The document presents information on phosphate fixation in soils. It discusses how phosphorus is an essential nutrient for plant growth but is limited in about 40% of the world's soils due to fixation reactions. These reactions reduce the solubility and availability of phosphorus by adsorbing phosphate ions onto soil particles like iron, aluminum, and calcium compounds. The degree of fixation depends on soil properties like mineral composition, pH, and calcium carbonate content. Phosphate can be temporarily or permanently fixed depending on the reaction conditions, reducing phosphorus efficiency in soils to 10-20%.

1. PRESENTATION ON-
PHOSPHATE FIXATION
)
PRESENTED BY
 STANLEY SAMUEL
 MSc Ag. 1st YEAR
 (DEPT. OF SOIL SCIENCE)
SOIL FERTILITY AND FERTILIZER USE
SOIL -502

3. Phosphate fixation
 RECOGNIZED BY : THOMAS WAY (1950)
 Phosphorus fixation - The reduction of solubility of
fertilizer P that is added to the soil.
 Phosphorous is element to plant growth ,metabolism and
reproduction and is a limiting factor to plant productivity
on an estimated 40 % of the world’ arable soil.
 The main inorganic forms of phosphorous are HPO42- and
H2po4-.(Orthophosphate anions)

4.  These anions are highly reactive and quickly engage in
adsorption reaction on the the surface of mineral particles
and organic matter in the soil.
 These “fixation” reactions decrease the availability of
soluble phosphates for plant uptake.
 The type of orthophosphate ions that exist in the soil are
primarily dependent on pH and soil type

5. SOIL SOLUTION-ORTHOPHOSPHATE FORM
(3-phases)
 Orthophosphate are mainly found in three phases or pools.
 It is the only pool from which plant take up P and is the only pool with
any measurable mobility.
 The active P pool is in a solid phase, formed through :
 1) Loosely held inorganic complexes and are usually held on the soil
surface.
 2)phosphates that react with the element such as iron ,aluminium or
calcium to form soluble solid.
 3)Organic P that is easily mineralized.

6. FATE OF SOLUBLE P
 phosphorus fixation reactions involving iron oxide and
aluminum oxide. While the adsorbed phosphate is labile
and considered to be in a reversible (desorbable)
condition, it may be months or years before biological or
chemical conditions exist to release the phosphate.
 2.The phosphate anion is not strongly adsorbed and may
return to the soluble phase over time. However, labile P is
only a very small fraction of total soil P.
 3.Adsorption reactions can also occur between phosphates
and organic matter.
 4.Organic solubilization is accomplished through microbial
mineralization.

7. FATE OF INSOLUBLE P(fixed pool)
 The Fixed pool of inorganic and organic phosphate
compounds are very insoluble and may remain in soils for
years without being made available to the soil.
 2.Some very slow conversion between the Fixed pool and
the Active P pool does occur in soils.
 1.Phosphate fixation reduces P efficiency to 10-20% --
either temporarily or permanently depending on the stage
of the fixation reactions.

8.  phosphate fixation reactionsbetween iron / aluminum and phosphate
through the creation of coordinate (chelation) bidentate linkages.
This results in very strong bonds that take a much longer time to
desorb.
 2.These bond configurations are much stronger than single atom
bonds (monodentate bonds). Processes that form these types of
adsorption complexes are often referred to as chemisorption,
specific adsorption, or ligand exchange.
 Phosphates trapped by a metal mantle coating are usually deemed as
permanently fixed and cannot be desorbed (released) under normal
soil conditions.
 3.This usually occurs under very acidic conditions.

9.  In alkaline soils, soluble phosphate ions quickly react with
calcium to form a sequence of products of decreasing
solubility.
 initial fixation reaction of calcium carbonate with
phosphoric acid resulting in the formation of Dicalcium
Phosphate.
 2.Continued reactions between calcium carbonate and
phosphate ions result in formation of even more insoluble
phosphate compounds.

10. FACTORS RESPONSIBLE FOR PHOSPHATE
FIXATION
 # 1. Nature and Amount of Soil Components:
Adsorption and desorption reactions are affected by
the type of surfaces contacted by phosphorus in the soil
solution.
(a)Hydrous Oxides of Iron and Aluminium:
When iron and aluminium oxide compounds is soil are less
crystalline, the phosphate fixing capacity of the soil be more
because of greater surface areas. However, crystalline
hydrous oxides are usually capable of fixing more phosphorus
than layer silicates

11. Fixation of phosphorus by oxides of Fe and Al
takes place over a wide pH range that is shown
below:

12. (b) Type of Clay
 Phosphorus is retained or fixed to a greater extent by 1: 1
than by 2: 1 clays and this may be due to the higher
amounts of hydrated oxides of Fe and Al associated with
1: 1 type clays. The phosphate fixing capacity of clay
minerals may be found in the following order:
 Montmorillonite > Vermiculite > Kaolinite > Muscovite.

13. Amount of clay
 (c) Amount of Clay:
 Soils containing large quantities of clay will fix more
phosphorus than that of soils containing small amount of
clay.
 (d)Amorphous Colloids:
 Amorphous alumina silicate minerals like allophane have a
large negative charge which is partly or entirely balanced
by complex aluminium cations. Phosphorus becomes
adsorbed by reacting with this aluminium.

14.  (e) Calcium Carbonate:
 Calcium carbonate (CaCO3) exerts significant influence on
phosphate fixation. Usually higher the content of CaCO3 in
soil, the higher is the fixation of phosphate. Phosphate
may be adsorbed on the surface of calcium carbonate
particles. The solubility of phosphatic minerals has also
been found to be depressed in the presence of CaCO3 (due
to formation of less/soluble tricalcium phosphate and
latter carbonate apatite).
 Exchangeable calcium has also found to be effective in
phosphate fixation (due to linkage between phosphate and
soil colloid via calcium).

15. #2 pH
 Soil pH has a profound influence on the amount and manner in which soluble
phosphorus becomes fixed. Between pH 4.0 and 8.0, H2PO4 and HPO4
– are the
principal phosphate ions in soil. Above pH 9.0, PO4
3- dominates but H2PO4
– is
still present. Different mechanisms of phosphate fixation operate at different
pH levels.
 At pH 2-5 the fixation is chiefly due to the formation of Fe and Al-phosphates.
Al pH 4.5-7.5, phosphates are fixed on the surface of the clay colloids and at
pH 6-10 phosphate is precipitated usually by Ca and Mg. So the fixation of
phosphate in relation to different soil pH

16. FIXATION OF PHOSPHOROUS IN RELATION
TO DIFFERENT PH

17. 2.pH:
Aluminum Precipitation at Low pH
 Form of available P at low pH: H2PO4
- (pH 3-6)
H2PO4
- combines with free Al3+ and Fe3+
Al3+ + + H2PO4
- + 2H20 = Al(OH)2H2PO4 + 2H+
Al(PO4) • H2O
Variscite

18. Calcium Binding in Basic Soils
CaCO3
CaCO3 + 2H2(PO4)- = Ca [H2(PO4)]2 + CO3
2-
CaHPO4
Ca5(PO4)3OH (Apatite mineral)
(higher calcium availability)
H2(PO4)- is the available form of P

19. REFERENCE:
1. https://www.ipni.net/ppiweb
2. pubs.acs.org/doi/pdf/10.1021/ie50385a00
3. www.soilmanagementindia.com › ... › Phosphate › Phosphate Fixation
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