Magnesium. As an element of plant food, magnesium is as essential as
calcium. It leaches out of the soil less readily, and there may be even
less need of its application as a plant food, though the need of calcium
applications for this
purpose is assumed to be small. In the correction
of soil acidity magnesium is more effective than calcium, 84 pounds of
the carbonate being equal to 100 pounds of calcium carbonate. It is a
curious fact, however, that there is widespread fear of magnesium as a
soil amendment. This is not traceable to any considerable experience by
practical farmers that inspires caution in its use, although immense
quantities of magnesian limestone and lime have been used. Neither is it
due to any weight of evidence against it in the experience or teachings
of soil chemists and experiments. The facts of the case appear to be as
1. An investigator found in his laboratory that a plant growing in a
water solution was injured when magnesium was added, and that the
injury was checked when calcium in equal amount was added to the water.
The theory was worked out that a soil should not contain a greater total
amount of magnesium than of calcium, and as the soil's supply of calcium
tends to leach out more readily than the supply of magnesium, it was
best to use a high-calcium lime. If this discovery of the laboratory had
been carried into the field, its significance would have dwindled to
zero in the case of normal soils, and a lot of exploitation would have
been rendered impossible. As it was, the discussion went merrily along
until it occurred to some one to test the matter in the soils where
plants grow, and one would now hear little of it if commercial interests
were not at stake.
2. Very much of our limestone supply is high in magnesium, and some men
who have limestone very low in magnesium and high in calcium have done a
good stroke of business for themselves by deepening the public's
impression, due to laboratory tests with water cultures, that magnesium
in lime is injurious.
3. Many people knew "lime," but had no knowledge of magnesia, and if it
was an impurity like clay or sand, cutting down value per ton, and if it
was worse because harmful, they wanted none of it.
The Fact's Importance. If every farm could get its supply of pure
calcium lime as cheaply as it can have magnesian lime, the truth
respecting the value of the latter would have small agricultural
importance, but as a great bulk of farm and commercial supplies of lime
is magnesian, financial injury has been done consumers who have paid
more than should have been paid for relatively pure calcium lime and
limestone, being afraid to use goods whose content of magnesium was not
small. It is poor policy to use either kind of burned lime in great
excess, but when rationally used on all soils except sandy ones, there
is no preference to be exercised that can be based upon performance. A
magnesian lime corrects as much acidity as a high calcium lime, and a
little more, and its use is to be recommended if there is any advantage
in the matter of price, except in the case of distinctly sandy soils.
Magnesian Limestone. Leading scientists making tests of limestone for
normal soils, use magnesian limestone freely. They recommend its use to
farmers wherever there is advantage in point of price. The advice is
safe that the limestone of a given fineness should be chosen whose total
percentage of carbonates of calcium and magnesium is the highest. The
example of these scientists, buying pulverized limestone for
agricultural colleges and experiment farms, and for their own farms,
should loosen the curious hold that the early warnings of a laboratory
experimenter took upon public imagination. The farmer should buy
limestone on a basis of ability to correct soil acidity, and make each
dollar do the most possible toward that end.
Most limestones contain some percentage of magnesium, and in the case of
a pure dolomite over 45% carbonate is present in combination with
calcium carbonate. A stone rich in magnesium slakes less readily than
one high in calcium, and therefore is preferred by manufacturers
shipping pulverized burnt lime to reach its destination before slaking.
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