Vegetable Crops in Relation to Soil Fertility — Calcium Contents of Green Leafy Vegetables
GREEN LEAFY VEGETABLES are recognized as important foods in the human diet. As providers of minerals and vitamins they are among the “protective” foods recommended by nutritionists. Attention has been called by Kohman, Sherman, and Wittwer to important nutritional differences between certain greens of the mustard family (kale, mustard greens, and turnip tops) and those of the goosefoot family (spinach, Swiss chard, beet greens, and New Zealand spinach). The superiority of greens of the mustard group may be ascribed to their contributions of calcium and ascorbic acid.
The importance of soil fertility as a determiner of “nutritive quality” in crops has been emphasized in a review by Beeson. More recent reports of Holmes, Crowley, and Kuzmeski; Lucas, Scarseth, and Sieling; Sheets, McWhirter, et al.; and Speirs, Anderson, et al. have continued to focus attention on this subject.
This report deals with comparative calcium values of some important green leafy vegetables, which were grown during the winter under controlled conditions in the greenhouse and with colloidal clay cultures.
Experimental Procedure
The clay-culture technique of growing plants, using variable levels of calcium and nitrogen, was utilized. For a source of colloidal material the clay subsoil of Putnam silt loam was selected. This native material, leached of its exchangeable nutrients, has an exchange capacity of 28 milliequivalents per 100 grams, 12 of which are hydrogen. By replacing the adsorbed hydrogen with cationic nutrients and blending the clay with pure white, quartz sand, a clay-sand mixture results having the semblance of natural soil. The details of preparing the clay and adding the nutrients have been adequately described by Albrecht and Schroeder.
In these studies a series of treatments was prepared by supplying calcium and nitrogen levels each of 5, 10, 20, and 40 milliequivalents (m.e.) with all possible combinations of the two nutrients. Other ions were held constant for all treatments. The nutrient salt combinations and quantities of clay used to provide the 16 nutrient levels are presented (Table 1). The pH values of the cultures approximated 6.8.
The vegetables were grown in one-gallon glazed crocks with 10 replicates for each treatment. Seedling plants were allowed to develop for a period of 60 to 90 days depending on the crop. At the proper stage of maturity the tops were harvested and the fresh and dry weights recorded. After being shredded in a Wiley mill and finely ground in a Merker mill, the dried material was suitable for analyses. Chemical determinations for calcium and magnesium were made according to the official A.O.A.C. methods. Oxalate was measured according to Pucher, Wakeman, and Vickery.
Results and discussion
The fresh weights of all the crops as influenced by the variable calcium and nitrogen are presented (Table 2). For all these vegetables the response to nitrogen was more marked than that for calcium. With New Zealand spinach and kale the calcium level in the cultures had practically no influence on the production of vegetation; whereas with spinach, chard, and mustard greens a pronounced interaction of calcium with nitrogen was noted. The amount of total vegetation produced varied with the crop and not with the botanical family to which it belonged. Yields of fresh material in spinach and beet greens were about half those for chard, turnip greens, New Zealand spinach, and kale. This would indicate that the latter collection of crops from the two families is not as exacting in the requirements for a high nutrient level in the soil as are spinach and beets, both of the goosefoot family. The yields, therefore, did not differentiate the two families.
The calcium contents of the crops, expressed in percentage compositions of dry weights, are assembled (Table 3) and portrayed graphically, as influenced by the calcium supplied in the clay cultures (Fig. 1). Marked differences in calcium values between greens of the mustard and goosefoot families are evident. In no case, regardless of the supply of exchangeable calcium in the soil, does the highest figure for the goosefoot greens (spinach in this case) equal the lowest graphical value for any of the mustard group. Thus the amounts of dietary calcium supplied by the greens of the two families are widely different.
Fig. 1. Comparative calcium contents of green leafy vegetables.
As a result of increasing the calcium supply in the soil there were corresponding improvements in concentrations of calcium in the plant tissue. However, the increase in nutritional value in this respect was more pronounced in the mustard family. Of significance in this group was the betterment of quality, as concentrations of calcium in these vegetables, that was possible by additions of this nutrient element to the soil without any obvious external change in the appearance of the crop. Kale was one of the most responsive plants to an increased calcium supply, in so far as this altered its chemical composition, yet was influenced the least in its vegetative growth by the same nutrient. The calcium content was almost doubled without any apparent change in vegetative growth or appearance (Fig. 2).
Fig. 2. Kale plants and their calcium contents when grown at variable levels of nitrogen and calcium. (Small numbers beneath each plant indicate the percentage in the crop on a dry-weight basis.)
These differences in calcium concentrations of the green leafy vegetables are greatly magnified when one considers differences in their nutritional availability. According to Fairbanks and Mitchell, Fincke and Sherman, Kohman, Speirs, and Tisdall and Drake the calcium of spinach, Swiss chard, beet greens, and New Zealand spinach cannot be utilized in the diet because of the large amounts of oxalic acid present which combine with the plants’ calcium and also with their magnesium to form insoluble and indigestible oxalates. In sharp contrast, according to the same investigators, the calcium of mustard greens, turnip tops, and kale is almost completely utilizable since these plants are practically free of oxalates.
Fig. 3. Probable disposition of oxalate in New Zealand spinach, Swiss chard, beet greens, and spinach when grown at variable levels of calcium.
With reference to spinach, Schroeder and Albrecht compared its nutritive quality when grown at variable levels of calcium in an acid soil (pH 5.2) and in another approaching neutrality (pH 6.8). The outstanding features of their experiments were the higher concentrations of oxalate, calcium, and magnesium shown to be in the plants grown on the soil at a pH of 5.2. For this acid soil at all calcium levels the two bases, added together, were present in the crop in more than sufficient quantities to neutralize all the oxalic acid. The plants grown in near neutral soils (pH 6.8), however, failed to absorb sufficient calcium and magnesium for complete neutralization of their oxalate contents. Under the neutral conditions, increased calcium applications to the soil also failed to alter appreciably the concentration of calcium in the plant. In the experiments reported herein the clay cultures were prepared with pH values comparable to those approaching neutrality as used by Schroeder and Albrecht. Therefore, high oxalate concentrations in relation to those of calcium and magnesium in the crops were anticipated.
The total oxalates produced, including those portions neutralizable by the plants’ calcium and magnesium as well as those in excess, are portrayed (Fig. 3) for the four crops of the goosefoot family. Expressed as milliequivalents per 10 grams of dry plant tissue, stoichiometrically the oxalate exceeded by several times the calcium at all fertility levels. Under the conditions of these experiments not one of the four crops contained sufficient calcium, or even enough calcium and magnesium combined, to neutralize all its oxalic acid. The condition of complete neutralization was most nearly approached in beet greens and spinach. According to the chart New Zealand spinach, Swiss chard, beet greens, and spinach could contribute no dietary calcium. In addition, some excess oxalate was always present beyond the quantities possible of neutralization by the plants’ bases. If one were to neutralize completely the oxalate of New Zealand spinach by calcium it would require from four to six times as much as the plant itself contains.
The effect on oxalate production of increasing the soil’s calcium supply was not appreciable. It has been shown by Wittwer, Albrecht, and Goff that altering the level of soil nitrogen does, however, influence oxalate synthesis. It is of interest that the undesirable effects of excess, soluble oxalates in the goosefoot family were not overcome by the mere addition of more calcium to the clay media, and that in no case could all the oxalate be neutralized even when both the plants’ magnesium and calcium were considered for that end.
Conclusions
The comparative calcium contents of spinach, Swiss chard, beet greens, New Zealand spinach, mustard greens, turnip tops, and kale were ascertained by analyzing the crops, each grown under controlled greenhouse conditions in colloidal clay cultures.
Increasing the calcium supply in the substrate enhanced, in general, the calcium concentrations in the crops.
The members of the mustard family (turnip greens, kale, mustard greens) had a much higher percentage of calcium than those of the goosefoot family (spinach, Swiss chard, beet greens, and New Zealand spinach). The differences in calcium contributions to the human diet by the two plant families were magnified by the high oxalate content in the goosefoot greens. When this oxalate was expressed on a chemically equivalent basis, it was present in sufficient quantities to neutralize and thereby make insoluble and indigestible all the calcium and magnesium in these greens and to leave excess oxalate for dietary removal of calcium derived from other foods consumed with them.
