POTASSIUM IN KENTUCKY SOILS
by Lloyd Murdock, and Kenneth Wells, Extension Specialists in Agronomy,
University of Kentucky College of Agriculture
Potassium in Kentucky Soils
Thirteen of the sixteen elements essential
for plant growth must come from the soil. And, except for nitrogen, potassium
is required by plants in much greater amounts than all the other soil-supplied
nutrients. For profitable crop production, there must be an adequate supply
of potassium within the root zone of the plants being grown, and this potassium
must be in a form which can be used by the plant.
The "total" potassium content in Kentucky
soils is much greater than that of nitrogen or phosphorus. Unfortunately
much of the "total" potassium is in a mineral form which is unavailable
for use by plants. So, even though total potassium content is high, "plant
available" potassium levels may be low.
Potassium Content of Kentucky Soils
The potassium content of Kentucky soils
can be attributed to two sources--native and fertilizer potassium.
As previously indicated, the quantity
of total potassium found in Kentucky soils is relatively high. Table 1
shows the total potassium content of the plow layer (the surface 7 inches)
of some typical Kentucky soils.
Table 1.-Total Potassium Content of the Surface 7 Inches of Soils
on Experiment Fields in Kentucky*
*Kentucky Agricultural Experiment Station Bulletin 397, "Soil Management
Experiments."(Out of print; copies available only at libraries.)
||Location of Experiment Field
|Maury silt loam
|Crider silt loam
|Tilsit silt loam
|Monongahela silt loam
|Welston silt loam
||Fariston (Laurel Co.)
|Bedford & Dickson silt loam
|Tilsit catena silt loam
|Grenada silt loam
Native Potassium -- This is the
potassium that was in the parent material (rocks) from which the soil was
formed. The potassium in the parent material is contained primarily in
two minerals, mica and feldspar. Where the parent material has a relatively
high content of mica, potassium is released to an available form at a more
rapid rate. On the other hand, where soils are derived from parent materials
relatively high in feldspar, as in the sandstone formations, the total
potassium will be released at a much slower rate.
Data from a greenhouse study, shown
in Table 2, illustrate differences in the rate of release of potassium.
On an Eden soil having 300 pounds of "available" potassium per acre, four
millet crops removed 1100 pounds of potassium, while on a Tilsit soil with
54 pounds of "available" potassium per acre, the four millet crops removed
only 50 pounds of potassium.
Table 2.--Potassium Uptake by Millet Grown on Six Kentucky Soils*
*Data from paper by Paul Sutton and W.A. Seay (1958), "Relationship
Between Potassium Removed by Millet and Red Clover and the Potassium Extracted
by Four Chemical Methods from Six Kentucky Soils." SSSAP22:110
Available Potassium (lb/A)
-----Soil Test Level-----
in 4 Millet Crops (lb/A)
||Calcareous Shale, Siltstone, and Limestone
||Sandstone and Shale
Alluvial (water transported) soil materials
may have been carried great distances from their area of origin and, because
of this, soils formed from such material may be quite variable in potassium
content, depending on the mineralogical composition of the parent material.
Fertilizer Potassium -- Potassium
fertilizer has been applied, sometimes in large amounts, to many Kentucky
soils. While these applications increase the level of "available" potassium,
particularly in and just below the plow layer, they have relatively little
affect on the total potassium content of the soil. Much of Kentucky's tobacco
land has had heavy applications of potassium fertilizer. On such soils
where the level of available potassium has been increased to a high level,
the soil will supply all the potassium needed for good plant growth. However,
on soils where the available potassium is low, the potassium requirement
by crops may be greater than the rate at which a particular soil can supply
it, and crop yields will be lowered without the addition of potassium fertilizers.
The soil test result of a representative soil sample is the best guide
to follow in determining the potassium fertilizer needs of a particular
Just as with all the other nutrients,
potassium must be present in the soil in a form that is available to plants.
Even though the total potassium content of most Kentucky soils is far above
the amounts required or removed by crops, many of these soils will not
release sufficient potassium for highest crop yields during the growing
season. This is because only a very small amount of the total potassium
is in the readily available form during a cropping season.
Terms commonly used by soil scientists
to describe the different categories of potassium in the soil are "nonexchangeable"
or "fixed," "exchangeable" or "available," and "readily available" (see
Exchange reactions in the soil are
such that a balance is maintained between the three categories shown in
Figure 1. As the growing crops remove readily available potassium from
the soil solution, which we may define as the soil water and the minerals
dissolved in it, some of the exchangeable potassium will move into solution.
And then some of the nonexchangeable or fixed potassium will move into
the exchangeable form. However, nonexchangeable potassium does not become
exchangeable as fast as exchangeable potassium becomes readily available.
This process is reversible and when the readily available potassium is
increased, for example, through fertilization, some of it will revert to
In addition to plant uptake of potassium
from the soil solution, some of the exchangeable potassium on the soil
colloids is also absorbed directly by roots which touch those colloids.
Plant roots possess a negative charge and attract the positively charged
potassium (K+) which is held on the clay mineral surfaces and
Soil tests reflect the amounts of exchangeable
and readily available potassium present when the soil sample is tested.
Tests do not reflect the rate at which the nonexchangeable or fixed potassium
can move into the exchangeable form. This explains why some soils may have
a relatively low test level and yet supply enough potassium for relatively
high crop yields. This is largely influenced by the mineral form of the
soil colloids; and, for this reason, consideration must be given to the
clay mineralogy of a particular soil in addition to recent potassium fertilization,
crop management, and soil test results when planning potassium fertilizer
Potassium Fixation and Release
As previously indicated, only a very
small percentage of the total potassium in a soil is in a form available
to plants at any given time because the reactions shown in Figure 1, are
constantly taking place. With the application of potassium fertilizer,
potassium first goes into the soil solution, soon after which much of it
goes into the exchangeable and some to the nonexchangeable forms. As crops
remove the readily available potassium, the reactions are reversed and
exchangeable potassium goes into the soil solution. As a result there is
constant fixation and release of potassium in the soil.
illustrates the potassium cycle in the soil. During weathering, physical,
chemical, and biological forces act on the parent materials and break them
down into finer fractions, largely sand, silt, and clay size particles.
This breakdown results in the release of several chemical elements, including
potassium, and the formation of different clay minerals.
Most of the total potassium inherited
from the parent material during the soil forming processes will be in the
nonexchangeable and exchangeable forms. Note in Figure 2 that both exchangeable
and nonexchangeable potassium are sources of readily available potassium
and that the process is reversible.
The relative amounts of sand, silt
and clay fractions found in a soil depend on the kind of parent material
(sandstone, limestone, shale or mica) from which the soil was derived.
Potassium fixation and release is greatly influenced by the relative amounts
of these fractions and the kinds of clay minerals present in the soil.
Sand and Silt Fractions -- The
sand and silt fractions of most soils are made up largely of quartz. Other
minerals, mainly feldspars, in these fractions may also contain potassium
and other nutrient elements but, since the particle size is relatively
large, the particles dissolve very slowly and the rate of potassium release
is low. Also, because of the physical and mineralogical nature of sand
and silt, their ability to fix potassium is low.
Clay Minerals -- Clay minerals
(the dominant materials in the clay or colloidal fraction) in a soil are
relatively active in fixing and releasing potassium. The different types
of clay minerals vary in their capacity to fix and release potassium.
Generally there are four dominant clay
minerals in Kentucky soils. Listed here in order of their abundance, they
are kaolinite, soil mica or illite, vermiculite, and montmorillonite. No
soil is composed of only one of these and, usually, a soil will contain
as many as three or four. Each clay mineral has its own characteristics
with respect to potassium fixation and release. In addition, each clay
mineral contains different amounts of native potassium, which is bonded
between the clay layers.
Because of their crystal structure
and the location and amount of negative charges within the crystals, illite
and vermiculite clays are capable of absorbing potassium from the soil
solution and entrapping it between layers of the clay particle (see
Fig. 3). The potassium cations are fixed or entrapped in this way because
of the relationship of their size to the hexagonal cavities in the silica
sheets of two adjoining mica or vermiculite layers. This fixed, or nonexchangeable,
potassium is not available to plants but is slowly released as the levels
of exchangeable and soil solution potassium become lower.
It will be noted in Figure 3 that kaolinite
does not have potassium entrapped between the layers. Soils containing
predominantly the kaolinite clay mineral have less exchangeable potassium
to release than soils which have a higher percentage of the mica and vermiculite
type clay minerals.
Few Kentucky soils, except those in
the Purchase Area and those occurring in slack water bottom positions,
contain appreciable quantities of montmorillonite clay. The montmorillonite
mineral can hold large amounts of exchangeable potassium, but will fix
only a small percentage of it. Therefore, most of the potassium held by
montmorillonite clay is in an available form.
Influence of Parent Material on
Types of Clay Minerals -- Soils derived from calcareous shales are
high in exchangeable potassium. These soils contain some illite and vermiculite
in their clay and fine silt fraction as well as some readily weatherable
Soils derived from the limestone formations
in Kentucky are medium in their ability to release potassium. This suggests
that they are composed of a mixture of the clay minerals, with kaolinite
predominating but with vermiculite and illite also present.
Soils derived from sandstone and acid
shales are low in their ability to release potassium. The major factors
being a low percentage of clay minerals in these soils and kaolinite being
the predominate mineral present.
The parent materials from which loessial
soils are derived are unknown. However, these soils contain montmorillonite
and illite clays, and are medium in their ability to release potassium.
Organic Matter -- Growing plants
obtain potassium from the soil for their nutrient supply. When the plant
residues are returned to the soil, the potassium they contain is readily
released and can then be adsorbed to the exchange sites in the soil (see
Figure 2). Highly decomposed organic matter is called humus. Because of
the negative charges on the humus particles, humus can also adsorb potassium
cations in much the same way as the clay minerals and hold them in an exchangeable
form for rapid release.
Ions with a positive (+) charge are
referred to as "cations," while those with a negative (-) charge are referred
to as "anions." The interaction of potassium and other cations, such as
calcium and magnesium, with the soil colloids is referred to as "cation
exchange." This is shown in Figure 4.
The importance of cation exchange capacity
(CEC) is that it prevents or reduces the leaching of fertilizer components
such as potassium, ammonium, magnesium, calcium, and other cations. Cation
exchange is a means by which the soil can store potassium and other cations
that may be released later to plants.
As pointed out previously, the soil
colloids with negative charges attract and hold the cations. The contribution
of the clay mineral fraction to the cation exchange capacity is dependent
on both the kinds and amounts of minerals in the soil. The contribution
of humus depends on the amount in the soil; though in most Kentucky soils
the humus content is, on a percentage basis, very low. While the clay minerals
and humus account for most of the CEC, the finer fractions of the silt
can also have a limited number of exchange sites.
Of the clay minerals, kaolinite has
the lowest CEC (5 to 15 me/100 grams). The CEC of illite is intermediate
(10 to 45 me/100 grams), while montmorillonite and vermiculite clay minerals
are relatively high (60 to 150 me/100 grams). The CEC of humus is about
140 me/100 grams. These values are for pure clay minerals or humus. The
sand and silt fractions account for roughly 75 to 85 percent of the weight
of silt loam soils and contribute little to the CEC. The 15 to 25 percent
of clay minerals in silt loam soil along with the smell amounts of humus
in the surface soil is largely responsible for the CEC. While CEC determinations
are not routinely made on soil samples tested in Kentucky soil testing
laboratories, most of the silt loams in Kentucky have a CEC of 8 to 12
Cations on the exchange sites are held
rather loosely on the edges of the clay mineral or humus particles and
are constantly being replaced by other cations. They occupy exchange sites
because they are balancing the negative charges of the clay minerals and
humus fractions in the soil. For this reason the reactions are reversible.
Crops require relatively large quantities
of potassium. On soils where potassium is not released within the plant
root zone at rates sufficient to meet the needs of a particular crop, applications
of potassium fertilizers are essential if high crop production is to be
maintained. The best guide to follow in planning a potassium fertilization
program is the soil test result from a good representative soil sample.
Soil test results along with past fertilization, cropping history, the
crop to be grown, and management of crop residues are most helpful in determining
if additional potassium is needed and how much should be applied.
The most common source of potassium
is muriate of potash (KCl or potassium chloride). This source is satisfactory
for all field crops grown in Kentucky except tobacco. A non-chloride source
of potassium such as the sulfate or nitrate form should be used on tobacco
because excessive amounts of chlorine lower the quality of tobacco and
can cause "white stem."
Because of the reactions previously
explained, all the potassium applied as fertilizer is not used by crops
the year in which it is applied. Even under ideal conditions, only 40 to
50 percent of the potassium applied will be recovered by the immediate
crop. The remainder-held in the soil - is slowly released to succeeding
crops, if erosion is controlled and there is no sediment loss. Except for
plant removal, erosion is about the only way potassium will be lost from
the rooting zone of silt loam and heavier-textured soils.
Time of Application -- Since
potassium is not subject to leaching except on very sandy soils, potash
fertilizers can be safely applied in the fail if erosion is controlled.
The advantages of applying needed potassium in the fall are: (1)
avoiding the rush season of spring planting, (2) soil conditions
are ideal for spreading, (3) fertilizer dealers are well-supplied
with material, and (4) a price discount can often be obtained for
fall purchases. On very sandy soils and where there is considerable risk
of surface soil losses from erosion during the winter months, potash fertilizer
should be applied nearer planting time.
Cation Balance -- The relative
concentration of potassium, calcium, magnesium, ammonium, and other positively
charged ions in the soil solution will influence the uptake of any particular
positively charged cation. With an excessively high concentration of any
one of these cations, the plant will take up large amounts of that particular
ion. Since the plant can utilize only a certain amount of cations in proportion
to anions, an excessive amount of one particular cation may result in inadequate
uptake of other essential cations even though there is an adequate concentration
in the soil solution. Very high concentrations of calcium in the soil solution
may reduce the uptake of potassium. On the other hand, a high concentration
of potassium may reduce magnesium uptake. So, even though it is important
to keep cation concentrations high enough for good crop growth, it is just
as important not to supply so much of any one cation that excessive amounts
will limit the uptake of other cations. An adequate supply of all of the
essential elements should be available for rapid root development and high
Rate of Application -- When
potassium fertilizer is applied to soils low in available potassium, some
of the applied potassium will revert to the exchangeable form and some
to the nonexchangeable form Heavy rates of application will build up the
levels of potassium in the soil to a point where less and less will revert
to the nonexchangeable form. At this point lighter applications of potash
fertilizers will be sufficient to supply crop needs and maintain a high
concentration of available K. As might be expected, a soil test is the
best guide for planning the potassium fertilization program.
Crop Management Practices -- A
100-bushel mature corn crop (stover and cobs included) contains about 100
pounds of elemental potassium. Only about 22 pounds of this will be in
the grain. That is, about 78 pounds of potassium will be returned to the
soil if the stover is left on the field. Likewise, a 40 bushel crop of
wheat contains 40-45 pounds of potassium, about 30 pounds in the straw
and only 10-15 pounds in the grain. It is obvious that the management of
these crop residues will influence the potassium fertilization on the succeeding
crops in the rotation. When the whole crop is harvested, as is the case
of corn or small grain silage, larger amounts of potassium are removed
and heavier applications of potassium fertilizer will be required to maintain
an adequate supply in the available form for the next crop.
Another consideration when planning
a potassium fertilization program is the intensity of the cropping system.
When double cropping is practiced, annual removal of potassium is increased,
requiring adjustments in the annual potassium fertilization practices.