t‘t t H t M t W t t t t L t T‘ in T“ W t M ‘t t t t t t t ‘t M \tt‘t H \l‘ "{\|_.\ 01 _(DCD\| PHOSPHORUS ADSORPTION AND MOVEMENT IN SOILS Dissertation for the Degree of Ph. D. MICHIGAN STATE UNIVERSITY ROBERT WALTER TAYLOR 1977 PLACE IN RETURN BOX to remove this checkout from your record. TO AVOID FINES return on or before date due. DATE DUE DATE DUE DATE DUE A (x [7 3/11192 (i I ABSTRACT PHOSPHORUS ADSORPTION AND MOVEMENT IN SOILS BY Robert Walter Taylor The movement of phosphorus (P) through heavily fertilized agri- cultural soil profiles into groundwater is potentially an important source of its enrichment in rivers and lakes. Therefore, it is desirable to understand the mechanism by which it is adsorbed and moved through soils. The Langmuir adsorption isotherm has been widely used to relate soil solution P to surface adsorbed P. However, deviations from the straight line predicted by the linear form of the equation have been reported for both soil and homogeneous soil minerals. Elucidation of the mechanism of P adsorption may explain the deviations and indicate whether the Langmuir equation accurately characterizes P adsorption by soils. Langmuir plots of P adsorption isotherms of four soils were shown to fit two intersecting lines. The adsorption data were also found to fit the BET equation. The monolayer capacities computed from the BET equation corresponded closely with the adsorption maxima com— puted from initial slopes of the Langmuir plots. Studies of P adsorption on energetically homogeneous sites of the anion exchange resins Dowex l-X8 and Dowex 2-X4 (Cl-forms) at 25 C Robert Walter Taylor gave results similar to those obtained with soils. Measurements of C1- released during adsorption indicated that initially 2 mMole of Cl- were released per mMole of P adsorbed, and this value decreased at higher equilibrium P concentrations eventually approaching 1 mMole of Cl-. Measurements of the pH of equilibrium P solutions suggested that there was deprotonation of H2P04- during the initial stages of adsorp- tion. Similar results were obtained when Dowex l-X8 was saturated with 103- and P adsorption on the resin studied. The differential isoteric heat of adsorption, KB) of P adsorp- tion on Dowex l-XB was computed between 11 C and 25 C. KB decreased sharply over the first region of the isotherm and became almost constant over the second. It was concluded that P was bonded by two points of attachment after deprotonation of H2P04- followed by one point of attachment during adsorption on the resin surface. This resulted in the devia- tion from linearity predicted by the Langmuir equation. This hypothesis for the mechanism of P adsorption is presented to explain the deviation observed on soil and homogeneous soil mineral surfaces. The vertical movement of P was studied by sampling agricultural soils at 15 cm intervals to a depth of 152 cm. Bray P1 extractable P measurements were made on all samples and Langmuir adsorption maxima determined from isotherms of the 0-15 cm samples. Significant down- ward movement of P was not discernible on loam soils. Sandy loam soils generally showed appreciable downward movement of P. Movement was not solely related to Bray P1 levels of surface soils. To determine if P movement was related to the fraction of adsorbed P at 1 ppm P in solution, ratios of Bray Pl extractable P to adsorbed P were calculated. Little relationship was found between this ratio and movement of P in soil profiles. PHOSPHORUS ADSORPTION AND MOVEMENT IN SOILS BY Robert Walter Taylor A DISSERTATION Submitted to Michigan State University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Crop and Soil Sciences 1977 DEDICATION to my parents, wife, and son ii ACKNOWLEDGEMENTS The author expresses appreciation to his major professor, Dr. Boyd G. Ellis, for his valuable guidance, patient assistance, and suggestions throughout the course of his studies and during the preparation of this manuscript. Gratitude is expressed to the members of his guidance committee, Dr. Clifford J. Pollard, Dr. Lee Jacobs, Dr. Maurice Vitosh, and Dr. James Tiedje, for their time and effort spent evaluating and constructively criticizing this manuscript. Appreciation is also expressed to other members of the Crop and Soil Science Department, especially Dr. Max Mortland, for aiding in his intellectual growth. The author deeply appreciates the financial aid provided by the EXperiment Station's NC98 regional research project. iii TABLE OF CONTENTS LIST OF TABLES O O O O O O O O C O O O O D O O O O O O O O . LIST OF FIGURES. . . . . . . . . . . . . . . . . . . . . . . INTRODUCTION . O O O O I O O O O O O O I O O O O O O O O O 0 CHAPTER I - A MECHANISM OF PHOSPHORUS ADSORPTION ON SOIL AND ANION EXCHANGE RESIN SURFACES . . . . . . . . . . . . Introduction. . . . . . . . . . . . . . . . . . . . . Materials and Methods . . . . . . . . . . . . . . . . Results and Discussion. . . . . . . . . . . . . . . . Literature Cited. . . . . . . . . . . . . . . . . . . CHAPTER II - VERTICAL MOVEMENT OF PHOSPHORUS AS RELATED TO AVAILABLE PHOSPHORUS IN SOIL PROFILES . . . . . . . . Introduction. . . . . . . . . . . . . . . . . . . . . Materials and Method. . . . . . . . . . . . . . . . . 'Soil Sample. . . . . . . . . . . . . . . . . . Soil Analyses. . . . . . . . . . . . . . . . . Langmuir Adsorption Isotherm . . . . . . . . . Results and Discussion. . . . . . . . . . . . . . . . Tuscola County . . . . . . . . . . . . . . . . Gratiot County . . . . . . . . . . . . . . . . St. Joseph County. . . . . . . . . . . . . . . CONCLUSIONS 0 O O O — O O O O O O O r O O O O O O O O O O O O O 0 Chapter I o o. o o o o o o o o o o o o o o o o o o o 0 Chapter II o o o o o o o o o o o o o o o o o o o o o 0 LITERATURE CITED 0 O O O O O C O O C O C O O O I O O O O C 0 iv Page vii lerIU‘l-b 28 28 3O 30 3O 31 31 31 33 41 47 47 47 48 Table LIST OF TABLES Cha ter I Physical and chemical properties of Computed Langmuir adsorption maxima capacities of soils . . . . . . . . Computed Langmuir adsorption maxima capacities of anion exchange resins Chloride released during adsorption soils used. . . . and BET monolayer and BET monolayer (Cl’fomS) o o o o of P on Dowex l-X8 and pH values of the equilibrium solutions. . . . . . Chloride plus iodate released during adsorption of P on Dowex 1—X8 (after partial replacement of C1. by 103) and pH values of the equilibrium solutions. . . . . . Chloride released during P adsorption at high initial P concentration on Dowex 1-X8 (Cl-forms) and pH values of the equilibrium solutions. . . . Chapter II Bray Pl extractable P and pH measurements of the loam soils from Tuscola County . . . . . Langmuir adsorption parameters for surface samples from Tuscola county. 0 I O O O O O O I O O O O O O O O O O O The ratio of Bray Pl extractable P to adsorbed P at 1 ppm P in soil solution for surface samples from Tuscola County. . . . . . . . . . . Bray P1 extractable P and pH measurements of the loam soils from Gratiot County . . . . . . . . . . . . . . . Langmuir adsorption parameters for surface samples from Gratiot County. . . . . . . . . . . The ratio of Bray P1 extractable P to adsorbed P at 1 ppm P in soil solution for surface samples from Gratiot county. I C C O O O O O C . Bray Pl extractable P and pH measurements of the sandy loam soils from St. Joseph County . Page 12 l3 17 20 21 32 34 35 36 38 40 42 Table Page Langmuir adsorption parameters for surface samples from St. Joseph County . . . . . . . . . . . . . . . . . . . . 44 The ratio of Bray Pl extractable P to adsorbed P at 1 ppm P in soil solution for surface samples from St. Joseph County . . . . . . . . . . . . . . . . . . . . 45 vi Figure 1 LIST OF FIGURES Langmuir isotherm for P adsorption on Locke sandy and Hillsdale sandy loam (data from Srisen, 23) BET isotherm for P adsorption on Locke sandy loam (co = 1.43 x 10-3 M p) and Hillsdale sandy loam (co = 2.08 x 10’3 M P) (data from Srisen, 23) . Langmuir isotherm for P adsorption on iodate and chloride saturated resins (Dowex l-XB). . . . . BET isotherm for P adsorption on the Cl- saturated resin (Dowex l-X8). . . . . . . . . . . . . . . Langmuir isotherm for P adsorption on the Cl- saturated resin (Dowex l-X8) at high equilibrium concentration. The isosteric differential heat of adsorption, EH; as a function of phosphate adsorbed on Dowex 1-X8. vii loam Page ll 14 ‘15 19 24 INTRODUCTION The liberal application of phosphate fertilizers is the most extensively used procedure for improving P fertility of soils. Since the efficiency of P fertilizers is low, it usually requires rela- tively large applications to produce maximum yields. Such applica- tions are generally profitable and are therefore adopted by some Michigan farmers. But these large applications over many years may result in gradual movement of P down through the soil profiles and into the groundwater. Once in the groundwater, P may enter streams, rivers and lakes resulting in pollution of these natural waters. Many of the rivers and lakes in Michigan are polluted by high levels of P from detergents in domestic and industrial wastewaters. However, since the source of these additions is known, their input into natural waters can be controlled. What is not generally recognized is the input from the groundwater resulting from heavy application of P fertilizers to agricultural soils. Because of this, there is a need to identify at what levels of available P and on which agricul- tural soils P movement into groundwater can be expected in Michigan. In addition, soils are increasingly being considered for treat- ment and renovation of agricultural, domestic and industrial waste— waters and materials. It is essential that we identify soils which would best serve this purpose for a satisfactory length of time. If a procedure is to be developed to determine when appreciable downward movement of P may occur, the establishment of a level, or 1 levels, of extractable soil P (measured in Michigan by the Bray Pl extraction) above which downward movement would be significant seems feasible. Phosphorus must be in solution to move, so some method must be devised to relate Bray Pl extractable P to concentrations of P in the soil solution. A number of Michigan soils showed a high linear correlation between Bray Pl extractable P and surface adsorbed P (Susuki et al., 1963). This correlation appeared only above 20 ppm Bray Pl extractable P. However, values below 20 ppm are not in the range where P move- ment would be expected. This suggests that Bray Pl extractable P could be used as a good measure of surface adsorbed P. The linear form of the Langmuir adsorption isotherm has been widely used to study P adsorption by soils. There are certain assump- tions that must be made when applying this equation to P adsorption by soils, and the equilibrium concentration of P determined experi- mentally is probably not precisely the same as the concentration in the soil solution at field capacity. A rigorous discussion of the applicability of the Langmuir isotherm to studies of soil phosphorus is given by Fried and Broeshart (1967). Deviations from the straight line predicted by the linear form of the Langmuir equation have been reported for both soil and homogeneous soil minerals. These devia- tions were fitted by two intersecting lines suggesting that two different reactions were controlling the soil P adsorption system--one that dominated at relatively high concentrations of P in solution and the other that dominated at much lower P concentrations. Elucidation of the mechanism of P adsorption on soil and soil mineral surfaces may explain the deviations observed and may indicate whether the Langmuir equation accurately characterizes P adsorption by soils. The latter should be determined before constants obtained from the Langmuir equation are used in the interpretation of P movement data. CHAPTER I A MECHANISM OF PHOSPHORUS ADSORPTION ON SOIL AND ANION EXCHANGE RESIN SURFACES Introduction The mechanism of P adsorption on soil and homogeneous soil mineral surfaces is not clearly defined in the literature. However, it is generally agreed that on clay and sesquioxide mineral surfaces, phosphate ions replace exposed OH groups and/or other adsorbed anions. Whether the bonds between the phosphate ions and the Fe (III) and Al (III) atoms in the colloidal surface are ionic, covalent, or coordinate covalent is not agreed upon. Stout (24) and other investigators (5,12), working with kaolinite, soils, and hydrated sesquioxides, respectively, suggested that phos- phate adsorption represents a physicochemical anion exchange equilibrium whereby phosphate ions replace exposed on ions from the colloidal materials. Dickman and Bray (6) presented clear evidence that the exposed OH groups of kaolinite are replacable. Lutz et al. (17). Muljadi et al. (18), and Kafkafi et al. (11) also suggested that P is adsorbed by exchange of edge OH groups in kaolinite and hydrated sesquioxide minerals. Kuo and Lotse (13) disagreed with this and suggested that a coordinate covalent bond is formed between A1 of the surface and O of the phosphate ion by replacement of coordinated H20 or another anion. The research of Rajan and Watkinson (22) indicated that, during P adsorption on an allophane clay, adsorbed $04, Si04, 5 H20 and OH groups were exchanged for phosphate. Parfitt et al. (21) and Atkinson et a1. (1), using infrared spectrosc0pic techniques, presented evidence of the formation of a binuclear surface complex of the type Fe-O-P(02)—O-Fe in which two of the O atoms of the phosphate ion are coordinated, each to a different Fe+3 ion, when phosphate reacts with Fe oxides. Although adsorption isotherms by themselves do not indicate the mechanism involved, they do illustrate the equilibrium relationship between the amounts of adsorbed and dissolved species at a given temperature. The Langmuir equation has been used to characterize the adsorption of P by soils (20) and soil minerals (4,10). Deviations from a single linear Langmuir relationship at high equilibrium P concentrations in soils (20) and soil minerals (4) as well as at low equilibrium concentrations (7,25) have been reported. Griffin and Jurinak (7) found that the deviation at low equilibrium P concentra- tion, when P interacted with calcite, could also be represented by the BET equation which gave a single straight line. The purpose of this research was to investigate the mechanism of P adsorption at low equilibrium concentrations. Materials and Methods The physical and chemical properties of the air-dried surface soils as well as the equilibrium phosphate data were obtained from Srisen (23). The necessary calculations were performed on the equili- brium data and the resulting isotherms plotted (Figures 1 and 2). The sorption of phosphate by Dowex l-X8 and Dowex 2-X4 was studied by shaking (on a wrist action shaker) l g of resin in 50 ml centrifuge tubes containing 20 ml of KH2P04 solution. The Cl-resin 6 was washed with deionized H20 until free of excess Cl- and dried over- night at 60 C in an oven. The concentrations of the initial solutions ranged from 4.0 to 25.0 ppm P for the low equilibrium range and from 120 to 190 ppm P for the high. The centrifuge tubes were covered with Parafilm and shaken in a constant temperature chamber at 25 C for 3 hours, filtered, and the filtrate analyzed for P content. Phosphate was determined by the method of Murphy and Riley (19), modified for use with a Technicon Auto Analyzer II. The P sorbed was calculated from the difference between initial and final concentrations of P in solution. Dowex l-X8 and Dowex 2-x4 had anion exchange capacities of 3.2 and 3.1 meq per g (dry weight), respectively. Chloride released during adsorption was measured using a specific ion Cl-electrode with a double-junction reference electrode. Iodate was measured by the method of Salcedo and Shields (unpublished) as follows: 5 ml of sample is added to 15 ml of acetonitrile, 1 ml of acetic acid, 0.2 ml of a supersaturated KI solution, and the volume made up to 25 ml with deionized H20. The resulting yellow color is measured at 360 mu and compared to a standard curve. Iodate saturated Dowex l-X8 was obtained by treating the resin successively with 1 liter of 1N K103, 0.1N K103, 0.01M K103 and finally with deionized H20 until the wash was free of excess 103- and Cl-. The drying procedure was the same as described above. The Langmuir (14) and BET (3) equations were used to interpret the equilibrium adsorption data. The linear form of the Langmuir equation is: [1] 010 l ——-+ x/m Kb 7 where c is the equilibrium P concentrations, x/m is the amount of P adsorbed per unit mass of adsorbent, b is the P adsorption maximum, and K is a constant related to the energy of adsorption. The BET equation in its linear form is: c __ l (k-l) c x/m (c -c) _ kx + kX c [2] o m m o where c is the equilibrium P concentration, cO is the maximum concen- tration of P that can exist in solution before precipitation can occur, x/m is the amount of P adsorbed per unit mass of adsorbent, k is related to the free energy of transfer of P from the bulk solu- tion to the surface of the adsorbent and is assumed to be constant (7). The monolayer capacity (Xm) may be calculated graphically using the above equation. The differential isosteric heat of adsorption, KHZ was obtained by collecting adsorption data at 11 C and 25 C and applying the Clausius-Clapeyron equation to the system where the surface coverage, 6, is maintained constant (7). C]_z§ [1_1 c1 9 2.303 R T1 T2 Log [ l [3] where c and c are the equilibrium concentrations at temperatures T l 2 1 and T2, respectively, and R is the molar gas constant. Results and Discussion Some of the physical and chemical properties of the soils used in this study are presented in Table l. The adsorption isotherms of two of the four soils used are shown in Figure l. The data represent averages of triplicates and were shown to fit a two slope Langmuir plot. Some workers (4,20) obtained a single linear Langmuir * Table 1. Physical and chemical properties of soils used Taxonomic Organic Bray Pl Soil type placement pH matter (%) (ppm) Hillsdale sandy Typic hapludalf 5.82 1.66 57.9 loam coarse-loamy Locke sandy loam Aquallic hapludalf 6.00 2.72 41.7 coarse-loamy Sims clay loam Mollic haplaquept 6.23 6.77 39.5 fine-mixed Hoytville clay Mollic ochraqualf 6.50 3.16 7.5 loam fine-illitic * Data taken from Srisen (23). 24 20. IS g—LOCKE DE 8 O —HILLSDALE IO 20 30 40 50 C x IO"J mole/liter Figure 1. Langmuir isotherm for P adsorption on Locke sandy loam and Hillsdale sandy loam. (data from Srisen, 23) 10 relationship over the same equilibrium P concentration range (up to 14 ug/ml). Syers et a1. (25), however, obtained a two slope Langmuir plot for three soils over this equilibrium P concentration range. They interpreted this as an indication of two populations of sites on the soil surface which have a widely differing affinity for P. Gunary (8) reported that P adsorption data for 24 soils fitted curvilinear lines. This was interpreted to signify that the soil will adsorb P with decreasing energies of binding. It should be noted that some two-slope Langmuir plots can be fitted to curvilinear lines and vice versa. The adsorption data of the four soils used in this study were also found to be described by the BET equation (curves for two soils are illustrated in Figure 2). The co term was computed from the pH of the soils and the pK of 6.66 for CaHPO assuming a pCa of 2.5 (15) 4 and by the use of the Debye-Huckel equation for estimating the activity of the individual ion species in an ionic strength of 0.01M CaC12. The data in Table 2 show that the monolayer capacities, Xm, computed by the BET equation, agree closely with the adsorption maxima, b , obtained from the initial slopes of the Langmuir isotherms. l Griffin and Jurinak (7) reported similar results. Although the BET equation is linear for the entire region, this is not a sufficient criterion to conclude that P adsorption on soil and soil mineral sur— faces is a multilayer phenomenon. But the linear fit does suggest that P is being adsorbed with at least two different energies of binding making no commitment as to the actual location of these sites with respect to the surface. 11 IS a —LOCKE o —- HILLSDALE 5 IO )5 20 25 (:/(:0 x lo: Figure 2. BET isotherm for P adsorption on Locke sandy loam (co = 1.43 x 10"3 M P) and Hillsdale sandy loam (co = 2.08 x 10'3 M P) (data from Srisen, 23). 12 Table 2. Computed Langmuir adsorption maxima and BET monolayer capacities of soils Langmuir adsorption BET monolayer Soil maximum* capacity ‘mg P/100 g of soil Hillsdale A 15.3 16.8 Locke A 13.3 13.4 Sims A 15.5 14.9 Hoytville A 15.0 17.6 * Correction made for adsorbed P assuming adsorbed P is equiva- lent to Bray Pl extractable P. The different slopes noted in Figure 1 may be due to a) adsorp- tion taking place at energetically different sites on the surface, b) adsorption occurring in layers on the surface, and c) precipitation of the phosphate ion. Griffin and Jurinak (7) and Muljadi et al. (18) reported that P is adsorbed with different energies of binding at low equilibrium concentrations on homogeneous soil mineral surfaces. The relatively low equilibrium P concentrations at which the deviations in those studies occurred should exclude precipitation as a factor. Also, the expected homogeneity of the surfaces suggests that another mechanism of adsorption other than the existence of energeti- cally different binding sites for P may be responsible for this effect. In order to investigate this mechanism, P adsorption on anion exchange resin surfaces, Dowex l-X8 and Dowex 2-X4, was studied. The anion exchange resin system should have energetically homogeneous 13 surface sites, should not precipitate P, and should have known replaceable anions which can be easily measured in equilibrium solutions. Langmuir adsorption isotherms for P adsorption on Dowex l-X8 (C1- and I03- forms) are shown in Figure 3. The data represent averages of duplicates and, like the soils data, fit two lines. The same result was obtained for Dowex 2-x4. A linear fit was obtained with the BET equation. However, in the case of the resin, a co term could not be calculated since no precipitation of P is expected in this system. A value for co was obtained by iteration until the best fit for the straight line was obtained. The Langmuir adsorption maxima and BET monolayer capacities for Dowex 1-X8 and Dowex 2-X4 are given in Table 3. As in the case of the soils, close Table 3. Computed Langmuir adsorption maxima and BET monolayer capacities of anion exchange resins (Cl-forms) Langmuir adsorption BET monolayer Resin maximum capacity mg P/100 g of resin Dowex 2-X4 31.0 33.9 Dowex l-X8 29.2 26.4 agreement was obtained. The fact that the data from the two slope Langmuir isotherm (Figure 3) can fit a single line BET isotherm (Figure 4) may indicate P adsorption on the resin surface with at least two different energies of binding in spite of the expected homogeneity of the resin surface sites. 14 .cfimou concusumm dunno“ may you meow onu one a can 0 .camou coucusumm moanedno 0:» mo .>Ho>wuoommmu .N can a m:0flmmu mum m can m .Amxla xo3oov mcfimmu concusumm mcfluoano cam oumcofl co coflumuomow m you EMOSHOmH stsmcmq .m mndmfim .2: :2: .0. x o m. m. m c - n . 2.8: 35.52.... 3.3.50...- “ _ Emma 935.33 2.307.. n ‘ . u x m m _ _ VA _ M” II _ . . i\ I 0‘ . Q U . u v nu II I. In I..I .I r..l wv A.I.I I ...r I I Iv m u < _ . .Amxla xm3oav cammu @mumusuMm IHU ms» :0 COHuQHOmom m Now EumnuOmfi 9mm .v musmflm o , «o. x ob on om o. 5 l l v k. w )3 ow a m ll x m N. ... m. 16 Computations derived from measurements made on the equilibrium P solutions of the isotherms in Figure 3 are presented in Tables 4 and 5. Measurements of the Cl- released during P adsorption (Table 4) indicate that over region 1 two Cl- ions were initially released per phosphate ion adsorbed. This value decreased and continued to decrease over region 2, approaching a 1:1 ratio. pH measurements of the equilibrium solutions showed a decrease from the 0 ppm to the 7 ppm initial P treatments. There was no significant change in pH from the 7 ppm to the 20 ppm initial P treatments. This suggests very little or no deprotonation at higher equilibrium concentrations. The above results suggest P adsorption initially as HPO4 (two point attachment) followed by adsorption as H P0; (one point attachment). 0 ll 2 / ///® 5) //PESI mm.h mo.h gm.® 55.0 m®.© ON.® mm.m ha.o mm o m N N 6.6 6.6 6.6 . 6.6 6.6 . 6.6 66 66 65666 66 6666 66 56.5 66.5 66.5 66.5 66.5 66.5 65.6 66.6 66.6 66.6 66 6.6 6.6 6.6 6.6 5.6 5.6 6.6 666 66 66 62666 66 6666 6 65.5 66.5 65.5 65.5 55.5 66.5 66.5 66.5 66.5 56.5 66 6.6 6.6 66.6 6.6 6.6 6.6 6.6 6.6 66 66 66666 66 6666 6 65.5 65.5 66.5 66.5 66.5 65.5 66.5 66.5 66.6 66.6 66 6.6 6.6 6.6 6.6 6.6 6.6 6.6 6.6 66 66 62666 66 6666 5 I 66.5 66.5 66.5 66.5 66.5 56.6 56.6 66.6 66.5 66 I 6.6 6.6 6.6 6.6 6.6 6.6 6.6 66 66 65666 66 6666 6 66.5 66.5 66.5 66.5 66.6 66.5 66.6 66.6 66.6 66.6 66 6.6 6.6 6.6 6.6 6.6 6.6 6.6 6.6 . 6.6 66 65666 66 6666 6 I I 66.5 56.5 66.5 66.5 66.5 66.5 56.5 66.5 66 I I 66.6 6.6 6.6 6.6 5.6 6.6 6.6 6.6 66666 66 6666 6 66.5 66.5 66.5 66.5 56.5 66.5 56.6 66.6 66.6 66.6 66 6.6 6.6 6.6 5.6 5.6 . 6.6 . 6.6 6.6 6.5 6.6 6366. 66 6666 6 65.5 66.5 66.5 66.5 66.5 66.5 66.5 66.5 66.5 66.5 66 6.6 6.6 6.6 6.6 6.6 6.6 6.6 6.6 6.6 6.6 65666 66 6666 6 I 65.5 66.5 66.5 66.5 66.5 66.6 65.6 66.6 66.6 66 I 66.6 6.6 6.6 5.6 5.6 6.6 6.6 6.6 6.6 6266. 66 6666 6 666I666 666I666 666I666 666I66 66I65 65I66 66I66 66I66 66I66 66I6 6566 66666 6666 >ucsoo uOHumuu 806m 66606 8606 0:6 mo 666056656668 mm 0cm 6 manmuomuuxm Hm wmum .6 magma 37 66.5 66.5 66.5 66.5 66.5 66.5 66.5 66.5 66.5 66.6 66 6.6 6.6 6.6 6.6 6.6 6.6 6.6 66 666 65 66666 66 6666 56 65.5 65.5 56.5 65.5 66.5 66.5 66.5 66.5 66.5 66.6 66 6.6 6.6 6.6 6.6 6.6 6.6 6.6 66 65 66 66666 66 6666 66 I I I 66.6 65.6 65.6 65.6 56.6 66.6 66.6 66 I I I 6.6 6.6 6.6 6.6 6.6 66 66 62666 66 6666 66 66.5 66.5 66.5 66.5 66.5 66.5 66.5 66.5 66.5 65.6 66 6.6 6.6 6.6 6.6 6.6 6.6 6.5 66 66 66 .6666 66 6666 66 65.5 66.5 65.5 66.5 56.5 66.5 66.5 56.5 56.6 66.6 66 66.6 6.6 6.6 6.6 6.6 6.6 6.6 6.6 66 66 66666 66 6666 66 66.5 66.5 66.5 66.5 66.5 66.5 66.5 66.5 66.6 66.6 66 6.6 6.6 6.6 6.6 6.6 6.6 6.6 6.6 66 66 65666 66 6666 66 65.5 66.5 66.5 66.5 56.6 66.5 66.6 66.6 65.6 66.6 66 6.6 6.6 6.6 6.6 6.6 6.6 6.6 6.6 66 66 66666 66 6666 66 666I666 666I666 666I666 666I66 66I65 65I66 66I66 66I66 66I66 66I6 6666 66666 6666 66666666666 6 66666 38 Table 5. Langmuir adsorption parameters for surface samples from Gratiot County Region 1 01 at 1 ppm P in Site K1 b1 soil solution X 10-4 mg/100 9 soil 1 8.3 12.0 0.73 2 3.9 11.0 0.56 3 1.0 14.0 0.25 4 2.5 19.3 0.45 5 4.1 13.3 0.57 6 50.0 10.0 0.94 7 1.0 17.0 0.25 8 2.0 15.8 0.39 9 2.7 14.2 0.46 10 2.1 14.5 0.41 11 2.6 10.1 0.45 12 1.4 17.0 0.31 13 2.7 11.6 0.47 14 5.1 14.0 0.62 15 9.0 23.2 0.74 16 2.0 14.7 0.39 17 2.3 17.5 0.42 39 P movement was indicated) the Bray Pl phosphorus levels of the 15-30 cm layer are higher (sometimes much higher) than those of the cor- responding 0-15 cm samples. This might be due to mechanical mixing of the surface soils during plowing followed by crop removal of some of the P from the 0-15 cm layers prior to soil sampling. That might also explain why the extractable P levels of sites 10, 11 and 14, being in a range where movement is not expected in a fine textured soil, do show a little movement. The ratio of Bray P1 extractable P to that fraction of the adsorption maximum which would give movement if the soil solution concentration was 1 ppm P are shown in Table 6. When the ratio is above 1.0 (sites 16 and 17) some movement is indi- cated. Movement is manifested in some cases when the ratio is approximately 0.5 (sites 10 and 14) and 0.6 (site 11), but other sites having the same (sites 7 and 12) and higher (site 13) ratios show no movement. This suggests that other factors (i.e., actual texture of entire soil profile, actual amount of water that moved through the soil profile, etc.) not taken into consideration here also affect P movement in these soils. A critical ratio value above which P moves and below which it does not move cannot be established from the data presented in Table 6. Generally, the soil profiles (sites) from Gratiot County have lower pH values (below 7.0) between 0-60 cm than the sites from Tuscola County; however, the pH increased between 60-152 cm depth to values (above 7.0) similar to those obtained from the Tuscola County soils (Tables 1 and 4). Any effect of pH on P movement cannot be discerned. 40 Table 6. The ratio of Bray Pl extractable P to adsorbed P at 1 ppm P in soil solution for surface samples from Gratiot County Bray Pl Movement Depth of Site b1 X 01 indicated movement (cm) 1 0.07 -* 2 0.11 - 3 0.25 - 4 0.11 - 5 0.19 - 6 0.17 - 7 0.48 - 8 0.34 - 9 0.34 - 6'. 10 0.49 + 30-45 1. 11 0.64 + 30-45 12 0.63 - 13 0.74 - 1. 14 0.48 + 30-45 15 0.31 - ’ + 16 1.08 + 30-45 17 1.03 + 30-45 * (-) indicates no discernible downward movement of P and (+) indicates discernible downward movement of P to a specified depth. *Apparent movement at this depth but little accumulation of P. 41 St. Joseph County Table 7 shows the Bray Pl extractable P and pH measurements of the sandy loam soils from St. Joseph County. Thirteen of the 20 sites show appreciable downward movement of P. Of these, movement advanced to the greatest depth in site 14, showing P enrichment of the 90-105 cm layer. One-fourth, or 5 sites (16, 17, 18, 19 and 20) have Bray Pl extractable P values above 100 ppm P, the level above which appreciable downward movement of P may occur in soils (Doll et al., 1972).1 One of the five (site 16) exhibits no movement while the others indicate definite movement. Again, some of the sites which manifest movement have higher Bray Pl levels in the 15-30 cm layers than the corresponding 0-15 cm layers. This is not observed in sites which show no movement. The same explanation as put forth for similar findings in the surface soils of Gratiot County is presented here. The Bray P1 extractable P data for these sandy loam soils reveal that at higher levels (above 80 ppm P), P movement is evident for most sites but at the lower levels (below 50 ppm P) some sites also exhibit movement. Generally, the soil profiles from St. Joseph County are more acid than those from the other two counties. However, the degree of movement seems to be determined more by soil texture rather than soil pH. The absence of a relationship between the ratio of Bray P1 extrac- table P to that fraction of the adsorption maximum which would allow movement if the soil solution concentration was 1 ppm P is shown in Table 9. A critical ratio value (explained earlier) for P movement 1Doll, E. C., J. F. Demeterio and R. P. White. 1972. Accumu- lation and movement of phosphorus in Michigan soils. Unpublished data, Michigan State University. 42 66.6 66.6 66.6 66.6 66.6 66.6 66.6 66.6 66.6 66 6.6 5.6 66 66 66 66 66 56 66 66666 66 6666 65.6 66.6 56.6 66.6 66.6 66.6 65.6 66.6 56.6 66 66 66 66 66 66 66 66 66 66 66666 66 6666 66.5 66.6 66.6 66.6 66.6 56.6 66.6 66.6 66.6 66 66.6 6.6 6.6 6.5 5.6 6.6 66 66 66 6666. 66 6666 56.6 66.6 66.6 66.6 66.6 66.6 66.6 66.6 66.6 66 66 66 66 66 66 66 56 66 66 66666 66 6666 I I I I 66.5 66.5 66.5 66.5 66.5 66 I I I I 56 66 66 65 66 66666 66 6666 55.6 56.6 65.6 65.6 66.6 66.6 66.6 66.6 66.6 66 6.6 6.6 66 66 66 66 66 66 66 66666 66 6666 66.6 66.6 66.6 65.6 66.6 66.6 66.6 66.6 66.6 66 6.6 5.6 6.5 6.6 5.6 6.6 66 66 56 66666 66 6666 65.6 56.6 66.6 66.6 65.6 66.6 66.6 66.6 66.6 66 6.66 6.6 6.6 6.6 66 66 66 66 66 66666 66 6666 66.6 66.6 65.6 66.6 66.6 66.6 56.6 66.6 66.6 66 6.6 5.6 6.6 6.6 6.6 66 66 66 66 66666 66 6666 66.5 66.5 66.6 65.6 66.6 66.6 65.6 66.6 66.6 66 6.6 6.6 6.6 6.6 6.6 6.6 6.5 6.6 66 66666 66 6666 666I666 666I666 666I666 666I66 66I65 65I66 66I66 66I66 66I66 66I6 6666 66666 hucsou smomOH .um 5066 66606 8606 >UCMm on» No mucoEmHSmmoE mm new 6 wanmuomuuxm Hm xmum 43 [W 66.6 56.6 66.6 56.6 66.6 66.6 66.6 56.6 66.6 66.6 66 66 66 66 66 56 66 66 666 666 666 66666 66 6666 66 66.6 66.6 66.6 66.6 55.6 66.6 65.6 66.6 66.6 66.6 66 6.6 5.5 5.5 6.6 6.6 6.5 66 65 666 666 66666 66 6666 66 66.6 56.6 65.6 65.6 66.6 65.6 66.6 56.6 56.6 56.6 66 6.6 6.5 6.6 6.5 6.6 66 56 66 666 666 66666 66 6666 66 66.6 66.6 66.6 66.6 56.6 66.6 66.6 66.6 66.6 66.6 66 66 6.5 6.6 66 66 66 66 66 666 666 66666 66 6666 56 56.6 65.6 66.6 66.6 66.6 66.6 66.6 66.6 66.6 66.6 66 66 66 66 66 6.6 66 6.6 66 66 566 66666 66 6666 66 66.6 66.6 66.6 66.6 66.6 66.6 56.6 66.6 56.6 56.6 66 6.6 6.6 6.6 6.6 6.6 6.6 6.6 66 65 66 66666 66 6666 66 66.6 66.6 66.6 66.6 56.6 66.6 56.6 66.6 66.6 66.6 66 66 66 66 66 66 666 666 56 666 66 66666 66 6666 66 66.6 66.6 66.6 66.6 66.6 66.6 66.6 66.6 66.6 66.6 66 6.6 66 66 66 66 66 66 66 66 65 66666 66 6666 66- 56.6 66.6 66.6 66.6 66.6 66.6 66.6 66.6 66.6 66.6 66 6.5 6.6 6.5 6.5 6.5 6.6 66 66 66 65 66666 66 6666 66 66.6 66.6 66.6 66.6 55.6 I 65.6 66.6 66.6 66.6 66 5.6 66 66 66 66 I 66 66 66 66 66666 66 6666 66 666I666 666I666 ‘666I666 666I66 66I65 65I66 66I66 66I66 66I66 66I6 6666 66666 6666 L 66666666666 5 66666 44 Table 8. Langmuir adsorption parameters for surface samples from St. Joseph County Region 1 01 at 1 ppm P in Site K1 b1 soil solution x 10’4 69/100 9 soil 1 8.9 14.0 0.74 2 2.3 12.5 0.43 3 4.2 20.0 0.57 4 6.3 11.3 0.67 5 4.4 12.5 0.58 6 7.1 14.0 0.69 7 7.3 13.7 0.70 8 3.7 13.7 0.54 9 5.4 13.2 0.63 10 10.7 15.5 0.77 11 17.5 19.0 0.84 12 12.5 20.0 0.80 13 5.5 15.0 0.63 14 17.5 19.0 0.84 15 6.0 16.7 0.65 16 3.6 20.0 0.53 17 5.8 21.4 0.65 18 1.7 25.0 0.35 19 6.6 25.0 0.68 20 4.9 40.5 0.61 45 Table 9. The ratio of Bray P1 extractable P to adsorbed P at 1 ppm P in soil solution for surface samples from St. Joseph County Bray P1 Movement Depth of Site bl x 01 indicated movement (cm) 1 0.23 -* 2 0.66 + 30-45 3 0.32 - 4 0.50 - 5 0.52 + 75-90 6 0.56 + 60-75 7 0.63 + 60-75 8 0.83 - 9 0.75 + 30-45 10 0.54 - 11 0.41 + 30-45 12 0.45 + 30-45 13 0.84 + 45-60 14 0.51 + 90-105 15 0.76 - 16 1.20 - 17 l 0.95 + 30-45 18 1.51 + 30-45 19 1.08 + 45-60 20 1.33 + 75-90 * (-) indicates no discernible downward movement of P and (+) indicates discernible downward movement of P to a specified depth. 46 cannot be established. Of the sites which show a ratio above 1.0, one of them does not display movement (unlike the soils from Gratiot County). Other factors (i.e., actual texture of the entire soil profile, amount of water which actually moved through the profiles, etc.) other than the ratio seem to control P movement. Generally, these sandy loam soils have higher Bray P1 extractable P levels in the 0-15 cm layer and show greater downward movement of P than the loam soils from Tuscola and Gratiot Counties. The higher levels of extractable P might be due to heavier application of P fertilizers to these soils. The above results confirm the supposition that appreciable downward movement of P is likely to occur on heavily fertilizes coarse textured soils. CONCLUSIONS Chapter I l). The Langmuir adsorption isotherm can be used to illustrate P adsorption on soil surfaces although two separate reactions seem to be involved even at low equilibrium P concentrations. These reactions yield two sloped isotherms when the linear form of the Langmuir equation is used. 2). It is hypothesized that the deviation from linearity observed on soil and homogeneous soil mineral surfaces when using the Langmuir equation is due to P adsorption initially by ”two point attachment" followed by "one point attachment.” Chapter II 1). Coarse textured Michigan soils, when heavily fertilized, will permit appreciable downward movement of P. Over many years, movement may actually progress as far as the groundwater. 2). Significant downward movement of P is not observed in heavily fertilized fine textured Michigan soils. 3). A level or levels of Bray P1 extractable P of surface soils above which P movement is expected cannot be established from the data obtained in this study. Levels of Bray Pl extractable P of surface soil does not seem to correlate well with P movement. 47 LITERATURE CITED LITERATURE CITED Atkinson, R. J., R. L. Parfitt, and R. St. C. Smart. 1974. Infrared study of phosphate adsorption on geethite. J. Chem. Soc. Faraday l, 70:1472. Barrow, N. J. and T. C. Shaw. 1975. The slow reactions between soil and anions: 3. The effect of time and temperature on the decrease in isotopically exchangeable phosphate. Soil Sci. 119:190-197. Brunauer, 8., P. H. Emmett, and E. Teller. 1938. Adsorption of gases in multimolecular layers. J. Am. Chem. Soc. 60:309-319. Cole, C. V., S. R. Alsen, and C. 0. Scott. 1953. The nature of phosphate sorption by calcium carbonate. Soil Sci. Soc. Amer. Proc. 17:352-356. Dean, L. A. and E. J. Robins. 1947. Anion exchange in soils: 1. Exchangeable phosphorus and the anion exchange capacity. Soil Sci. 63:377-387. Dickman, S. R. and R. H. Bray. 1941. Replacement of adsorbed phos- phate from kaolinite by fluoride. Soil Sci. 52:263-275. Doll, E. C., A. L. Hatfield, and R. J. Todd. 1959. Vertical distribu- tion of topdressed fertilizer phosphorus and potassium in rela- tion to yield and composition of pasture herbage. Agron. J. 51:645-648. Doll, E. C., J. C. Shickluna, and J. F. Demeterio. 1972. Levels and changes in soil test in lower Michigan 1962-1971. Mich. Exp. Sta. Res. Rep. 197. Ellis, B. G. 1975. Phosphorus adsorption and movement as related to soil series. Soil Sci. Southern Africa Proc. 65h Congress 58-66. Erickson, A. E. and B. G. Ellis. 1971. The nutrient content of drainage water from agricultural land. Mich. Agr. Exp. Sta. Res. Water Resources Commission. Fried, M. and H. Broeshart. 1967. The soil-plant system in relation to inorganic nutrition. Academic Press, Inc., 111 Fifth Avenue, New York. 358 pp. 48 49 Griffin, R. A. and J. J. Jurinak. 1973. The interaction of phosphate with calcite. Soil Sci. Soc. Amer. Proc. 37:847-850. Helfferick, F. 1962. "Ion Exchange." p. 166. McGraw-Hill, New York. Hsu, P. H. and D. A. Rennie. 1962. Reaction of phosphate in aluminum systems: 1. Adsorption of phosphate by X-ray amorphous "aluminum hydroxide.” Can. J. Soil Sci. 42:179-209. Kafkafi, U., A. M. Posner, and J. P. Quirk. 1967. Desorption of phosphate from kaolinite. Soil Sci. Soc. Amer. Proc. 31:348-353. Kelly, J. B. and A. R. Midgley. 1943. Phosphate fixation - an exchange of phosphate and hydroxyl ions. Soil Sci. 55:167-175. Kao, C. W. and R. W. Blanchar. 1973. Distribution and chemistry of phosphorus in an Albaqualf soil after 82 years of phosphate fertilization. J. Environ. Quality 2:237-240. Kuo, S. and E. G. Lotse. 1972. Kinetics of phosphate adsorption by calcium carbonate and Ca-kaolinite. Soil Sci. Soc. Amer. Proc. 36:725-729. Langmuir, I. 1918. The adsorption of gases on plane surfaces of glass, mica, and platinum. J. Amer. Chem. Soc. 40:1361-1403. Lindsay, W. L. and E. C. Moreno. 1960. Phosphate phase equilibria in soils. Soil Sci. Soc. Amer. Proc. 24:177-182. Low, P. F. and C. A. Black. 1950. Reactions of phosphate with kaolinite. Soil Sci. 70:273-290. Lutz, J. F., R. A. Pinto, R. Garcia-Lagos, and H. G. Hilton. 1966. Effect of phosphorus on some physical properties of soil: II. Water retention. Soil Sci. Soc. Amer. Proc. 39:433-437. Muljadi, D., A. M. Posner, and A. M. Quirk. 1966. The mechanism of phosphate adsorption by kaolinite, gibbsite, and pseudoboehmite: 1. The isotherms and the effect of pH on adsorption. J. Soil Sci. 17:212-229. Murphy, J. and J. P. Riley. 1962. A modified single solution method for the determination of phosphate in natural waters. Anal. Chim. Acta 27:31-36. Neller, J. R. 1946. Mobility of phosphates in sandy soils. Soil Sci. Soc. Amer. Proc. 11:227-230. Olsten, S. R. and F. S. Watanabe. 1957. A method to determine a phosphorus adsorption maximum of soils as measured by the Langmuir isotherm. Soil Sci. Soc. Amer. Proc. 21:144-149. Parfitt, R. L., R. J. Atkinson, and R. St. C. Smart. 1975. The mechanism of phosphate fixation by iron oxides. Soil Sci. Soc. Amer. Proc. 39:837-841. 50 Rajan, S. S. S. and J. H. Watkinson. 1976. Adsorption of selenite and phosphate on an allophane clay. Soil Sci. Amer. Proc. 40:51-54. Spencer, W. F. 1957. Distribution and availability of phosphates added to a Lakeland fine sand. Soil Sci. Soc. Amer. Proc. 21:141-144. Srisen, M. 1974. Adsorption of phosphorus by five Michigan soils under anaerobic conditions. Ph.D. Thesis, Michigan State University, East Lansing, Michigan. Stout, P. R. 1939. Alterations in the crystal structure of clay minerals as a result of phosphate fixation. Soil Sci. Soc. Amer. Proc. 4:177-182. Susuki, A., K. Lawton, and E. C. Doll. 1963. Phosphorus uptake and soil tests as related to forms of phosphorus in some Michigan soils. Soil Sci. Soc. Amer. Proc. 27:401-403. Syers, J. K., M. G. Browman, G. W. Smillie, and R. B. Corey. 1973. Phosphate sorption by soils evaluated by the Langmuir adsorp- tion equation. Soil Sci. Soc. Amer. Proc. 37:3538-363. 45 Table 9. The ratio of Bray P1 extractable P to adsorbed P at 1 ppm P in soil solution for surface samples from St. Joseph County Bray P1 Movement Depth of Site b1 X 01 indicated movement (cm) 1 0.23 -* 2 0.66 + 30-45 3 0.32 - 4 0.50 - 5 0.52 + 75-90 6 0.56 . + 60-75 7 0.63 + 60-75 8 0.83 - 9 0.75 + 30-45 10 0.54 - 11 0.41 + 30-45 12 0.45 + 30-45 13 0.84 + 45-60 14 0.51 + 90-105 15 0.76 - 16 1.20 - 17 i 0.95 + 30-45 18 1.51 + 30-45 19 1.08 + 45-60 20 1.33 + 75-90 * (-) indicates no discernible downward movement of P and (+) indicates discernible downward movement of P to a specified depth. 46 cannot be established. Of the sites which show a ratio above 1.0, one of them does not display movement (unlike the soils from Gratiot County). Other factors (i.e., actual texture of the entire soil profile, amount of water which actually moved through the profiles, etc.) other than the ratio seem to control P movement. Generally, these sandy loam soils have higher Bray Pl extractable P levels in the 0-15 cm layer and show greater downward movement of P than the loam soils from Tuscola and Gratiot Counties. The higher levels of extractable P might be due to heavier application of P fertilizers to these soils. The above results confirm the supposition that appreciable downward movement of P is likely to occur on heavily fertilizes coarse textured soils. CONCLUSIONS Chapter I 1). The Langmuir adsorption isotherm can be used to illustrate P adsorption on soil surfaces although two separate reactions seem to be involved even at low equilibrium P concentrations. These reactions yield two sloped isotherms when the linear form of the Langmuir equation is used. 2). It is hypothesized that the deviation from linearity observed on soil and homogeneous soil mineral surfaces when using the Langmuir equation is due to P adsorption initially by "two point attachment" followed by ”one point attachment." Chapter II 1). Coarse textured Michigan soils, when heavily fertilized, will permit appreciable downward movement of P. Over many years, movement may actually progress as far as the groundwater. 2). Significant downward movement of P is not observed in heavily fertilized fine textured Michigan soils. 3). A level or levels of Bray P1 extractable P of surface soils above which P movement is expected cannot be established from the data obtained in this study. Levels of Bray Pl extractable P of surface soil does not seem to correlate well with P movement. 47 LITERATURE CITED LITERATURE CITED Atkinson, R. J., R. L. Parfitt, and R. St. C. Smart. 1974. Infrared study of phosphate adsorption on geethite. J. Chem. Soc. Faraday l, 70:1472. Barrow, N. J. and T. C. Shaw. 1975. The slow reactions between soil and anions: 3. The effect of time and temperature on the decrease in isotopically exchangeable phosphate. Soil Sci. 119:190-197. Brunauer, S., P. H. Emmett, and E. Teller. 1938. Adsorption of gases in multimolecular layers. J. Am. Chem. Soc. 60:309-319. Cole, C. V., S. R. Alsen, and C. 0. Scott. 1953. The nature of phosphate sorption by calcium carbonate. Soil Sci. Soc. Amer. Dean, L. A. and E. J. Robins. 1947. Anion exchange in soils: 1. Exchangeable phosphorus and the anion exchange capacity. Soil Sci. 63:377-387. Dickman, S. R. and R. H. Bray. 1941. Replacement of adsorbed phos- phate from kaolinite by fluoride. Soil Sci. 52:263-275. Doll, E. C., A. L. Hatfield, and R. J. Todd. 1959. Vertical distribu- tion of topdressed fertilizer phosphorus and potassium in rela- tion to yield and composition of pasture herbage. Agron. J. 51:645-648. Doll, E. C., J. C. Shickluna, and J. F. Demeterio. 1972. Levels and changes in soil test in lower Michigan 1962-1971. Mich. Exp. Sta. Res. Rep. 197. Ellis, B. G. 1975. Phosphorus adsorption and movement as related to soil series. Soil Sci. Southern Africa Proc. 65h Congress 58-66. Erickson, A. E. and B. G. Ellis. 1971. The nutrient content of drainage water from agricultural land. Mich. Agr. Exp. Sta. Res. Water Resources Commission. Fried, M. and H. Broeshart. 1967. The soil-plant system in relation to inorganic nutrition. Academic Press, Inc., 111 Fifth Avenue, New York. 358 pp. 48 49 Griffin, R. A. and J. J. Jurinak. 1973. The interaction of phosphate with calcite. Soil Sci. Soc. Amer. Proc. 37:847-850. Helfferick, F. 1962. ”Ion Exchange." p. 166. McGraw-Hill, New York. Hsu, P. H. and D. A. Rennie. 1962. Reaction of phosphate in aluminum systems: 1. Adsorption of phosphate by X-ray amorphous "aluminum hydroxide.” Can. J. Soil Sci. 42:179-209. Kafkafi, U., A. M. Posner, and J. P. Quirk. 1967. Desorption of phosphate from kaolinite. Soil Sci. Soc. Amer. Proc. 31:348-353. Kelly, J. B. and A. R. Midgley. 1943. Phosphate fixation - an exchange of phosphate and hydroxyl ions. Soil Sci. 55:167-175. Kao, C. W. and R. W. Blanchar. 1973. Distribution and chemistry of phosphorus in an Albaqualf soil after 82 years of phosphate fertilization. J. Environ. Quality 2:237-240. Kuo, S. and E. G. Lotse. 1972. Kinetics of phosphate adsorption by calcium carbonate and Ca-kaolinite. Soil Sci. Soc. Amer. Proc. 36:725-729. Langmuir, I. 1918. The adsorption of gases on plane surfaces of glass, mica, and platinum. J. Amer. Chem. Soc. 40:1361-1403. Lindsay, W. L. and E. C. Moreno. 1960. Phosphate phase equilibria in soils. Soil Sci. Soc. Amer. Proc. 24:177-182. Low, P. F. and C. A. Black. 1950. Reactions of phosphate with kaolinite. Soil Sci. 70:273-290. Lutz, J. F., R. A. Pinto, R. Garcia-Lagos, and H. G. Hilton. 1966. Effect of phosphorus on some physical properties of soil: II. Water retention. Soil Sci. Soc. Amer. Proc. 39:433-437. Muljadi, D., A. M. Posner, and A. M. Quirk. 1966. The mechanism of phosphate adsorption by kaolinite, gibbsite, and pseudoboehmite: l. The isotherms and the effect of pH on adsorption. J. Soil Sci. 17:212-229. Murphy, J. and J. P. Riley. 1962. A modified single solution method for the determination of phosphate in natural waters. Anal. Chim. Acta 27:31-36. Neller, J. R. 1946. Mobility of phosphates in sandy soils. Soil Sci. Soc. Amer. Proc. 11:227-230. Olsten, S. R. and F. S. Watanabe. 1957. A method to determine a phosphorus adsorption maximum of soils as measured by the Langmuir isotherm. Soil Sci. Soc. Amer. Proc. 21:144-149. Parfitt, R. L., R. J. Atkinson, and R. St. C. Smart. 1975. The mechanism of phosphate fixation by iron oxides. Soil Sci. Soc. Amer. Proc. 39:837-841. 50 Rajan, S. S. S. and J. H. Watkinson. 1976. Adsorption of selenite and phosphate on an allophane clay. Soil Sci. Amer. Proc. 40:51-54. Spencer, W. F. 1957. Distribution and availability of phosphates added to a Lakeland fine sand. Soil Sci. Soc. Amer. Proc. 21:141-144. Srisen, M. 1974. Adsorption of phosphorus by five Michigan soils under anaerobic conditions. Ph.D. Thesis, Michigan State University, East Lansing, Michigan. Stout, P. R. 1939. Alterations in the crystal structure of clay minerals as a result of phosphate fixation. Soil Sci. Soc. Amer. Proc. 4:177-182. Susuki, A., K. Lawton, and E. C. Doll. 1963. Phosphorus uptake and soil tests as related to forms of phosphorus in some Michigan soils. Soil Sci. Soc. Amer. Proc. 27:401-403. Syers, J. K., M. G. Browman, G. W. Smillie, and R. B. Corey. 1973. Phosphate sorption by soils evaluated by the Langmuir adsorp- tion equation. Soil Sci. Soc. Amer. Proc. 37:3538-363. 5! IX. "Irmill‘uawWEED