Potassium (K) fertilizer is often referred
to as "potash," a term coined by early American
settlers who produced potassium carbonate by evaporating
water filtered through wood ashes. The ash-like crystalline
residue remaining in the large iron pots was called "pot
ash," and was used in making soap. This process of
making potash is registered as U.S. Patent No. 1.
The source of most of the K used for plant food
today is potassium chloride (KCl), which is also called muriate
of potash or potash. Most of the
world reserves of K were deposited as sea water from ancient
inland oceans evaporated, and the K salts crystallized into beds
of potash ore being mined today. The deposits are a
naturally-occurring mixture of KCl and sodium chloride (NaCl),
better known as common table salt. Over time, as the surface of
the earth changed, these deposits were covered by thousands of
feed of soil.
Thus, most potash mines today are deep shaft mines
as much as 3,300 feet underground. In above-ground processing
plants, the KCl is separated from the mixture to produce a high
analysis natural K fertilizer. Other naturally occurring K salts
can be separated by various procedures, resulting in potassium
sulphate and potassium-magnesium sulphate.
Nearly 95 percent of the KCl produced is used in
agriculture. The remaining 5 percent is used for industrial and
home uses.
About three-fourths of the potash used in U.S.
crop production comes from vast, high quality potash deposits in
western Canada. Commercial production of potash in the U.S. has
largely been centered around ore deposits near Carlsbad, New
Mexico, and at Moab, Utah. Potash is also produced by evaporation
of brines . . . such as those from the Great Salt Lake . . . in
Utah and California.
Fertilizer potash is applied to supply the K needs
of growing crops. Some important functions of K in plants are:
Potassium as a nutrient for plants is available
from several sources. Following are some facts on various forms.
Potassium chloride, or muriate of potash,
has analyses ranging from 0-0-60 to 0-0-63. It is a crystalline,
water-soluble material containing 50 to 52 percent K (60 to 63
percent K2O) and 46 to 47 percent chlorine (as the
chloride ion). It is found in white or red colors, depending upon
presence of trace amounts of other minerals such as iron. The K
from either white or red forms has the same agronomic value.
Potassium chloride has higher water solubility than other K
sources.
The chloride (Cl) portion of KCl is also an
essential plant nutrient, although there is usually enough
available in most soils that it is not limiting to crop yields.
Research and farmer experience in the Great Plains and Far West
have shown positive benefits from the chloride portion of KCl.
The chloride reduces the incidences of some crop diseases in
small grains, corn and many other crops. In these situations, KCl
is sometimes recommended even through there may not be a need for
K.
It is important to distinguish the chloride in
potash (KCl) from the chlorine used as a disinfectant. While both
are derived from the same element, their chemical characteristics
and biological activities are dramatically different:
Chlorine is a greenish yellow gas with
a sharp, disagreeable odor but has many important uses. It is
most commonly know as a highly toxic chemical used as
a bleaching agent and disinfectant. When a chemical such as
calcium hypochlorite is mixed with water, hypochlorous acid
is formed --- a powerful oxidizing agent. Many other chlorine
compounds are used in industrial and home applications for
everything from solvents (carbon tetrachloride) to synthetic
materials such as polyvinyl chloride (PVC).
Chloride (Cl-), the
negatively-charged ionic form found in potash, is relatively
non-reactive in the soil, and is not toxic to soil organisms
or to higher plants. (Some vegetable and fruit crops are
sensitive to excessive chloride in the soil as seedlings). Corn,
soybeans, wheat, alfalfa, cotton and most other major crops
are not particularly sensitive to chloride, even under high
potash fertilizer application rates. (Chloride can
accumulate, along with other salts, to toxic levels in soils
with severely limited internal drainage. These saline
soils are found mostly in isolated spots in semi-arid
regions).
Animals, including humans, require chloride in
their diets. Usually it is supplied by table salt (NaCl). But
for patients who must restrict their intake of sodium,
doctors recommend "lite salt" (KCl), which is the
same potash used as fertilizer.
Since it carries a negative charge, chloride
is not held tightly in the soil, and readily moves downward
in drainage water, so it does not accumulate in the root zone
in most soils, even under high fertility systems.
Misunderstanding of the differences in chemical
and biological activity of chloride and chlorine have led to
misguided concerns that using potash fertilizer will lead to the
production of toxic chlorine in the soil. This does not
happen. The chloride in the soil water does not in any way
damage the environment.
Potassium sulphate (K2SO4)
has an analysis of 0-0-50. It is a white crystalline salt
containing 42 to 44 percent K (50 to 53 percent K2O),
18 percent sulphur (S) and less than 2.5 percent chloride. It is
produced primarily for use on certain crops that require a lower
chloride content (such as tobacco, chipping potatoes), or where K
and S are deficient in crop production.
Potassium-magnesium sulphate (K2SO4
. 2MgSO4) has an analysis of 0-0-22. It
contains 18 percent K (22 percent K2O), 11 percent
magnesium (Mg) and 22 percent S. It is useful where K, Mg and S
are needed because it provides a readily soluble source of all
three nutrients.
Potassium nitrate (KNO3)
has an analysis of 13-0-44. It contains 37 percent K (44 percent
K2O) and 13 percent nitrogen (N). Originally obtained
from Chile as a by-product of the production of nitrate of soda,
potassium nitrate is now produced chemically by reacting
potassium salts with nitric acid.
Potassium hydroxide (KOH) has an analysis
of 0-0-75. It contains 62 percent K (75 percent K2O)
and is a highly caustic, highly soluble source of K. However, its
high cost makes it useful only for special needs, such as
specialty liquid fertilizers, where high analysis of K is
desired.
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The article was published in the Spring 1990 Better Crops
with Plant Food by the Potash & Phosphate Institute
(PPI), 655 Engineering Drive, Suite 110, Norcross, GA 30092-2843,
Phone: 770-447-0335; Fax: 770-448-0439