| Diagnosing
a micronutrient deficiency can be a difficult and time
consuming process. To identify a micronutrient deficiency
follow these steps: |
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Ensure
that poor crop growth is not the result of a macronutrient
deficiency, drought, salinity, disease or insect
problem, herbicide injury or some physiological
problem. |
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Find
out if a micronutrient deficiency has been identified
before in a particular crop or soil type in the
area. |
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Examine
the affected crop for specific micronutrient deficiency
symptoms. |
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Take
separate soil samples from both the affected and
unaffected areas for complete analysis, including
micronutrients. |
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Send
plant tissue samples from both the affected and
unaffected areas for complete analysis that includes
tests for micronutrient levels. |
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If
all indications point to a micronutrient deficiency,
apply the micronutrient to a specific, clearly marked
out affected area of land to observe results in
subsequent seasons. |
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| M
a c r o n u t r i e n t s |
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| Nitrogen
is probably the nutrient that most often limits plant
growth. The bulk of soil N is found within 2 feet of the
surface. Soil N is present in three major forms. |
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found
in a gaseous form in the soil atmosphere, is of
direct significance to plants only as it may be
involved in bacterial fixation. (e.g., symbiotic
N fixation associated with a legume plant or small
amounts of N fixation by free-living bacteria).
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makes
up about 5 percent of the soil organic matter (humus)
by weight and about 98 percent of the total soil
N. Although organic N is not available to plants,
soil organisms convert a portion of it each year
to inorganic forms (ammonium and nitrate) that are
readily used by plants. Organic N fertilizers (e.g.,
manures and biosolids) are popular for lawns and
gardens because of their "slow release"
and long-lasting properties. The relatively low
concentration of N in organic materials means several
tons per acre are required to supply sufficient
N for commercial field crop production. The economics
of transporting these bulky materials are a major
factor when considering their use as an N source.
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for
agricultural crops is largely inorganic N consisting
of three types: ammonium (NH 4 + ), nitrate (NO
3 - ), and urea (CO(NH 2 ) 2 ). Although urea is
an organic N fertilizer, it is rapidly converted
to the ammonium form within a short time after exposure
to moist, aerated soil. Therefore, under most conditions
urea acts more like inorganic ammonium fertilizers
than like natural organic fertilizers. |
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In
warm, moist soils with a pH above 5.0, the majority
of ammonium N is converted to nitrate N by soil
organisms rather quickly (within days). Therefore,
most N taken up by plants is in the NO 3 - form,
although NH 4 + is taken up when present in the
soil solution. Thenitrate ion (NO 3 - ) carries
a negative charge which prevents its retention by
the negatively- charged soil colloids. Since it
is soluble and mobile, the nitrate ion is readily
and easily available to plants. |
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Nitrate
moves in the soil solution and can be leached below
the plant root zone when soil moisture is excessive.
The loss of nitrate by leaching is a common problem
on coarse-textured, sandy soils of Florida. Leaching
losses of fertilizer N are minimal when rates of
application conform to recommendations consistent
with the yield potential for the crop and soil in
question. Nitrate N is also subject to denitrification,
a process in which the nitrate ion (NO 3 - ) is
reduced through several intermediate steps to a
gaseous N oxide or to elemental N. |
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is
a constituent of all living cells and is a necessary
part of all proteins, enzymes and metabolic processes
involved in the synthesis and transfer of energy.
Nitrogen is a structural part of chlorophyll, the
green pigment of the plant that is responsible for
photosynthesis. The energy of light is combined
with water and carbon dioxide through the process
of photosynthesis to form simple carbohydrates essential
for plant growth. Other functions of N include stimulating
plants into rapid, vigorous growth, increasing seed
and fruit yield and improving the quality of leaf
and forage crops. |
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Like
N, phosphorus (P) is an essential part of the process
of photosynthesis. Plants use the energy of sunlight,
and P must be present in the active portions of
the plant for this energy transfer to be made and
for photosynthesis to occur. |
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The
immediate source of P for plants is that which is
dissolved in the soil solution. Plants absorb P
primarily as the H 2 PO 4 - and HPO 4 = ions which
are predominant in most soils. The H 2 PO 4 - ion
is more readily absorbed than the HPO 4 = by most
plants. A soil solution containing only a few parts
per million of phosphate ions is usually considered
adequate for plant growth. Concentrations of phosphate
ions in the soil solution may be as low as 0.001
parts per million. Phosphate ions are absorbed from
the soil solution and used by plants. The soil solution
is replenished fromsoil minerals, soil organic matter
decomposition or applied fertilizers. |
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In
young plants, P is most abundant in tissue at the
growing point. It is readily translocated (moved
about) from older tissue to younger tissue, and
as plants mature, most of the element moves into
the seeds and/or fruits. P is responsible for such
characteristics of plant growth as utilization of
starch and sugar, cell nucleus formation, cell division
and multiplication, fat and albumin formation, cell
organization, and transfer of heredity. |
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is
absorbed by plants in larger amounts than any other
mineral element except N and, in some cases, Ca.
Potassium is supplied to plants by soil minerals,
organic materials, and inorganic fertilizer. Due
to the highly weathered status of Florida soils,
their K supplying power is quite low in most cases.
Potassium occurs in the soil solution as a monovalent
cation (K + ). The cation exchange capacity (CEC)
of the soil controls the retention of K + and in
very sandy soils (low cation exchange capacity)
under high rainfall, K is subject to leaching losses.
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Potassium,
unlike N and P, is not found in organic combination
with plant tissues. Potassium plays an essential
role in the metabolic processes of plants and is
required in adequate amounts in several enzymatic
reactions, particularly those involving the adenosine
phosphates (ATP and ADP), which are the energy carriers
in the metabolic processes of both plants and animals.
Potassium also is essential in carbohydrate metabolism,
a process by which energy is obtained from sugar.
There is evidence that K also plays a role in photosynthesis
and protein synthesis. |
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Calcium
(Ca) occurs in the soil solution as a divalent cation
(Ca ++ ). It is supplied to plants by soil minerals,
organic materials, fertilizers, and by liming materials.
There is a strong preference for Ca ++ on the cation
exchange sites of most soils and it is the predominant
cation in most soils with a pH of 6.0 or higher.
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Calcium,
an essential part of plant cell wall structure,
provides for normal transport and retention of other
elements as well as strength in the plant. It is
also thought to counteract the effect of alkali
salts and organic acids within a plant. Calcium
is absorbed as the cation Ca ++ and exists in a
delicate balance with Mg and K in the plant. Too
much of any one of these elements may cause insufficiencies
of the other two. |
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Soil
minerals, organic material, fertilizers, and dolomitic
limestone are sources of magnesium (Mg) for plants.
Magnesium occurs as a divalent cation (Mg ++ ) and
is held on the exchange sites like calcium (Ca ++
) and potassium (K + ). |
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Magnesium
is part of the chlorophyll in all green plants and
essential for photosynthesis. It also helps activate
many plant enzymes needed for growth. Magnesium,
a relatively mobile element in the plant, is absorbed
as the cation Mg ++ and can be readily translocated
from older to younger plant parts in the event of
a deficiency. |
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In
most soils, sulfur (S) is present primarily in the
organic fraction which becomes available upon decomposition
of organic matter and crop residues. The available
sulfate (SO 4 = ) ion remains in soil solution much
like the nitrate (NO 3 - ) ion until it is taken
up by the plant. In this form it is subject to leaching
as well as microbial immobilization. In water-logged
soils, it may be reduced to elemental S or other
unavailable forms. Sulfur may be supplied to the
soil from the atmosphere in rainwater. It is also
added in some fertilizers as an impurity, especially
the lower grade fertilizers. The use of gypsum (CaSO
4 ) also increases soil S levels. |
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Sulfur
is taken up by plants primarily in the form of sulfate
(S0 4 = ) ions and reduced and assembled into organic
compounds. It is a constituent of the amino acids
cystine, cysteine, and methionine and, hence, proteins
that contain these amino acids. It is found in vitamins,
enzymes and coenzymes. |
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Sulfur
is also present in glycosides which give the characteristic
odors and flavors in mustard, onion, and garlic
plants. It is required for nodulation and Nfixation
of legumes. As the sulfate ion, it may be responsible
for activating some enzymes. |
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Of
the 16 elements known to be essential for plant
growth, seven are required in such small quantities
that they are referred to as "micronutrients".
These are Fe, Mn, Zn, Cu, B, Mo, and Cl. |
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Micronutrient
deficiencies are most apt to limit crop growth under
the following conditions: (1) highly leached acid
sandy soil, (2) muck soils, (3) soils high in pH
or lime content, and (4) soils that have been intensively
cropped and heavily fertilized with macronutrients.
Four of the micronutrients occur predominantly as
cations in the soil solution. They are iron (Fe
+++ ), copper (Cu ++ ), manganese (Mn ++ ), and
zinc (Zn ++ ). Two occur predominantly as anions.
These are molybdenum (MoO 4 = ) and chlorine (Cl
- ). Boron occurs as the neutral species H 3 BO
3 . |
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