RESEARCH ON PHOSPHORUS SORPTION AND SILICATE APPLICATION TO IMPROVE PLANT AVAILABLE PHOSPHORUS CONTENTS IN RICE SOILS IN SOUTHERN VIETNAM

MINISTRY OF EDUCATION
AND TRAINING
MINISTRY OF AGRICULTURE
& RURAL DEVELOPMENT

VI VIETNAM ACADEMY OF AGRICULTURAL SCIENCES C



TRAN THI TUONG LINH


RESEARCH ON PHOSPHORUS SORPTION AND
SILICATE APPLICATION TO IMPROVE PLANT
AVAILABLE PHOSPHORUS CONTENTS IN RICE SOILS
IN SOUTHERN VIETNAM


Subject: Soil Science
Code: 62 62 01 03



SUMMARY OF DOCTORAL THESIS
IN AGRICULTURAL SCIENCE

Ho Chi Minh city - 2014



This thesis is completed at:
VIETNAM ACADEMY OF AGRICULTURAL SCIENCES


Supervisors: 1. Prof. DSc. PHAN LIEU
2. PhD. VO DINH QUANG



Reviewer 1: PGS. TS TRAN KIM TINH
Reviewer 2: PGS. TS PHAM VAN HIEN
Reviewer 3: TS LE XUAN ĐINH



The thesis will be protected against the Scientific Commitee at
institution level at Institute of Agricultural Science for Southern
Vietnam, ………………………………………………………….



This thesis can be refered at:
1. The national library
2. The library of Vietnam Academy of Agricultural Sciences
3. The library of Institute of Agricultural Science for Southern
Vietnam

1

Chapter 1. INTRODUCTION

1.1. BACKGROUND
Phosphorus (P) deficiency is considered to be the main yield
limiting factor and P supply is one of the resolutions to contribute
to improving crop yields in many areas today. Specially on large
areas of acid sulphate soils have high P sorption capacity, P
deficiency phenomenon on plants occurs not only due to P poor
soil but also by the majority of phosphorus in soil hold with
soluble iron, aluminum compounds that plants can not absorb.
Phosphorus adsorption and desorption characteristics are decisive
factors in P supply abilities of soils as well as doses and effects of P
fertilizers for rice.
In some countries (USA, India, Japan), the use of fertilizers
and slags containing silicate shown to improve nutritional status of
phosphorus in soil and crop yields. In Vietnam, silicate application
to increase concentrations of phosphorus in soil has hardly been
studied. Silicate anions that are capable of competing strongly
with phosphate anions on adsorption sites of iron, aluminum oxide
therefore reduce the amount of phosphorus adsorbed in soil.
Researching on application of silicon in rice cultivation in order that
blending to diversify phosphate fertilizer products or create multi-
element fertilizers containing P and Si suitable for soil conditions and
needs of rice in the South is a feasible direction. Basing on the
scientific basis and practical needs, thesis: “Research on
phosphorus sorption and silicate application to improve plant
available phosphorus contents in rice soils in southern Vietnam”
was made.

2

1.2. OBJECTIVES
- To evaluate the P sorption capacities of some major rice soils in
the South, identifying main factors deciding the P sorption
capacities in order to research on solutions inhibiting to reduce the
P fixing ability.
- To evaluate the applicability of adsorption competition anions
for inhibiting the P fixing ability, as the same time promoting
plant available P contents in rice soils.
1.3. SUBJECTS AND SCOPE
The study was carried out on 20 soils from main rice areas in
the Mekong Delta and the suburbs of Ho Chi Minh city, these
soils belong three groups: Alluvial soils (Fluvisols); acid sulphate
soils (Thionic Fluvisols) and grey soils (Acrisols). Researching on
silicon application in order to inhibit the P sorption capacity was
conducted through P sorption and desorption experiments in
laboratory, rice cultural trials on field and in green house.
1.4. MEANING OF SCIENCE AND PRACTICE
- Meaning of science: Contributing to the theoretical basis of the
phosphorus nutrient in submerged soils in southern Vietnam and
the tropical; pointing out the factors affecting the P sorption
capacity of soils; contributing to clarifying the role and the
influence of organic matter, silicofluoride and silicate anions for P
sorption capacity of soils. The thesis also found that the results of
using anionite to extract P released amounts from soils were
consistent with the capacity of soils in providing plant available P.
- Meaning of practice: Evaluating the main factors deciding
sorption capacities in rice soil to create a scientific basis in order
to find solutions to reduce P fixed amounts and raise the efficiency

of phosphorus fertilizer use. Finding out the role of silicon in
reducing P adsorption capacity, improving P released amounts in
soils is a good basis for the application of silicon in enhancing the
efficiency of phosphorus fertilizers. Detecting the closed
3

correlation between the intensity of P release determined by
anionite to P uptake amounts by plants is useful in
recommendation of anionite application for evaluating the ability
of soil in providing plant available P.
1.5. NEW FINDING OF THE STUDY
- Detecting a closed inversely correlation between P adsorption
and desorption, finding main soil characteristics that effect on P
desorption capacity of rice soil in southern Vietnam.
- Demonstrating the method of using anionite to determinte P
released amounts should be able to use for assessing the capacity
of soils in providing plant available P of the studied soils.
- Showing the effect of sodium silicate and silicofluoride on
inhibiting P sorption capacity, as the same time promoting plant
available P contents in soils.

Chapter 2. OVERVIEW

2.1. PHOSPHORUS ADSORPTION AND PRECIPITATE IN
SOIL
2.1.1. Mechanism of P adsorption and precipitate
The adsorption phenomenon is considered to be the major
cause of reducing soluble P concentrations in solution. The higher
the soil's P adsorption capacity, the lower the soil's P supply
capacity to plants. The adsorption process based on exchange

mechanism with OH
-
ion does not affect the surface charge of clay
minerals; in contrast, the adsorption process based on exchange
mechanism with OH
2
+
group reduces the positive charge on the
surface. Even when surface charge of colloidal clay is negative,
then it is able to adsorp anions (Smyth J. & Sanchez P. A., 1980).
Besides the chemical - physic adsorption process of positive
colloids, the chemical adsorption process of anions such high
valence phosphate anions in soil is very high. Also adsorbed
4

phosphate anions after the exchange can swap adsorbed
unexchangeable forms (Sakurai K., Ohdate Y. & Kyuma K.,
1989). Phosphate precipitation phenomenon is one of the process
that reduces the concentration of phosphorus in soil solution. The
adsorption process mainly occurs at low concentrations of
phosphate, phosphate deposition process mainly occurs in high
concentrations of phosphate.
2.1.2. Factors affecting phosphorus sorption capacity
The P adsorption capacity of soils depends on the following
factors: i) soil pH; ii) The activity and area surface of adsorbents;
iii) The ability to "lock" phosphorus; iv) Effects of cations; v) The
presence of anions competitive adsorption positions with
phosphate anions; vi) The temperature and reaction time.
2.1.3. Adsorption isotherms
2.1.3.1. Langmuir adsorption isotherm

Q
=
Q
max
. k . C

(1 + k . C)
Q anh C are respectively the quantity of sorbed P and the P
concentration in solution at the end of the experiment; Q
max
, k are
regression coefficients; the Q
max
parameter is assimilated to the
maximum sorption capacity; the k parameter is related to the
bonding energies.
2.1.3.2. Binary Langmuir adsorption isotherm
Q
=
Q
max1
. k1 . C + Q
max2
. k2 . C

(1 + k1 . C) (1 + k2 . C)
Q anh C are respectively the quantity of sorbed P and the P
concentration in solution at the end of the experiment; Q
max1
,

Q
max2
, k
1
and k
2
are regression coefficients; the Q
max
parameters are
assimilated to the maximum sorption capacities at sites 1 and sites
2; the k
1
and k
2
parameters are related to the bonding energies of
the two classed sites.
5

2.1.3.3. Freundlich adsorption isotherm
Q = k.C
n

Q anh C are respectively the quantity of sorbed P and the P
concentration in solution at the end of the experiment; k is the
quantity of sorbed P at the P concentration in solution by one unit;
the n parameter is related to the bonding energies.
2.1.3.4. Tempkin adsorption isotherm
x
RT
ln Ac

b
a
x and c are respectively the quantity of sorbed P and the P
concentration in solution at the end of the experiment; A and a are
coefficients; R is the universal gas constant; b is the Langmuir
adsorption maximum.
2.2. DYNAMIC OF PHOSPHORUS IN SUBMERGED
SOILS
2.2.1. Changes in phosphorus adsorption capacities
During submerged process, the P sorption capacity of soil
increases due to hydration or hydrolysis processes of crystalline
oxidhydroxide Fe (III) transformed into amorphous ferrihydrite
with large surface area having high P sorbed acpacities.
Submerging processes help release P from unsustainable iron
oxide but also new processes precipitated iron compounds occur
resulting amounts of P adsorbed more than one released.
2.2.2. Phosphorus transformations in soil
During submerging process concentrations of Fe-P forms
increase due to a part of Al-P in the variscite form can be
converted into Fe-P in the vivianite form. Contents of Ca-P forms
are less likely to change during submerging. On neutral and
alkaline soils, organic matter decompositions may release CO
2
to
6

form H

2
CO
3
that has the ability to increase the solubility of Ca-P
minerals. In submerged soils, goethite reduction processes inform
vivianite to be able absorbed by rice.
2.2.3. Phosphorus releases in submerged soils
In submerged conditions, contents of plant available P of
native P from soil and P added from fertilizer increased due to
followings: i) Reducting and transfering soluble strengite, variscite
into more soluble vivianite; ii) The increase of pH values due to
reduction promotes the ability of strengite and variscite hydrolysis
in acidic soils; iii) The release of phosphate anions in Fe-P and Al-
P compounds; iv) Reducing pH values due to decompositions of
organic matters leads to increasing the solubility of Ca-P
compounds in soil rich in calcium; v) Due to the acidity decreases,
the OH
-
anions exchange with phosphate anions adsorbed on
surfaces of soil colloids; vi) The release of phosphorus locked in
amorphous iron oxides; vii) Phosphorus diffusions increase.
2.3. RESOLUTIONS TO IMPROVE THE EFFICIENCY OF
USING PHOSPHORUS FERTILIZERS
Some technical resolutions to improve the efficiency of
phosphorus fertilizer use include: i) Applying the balance between
nitrogen and phosphate fertilizers; ii) Improving soil acidity (pH);
iii) Applying adsorption competition of anions; iv) Managing
water regimes.
2.4. RELATIONS BETWEEN SILICON (Si) AND
PHOSPHORUS (P)

2.4.1. Silicon in soil
2.4.1.1. Total silicon
Total Silicon contents (SiO
2
) accounts about 60-90% of soils
in the form of amorphous silicate, aluminosilicate mineral, in
organic matter and organic compounds - mineral (Samuel L. T. et
7

al., 1993). Through weathering process a part of liberated Si
contents can be converted into acid silisic (H
4
SiO
4
), an other one
can be transformed into colloidal silica (SiO
2
.nH
2
O). Silisic acid
can combine with the hydroxide or soluble salt of the metal has
been released to form the silicate salt. In weak base conditions
silisic acid created with K and Na into the soluble silicate. If acid
reactive environment prevails, Si transforms into free silisic acid,
easy washed down deep and moving.
2.4.1.2. Soluble silicon
In the wide pH range (2-9) Si dissolved in the soil solution
mainly H
4
SiO

4
0
form and in equilibrium with silica (SiO
2
)
amorphous with the equilibrium concentration of about 2 mmol; at
pH> 9, H
4
SiO
4
releases proton. When the Si concentration in
liquid is high, H
4
SiO
4
0
molecules coincidence SiO
2
precipitate. In
normal soil, Si concentrations in solution range from 3-37 ppm.
The concentration of H
4
SiO
4
in solution largely dominated by
adsorption reactions depending on the pH on the surface of
sesquioxides (Samuel L. T. et al., 1993).
2.4.1. The relationship between Si and P
The -OH group of silisic acid and -OH groups of the sugar
(and other molecules) can condense like -OH groups of

phosphoric. Unlike P, Si is not able to form double bonds.
Phosphorus needs of plants can be partly met by Si due to
adsorption competitions of silisic acid ions with phosphate ions in
soil.

8

Chapter 3. CONTENTS AND METHODS

3.1. CONTENTS
- Researching on the phosphorus adsorption capacity of soils by
the application of isotherm equations.
- Studying the phosphorus desorption capacity of soils.
- Studying the effect of organic matter (as humic acid and oxalate)
on the phosphate adsorption capacity of soils.
- Researching on the application of sodium silicate (Na
2
SiO
3
) and
sodium silicofluoride (Na
2
SiF
6
) to inhibit the phosphorus
adsorption capacity, improve the content of plant available
phosphorus in rice soils.
3.2. METHODS
3.2.1. Researching on the phosphorus adsorption capacity of
soils by the application of isotherm equations

The experiment 1: The study was carried out on 20 soils that from
main rice areas in the Mekong Delta and the suburbs of Ho Chi
Minh city include: 12 Thionic Fluvisol samples, 5 Fluvisol
samples and 3 Acrisol samples. The soil samples were collected in
the 0-20 cm layer, air dried at room temperature, crushed and
passed through a 2-mm sieve. Duplicate 1.5 g subsamples were
weighed in centrifuge tubes with 25 ml 0.01 M KCl at various
KH
2
PO
4
concentrations (0-134 mg P/l). The suspensions were
shaken end-over-end for 24 h, then centrifuge and filter to clear
extracts. Determine the amount of P remaining in solution, the
amount of P adsorbed in soil is calculated through the difference
between the amount of added P and P remaining in solution.
Phosphorus sorption capacities of soils were calculated by using
Langmuir and Freundlich equations.
9

3.2.2. Studying phosphorus desorption capacities of soils
3.2.2.1. Studying the phosphorus desorption capacity of soils by
the method using electrolyte solution to extract soil samples
The experiment 2: The implementation was carryed out on 20 soil
samples as in the Experiment 1. Shake 1.5 g soil with 25 mL 0.01
M KCl containing 160 mg P/l (as KH
2
PO
4
) for 24 hours;

centrifuge and filtrate to clear the suspensions, determine the
amount of P adsorbed in soil. The soil crusts remaining in the
centrifuge tube was washed with alcohol, and air dried to keep
shaking soil samples with 25 ml of 0.01 M KCl in 24 hours,
centrifuged and filtered. Determine the amount of P in the filtrate.
Experiment 3 replicates, calculating the linear correlation between
the amount of P released and adsorption parameters, soil physical
and chemical properties.
3.2.2.2. Research phosphate release speed by anionite
The experiment 3: The implementation was performed on alluvial
soils and grey soils. Speeds of P release in soil samples adsorbed P
and in the initial soil samples (not adsorbed P) were determined by
anion Cl
-
form DOWEX 1. Determine the amount of P released,
established the correlation between the amount of P released, P
release speeds and the interact time (t) between the soil with
anionite according to Cooke equation (1958 ): y = R√ t + B;
where: y: The amount of P released; R: P the P release speed; B:
The P concentration in equilibrium solution before putting
anionite into solution.
The experiment 4: The implementation was performed on 8 of 20
soils in the Experiment 1. Duplicate subsamples were weighed in
pots, then soils in these pots were submerged in 3 cm deep. After 5
days of submerge, use 1 pot (for each soil type) to determine the
amount of P released by anionite. On each soil type, sowing rice
into 3 pots then after 15 days determine amounts of P uptake. The
10

relations between P uptake amounts with P amounts released by

anion and between P uptake amounts with plant available P (by
Onioani method) in soil samples before planting rice were analyzed.
3.2.3. Study the effect of organic matter on phosphate
adsorption capacity of soils
3.2.3.1. Study the effect of the remove of organic matter in soil
on phosphate adsorption capacity of soils
The experiment 5: The implementation was performed on 20 rice
soils as in the Experiment 1. Destroy a part of soil organic matter
by hydrogen peroxide (H
2
O
2
). Determine P adsorption capacity of
the soils removed organic matter and compare the results with the
P sorption capacity of the initial soils.
3.2.3.2. Study the effect of humic acid on P adsorption capacity
of an iron hydroxide
The experiment 6: Determine the P sorption capacities of humic
acid samples, an amorphous iron hydroxide, the composition of
amorphous iron hydroxide and humic acid mixed for 10 hours, and
the same composition mixed for 15 days.
3.2.3.3. Study the effect of oxalate on phosphorus adsorption
capacity of oils
The experiment 7: The implementation was carryed out on an acid
sulphate soil, an alluvial soil and an ancient alluvial grey soil, (in
the 0-20 cm layer, through a 2-mm sieve).
a) Directly adding oxalate into the electrolyte solution containing
P: Directely adding (NH
4
)

2
C
2
O
4
at concentration levels 0 and
2.818 mg/l into 0.01 M KCl solution containing KH
2
PO
4
(0-400
mg P/l), determine P adsorption capacity.
b) Adding oxalate in submerged soils before interacting soils with the
electrolyte solution containing P: Apply 6g (NH
4
)
2
C
2
O
4
in 200g soil,
submerge soils in 3cm deep. Determine the amount of P adsorbed in
fresh soil samples after 1, 14 and 45 day after submerging (DAS).
11

3.2.4. Research on using Na
2
SiO
3

and Na
2
SiF
6
in limiting
phosphorus adsorption capacity, improve plant available
phosphorus contents in soil
3.2.4.1. Study the effect of Na
2
SiO
3
and Na
2
SiF
6
on phosphorus
sorption capacity of soil
The experiments were performed on an acid sulphate soil, an
alluvial soil and a grey soil. The soil samples were in the 0-20 cm
layer, crushed through a 2-mm sieve).
The experiment 8: Shake 5 g soil with 25 mL 0.01 M KCl solution
containing P (0-400 mg P/l) supplemented with Na
2
SiO
3
and
Na
2
SiF
6

(the concentration levels at 0, 200, 500, 800 mg SiO
3
or
SiF
6
/l) for 24 h, filtrate and centrifuge to clear the suspensions.
Determine P sorption capacities of soils in two cases: i) Without
adjusting original pH of the initial solutions; ii) Adjusting pH of
the initial solutions into 4.7.
The experiment 9: Duplicate 200 g soil samples into plastic
bottles, add Na
2
SiO
3
and Na
2
SiF
6
at the level of 180 mg or 180 mg
SiF
6
or SiO
3
per kg dry soil, submerge soil samples. Determine P
adsorption capacity in fresh soil samples at 1, 14 and 42 DAS.
The experiment 10: Shake 5 g dry soil with 25 mL 0.01 M KCl
solution containing 200 mg P/l (as KH
2
PO
4

) supplemented with
Na
2
SiO
3
and Na
2
SiF
6
(200 mg SiO
3
or SiF
6
/l), centrifuge and
filtrate, determine the amount of P adsorbed. Wash soil samples in
the centrifuge tubes with alcohol, add 25 mL of 0.01 M KCl
containing Na
2
SiO
3
and Na
2
SiF
6
(200 mg SiO
3
or SiF
6
/l) then
shake the suspensions for 24 hours, centrifuge and filtrate,

determine the amount of P released into the solution.
3.2.4.2. Study the effect of Na
2
SiO
3
and Na
2
SiF
6
on the
efficiency of phosphorus fertilizers on rice culture
The trial was carried out on an acid sulphate soil, an alluvial
soil and a grey soil.
The experiment 11: The paddy field trial was conducted in the
12

Summer Autumn season in 2001 and the Winter Spring season
from 2001-2002; rice varieties: VND 404 (on the acid sulphate
soil and grey soil), VNĐ 361 (on the alluvial soil).
- Treatments:
Treatm.1: Controls (N, K)
Treatm.4: (N, K) + P
Treatm.2: (N, K) + Na
2
SiO
3

Treatm.5: (N, K) + P + Na
2
SiO

3

Treatm.3: (N, K) + Na
2
SiF
6

Treatm.6: (N, K) + P + Na
2
SiF
6

- Fertilizers (kg/ha/crop): On the acid sulphate soil: 100 N+90
P
2
O
5
+30 K
2
O+180 SiO
3
or SiF
6
; on the alluvial soil: 100 N+60
P
2
O
5
+30 K
2

O+180 SiO
3
or SiF
6
; on the grey soil: 100 N+60
P
2
O
5
+60 K
2
O+180 SiO
3
or SiF
6
.
- Target tracking: Grain yield; Test Design: RCBD, 3 replications;
plot area: 30 m
2
.
The experiment 12: The rice planting experiment was conducted
in a greenhouse from April - May/2002; rice varieties: VND 404.
- Treatments:
Treatm.1: Controls (N, K)
Treatm.4: (N, K) + P
Treatm.2: (N, K) + Na
2
SiO
3

Treatm.5: (N, K) + P + Na
2
SiO
3

Treatm.3: (N, K) + Na
2
SiF
6

Treatm.6: (N, K) + P + Na
2
SiF
6


- Fertilizers (mg/kg soil): On the acid sulphate soil: 100 N+90
P
2
O
5
+30 K
2
O+180 SiO
3
or SiF
6
; On the grey soil: 100 N+90
P
2

O
5
+30 K
2
O+180 SiO
3
or SiF
6
.
- Target monitoring and analysis: Plant height, biomass weight,
number of branches/plant on rice at 25 and 45 day periods after
sowing. Analysis of dried plant samples: Contents of total P, Si,
N, Fe, Al in rice plants at 25 and 45 days after sowing.
- Test Design: CRD, 3 replications.
3.2.5. Method of analysis
According to the guidance of ISRIC, Soils and Fertilirzers
Research Institute, TCVN and TCN.
13

3.2.6. Data processing
According to the method of linear correlation analysis,
analysis of variance, least significant difference test and Duncan’s
Multple test.

Chapter 4. RESULTS AND DISCUSSION

4.1. PHOSPHORUS ADSORPTION CAPACITIES OF RICE
SOILS IN SOUTHERN VIETNAM
A part of this work has been published with co-author Vo
Dinh Quang on European Journal of Soil Science (3/1996, No. 47,

pp. 112-123); in which the binary Langmuir equation applied to
determine P adsorption capacities of the soils. In the framework of
the subject, P adsorption capacities of the soils are assessed
according to application of the single Langmuir and Freudlich
equations; results were compared with P adsorption capacities of
the soils calculated by the binary Langmuir equation.
4.1.1. Phosphorus adsorption capacities of soils determined by
the method of applying isothermal equations
4.1.1.1. Phosphorus adsorption capacities of soils determined by
the single Langmuir equation
Maximum adsorbed phosphorus amount of the study soils
were classified under three groups descending order: Acid
sulphate soils (Q
max
= 1.498 mgP/kg)> Alluvial soils (Q
max
= 824
mgP/kg)> Grey soils (Q
max
= 297mgP/kg). The amount of P
adsorbed at solution equilibrium concentration of 0.2 mg P/l (P
0,2
)
on three soil groups according to the descending order: Acid
sulphate soils (P
0,2
: 274 mgP/kg)> Alluvial soils: (P
0,2
: 92
mgP/kg> Grey soils (P

0,2
: 4 mgP/kg). To maintain the
concentration at 0.2 mg P/l, the amount of P fertilizers needed as
follows: Acid sulphate soils: 549 kg P/ha, Alluvial soils: 184 kg
P/ha, Grey soils: 8 kg P/ha.
14

4.1.1.2. Phosphorus adsorption capacities of soils determined by
Freundlich equation
The results of k coefficients of the three soil descending
order: Acid sulphate soils (k = 612 mg P/kg) > Alluvial soils (k =
312 mg P/kg) > Grey soils (k = 40 mg P/kg). The values of P
0,2
in
the following order: Acid sulphate soils (P
0,2
: 122 mg P/kg, P
fertilizers needed as 245 kg P/ha) > Alluvial soils (P
0,2
: 62 mg
P/kg, P fertilizers needed as 125 kg P/ha) > Grey soils (P
0,2
: 8 mg
P/kg, P fertilizers needed as 16 kg P/ha) .
General comments: There is a very tight positive correlation
between the P adsorption parameters of the 20 soils calculated by
three equations. The single Langmuir and Freudlich equations are
well illustrated adsorption process and not inferior to dual the
binary Langmuir equation. Although the correlation coefficients
obtained by the binary Langmuir and Freundlich equations are

quite good, but to calculate the parameters it required having a
special software so that these two equations should not common.
The single Langmuir equation shows most appropriate to describe
the phenomenon of P adsorption in the study soil groups because
of it has described well by adsorption phenomena, especially in
areas close to the actual field conditions, obtains good correlation
coefficients and is calculated simply by switching to a linear form.
4.1.2. The relationship between phosphorus adsorption
parameters and soil physical and chemical properties
The acid sulphate soil group is quite acid (pH
H2O
: 3.5-4.6), the
alluvial soil group is less acid (pH
H2O
: 4.5-5.8) and grey soil group
is near-neutral (pH
H2O
: 6.1-6,9). The grey soil group is poor in clay
(18.1-35.9%); alluvial soil and acid sulphate soil groups’ clay
contents range 20.6-75.6%, mostly quite rich clay. In the acid
sulphate soil group, the contents of P are low, almost the total P
contents are about 0.175-0.445 g P/kg (0.04-0.10% P
2
O
5
). Total P
15

contents in the grey soil group is also low, about 0.114-0.218 g
P/kg (0.03 to 0.05% P

2
O
5
). Total P contents in the alluvial soil
group range from the poor to the rich (0.184-0.521 g P/kg, 0.04 to
0.28% P
2
O
5
). In the acid sulphate soils, total iron contents are high
(12.8-90.0 gFe
2
O
3
/kg); of which: Free iron 5.22-39.84 g Fe
2
O
3
/kg,
amorphous iron 2.25-14.81g Fe
2
O
3
/kg, iron crystals 0.40-28.03 g
Fe
2
O
3
/kg. Total iron contents in alluvial soils range 38.4-125.0 g
Fe

2
O
3
/kg; of which: Free iron 6.06-28.53 g Fe
2
O
3
/kg, amorphous
iron 3.60-15.62 g Fe
2
O
3
/kg, iron crystals 0.43-22.93 gFe
2
O
3
/kg.
The grey soils are very poor in total iron contents (4.4-7.2g
Fe
2
O
3
/kg). The alluvial soils are rich in total aluminum contents
(102.6-180.2g Al
2
O
3
/kg); in which, free aluminum 0.81-3.08 g
Al
2

O
3
/kg, amorphous aluminum 1.36-2.49g Al
2
O
3
/kg. The total
aluminum contents in acid sulphate soils are rich, most about 30.5-
179.5g Al
2
O
3
/kg; include: Free aluminum 0.85-11.62g Al
2
O
3
/kg,
amorphous aluminum 0.95-13.61g Al
2
O
3
/kg. The grey soils has
low levels of aluminum: Al total 13.9-25.2g Al
2
O
3
/kg, free
aluminum 0.17-0.59g Al
2
O

3
/kg, amorphous Al 0.40-1,12g
Al
2
O
3
/kg. The amounts of organic matter in acid sulphate soils are
quite high (1.79-10.06%), in the alluvial soils: 0.87-2.20 % , in the
grey soil: 0.52-0.87 %.
- There is a very tight inverse correlation between the values of soi
pH
KCl
, pH
H2O
with Q
max
parameter results based on the single
Langmuir equation and k parameter of the Freundlich equation.
- There is a very tight positive correlation between clay contents with
P adsorption parameters as Q
max
of single Langmuir equation, Q
max2

and ∑Q
max
of the binary Langmuir equation and k of the Freundlich
equation. The correlation between Q
max1
and P

0, 2
of the binary
16

Langmuir equation based on the clay contents has not reached a
probability p <0.05. The amount of P
0,2
calculated by the single
Langmuir and Freundlich equations correlated with the clay contents.
- The value of Q
max
according the single Langmuir equation does not
correlate with total iron and crystal iron contents but significantly
positively correlate with free Fe contents and specially correlates very
closely with amorphous Fe contents. A similar rule was detected
between the Freundlich k coefficient and iron components. These
results coincide well with the calculated correlation rule to Q
max1
,
Q
max2
and ΣQ
max
accding to the binary Langmuir equation.
- There is no relationship between total aluminum contents with
Q
max
values of the single Langmuir equation and k values of the
Freundlich equation. Free Al contents were found only
correlations significant at p <0.05 with the single Langmuir Q

max
.
In contrast, there is a close correlation between the amorphous Al
contents and the single Langmuir Q
max
(p <0.01) and Freundlich k.
- There is no relationship between total aluminum contents and
single Langmuir Q
max
, Freundlich k parameters. For free Al
contents, the correlation significant at P <0.05 is found only to the
single Langmuir Q
max
. In contrast, there is a close correlation
between the amorphous Al contents to the single Langmuir Q
max

(p <0.01) and Freundlich k values.
- No detectable correlation between total P and P Bray 2 contents
with P adsorption parameters.
- There are quite a tight positive correlation between P adsorption
parameters and organic matter contents.
17

4.2. PHOSPHORUS DESORPTION CAPACITIES OF RICE
SOILS IN SOUTHERN VIETNAM
4.2.1. Research results of phosphorus desorption capacities of
soil by method using electrolyte solution to extract soil samples
4.2.1.1. The relationship between the amount of phosphorus
released with the phosphorus adsorption capacity

The P amount released in acid sulphate soils 21-33 mg P/kg,
an average of 31 mgP/kg. In alluvial soils, the P amount released
about 27-95 mgP/kg, an average of 58 mg P/kg. Grey soils had the
highest amount of P released ranged 99-110 mgP/kg, an average
of 105 mgP/kg. There is a very tight negative correlation between
P adsorption parameters and the amount of P released.
4.2.1.2. The relationship between the amount of P released with
soil physical and chemical properties
The amount of phosphate released was inversely correlated
tightly with clay contents, organic matter contents, amorphous iron
and aluminum contents, quite tightly correlated inversely with total
and free aluminum contents, very tightly correlated with soil pH.
4.2.2. Results of researching the release speed of phosphorus
by anionite
In soil no P added, the grey soil has highest P released speed
(R = 1.91 mg P/√minute, followed by the alluvial soil (R = 1.62
mg P/√minute) and the lowest is based on acid sulphate soil (R =
0.99 mg P/√minute). Results on soil added P has the same rules as
above, the grey soil has the highest P released speed (R = 8.37 mg
P/√minute), P release rate is 21,1%; the next is the alluvial soil (R =
4.67 mg P/√minute), P release rate rate is 35.9%; the acid sulphate
18

soil has the lowest P released speed (R = 1.81 mg P/√minute) and P
release rate is also lowest (4.3%).
The correlation between the amounts of P uptake by rice
plants with the amounts of plant available P as P Onioani has no
statistical significance. In contrast, there is a relatively tight
positive correlation between the amounts of P extracted by
anionite with P uptake by plants. This suggests that the method of

determining the amount of P extracted by anionite reflects the P
supply ability of the soils better than the extraction methods
according Onioani to determine plant available P.
4.3. INFLUENCES OF ORGANIC CHEMICALS ON
PHOSPHORUS ADSORPTION CAPACITIES OF RICE
SOILS IN SOUTHERN VIETNAM
4.3.1. Effect of removing organic matter contents on
phosphorus adsorption capacities of soils
After treatmenting soil samples with H
2
O
2
solution, the
organic matter contents decline leading to reduced P adsorption
capacities of the soils, but did not detect a correlation between the
amounts of organic matter loss to the decrease of P adsorption
capacities; while fairly tight correlation between the concentrations of
organic matter with amorphous iron, aluminum contents.
4.3.2. Effects of humic acid on P adsorption capacity of iron
hydroxide
The amorphous hydroxide compounds have Q
max
= 2.652 mg
P/mmol Fe, separate humic acid have Q
max
= 244 mg P/g; mixing
humic acid and iron hydroxide for 10 hours reduced P adsorption
capacity of iron hydroxide (Q
max
= 1.528 mg P/mmol Fe).

However, humic acid interactions with iron hydroxide for 15 days
19

showed a strong increase in Q
max
of iron hydroxide (Q
max
= 9,761 mg
P/mmol Fe), much larger than the sum of the separate adsorption of
iron hydroxide and humic acid (2,652 + 244 = 2,896 mg P).
4.3.3. The influence of oxalate on P adsorption capacity of soil
4.3.3.1. The impact directly adsorption competition of oxalate on
P adsorption capacity of soil
Directly adding oxalate into the adsorption solutions (0.01 M
KCl containing P), the acid sulphate soil has Q
max
= 1.532 mg
P/kg, down 19% compared to the control (Q
max
= 1.886 mg P/kg),
the alluvial soil has Q
max
= 446 mg P/kg, down 65% compared to
the control (Q
max
= 1.261 mg P/kg), the grey soil has Q
max
= 93 mg
P/kg, down 76% compared to the control (Q
max

= 338 mg P/kg).
4.3.3.2. The influence of oxalate on P adsorption capacities of
soil in submerge conditions
a) On the acid sulphate soil: Q
max
of soils added oxalate decreased
9% (Q
max
= 1.918 mg P/kg) compared to the control (Q
max
= 2.109
mg P/kg) at 1day period; 4-5% reduction in 14 and 45 DAS.
b) On the alluvial soil: Q
max
decrease 27% and 16% respectively
under the effect of addition of oxalate in the 1 day period after
submerging (Q
max
= 879 mg P/kg) and 45 DAS (Q
max
= 1216 mg
P/kg); except for the period of 14 DAS (Q
max
= 1288 mg P/kg),
Q
max
increased by 4% compared to the control.
c) On the grey soil: Q
max
of the soils added oxalate in period 1, 14

and 45 DAS ranges 74-315 mg P/kg, reduced 45-82% compared to
the control (Q
max
: 417-568 mg P/kg). At low P concentrations,
under the influence of oxalate virtually no phenomenon of P is
adsorbed but on the contrary P in soils dissolves into soil solution.
20

4.4. THE USE OF SODIUM SILICATE (Na
2
SiO
3
) AND
SODIUM SILICOFLUORIDE (Na
2
SiF
6
) IN LIMITING
PHOSPHORUS ADSORPTION CAPACITIES OF SOILS,
PROMOTING PLANT AVAILABLE PHOSPHORUS
CONTENTS IN RICE SOILS IN SOUTHERN VIETNAM
4.4.1. The effect of Na
2
SiO
3
and Na
2
SiF
6
on P sorption

capacities of soils
4.4.1.1. The impact directly competitive adsorption of Na
2
SiO
3

and Na
2
SiF
6
on P adsorption capacities of soils
The adsorption solutions have initial pH about 4.7-5.5;
Na
2
SiO
3
addition increases pH up 4.7-11.8; Na
2
SiF
6
supplement
reduces pH (3.5-5.5). Do not adjust the pH of the adsorption
solutions, P adsorption capacities of soils under the influences of
competing anions (SiO
3
2-
, SiF
6
2-
) and pH; adjusted to pH at 4.7 to

eliminate the influence of pH.
a) On the acid sulphate soil: Not adjust pH of the adsorption
solutions, the effect of Na
2
SiO
3
and Na
2
SiF
6
reduced 6-21% Q
max
,
41-73% P
0,2
. Because of it is unable to apply a huge amount of P
fertilizers to reach plant P saturation levels, but if a reasonable
amount of P fertilizers is applied that can maintain the
concentration of P in the soil solution to meet the needs of crops is
a practical technical resolution. In the case adjusting pH of
adsorption solutions at 4.7, the impact of the decline in Q
max
due to
Na
2
SiF
6
application equal to the results in the case without
adjusting pH, whereas the effect of Na
2

SiO
3
on limiting Q
max
is
negligible. Effects of Na
2
SiO
3
on the P adsorption capacity mainly
due to the mechanism alkaline environmental pH, this leads to
reducing the negative charge on the surface adsorption and the
affinity of soil adsorbent complexes to phosphorus. The effect of
Na
2
SiF
6
on reduction of Q
max
mainly depends on the mechanism of
anion adsorption competition. The value P
0,2
reduced 25-50% by
additional Na
2
SiO
3
, down 55-74% due to additional Na
2
SiF

6
.
21

b) On the alluvial soil: The decline in Q
max
by Na
2
SiO
3
in both
cases no pH adjustment (Q
max
decreased 2-15%) and pH
adjustment (Q
max
decreased 4-14%) is not much. Under the
influence of Na
2
SiF
6
, Q
max
decreased 1-50% in the case of pH not
adjusted, decreased 17-36% in the case of pH adjusted. Additional
Na
2
SiF
6
and Na

2
SiO
3
reduced 82-98% P
0, 2
.
c) On the grey soil: Without pH adjustment, Na
2
SiO
3
reduced 10-
96% Q
max
, Na
2
SiF
6
reduced 11-56% Q
max
. There is not a difference
in the impact of two silicon compounds studied on the P
adsorption capacity of soil in two cases of adjusting or not
adjusting pH of initial adsorption solutions. The maximum P
adsorption capacity of the grey soil is low there for P
0,2
values are
almost negligible.
4.4.1.2. The effect of Na
2
SiO

3
and Na
2
SiF
6
on P adsorption
capacities in submerged soils
a) On the acid sulphate soil: At 1, 14 and 42 day periods, Q
max

values of soils added Na
2
SiO
3
is not much different compared to
the control, P
0,2
reduces 7-30% compared to the control. The
addition of Na
2
SiF
6
in soil decreases 2-20% Q
max
, 18-74% P
0,2

compared to the control.
b) On the alluvial soil: The Na
2

SiO
3
application does not reduce
Q
max
, whereas reduces 40-61% P
0,2
. Adding Na
2
SiF
6
reduces 4-6%
Q
max
in 1 and 14 day periods, but can not reduce Q
max
at 42 day
period. The P
0,2
values lessen 39-68%.
c) On the grey soil: Appling Na
2
SiO
3
reduces 10-23% Q
max
at 1
and 14 days after submerging, but increased 3% Q
max
at 42 day

period. During submerge duration P
0,2
values decrease 4-70%
compared to the control. The application of Na
2
SiF
6
reduces 11-
58% Q
max
, 12-56% P
0,2
compared to the control.
22

4.4.1.3. The effect of Na
2
SiO
3
and Na
2
SiF
6
on the desorption
capacity of soils
Under the influence of Na
2
SiO
3
or Na

2
SiF
6
6, the calculation
based on the amount of P adsorbed in the initial soil samples, the
amount of P released ranges 24-29% on the acid sulphate soil (the
control: 4%); 24-29% on the alluvial soil (the control: 18%); and
62-74% on the grey (control: 43%).
4.4.2. The effect of Na
2
SiO
3
and Na
2
SiF
6
on the effective of
phosphate fertilizers on rice on the field
4.4.2.1. The effect of Na
2
SiO
3
and Na
2
SiF
6
on grain yield
On the acid sulphate soil and the grey soil, Na
2
SiO

3
or Na
2
SiF
6

addition without P applycation increases yield 5-13% compared to
control; using Na
2
SiO
3
or Na
2
SiF
6
in

combination with P fertilizers
increases yield 10-18% compared with controls, but no significant
difference compared with the P added treatmen. On the alluvial
soil, Na
2
SiO
3
or Na
2
SiF
6
does not bring yield responses.
4.4.2.2. The effect of Na

2
SiO
3
and Na
2
SiF
6
on the growth and
nutrient uptake of rice in the greenhouse
On the acid sulphate soils, separately using Na
2
SiO
3
or
combined Na
2
SiO
3
or Na
2
SiF
6
with P fertilizers increase biomass,
number of branches/plant and plant height. The correlation
between Si and P in plants has a positive effect on the absorption
and metabolism of nutrients as P, Si and N. Applying separately or
combining Na
2
SiO
3

or Na
2
SiF
6
tends to reduce the iron and
allumium contents this lead to increasing P/Fe and P/Al in plants.
On the grey soil, the growth of rice plants between phosphorus
and silicon treatments are not significantly different as compared
to control in the experiment on the acid sulphate soil. In the
treatments using separately Si there are not clear differences in
plant height, biomass and number of branches/plant compared to
control. The combination use of Si and P fertilizers slight
increases plant height, biomass, the amount of P and N uptake.
23


Chapter 5. CONCLUSIONS AND
RECOMMENDATIONS

5.1. CONCLUSIONS
1) Three single Langmuir, binary Langmuir and Freundlich
isotherm equations favorablely illustrate the relationship between
P concentration in equilibrium solution and the amount of P
adsorbed in study soils. Depending on the purpose of the study we
can use one of three isotherm equations to calculate the P
adsorption capacity of rice soils in southern Vietnam. There is a
very tight correlation between the adsorption parameters
calculated according to three equations. Phosphorus adsorption
capacities of soils determined by three equations with the same
rules and can be ranked in the following order: Acid sulphate soils

> Alluvial soils > Grey soils. Phosphorus adsorption capacities of
soils are tightly correlated with organic matter contents, clay
contents and amorphous iron and aluminum contents; correlated
with lower levels of iron and aluminum free contents; tightly
correlated inversely with soil pH; found no correlation with total P
contents, P Bray 2 contents and total aluminum and iron contents.
2) Increase in soil organic content leads to an increase in content
of amorphous allumium and iron oxyhydroxide in soil. After
removing organic matters in soil, P maximum adsorption capacity
(Q
max
) of soils fell 44-67% in comparison with initial sois. Both
simultaneously supplementing organic matter as oxalate and
phosphorus into solution or supplementing oxalate in submerged