Transcript Chapter 1 Introduction - Oklahoma State University

Chapter 9
Soil and Fertilizer S, Ca, and Mg
Soil and Fertilizer S
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Source of soil S (total content in soils may range from a
few 100 to several thousand lb S/acre)
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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
Atm.
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SO2 + H2O  2H + SO4
(in rain)
2-
Plant residue
2+
2-
CaSO4  Ca + SO4
(in arid and semi-arid soils)
Plant uptake
Anion
exchange
(in tropic
soils)
SO4
2-
Soil
Organic
Matter
Mineralization
Leaching
Solution S
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Present as SO4= in amounts ranging from
between 1 to 100 ppm
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In equilibrium with solid forms, like gypsum in
arid and semi-arid soils
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Form absorbed by plants
Concentration changes rapidly depending upon
uptake and leaching
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
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Leaches in conjunction with cations like K+, Na+,
Ca2+, and Mg2+
Exchangeable SO4
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Exchangeable SO42Most important in highly weathered acid soils that
have a high (or significant) anion exchange capacity
Organic S
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In non-calcareous soils, S behaves in soils like N, and
organic matter accounts for >90% of total soil sulfur
C, N, and S are closely related in soil organic matter,
with common ratio of about 12:1:0.14, and N:S of
about 7:1
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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.
Precipitated sulfate
compounds
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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
S oxidation - reduction
reactions
H2S lethal
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Anaerobic environments produce H2S (rotten egg
smell) S response? # of years?
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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 ========> H2SO4 ===> 2 H+ + SO42-
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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
Soil Test S
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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
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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)
Crop Use and Deposition
• SO4 08/09
Cl 08/09
Ca 08/09
Soil and Fertilizer Calcium
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Soil Ca
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Content depends upon mineralogy, rainfall and
CEC to a greater degree than for K and Mg
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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.
Soil and Fertilizer Calcium
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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
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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).
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Soil test by determining exchangeable Ca, similar to that for K.
Fertilizer Ca
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Use lime or gypsum
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++).
Magnesium
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Soil Mg
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Content varies with parent material and climate (rainfall)
under which soil developed.
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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
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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.
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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.
Magnesium
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Fertilizer Mg
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K-Mag (11% Mg)
Aglime (most aglime contains some MgCO3)
Dolomitic lime (contains significant amounts of MgCO3)
MICRONUTRIENTS
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Fe, Zn, Mn, and Cu
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All are absorbed by plants as the metal
cation
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All are immobile in soils
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All form relatively strong chelates, both
naturally and synthetically
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strength of formation (strength with which the
metal ion is held) is in the order Cu>Fe>Zn, Mn
Iron
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Soil Fe
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Total content
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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
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Amount of Fe2+ and Fe3+ in soil solution is
extremely small in all normal soils and is
governed by the following reactions
Iron
Fe2+ + O2 =======> Fe3+, in cultivated soils, except flooded rice, Fe in solution is
present as Fe3+. The amount of Fe3+ in the soil solution is governed by the
equilibrium reaction
Fe(OH)3 <======> Fe3+ + 3 OHThe equilibrium condition is described by
(Fe) (OH)
Fe(OH ) 3
3
= K eq
and since so little of the Fe (OH)3 actually dissolves the amount remaining is equal to 1.0
for practical purposes and the equilibrium can be described as
(Fe) (OH)3 = Ksp (solubility product constant)
For iron oxide, Fe (OH)3, Ksp = 10-39
2 Fe3+ + 3 O2- + 3 H2O =  Fe2 O3 . 3H2O (rust)
2 Fe(OH)3
Iron
Fe(OH)3 <======> Fe3+ + 3 OH-
In a soil at pH 5, the concentration of Fe3+ in the soil solution can be calculated as
follows:
at pH 5 the OH- concentration is 10-9 because (OH) (H+) = 10-14, and since H+ is 10-5 at
pH 5, the (OH-) must be 10-14/105 = 10-9
Using the equilibrium equation for Fe (OH)3 from above and rearranging terms to solve
for Fe3+, we have
(Fe3+) (OH)3 = Ksp = 10-39,
Fe3+ = 10-39/(OH-)3
At pH of 7.0
=Fe+++ = 10-39/(10-7)3
=10-39/10-21
=10-18 moles/liter
with 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 aren’t plants deficient at pH 5?
Iron
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Plant uptake.
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chelates (claw-like organic chemical structures
that hold metal ions tightly, e.g. Fe in heme, Mg in
chlorophyll)
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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
Iron
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Diffusion of
CHEL-Fe
CHEL-Fe

CHEL
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Fe(OH)3 Fe3+ + 3OH-
Diffusion
of CHEL
CHEL-Fe

Fe3+
+
CHEL
ROOT
Fe-R
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.
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increased production of organic acids
increased production of chelates
Fe Soil test.
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Most common is extraction of soil using a
synthetic chelate, DTPA.
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critical level is 4.5 ppm.
Iron
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Crop deficiencies
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Crop specific, usually only on high pH soils (>7.5)
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Sorghums and sorghum-sudan are most sensitive
Fertilizer
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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
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chelates may be economical for high value crops
(horticultural)
Foliar application is temporarily effective
Zinc
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Zn
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Deficiencies are uncommon
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Often a result of high pH, low soil organic matter
corn is most sensitive cultivated crop
Soil test
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DTPA
Critical level depends upon crop
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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.
Manganese and Copper
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Mn, Cu: Deficiencies are rare
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Cu deficiency most common in high organic matter soils.
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Mn toxicity may be more common than deficiency
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Strong chelate complex formed between organic matter
and Cu.
Low pH soils (<4.5)
Frequently flooded conditions (rice).
DTPA soil test
Fertilize using sulfate salt
Cloride
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Cl: Deficiency extremely rare
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Response to Cl is often confounded with disease suppression
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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
Boron
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B: Deficiency is limited to sandy, low organic matter soils
in areas of high rainfall
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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)
Molybdenum
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Mo: Deficiency is limited to areas of low soil Mo
or where soils are highly weathered and acidic.
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availability strongly linked to soil pH.
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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