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Soil and Applied Iron _A3554_


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        Soil and Applied Iron
E.E. Schulte

   ron (Fe) is the fourth most-abundant     soil conditions. Iron deficiencies occur     Plant Adaptations
   element on earth, mostly in the form     most frequently in cool, wet soils early          Some plants cope with low iron
of ferromagnesium silicates. Soils          in the growing season, when microbial        availability in soil by excreting
typically contain 1–5% total iron, or       activity and root growth are limited. As     hydrogen ions (H+) from roots, which
20,000–100,000 lb/a in the plow layer.      the soil warms, microbial activity and       lowers the pH at the root interface and
Most of the iron in soil is found in        root proliferation increase, allowing        increases the solubility of iron from
silicate minerals or iron oxides and        plants to absorb more iron. If microbial     iron hydroxides. Other plants excrete
hydroxides, forms that are not readily      activity is sufficient to decrease the       organic compounds that reduce ferric
available for plant use.                    oxygen supply in acid soils, some ferric     iron to the more soluble ferrous forms.
     The iron oxides and hydroxides in      iron oxides and hydroxides will be           Alfalfa, corn, and small grains are
soil are responsible for its reddish and    transformed to more soluble ferrous          tolerant of low iron availability; most
yellowish colors. The red soils of          forms. On the other hand, in alkaline        fruits and some ornamentals are
eastern Wisconsin owe their color to        soils rapid microbial respiration may        sensitive to low iron availability.

                                                                                         FERTILIZER SOURCES OF IRON
iron oxide or hydroxide coatings on soil    produce sufficient carbon dioxide to
clay minerals. Iron is also indirectly      react with water to form bicarbonate
responsible for much of the green color     ions. Plant-absorbed bicarbonate ions
                                                                                           ron deficiency is difficult to correct
of growing plants, because of its role in   immobilize iron within plants, resulting
                                                                                           because of the rapid transformation
                                             Organic Matter
the production of chlorophyll.              in deficiency.
                                                                                        of iron contained in fertilizer to
                                                                                        unavailable forms in soil. (See Table 1

                                                  Organic matter improves iron          for a list of several fertilizer sources of
                                             availability by combining with iron,       iron.) Iron chelates (iron in association
   ron in soil exists in ferrous (Fe++)      thereby reducing chemical fixation or      with an organic ligand) applied to the
   and ferric (Fe+++) forms. Soil pH         precipitation of iron as ferric hydroxide. soil have been successful in some cases.
and the aeration status of the soil          This reduction in fixation and             However, not all chelates remain stable
determine which form predominates.           precipitation results in higher concen-    over a wide range of soil pH, and the
Ferric iron compounds have low               trations of iron remaining in the soil     presence of other elements such as
solubility in the soil solution, and         solution, available for root absorption.   calcium influences different chelates to
conditions that favor formation of these Organic matter can also affect iron            different degrees. Iron EDDHA
compounds decrease iron availability.        availability by acting as an energy        maintains iron in a soluble form from
Soil pH, soil aeration, reactions with       source for microorganisms that use up      pH 4.0 to 9.0 and is a good choice for
organic matter, and plant adaptations        oxygen under waterlogged conditions.       use on calcareous soils. Iron DTPA is
                                             When microorganisms decompose
Soil pH
influence iron availability.                                                            selective for iron up to pH 7.5. The
                                             organic matter, iron
                                             previously tied up in Table 1. Fertilizer sources of iron.
     The concentration of iron in the soil
                                                                      SOURCE FORMULA                      PERCENT METHOD OF
                                             organic compounds is
solution decreases sharply as the soil pH
                                                                                                            IRON APPLICATION
                                             released in forms
increases, with a minimum around pH
                                             available for plant
                                             uptake. Finally, many Chelates:        FeDTPA                     10            Soil, foliar
7.4–8.5. It is in this range that most cases

                                                                                    FeEDTA                    9–12           Soil, foliar
of iron deficiency occur. While this is
                                             organic materials can
                                                                                    FeEDDHA                     6            Soil, foliar
not widespread in Wisconsin, it can
                                             also be sources of iron.
occur in calcareous soils.
                                                                      Ferrous       (NH4)2Fe(SO4)2•6H2O 14                     Foliar
                                             Livestock manure
                                             contains 0.5–5.0
     Poor soil aeration, or reduced          lb/ton of iron,
oxygen level, is caused by flooding or       depending on the
                                                                      Ferrous       FeSO4•7H2O                 20              Foliar
compaction. It can increase or decrease manure source.
iron availability depending on other
EDTA chelate is least effective and will           a more immediate response, use battery       ensure that they are not contaminated
only remain stable up to pH 6.3, where             acid (sulfuric acid) in a similar manner.    with soil. Also, some physiologists feel

                                                   DIAGNOSTIC TECHNIQUES
iron deficiencies normally do not exist.                                                        that “total” iron in tissue is a poor
The chelates should be applied to the                                                           indicator of plant needs. Table 2 gives
                                                   Deficiency Symptoms
soil before or at planting at the rates                                                         guidelines for interpreting iron
suggested by the manufacturer, usually                                                          concentrations in some field crops. If
about 0.5–2.0 lb/a of iron.                              Iron is immobile in plants.            visible symptoms are not apparent,
     Foliar applications require only              Therefore, deficiency symptoms appear        response to iron is unlikely.
                                                   first on the youngest leaves. Plants need
                                                                                                ADDITIONAL INFORMATION
0.10–0.15 lb/a of iron, but several
applications may be necessary to                   iron to produce chlorophyll. Lack of
correct severe deficiency. Ferrous                 iron results in yellowing (chlorosis) of
                                                   the younger leaves. Mild iron deficiency         These publications in the
sulfate and ferrous ammonium sulfate                                                          Understanding Plant Nutrients series are
are used as foliar sprays. Typically, a            appears as interveinal chlorosis and is
                                                   often confused with manganese              available from your county Extension
2–3% solution of either material is                                                           office:
                                                   deficiency. If the deficiency is severe
                                                                                              Soil and Applied Boron          (A2522)
applied at a rate of 20 gal/a. Ferrous
                                                   and prolonged, each new leaf emerges
                                                                                              Soil and Applied Calcium        (A2523)
sulfate has given some success as a soil

                                                   Soil Analysis
application to alleviate iron deficiency           lighter in color than the preceding leaf.
in turf. The limited soil contact that                                                        Soil and Applied Chlorine       (A3556)
                                                                                              Soil and Applied Copper         (A2527)
occurs with foliar applications reduces                  Iron deficiency is extremely rare in

                                                   no soil test for iron has been developed. Soil and Applied Iron            (A3554)
fixation, and its acidifying effect on soil        Wisconsin field crops. For this reason,

                                                                                              Soil and Applied Magnesium      (A2524)
improves uptake.
     For ornamentals, shrubs, and trees,           If iron deficiency is suspected, plant
                                                                                              Soil and Applied Manganese      (A2526)
                                                   Plant Analysis
local acidification of calcareous soils is         analysis can verify the deficiency.
sometimes effective in correcting iron                                                        Soil and Applied Molybdenum     (A3555)
                                                                                              Soil and Applied Nitrogen       (A2519)
deficiency. Sulfur bacteria in the soil                  Analysis of plant tissue for iron is

                                                                                              Soil and Applied Phosphorus     (A2520)
oxidize elemental sulfur to sulfuric acid;         not as reliable as it is for other
the sulfuric acid dissolves enough iron            nutrients, because plant tissue
oxide to supply the plant with iron.               commonly is contaminated with iron-        Soil and Applied Potassium      (A2521)
Pour elemental sulfur into holes made a            containing soil from dust or from soil     Soil and Applied Sulfur         (A2525)
                                                                                              Soil and Applied Zinc           (A2528)
foot deep with a soil probe and spaced             splashed on by raindrops. Use special
about 2 ft apart under the drip line. For          care when collecting these samples to

Table 2. Iron plant-analysis interpretations for common Wisconsin crops.
                                                                        ———————— INTERPRETATION ————————
                 CROP           PLANT PART TIME OF                       DEFICIENT             LOW        SUFFICIENT          HIGH
                                SAMPLED    SAMPLING
                                                                              —————————- ppm ——————————
                 Alfalfa        Top 6 inches            Bud                  <20               20–30          31–250           >250
                 Corn           Earleaf                 Silking              <10               10–50          51–250           >250
                 Oat, wheat     Top leaves              Boot stage             —                <20           20–250           >250
                 Potato         Top leaves              Flowering              —                <11           11–300           >300
                 Soybean        First trifoliate        Early flower         <30               30–50          51–350           >350

Author: E.E. Schulte is professor emeritus of soil science, College of Agricultural and Life Sciences, University of Wisconsin-Madison
and University of Wisconsin-Extension, Cooperative Extension. The author wishes to thank P.D. Ehrhardt for editorial assistance.
Produced by Cooperative Extension Publications, University of Wisconsin-Extension.
University of Wisconsin-Extension, Cooperative Extension, in cooperation with the U.S. Department of Agriculture and Wisconsin
counties, publishes this information to further the purpose of the May 8 and June 30, 1914 Acts of Congress. An Equal Employment
Opportunity/Affirmative Action employer, University of Wisconsin-Extension provides equal opportunities in employment and
programming, including Title IX requirements.
This publication is available from your Wisconsin county Extension office or from Cooperative Extension Publications. To order, call
toll free 877-WIS-PUBS (947-7827) or visit cecommerce.uwex.edu
A3554 Soil and Applied Iron                                                                                          RP 08-2004 (I 09/92)

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