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Variation in Mineral Composition of Vegetables

The Firman Bear Report

The percentages of ash and of each constituent in the ash of any given species of plant are known to vary widely. They vary with the variety and with the age of the plant and the environmental conditions under which it was grown. As Sims and Volk have pointed out (9)3, such variation is of considerable significance to animals and man, since these creatures depend upon plants for most of the mineral matter they require.

Recent studies of plant ash have confirmed Liebig's century-old concept (5) that "the species of one and the same family will contain the same number of basic equivalents combined with vegetable acids." This principle would now be stated as follows:

Under uniform conditions for growth, except for limited variations in the relative amounts of the several cations in the nutrient media, the sum of the Ca, Mg K, and Na, expressed in milliequivalents per unit weight of dry matter, is a constant for any given plant variety.

Recognition that this principle applies in plants was delayed because chemists have long been reporting analyses of plant ash in terms of percentages of the constituent elements, rather than as their equivalents. Within recent years, however, a number of workers have presented their data in equivalent form, and the principle has been adequately confirmed (2, 6, 7). The highest degree of constancy is found in the terminal leaves (10).

Although Ca is the dominant cation in the exchange complex of normal agricultural soils, its rate of movement into the plant is relatively slow in comparison with that of K. Thus, in an experiment with alfalfa (3), it was found that, with a Ca-K equivalent ratio of 32:1 in the exchange complex of the soil, the ratio of these cations in the plants which grew on that soil was only a little over 3:1. This tendency of plants to take up K is such that much larger amounts of it are often absorbed from the soil than are required for optimum crop yields. When this occurs, the absorption of Ca, Mg, and Na is correspondingly reduced. This may be to the disadvantage of the consuming animal and to man.

The principle of constancy also appears to apply to the mineral anions in plants. For example, Nightingale pointed out (8) that application of nitrate results in the reduction of phosphate uptake in pineapples. When soil fumigants were employed and the ammonia forms of nitrogen were not changed to nitrate for a considerable period of time, phosphate absorption was increased.

In a series of alfalfa plants that were grown in our greenhouse under standardized conditions, except for wide variations in the individual anion values in the nutrient media, the sums of the N, S, Cl, and P absorbed, per unit of dry matter, were essentially constant. It should be noted in this connection that the pH values of the nutrient media were kept uniform. This is important in both cation and anion studies that have to do with this point.

Percentages of ash and summation values are known to be subject to wide variations, depending upon the extent to which the dilution factor of carbohydrate production operates. They tend to be considerably higher in the irrigated arid and semi-arid regions than in the more humid regions. This is in conformity with Albrecht's concept of high-carbohydrate versus high protein-and-mineral vegetation regions of the United States (1).

It is apparent from the foregoing that the mineral cation and anion values in plants are an expression of the environment in which the plants were grown. The environmental factors that seem to exert the greatest influence are soil type, fertilizer practice, and climate.

Wide variation in these three environmental factors is readily found as one proceeds from south to north and from east to west in the United States. An opportunity was recently provided4 to obtain samples of vegetables from a line of states extending northward from Georgia to New York (Long Island) along the Atlantic Coast and from another line of states that extended as far west as Colorado. It is the purpose of this paper to present the results of a study of the mineral composition of the vegetables so selected.

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