We know that the essential positively-charged nutrient elements, or cations--calcium, magnesium, potassium. sodium, manganese, zinc, copper and others--are taken out of solution and adsorbed by colloidal clays and humus of the soil, but are, nevertheless, exchangeable to plant roots offering the non-nutrient hydrogen in trade. We need to consider just what part of that adsorption-exchange capacity by each nutrient element will offer a balanced diet for the healthy growth and multiplication of each desired plant species.
Contribution from Scotland
A contribution to the answer to this question was made in Scotland by a study of what increasing amounts of magnesium do to rhododendron plants. This species is erroneously believed to require acid soil; it really requires one of low calcium content. It does well on a high-magnesium soil, and consequently served well to study what, for most commonly cultivated crops, would be an excess of magnesium.
For testing the growth of the rhododendron, the researchers used a very acid soil (pH 5.0) in which three stages of acidity reduction (above pH 5.0, above 7.0 and near 8.0) were brought about by increments of magnesium carbonate. The reduction of the soil acidity from roughly pH 5.0 to 8.0 caused the plants to grow better. This fact tells us that this species does not grow well on soil with an acid or hydrogen-saturated clay-humus. Instead, it requires a soil with the exchange capacity of that fraction of the soil highly loaded with magnesium. The rhododendron is a magnesiphile and a calciphobe; that is, it is magnesium-loving and calcium-hating. In experiments, it grew best when magnesium carbonate (not calcium carbonate) had increased the pH roughly from 5.0 to near 8.0.
Graph shows effects
Just what this high degree of magnesium saturation did to the plant's chemical composition is shown most simply in the accompanying graph, in which the concentrations of nitrogen (N), calcium (CaO), potassium (K2O) and magnesium (MgO) are shown on the scale on the left as percent of dry matter; the phosphorus (P2O5) is shown similarly by the scale at lower right; and the manganese is given as parts per million in the scale at the upper right.
DESCRIPTION . . . Decreasing the soil acidity (raising the pH) by using increasing amounts of calcium carbonate augmented the nitrogen (N), magnesium (MgO) and phosphorus (P2O5) in the rhododendron plants, but decreased the amount of calcium (CaO), potassium (K2O) and manganese (Mn).
The significant results show: (1) the adverse effects of high magnesium in the soil on the movement of calcium, potassium and manganese into the plant; (2) the favorable effects on the movement of nitrogen and phosphorus into the plant as a result of saturating the soil with magnesium; and (3) the very large increase in the concentration of the magnesium in the plants when the magnesium in the soil was increased.
The "antagonistic" effect by the magnesium on the calcium is an almost directly inverse one. The graph shows that the line for the concentrations of calcium goes downward at an angle about equal to that of the line showing rising concentrations of magnesium. This has been a well-known fact for many years. Similarly, there is the antagonistic reduction of potassium in the plant by the increased magnesium in the plant due to that in the soil, when, at the same time, its carbonate reduced the degree of soil acidity. Also, there was a very significant reduction in the concentration of the manganese in the plant. Relatively speaking, this latter was one of the largest reductions in the elements for which analysis was made.
Perhaps the most surprising result was the increase in the amount of phosphorus taken into the plants when magnesium in the soil was increased. In the quantitative determination of phosphorus in the laboratory, it is common practice to precipitate it as magnesium phosphate, a most insoluble compound. Yet, contrariwise, putting more magnesium into the soil mobilized more of the soil's phosphorus into the rhododendron plants. This tells us that chemical analysis of the soil gives by no means the same values we get when the values are determined by the biochemistry by root contact in the soil.
Increase in the nitrogen of the plants was as expected, since it is the constituent of protein, the chemical compound carrying life, and its increase goes with increase of growth and the factors bringing it about.
All this clarifies the interrelations (all too poorly comprehended) between the nutrient elements in the soil and the different crops created by these elements' quantitatively different roles. It explains the variations in the chemical compositions of any single crop as the result of its diet varying according to the exchange capacity of the colloidal clay and the soil organic matter.
Let's Live Magazine, June, 1965