Soil analysis and plant tissue tests both have their value, but also
their limitations. Many laboratories and experiment stations are
equipped to make rapid soil tests, and some engage in leaf analysis. It
is important that they be correctly interpreted. For instance, at the
Citrus Experiment Station, Riverside, California[22], bark and leaves
were collected from healthy and diseased Persian Walnuts. They were
analyzed for calcium, magnesium, inorganic phosphate, manganese and
iron. A higher percentage of ash was found in the diseased than in the
healthy bark, and calcium, magnesium, manganese and inorganic phosphates
were also generally higher.
It would be a fallacy I think to conclude therefrom that these elements
were not necessary, or were present to excess. They were probably
present because they had failed to function properly, due to changes in
weather, excessive rains or droughts, and could not eliminate
themselves.
We must consider the results from the functions of the essential
elements, and discard the popular belief that inorganic nutrients in
soils are transmitted from soils to plants, and therein contained for
the express purpose of satisfying the need of animals and humans[23].
The plant has only one purpose to perform which is to grow and to
reproduce itself, and such is the case with all other forms of life.
Plants contain very often inorganic elements in a form in which they
cannot be utilized. It is therefore quite easy to mistake their presence
either as a toxicity symptom or as a high requirement, when as a matter
of fact these elements are present due to conditions unfavorable to
metabolism, and they remained in bark and leaves as end products, in an
inert form. Rather than being transmitted from soils to plant, their
functions may consist of the formation of enzymes, proteins, hormones,
chlorophyll, antibodies, vitamins, in carbon assimilation. When they
have served such purposes they are not likely to be present in plants in
anything like the amounts or forms as present in soils. They may come
into question as catalysts or bio-catalysts, as sources of energy for
microorganism, from which their optimum effects have been secured when
they are not transmitted at all, causing changes, but remaining
themselves unchanged. They are essential in the sense that the elements
composing soils, sea, atmosphere are constantly energized, changed and
used over and over again to create plant, animal and human life. In this
cycle nothing is lost, only changed from old to new generations.