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: Prof DSc PHAN LIEU 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: The national library The library of Vietnam Academy of Agricultural Sciences The library of Institute of Agricultural Science for Southern Vietnam Chapter 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 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 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 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 and sites 2; the k1 and k2 parameters are related to the bonding energies of the two classed sites 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 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 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 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 Chapter 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, Fluvisol samples and 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 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: 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, of the binary 16 Langmuir equation based on the clay contents has not reached a probability p