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Petiole Analysis
– An Important Tool in Cotton |
8
Cotton growers enrolled in the BAISC program for 2002 were
provided with two petiole nitrate samplings. The first sample
was selected and tested in early July and the second in
mid-August. Research has shown that petiole analysis can
provide a way to monitor the nutritional status of your
cotton.
The Right Amount?
For high yields, cotton must have the right amount of nitrogen
during all phases of growth and fruit development. But the
questions for farmers always remains what is the right amount
of nitrogen to apply? Depending on the soil and climatic
conditions it is easy to have too much or too little nitrogen
available for cotton. Due to the nature of the soil nitrogen
and the manner in which cotton responds to it, nitrogen
has a greater potential for increasing or decreasing yield
than any other nutrient.
Too little nitrogen leads to small stalks,
pale green to yellow leaves, small bolls, fruit shed and
low yields. If low nitrogen is detected early enough during
the growing season, proper nitrogen application can mean
sizeable yield increases.
Too much nitrogen delays maturity, causes
rank growth, encourages diseases and increases the risk
of boll rot and reduced lint quality. Also at critical periods,
excessive nitrogen can cause cotton plants to shed pinhead-sized
squares.
Seasonal Variations
Cotton response to nitrogen fertilization varies from one
year to another, primarily due to climate changes. An optimum
amount of preplant fertilizer nitrogen in a “normal”
year may be excessive in a “wet” year and deficient
in a “dry” year. Researchers have found that
by monitoring the nitrate-nitrogen content of cotton peitioles
(leaf stems), adjustments can be made in the nitrogen fertilization
program during the growing season to compensate for seasonal
variations. Such adjustments aid in achieving top yields
by avoiding excess nitrogen in early and late season and
insuring adequate nitrogen during crucial fruiting periods.
Monitoring
BASIC field scouts began about a week before the first bloom,
just when the white tip of the first blooms were seen emerging
from the oldest squares. They sampled only primary leaves
on the main stem and avoided taking leaves from fruiting
or vegetative branches. 25 to 35 leaf petioles (leaves are
discarded) were taken from four different quadrants in the
field. They were labeled and taken to Denele Agri-Link Laboratory
for analysis. Analysis consists of chemically monitoring
the nitrate-nitrogen and phosphorus content of cotton petioles.
Growers then received a computer printout which graphically
shows the nitrogen and phosphorus content along with any
appropriate recommendations.
Petiole nitrate-nitrogen and phosphorus levels serve as
indicators of the relative amounts of unused nitrogen and
phosphorus in the plants. Both elements are potential components
of amino acids and proteins. In densely fruiting cotton
through about the fifth week of blooming, there is an inverse
relationship between the nitrate and phosphorus levels.
As nitrates increase, phosphorus tends
to decrease. As phosphorus increases, nitrates tend to decrease.
For example:
- When nitrates are decreasing and phosphorus
in increasing, this is an indication of adequate moisture,
heavy fruiting and rapid use of nitrogen.
- When both nitrates and phosphorus are decreasing,
this is an indication of drought stress.
- When there is a sharp increase in both nitrates
and phosphorus, this is a response to above normal moisture
conditions. Fruiting may or may not be good. Conditions
are conducive to insect damage.
- When nitrates are increasing and phosphorus
is decreasing, this indicates that moisture is adequate,
fruiting is poor, and fruit loss is possible.
Petiole analysis will indicate a need for
nitrogen about two weeks prior to the appearance of plant
symptoms. If petiole nitrate-nitrogen is low during the
first three weeks of blooming, a soil application, foliar
application or both would be recommended. Urea has been
found to be an effective and safe source of nitrogen to
apply to a developing cotton plant. Leaf and petiole analyses
are most reliable when moisture and other stress-related
factors are not influencing growth. Care should be taken
to note recent growing conditions along with sampling
(BASIC does this)
This article was prepared by C. Owen Plank,
Extension Agronomist, University of Georgia Extension Service.
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