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

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

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

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

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

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

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 OH2

+ 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

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

2.1.3.2 Binary Langmuir adsorption isotherm

2.1.3.3 Freundlich adsorption isotherm

Q = k.Cn

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 CO2 to

form H2CO3 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 (SiO2) 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

al., 1993) Through weathering process a part of liberated Si contents can be converted into acid silisic (H4SiO4), an other one can be transformed into colloidal silica (SiO2.nH2O) 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 H4SiO4

0 form and in equilibrium with silica (SiO2) amorphous with the equilibrium concentration of about 2 mmol; at pH> 9, H4SiO4 releases proton When the Si concentration in liquid is high, H4SiO4

0 molecules coincidence SiO2 precipitate In normal soil, Si concentrations in solution range from 3-37 ppm The concentration of H4SiO4 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

Chapter 3 CONTENTS A ND 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 (Na2SiO3) and sodium silicofluoride (Na2SiF6) to inhibit the phosphorus adsorption capacity, improve the content of plant available phosphorus in rice soils

KH2PO4 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

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 KH2PO4) 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

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 (H2O2) 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 (NH4)2C2O4 at concentration levels 0 and 2.818 mg/l into 0.01 M KCl solution containing KH2PO4 (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 (NH4)2C2O4 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)

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 Na2SiO3 and

Na2SiF6 (the concentration levels at 0, 200, 500, 800 mg SiO3 or SiF6/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 Na2SiO3 and Na2SiF6 at the level of 180 mg or 180 mg SiF6 or SiO3 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 KH2PO4) supplemented with

Na2SiO3 and Na2SiF6 (200 mg SiO3 or SiF6/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 Na2SiO3 and Na2SiF6 (200 mg SiO3 or SiF6/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

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) + Na2SiO3 Treatm.5: (N, K) + P + Na2SiO3Treatm.3: (N, K) + Na2SiF6 Treatm.6: (N, K) + P + Na2SiF6

- Fertilizers (kg/ha/crop): On the acid sulphate soil: 100 N+90

P2O5+30 K2O+180 SiO3 or SiF6; on the alluvial soil: 100 N+60

P2O5+30 K2O+180 SiO3 or SiF6; on the grey soil: 100 N+60

P2O5+60 K2O+180 SiO3 or SiF6

- Target tracking: Grain yield; Test Design: RCBD, 3 replications;

plot area: 30 m2

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) + Na2SiO3 Treatm.5: (N, K) + P + Na2SiO3Treatm.3: (N, K) + Na2SiF6 Treatm.6: (N, K) + P + Na2SiF6

- Fertilizers (mg/kg soil): On the acid sulphate soil: 100 N+90

P2O5+30 K2O+180 SiO3 or SiF6; On the grey soil: 100 N+90

P2O5+30 K2O+180 SiO3 or SiF6

- 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

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 (Qmax = 1.498 mgP/kg)> Alluvial soils (Qmax = 824 mgP/kg)> Grey soils (Qmax = 297mgP/kg) The amount of P adsorbed at solution equilibrium concentration of 0.2 mg P/l (P0,2)

on three soil groups according to the descending order: Acid sulphate soils (P0,2: 274 mgP/kg)> Alluvial soils: (P0,2: 92 mgP/kg> Grey soils (P0,2: 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