A few oddities may also be mentioned for anyone inclined to experiment:
From Holland it is reported that an avenue of large handsome shade trees
close to a century old, all died in one year, except where a junk dealer
had stacked a pile of old metals. The trees had exhausted the inorganic
nutrients within reach of their roots in the soil, but the junkpile had
replenished them sufficiently, so that those within reach of it kept
alive to this day, twenty years later.
A rock mulch is reported to have improved the growth of lime and lemon
trees considerably[17], and it would seem that similar experiments
should be made on young nut trees, just before bearing age in a
comparative test with a check planting. Stones can be selected for the
nutrients they contain, and a geologist can easily point out those
containing the greatest number of elements. No one could go wrong in
placing a few rocks of limestone or dolomite near the base of a tree,
and let rain and sunshine, heat and frost attend to the fertilizing in a
slow but perpetual manner.
Maple sugar contains manganese[18], showing this as a distinct quality
over cane sugar. Manganese and other essential nutrients are known to
facilitate the production of proteins[19], and the question of better
quality nut production may well be examined from the viewpoint of the
indirect effect from activities of soil microbiology by manganese,
copper, cobalt and zinc. Some of these elements have also been classed
as inorganic plant hormones[20]. "Chlorosis," the yellowing of leaves,
may not only be a deficiency symptom of manganese, but also one of iron,
copper and magnesium. Lack of manganese can cause a decrease in
photosynthesis[21], so much so that in manganese deficient leaves the
CO2 assimilation may be reduced to half of normal. Herein, too, may lie
the cause of low yields, smaller roots and lowered resistance of those
roots to invading detrimental organism.
Contemporary work on soil microbiology may show that manganese and other
essential nutrients are perhaps most important in their functions for
the preservation and balancing of microbial life and actions in soils.
There is where tree nutrition must begin; whatever is neglected in soils
can at best only temporarily be adjusted afterwards. After all,
deficiency symptoms on foliage show lack of soil fertility, and while we
should welcome them for their diagnostic value, our corrective measures
to be most economical must be taken on soils.