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Fertilization of
Field & Forage Crops
E. B. Whitty and C. G.
Chambliss
SOIL NUTRIENT RECOMMENDATIONS
Nitrogen
Recommendations
for N fertilization are not based on soil tests. The complex
nature of soil N precludes the use of rapid, inexpensive and
reliable tests to measure this element. Fertilizer N is not
normally needed for field crops grown on organic soils.
Nitrogen can be leached from sandy soils by heavy rainfall. If
this occurs, it may be desirable to apply more N as an
adjustment to leaching or to use smaller, but more frequent
applications.
Phosphorus
Many fields
used for agronomic crops have received annual applications of
P-containing fertilizers for many years. As a result, the P
levels of these soils are high or very high as measured by
soil test. Since crop responses to even more P is unlikely,
this element may not be needed until soil tests indicate that
the soil levels have declined to the medium category.
Potassium
Potassium can
be leached below the root zone of agronomic crops grown on
sandy soils if excessive rainfall occurs. If so, it would be
desirable to replace the leached nutrient or to make smaller,
but more frequent applications. Do not apply K-containing
fertilizer to emerged peanuts, as high levels in the soil can
interfere with the uptake of calcium by the peanut pods.
Magnesium
If the
magnesium soil test is below the target level and dolomitic
limestone was not applied, soluble Mg should be included in
the basic application of fertilizer for most field crops.
Application of at least 10 lb/A of Mg is recommended. Since
lime is seldom used for flue-cured tobacco, the fertilizer for
this crop should contain at least 20 lb/A Mg.
Sulfur
Indications are
that at least 16 lb/A of S should be applied annually in
either the fertilizers or the pesticides used in field crop
production programs. Except for areas close to paper mills and
certain other industrial establishments, very little sulfur is
added to Florida soils in rain-water. Also, high-analysis
fertilizers usually contain little or no sulfur. Fertilizer
materials that contain relatively high percentages of sulfur
include ammonium sulfate, ordinary superphosphate, sulfate of
potash, sulfate of potash-magnesia, agricultural gypsum, and
magnesium sulfate.
Calcium
If the soil pH
is appropriate for the field crop in question, there will be
sufficient Ca for proper plant nutrition. Peanuts are an
exception since that crop requires relatively high levels of
Ca in the pegging zone for proper kernel development and
subsequent germination.
Micronutrients
Iron (Fe),
copper (Cu), manganese (Mn), zinc (Zn), boron (B), and
molybdenum (Mo) are the essential micronutrients. Deficiencies
of one or more of the micronutrients have been noted on some
crops in all sections of Florida. Perennial grasses and
clovers on certain mineral soils in central and southern
Florida, and all field crops on organic soils in the same
area, have responded to applications of Cu, Mn, Fe, Zn, and B.
Small grains and peanuts on phosphatic soils in central and
northern Florida have responded to applications of Cu. Zn has
controlled "white bud" of corn. Peanut "hollow
heart" has been prevented by applications of B. Chlorosis
of perennial grasses, generally on new growth in the spring on
soils with a high pH or on light gray sands in central and
southern Florida has been overcome by applications of Fe. As a
seed treatment, Mo has improved the nodulation of soybeans
grown on unlimed or inadequately limed soils. There are
numerous other instances where one or more of the
micronutrients have limited crop production.
Soil tests for
micronutrients are expensive and difficult to interpret.
Fertilization studies with micronutrients have not always been
conclusive because many factors affect the response of plants
to these elements. Therefore, local experience should serve as
the primary guide as to the need for these nutrients.
Micronutrient deficiencies are most likely to occur on organic
soils, soils with pH >6.0, and virgin flatwood soils.
For new
plantings of forages on flatwood soils, 3 lb/A of copper
should be applied with the initial fertilization.
In instances
where deficiencies are probable, micronutrient applications
serve as insurance against reduced yields. Some growers of
peanuts and other high-value crops make routine applications
of B and other micronutrients, especially when the soil pH is
6.0 or above. Soybean and some other legumes grown on soils
with pH >6.0 often show Mn or other deficiency symptoms
which can be corrected with periodic applications of
micronutrients. Corn has shown manganese deficiency symptoms
at early growth stages that tend to disappear as the root
system develops.
Cu, Zn, Mn, and
Fe may be supplied as oxides or sulfates. B is usually applied
as borax or other borates. Plants respond equally well to all
of these sources, but oxides are usually more economical to
use than sulfates. Since there is a very small difference
between sufficient and toxic amounts of B, care must be
exercised when applying this micronutrient. Recommended
amounts should not be exceeded.
When forages
are grazed there is recycling of nutrients within the
soil-plant-animals system. Livestock mineral mixtures
containing micronutrients make small but important
contributions of these elements to the soil. Harvest of hay or
silage increases the removal of all elements from the soil.
Under grazing
conditions, the metallic micronutrients should be applied once
every 5 to 10 years unless deficiencies are confirmed by soil
and/or plant analyses. Boron should be applied to legume
pastures every 3 to 4 years. Where forage is removed as silage
or hay it may be necessary to shorten this interval.
APPLICATION OF
MICRONUTRIENTS
Micronutrients
may be applied to the soil or to the growing plant. Mo may
also be used as a seed treatment. Soil applications usually
involve mixing and then applying the micronutrients with
fertilizers, or pesticides. Since small amounts of
micronutrients are used, the mixtures can be applied more
uniformly and economically than in a separate operation.
Foliar applications may be made in sprays or dusts.
Compatibility must be known before mixing elements with
foliar-applied pesticides. Boron and other micronutrients may
be mixed with gypsum before it is applied to growing peanuts.
Soluble micronutrients can also be applied as a tank mix with
herbicides that will be incorporated with soil. Availability
and characteristics of materials and equipment and the crop
conditions determine the most suitable method of application.
Legume Seed
Treatment
Molybdenum may
be added to soybean and other legume seed to be planted on
mineral soils that have not been limed to a pH of 6.0. To
treat a bushel of soybean seed with Mo and bacterial inoculant,
first dissolve 1/2 oz of either ammonium molybdate or sodium
molybdate in 1/2 pt of hot water and then add a few drops of
syrup or molasses. Cool, and mix the solution with the seed
and then add the inoculant. plant before the seed coats are
completely dry.
Soil
Applications
Solubility of
Mn, Zn, Cu, and Fe decreases rapidly as pH increases above
6.0. This has important consequences. A deficiency of one of
these nutrients is often caused or aggravated by an elevated
soil pH. Thus over liming can provoke micronutrient
deficiencies. Also, when soluble forms of Mn, Zn, Cu, or Fe
are applied to deficient soil, the nutrients are rather
rapidly changed to insoluble forms. This effect is much worse
when micronutrients are broadcast than when they are banded.
The best method of soil application of these nutrients is in a
band together with elemental sulfur. The resulting low pH in
the band will keep the nutrients soluble through the growing
season. Borax to supply B and sulfates of Cu, Mn, and Zn are
common soluble sources of micronutrients that may be mixed
with fertilizer for soil applications. Care must be taken not
to apply excess amounts of the materials or toxicities may
result.
Micronutrient
Mixtures
There has been
a tendency in the past to recommend commercially available
mixes of micronutrients. These were used in a
"shotgun" approach to fertilization -- throw a bit
of all the micronutrients at the plant, and if one of the
elements is limiting yield you application will do some good.
This approach should be discouraged for the following reasons:
1) It is an added cost with questionable returns; 2) repeated
additions could result in build-up to toxic levels of some
metals; 3) if a deficiency of a particular micronutrient does
exist, the quantity applied in the mix will probably not be
sufficient to completely correct the problem and may mask a
serious deficiency. Producers should watch for signs o
deficiency, determine which nutrient is needed (by responses
to single element applications), and subsequently apply the
needed element.
Foliar
Applications
Foliar
applications of micronutrients will normally result in a rapid
response if a deficiency exists. Also, a response can be
obtained with less material than in soil applications.
However, there is a danger of burning the foliage if excessive
rates are applied. Only meager information is available on the
tolerance of agronomic plants to foliar sprays. Species, age,
and condition of the plants will greatly affect tolerance to
nutrient sprays. Since there is often a synergistic effect
between certain pesticides and nutrient sprays should be
applied separately. In general, fine mists that do not result
in run-off liquid, will cause less damage to leaf tips and
margins than will coarse sprays.
Rates of Cu,
Fe, Mn, or Zn sulfates that should be effective and safe for
agronomic crop plants are about 1 lb of material in sufficient
water to cover one acre. Borax and other soluble forms of B
should be used at this same rate. If more than one nutrient is
applied, do not exceed a total of 2 lb/A of salts. Follow
label directions if commercial mixtures of micronutrients are
used.
Chelates are
generally more effective on firm and thick leaves of trees
than on the soft and succulent foliage of herbaceous plants.
Other sources of micronutrients are available for foliar
application and may be satisfactory.
TIMING AND
PLACEMENT OF FERTILIZER
Mixing seed
with fertilizer and broadcasting both in the same operation
has given fair to good stands of ryegrass, small grains (rye,
oats, and wheat), pearlmillet, sorghum-sudangrass, bahiagrass,
and other crops. Legume seed should not be mixed with
fertilizers since the inoculant can be killed or its
effectiveness greatly reduced by these materials.
Seed
distribution has been a problem in many cases. Conventional
seeding method, drilling or broadcasting on a well-prepared
seedbed, are preferred since seed can be more uniformly
distributed. Broadcasting operations should be followed with a
light disk and/or cultipacker.
This document
is SS-AGR-152, one of a series of the Agronomy Department,
Florida Cooperative Extension Service, Institute of Food and
Agricultural Sciences, University of Florida. First printed
December 1992. Revised May 1998. Please visit the FAIRS Web
site at http://hammock.ifas.ufl.edu.
E. B. Whitty, professor, Agronomy Department, and C. G.
Chambliss, associate professor, Agronomy Department,
Cooperative Extension Service, Institute of Food and
Agricultural Sciences, University of Florida, Gainesville,
32611. The use of trade names in this publication is solely
for the purpose of providing specific information. It is not a
guarantee or warranty of the products named and does not
signify that they are approved to the exclusion of others of
suitable composition.
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