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

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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 multielement 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 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 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 = Qmax . 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; Qmax, k are regression coefficients; the Qmax 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 = Qmax1 . k1 . C + Qmax2 . 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; Qmax1, Qmax2, k1 and k2 are regression coefficients; the Qmax parameters are assimilated to the maximum sorption capacities at sites 1 and sites 2; the k1 and k2 parameters are related to the bonding energies of the two classed sites. 5 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 6 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 AlP 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 7 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 H4SiO40 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, H4SiO40 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. 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 (Na2SiO3) and sodium silicofluoride (Na2SiF6) 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 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. 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 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 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 (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). 11 3.2.4. Research on using Na2SiO3 and Na2SiF6 in limiting phosphorus adsorption capacity, improve plant available phosphorus contents in soil 3.2.4.1. Study the effect of Na2SiO3 and Na2SiF6 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 Na2SiO3 and Na2SiF6 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) + Na2SiO3 Treatm.5: (N, K) + P + Na2SiO3 Treatm.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 + Na2SiO3 Treatm.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. 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 (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. 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 P0,2 in the following order: Acid sulphate soils (P0,2: 122 mg P/kg, P fertilizers needed as 245 kg P/ha) > Alluvial soils (P0,2: 62 mg P/kg, P fertilizers needed as 125 kg P/ha) > Grey soils (P0,2: 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 (pHH2O: 3.5-4.6), the alluvial soil group is less acid (pHH2O: 4.5-5.8) and grey soil group is near-neutral (pHH2O: 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% P2O5). 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% P2O5). 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% P2O5). In the acid sulphate soils, total iron contents are high (12.8-90.0 gFe2O3/kg); of which: Free iron 5.22-39.84 g Fe2O3/kg, amorphous iron 2.25-14.81g Fe2O3/kg, iron crystals 0.40-28.03 g Fe2O3/kg. Total iron contents in alluvial soils range 38.4-125.0 g Fe2O3/kg; of which: Free iron 6.06-28.53 g Fe2O3/kg, amorphous iron 3.60-15.62 g Fe2O3/kg, iron crystals 0.43-22.93 gFe2O3/kg. The grey soils are very poor in total iron contents (4.4-7.2g Fe2O3/kg). The alluvial soils are rich in total aluminum contents (102.6-180.2g Al2O3/kg); in which, free aluminum 0.81-3.08 g Al2O3/kg, amorphous aluminum 1.36-2.49g Al2O3/kg. The total aluminum contents in acid sulphate soils are rich, most about 30.5179.5g Al2O3/kg; include: Free aluminum 0.85-11.62g Al2O3/kg, amorphous aluminum 0.95-13.61g Al2O3/kg. The grey soils has low levels of aluminum: Al total 13.9-25.2g Al2O3/kg, free aluminum 0.17-0.59g Al2O3/kg, amorphous Al 0.40-1,12g Al2O3/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 pHKCl, pHH2O with Qmax 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 Qmax of single Langmuir equation, Qmax2 and ∑Qmax of the binary Langmuir equation and k of the Freundlich equation. The correlation between Qmax1 and P0, 2 of the binary 16 Langmuir equation based on the clay contents has not reached a probability p <0.05. The amount of P0,2 calculated by the single Langmuir and Freundlich equations correlated with the clay contents. - The value of Qmax 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 Qmax1, Qmax2 and ΣQmax accding to the binary Langmuir equation. - There is no relationship between total aluminum contents with Qmax 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 Qmax. In contrast, there is a close correlation between the amorphous Al contents and the single Langmuir Qmax (p <0.01) and Freundlich k. - There is no relationship between total aluminum contents and single Langmuir Qmax, Freundlich k parameters. For free Al contents, the correlation significant at P <0.05 is found only to the single Langmuir Qmax. In contrast, there is a close correlation between the amorphous Al contents to the single Langmuir Qmax (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 H2O2 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 Qmax = 2.652 mg P/mmol Fe, separate humic acid have Qmax = 244 mg P/g; mixing humic acid and iron hydroxide for 10 hours reduced P adsorption capacity of iron hydroxide (Qmax = 1.528 mg P/mmol Fe). However, humic acid interactions with iron hydroxide for 15 days
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