Chapter 9: Plant Nutrients

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Chapter 9 Plant Nutrients 91 Important Facts
to Know

• Essential plant nutrients and their importance
  to plant growth.
• Importance and characteristics of key nutrient
  cycles.
• Behavior and management of primary and secondary
  plant nutrients (i.e., macronutrients).
• Behavior and management of micronutrients
  importance to plant growth and environmental
  significance.

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Homework Chapter 9
Questions 1, 2, 4 and 5, 8, 15 _at_ 2 pts 12
pts Due Monday 1 November 2010
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92 Essential Elements

• Plants need at least 17 essential elements C, H
  and O from CO2 and H2O six others are called
  macronutrients (3 primary, 3 secondary), 8 more
  are micronutrients.
• Several others are either necessary or
  beneficial (Table 9-1).
• Night Bulletin CHOPKNS CaFe
• CuMg cuisine mighty-good
• (with) ZnMn zinc and manganese
• ClMo closed Mondays

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93 Mechanisms of Nutrient Uptake
Prior to absorption, nutrients reach the root by
3 mechanisms Mass flow movement with the
water flow. Most prominent. Diffusion movement
in response to a concentration gradient.
Slow. Root interception root extension. Very
important to find new nutrient sources.
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Absorption into roots.
Passive Uptake Some ions such as nitrate, can
move passively through the outer membrane of the
root surface along with water in the
transpiration stream. Active Uptake Not well
understood, but many nutrients (e.g., K and
H2PO4-) must somehow bond with an ion-specific
carrier (see Insight p.270). Maintaining an
Electrical Balance As cations are absorbed H is
excreted or organic anions are produced. As
anions are absorbed HCO3- is excreted or
compensating cations are absorbed.
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Absorption through leaves
Stomatal Absorption Rapid absorption of soluble
ions from nutrient enriched water. Used mostly
for the immediate correction of critical nutrient
deficiencies. Most efficient for the
micronutrients. Does not build soil fertility.
Danger of phytotoxic effects if over applied.
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94 Soil Nitrogen Gains and Transformations

• N is unique in several ways
• No mineral source (usually) SOM stores nearly
  all N (90). See Figure 9-3 for the N-Cycle.
• Atmosphere is main reserve, but unavailable
  must be fixed.
• Very low soil available pools relative to
  uptake.
• Volatile phases (NH3, N2O, N2).
• Can be taken up as cation (NH4) or anion (NO3-)
  form. Assimilating NH4 costs 2-5 of plant
  energy, NO3- costs 15 forms proteins amino
  acids.
• Deposition can be major input in polluted areas.

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Nitrogen Cycle Fig 10-3 This figure lacks
atmospheric deposition, which is important in
wildland systems, especially those without fixers
and in polluted areas!!!!
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Nitrogen Cycling in soils Biologically controlled
Atmospheric Deposition
N2 fixation
N2, N2O
Plant N
Litterfall Root turnover
Uptake
Denitrification
Uptake
Mineralization
Organic N
NH4
NO3-
Nitrification
Immobilization
Leaching
Immobilization
Clay-fixed NH4
Includes mostly microbial (biotic) but also some
abiotic immobilization
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Nitrogen Fixation

• Requires great energy see Insight on p. 271
• Mechanism may be strictly symbiotic,
  non-symbiotic (free living), or associative
  symbiotic.
• 12 g organic carbon per g N.
• Amounts vary enormously as low as 1-2 kg ha-1
  yr-1 for lichens in Douglas-fir canopies to over
  300 kg ha-1 yr-1 for alders (not just 170 kg ha-1
  yr-1, as in book)
• Non-symbiotic N fixations is usually not
  important, except possibly in desert crusts but
  no measurements of the latter are available.

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Nitrogen Mineralization

• Release from SOM by the conversion of organic N
  into inorganic N terminal group aminization,
  de-aminization, ammonification
• Highly dependent on CN ratio (occurs at values
  lt 20-30)
• Approximately 1-5 of the total organic N pool
  per year
• Mineralization is critical to N cycling and
  plant growth because it is the point at which N
  is converted from organic to NH4 form, the
  latter of which is available to plants and
  nitrifiers

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Nitrification of ammonium.

• Nitrification
• Conversion of NH4 to NO3- H (review)
• Two-stage process (p. 272)

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Other kinds of fixation

• Immobilization
• Organic tie-up of NH4 and/or NO3- by microbes
• Opposite of mineralization
• Highly dependent on CN ratio (occurs at values
  gt20-30)
• Can have abiotic immobilization (chemical
  reactions between NH4 and/or NO3- and soil
  organic matter) in some cases
• Ammonium Fixation
• Adsorption and collapse within the crystal
  lattice structure (e.g., illites)

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95 Nitrogen Losses

• Leaching
• Only the NO3- form is important NH4 adsorbs
  strongly
• Nitrification is key to facilitating leaching
• Pollutes water, wastes N, and acidifies soil
• Nitrification inhibitors are sometimes used to
  prevent it after fertilization
• Great need to get fertilizer timing and amounts
  correctly to minimize this effect.

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Gaseous Losses

• Denitrification
• Microbial conversion of NO3- to N2O and N2.
• Anaerobic - may occur in microsites in aerobic
  soils (Fig 9-5)
• Requires energy (organic matter)
• Usually less important loss than leaching in
  aerobic soil
• Ammonium volatilization
• Chemical, not biological process
• Occurs only at high pH (8 or above) NH4 OH-
  ? NH3 H2O
• Usually not important in mesic soils because pH
  is too low
• Exception may be after urea fertilizer, which
  creates high pH and NH4

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96 Nitrogen Balance
Fig 9-6 shows an N cycle/balance for a pasture
system. Table 9-3 shows some values for N
balances. Note The term sedimentation in Table
9-3 is unclear. Does it mean adsorbed N as
ammonium, or is it really in reference to
atmospheric deposition. Note Values can differ
widely from these, considerably for forests
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Properties of Macronutrient Cycles in Forest Soils
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• Note from the previous table that
• Usually leaching lt deposition (net gain)
• Due to biological uptake, not chemical
  adsorption and precipitation
• Reflects N limitation in most cases
• Exceptions where N inputs are high
• Deposition varies widely
• Soil organic pools are large
• Soil available pools are small
• Leaching varies with input (deposition or
  fixation)

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97 Materials Supplying N
Table 9-4
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Important note on N fertilization.

• You never have long-term increases in soil
  exchangeable NH4 or NO3-
• Soil NH4 to NO3- will not remain high in
  soils after fertilization gt 1 year
• NH4 does not leach much, but nitrifies to NO3-
  
• NO3- in excess of plant and microbial supply
  will leach
• Fertilization with N in crops must be repeated
• This creates major problems with groundwater
  NO3- and soil acidification.
• In forests, N fertilization can be every 5 years
  because of cycling in plant.

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• Anhydrous ammonia
• Highest N (82)
• Injected with chisels (ag use only)
• Dangerous

• Urea
• Cheapest solid N fertilzer because highest N
  (45-46)
• Volatilization losses of ammonium can be high
• In forests, abiotic immobilization can be high

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• Ammonium sulfate
• Relatively expensive (21N)
• Ammonium nitrate
• Relatively cheap (33.5N)
• Explosive (mixed with diesel to make bombs -
  Oklahoma City was this)
• Nitrate in it can leach readily
• Slow-release fertilizers
• Idea is not to flood soil with NH4 immediately

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Time
lt 1 yr
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98 Soil Phosphorus

• Second most commonly limiting (second on fert
  bag)
• Taken up in anion form (H2PO4- or, at higher pH,
  HPO42- )
• Both soil mineral (apatite) and organic sources
  are important
• Deposition is unimportant over the short-term
  except perhaps the Lake Tahoe Basin
• Strongly retained by adsorption in acid soils
  and by precipitation with Ca (forming apatite) in
  alkaline soils
• Forms in plants ADP, ATP (plant energy
  currency)
• Most uptake is thought to be by diffusion root
  exploration it therefore critical

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99 The Phosphorus Problem
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Important Features of Phosphorus Cycling

• Atmospheric deposition unimportant (?)
• Leaching is a small flux due to biological
  uptake when P is limiting, due to adsorption and
  precipitation in every case
• Erosion is major loss pathway
• No major volatilization pathway
• Large, inorganic adsorbed or apatite pools
• Small or large "available" soil pools -
  fertilization can cause P buildup
• CP ratios mineralization when lt2001,
  immobilization when gt3001
• Examples of P budgets in crops Table 9-7

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910 Managing Soil Phosphorus
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Properties of Macronutrient Cycles in Forest Soils
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911 Materials Supplying Phosphorus

• Often must add bands or dollops or spikes in
  P-fixing soils
• Excess P is retained in soil does not leach
  like N
• P fertilization can enhance soil P availability
  for decades (unlike N)
• Excess P can fill anion adsorption sites and
  cause problems with sulfate retention
• P fertilizers - Table 9-8

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P Fertilization

• Also note the N-P fertilizers diammonium
  phosphate (DAP), monoammonium phosphate (MAP)
• Note most P concentrations in fertilizers are
  reported as P2O5
• This is very old-fashioned and misleading,
  because P2O5 is only 44 P (P2O5 weighs 2 x 31
  5 x 16 144 P 62/142).
• Book discusses agronomic P fertilization
• In forests, P cycling is a major factor allowing
  long-term responses
• Also can have long-term increases in soil
  adsorbed P

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912 Soil Potassium

• Second most in terms of plant use.
• No volatile phases.
• Mineral sources in soils are important.
• Organic sources in soils are not important.
• Taken up as cation (K )
• Often limiting and often added third number on
  fertilizer bag
• Roles in plants stomatal control, cell
  division, translocation of sugars, enzymes

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• Highly soluble
• Mineral phases (micas and orthoclase feldspar)
  are highly insoluble
• Soil total K is often very large
• K can be "fixed" between 21 clays this form
  slowly available to plants
• Exchangeable K is the major form available to
  plants and is only a fraction of the total.
• Smaller than exchangeable Ca2 and Mg2
• But much larger than exchangeable NH4

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Important features of the K cycle

• Atmospheric deposition is moderately important
• No gaseous phases
• Usually leaching gt deposition (net loss from the
  system)
• Foliar leaching is important - can equal or
  exceed litterfall (See Table from IFS on next
  slide)
• Soil Total Pools are very large
• Exchangeable pools can be larger relative to
  vegetation than is the case for N
• Fertilizer K can result in long-term increases
  in exchangeable K pools (unlike N)

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Properties of Macronutrient Cycles in Forest Soils
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K Management

• Book discusses crop K fertilization.
• In forests, K cycling is a major factor allowing
  long-term responses
• Also can have long-term increases in soil
  exchangeable K
• K Fertilizers Table 9-10.
• Added as ionic K form with various anions.
• Note also that often expressed as K2O.
• This is 39.12 16 94.2 K in this is 83

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913 Soil Calcium
Ca2 and Mg2 have many similarities in
soils Both divalent Primary minerals are the
major source for both Abundant in most soils
(except acid soils) Mass flow dominates plant
uptake Difference Ca2 is immobile in plants
(pectates, etc) Mg2 is mobile (chlorophyll)
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• Present in many primary minerals and very
  abundant in soils
• Forms secondary minerals (calcite, CaCO3
  gypsum, CaSO4)
• Dominates the exchanger in non-acidic soils
• Ca2 dominates exchange sites and soil solution
  in non-acidic, non-serpentine soils
• Ca2 also usually dominates the base cation
  component in acidic soils (where H and Al3
  dominate the exchange sites)
• Ca is a component of cell wall material in
  plants very immobile in plants

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• Mass flow is usually adequate for transport to
  roots
• Very rarely limiting in nature and when so often
  indistinguishable from Al toxicity
• Very large variation in plant demand for Ca
• In forests we have high Ca uptake trees (oaks,
  hickories, aspen, cedars)

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Calcium Deficiencies

• Rare, because soils low in Ca are usually
  extremely acidic and have Al3 or H toxicity
  first
• Large quantities of Ca2 are added in lime
• Can also add gypsum or CaCl2

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914 Soil Magnesium

• Present in many primary minerals
• Mg2 form
• Adsorbed to exchange sites about equal to Ca2
• Usually second most abundant in non-acid soils
  and soil solutions
• Forms less soluble carbonates and sulfates than
  calcium does
• Mass flow dominates uptake mechanism

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• Role in chlorophyll in plants mobile in plants
  less variable in plant uptake than Ca
• Deficiencies common in acidic soils
• Fertilizers dolomitic lime, Mg,K2 -SO4, MgSO4
  (epsom salts)

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The book does not show Ca and Mg cycles

• Schematic on the next slide shows salient
  features
• Primary mineral source via weathering
• Soil organic matter not important source or sink
  (we assume)
• Atmospheric deposition can be important
• Ecosystems usually show a net loss (soils
  acidify)
• No internal translocation for Ca but there is
  for Mg

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Properties of Macronutrient Cycles in Forest Soils
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Properties of Macronutrient Cycles in Forest Soils
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915 Soil Sulfur
Many similarities to N Atmosphere a major
source Role in three plant proteins Mobile in
plants (translocates) Gaseous phases Few
mineral phases (sulfides) CS and NS ratio can
control mineralization Oxidized and
reduced forms
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• Major differences from N
• Inorganic form (SO42-) can be a major form in
  plants and soils
• Some soil mineral sources
• Inorganic soil SO42- pools can be quite large
• Not limiting as often (air pollution)
• Required by plants in much lower quantities

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• Excess S in plants present as SO42- (not in
  book)
• Some foliar SO42- appears to be necessary
• Foliar SO42- is a good diagnostic for S
  deficiency in trees
• Anionic form in aerobic soils (SO42-)
• "easily leached" according to book not always
  true
• Maybe very immobile in acid soils
• Adsorbed to Fe, Al hydrous oxides
• Some acid forest soils retain 50-80 of
  atmospherically-deposited S (See next table from
  IFS study)

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Properties of Macronutrient Cycles in Forest Soils
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• Elemental S or sulfide S (FeS, PbS) is oxidized
  by Thiobacillus to sulfuric acid in aerobic soils
  (Insight on p. 297)
• Sulfate is reduced to sulfide (S2-) by
  S-reducing bacteria in anaerobic soils
• This further forms either H2S (hydrogen sulfide
  gas, rotten egg smell in swamps) or FeS

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S Management

• S is supplied from
• Natural sources
• Soil organic matter (major reservoir in non-acid
  soils)
• Atmospheric deposition
• Minor except in volcanic and polluted areas But
  may be major source over the very long term - no
  S fixation
• Some soluble minerals (FeS in acid soils, CaSO4
  in arid soils)

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• S is supplied from
• Anthropogenic sources
• Air pollution - atmospheric deposition (acid
  rain, SO2)
• P Fertilizers
• Pesticides

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• S deficiencies are becoming more common in crops
  because of reduced S emissions, reduced use of S
  in P fertilizer and pesticides
• S deficiencies in forests are very rare
  (unpolluted areas like Australia, PNW US)
• S fertilizers ammonium sulfate, potassium
  sulfate, ammonium thiosulfate (NH4)2S2O3,
  gypsum, elemental S

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The Micronutrients

• B, Fe, Mn, Zn, Cu, Cl, Mo
• All essential for plant growth, but taken up in
  very small quantities
• Except for Cl, the dominant role of micros is
  enzyme activation
• Role of B not well understood seems related to
  shoot growth and water
• Most come from soil primary minerals (but not
  usually Cl)

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916 Soil Boron

• Essential for growth of new cells
• Not mobile in the plant
• Forms a weak acid in soils (boric acid, H3BO3)
• At pHgt8.5, forms borate B(OH)4- thus mostly
  in un-dissociated form and easily leached in
  soils
• H3BO3 H2O ? B(OH)4- H

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• Non-metal sources include
• Primary minerals (as trace element) SOM
  Adsorbed to Fe, Al hydrous oxides
• Exists as H3BO4 or B(OH4)- in soil solution
  Commonly deficient in forests of PNW, Australia,
  Scandinavia
• Fertilizer Borax (sodium tetraborate, Na2BO4O7
  . 5H2O 14 B)
• Problems with leaching away too fast Must use
  great care not to over-fertilize toxic (we have
  this at Steamboat area) (Tables 9-12 and 9-13)

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917 Soil Chloride

• Exists as Cl-
• Highly soluble at all pH's
• Cycles rapidly
• Poorly adsorbed to soils often used as a tracer
  for water
• Believed to have a role in osmosis
• About the same concentrations as S (0.2)
• Salt tolerant plants may contain 10
• No known deficiencies, but may be disease related

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918 Soil Copper

• Deficiencies
• Organic amendments and organic soils
• Sandy soils (low total Cu contents)
• Calcareous soils
• Common in forest nurseries
• Can be toxic (boat bottoms)
• Add very little - 1.2-2.5 kg ha-1 can supply
  plants for many years
• Types of fertilizer (Table 9-14)

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• Plant enzymes
• Exists at cupric (Cu2) and less as cuprous
  (Cu) ions
• Plants absorb Cu2, but CuOH is common in less
  acid soils
• Cu(OH)2 at neutral to alkaline pH's
• Most common copper mineral is CuFeS2
  (chalcopyrite)
• Strongly bonds with OM

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919 Soil Iron

• In soils
• Least soluble in high pH and aerobic soils
  (mainly Fe3, as FeOH3)
• For that reason, liming often causes iron
  chlorosis
• Most heavily weathered soils are rich in Fe(OH)3
• Problem is not supply of iron, but how to keep
  it soluble for plant uptake
• In anaerobic soils, Fe2 is more soluble and can
  leach

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• Chelates and solubility
• Soil solution pH is important for Fe solubility
  ( Fig 9-13)
• Fe is soluble enough for plant needs at pH 3
  however, this is too low for most plants and many
  other nutrients
• Solubility decreases 1000 X per unit pH rise
• Chelation is what keeps iron available organic
  "claw" around Fe atom, keeping it in solution
  (Fig 9-14)

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• Ligand the organic molecule
• Chelate Fe organic
• Some plants can produce these chelates when Fe
  deficient (siderophores, iron chelate reductases)
• Some plants produce too much siderophores
  (mutant pea) and creates Fe toxicity
• Chelation potential Fe3gtAl3gtCu2gtCo2gtZn2gtFe2
  gtMn2gt
• Ca2Mg2

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• Deficiencies, Fe fertilization
• High pH calcareous soils, arid soils
• Rare in forests except nurseries
• Some plants have high Fe demand (VG Table 11-3)
• Visual general extreme yellowing, even
  whitening (iron chlorosis)
• Fertilize with chelates commonly also foliar
  sprays (but these do not last long - 3-4 times/yr)

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920 Soil Manganese

• Many enzyme reactions in plants and electron
  transport
• Occurs as Mn2 in soil solution
• In aerobic soils, precipitates as MnO2 (Mn4)
• OM decomposition furnished electrons to reduce
  Mn4 to Mn2
• Deficiencies
• Sandy soils, organic soils, high pH soils
• Interveinal chlorosis of younger leaves
• Mn toxicity can be a problem in acidic soils
  liming can control this

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921 Soil Molybdenum

• Occurs as molybdate anion (MoO4-)
• Many of the same reactions as phosphate - ads to
  Fe and Al hydrous oxides
• More available as pH increases because of this
• May be toxic to grazing animals
• Very low amounts
• Very low amounts needed 0.04 to 0.4 kg ha-1

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922 Soil Zinc

• Essential for enzyme systems
• No oxidation reduction reactions as for Fe
• Occurs as Zn2, and above pH 7.7, as Zn(OH)
• Does not precipitate to Zn(OH)2 until pH 9.1
• ZnS is only major insoluble form in soils
• Less soluble in anaerobic than in aerobic soils

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• Deficiencies
• Basic soils (limed, naturally high pH)
• High Zn plants (corn, onions, fruit trees)
  (Table 11-6)
• Commonly deficient in forest nurseries and
  occurs in plantation forests
• Interveinal chlorosis in both younger and older
  leaves
• Solubility increases 100 X with each unit pH
  decrease
• Zn fertilizers (Table 9-21)
• Foliar sprays also used, but not efficient

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