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Efficient Fertilizer Use -- Micronutrients: Dr. John Mortvedt


SECTION CONTENTS:
· Introduction
· Micronutrient Nutrition
· Micronutrient Fertilizer Sources
· Application with Mixed Fertilizers
· Foliar Sprays
· Micronutrient Rates & Example Results

Micronutrients Micronutrients are elements which are essential for
plant growth, but are required in much smaller amounts than those of
the primary nutrients, nitrogen, phosphorus and potassium. The
micronutrients are boron (B), copper (Cu), iron (Fe), manganese (Mn),
molybdenum (Mo), zinc (Zn), and chloride (Cl). While chloride is a
micronutrient, deficiencies rarely occur in nature, so discussions on
supplying micronutrient fertilizers are confined to the other six
micronutrients.

Deficiencies of micronutrients have been increasing in some
crops. Some reasons are higher crop yields which increase plant
nutrient demands, use of high analyses NPK fertilizers containing
lower quantities of micronutrient contaminants, and decreased use of
farmyard manure on many agricultural soils. Micronutrient deficiencies
have been verified in many soils through increased use of soil testing
and plant analyses.

Micronutrient Nutrition A brief discussion of micronutrient functions
and nutrient deficiency symptoms in plants and soil conditions
affecting micronutrient availability serves to help understand their
importance in crop production and to recognize symptoms of possible
deficiencies.

Boron

A primary function of boron is related to cell wall formation, so
boron-deficient plants may be stunted. Sugar transport in plants,
flower retention and pollen formation and germination also are
affected by boron. Seed and grain production are reduced with low
boron supply. Boron-deficiency symptoms first appear at the growing
points. This results in a stunted appearance (rosetting), barren ears
due to poor pollination, hollow stems and fruit (hollow heart) and
brittle, discolored leaves and loss of fruiting bodies.

Boron deficiencies are mainly found in acid, sandy soils in regions of
high rainfall, and those with low soil organic matter. Borate ions are
mobile in soil and can be leached from the root zone. Boron
deficiencies are more pronounced during drouth periods when root
activity is restricted.




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Efficient Fertilizer Use -- Micronutrients: Dr. John Mortvedt


Copper

Copper is necessary for carbohydrate and nitrogen metabolism, so
inadequate copper results in stunting of plants. Copper also is
required for lignin synthesis which is needed for cell wall strength
and prevention of wilting. Deficiency symptoms of copper are dieback
of stems and twigs, yellowing of leaves, stunted growth and pale green
leaves that wither easily.

Copper deficiencies are mainly reported on organic soils (peats and
mucks), and on sandy soils which are low in organic matter. Copper
uptake decreases as soil pH increases. Increased phosphorus and iron
availability in soils decreases copper uptake by plants.

Iron

Iron is involved in the production of chlorophyll, and iron chlorosis
is easily recognized on iron-sensitive crops growing on calcareous
soils. Iron also is a component of many enzymes associated with energy
transfer, nitrogen reduction and fixation, and lignin formation. Iron
is associated with sulfur in plants to form compounds that catalyze
other reactions. Iron deficiencies are mainly manifested by yellow
leaves due to low levels of chlorophyll. Leaf yellowing first appears
on the younger upper leaves in interveinal tissues. Severe iron
deficiencies cause leaves to turn completely yellow or almost white,
and then brown as leaves die.

Iron deficiencies are found mainly on calcareous (high pH) soils,
although some acid, sandy soils low in organic matter also may be
iron-deficient. Cool, wet weather enhances iron deficiencies,
especially on soils with marginal levels of available iron.  Poorly
aerated or compacted soils also reduce iron uptake by plants. Uptake
of iron decreases with increased soil pH, and is adversely affected by
high levels of available phosphorus, manganese and zinc in soils.

Manganese

Manganese is necessary in photosynthesis, nitrogen metabolism and to
form other compounds required for plant metabolism. Interveinal
chlorosis is a characteristic manganese-deficiency symptom. In very
severe manganese deficiencies, brown necrotic spots appear on leaves,
resulting in premature leaf drop. Delayed maturity is another
deficiency symptom in some species. Whitish-gray spots on leaves of
some cereal crops and shortened internodes in cotton are other
manganese-deficiency symptoms.

Manganese deficiencies mainly occur on organic soils, high-pH soils,
sandy soils low in organic matter, and on over-limed soils. Soil
manganese may be less available in dry, well-aerated soils, but can
become more available under wet soil conditions when manganese is
reduced to the plant-available form. Conversely, manganese toxicity
can



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Efficient Fertilizer Use -- Micronutrients: Dr. John Mortvedt


result in some acidic, high-manganese soils. Uptake of manganese
decreases with increased soil pH and is adversely affected by high
levels of available iron in soils.

Molybdenum

Molybdenum is involved in enzyme systems relating to nitrogen fixation
by bacteria growing symbiotically with legumes. Nitrogen metabolism,
protein synthesis and sulfur metabolism are also affected by
molybdenum. Molybdenum has a significant effect on pollen formation,
so fruit and grain formation are affected in molybdenum-deficient
plants. Because molybdenum requirements are so low, most plant species
do not exhibit molybdenum-deficiency symptoms. These deficiency
symptoms in legumes are mainly exhibited as nitrogen-deficiency
symptoms because of the primary role of molybdenum in nitrogen
fixation. Unlike the other micronutrients, molybdenum- deficiency
symptoms are not confined mainly to the youngest leaves because
molybdenum is mobile in plants. The characteristic
molybdenum-deficiency symptom in some vegetable crops is irregular
leaf blade formation known as whiptail, but interveinal mottling and
marginal chlorosis of older leaves also have been observed.

Molybdenum deficiencies are found mainly on acid, sandy soils in humid
regions.  Molybdenum uptake by plants increases with increased soil
pH, which is opposite that of the other micronutrients. Molybdenum
deficiencies in legumes may be corrected by liming acid soils rather
than by molybdenum applications. However, seed treatment with
molybdenum sources may be more economical than liming in some areas.

Zinc

Zinc is an essential component of various enzyme systems for energy
production, protein synthesis, and growth regulation. Zinc-deficient
plants also exhibit delayed maturity. Zinc is not mobile in plants so
zinc-deficiency symptoms occur mainly in new growth. Poor mobility in
plants suggests the need for a constant supply of available zinc for
optimum growth. The most visible zinc-deficiency symptoms are short
internodes (rosetting) and a decrease in leaf size. Chlorotic bands
along the midribs of corn, mottled leaves of dry bean and chlorosis of
rice are characteristic zinc-deficiency symptoms. Loss of lower bolls
of cotton and narrow, yellow leaves in the new growth of citrus also
have been diagnosed as zinc deficiencies. Delayed maturity also is a
symptom of zinc-deficient plants.

Zinc deficiencies are mainly found on sandy soils low in organic
matter and on organic soils. Zinc deficiencies occur more often during
cold, wet spring weather and are related to reduced root growth and
activity as well as lower microbial activity decreases zinc release
from soil organic matter. Zinc uptake by plants decreases with
increased soil pH. Uptake of zinc also is adversely affected by high
levels of available phosphorus and iron in soils.




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Efficient Fertilizer Use -- Micronutrients: Dr. John Mortvedt


Chloride

Because chloride is a mobile anion in plants, most of its functions
relate to salt effects (stomatal opening) and electrical charge
balance in physiological functions in plants.  Chloride also
indirectly affects plant growth by stomatal regulation of water
loss. Wilting and restricted, highly branched root systems are the
main chloride-deficiency symptoms, which are found mainly in cereal
crops.

Most soils contain sufficient levels of chloride for adequate plant
nutrition. However, reported chloride deficiencies have been reported
on sandy soils in high rainfall areas or those derived from
low-chloride parent materials. There are few areas of chloride-
deficient soils in the U. S., so this micronutrient generally is not
considered in fertilizer programs. In addition, chloride is applied to
soils with KCl, the dominant potassium fertilizer. The role of
chloride in decreasing the incidence of various diseases in small
grains is perhaps more important than its nutritional role from a
practical viewpoint.

Plants differ in their requirements for certain micronutrients. The
following table shows the estimate of the relative response of
selected crops to micronutrients. The ratings of low medium and high
are used to indicate the relative degree of responsiveness.




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Efficient Fertilizer Use -- Micronutrients: Dr. John Mortvedt


Crop Response to Micronutrients

Table 9.1: Relative Responsiveness of Selected Crops to Micronutrients

Crop                 B          Cu         Mn         Zn
Alfalfa              High       Med        Low        Med
Apples               High       Med        Low        Med
Sugar Beet           High       Low        Low        Med
Cabbage              Med        Low        Med        Low
Citrus               Med        High       Med        Med
Clover               Med        Med        Low        Med
Corn                 Med        Low        Low        High
Cotton               High       Low        High       Med
Grain Sorghum        Low        Med        Med        High
Grass                Low        Low        Low        Med
Lettuce              Med        High       High       Med
Oat                  Low        Med        High       Med
Onion                Low        High       High       High
Peach                Med        Med        Med        High
Peanut               High       Low        Med        Low
Pecan                Med        Low        Low        High
Potato, Irish        Low        Med        Med        High
Potato, Sweet        High       Low        High       Med
Rye                  Low        Low        Low        Med
Soybean              Low        Med        High       Med
Tobacco              Med        Low        Med        Med
Tomato               High       High       Med        Med
Wheat                Low        High       High       Low


Micronutrient Fertilizer Sources
Micronutrient sources vary considerably in their physical state, chemical 
reactivity, cost,
and availability to plants. Some of the commonly used micronutrient sources 
are shown
in Table 9.2.




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Efficient Fertilizer Use -- Micronutrients: Dr. John Mortvedt


Table 9.2: Micronutrient Fertilizer Sources

Source                    Solubility in H2O     Percent Element
                               Boron
                                                       17
H3BO3                          Soluble
                                                       20
Na2B4O7.5H2O                   Soluble
                                                       11
Na2B4O7.10H2O                 Soluble
                                                       10
Ca2B6O11.5H2O             Slightly soluble
                              Copper
                                                       25
CuSO4.5H2O                     Soluble
                                                      50-75
CuO                           Insoluble
                                 Iron
                                                       20
FeSO4.7H2O                     Soluble
                                                       5-9
FeHEDTA                        Soluble
                                                        6
FeEDDHA                       Soluble
                             Manganese
                                                       24
MnSO4.4H2O                     Soluble
                                                      41-68
MnO                           Insoluble
                                                      30-50
Mn oxysulfate                 Variable
                            Molybdenum
                                                       39
Na2MoO4.2H2O                 Soluble
                                                       49
(NH4)2MoO4                     Soluble
                                                       66
MoO3                           Soluble
                                Zinc
                                                       36
ZnSO4.H2O                      Soluble
ZnSO4 - NH3                                           10-15
                               Soluble
complex
                                                      60-78
ZnO                           Insoluble
                                                      18-50
Zn oxysulfate                 Variable
                                                      6-14
ZnEDTA                         Soluble

The four main classes of micronutrient sources are:

       inorganic products
   ·
       synthetic chelates
   ·
       natural organic complexes
   ·
       fritted glass products (frits).
   ·




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Efficient Fertilizer Use -- Micronutrients: Dr. John Mortvedt


Inorganic Sources

Inorganic sources include oxides and carbonates, and metallic salts
such as sulfates, chlorides, and nitrates. The sulfates are the most
common of the metallic salts and are sold in crystalline or granular
form. An ammoniated ZnSO4 solution also is used in polyphosphate
starter fertilizers. Oxides of manganese and zinc also are commonly
used, and are sold as fine powders and in granular form. Because
oxides such as ZnO and MnO are water insoluble, their immediate
effectiveness for crops is rather low in granular form. Also, the
available divalent form of manganese in MnO will oxidize to the
unavailable tetravalent form of manganese, so there is very little
residual availability of manganese fertilizers for succeeding
crops. Thus, agronomic effectiveness of granular MnO may be rather
low. Since manganese in MnO2 already is in the unavailable form, it
should not be used as a manganese fertilizer.

Oxysulfates are oxides, usually industrial by-products, which have
been partially acidulated with sulfuric acid, and generally are sold
in granular form. The percentage of water-soluble manganese or zinc in
oxysulfates is directly related to the degree of acidulation by
sulfuric acid. Research results have shown that about 35 to 50 percent
of the total zinc in granular zinc-oxysulfate should be in
water-soluble form to be immediately effective for crops. Similar
results would be expected for manganese- oxysulfate. Inorganic sources
usually are the least costly sources per unit of micronutrient, but
they may not always be the most effective for crops.

Synthetic Chelates

These sources are formed by combining a chelating agent with a metal
through coordinate bonding. Stability of the metal-chelate bond
affects availability to plants of the micronutrient metals --- copper,
iron, manganese, and zinc. An effective chelate is one in which the
rate of substitution of the chelated micronutrient for other cations
in the soil is quite low, thus maintaining the applied micronutrient
in chelated form. Relative effectiveness for crops per unit of
micronutrient as soil-applied chelates may be from two to five times
greater than that of inorganic sources, while chelates costs per unit
of micronutrient may be five to 100 times higher. Several types of
chelates are sold, so relative effectiveness values depend on the
sources of chelates and inorganic products being compared.

Natural Organic Complexes

These complexes are made by reacting metallic salts with some organic
by-products of the wood pulp industry or related industries. Several
types of these complexes are the lignosulfonates, polyflavonoids and
phenols. The types of chemical bonding of the metals to the organic
components are not well understood. Some bonds may be coordinate as in
the chelates, but other types of chemical bonds also may be present.
While natural organic complexes are less costly per unit of
micronutrient, they usually are less effective than synthetic
chelates. They also are more readily decomposed by



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Efficient Fertilizer Use -- Micronutrients: Dr. John Mortvedt


microorganisms in soil. These sources are more suitable for foliar
sprays and mixing with fluid fertilizers.

Frits

Fritted glassy products (frits) in which solubility is controlled by
particle size and changes in matrix composition. Micronutrient
concentrations vary from 2 to 25 percent, and more than one
micronutrient may be included in a fritted product. Fritted
micronutrients generally are used only on sandy soils in regions of
high rainfall were leaching occurs. This class of materials is more
appropriate for maintenance programs than for correcting severe
micronutrient deficiencies. Therefore, frits only have a small share
of the micronutrient market.

Application With Mixed Fertilizers The most common method of
micronutrient application for crops is soil application.  Recommended
application rates usually are less than 10 lb/acre (on an elemental
basis), so uniform application of micronutrient sources separately in
the field is difficult.  Therefore, both granular and fluid NPK
fertilizers are commonly used as carriers of micronutrients. Including
micronutrients with mixed fertilizers is a convenient method of
application and allows more uniform distribution with conventional
application equipment. Costs also are reduced by eliminating a
separate application. Four methods of applying micronutrients with
mixed fertilizers are:

        Incorporation during manufacture · Bulk blending with granular
   fertilizers · Coating onto granular fertilizers · Mixing with fluid
   fertilizers ·

Incorporation With Granular Fertilizers

Incorporation during manufacture results in uniform distribution of
micronutrients throughout granular NPK fertilizers. Because the
micronutrient source is in contact with the mixed fertilizer
components under conditions of high temperature and moisture, the rate
of chemical reactions which may reduce the plant availability of some
micronutrients is increased. For example, acid decomposition of ZnEDTA
or any synthetic chelate may occur if they are mixed with phosphoric
acid before ammoniation during manufacture, which results in reduced
plant availability of the micronutrient.  Immediate plant availability
of applied zinc in granular ammoniated phosphates also decreases with
the level of water-soluble zinc in these products.

Bulk Blending With Granular Fertilizers

Bulk blending of micronutrients with granular NPK fertilizers is a
common practice in the U. S. The main advantage is that fertilizer
grades can be produced which will provide



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Efficient Fertilizer Use -- Micronutrients: Dr. John Mortvedt


the recommended micronutrient rates for a given field at the usual
fertilizer application rates. The main disadvantage is that
segregation of nutrients can occur during the blending operation and
with subsequent handling. Segregation results in nonuniform
application, which is critical with micronutrients since their
application rates are quite low.

Segregation can be minimized by properly matching particle sizes of
micronutrient sources with those of the NPK components of the
blend. Mechanical devices to minimize coning and segregation of the
materials during handling and storage are available. Blending of
various sized fertilizer particles results in nonuniform application
because of segregation in the applicator during transport and
spreading operations.

Coating Granular Fertilizers

Coating powdered micronutrients onto granular NPK fertilizers
decreases the possibility of segregation, which is the main
disadvantage of bulk blending micronutrients with mixed
fertilizers. Fertilizer solutions are preferred as binding agents
because the fertilizer grade is not decreased so much as with use of
water, oils and waxes. Some binding materials are unsatisfactory
because they do not maintain the micronutrient coatings during
bagging, storage, and handling. This results in segregation of the
micronutrient sources from the granular NPK components.

Agronomic effectiveness of micronutrients coated onto soluble granular
NPK fertilizers should be similar to that with incorporation during
manufacture. This method of micronutrient application is not commonly
used because of the extra costs associated with coating.

Fluid Fertilizers

Mixing micronutrients with fluid fertilizers has become a popular
method of application, especially in the U. S. Clear liquids are
commonly used as starter fertilizers for row crops and some
micronutrients, especially zinc sources, are easily applied with these
fluids. Solubility of some micronutrient sources is higher in
polyphosphate fertilizers such as 10-34-0 than in orthophosphate clear
liquids. Micronutrients also may be applied with nitrogen solutions
such as UAN, but solubility of many sources is rather
low. Compatibility tests should be made before tank mixing operations
of micronutrients with fluid fertilizers are attempted; otherwise,
problems could occur when incompatible sources are mixed. Suspension
fertilizers also are used as micronutrient carriers.  Oxides also can
be applied with suspensions since complete solution is not required.

Foliar Sprays Foliar sprays are widely used to apply micronutrients,
especially iron and manganese, for many crops. Soluble inorganic salts
generally are as effective as synthetic chelates in foliar sprays, so
the inorganic salts usually are chosen because of lower costs.
Suspected micronutrient deficiencies may be diagnosed with foliar
spray trials with one


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Efficient Fertilizer Use -- Micronutrients: Dr. John Mortvedt


or more micronutrients. Correction of deficiency symptoms usually
occurs within the first several days and then the entire field could
be sprayed with the appropriate micronutrient source. Inclusion of
sticker-spreader agents in the spray is suggested to improve adherence
of the micronutrient source to the foliage. Caution should be used
because of leaf burn due to high salt concentrations or inclusion of
certain compounds in foliar sprays.

Advantages of foliar sprays are: (1) application rates are much lower
than for soil application; (2) a uniform application is easily
obtained; and (3) response to the applied nutrient is almost immediate
so deficiencies can be corrected during the growing season. Low
residue foliar sprays of manganese and zinc have been used to correct
deficiencies of citrus and other fruit crops, but sprays which will
discolor the fruit should be avoided.

Disadvantages of foliar sprays are: (1) leaf burn may result if salt
concentrations of the spray are too high; (2) nutrient demand often is
high when the plants are small and leaf surface is insufficient for
foliar absorption; (3) maximum yields may not be possible if spraying
is delayed until deficiency symptoms appear; and (4) there is little
residual effect from foliar sprays. Application costs will be higher
if more than one spray is needed, unless they can be combined with
pesticide spray applications.

Micronutrient Rates And Example Results Boron

Recommended application rates of boron are rather low (0.5 to 2
lb/acre), but should be carefully followed because the range between
boron deficiency and toxicity in most plants is narrow. Uniform
application of boron in the field is very important for the above
reason. Boronated NPK fertilizers (those containing boron sources
incorporated at the factory) will insure a more uniform application
than most bulk blended fertilizers. Foliar sprays also insure a rather
uniform application, but costs generally are higher.

Table 9.3: Effect of Soil pH on Boron Availability and Cotton Yields


                                                       In an experiment in 
Arkansas, cotton
   SOIL pH           0 lb/acre B 0.5 lb/acre B
                                                       yields were increased 
by 490 and 584
                  ---------- yield, lb/acre ---------- lb/acre at boron rates 
of 0.3 and 0.5
       5.7                   1,759        2,100        lb/acre, respectively. 
Response of
                                                       cotton in Tennessee to 
boron
       7.3                   1,453        1,784
                                                       application is shown 
below. Without
       7.5                     862        2,062        applied boron, cotton 
yields decreased
with increasing soil pH. Yields were increased at all soil pH levels when 
boron was
applied at a rate of 0.5 lb/acre.




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Efficient Fertilizer Use -- Micronutrients: Dr. John Mortvedt


Soil tests should be included in boron fertilization programs, first
to assess the level of available boron and later to determine possible
residual effects (buildup). The most common soil test for boron is the
hot-water-soluble test. This test is more difficult to conduct than
most other micronutrient soil tests, but most boron response data have
been correlated with it.

Copper

Recommended copper rates range from 3 to 10 lb/acre as CuSO4 or finely
ground CuO.  Residual effects of applied copper are very marked, with
responses being noted up to eight years after application. Because of
these residual effects, soil tests are essential to monitor possible
copper accumulations to toxic levels in soils where copper fertilizers
are being applied. Plant analyses also can be used to monitor copper
levels in plant tissues. Copper applications should be decreased or
discontinued when available levels increase beyond the deficiency
range.

Iron

Soil applications of most iron sources generally are not effective for
crops, so foliar sprays are the recommended application method. Spray
applications of a 3 to 4% FeSO4 solution at 20 to 40 gallons/acre are
used to correct iron deficiencies. The application rate should be high
enough to wet the foliage. More than one foliar application may be
required for correction of iron chlorosis. Inclusion of a sticker-
spreader agent in the spray is suggested to improve adherence of the
spray to the plant foliage for increased iron absorption by the plant.

Manganese

Recommended application rates range from 2 to 20 lb/acre of manganese,
generally as MnSO4. Application rates of MnO would be similar if
applied as a fine powder or in NPK fertilizers. Band application of
manganese sources with acid-forming fertilizers results in a more
efficient use of applied manganese because the rate of oxidation of
applied manganese to the unavailable tetravalent form (as in MnO2) is
decreased. There are no residual effects of applied manganese for the
same reason, so annual applications are needed. Foliar spray
applications of MnSO4 also are used and require lower rates than soil
applications.


Table 9.4: Soybean Response to Manganese

                                                    Results given
                                                    below show that
                                                    soybean
  lb/acre Mn                       yield, bu/acre   yields were more than 
doubled when 10
                                                    lb of manganese/acre was 
applied as
        0                                13
                                                    MnSO4 to a poorly drained 
sandy soil in
      2.5                                24         Georgia.
      5.0                                26
       10.0                             33

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Efficient Fertilizer Use -- Micronutrients: Dr. John Mortvedt


Cotton also responded to applied Manganese in Georgia, as shown below.

Table 9.5: Cotton Response to Manganese

   lb/acre Mn        yield, lb/acre     increase, lb/acre
        0                2,657                ----
       2.5               2,857                200

Molybdenum

Recommended molybdenum rates are much lower than those for the other
micronutrients, and uniform application is very important. Broadcast 
application of
molybdenized phosphate fertilizers prior to planting or to pastures has been 
used to
correct molybdenum deficiencies. Soluble molybdenum sources also can be 
sprayed on
the soil surface before tillage to obtain a uniform application.

Table 9.6: Soybean Response to Molybdenum

                      Mo applied, oz/acre
                                                 Seed treatment is the most 
common
                      0
   soil pH                            1         method of molybdenum 
application.
                      --- yield, bu/acre ---    Molybdenum sources are coated 
onto
                                                the seed with a sticking 
agent and/or
      5.3            25.7              45.0
                                                conditioner. This method 
insures a
                     34.3
      5.7                              45.0
                                                uniform application and 
sufficient
                     41.8
      6.3                              46.1     amounts of molybdenum can be 
seed
                                                coated to provide sufficient
molybdenum. Data in the following table show the effectiveness for soybean in 
Georgia
of seed-coated molybdenum at a rate of one ounce of molybdenum/acre. Soybean
yields without applied molybdenum increased with increases in soil pH, but 
not as high
as those with seed-applied molybdenum at each soil pH level.

Zinc

Recommended rates of zinc generally range from 1 to 10 lb/acre. Band
or broadcast applications are used, but foliar applications also are
effective. Band applications of zinc sources with starter fertilizers
is a common practice for row crops. Foliar sprays of a 0.5% ZnSO4
solution applied at a rate of 20 to 30 gallons/acre also will supply
sufficient zinc, but several applications may be necessary. As with
copper, residual effects of applied zinc are substantial, with
responses found at least 5 years after application.  Because of these
residual effects, soil test levels of available zinc generally
increase after several applications. Many states have reduced their
recommended zinc application rates because of these residual effects.




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Efficient Fertilizer Use -- Micronutrients: Dr. John Mortvedt


Crop response to several zinc sources each banded with a 10-34-0
starter fertilizer at zinc rates up to 3 lb/acre for corn in Nebraska
is shown below. Results show that ZnEDTA was much more effective at
the lower zinc rates, but all zinc sources were about equally
effective at the highest zinc rate.

Table 9.7: Corn Response to Zinc

                                  Zn Applied, lb/acre
Zn Source                0       0.1     0.3     1.0      3.0
                         -------- Grain Yield, bu/acre --------
ZnSO4                    62 132         141     131      144
ZnO                             124     124     134      144
ZnSO4-NH3 complex               118     138     134      140
ZnEDTA                          138     139     155      141

Summary

Micronutrients are as important as the primary and secondary nutrients
in plant nutrition.  However, the amounts of micronutrients required
for optimum nutrition are much lower.  Micronutrient deficiencies are
widespread because of increased nutrient demands from the more
intensive cropping practices. Soil tests and plant analyses are
excellent diagnostic tools to monitor the micronutrient status of
soils and crops. Visual deficiency symptoms of these nutrients also
are well recognized in most economic crops.  Micronutrient
recommendations are based on soil and plant tissue analyses, the type
of crop and expected yield, management level, and research results.

Numerous micronutrient fertilizers are on the market. These sources
are classified as inorganic, synthetic chelates, natural organic
complexes, and fritted glasses. Some industrial by-products are used
as micronutrient fertilizers because of their lower cost.
Micronutrient sources vary considerably in physical form, chemical
reactivity, cost, and relative availability to plants. While most
micronutrient fertilizers are applied to soils, foliar sprays also are
used on tree crops and some vegetables. Because recommended rates
usually are low, most micronutrients are applied with NPK fertilizers
by incorporation during manufacture or bulk blending with granular
fertilizers, or by mixing with fluid fertilizers just before
application to soil. Choice of micronutrient source depends on the
method of application, compatibility with the NPK fertilizer,
convenience of application, and the relative agronomic effectiveness
and cost per unit of micronutrient.


Links to other sections of the Efficient Fertilizer Use Manual History
· Mey · Soil · pH · Nitrogen · Phosphorus · Potassium · Secondary ·
Fertigation · Fluid-Dry · Sampling · Testing · Site-Specific · Tillage
· Environment · Appendices · Contributors




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-- 
Chad Knepp
python -c 'import base64;print base64.decodestring("cHlnQGdhbGF0ZWEub3Jn")'