Soil and Fertilizer S, Ca, and Mg

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Chapter 8

• Soil and Fertilizer S, Ca, and Mg

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Soil and Fertilizer S

• Source of soil S (total content in soils may
  range from a few 100 to several thousand lb
  S/acre)
• Metal sulfides (e.g. FeS, pyrite)
• Gypsum, CaSO4 . 2H2O (neutral salt)
• Elemental S
• Atmosphere, SO2
• Contributes about 6 lb S/acre/year by rainfall in
  Oklahoma
• Most (70) is from natural causes, such as
  volcanoes
• Ocean, 2700 ppm SO4 (0.27)
• Irrigation additions for every 1 ppm of SO4-S
  (or anything else, for that matter) in the
  irrigation water, there will be added 2.7 lb/acre
  to the soil with each acre-ft of irrigation.
• 2.7 X ppm S lb S/acre foot of irrigation

1 acre foot 325,851 gallons Water 8.34
lbs/gallon 8.34 325,851 2,717,597 lbs
U-Cal
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Solution S

• Present as SO4 in amounts ranging from between 1
  to 100 ppm
• Form absorbed by plants
• Concentration changes rapidly depending upon
  uptake and leaching
• In equilibrium with solid forms, like gypsum in
  arid and semi-arid soils
• Saturated gypsum (CaSO4 ? 2H2O) solution contains
  about 2,410 ppm gypsum, or about 450 ppm SO4-S.
  This is more than enough to meet the needs of
  vigorously growing plants.
• Relatively mobile in soils
• Leaches in conjunction with cations like K, Na,
  Ca2, and Mg2

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Exchangeable SO4

• Exchangeable SO42-
• Most important in highly weathered acid soils
  that have a high (or significant) anion exchange
  capacity
• Organic S
• In non-calcareous soils, S behaves in soils like
  N, and organic matter accounts for gt90 of total
  soil sulfur
• C, N, and S are closely related in soil organic
  matter, with common ratio of about 1210.14, and
  NS of about 71
• For every 7 lb of N mineralized there may be an
  associated 1 lb of S mineralized
• Mineralization and immobilization of S is similar
  to that for N as far as factors affecting the
  processes, the end product, and effect of the
  processes relative to plant available S.

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Precipitated sulfate compounds

• In calcareous soils SO42- precipitates as
  sparingly soluble gypsum and epsomite (MgSO4 .
  7H2O).
• Equilibrium solution SO42- concentration far
  exceeds that required to meet plant requirement
• Subsoils, even in humid climates, usually are
  much higher in SO4 concentration than surface
  soil, especially if there is an accumulation of
  clay in the B horizon

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S oxidation - reduction reactions

• Anaerobic environments produce H2S (rotten egg
  smell) (S link)
• S response? of years?
• S emissions
• Elemental So can be oxidized by thiobacillus sp.
  in the presence of oxygen as described by the
  general reaction
• So 1½ O2 H2O gt H2SO4 gt 2 H
  SO42-
• This provides a source of available SO42- as well
  as an acidifying effect on the soil
• The reaction is slow and usually requires several
  weeks/months to affect a change in soil pH

H2S lethal
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Soil Test S

• Usually measures water soluble and easily
  exchangeable SO42- - S
• Saturated calcium phosphate
• Ammonium acetate
• Only of importance in humid regions, even then
  soil test is of questionable value
• Often recommend blanket S fertilizer for sandy,
  low organic matter soils
• Fertilizer S
• Many minor formulations, most economical is
  gypsum (17 S)
• K-Mag (22 S)
• Ammonium Sulfate (21-0-0, 24S)
• Slow release fertilizer forms include
• animal waste
• gypsum
• S-coated urea (about 25 S)

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Soil and Fertilizer Calcium

• Soil Ca
• Content depends upon mineralogy, rainfall and CEC
  to a greater degree than for K and Mg
• Ca bearing minerals, except for carbonates and
  sulfates, are too slowly weathered to supply crop
  needs as a sole source. However, this is seldom
  a circumstance.
• High rainfall leaches Ca out of soil over
  geologic time, however, plant growth and the
  consequent recycling (most plants contain
  relatively high amounts of Ca (.5)) continually
  replenishes the surface where Ca is held on CEC.

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Soil and Fertilizer Calcium

• Soil solution
• May contain from 30-300 ppm. For corn 15 ppm Ca
  in soil solution is related to max yield.
• Mass flow (because solution concentration is
  usually high) and root interception are major
  uptake mechanisms.
• Deficiency is uncommon
• Low supply of available Ca (Ca 2 ) is associated
  with very acid soil. Correction of acidity
  (addition of CaCO3) usually supplies more than
  enough available Ca.
• Some crops may have difficulty getting enough Ca
  translocated to plant parts with high demand
  under certain special circumstances (e.g.
  peanuts).
• Soil test by determining exchangeable Ca, similar
  to that for K.
• Fertilizer Ca
• Use lime or gypsum

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Soil and Fertilizer Mg

• Magnesium behavior in soils is more like calcium
  than any other element. As a general rule, Mg
  salts (compounds) are usually slightly more
  soluble than Ca salts (also, Mg2 is less tightly
  held on exchange complex than Ca).

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Magnesium

• Soil Mg
• Content varies with parent material and climate
  (rainfall) under which soil developed.
• Ranges from a few 1000 ppm to a percent or more.
• Acid, highly leached soils are lowest and most
  likely to be deficient.
• Exchangeable Mg is most important available form
• Deficiency is more common than for Ca
• May be a result of low CEC, high rainfall, and
  abundant Ca.
• Grass tetany (hypomagnesmia) is a disease or
  malady of livestock that have low Mg blood levels
  relative to K and Ca.
• Most economical remedy is to supply Mg supplement
  free choice to livestock.
• Soil test is determination of exchangeable Mg,
  using same extraction as for K and Ca.

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Magnesium

• Fertilizer Mg
• K-Mag (11 Mg)
• Aglime (most aglime contains some MgCO3)
• Dolomitic lime (contains significant amounts of
  MgCO3)

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MICRONUTRIENTS

• Fe, Zn, Mn, and Cu
• All are absorbed by plants as the metal cation
• All are immobile in soils
• All form relatively strong chelates, both
  naturally and synthetically
• strength of formation (strength with which the
  metal ion is held) is in the order CugtFegtZn, Mn

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Solubility (review)

• Solubility of a substancequantity that
  dissolves to form a saturated solution (g of
  solute/L)
• Solubility product
• Equilibrium constant for the equilibrium
  between an ionic solid and its saturated solution

Solid AgCl is added to pure water at 25C. Some
of the solid remains undissolved at the bottom of
the flask. Mixture stirred for 2 days to ensure
an equilibrium is reached. Ag conc. Determined
to be 1.34x10-5M. What is Ksp (solubility
product constant) for AgCl?
AgCl ?? Ag Cl- Ksp AgCl-
At equilibrium, conc of Ag 1.34 x 10-5
conc of Cl- 1.34 x 10-5
Ksp (1.34 x 10-5)(1.34 x 10-5) 1.80 x
10-10
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Aluminum

• The apparent solubility product constant (Ksp)
  for Al(OH)3 in soils is about 10-30. From this,
  the concentration of Al in the soil solution
  and its change with change in pH can be
  calculated.

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Solving the above at pH of 5, OH- would be equal
to 10-9
The concentration of Al (10-3) is moles/liter.
Since the atomic weight of Al is about 27, a
mole/liter would be 27 grams/liter (g/L) and the
concentration of 10-3 is equal to 0.027 g/L, or
27 ppm. 27 ppm at a pH of 5
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Solubility

• Critical to the management and growth of plants
  in acid soils is the knowledge that Al in the
  soil solution increases dramatically with
  decrease in pH below about 5.5. When solved for
  a soil pH of 4.0 (OH- is equal to 10-10), we have

A concentration of 1.0 mole/L is equal to 27 g/L
or 27,000 ppm. While there may not be a
1000-fold increase in soil solution Al 3
concentration when pH changes from 5.0 to 4.0,
these calculations should make it clear why Al 3
concentrations may be significant at pH 4.5, for
example, and immeasurable at 5.5.
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Al toxicity

• Soluble Al is toxic to winter wheat at
  concentrations of about 25 ppm.
• Adverse effect of soil acidity on non-legume
  plants is usually a result of Al and Mn toxicity.
  
• In winter wheat, Al toxicity inhibits or prunes
  the root system and often causes stunted growth
  and a purple discoloration of the lower leaves.
• These symptoms are characteristic of P
  deficiency, and are likely a result of the plants
  reduced ability to extract soil P.

Laboratory exercise, applying P to decrease Al
toxicity?
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Iron

• Soil Fe
• Total content
• ranges from 20,000 to 100,000 lb Fe/acre.
• most is present as Fe2O3 ? 3H2O, which may also
  be written as 2 Fe(OH)3.
• Soil solution Fe
• Amount of Fe2 and Fe3 in soil solution is
  extremely small in all normal soils and is
  governed by the following reactions

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Iron
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Iron
At pH of 7.0Fe 10-39/(10-7)310-39/10-21
10-18 moles/literwith an atomic weight of 55.85
(Fe)Conc. in ppm 55.85 g/mole 1000mg/g
10-18 moles/l 55.85 x 10-15 mg/l 55.85 x
10-15 ppm
pH of 5.0 55.85 x 10-7 ppm (critical amount
10-6) why arent plants deficient at pH 5?
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Iron

• Plant uptake.
• chelates (claw-like organic chemical structures
  that hold metal ions tightly, e.g. Fe in heme, Mg
  in chlorophyll)
• Chelates improve the mobility of metal ions
  because the metal-chelate complex is water
  soluble.
• Chelates are naturally occurring in soils.
  (fulvic and humic acids)
• Synthetic chelates are sometimes used as
  fertilizer.
• Chelate acts like conveyor belt between Fe(OH)3
  and plant root surface

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Iron
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Iron

• Many plants are capable of causing Fe to become
  more available if they experience a deficiency.
  This is most common in dicots and is called
  adaptive response mechanism, triggered by Fe
  deficiency.
• increased production of organic acids
• increased production of chelates
• Fe Soil test.
• Most common is extraction of soil using a
  synthetic chelate, DTPA.
• critical level is 4.5 ppm.

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Iron

• Crop deficiencies
• Crop specific, usually only on high pH soils
  (gt7.5)
• Sorghums and sorghum-sudan are most sensitive
• Fertilizer
• Increase natural chelation by adding organic
  matter to soil (feedlot manure, rotten hay, etc.)
  is most effective long-term remedy.
• Soil applied compounds quickly becomes
  unavailable if they are soluble inorganics (e.g.
  Fe SO4) or are too expensive if they are
  synthetic chelates
• chelates may be economical for high value crops
  (horticultural)
• Foliar application is temporarily effective

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Zinc

• Zn
• Deficiencies are uncommon
• Often a result of high pH, low soil organic
  matter
• corn is most sensitive cultivated crop
• Soil test
• DTPA
• Critical level depends upon crop
• 2 ppm for pecans
• 0.8 ppm for corn
• 0.3 ppm for other sensitive crops
• 0.0 for wheat!
• Fertilize using ZnSO4, 2-6 1b Zn/ac.

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Manganese and Copper

• Mn, Cu Deficiencies are rare
• Cu deficiency most common in high organic matter
  soils.
• Strong chelate complex formed between organic
  matter and Cu.
• Mn toxicity may be more common than deficiency
• Low pH soils (lt4.5)
• Frequently flooded conditions (rice).
• DTPA soil test
• Fertilize using sulfate salt

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Cloride

• Cl Deficiency extremely rare
• Response to Cl is often confounded with disease
  suppression
• Limited to regions that do not receive Cl in
  rainfall or use KCl fertilizer for correcting K
  deficiency.
• Soil test is water extraction of Cl-, critical
  level is about 40 lb/ac 2 ft. deep
• Fertilizer is 0-0-62, KCl

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Boron

• B Deficiency is limited to sandy, low organic
  matter soils in areas of high rainfall
• H3 BO3 is mobile in soils.
• Shallow rooted crops are most sensitive to
  deficiency (e.g. peanuts)
• Soil test is hot-water soil extraction
• Fertilizers are sodium and calcium borate (borax)

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Molybdenum

• Mo Deficiency is limited to areas of low soil Mo
  or where soils are highly weathered and acidic.
• availability strongly linked to soil pH.
• deficiencies can often be corrected by liming
• Large seeded legumes can receive adequate supply
  from normal seed to meet season requirement.
• Deficiencies are so rare that a reliable soil
  test has not been developed
• Fertilization requirement is extremely small
• Seed coating of ammonium molybdate is adequate