Soil fertility and Fertility use

1. Indira Gandhi krishi vishwavidyalaya ,Raipur(C.G.)
Assignment on – 2022-23
Topic – Phosphorus reactions in Acid and Alkali soils, Factor affecting Phosphorus availability in
soils, Phosphatic Fertilizer Behaviour in soils and management under field conditions.
Course title – Soil Fertility and Fertility use.
Course no – SOILS- 502
Course credit – 3+1
Department - Agronomy
Submitted To – Submitted By -
Dr. Yusma Sao Mr. Mohan Sahu
Assistant Professor of Soil Science M.Sc.1st Sem & 1st year

2. Contents
 Problems of phosphorus in Acid and Alkaline Soil.
 Reactions of phosphorus in soils.
 Types of fixation of phosphorus.
 Phosphate Fertilizer Behaviour in soils.
 Factors affecting Phosphorus availability.
 Management of Phosphorus in field Conditions

3. Phosphorus is an essential macro-element, which require to
photosynthesis, energy transfer and synthesis and
breakdown of carbohydrates for plant.
Phosphorus presents in soil as a form
of phosphates (PO43−), mono-hydrogen phosphate
(HPO42−), and di-hydrogen phosphate (H2PO4−)
Phosphorus is most available to plants when soil is at pH
range between 6.5-7.5.

4. Problems of phosphorus (Acid & alkaline soil)
IN ACID SOIL
 Acid soils formed because of the results continuous additions of acid-
forming fertilizers.
 Naturally acid soils are usually found within the tropical, by the results
of thousands of years excessive weathering of soil minerals.
 Year-around high temperatures and high rainfall annual precipitation >
600–800 mm leaches all basic cations (such as Na, Ca, Mg, and K) and
pH buffering minerals (such as carbonates).
 Climatic condition give promotes to transform and subsequent leaching
of Si from Si-based minerals, leaving acidic iron and aluminum oxides
minerals.

5. IN ALKALI SOIL
 Saline and sodic soil is characterized by their electrical
conductivity (Ec), soil pH, and exchangeable Na%. When
exchangeable Na > 15%, soil aggregates disperse, reducing
permeability to water.
 In saline soils soluble salt concentration restricts the plant
growth, though salt tolerance differs with plant species.
 In sodic soils, excess Na disperses clays and may create
nutritional disorders. Generally saline-sodic soils content high
both salt and Na.

6. REACTIONS OF PHOSPHORUS IN SOILS
 ADSORPTION
In this reaction phosphorus reacts with the surface of minerals (e.g., iron and
aluminium oxide and hydroxide minerals, silicate clay minerals, calcite) if
phosphate concentration in soil solution is low. Thus, phosphorus is held on the
mineral surface. Adsorption reaction occurs in acid, neutral, alkaline and
calcareous soils.
Adsorption in acid soils The above discussion indicates that in acid soils
adsorption occurs on the surface of iron and aluminium oxide and hydroxide
minerals silicate clay mimirals.

7. 1.Adsorption on surface of iron and aluminium oxide and
hydroxide minerals
This adsorption is caused by two mechanisms as follows:
1. These minerals predominate in acid soils. They have a net positive charge
(although they have both positive and negative charges) on their surface.
These positive charges attract primary orthophosphate ions (H2PO4). As the
soils are acidic, primary orthophosphate ions HPO) predominate in soil
solution. Secondary orthophosphate ions (HPO4) do not predominate.
Hence, primary orthophosphate ions (H,PO) are attracted.
2. The hydroxyl anions (OH-) are held on the surface of the above mentioned
minerals by electrostatic attraction between negative charges of the anions
(OH) and positive charges on the surface of minerals. These hydroxyl
anions (OH) are exchanged with primary orthophosphate anions (H₂PO)
present in soil solution. Thus, phosphates are adsorbed.

8. 2. Adsorption by Soluble Aluminium
In strongly acid soils, soluble aluminium (Al) predominates. These aluminium ions (A) undergo hydrolysis
forming aluminium hydroxy cations.
The aluminium hydroxy cations [Al(OH)21 react with soluble orthophosphate ions(H2PO4) forming
insoluble aluminium hydroxy phosphates [Al (OH)2H2PO4).
Thus, phosphorus is adsorbed. When concentration of primary orthophosphate ions(HPO is HIGH,
phosphorus is precipitated by the same way.
3. Adsorption on Surface of Silicate Clays
This adsorption is caused by the following mechanisms:
1. Kaolinite (1: 1 silicate clays) acquires positive charges on broken edges in acid condition. It can be
represented as follows:OH Si
These positive charges depend on pH. They exist in acid condition. In alkaline condition, they disappear
and negative charges appear. At neutral condition no charge exists.

9. 2. The silicate clay minerals, particularly the 1: 1 minerals e.g., kaolinite have
exposed hydroxyl anions (OH) at their broken edges. These anions (OH) are
primary orthophosphate anions (H2PO4). Thus, phosphates are adsorbed.
Exchanged
3. The silicate clay minerals, particularly the 2: 1 minerals have net negative
charges in slightly acidic soils (pH slightly less than 6.5). These negative
charges are neutralized by a part of positive charges of a divalent or polyvalent
metallic cation (e.g., Ca 2+/Al 3+ / Fe3+). The other part of positive charges of
the cation is neutralized by the negative charge of primary orthophosphate
anions (H2PO4).

10. 4.Adsorption in neutral, alkaline and calcareous soils
Primary or secondary orthophosphate anions (H₂PO, or HPO) replace carbonate anion (CO) on
the surface of calcium carbonate (CaCO3) that predominates in neutral, alkaline or calcareous
soils and thus, they are adsorbed on the surface of calcium carbonate (CaCO3).
PRECIPITATION
In this reaction, phosphorus reacts with iron and aluminium oxide and hydroxide minerals or
with calcium carbonate mineral forming precipitates of hydrated or hydroxy phosphates of iron
and aluminium and of calcium phosphate, if the phosphate concentration in soil solution is
HIGH. Precipitation occurs in (i) acid and (ii) neutral, alkaline or calcareous soils.

11.  Precipitation in acid soils In acid soils,
iron and aluminium oxide and hydroxide minerals predominate.
Whenphosphate (H2PO4) concentration in soil solution is high, phosphate
(H2PO4) reacts with these minerals forming precipitates of iron and
aluminium hydroxy phosphates.
In this reaction, the hydroxyl anion (OH) of iron hydroxide mineral is
replaced by primary orthophosphate anion (H2PO4).

12.  Precipitation with Aluminium Hydoxide Mineral
In this reaction, the hydroxyl anion (OH)of aluminium hydroxide mineralis replaced by primary
orthophosphate anion (H2PO4).
In strongly acid soils, iron and aluminium cations (Fe3+ and Al 3+) predominate in soil solution. They react
with primary orthophosphate ions (H2PO4) forming precipitate of their hydroxy phosphates. These
reactions may be demonstrated as follows:
 Precipitation with iron

13.  Precipitation with Aluminium
 Precipitation in neutral, alkaline and calcareous soils
In these soils, soluble calcium (Ca2+) that predominates reacts with orthophosphate anions (H2PO,
and HPO forming precipitates of secondary calcium phosphate minerals. The reactions that occur may
be suggested as follows:
. Reaction I shows that calcium cation (Ca) reacts with secondary orthophosphate anion (HPO) that
predominates in neutral, alkaline and calcareous soils forming precipitates of dicalcium phosphate
(CaHPO4). It takes few weeks for completion. Reaction 2 shows that calcium cation (Ca) reacts with
primary orthophosphate anion (H2PO4) that is present in small amount forming monocalcium
phosphate [Ca (H2PO4)21.

14. Types of Reactions/Fixation of Phosphorus
Types of Reactions of PhosphorusThe reactions of phosphorus in soils may be of several types as
follows:
 Precipitation
Phosphorus is precipitated as iron and aluminium hydroxy phosphates or as calcium phosphates on the
surface of solid particles of hydrous oxides of iron and aluminium or of silicate clay minerals or of
calcium carbonate
 Adsorption
Phosphates are adsorbed (or held) on the surface of solid particles of hydrous oxides of iron and
aluminium or of silicate clay minerals or of calcium carbonate
 Absorption or Chemisorption
Phosphates previously adsorbed (or held) on the surface of solid particles of either hydrous oxides of
iron and aluminium or calcium carbonate penetrate into these particles.

15. Labile and Non- Libile Phosphorus
Based on the nature of sorption in acid soils high in hydrous oxides of iron
and aluminium [Fe(OH)3 and Al (OH)3) phosphorus may, sometimes, be
labile (readily releasable) and sometimes, be nonlabile (hardly releasable).
Labile - The labile phosphorus is loosely held (adsorbed) by the hydrous
oxides of iron and aluminium [Fe(OH)3 and Al (OH)3] and can readily be
released (desorbed) into soil solution.
Non-Libile - the nonlabile phosphorus is tightly held (chemisorbed) by the
hydrous oxides of iron and aluminium [Fe(OH)3 andAl (OH)3) and can
hardly (with difficulty) be released (desorbed) into soil solution.

16. Phosphorus Fixing Soils/Soils Having High Phosphorus Fixing Capacity
(Power) (High Retention/Adsorption/Sorption)
 Highly weathered acid soils containing large quantity of hydrous oxides (oxides
and hydroxides) of iron and aluminium.Acid organic soils containing small
quantity of hydrous oxide (oxide and hydroxide) of iron and aluminium may not
fix (adsorb/ precipitate) large quantity of phosphorus.
 Neutral or alkaline soils containing large quantity of calcium carbonate
(CaCO₂).
 Alkaline soils high in soluble calcium (Ca2+).
 Fine-textured soils (ie., heavy soils) high in clay content, such as clay soils
 Soils containing more 1: 1 silicate clays (e.g., kaolinite) than 2 : 1 silicate
clays(e.g., montmorillonite).

17. Phosphate Fertilizer Behaviour in soil
 Fates of Single super phosphate (SSP) In Soils
When SSP is added to soils, a series of chemical reactions takes place which is discussedhereunder.
Reactions of Monocalcium Phosphate (MCP)
When added to a soil, undergoes reactions. 1When added to a soil (of any pH), the droplets (or
granules or pellets or bands) of thefertilizer absorb soil water (or moisture). This water reacts with
MCP in the fertilizer (hydrolysis) to yield phosphoric acid (H3PO,) and dicalcium phosphate dihydrate
(DCPD) [CaHPO2H₂O].

18.  The relative amount of dicalcium phosphate dihydrate (CaHPO4.2H2O) and dicalcium
phosphate anhydrous (CaHPO4) depends on the moisture content of soil. At high soil
moisture content dicalcium phosphate dihydate (CaHPO4.2H2O) is formed while at
low soil moisture content dicalcium phosphate anhydrous (Ca HPO4), is formed. This
reaction occurs rapidly. Phosphoric acid (H3PO4) and dicalcium phosphate form in few
days or weeks, even in less than 1 hour.
 Fates of orthophosphate anions
Plant uptake -
Leaching-
Fixation-
Immobilization -
Volatilization-
 Plant uptake -As mentioned earlier, a portion of orthophosphate anions (H₂PO HPO2)
is assimilated by plants.

19.  Immobilization- orthophosphate anions (H2PO4, HPO2) is assimilated by
soil microorganisms, if large quantity of undecomposed (raw/residues)
organic matter low in A portion phosphorus (< 0.2%) but high in other
nutrients is added to soil. This phenomenon is similar to the assimilation of
ammonium (NH4) and sulphate (SO,2) by soil microorganisms.
 Leaching- A portion of orthophosphate anions (H2PO4 HPO4) may be
subjected to leaching.
 Volatilization-A portion of orthophosphate anions (H2PO4, HPO42) may be
volatilized.
 Fixation – A portion of orthophosphate anions (H2PO4, HPO42) is adsorbed
or precipitated in soils (See

20.  Reactions of Gypsum
The sparingly soluble gypsum (CaSO4. 2H2O) present in large amount (about 50%) in single
superphosphate very slowly dissolves in soil to yield calcium and sulphate ions (Ca2+and SO,2).
Both the ions (Ca2+, SO,2) are assimilated by plants or subjected to other reactions. Thus,not
only phosphorus but also calcium and sulphur are supplied by single superphosphate to plants.
FATES OF AMMONIUM PHOSPHATES IN SOILS
When added to a soil the droplets (or granules or pellets or bands) of the fertilizer absorb soil
water. This water reacts with the fertilizer to yield phosphoric acid. The reactionsare probably as
follows.

21.  FATES OF AMMONIUM POLYPHOSPHATE IN SOILS
When added to soils, this fertilizer undergoes the reactions as follows:
Hydrolysis
Adsorption
Sequestration
Precipitation
 Hydrolysis
As stated above, ammonium polyphosphate fertilizer contains ammonium orthophosphate,
ammonium pyrophosphate and other long-chained ammonium polyphosphates. When added to
soil, ammonium pyrophosphate and other long-chained ammonium polyphosphates fractions of
ammonium polyphosphate fertilizer undergo hydrolysis.
Biological hydrolysis needs the activity of PHOSPHATASE enzyme. This enzyme is
contributed by plant roots and soil microorganisms thriving in the rhizosphere. Hence,
biological hydrolysis occurs mostly in rhizosphere.

22. Chemical hydrolysis is very slow. It takes probably few weeks or few months
for completion. It is caused by silicate clays and hydrous oxides, such as iron
oxides. The pyrophosphate is strongly adsorbed on soil surface. In this
situation, pyrophosphate is not hydrolysed.
 Adsorption - Ammonium pyrophosphate may be adsorbed on soil surface.
 Precipitation
In acid soils, ammonium polyphosphate reacts with iron or aluminium to form
insolublecompounds. Probably, the orthophosphate component of ammonium
polyphosphate reacts with iron and aluminium to form insoluble compounds.
In alkaline or calcareous soils, ammonium pyrophosphate reacts with calcium
to form insoluble calcium-ammonium pyrophosphate. Also, it is likely that the
orthophosphatecomponent of ammonium polyphosphate reacts with calcium to
form insoluble calcium phosphates. About 25% of added phosphorus may be
precipitated within first week.

23. Types of phosphorus-
Soil phosphorus is two typesone organic and another inorganic.
 Organic phosphorus is found in plant residues, manures and microbial
tissues.
In soils low organic matter contain about 3% of total phosphorus in the
organic form and high-organic-matter content soils contain about 50% or
more of their total phosphorus content in the organic form.
 Inorganic forms of soil phosphorus consist of apatite (the original source
of all phosphorus), complexes of iron and aluminum phosphates, and
phosphorus absorbed onto clay particles.
The solubility of these phosphorus compounds as well as organic
phosphorus is extremely low, and only very small amounts of soil
phosphorus are in solution at any one time.

24.  Most soils contain less than a pound per acre of
soluble phosphorus, with some soils containing
considerably less.
By application satisfactory phosphorus fertilizer and
crop management practices, soil solution phosphorus
can be replaced quickly enough for optimum crop
production.

25. Factors Affecting Phosphorus Availability In Soils
While the processes such as weathering,
dissolution, mineralization, and desorption
increase phosphorus availability in the soil for
plant uptake, processes such as immobilization,
adsorption, precipitation, runoff, and erosion
decrease the phosphorus availability.

26.  Organic Matter
Organic matter is an important factor in controlling phosphorus availability.
With the addition of organic matter, availability of phosphorus increase
Mineralization of organic matter releases plant- available forms of
phosphorus into soils.Organic molecules will compete with phosphate
adsorbed to soil surfaces and will reduce phosphorus retention. This process
will increase availability of phosphorus.
 Clay Content
Soils with higher clay content have high phosphorus retention
capacity because clay particles have very large surface area per unit
volume, which can adsorb phosphorus easily.

27.  Soil Mineralogy
The mineral composition of the soil influences the phosphorus adsorption capacity. For
example, soils with a high content of Al3+ and Fe3+also tend to have the greatest
phosphorus adsorption capacity.
 Soil pH
Optimum soil pH between 6 and 7 will result in maximum phosphorus availability. At
low pH (acidic soils), soils have greater amounts of aluminum and iron, which form
very strong bonds with phosphate. At high pH when calcium is the dominant cation,
phosphate tends to precipitate with calcium.
 SOIL TEMPERATURE /AERATION /MOISTURE AND COMPACTION
Phosphorus absorption by the plant is decreased by low soil temperature and poor soil
aeration. Starter fertilizers containing water-soluble phosphorus are much more likely
to increase crop growth during cool weather. Excessive soil moisture or soil
compaction reduces the soil oxygen supply and decreases the ability of the plant roots
to absorb soil phosphorus. Compaction reduces aeration and pore space in the root
zone. This reduces phosphorus uptake and plant growth. Compaction also decreases the
soil volume that plant roots penetrate, limiting their total access to soil phosphorus.

28.  SOIL TEST PHOSPHORUS LEVELS
Crop responses to fertilizer phosphorus will be greater and occur more frequently on
soils testing low in phosphorus than on high-testing soils. However, yields on soils with
high P soil test levels usually are higher. The response to phosphorus fertilizer on high-
testing soils is increasing, and it is important to maintain high soil phosphorus levels to
support optimum crop production.
 APPLICATION TIMING
Fixation of soil phosphorus increases with time of contact between soluble phosphorus
and soil particles. Consequently, more efficient utilization of fertilizer phosphorus is
generally obtained by applying the fertilizer shortly before planting the crop. This
practice is especially effective on soils with high phosphorus-fixing capacities. On
coastal plain areas, fertilizers may be applied several months before planting with little
or no decrease in availability of the fertilizer phosphorus to the crop. Banding of
fertilizer for row crops is also much more likely to increase the efficiency of fertilizer
phosphorus on soils of high phosphorus-fixing capacity than on soils of low phosphorus-
fixing capacity.

29. Management of Phosphorus in Field Conditions
 Phosphorus is particularly important in early growth stages.
 It is mobile within the plant and promotes root development (Particularly
the development of fibrous roots),tillering and early flowering.
 Addition of mineral P fertilizer is required when the rice plants root
system is not yet fully developed and the native soil P supply is
inadequate.
 Phosphorus is remobilized within the plant during later growth stages if
sufficient P has been absorbed during early growth.
 It also increases resistance to disease and strengthens the stems of cereal
plants, thus reducing their tendency to lodge. It offsets the harmful effects
of excess nitrogen in the plant.

30.  Application of P fertilizer based on soil test value for P is important to avoid
excess rate of application that cause environmental and economic hazards.
Soils testing ‘high’ will not respond to addition of P fertilizers and hence P
fertilization could be done only in soils with ‘low’ to ‘medium’ P soil test
values
 Using larger fertilizer P particles, which are subject to less fixation, is
sometimes more effective than using finely ground P fertilizer
 Correct placement of fertilizers in the plant root zone will improve P use
efficiency and seedling vigor, and reduce the amount of P loss through surface
runoff and erosion. Band application of P in the root zone reduces surface loss
potential and enhances P availability. By banding, fertilizer P comes into less
direct contact with soil, reducing the opportunity for fixation of soluble P.
 It also increases resistance to disease and strengthens the stems of cereal
plants, thus reducing their tendency to lodge. It offsets the harmful effects of
excess nitrogen in the plant.

31.  Dipping the rice seedlings before transplanting in a slurry of soil and
phosphatefertilizer can increase the efficiency of fertilizer P and economize on
phosphate use.
 Fixation of soil phosphorus increases with time of contact between soluble
phosphorus and soil particles. Consequently, more efficient utilization of
fertilizer phosphorus is generally obtained by applying the fertilizer shortly
before planting the crop. This practice is especially effective on soils with high
phosphorus-fixing capacities.
 Use of Vesicular Arbuscular Mycorrhizae (VAM) for increasing root extraction
of P Extensive hyphal growth of VAM reduces the distance for diffusion of P in
soil and thereby increases P uptake by plants VAM may chemically modify the
rhizosphere through the exudation of chelating compounds or phosphatases,
which help in solubilizing slowly soluble soil P.
 Use of Phosphobacterium (Bacillus megatherium var. phosphobacterium) could
be used for increasing the availability of native soil P

32. Reference -
 Basak Ranjan Kumar ( 2007) A Text Book of Fertilizer ,Kalyani publisher,New
Delhi.
