The delayed appreciation of the significance of calcium in
plant nutrition may be laid at the doorstep of a confused thinking about liming
and soil acidity. The absence of lime in many soils of the non-temperate zone
has long been known. Lime in different forms such as chalk, marl, gypsum or
land plaster, has been a soil treatment for centuries. Lime was used in Rome in
times B.C., and the Romans used it in England in the first century A.D.
Chalking the land is an old practice in the British Isles. The calcareous
deposits like “The White Cliffs of Dover” were appreciated in soil improvement
for centuries before they were commemorated in song. Liming the soil is a very
ancient art, but a very recent science of agriculture. It was when Liebig,
Lawes and Gilbert and other scientists began to focus attention on the soil as
source of chemical elements for plant nutrition that nitrogen, soluble
phosphate, and potassium became our first fertilizers. It was then that the
element calcium and the practice of liming were put into the background.
Unfortunately for the wider appreciation of calcium, this element in the form
of gypsum was regularly a large part of the acid phosphate that was applied
extensively in fertilizer to deliver phosphorus. Strange as it may seem,
superphosphate fertilizer carries more calcium than it does phosphorus, and
consequently calcium has been used so anonymously or incidentally that its
services have not been appreciated. Fertilizers have held our thought. Calcium
was an unnoticed concomitant. It has been doing much for which the other parts
of the fertilizers were getting credit. Appreciation of the true significance
of calcium in plant nutrition was therefore long delayed.
More recently soil acidity has held attention. This again has
kept calcium out of the picture. Credit for the service of liming has been
going to the carbonates with which calcium is associated in limestone. It was a
case of the common fallacy in reasoning, namely the ascribing of causal
significance to contemporaneous behaviors. Here is the line of reasoning: “Limestone
put on the soil lessens the acidity, and limestone put on the soil grows
clover. Therefore the change in acidity must be the cause of the growing
clover.” Therefore the change in acidity must be the cause of the growing
clover. At the same time, there was disregarded the other possible deduction,
namely: “Limestone put on the soil applies the plant nutrient calcium.
Therefore the applied calcium must be the cause of the growth of clover.”
The labeling of calcium as fertilizer element of first
importance was delayed because scientists, like other boys, enjoyed playing
with their toys. The advent of electrical instruments inducement to measure
soil acidity everywhere. The pH values were determined on slight provocations
and causal significance widely ascribed to them, when as a matter of fact the
degree of acidity like temperature is a condition and not a cause of many soil
chemical reactions. Because this blind
alley of soil acidity was accepted as a thoroughfare so long and because no simple
instrument for measuring calcium ionization was available, it has taken
extensive plant studies to demonstrate the hidden calcium hungers in plants
responsible in turn for hidden but more extensive hungers in animals.
Fortunately, a truce has recently been declared in the fight on soil acidity.
What was once considered a malady is now considered a beneficial condition of
the soil. Instead of a bane, soil acidity is a blessing in that many plant nutrients
applied to such soil are made more serviceable by its presence, and soil
acidity is an index of how seriously our attention must go to the declining
soil fertility.
Now we face new concepts of the mechanisms of plant
nutrition. By means of studies using only the colloidal, or finer, clay
fraction of the soil, it was learned that this soil portion is really an acid.
It is also highly buffered or takes on hydrogen, calcium, magnesium, and any
other cations in relatively large quantities to put them out of solution and
out of extensive ionic activities. It demonstrated that because of its
insolubility, it can hide away many plant nutrients so that pure water will not
remove them, yet salt solutions will exchange with them. This absorption and
exchange activity of clay is the basic principle that serves in the plant
nutrition. This concept comes as a by-product of the studies of calcium in
relation to soil acidity.
Imagine that a soil consists of some calcium-bearing minerals
of silt size mixed with acid clays. The calcium-bearing mineral interacts with
the hydrogen of the acid clay. The hydrogen goes to the mineral in exchange for
the calcium going to the clay. Imagine further that the plant root enters into
this clay and mineral mixture. It does so more readily because of the presence
of the clay. It excretes carbon dioxide (possibly other compounds) into this
moist mixture to give carbonic acid with its ionized hydrogen to carry on
between the root acid and the clay particle and the mineral. The hydrogen from
the root exchanges with the calcium absorbed on the clay in close contact.
Thus plant nutrition is a trading business between root and
mineral with the clay serving as the jobber, or the “go-between”. The clay
takes the hydrogen offered by the root, trades it to the silt minerals for the
calcium and then passes the calcium to the root. Thus nutrients, like calcium,
and other positive ions as well, pass from the minerals to the clay and to the
root, while hydrogen or acidity, is passing in the opposite direction to
weather out of the soil its nutrients mineral reserve and leave finally the
acid clay mixed with unweatherable quartz sand. Acid soils are, then, merely
the indication of nutrient depletion.
- Excerpt from Albrecht's Foundation Concepts - The Albrecht Papers Vol. 1 - pgs. 149-150
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