Soil Alters Calcium Digestibility in Leafy Greens
THAT THE GREEN LEAFY vegetables of the goosefoot family (spinach, Swiss chard, beet greens, and New Zealand spinach) do not increase the concentration of their calcium according as the soil is more heavily limed—as is true for the leafy vegetables of the mustard family (kale, mustard greens and turnip greens)—has been previously pointed out. Nor do those of the goosefoot family carry as high a concentration of calcium as do any of the mustard family group. The difference in the calcium concentrations between these two families are much more magnified when one considers differences in their nutritional availability of this essential inorganic element. The calcium in the four kinds of goosefoot greens cannot even be digestively utilized in the diet, according to good authorities, because of the large amounts of oxalic acid formed and present in these plants.
This organic compound of plant origin combines with the plant’s calcium and also with the magnesium to convert these into insoluble and indigestible oxalates. In sharp contrast, according to these authorities, the calcium, for example, in the mustard greens, turnip tops and the kale is almost completely usable since these of the mustard family are practically free of the oxalates which make the calcium and magnesium indigestible. In some experiments using soils controlled as to both their available calcium and their degree of acidity (pH values) while growing spinach, we made the startling discovery that the spinach grown on the more acid soil had higher concentrations of the inorganic elements, calcium and magnesium, and also of the organic compound, oxalate, than were those found in the spinach grown on the less acid or nearly neutral soil.
But even at these higher concentrations, those amounts of the calcium and the magnesium added together were more than sufficient to neutralize the oxalic acid by forming oxalates and to leave some extra calcium and magnesium in other forms than this indigestible combination of them. The spinach plants on the nearly neutral soil failed to take enough calcium and magnesium from the same soil’s supply to offset the oxalic acid produced within the plants. Therefore, they could offer no digestible calcium and magnesium when these greens were put into the diet. Also, on the nearly neutral soils, the application of varied amounts of extra calcium to the soil failed to change the concentrations of calcium within this green leafy vegetable significantly.
Probable disposition of oxalate in New Zealand spinach, Swiss chard, beet greens, and spinach when grown at variable levels of calcium.
When the four goosefoot greens were grown on nearly neutral soils and chemical analyses made of them for their (a) calcium, (b) magnesium, and (c) oxalate, their high concentrations of oxalates were most noticeable. Even though the calcium in the soil was increased through units of 5, 10, 20, and 40, while the magnesium in the soil was held constant, these four leafy greens each produced more than enough oxalic acid to make both the calcium and the magnesium insoluble and indigestible. This is quite different from what would have been the case had the soil been varied in its available calcium while kept at a more acid reaction, which results when calcium in the gypsum rather than in the limestone or carbonate form is used. The chemical results of these greens as carriers of calcium and magnesium in combination with the oxalates when grown on nearly neutral soil under variable calcium supply are shown in the accompanying chart.
In only the best greens was the combination of calcium and magnesium almost enough to overcome the detrimental effect of the oxalic acid in making these two essential elements wholly insoluble and indigestible. These data tell us that not only the amount of calcium in the soil but even the degree of soil acidity comes in to determine whether the green leafy vegetables will give us the mineral elements, calcium and magnesium, in a digestible form. Only slowly are we coming to realize that the condition of the soil, as well as the kind of crops, determine the nutritional value of what we grow and eat.