimental to crop quality or crop yield. That such a generalization might be untenable is suggested when we note that the maximum human population has come to support itself in the humid temperate zone, where food production, in consequence, must be high; yet it is in this region that the clay fraction of the soil is of such type as to permit the largest relative hydrogen adsorption and therefore the highest degree of soil acidity. If cognizance is taken of the evolutionary aspect of crop adaptation to this environment, most plant species have long been growing on soils with hydrogen present on the soil complex.

The very process of soil development, with carbonic acid as the dominating agency, demands an active role by hydrogen ions in the process and anticipates an end-product containing them. Should it not be a fair assumption, then, that the plant root itself has been evolved as a physicochemical system in approximate equilibrium with an environment representing some significant concentration of hydrogen ions? Since the examples of crop detriment by soil neutrality are accumulating, it would seem reasonable to believe that the movement of nutrient ions from the soil into the plant–and therefore plant growth itself–is at an optimum in the presence of at least some significant degree of hydrogen-ion concentration. The following study presents some evidence in support of this hypothesis.

Historical

Calcium adsorbed on the colloidal complex of the soil is displaced, by simple chemical exchange, to a higher degree when it is associated with hydrogen than when associated with corresponding equivalents of barium, magnesium, or sodium, according to Jarusov.⁷ That this increased calcium activity in the presence of hydrogen is transmitted to plant growth activity is suggested in some work by Gedroiz.⁴ He points out that “in a soil fully saturated with hydrogen, the oats did not develop at all in the absence of calcium carbonate. Two causes prevented their growth; the absence in such soil of available calcium, and the acid reaction.” Of his tests with the same soil saturated completely with each of the separate sixteen cations by a method that allowed the growth of oats, he remarked, “The crops gave the same yield as in the original chernozem only in the soil saturated with calcium.” According to him, complete calcium saturation was no disturbance in terms of crop yields. He reports further, “The plants did not grow at all in the soils saturated with each of the remaining bases (except strontium) both without fertilization, and with nitrogen and phosphorus fertilizer. When, however, calcium carbonate was added to the soil, in addition to nitrogen and phosphorus, a normal crop similar to that on the original soil, was obtained only in the soil saturated [originally] with the hydrogen ions.” According to him, then, the small amounts of calcium carbonate, of nitrogen, and of phosphorus were effective for normal plant growth when added to a soil originally saturated with hydrogen but not when put on a soil that was saturated with any other cation maintaining these soils neutral in reaction. This suggests therefore that, in terms of plant growth, calcium is as effective in small amounts and in the presence of hydrogen ions as it is in large amounts represented by clay saturation with it and consequently in the absence of hydrogen ions.

That calcium is more active when associated with hydrogen and its acidity than with barium and its neutrality is indicated by the work of Hutchings.⁶ Using colloidal clay as a nutrient medium to provide varying total amounts of calcium and varying degrees of saturation by either hydrogen or barium accompanying the calcium he has shown that significantly larger amounts of calcium and higher percentages of that exchangeable from the soil moved into the soybean crops when it was accompanied by hydrogen than by barium. Portions of his data are assembled in table 1.

In general, the soybean crops carried a higher calcium concentration and a higher total calcium content, and used the exchangeable calcium with a greater efficiency when this nutrient adsorbed on the clay was accompanied by hydrogen rather than by barium.

In a similar study, Horner⁵ provided constant total amounts of exchangeable calcium at varying degrees of calcium saturation and reciprocally varying degrees of saturation by either hydrogen or barium. Portions of his data are assembled in table 2.

Again according to Horner’s data, there were larger concentrations of calcium within the plants, larger total amounts taken by the crops, and a greater relative utilization by the crop of the constant amount of exchangeable calcium offered in every case where calcium was associated with the hydrogen ion rather than with the barium ion.

Table 1–Calcium contents of the soybean crops according as hydrogen or barium was associated with the calcium on the colloidal clay.
Data from Hutchings

Degree of Saturation By		Calcium in the Crop
Calcium	Reciprocal	Concentration	Total	Efficiency
		per cent	mgm.	per cent
Calcium and hydrogen
2550 75	(Hydrogen) 75 50 25	0.270.55 0.77	40.2785.54 122.40	40.240.7 40.8
Calcium and barium
2550 75	(Barium) 75 50 25	0.290.31 0.66	31.2045.54 104.84	31.222.7 34.9

Table 2–Calcium contents of the soybean crops according as hydrogen or barium was associated with constant total amounts of exchangeable calcium but varying degrees of saturation of the colloidal clay.
Data from Horner

Degree of Saturation By		Calcium in the Crop
Calcium	Reciprocal	Concentration	Total	Efficiency
		per cent	mgm.	per cent
Calcium and hydrogen
4060 75 87.5	(Hydrogen) 60 40 25 12.5	0.5070.651 0.702 0.764	25.244.8 50.9 57.1	12.622.4 25.4 28.5
Calcium and barium
4060 75 87.5	(Barium) 60 40 25 12.5	0.3860.594 0.672 0.707	23.938.0 47.0 56.2	11.919.0 23.5 28.1

 

According to these data by Gedroiz, by Hutchings, and by Horner, the greater utilization by the plant of calcium when associated with hydrogen rather than with barium, or other cations, is in agreement with the simple chemical behavior suggested by Jarusov. Since the plant behavior is in agreement with the chemical behavior, the failure of the plant to absorb as much calcium when this is associated with barium as when associated with hydrogen, seems less a case of so-called barium “toxicity.” It is highly suggestive that it represents increased calcium activity through its association with hydrogen, whether in soil-plant performances or in the laboratory test tube.

Plan and Methods

If the hydrogen ion exerts beneficial effects in “mobilizing” calcium into legumes, which are considered calcophiles–or what has been more commonly interpreted as “acidophobes”–might not the hydrogen of the soil be an agent helping non-legumes to obtain ample calcium from the soil by making the small supply more active toward entrance into the crop? In order to test this question, spinach and potatoes, both non-legumes of dietary importance, were selected. The former was chosen in consideration of its extensive garden use without regard to soil requirements, its general inclusion as a leafy green vegetable in recommended diets for its mineral contents of antirachitic value, and the controversial value of its calcium content as related to the extent of possible precipitation of calcium within the plant as an oxalate. The potatoes were chosen because of the commonly recommended practice that they be grown on acid soils.

The soil for spinach production was prepared in an attempt to supply all the required nutrients accompanied by (a) significant amounts of hydrogen, or in a soil at pH 5.2 (as determined by means of the glass electrode), and (b) almost no hydrogen, or in the same soil at pH 6.8. In the following discussion, the former will be considered as the “acid” series and the latter as the “neutral” series. Additions of calcium to the soil were varied by increments of 3 m.e. per plant, through a series from 0 to 12 m.e., in order to study the influence of the variation of amounts of this nutrient in the presence or absence of the hydrogen ion. The other nutrient additions to the soil were constant and included 3 m.e. of magnesium, 6 m.e. of potassium, and 6 m.e. of phosphorus. The soil was given the amounts of cations and anions as set forth in table 3.

The soil given these modifying treatments was an acid, clay subsoil of Putnam silt loam. This clay had a total exchange capacity of 28 m.e. per 100 gm. Of this capacity, 12 were taken by calcium, 12 by hydrogen, and 4 by other cations. The original soil thus carried exchangeable calcium to the extent of 43 per cent of possible saturation. The additions of calcium did not carry the total exchangeable calcium beyond 67 per cent of saturation. The total amounts of all cations added were such as should have been completely adsorbed on the colloidal complex.

In providing the soil with variable calcium levels but of neutral reaction, the cations were added mainly as oxides and hydroxides. The additions of the amounts indicated in table 3 resulted in a soil with a pH of 6.8.

In providing the soil with variable amounts of calcium but of acid reaction, the cations were added as neutral salts according to the amounts in table 3. This acid soil carried, therefore, such anions as nitrates, chlorides, and sulfates. No acid was added, but these additions of salts resulted in a pH of 5.2. Lesser amounts of clay were used in preparing this acid series than the neutral series; hence less of the original soil calcium was offered in the former than in the latter series, as shown in table 3. At any single calcium level, the difference in amount of soil between the acid and the neutral soil was no greater than 50 gm. With no calcium applied, the difference was no greater than 35 gm., as contrasted to the maximum total amount of soil at 225 gm. per pot.

 

Table 3–Nutrients added to the soil to provide variable calcium levels under low and high pH levels.

	Nutrients Per Plant							Clay Per Plant	Resulting pH
	Ca	N	P	K	Mg	S	Cl
Low pH, or “acid” soil	m.e 0 3 6 9 12	m.e 6 6 6 6 6	m.e 6 6 6 6 6	m.e 6 6 6 6 6	m.e 3 3 3 3 3	m.e 3 3 3 5 9	m.e 2 1 2 3 4	91.5 100.0 125.0 150.0 175.0	5.2 5.2 5.2 5.2 5.2
High pH, or “neutral” soil	0 3 6 9 12	6 6 6 6 6	6 6 6 6 6	6 6 6 6 6	3 3 3 3 3	3 3 3 3 3	…. …. …. …. ….	125.0 150.0 175.0 200.0 225.0	6.8 6.8 6.8 6.8 6.8

 

Because of the variable amounts of clay, the increase of which made possible the provision of the increasing amounts of calcium while adding constant amounts of the other cations, a variation in the degree of saturation by the different cations naturally resulted. The saturation degree for calcium increased with increasing amounts of clay from 43 to 67 per cent for the acid series and from 43 to 62 per cent for the neutral series. The saturation degrees for the potassium and the magnesium decreased with increasing amounts of clay. For the former the shifts were from 23 to 12 per cent and from 17 to 9.5 per cent for the two series respectively. For the latter the figures changed from 11 to 6 and from 8.5 to 4.7 per cent accordingly. Thus while the saturation for calcium was increasing in the series, it was decreasing for potassium and magnesium. These variable amounts of clay were of no significance in disturbing the texture of the sand-clay mixture in which they were used to fill the pots. In all instances, the mixture was of a sandy texture with decided dominance of sand.

Whether the cations in the acid soil with their accompanying different anions have the same relation to the plant as when the cations were applied principally as oxides or hydroxides, involves the questions of their degree of adsorption and of their presence possibly in solution within the soil. The sulfate ion was used in both the neutral and the acid soil, though in larger amounts in two cases of the latter. The chloride and the nitrate anions were present at the outset only in the acid soil. Nitrogen, representing both cation and anion forms, in ammonium nitrate was added during the growth of the plants on both the neutral and the acid soils. This amounted to 3.5 m.e. of each ion form, or a total of 7 m.e. of nitrogen. With the consumption of the nitrate ion by the plant, this removal should tend toward the induction of alkalinity. The chloride ion was, therefore, in the main, the differing ion between the neutral and the acid soils. Since variable amounts of this anion were used even within the acid soil series, variations in plant response in accordance with them should be manifest if the chloride ion is a significant factor.

These variations in anions between the acid soil and the neutral soil are an inescapable experimental condition, since the introduction of acidity is impossible when the amounts of cations are held constant without the incorporation of the anions. Such is the occurrence in practice where the application of fertilizers involves the use of salts, hence the conditions were accepted for experimental purposes in these trials. As the data reveal, no significant influence could be ascribed to any of the individual anions introduced as possible irregularities into the plan of the experiment.

The variety of spinach used was Bloomsdale Long-standing. Four seeds per 5-inch clay pot were planted and thinned later to one plant per pot. Forty plants represented each of the ten different treatments. The season of growth extended from February 10 to April 15. The usual careful control of greenhouse conditions was maintained. The tops of the plants were harvested, washed, dried at 65°C., weighed, and ground for chemical analyses.

The soil for the potato production was prepared in the same manner as that used for the spinach. The same two degrees of hydrogen-ion concentration, namely, pH 5.2 and 6.8, were used. In these, however, the amount of treated clay taken was larger than that used for the spinach in order to accommodate the larger potato plants. In both series, the allotment of potassium was a constant at 60 m.e. per plant, whereas the calcium was used at 30, 60, and 90 m.e. The amounts of the other elements were also larger, as represented by 60 m.e. each of nitrogen and of phosphorus, and 6 m.e. each of magnesium and sulfur per plant.

An additional potato series at pH 5.5 was introduced in order to test the influence of a wider variation in the amount of potassium through 10, 50, and 100 m.e. per plant when the calcium was held constant at 60 m.e. All other nutrient levels in these series were the same as in the other series for potatoes.

These three series with the potatoes permitted observation of pathological behaviors manifested by the incidence of potato scab as premised on controlled physiological conditions.

The variety of potatoes used was the Red Warba. Uniform apical cuttings of 1 ½ – 2 ounces in weight were used as seed stock in the soils at pH 5.2 and 6.8. Basal cuttings of the same size served in the series at pH 5.5. The season of potato growth extended from March 10 to May 15, or a total growth period of 66 days.

Experimental Results

Yield of crops

The weight of the spinach crop produced was larger, in general, on the soil nearer the neutral point than on the acid soil throughout the series of calcium increments, except where 9 m.e. was applied (The complete date as to crop yields and chemical composition have been assembled elsewhere.⁸). For the neutral series this was a relative decline in the curve, as shown in figure 1. There was also one recession in the corresponding curve for the acid soil, but this occurred where 6 m.e. of calcium was added. The summation of all the dry crop weights of the entire series on the acid soil gave 259 gm. The corresponding figure for the neutral soil was 279 gm. Since 200 plants were involved in each series, the difference in weights amounts to 0.1 gm. per plant increase over a mean individual plant weight of 1.295 gm., or an increase of 7.8 per cent because of the neutral soil.

Fig. 1. Weights of Spinach in Relation to Exchangeable Calcium Applied to Soil at Different Degrees of Acidity

 

These effects of the increasing amounts of calcium offered within each series are also reflected by increased crop yields. The crop yield for the maximum calcium addition was greater by 12.96 gm. than that of the no-calcium treatment in the acid soil series. The corresponding spread in yield was 15.90 gm. in the neutral soil. The figures represent yield increases in consequence of calcium additions, of 29 and 33 per cent, respectively, over the soil given the same other nutrient additions but no calcium. Though the yield increases due to calcium increments were not straight-line functions of these, all calcium treatments, regardless of amounts, resulted in yield increases of the spinach crop.

The weights of the potato crop are given in terms of the yield of tubers and of tops in table 4. It is significant that the potato crop–quite the reverse of spinach–was greater on the acid soil at pH 5.2 than on the more nearly neutral soil at pH 6.8. A total of 622 gm., dry weight, of tubers was produced by the soil at pH 5.2 with the three different calcium levels, and only 543 gm. with the same potassium-calcium series at pH 6.8. This is an increase of 14 per cent as a result of the presence of more hydrogen in the soil medium. It is significant that the yield of tops, also, was 28 per cent higher as the result of the more acid reaction in the soil.

The yields at pH 5.5 are not strictly comparable with the others because they were from plantings of only basal pieces. Nevertheless, they illustrate a growth equivalent of that on the soil with the reaction of pH 6.8, even though the potassium applications in two of the three cases were lower than those on any of the other trials. This suggests a high degree of efficiency for the lower potassium supply on the soil with this low figure for pH.

Table 4–Yields of potatoes, tubers and tops, and the numbers of scab areas in relation to variable calcium with constant potassium and vice versa at different degrees of soil acidity.

Degree of Acidity	Applications of		Yields, Dry Weight		Scab Areas
	Calcium	Potassium	Tubers	Tops
pH 5.2	m.e 30 60 90	m.e 60 60 60	m.e 224 200 198	m.e 58 58 58	m.e 42 6 19
6.8	30 60 90	60 60 60	181 192 170	44 47 45	41 8* 46†
5.5	60 60 60	10 50 100	137 234 175	29 46 44	9 20= 84§

* Two potatoes had one fourth of their entire surfaces covered with scab.

† One potato had half of its entire surface covered with scab.

= One potato had one third of its entire surface covered with scab.

§ Two potatoes had their entire surfaces covered with lesions.

 

Plant composition

Calcium. In terms of the total calcium taken from the soil by the spinach crop, perhaps the outstanding revelation of the study is the fact that the amounts were greatest, and in closest agreement with the calcium increments as soil treatments, in the acid soil. The graph for this soil in figure 2 is almost a straight line from 406.3 mgm. of calcium in the crop with no treatment to 795.5 mgm. where 12 m.e. of calcium was added to the soil. For the neutral soil the relation of calcium increase within the crop to those increments added to the soil is not so consistent. The calcium ranges from 256.9 to 424 mgm. In only one treatment on the neutral soil was the total calcium of the crop higher than that in the acid soil with no calcium additions. This difference amounted to only 17.7 mgm. or about 4 per cent. In the acid soil the larger calcium application increased the total calcium in the crop by about 96 per cent; in the neutral soil, by 65 per cent. In terms of the total amounts of calcium taken from all the soils by all the 200 plants, that on the neutral soil was 1.671 gm., and that on the acid soil, 3.004 gm. This represents an increase of 1.333 gm. of calcium, or 79 per cent more taken from the same calcium addition to a soil at pH 5.2 than from the same soil at pH 6.8. Rather than producing a detrimental effect, soil acidity provided an increased efficiency–in terms of use by plants–of the calcium supplied in an available, or exchangeable, form on the clay complex of the soil.

Fig. 2. Calcium in Spinach in Relation to Exchangeable Calcium Applied to Soil at Different Degrees of Acidity

These facts are particularly significant when viewed in terms of the differences in concentration of the calcium within the crop. When 3 m.e. of calcium was applied, the spinach grown on the acid soil contained 0.97 per cent of this element, and on the corresponding neutral soil, but 0.60 per cent. When 12 m.e. was applied, the crop concentration of calcium on the acid soil was 0.73 per cent greater than that on the neutral soil. Thus, on the acid soil, the smallest addition of calcium reflected itself as an increase of 60 per cent, and the largest addition as an increase of 110 per cent in the concentration of this nutrient within the crop.

The significance of the increments of calcium within each series was outstanding in the acid soil, but much less so in the neutral soil. The introduction of these increments into the soil did not increase the calcium concentration in the crop as a straight-line function, though it was more nearly so in the acid than in the neutral soil. In the acid soil the concentration rose from 0.92 to 1.39 per cent in the crop, on a dry-weight basis. In the neutral soil the corresponding rise was from 0.53 to 0.66 per cent. The maximum calcium application to the acid soil gave a calcium concentration in the crop more than twice as great as that in the crop with the same calcium treatment on the neutral soil and 2.6 times that of the crop with no treatment on the neutral soil.

In terms of its mobilization into the crop, the addition of calcium was much more efficient on the acid soil than on the neutral soil. Of the total calcium applied throughout the series on the neutral soil, but 1.6 per cent was recovered in the crop. On the acid soil the corresponding efficiency figure was 4.05 per cent. Thus, the efficiency of a calcium application in terms of its recovery in the spinach crop was 2 ½ times as great on the acid soil as on the neutral soil.

The crop composition and the crop yields reflected the calcium treatments quite differently. The crop was 7 per cent larger, as a mean figure, on the neutral soil than on the acid soil. But in terms of calcium concentrations and calcium totals within the crop these were approximately 250 per cent greater on the acid than on the neutral soils. This indicates a far greater activity of the calcium in the soil, or greater availability to the plant, when a significant amount of hydrogen is present than when a soil is of neutral reaction. It raises the question whether nutrient cation mobility is not increased for plant growth improvement by the presence of hydrogen ions rather than by their complete exclusion from the soil.

Magnesium. There was a close similarity between the total magnesium moved into the spinach crop and the calcium so mobilized, even though the soil additions of magnesium were constant while those of calcium represented increasing amounts. On the acid soil, the total magnesium in the crop increased almost regularly as the soil was given more calcium, as is shown in figure 3. On the neutral soil the total magnesium in the crop for each calcium treatment was higher than that for no calcium addition, but there was no agreement between the magnesium and the calcium amounts, as was suggested by the acid soil. The graphs showing variations in the total magnesium reflects a similarity to the graphs for the total calcium in figure 2, but the variations from a straight line are greater. The highest calcium treatment on the neutral soil gave a total magnesium content but 54.1 mgm. higher than that on this soil given no calcium treatment, and lower than that for any of the calcium treatments on the acid soil.

Fig. 3. Magnesium in Spinach in Relation to Exchangeable Calcium Applied to Soil at Different Degrees of Acidity

The concentration of magnesium in the crops on the neutral soil was in no case so great as that on the acid soil with no calcium treatment, as is shown in figure 3.

It is significant to note the close similarity in behavior of the magnesium in the crop to that of calcium, despite the fact that variable amounts of calcium were applied to bring about corresponding calcium behavior and that the amounts of magnesium applied were constant. Magnesium, also a divalent ion chemically akin to calcium, seems closely allied with the latter when recovery of the two as totals and concentrations within the crop run so closely parallel. This occurred when in the series of soils the increasing calcium meant increasing calcium saturation but decreasing magnesium saturation.

Strontium. No applications of strontium were made to the soil; nevertheless, both the total and the concentration of strontium in the spinach crop increased as the acid soils carried more exchangeable calcium. On the neutral soil in no case was either the total or the concentration of strontium as high as on the acid soil. The neutral soil, however, delivered slight increases in total and in concentration of strontium with increasing applications of calcium, as is shown in figure 4.

Fig. 4. Strontium in Spinach in Relation to Exchangeable Calcium Applied to Soil at Different Degrees of Acidity

Manganese. Again, with no addition of manganese to the soil, both the total and the concentration of this micronutrient within the spinach crop manifested a behavior similar to that of strontium. On the soil at pH 6.8, however, the increments of calcium in the soil were without effect. On the acid soil, as shown in figure 5, the relative effects by these increments were even more pronounced than for the strontium. The relation between calcium, manganese, and hydrogen (soil reaction) is particularly interesting since it was previously shown¹ that calcium, through its service as a plant nutrient, functions in mobilizing manganese into the crop; this is more pronounced when it does not serve as a reducer of hydrogen concentration. The data for the utilization of manganese by spinach suggest that if calcium is to function in this nutritional role, then the presence of hydrogen along with the calcium is a requisite.

The possible influence of the exchangeable calcium on the total and the concentration of magnesium, strontium, and manganese in the crop deserves particular notation. With increments of calcium, but not of the other three ions, there were increases as totals and as concentrations of the other three, with the exception of manganese in the neutral soil. These influences by the calcium on the other elements were always far greater in the acid soil. If these are the facts, liming of a soil demands caution lest one apply sufficient carbonate to remove completely the beneficial hydrogen.

Fig.. 5. Manganese in Spinach in Relation to Exchangeable Calcium Applied to Soil at Different Degrees of Acidity

 

Phosphorus. The total amounts of phosphorus taken from the soil by the spinach crop differed but little for the two different degrees of soil acidity. The acid soil gave a slightly higher concentration of the phosphorus in the spinach where 3, 6, and 12 m.e. of calcium were used, but these differences were not relatively large. These two conditions are revealed in figure 6.

It is interesting to note that with constant applications of phosphorus to the soil, the difference in degree of acidity does not give significant difference in the total phosphorus in the crop, as shown by the close proximity of the two curves in the figure. It is significant, however, to note the influence of the variation in exchangeable calcium on the total and on the concentration of phosphorus in the crop. This suggests that the calcium in the soil, rather than the reaction degree, determined the amount of phosphorus that went into the crop. That the increments of 3 and 6 m.e. of calcium should have given more phosphorus in the crop that did 9 and 12 m.e. when the total crop growth of the former two was less than that of the latter two, is an interesting observation. There is a similar situation for the concentration of phosphorus in the crop.

Fig. 6. Phosphorus in Spinach in Relation to Exchangeable Calcium Applied to Soil at Different Degrees of Acidity

Potassium. The behavior of total potassium in the crop, like that of the phosphorus, depended, not on the degree of acidity in the soil, but on the application of calcium. The concentration of potassium in the crop was influenced markedly by the calcium on the neutral soil, but not on the acid soil, as revealed in figure 7. It is significant that the concentration in the crop was highest on the acid soil when but 3 m.e. of calcium was applied but was not highest on the neutral soil until 12 m.e. was applied. It may be significant that, as previously mentioned, the concentration of calcium was higher by almost 50 per cent when 3 m.e. was added to the acid soil than when 12 m.e. was applied to the neutral soil. This suggests that on the neutral soil an increasing concentration of calcium in the crop was still possible without disturbing the potassium concentration. But on the acid soil, with its more effective delivery of calcium, there was sufficient increase in concentration within the plants, even at this low calcium offering, to suppress the concentration of potassium, if credence is to be given to the commonly accepted calcium-potassium “antagonism,” or probably more properly, the “Kalk-Kali Gesetz” of Ehrenberg. In the interrelationships of the different nutrient cations, both within the soil and within the plant, the non-nutrient cation hydrogen must also be considered, particularly when most biological activities are so commonly associated with acid reactions.

Fig. 7. Potassium in Spinach in Relation to Exchangeable Calcium Applied to Soil at Different Degrees of Acidity

The fact that the quantities of phosphorus and potassium absorbed by spinach are not so nearly straight line functions when related to the calcium increments as was the case for the other elements is an interesting but not a wholly unexpected phenomenon. Davidson³ pointed out that the “effect of hydrogen-ion concentration on the absorption of phosphorus and potassium is physiological in nature,” and that the same causes were operative in the field as in water cultures. As a possible explanation, he pointed to the “iso-electric relations of the ampholytes of the living cell.”²

In discussing his field results, Davidson³ points to the possibility of

“… a wide range in the iso-electric points of the plant ampholytes, allowing the occurrence of both electro-positive and electro-negative ampholytes within certain limitations of hydrogen ion concentration. This makes possible the simultaneous accumulation of cations and anions. A change in the reaction of the medium may modify, to some extent, the reaction within the plant cells and thus cause a shift of some of the electropositive ampholytes to the negative side and vice versa. This may cause an increased absorption of cations or anions, respectively, which is the explanation offered for the effect of hydrogen ion concentration on the absorption of potassium and phosphorus by wheat plants in the parallel experiments with water cultures and under field conditions.”

According to the data of the studies reported herein, the increased hydrogen-ion concentration in the medium brought movement.into the plant of larger quantities of the dibasic cations, calcium, magnesium, and strontium, and of the septibasic cation manganese; but for the monobasic cation, potassium, and for the anion phosphorus, the hydrogen-ion concentration of the medium was without such effect. There is a decided physiological significance, however, in the fact that the behaviors of the potassium and the phosphorus, one a cation and the other an anion, suggest a precipitation-peptization curve for colloids like protein and at each of the two different degrees of acidity. They do not indicate acidity neutralizations. Rather, they suggest calcium as a factor in modifying the physiology of the plant which in its behavior like an ampholyte gives minima for phosphorus at 0 and 9 m.e. of calcium, and maxima at 3-6 and 12 m.e. For potassium utilization, the minima were at 0 and 6 m.e. and the maxima at 3 and 12 m.e. of calcium provided by the media in both the acid and neutral reactions. For both phosphorus and potassium, then, there were two minima and two maxima, for the total and for the concentration whether at pH 5.2 or 6.8, save for the concentration of phosphorus at the latter pH figure.

These performances by the phosphorus and the potassium, both of which were disturbed but little by the hydrogen-ion differences but followed a curve suggesting ampholytic behavior as induced by the calcium increments, give emphasis in this study to the physiological behaviors of all the ions more than to simple neutralizations and common reaction differences. The influence by the hydrogen ion in “mobilizing” more of the dibasic cations into the plant might, at first thought, be viewed simply in the light of the neutralization concept. But when the hydrogen concentration difference is without effect on the behavior of potassium–a similar but more active monobasic cation–and on the behavior of phosphorus–an anion–and when both of these apparently follow the suggested effects by calcium on an ampholyte, then the behavior of all the ions considered moves into the physiological sphere of the plant rather than remaining in the simple acidity concept.

Disease and nutrition

That the role of the hydrogen ion in its inimical effects on potato scab may be exercised physiologically through its influence on the other nutrient cations, is suggested by the observations of the scabbiness of the potatoes in this experiment. The amount of scab increased with increments of exchangeable potassium in the soil over the constant level of calcium, and also with increments of exchangeable calcium over the constant level of potassium at the three different soil reactions, viz., pH 5.2, 5.5, and 6.8. It is significant that either of these distortions of the nutrient levels reflected itself as a metabolic disturbance in the plant by increase in scabbiness far more severely on the neutral than on the acid soil. Even though this suggests that the infestation by the potato scab organism is more a matter of proper plant nutrition than merely of soil reaction, it points to the hydrogen as an agency in making for better nutrition. If hydrogen plays this apparent role of mobilizing the exchangeable nutrients in the soil irrespective of their levels, it suggests that hydrogen may be contributing a service in guaranteeing the large variety of nutrients requisite for plant growth. It suggests that it may be fulfilling in a large measure the condition suggested by True⁹ when he says, “The larger the variety of ions present the greater may be the absorption of all the electrolytes and the less marked the importance of the proportional concentration between ions.”

Summary

Studies with spinach and potatoes at constant and controlled levels of exchangeable nutrients in the soil but at different degrees of acidity have shown far greater mobilization of the nutrients into the crop when the soil was acid than when it was neutral. In some cases this reflected itself as increased crop yields, but in all cases of the chemical analyses it was evident in the form of increased concentration of many of the different elements in the plant tissue. The results point forcefully to the possible role by the hydrogen ion in bringing about a greater availability of the exchangeable nutrients in the soil so that the presence of this ion may be serving as a benefit rather than as a detriment.

Of the possible cations taken by plants, calcium, magnesium, strontium, and manganese were moved into the spinach crop in greater amounts and greater concentrations in the presence of significant hydrogen concentration than in the soil that was nearly neutral. The only other nutrient ions considered in the study, potassium and phosphorus, manifested a different behavior. Though their utilization by the crop was not widely different according to the presence or absence of hydrogen ions, yet both the totals and the concentrations suggested a precipitation-peptization behavior according to the amounts of exchangeable calcium in the soil.

All of these facts serve to emphasize the significance of hydrogen and calcium in the soil to plant physiology much more than as opposing forces in determining soil reaction.

 

References Cited:

1. Albrecht, W. A., and Smith, N. C.: “Calcium and phosphorus as they influence manganese in forage crops.” Bul. Torrey Bot. Club, 68: 372-380, 1941.
2. Davidson, J.: “The possible effect of hydrogen-ion concentration on the absorption of potassium and phosphorus by wheat plants under field conditions.” Jour. Agr. Res., 46: 449-453, 1933.
3. Davidson, J.: “Effect of hydrogen ion concentration on the absorption of phosphorus and potassium by wheat seedlings.” Jour. Agr. Res., 35: 335-346, 1937.
4. Gedroiz, K. K.: “Exchangeable cations of the soil and the plant: I. Relation of plant to certain cations fully saturating the soil exchange capacity.” Soil Sci., 32: 51-62, 1931. (Translation by S. A. Waksman.)
5. Horner, G. M.: “Relation of the degree of base saturation of a colloidal clay by calcium to the growth, nodulation and composition of soybeans.” Missouri Agr. Exp. Sta. Res. Bul., 232, 1936.
6. Hutchings, T. B.: “Relation of phosphorus to growth, nodulation and composition of soybeans.” Missouri Agr. Exp. Sta. Res. Bul., 243, 1936.
7. Jarusov, S. S.: “On the mobility of exchangeable cations in the soil.” Soil Sci., 43: 285-303, 1937.
8. Schroeder, R. A.: “Some effects of calcium and pH upon spinach.” Amer. Soc. Hort. Sci. Proc., 38: 482-486, 1940.
9. True, R. H.: “The function of calcium in the nutrition of seedlings.” Jour. Amer. Soc. Agron., 13: 91-107, 1921.