HIDDEN IDEAS IN UNOPENED BOOKS
Magnesium . . .
Its Excess, According to Plant Species
by William A. Albrecht, B.A., B.S., M.S., Ph.D.
Prof. Emeritus of Soils, College of Agriculture, University of
Missouri, Columbia, Mo.
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
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
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