Soils 502 lecture no 18 22 phosphorus

Dr.AB Jadhav, SSAC,AC, Pune
Phosphatic Fertilizers
Classification
Production processes of different fertilizers
Behavior and management of P fertilizers
under field conditions
Soil fertilizer Phosphorus-forms of soil P,
Solubilization
Immobilization
Mineralization
Factors affecting the P availability in soil
Dr.AB Jadhav,
SSAC, AC, Pune

1. INTRODUCTION
 The phosphorus is the second most abundant
macronutrient in plants after nitrogen.
 In plants , the concentration of phosphorus ranges from
0.1 to 0.5 %
 Globally 5.7 Billion ha of cultivated land is deficient in
Phosphorus for achieving optimum crop production due to
low diffusion and high fixation rate.
Phosphorus is one of the major plant nutrients and is
directly or indirectly affects plant physiological processes.
2

3
 It has a significant role in sustaining and
building up soil fertility, particularly under
intensive system of agriculture.
 It is key component of energy metabolism and
biosynthesis of nucleic acid.
 Also it plays vital role in photosynthesis in plants
and energy transformation in all forms of life

4
 The total P content is generally highest in soils
developed from granite and gneiss followed by
shales.
 Phosphorus is taken up by plant in the form of
H2PO4, HPO4 through diffusion .

5
2 . FORMS OF SOIL P
Soil P exist in various chemical forms including
inorganic P (Pi) and organic P (Po)
I. Inorganic P (Pi) :
 Inorganic phosphorus consist two forms :
a. Ionic: H2PO4 , HPO4
b. Combine form :
Apatite (Ca5(PO4)3(F,Cl,OH))

6
 Primary P minerals including apetite , strengite
( FePO4 · 2H2O) , and variscite (AlPO4•2H2O) are very
stable, and the release of available P from these
minerals by weathering is generally too slow to meet
the crop demand.
Variscite Strengite
Through direct application of phosphate rocks (i.e.
Apetite ) has proved relatively efficient for crop growth

7
 Secondary P minerals including Calcium (Ca), Iron (Fe)
,and aluminium (Al) phosphates vary in their dissolution
rates, depending on size of mineral particles .
( Reference : Plant Physiol. 2011 Jul; 156(3): 997–1005 Pierzynski et al.,
2005; Oelkers and Valsami-Jones, 2008
II. Organic P (Po) :
 Po generally accounts for 25 to 80 % of total P in
soils.
 Organic P exist in Stabilized forms as inositol
phosphate(C6H12O6)

8
Inositol
 The Po can be released through mineralization
processes mediated by soil organisms and plant roots
in association with phosphatase secretion.
Reference : (Plant Physiol. 2011 Jul; 156(3): 997–1005.Turner et al., 2007).

9
 Role of Phosphorus
 Energy Storage and transfer (ATP)
 Protein biosynthesis
 Stimulate formation and growth of roots.
 Necessary for proper photosynthesis.
 Improves germination.
 Increases biological activity of soil.

 The phosphorus (P) nutrient of all phosphatic
fertilizers is expressed as P2O5.
 In soil, P is present as
 (i) Organic P Inositol phosphate/Soil flora and fauna
 (ii) Inorganic P. H2PO-
4 ; HPO-2
4 ; and PO-3
4 ;
 Out of which, H2PO4 and HPO4 ions are available to
plant. In soil, water in is changed to HPO-2
4 and
 PO-3
4 ions with increase in pH.
-H+ -H+
H2PO-
4 HPO-2
4 PO-3
4
11

12
 Firstly, the P in soil is immobile or slightly mobile around
one cm diameter and therefore, they should be applied in
root zone.
 Secondly, the requirement of P is maximum in the initial
stages. The crop takes up 2/3 of total P when the crop gains
1/3 of total dry matter and hence, the entire quantity should
be applied at one time that is at the time of sowing as a basal
dose.
 Thirdly, water soluble-P is changed to insoluble form as Fe
and Al –PO4 (Phosphate) under acidic and calcium
phosphate in calcareous or high Ca content or in higher pH
soils and hence, there is no danger for the loss due to
leaching and volatilization. The applied P remain as in
available form in less quantity while greater quantity is
changed to insoluble form.

13
Chemistry of P compounds:
 Phosphorus when burns gives P2O5 and with water, it forms HPO3
(Metapohosphoric acid) and H3PO4 (orthophosphoric acid)
 P2O5 + H2O=2 HPO3; HPO3 + H2O = H3PO4.
 These H3PO4 is important in agriculture as it forms three
compounds (salts) by replacing one hydrogen every time.
-H+ -H+
H3PO4 H2PO-
4 HPO-2
4 PO-3
4

14
When H3PO4 combines with calcium, it forms
three salts.
They are
i) Ca(H2PO4
=)2 Monocalcium phosphate
ii) Ca2HPO-
4 Dicalcium phosophate
iii) Ca3 (PO4)-
2 Tricalcium phosophate
Classification of phosphatic fertilizers:
 The phosphatic fertilizers are classified into three
classes depending on the form in which H3PO4
combined with Ca.

1) Water soluble P
containing
 Super phosphate (SSP)
(16 to18% P2O5)
 Double Super
phosphate (DSP)
(32 to 36% P2O5)
 Triple Super
phosphate (TSP)
(46 to 48% P2O5)
 Mono ammonium
phosphate
(20% N and 20% P2O5)
 Diammonium
phosphate
(18% N and 46% P2O5)
3) Citrate and water
insoluble-P
containing
• Rock phosphate
(20 to 40% P2O5)
• Raw bone meal
(20 to 25% P2O5
and 3 to 4% N)
2) Citric acid soluble P
containing
• Basic slags
(14 to 18% P2O5)
• Dicalcium phosphate
(34 to 39% P2O5)
• Rhenania phosphate
(23 to 26% P2O5)
• Steamed bone meal
(22% P2O5)
(Part of P2O5 soluble in
citric acid)
15
Phosphatic fertilizers
Rhenania P: Made by heating ground phosphate rock, alkali carbonate, and silica at a high
temperature (12001400°C).

In the early days P-fertilizers
were manufactured from bones.
P-fertilizers are now
manufactured from rock-
phosphate mineral source found
in igneous, sedimentary and
metamorphic
Minerals bearing P are
apatite (Major), quartz,
silicates, carbonates,
sulphates, sesquioxides, etc.
Two imp. Material required
for P-fertilizer production
are rock phosphate (32%
P2O5) and H2SO4
Commercial rock phosphates
types
1) Fluorapatite [Ca10(PO4)6F2)]
2) Hydroxyapatite [Ca10(PO4)6
(OH2)]
3) Chloroapatite [Ca10(PO4)6 (Cl2)]
Now SSP is being replaced by
DAP as high analysis
fertilizers
Phosphatic Fertilizers

Rock Phosphate
Direct application
Single Super
phosphate
Triple super phosphate
Nitrophosphate
Dicalcium phosphate

Furnace (Thermal)
Process
Apatite is heated to above 14000C in presence of Si and C and
reduced to elemental P in a furnace. It is then oxidized to P2O5 and
dissolved in water to form H3PO3
Heating
Ca10(PO4)6F2)] + 6 SiO2 + 15 C ===== 3 (3CaO.2SiO2) + 1.5 P4 + CaF2 +
15 CO
Apatite 14000C
P4 + 5 O2 ====2 P2O5
P2O5 + 3 H2O ==== 2 H3PO4 (Phosphoric acid)
Phosphoric acid is an important product to produce P
fertilizers
H3PO3 is produced by two processes

Wet Process (Acidulation
route)
Mineral acids like HNO3 or H2SO4 or HCl are reacted with apatite
to produce phosphoric acid. 90.2 % of H3PO4 comes from this
processes.
Cost of production is very less as against furnace processes.
Ca10(PO4)6F2)] + 10 H2SO4 + 20 H2O ==== 6 H3PO4 + 10
CaSO4.2H2O + 2HF

Manufacturing of Superphosphates
In 1842, John Benet Lawes first time produced fertilizer
called “Superphosphate” in England and took a patent in
1843 by treating Coprolit (Fossiized dung) + H2SO4
The name “Superphosphate” applies to fertilizer obtained
by treating finely ground rock phosphate with H2SO4 or
H3PO4
“Superphosphate” contains 16 % P2O5 and referred as normal
single super phosphate (SSP). (11% Sulphur and 21% Ca)
The product made through H3PO4 contains 46 % P2O5 called
as triple super phosphate (TSP)

In India first time SSP
was produced by EID
Parry (India) Ltd. At
Ranipet (Near chennai)
They started with bones +
H2SO4 to produce SSP and
then used rock phosphate
Production tech.simple,
supplied P @ 13% and Ca @
23%
Low analysis fertilizer ,
high cost of transportation,
and application per unit of
the nutrient
Market share is only < 20%
due to low P content. Now a
day DAP is more used.
SSP in India

Finely ground rock phosphate is treated with H2SO4 in the
manufacturing of SSP and TSP
SSP
Ca10(PO4)6F2)] + 7 H2SO4 + 17 H2O ==== 3 Ca (H2PO4)2.H2O) + 7 CaSO4.2H2O
+ 2 HF
Monocalium phosphate
TSP
Ca10(PO4)6F2)] + 14 H3PO4 + 10 H2O ==== 10 Ca (H2PO4)2.H2O) +
2 HF

Specifications of SSP and TSP
Specifications SSP TSP
Moisture content
(maximum)
12 % 12%
Free phosphoric acid
(Maximum)
4% H3PO4
(P2O5)
3 % H3PO4 (as
P2O5 )
Water soluble P
(Minimum)
16 % P2O5 42.5 % P2O5

These fertilizers are produced by
reacting NH3 and H3PO4
Cost of production, transportation and
application per unit nutrient is much
less
Most important
complex fertilizers
are MAP and DAP
In india, DAP was
produced in 1967 by
Gujarat State
Fertilizer
Corporation (GSFC)
MAP is acidic pH is
4.0 and while DAP is
neutral pH 7.0
Grades are 11:52:0 and
18:46:0
Water soluble, good
physical properties
Complex fertilizer- Ammonium Phosphate
(MAP/DAP)

Manufacturing of MAP and DAP
H3PO4 + NH3 ======= NH4H2PO4
Monoammonium phosphate
H3PO4 + 2NH3 ======= (NH4)2HPO4
Diammonium phosphate

P - Transformations in Soil
Predominantly two forms of P in soil
Acid soil Neutral to alkaline soil
Iron-phosphate Ca-phosphate
Al-phosphate
Organic P normally declines sharply
down the soil profile
Content is higher in clayey soil than
coarse textured soils
Poor drainage, High pH and cultivation
practices adversely affects organic P.
Organic P
Inositol phosphate
Comprising 60% of soil
organic P
Nucleic acids
Phospholipid
s

Mineralization
Affected by soil
temperature, moisture,
tillage, OM, etc
Net mineralization of organic P is likely if the organic
residues added to soil have a C/P ratio below 200:1, while net
immobilization of soluble P is likely to occur at the C:P ratio
above 300:1
• Organic P
Microbes
• H2PO4
-
Fe3+,Al3+, Ca2+
• Fe, Al, Ca
Phosphate
Mineralization

Phosphate Adsorption
Phosphate + Surfaces of minerals (Fe and Al oxide and hydroxide minerals,
aluminosilicates clays, calcite)
Phosphate adsorption by minerals of Fe and Al hydroxides
• +
• +
• +
Minerals
of Fe and
Al and
hydroxides
3H2PO
4
+
+
+
• +
• +
• +
Minerals
of Fe and
Al
hydroxide
s
H2PO4
H2PO4
H2PO4
• +
• +
• +
Minerals
of Fe and
Al and
hydroxide
s
3H2PO
4
O
H
O
H
O
H
• +
• +
• +
Minerals
of Fe and
Al and
hydroxid
es
3OH
H2PO4
H2PO4
H2PO4

Phosphate Adsorption on Clay Surfaces
1:1
Aluminosil
icate clays
 Kalonite acquire + ve charges at the brocken edges
in acid
soil
 These + ve charges attracts H2PO4 present in soil
solution
and adsorb them.
 In alakline conditions, positive charges disappear
and - ve
charges appear while at neutral pH no charges exists
1:1 Clays
kaolinite
OH at the broken edges exchange with H2PO4 in the
soil solution and thus H2PO4 ions adsorbed through
the ligand exchange process
2:1
Aluminosil
icate clays
These have a net –ve charge in slightly acidic soils (pH
< 6.5). These – ve charges are neutralized by a part of +
ve charged divalent or polyvalent metallic cations
(Ca2+, Al3+ and Fe3+). The other part of +ve charges of
the cations is neutralized by the negative charges of

i) Fe and Al-oxides and
hydroxides
ii) Clays mineral
iii) pH
iv) Organic
matter
v) Redox state
vii) Role of
microbs
Factors Affecting P Availability
in Soil

i) Fe and Al oxides and hydroxides
Fe and Al oxides present in acid soils have capacity to adsorb large amount
of solution-P rendering it unavailable to plants
The P-fixing capasity is high in soils where amorphous Fe and Al oxides
contents are higher than their cystalline counterpart because of higher
specifi9c surface area.
Thus in highly weathered soils (rich in kaolinite clays with high Fe and Al
oxides);;;;;low P availability
ii) Clay mineral
Black soils /Vertisols will fix more P than do soils with low clay content (Red
and lateritic soils, Alfisols and Ultisols)
Among the clya minerals 1:1 type (Kaolinite) adsorb more P than do 2:1 clays
(montmorillonite).
Cation saturation on clays also affects P availability like
Clay saturated with Ca2+ retain larger amounts of P than those saturated
with Na or monovalent cations.

pH Form
dominat
es
Fixation
At low pH H2PO4
- P is fixed as Fe phosphate (strengite): FeIII PO4.H2O and
Al phosphate (vericite): AlPO4.2H2O
At
moderate
soil acidity
Acidic soil
HPO4
2- &
H2PO4-
At neutral
pH to
slightly
alkaline pH
HPO4
2- Soluble P is converted gradually to dicalcium phosphate
(CaHPO4.2H2O), octacalcium phosphate
[(Ca8H2(PO4)6.H2O], Tricalcium phosphate [Ca(PO4)2] and
hydroyl apatite 3[Ca3(PO4)2] which decreases solubility
abd P availability decreases.
Ca-P > Al-P > Fe-P
iii) pH

iv) Organic matter
OM enhances sorption (a phenomenon of fixation or capture ) of P in soil
and decrease P availability to plants due to associate ion with sesquioxides
and hence possible stabilization of the SOM by the sesquioxides.
The organic matter, notably the humic colloids, competes with P for the
common sorption sites in soil.
The humic and fulvic acid fraction in soil compete strongly with P for P
adsorption sites on geothite (iron oxyhydroxides) and gibbsite (the ultimate
end product of weathering”, and is usually present in high concentrations in
old and weathered soils such as Oxisols and Ultisols. Gibbsite is also found
commonly in young soils, i.e. Inceptisols) at low pH.
This will cause reduced P sorption and enhanced P availability to plants
plus citrate, tartarate and acetate are also effective in this order in reducing
P sorption by soils.
v) Redox state
In submerged soil low redox potential, the available P increases initially
because of reduction of Fe and Mn compounds.
Corresponding ferrous and manganeous phosphates are relatively soluble
compared he higher valent-ferric and manganic phosphates respectively.

vi) Role of microbes
Bacteria, fungi and actinomycets secrete organic acids (lactic, glycolic acids
etc.) and in the solubilization immobilized inorganic phosphates…therby
enhances P availability
Bacteria oxidizing ammonium to nitric acid and sulphur to sulphuric acid also
causes partial and gradual dissolution of phosphate rock and /or the insoluble
calcium phosphates (tricalcium phosphate) thereby enhancing P availability
for plants.

Thank You

Soils 502 lecture no 18 22 phosphorus

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Soils 502 lecture no 18 22 phosphorus