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In general there are two types of soil, or dirt, on this planet, what we may
call mountain soils and meadow soils. Although this is an over simplification, it
will serve as a basis for discussion.
What happens in the mountains is that the rain water washing through the
rotting organic matter at the surface of the ground picks up carbon dioxide to
form carbonic acid which tends to dissolve the calcium carbonate as it washes
through the underlying rocks. This dissolved calcium, as calcium bicarbonate,
goes down in the ground water to lower elevations.
During the summer months this ground water, laden with calcium
bicarbonate, evaporates at the surface of the meadows and re-precipitates the
calcium as calcium carbonate, tending to make the surface dirt more alkaline.
Rhododendrons, azaleas, camellias and many other plants apparently
prefer what we may call upland forest soils where the fertility of the surface dirt
depends largely on the ability of the acids to release the minerals of which the
dirt is made. Most of our orchards, including apples, pears, cherries etc. from the
rose family, and most of our vegetables, beans and grains like what we may call
valley or meadow soils where the fertility depends largely on the ability of the
calcium carbonate to hold the necessary minerals in the surface dirt where they
are available to the plant's roots.
Plants have a limited ability to select what they need from their available
resources by putting out more roots in the areas where they find what they want.
The fertility of garden soils may sometimes be gauged by dripping
hydrochloric acid on a sample. If the acid fizzes, the dirt has enough calcium
carbonate to hold the necessary nutrients.
The fertility of garden soils may be greatly improved by digging in organic
matter, leaves, manure and whatnot, and allowing it to rot, then treating it with
hydrated lime, fertilizer and whatever minerals may be needed.
The hydrated lime will be caught as calcium carbonate by the carbon
dioxide from the rotting organic matter. Then the phosphates of the fertilizer will
be caught by the calcium carbonate. Since calcium ammonium phosphate is
insoluble in water, even some of the nitrogen may be caught.
If, after digging-in the organic matter, the soil is allowed to dry before
sprinkling-in a 'light snowfall' of hydrated lime, the calcium will be caught. And if,
after liming, the soil is allowed to dry, before watering-in the fertilizer, the
phosphate will be caught and be available to hold most of the needed minerals
where the plants can get at them.
Some of the rules you learn in chemistry are that all nitrates are soluble,
and that most sodium, potassium and ammonium salts are soluble. But there are
a few exceptions. Potassium calcium sulfate is only sparingly soluble, so you can
even precipitate potassium if you have some sulfates in your fertilizer. Also,
magnesium ammonium phosphate is only sparingly soluble, and calcium
ammonium phosphate is insoluble, so you can precipitate nitrogen as ammonium
salts, and let the nitrifying bacteria oxidize the nitrogen to nitrate for the plants.
One of the advantages of letting the soil dry before sprinkling it with lime
and watering it in is that it allows the earthworms to go down out of harms way.
If you don't happen to live on a delta, where the mineral base of your dirt
is silt, brought down by the river from various sources, and if you don't happen to
live in south China, where the mineral base of your dirt is thirty feet deep, brought
down by dust storms from the Gobi Desert, you may want to up grade the
mineral base of your garden. Powdered granite from the aspirator at the
tombstone cutter's shop, which splashes in the bucket like water, is probably as
good as you can get. Failing that, you might try fine granite sand from the quarry.
Or you might add some dolomite number ten, or some oyster shell meal.
(In the nineteen sixties, when I was grinding telescope mirrors against a
cast iron tool, with granite sand for my abrasive, I poured the wet slurry around
the plants at night, and by morning I could see that they loved it.)
There are quite a number of minerals that are used in small quantities by
plants, magnesium, iron, manganese, copper, zinc, boron, and even
molybdenum. Salts of these minerals, dissolved in extremely dilute sulfuric acid,
may be sprinkled into the surface dirt occasionally to up grade the mineral base
of poorer soils.
John Dobson July 4, 2002
4135 Judah Street, San Francisco CA 94122
(415) 665-4054