Potassium Nutrition
K. Raja Reddy
Plant Physiologist
Department of Plant and Soil Sciences
Mississippi State University
Harry F. Hodges
Professor Emeritus
Department of Plant and Soil Sciences
Mississippi State University
Jac Varco
Professor
Department of Plant and Soil Sciences
Mississippi State University
For more information, contact Dr. Reddy at (662) 325-9463 (phone), (662) 325-9461 (fax), or
krreddy@ra.msstate.edu (email). Bulletin 1094 was published by the Office of Agricultural Communications,
a unit of the MSU Division of Agriculture, Forestry, and Veterinary Medicine. It was edited and
designed by Robert A. Hearn, publications editor. The cover was designed by Betty Mac Wilson,
designer/graphic artist. March, 2000
Bulletin 1094 March 2000
Knowledge of potassium (K) requirements for cotton
growth and development is needed for efficient production.
It makes little sense to limit production and profitability
with late-season K starvation. However,
late-season K deficiency symptoms can be found routinely
in cotton throughout the U.S. Midsouth. Many
modern varieties flower early and require a readily available
supply of nutrients during the fruiting period.
Approximately two-thirds of the total K uptake occur during
a 6-week period beginning at early flowering.
As site-specific agriculture becomes more widely
practiced, there will be greater interest in understanding
why certain areas produce less than others. It is well
known that high yields require good growing conditions.
Knowing the plant nutrient requirements needed to sustain
highly productive growth throughout the season will
be essential to managing the crop for overall greater productivity.
Fertilization of cotton with K is a rather complex
issue, because soils vary widely in terms of K-supplying
capacity and K fertilizer adsorption. Potassium exists as a
constituent of some primary minerals from which many
soils were originally formed. It is a part of the interlayer
of clay minerals such as hydrous mica, and it may become
available due to freezing and thawing or wetting and drying.
Potassium dissolved in soil solution is in equilibrium
with K
+ attached or bound electrostatically to organic matterand the surface of clay particles. Thus, only a portion
of total soil K is soluble, in an exchangeable form, and
readily available to plant roots. However, at other times K
held in a nonexchangeable form in soil minerals can
become exchangeable. When K fertilizer is applied to
soil, some fertilizer may be bound or trapped within soil
minerals so that part of it is either not available or slowly
available to plants. Routine soil tests primarily account for
exchangeable and soluble K forms.
Potassium concentrations in cotton leaves are highly
correlated with extractable soil K (Hsu 1976). If K is readily
available to the roots, it accumulates in cotton leaves
and other plant parts. This trait allows the crop to “bank”
a small portion of its total seasonal K requirement during
vegetative growth and use these reserves later in the growing
season when nutrient requirements are high or uptake
by roots cannot keep up with growth needs.
Plant tissue analyses have been used to diagnose the
nutrient status of plants and to guide fertilizer recommendations.
The use of chemical analysis of plant material for
diagnostic purposes in farmer fields is based on the
assumption that causal relationships exist between crop
growth rates and nutrient concentrations. The results presented
in Figure 1 support and reinforce this assumption.
To improve plant tissue analysis as an aid in making
fertilizer recommendations or diagnosing plant nutrient
status, the relationship between plant nutrient composition
and crop yield needs to be better defined. The relationship
between plant nutrient status and soil test K
levels is also useful information. Often, however, the correlation
between plant nutrient status and yield is relatively
low because many other factors can limit crop
yields (Adeli 1994). This fact could diminish growers’
enthusiasm for tissue analysis. Unless a crop has adequate
nutrients, yield can be limited and much of the other cropproduction
expenses will be wasted. In an attempt to diagnose
spatial variability in yields, the crop nutrient status
should be determined and deficiencies eliminated.
This bulletin has four main goals: (1) to show the
effects of leaf K concentrations on plant-growth-related
processes; (2) to show and describe leaf K deficiency
symptoms in the absence of other nutritional, water, or
disease stresses; (3) to discuss the relationship between
leaf K concentrations and root uptake; and (4) to discuss
tissue sampling for K analysis.
Mississippi Agricultural and Forestry Experiment Station 1
I
NTRODUCTIONPotassium Nutrition
of Cotton
We conducted an experiment in which all the known
factors limiting cotton growth were eliminated. The
Btresistantcotton variety, DP NuCOTN 33B, was grown in
a medium-fine sand, and plants were watered with a halfstrength
nutrient solution three times per day (Hewitt
1952). The plants were grown under natural solar radiation
conditions in temperature-, water-, and nutrient-controlled
growth chambers at the optimum temperature for
growth, 86°F (day) and 72°F (night). All other essential
nutrients (those known to be beneficial) were supplied in
a nutrient solution. The plants were grown under these
optimum conditions until first square. At that stage, the
solution was changed to provide varying amounts of K.
Removing or reducing K from the nutrient solution
resulted in dilution of K in the plant tissues because of
subsequent growth.
Leaf K concentration from the recently fully
expanded topmost leaves was measured weekly and interpolated
so that actual K in the leaves was estimated on a
daily basis. As the concentration of K in the plant tissues
changed, the growth rates of leaves and stems were measured.
Also, photosynthesis was measured throughout each
day, and the rate was related to the K concentration of the
leaves. The data collected in this manner provided information
on the rate of various production-related
processes, including leaf and stem growth, leaf addition
rate, and photosynthesis, as functions of leaf K concentration
when other growth-limiting factors were kept at an
optimum. This is a very sensitive methodology to measure
cotton responses to K, because other growth-limiting
factors were eliminated. Pictures were taken of plants and
individual leaves at various stages of K deprivation.
In a concurrent study, cotton plants were grown outdoors
in 26-inch-deep pots containing sand and were
watered three times per day with a nutrient solution containing
all the essential nutrients for plant growth. At first
square, one set of plants was deprived of K. Two sets of
plants were allowed to grow with full nutrients until
almost first flower and then deprived of K for either 12
days or 29 days. Following those periods of K deprivation,
the full-strength solution was restored to the plants.
As in the first experiment, the plants received all of the
other essential nutrients and water, so that only K was
deficient during the period of K deprivation. This experiment
provided K to plants that were in varying stages of
K deficiency, therefore providing information on recovery
from deficient conditions.
The information provided by these studies should be
particularly useful to those who are attempting to diagnose
the reasons for crops not performing as well as
expected. Fields in which yields are being monitored will
have considerable variability, and the reasons for the variability
often will not be apparent. Leaf K content is one
factor that should be checked as a yield-limiting variable.
Soil fertility status is important, but soil test results may
not provide all the needed answers because of the interaction
of K availability and other factors limiting the plant’s
ability to take up the nutrient. Tissue analysis reflects an
integration of all factors influencing growth and nutrient
uptake.
2 Potassium Nutrition of Cotton
A Closer Look at Potassium Deficiency Symptoms in Cotton
When K was withheld from the nutrient solution after
first square, the plants continued to grow but at a progressively
slower rate (Figure 1). The leaf K concentration
became progressively lower as K content was diluted by
production of dry matter. As K became more limiting,
much of the K in older leaves was translocated to younger
actively growing structures, but even with this reuse of K,
it became limiting within a few days. The old leaves
remained green and appeared healthy, but measurements
of photosynthesis on individual leaves showed that older
leaves with much of the K removed were essentially nonfunctional.
Older leaves expressing deficiency symptoms
were produced with inadequate available K. If leaves
were produced in an adequate-K environment, they apparently
did not develop deficiency symptoms even though
most of the K was subsequently translocated out of those
mature leaves. This finding illustrates the widely held
view that “hidden hunger” can indeed be a production
problem. Marginal K concentration in the upper leaves is
hardly detectable by visual symptoms.
The crop needs to have high K concentrations in its
leaves early in the season, because having only sufficient
K at that time could result in shortages during boll formation.
Later in the season, it is difficult to maintain adequate
K in the leaves because of the heavy requirements
for boll growth. Plant development at this stage is also
M
ATERIALS AND METHODSR
ESULTSMississippi Agricultural and Forestry Experiment Station 3
Figure 1. Potassium Nutrition and Cotton Growth -
Appearance of cottonleaf K deficiency symptoms, related K levels, and the relative rates of growth or
development expressed as a percentage of optimum K (
≥ 3%) in the leaves.Leaf K content Leaf development Leaf growth Stem growth Photosynthesis
≥
3.05% 100% 100% 100% 100%Leaf K content Leaf development Leaf growth Stem growth Photosynthesis
1.9% 90% 86% 100% 93%
Leaf K content Leaf development Leaf growth Stem growth Photosynthesis
1.15% 88% 66% 100% 85%
Leaf K content Leaf development Leaf growth Stem growth Photosynthesis
0.94% 85% 59% 98% 80%
Leaf K content Leaf development Leaf growth Stem growth Photosynthesis
0.39% 83% 37% 42% 45%
Leaf K content Leaf development Leaf growth Stem growth Photosynthesis
0.30% 82% 32% 5% 25%
The rate of leaf area expansion is reduced by K deficiency.
The rate of leaf area expansion increases with
increasing leaf K concentration to a maximum that occurs
at about 3% K (Figures 1 and 3). Although higher concentrations
of leaf K were observed in some conditions,
there was no additional advantage as far as leaf area
expansion rate was concerned. In a production environment,
there may be some advantage in early or mid-season
accumulation of K in plant tissue since it is mobile
and can be utilized later in the season to support additional
growth if drought interferes with absorption. Although the
total amount of K that can be stored in this way is relatively
small, the available K in the plant could provide a
buffer during stress periods.
Leaf growth rates were 14% lower in plants that had
only 1.9% K in the leaves compared to fully fertilized
plants with 3% K in the leaves. Leaf expansion rates
declined even more as K concentration decreased and
were only 59% as great in plants that had approximately
1% leaf K.
The rate of photosynthesis was 7% less in cotton
plants that had 1.9% leaf K compared with well-fertilized
plants (Figures 1 and 3). Plants with 1.9% leaf K looked
as healthy as plants containing more K. It has long been
assumed that 2% leaf K concentration in plants was sufficient;
however, this information suggests that this concentration
is marginal. Both canopy photosynthesis and
leaf growth were lower in plants with 1.9% leaf K compared
with plants with higher concentrations of K. The
young, fully expanded leaves that had only 1% K had
4 Potassium Nutrition of Cotton
complicated by limited root growth relative to the size of
the total plant (Figure 2). As cotton plants age, the ratio of
roots to above-ground parts decreases. This ratio appears
to continue after flowering and contributes to the difficulty
associated with meeting K uptake needs during the
fruiting period. Other studies have found that most nutrients
are absorbed by young, recently formed roots. As
roots age, they become more coarse, heavily suberized,
and lignified. Highly functional roots must continuously
grow new absorbing surfaces, but during the boll-producing
period, root growth slows and nutrient absorption cannot
keep up with the demands.
Figure 1 shows the appearance of
cotton leaves, the percent K in those
leaves, and the relative rate of growth
or development functions expressed as
percentages of the maximum rates
attainable. As leaves on plants developed
in low-K media, their appearance
was influenced by their K content. Progressively
more K-deficient leaves
have the following symptoms:
(1) Early evidence of K deficiency
was a downward curling or cupping of
the upper leaves;
(2) This symptom was followed by
mild mottling and eventually a severe
interveinal chlorosis. It should be noted
that these symptoms appeared in the
absence of disease organisms or deficiencies
of other nutrients;
(3) Necrotic areas at the margins of
the leaves did not appear until the plants
were in an extremely low-K condition for an extended
period; and
(4) Severely deficient cotton leaves without disease
have nearly yellow interveinal areas with pale-green
veins. The margins are often brown. Diseases did not
appear because these plants were isolated, but plants
grown on similarly low-K media in the natural environment
abscised most of their leaves because of foliar diseases.
Potassium deficiency symptoms may be
confounded with disease symptoms of various kinds
because K-deficient plants have increased susceptibility
to infection by microorganisms.
Potassium Effects on Physiology and Growth
Figure 2. Seasonal trends in root:shoot ratio of cotton. Vertical lines indicate the beginning
of squaring, flowering, and boll opening from left to right, respectively, for plants
grown outdoors in large pots.
20% lower canopy photosynthesis rates relative
to the well-fertilized plants (Figure 3).
Research in Arkansas found seedling cotton
leaves had maximum photosynthesis with
approximately 1% K (Oosterhuis and Bednarz
1997). They measured photosynthesis of
individual leaves, whereas we measured photosynthesis
of plants in a closed canopy.
The rates at which main stem leaves and
nodes were added were less in plants with
lower K concentrations (Figure 1). Leaves
were added to the main stem only 90% as fast
in plants whose sampled leaves contained
1.9% K compared with plants with higher K
concentrations. Cotton leaf area expansion
and development, however, are very sensitive
to temperature when water and nutrients are
not limiting (Hodges et al. 1993; Reddy et al.
1996). This small difference in association
with lower K concentration would not be
detected in most field situations, which is
another example of plants’ “hidden hunger” being masked
by other environmental factors.
Other research has found that in well-fertilized fields
that were not irrigated, the K concentration of cotton
leaves increased until about 3 weeks after flowering, then
declined rapidly (Hsu 1976). However, irrigated cotton
that was well-fertilized did not show a decline in leaf K
concentration throughout the fruiting period (Bennett et
al. 1965). We observed a similar phenomenon in a wellfertilized
crop grown with optimum nutrient solutions. In
crops exposed to drought, K nutrition during the fruiting
period is apparently closely linked to water supply. Potassium
uptake is associated with total root length and root
growth rates. During the fruiting period, there is severe
competition between roots and the increasing boll load for
both K and photosynthates. If water supply becomes limited
during that period, the uptake of K is insufficient to
meet fruit-load needs. This deficit causes a withdrawal of
K from leaves and a related slowing of growth processes.
In a non-limiting nutrient solution environment, K
concentration in the leaves remained stable during growth
even to maturity. This finding was true even with a large
population of bolls on all the plants. However, under field
conditions in which dry soil is sometimes a problem, the
K concentration in leaves changes during the season.
When leaf concentrations of potassium are low during dry
conditions, interpretation of fertility needs becomes more
complicated; it is unclear whether low leaf K is caused by
low soil K or drought. According to Hsu (1979), fieldgrown
cotton leaf K concentrations increased with age
until 3 weeks after flowering began. At that time, the
nutritional requirements of the bolls exceeded the ability
of the plants to take up nutrients, causing the leaf K concentration
to decrease. Presumably, this phenomenon was
caused by a combination of less available water and the
fact that there were fewer fine roots to support the aboveground
plant. As the boll load increased, there were fewer
energy-bearing compounds available to support growth of
new roots capable of taking up the nutrients needed.
The relative amount of roots compared with the
whole plant weight decreases throughout the season (Figure
2). As K becomes limiting, its concentration in the
mature leaves (both old and young) decreases. The K concentration
in the immature leaves follows the same trend.
One possible cause for this trend was seen in our solutionculture-
grown plants when K was not provided. In those
plants, when uptake was low, K was removed from the
leaves to support growth, causing leaf concentration to
decline.
Lower leaf K concentration affects several physiological
processes (Figures 2 and 3). At the time mineral nutrients
are being depleted in the leaves, the need for
photosynthetically produced sugar is also reaching a peak
for the same reason: energy is needed to support fruit
growth. Additional flowers and bolls require both minerals
and sugars. As their requirements exceed the supply,
less K is available to support growth and functioning of
roots. Healthy root cell membranes are effective barriers
to nutrient passage, and therefore energy is also required
for K uptake. Thus, energy provided by the respiration of
Mississippi Agricultural and Forestry Experiment Station 5
Figure 3. Cotton canopy net photosynthesis and leaf area expansion as a
function of leaf K concentration. Plants were grown in optimum day/night
(86/72°F) temperatures in a range of K-deficient conditions but with sufficient
water and other macro and micro nutrients. Potassium was determined in the
uppermost fully expanded leaves.
sugars is essential for K movement from the soil into the
root.
As rapidly growing cotton plants develop more bolls,
it becomes progressively more difficult to support their
growth. The reproductive parts appear to have a higher
priority for available carbon and other nutrient resources.
They survive at the expense of roots and other vegetative
plant parts. Stem growth becomes incrementally slower.
The addition of new leaves slows, and the leaves also
become progressively smaller due to the lack of nutrients
(both minerals and sugar). This process is a natural maturing
of cotton known as “cutout.” The cutout occurs earlier
when nutrients or water is limiting, causing fewer young
bolls to survive. Excessively hot weather also compounds
nutrient-deficit problems in cotton. Hot weather causes
bolls to form faster, which causes cutout to occur earlier.
Higher temperatures also increase respiration, which
requires the plant to consume even more energy. However,
excessively hot weather causes young bolls to drop
because of high-temperature injury, and the available
energy may go into producing vegetative growth.
To determine the potential dynamics of K concentration
in cotton plants, we also grew plants outdoors in sand
with nutrient solution added daily. These studies showed
that increasing K concentration above the amount that
resulted in additional growth caused increased concentration
of K in the leaves (Figure 4). This phenomenon is
called “luxury consumption.” When K was withheld from
the solution, the K concentration in the
leaves decreased in relation to the degree
and length of time of K starvation. If K was
completely removed from the solution, the
K concentration in the leaves decreased to
30% of the well-fertilized status within 10
days. If the starvation was continued for 29
days, the leaf K concentration decreased to
only 17% of the well-fertilized condition.
When K was restored to the nutrient media,
the plants required about the same time to
recover (10 or 29 days) as they did to
deplete K in the starved environment (Figure
4). This finding suggests that if plants
have a marginal K nutritional status and the
growing conditions become worse, an
immediate management action such as irrigation
is needed.
These findings illustrate the need for
timely tissue sampling for K analysis and
the appropriate interpretation of the results.
Our opinion is that leaves should be checked
for K concentration at the beginning of
flowering. Young mature leaves may require as much as
3% K at that time. This amount is higher than the critical
values stated elsewhere in this bulletin (2.1% for photosynthesis
and 2.5% for leaf growth), but 3% is considered
a more reasonable value to have at flowering, because leaf
K concentration decreases with age and boll growth.
Oosterhuis (1993), extracting results from several published
sources, reported petiole K concentrations
decreased from 4% at first flower to 3% at peak flowering,
2% at first open boll, and 1% just before harvest.
These petiole values are equivalent to only 1.38% leaf K
at first flower, 0.99% at peak flowering, 0.71% at first
open boll, and 0.56% at harvest calculated from the relationships
shown in Figure 6. Obviously, metabolic rates at
these lower values will be seriously limited by K late in
the season. If plants are sufficiently well-fertilized and
well-irrigated, they will not go through “cutout” unless
insufficient sugar is available to support all the possible
growth.
Potassium-deficient plants sometimes die prematurely,
because they are more susceptible to diseases and
nematodes. Inadequate K also limits growth, which may
cause early “cutout” and give the appearance of early
maturity. We determined the length of time required from
flowering to open bolls and found no delays caused by
high-K nutrition. However, plants grown under high-K
nutrition continue to produce flowers and set bolls over a
longer period, requiring more time to mature all the bolls.
6 Potassium Nutrition of Cotton
Figure 4. Influence of potassium nutrition on leaf K concentrations for plants
grown in pots outdoors. The plants were grown in optimum water and nutrient
solution culture up to flowering, and then potassium from the nutrient solution
was withdrawn at various stages. The arrow on the left indicates the beginning
of K starvation for treatments 2 and 3. The arrows at the middle and at the right
indicate the restoration of K to normal nutrient solution for treatments 2 and 3,
respectively.
Since plant structures vary in the amounts of K they
contain, selecting an appropriate tissue for analysis is an
important issue. Generally, young mature leaves have
higher K concentrations than old leaves or leaves that are
still growing. As plants grow and the amount of roots relative
to above-ground parts decreases, the percentage of K
in the above-ground parts increases. If the available K is
high, the concentration in above-ground parts may
remain about constant as plants age. The interpretation
of plant analysis results depends on both the time
of testing the samples and the parts analyzed.
Since the leaf blades are so important to light
interception and dry matter production, we concluded
that leaf nutritional status and function are of
primary concern. Therefore, young mature leaves
near the top of the plant should be sampled to represent
the nutritional health of the crop. The fourth or
fifth leaf from the main stem terminal is usually the
youngest fully expanded leaf, is physiologically the
most active leaf, and is therefore the appropriate leaf
to sample. Main stem leaves reach full size after
about 16 days (the time required for four to five additional
leaves to be added from the main stem).
This conclusion is also in agreement with many
other research reports, but it does conflict with the
conventional wisdom. Previous research showed that
leaf petioles have higher concentrations of K than
leaf blades and that they have the widest range of K
concentration of any structures (Hsu 1979). Based on
this finding, many people use petiole samples to
determine the K nutritional status of their crops. Petioles
function as a nutrient conduit and apparently temporarily
store small amounts of K. Indeed, there is a close correlation
between leaf petiole K and leaf blade K concentrations
(Figure 6). Potassium concentration in young mature
leaves has been shown to decrease earlier than in old
leaves in K-deficient, field-grown plants (Hsu 1979).
Bennett et al. (1965) reported on a study in
which cotton was grown with irrigation and
six levels of K fertilization. The land had been
subsoiled, fumigated to control nematodes,
and fertilized with other elements to avoid
deficiencies. Cotton was seeded early, sampled
for K at approximately first flowering and
30 days after first flowering, and hand picked
in early September. They found a close relationship
between whole-plant K and yield
(Figure 5). Even with the high yields (four
bales), they reported severe K deficiency
symptoms in leaves containing less than 1.5%
K. Their results are very consistent with the
responses we observed between leaf K and
photosynthesis (Figure 3).
Leaf K and Cotton Yields
Mississippi Agricultural and Forestry Experiment Station 7
When and What Plant Parts to Sample
Figure 5. Relation between yield and potassium content of above-ground
cotton plants (redrawn from Bennett et al. 1965).
Figure 6. The relationship between cotton leaf blade K concentration
and leaf petiole K concentration. The solid line represents data collected
from plants grown in nutrient solutions (y = 0.174 + 3.476 * X -
0.437 * X
2, r 2 = 0.89). The broken line represents 372 data points from57 locations in 2 years of field-grown plants (y = 0.149 + 3.1974 * X -
0.434 * X
2, r 2 = 0.61, Hsu, 1979). The field data points are not shownto avoid complexity of presentation.
Thurow (1997) points out that the lack of “onthe-
go” soil sensors for nutrient management
remains an important void in precision agriculture.
New electronic field diagnostic tools are being
developed for use in nutrient management, but there
is no substitute for a knowledgeable person to scout
fields for crop health. The chlorophyll meter
(SPAD-502) was developed by Minolta Company
as a tool to manage N status of crops. Several
researchers found a strong correlation between the
meter’s leaf chlorophyll measurements and leaf N
content. We compared SPAD readings on plants
varying widely in K content (Figure 7) and found
the SPAD meter readings are not sufficiently sensitive
to detect K deficiency symptoms in cotton. This
instrument detects differences only when leaf K
concentrations are below 1%. At that concentration,
it is too late to correct the problem. A person can
visually detect the nutritional problem before the
leaves reach such a low concentration.
High yield and quality of cotton requires healthy vigorous
plants throughout the season. Several investigators
have reported an association of K deficiency symptoms
and the incidence of verticillium wilt (Adeli 1994 and references
cited therein). Cassman (1994) points out that the
symptoms caused by K deficiency are sometimes mistakenly
attributed to verticillium wilt. He believes the two
symptoms are distinct and identifiable. Potassium deficiency
symptoms are often recognized as bronze-colored
leaves with necrosis occurring along the margins without
a clear border (Figure 1). Conversely, verticillium wilt
causes necrotic lesions with well-defined borders and rich
brown color between leaf veins. Also, when the stems are
split open with a knife, brown staining of the interior
xylem indicates verticillium wilt. Broadcasting K fertilizer
reduced verticillium wilt symptoms in one Mississippi
Delta study. In our enclosed chambers, we did not
find any disease symptoms even on severely K-deficient
plants (Figure 1). Plants with similar K levels grown outdoors,
however, prematurely lost all of their leaves due to
foliar disease.
8 Potassium Nutrition of Cotton
Figure 7. The relationship between SPAD readings and leaf
K concentrations for plants grown in optimum day/night temperatures
and at a range of potassium-deficient conditions.
Is a Chlorophyll Meter Right for Detecting K Deficiency?
Potassium Deficiency and Diseases
Potassium deficiency symptoms of cotton are often
seen late in the growing season. It is well known that yield
responses to other agronomic inputs are limited if any
essential nutrient is insufficient. Since K is required by
cotton in relatively large quantities and deficiency symptoms
are common, a clear definition of crop sensitivity to
tissue K concentration seems important. Also, as we
become more concerned with identifying reasons for
lower production in certain areas of a field, we will need
to know if K is a limiting factor.
Recent advances in remote spectral imaging of crops
should improve our capability for mapping K-deficient
areas within fields. This information may be coupled with
variable rate fertilizer applications to increase precision in
fertilization. This study defined physiological processes
as a function of leaf K concentration when other production
factors were not limiting.
It is reasonable to expect crop productivity to closely
reflect an integrated status of the various processes during
the growing season. Reporting yield data directly from
this study seems inappropriate, because the treatment area
was small and the experiment was canceled before many
healthy bolls reached maturity. However, in both experiments,
boll parameters (size, seed, and lint weight per
boll), boll numbers, boll weight per plant, and percent boll
retention were closely related to the K nutrition treatments
(data not shown). Therefore, it seems reasonable to interpret
the vigor of the physiological processes and growth
parameters directly influenced by leaf K concentration to
yield. We reached the following conclusions:
Mississippi Agricultural and Forestry Experiment Station 9
S
UMMARY AND CONCLUSIONS•
Growth processes are limited when leaf K concentrations arebelow 2%, and visual symptoms of K deficiencies are difficult to
identify. Critical foliar K concentration required for optimum photosynthesis,
and thus the productivity of cotton (95% of the
maximum), is 2.1%. Several processes are severely affected
below that critical foliar K level.
•
Early evidence of K deficiency is a downward cupping of theupper leaves and a mild mottling of those leaves.
•
Leaf growth is the most sensitive physiological process to K-deficientconditions, and it increased as foliar K increased up to 3%.
However, 2.5% is the critical foliar K concentration, a requirement
for optimum leaf growth and thus canopy development
(95% of the maximum).
•
The most practical way to avoid K deficiency is to provide adequatefertilizer to the soil. Only small amounts can be supplied
by foliar feeding. Foliar feeding is essentially a stop-gap procedure
that may be used in an emergency.
•
Potassium-deficient plants are more susceptible to plant diseasesand symptoms may be readily confused.
Appreciation is expressed for the excellent technical
assistance provided by Gary Burrell, Kim
Gourley, Wendell Ladner, and Sam Turner. We thank
Dr. Lyle E. Nelson for review and comments on the
manuscript. Part of the research was funded by the
U.S. Department of Energy National Institute for
Global Environment Change through the South Central
Regional Center at Tulane University (DOE
cooperative agreement no. DE-FCO3-90ER 61010)
and the National Aeronautical and Space Administration-
funded Remote Sensing Technology Center
at Mississippi State University (NASA grant number
NCC13-99001).
10 Potassium Nutrition of Cotton
A
CKNOWLEDGEMENTSR
EFERENCESAdeli, A.
1994. Potassium management effects on cotton yield, nutrition and soil test level. Ph.D. dissertation,Mississippi State University, Mississippi State, Mississippi.
Bennett, O.L., R.D. Rouse, and D.A. Ashley.
1965.Yield, fiber quality and potassium content of irrigatedcotton plant as affected by rate of potassium. Agron. J. 57: 296-299.
Cassman, K.G.
1994. Cotton. In: Nutrient Deficiencies and Toxicities in Crop Plants, ed.W. F. Bennett,p. 111-119. APS Press, The American Phytopathological Society, St. Paul, Minnesota.
Hewitt, E.J.
1952. Sand and water culture methods used in the study of plant nutrition. In: C.A.B. Tech.Commun. 22, p. 189. Commonwealth Agric. BAR., Farnham Royal, England. U.K.
Hodges, H.F., K.R. Reddy, J.M. McKinion, and V.R. Reddy.
1993. Temperature effects on cotton. Bulletin990, Mississippi Agricultural and Forestry Experiment Station, Mississippi State, Mississippi.
Hsu, H.H.
1976. Potassium soil test calibration for cotton. Master’s thesis, Mississippi State University,Mississippi State, Mississippi.
Hsu, H.H.
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