Plant Nutrition and the Hydrogen Ion: III. Soil Calcium and the Oxalate Content of Spinach

With the fuller recognition of the fact that the increase of hydrogen on the exchange complex, or colloidal fraction, of the soil is the approximate reciprocal of the loss therefrom of plant nutrient cations, the wide range in soil acidity should augur a wide range in the chemical composition of plants. This range may be expected both between plant species and within the same species. Some species reflect the soil nutrient shortage by reduced growth. Others on the same soil do not manifest lowered mass production as the reflection of the soil fertility shortage. They must therefore have lowered concentrations or totals relative to those soil-given nutrients that are deficient for the other plants.

Among the garden vegetables, spinach is a crop that usually makes ample growth over a wide range of soil differences. Its composition in ash constituents is widely variable.^6,2 Its content of oxalate has also been called to attention for its wide fluctuation.⁵ Because the oxalate is commonly considered a metabolic by-product, the variable amounts of this carbon compound in the crop as related to different levels of soil calcium and to different degrees of soil acidity may shed some light on the interrelations of all these in the plant processes. The dietary importance of spinach as a carrier of calcium, magnesium, and other nutrients, subject to fluctuations caused possibly by soil variations, encouraged the following study of the oxalate content of spinach in relation particularly to the exchangeable calcium in the soil.

Plan of the Experiment

As a means of controlling the nutrient supply in the substrate, the subsoil of the Putnam silt loam with its high content of colloidal clay was used as a carrier of the exchangeable nutrients. Since not only the clay fraction¹ but also the silt fraction of this soil type are almost inert toward plant growth,³ the coarser fractions were not removed from the finer colloidal ones. The subsoil clay used had an exchange capacity of roughly 28 M.E. per 100 gms. Twelve of the 28 M.E. were hydrogen, twelve were calcium, and the balance of the clay’s capacity was taken by other elements in minor amounts.

A series of different amounts of calcium was provided by putting specific amounts of it on the clay and using more or less of the treated clay so as to offer 0, 3, 6, 9, and 12 M.E. per plant. The other nutrients similarly supplied in constant amounts included: nitrogen 10 M.E., potassium and phosphate 6 M.E. each, and 3 M.E. each of magnesium and sulfate per plant.

One series was made up by adding oxides and hydroxides of the elements to the acid colloidal clay to give a nearly neutral condition with a final pH of 6.8. The second series was made up by adding them as salts, namely chlorides and sulfates, so that an acid soil at pH 5.2 resulted. These series will be spoken of as the “neutral” and the “acid” series respectively. The clay carrying its supply of exchangeable ions was mixed with quartz sand; put into porous clay pots; planted to a final crop of one plant per pot; and replicated as 40 pots in a single treatment. The amounts of nutrients and of clay per plant were those given in table 1. The plants were grown from February 10 to April 15, or a period of 64 days, after which the tops were harvested; dried at 65 ° C.; weighed; and prepared for chemical analyses.

Table 1–Nutrients added to the soil to provide variable calcium levels in “acid” and “neutral” soils

Results

The growth of the crop as related to increments of exchangeable calcium on the two soils of different degrees of acidity is shown in figure 1. The neutral soil produced slightly more spinach than the acid soil, a mean increase amounting to 7.8 per cent. In its response to calcium, this crop demonstrated the beneficial influence of the increasing amounts of calcium applied in the exchangeable form, regardless of whether the soil was at pH 5.2 or 6.8.

Fig. 1. Weights of spinach (dry and fresh) in relation to the exchangeable calcium applied to the soil at different degrees of acidity.

The chemical analyses included determinations of the oxalate, and of calcium, magnesium, strontium, manganese, and potassium. Some of these determinations were made by regular laboratory methods recognized as standard procedures, while some were spectrographic analyses. The results of these chemical analyses, in terms of concentrations, are assembled in table 2. They are given in terms of totals in the crop in table 3.

Table 2–Concentrations (percentage) in the crop of spinach when grown at pH 5.2 and at pH 6.8 with variable amounts of exchangeable calcium offered

Table 3–Totals (M.E) in the crop of spinach when grown at pH 5.2 and at pH 6.8 with variable amounts of exchangeable calcium offered

One of the outstanding features of the chemical composition of the spinach crop is its higher concentrations and higher total contents of the oxalate, and of calcium, magnesium, strontium, manganese, and potassium, when grown on the soil originally at pH 5.2 than when on the soil at pH 6.8. In addition, these different concentrations and totals increased, in general, with the increments of calcium in the acid soil, but not so much so on the neutral soil. These increases were apparently associated with increased metabolic performances by the plant as indicated by the larger amounts of oxalate. The concentrations of the oxalate in the spinach were higher on the acid than on the neutral soil, but not significantly related to the amounts of applied calcium at either pH as is shown in figure 2. That the total amounts of the oxalate in the spinach crop were related to the increments of applied calcium on the acid soil is very evident from the uppermost graph. On the neutral soil the amounts are lower and less closely related to the calcium applications.

Since the oxalate in chemical combination with calcium forms a very insoluble compound, and likewise when combined with magnesium, it will be interesting to note the amounts of oxalate as they are excessive or deficient for complete precipitation of the calcium and magnesium and thus put these two nutrients into possibly inert forms so far as ordinary digestive processes are concerned. These possible relations between the amounts of oxalate in contrast to those of calcium and magnesium combined are best shown in figure 3 for the acid soil and in figure 4 for the neutral soil.

Fig. 2–Concentrations and total amounts of oxalate in spinach as related to the exchangeable calcium in the soil. 

Fig. 3. Amounts (M.E.) of oxalate, calcium, magnesium, and of the latter two combined, in the spinach crop in relation to the exchangeable calcium in an acid soil, pH 5.2.

Fig. 4–Amounts (M.E.) of oxalate, calcium, magnesium, and of the latter two combined, in the spinach crop in relation to the exchangeable calcium in a neutral soil, pH 6.8.

For the acid soil, the combined amounts of calcium and magnesium were increasingly greater than their equivalent in oxalate as more calcium was applied. The calcium treatment may thus be considered as providing both calcium and magnesium in the crop in amounts beyond those which would be precipitated by the oxalate. There would then be available this surplus of these bases, calcium and magnesium, of possible nutritional value even if significant amounts of them had been made insoluble through precipitation by the larger amounts of oxalate. This surplus of bases is relatively larger as more calcium is applied. This is shown by the ratio of the oxalate to the equivalent of calcium, which ratio became narrower, or shifted from 2.4 to 1.6, as more calcium was added to the soil at the outset. It raises the question whether additional offering of calcium might not have given a ratio of one or less, or enough calcium so that it alone might serve to precipitate all the oxalate.

Though the magnesium was applied in constant, exchangeable amount to the soil, the total amounts of it in the crops increased with the increments of calcium applied to the soil. In all cases the magnesium in the crop was larger than the calcium by 25-40 per cent on the acid soil series, and by 50-60 per cent on the neutral soil. But even then in the latter case, or on the neutral soil, the totals of magnesium were less, in general, than those of calcium on the acid soil. Hydrogen ion absence had reduced the calcium intake by the crop more than the intake of magnesium, or the presence of the hydrogen ion activates the calcium for crop use relatively more than it does the magnesium.

The order of these three ions, viz., hydrogen, calcium, and magnesium, in their decreasing degree of adsorption on the clay colloid, may be suggestive. According to this lyotropic series⁴ hydrogen is adsorbed to a greater degree than calcium, and calcium to a greater degree than is magnesium. Accordingly, in the presence of hydrogen to place calcium and magnesium as second and third in the series, the difference in their migrations into the plant is less than when in the absence of hydrogen they are first and second in the adsorption series. Such visualization of these differences may not be the facts, but it is suggestive and helpful in connection with the differences in mobilization of these two bases into the plants in the absence and in the presence of the hydrogen ion on the clay.

In the case of the neutral soil, the combined amounts of calcium and magnesium were not the equivalent of the amounts of oxalate, except where 12 M.E. of calcium were offered per plant. At this high level of calcium offered on the neutral soil, it is important to note that both the oxalate and the calcium in total were just close to the equivalent of that in the crop on the acid soil given no exchangeable calcium. There was then an excess of oxalate which might be expected to precipitate all of the calcium and magnesium and lead us to believe these mineral elements in the crop unavailable for human digestive use. The crop grown under such conditions would not provide these minerals as a vegetable of this type is commonly believed to do.

With the oxalate as one of the plants’ metabolic by-products, then, it is evident that the plants were physiologically more active when grown on the acid soil, pH 5.2, than when on the neutral soil, pH 6.8. This is indicated by the higher concentration of oxalate as well as of nutrient cations in the plants on the former than on the latter. This difference in the oxalate amounted as a maximum to 2.0 per cent, and as a minimum to 0.52 per cent, as concentrations in the dry matter. As a mean, the percentage content of oxalate was one-third higher in the former because of the acid condition of the soil. Even though the concentration of oxalate was higher, there was more than enough of the oxalate-precipitating bases taken by the plants from the soil under acid conditions to combine it into the less soluble forms. On the neutral soil, the concentration of oxalate was lower than that in the acid soil by one fourth, as a mean. But even then, because of the deficiency in the total of the two oxalate-precipitating bases which were less than their equivalent in oxalate, this excess of the oxalate could precipitate and make unavailable some of the calcium contributed to the human digestive mix from other food sources. Thus the spinach might not only be valueless as a calcium contributor, but might even be damaging to the effects by other foods serving this purpose.⁷

Whether such views are the actual facts of human digestion, and whether the soil fertility supporting their growth determines the degree to which vegetables are digestive satisfactions or disappointments in our dietary, remains to be more fully determined. They suggest, however, that it might be well to test this hypothesis by means of some assays with smaller animals as help in appreciating the possible situation in human nutrition. Only by such means can the final facts be fully established even though plant composition differences in relation to fertility variations suggest that human deficiency troubles may find their origin in the deficient calcium and magnesium of the soil on which such vegetables and possibly other products are produced.

Summary

By using the oxalate concentration in the spinach crop as an index of the plants’ performances on soils with increasing amounts of calcium under pH of 5.2 and of 6.8, it was shown that the concentration of oxalate in the crop on the acid soil was higher than on the neutral soil. The total in the crop was similarly higher. When the total oxalate was related to the totals of calcium and magnesium it was shown that on the acid soil the amounts of calcium and magnesium taken together were more than the equivalent of the oxalate. With larger additions of calcium to the soil, these two basic cations were present as an increasing surplus over the oxalate.

On the neutral soil the combined amounts of these two cations were not the equivalent of the oxalate except in the case of the soil with the highest calcium application. This soil gave a spinach crop with an excess of the oxalate. Such a result raises the question whether this excess of oxalate may not represent a disturbing condition in the form of calcium deficiency for the plant functions as normal growth.

If the oxalate is considered as removing the two bases, calcium and magnesium, out of reaction in the digestive processes of animals, these results suggest that calcium as a soil treatment for spinach on an acid soil may provide the crop with calcium and magnesium as surpluses over that combined with the oxalate and therefore may contribute some of these minerals of nutritional use. If this soil treatment is applied on a neutral soil, or as a carbonate that should result in neutrality, there is the suggestion that the crop may be so deficient in bases with respect to its oxalate content that this excess of oxalate may not only make the spinach of no nutritional value as a provider of these bases, but may even make it injurious as this excess of oxalate disturbs the calcium coming from other food sources. The composition data of the spinach suggest the need for attention to both the calcium and the magnesium supplies, and to the reaction of the soil growing this crop if the fullest values in terms of its content of these nutrient bases are to be realized.

References Cited:

1. Albrecht, Wm. A., Graham, Ellis R., & Ferguson, Carl E.: “Plant growth and the breakdown of inorganic soil colloids.” Soil Sci., 47: 455-458, 1939.
2. Albrecht, Wm. A. & Schroeder, R. A.: “Plant nutrition and the hydrogen ion. I. Plant nutrients used most effectively in the presence of a significant concentration of hydrogen ions.” Soil Sci., 53: 313-327, 1942.
3. Graham, Ellis R.: “Soil development and plant production. I. Nutrient delivery to plants by the sand and silt separates.” Proc. Soil Sci. Soc. Am., 6: 259-262, 1941.
4. Jenny, Hans: “Kationen-und Anionennumtausch an Permutitgrenzflächen.” Kolloidchem. Beih., 33: 428-472, 1926.
5. Kohman, E. F.: “Organic acids and the acid-base relationship: oxalic acid in foods.” Jour. Am. Diet Assoc., 10: 100-106, 1934.
6. Schroeder, R. A.: “Some effects of calcium and pH upon spinach.” Proc. Am. Soc. Hort. Sci., 38: 482-486, 1940.
7. Shields, J. B., Fairbanks, B. W., Berryman, G. H., & Mitchell, H. H.: “The utilization of calcium in carrots, lettuce, and stringbeans in comparison with the calcium in milk.” Jour. Nutr., 20: 263-278, 1940.