Phosphate Sorption Isotherms Research Articles

Life at the extreme: Plant-driven hotspots of soil nutrient cycling in the hyper-arid core of the Atacama Desert

The hyperarid core of the Atacama Desert represents one of the most intense environments on Earth, often being used as an analog for Mars regolith. The area is characterized by extremes in climate (e.g., temperature, humidity, UV irradiation) and edaphic factors (e.g., hyper-salinity, high pH, compaction, high perchlorates, and low moisture, phosphorus and organic matter). However, the halophytic C4 plant Distichlis spicata appears to be one of the few species on the planet that can thrive in this environment. Within this habitat it captures windblown sand leading to the formation of unique structures and the generation of above-ground phyllosphere soil. Using a combination of approaches (e.g., X-ray Computed Tomography, TXRF, δ13C/δ15N isotope profiling, microbial PLFAs, 14C turnover, phosphate sorption isotherms) we examined the factors regulating the biogeochemical cycling of nitrogen (N), phosphorus (P) and carbon (C) in both vegetated and unvegetated areas. Our results showed that D. spicata rhizomes with large aerenchyma were able to break through the highly cemented topsoil layer leading to root proliferation in the underlying soil. The presence of roots increased soil water content, P availability and induced a change in microbial community structure and promoted microbial growth and activity. In contrast, soil in the phyllosphere exhibited almost no biological activity. Organic C stocks and recent C4 plant derived input increased as follows: phyllosphere (1941 g C m−2; 85% recent) > soils under plants (575–748 g C m−2; 55–60%) > bare soils (491–642 g C m−2; 9–17%). Due to the high levels of nitrate in soil (>2 t ha−1) and high rates of P sorption/precipitation, our data suggest that the microbial activity is both C and P, but not N limited. Root-mediated salt uptake combined with foliar excretion and dispersal of NaCl into the surrounding area indicated that D. spicata was responsible for actively removing ca. 55% of the salt from the rhizosphere. We also demonstrate that NH3 emissions may represent a major N loss pathway from these soil ecosystems during the processing of organic N. We attribute this to NH3 volatilization to the high pH of the soil and slow rates of nitrification. In conclusion, we demonstrate that the extremophile D. spicata physically, chemically and biologically reengineers the soil to create a highly bioactive hotspot within the climate-extreme of the Atacama Desert.

Lanthanum molybdate/magnetite for selective phosphate removal from wastewater: characterization, performance, and sorption mechanisms.

Lanthanum molybdate/magnetite (M-La2(MoO4)3) with various LaCl3/Fe3O4 mass ratios was synthesized and optimized for selective phosphate removal from wastewater. M-La2(MoO4)3 (2:1) was selected on the basis of phosphate sorption capacity for further experiments and characterized by a variety of methods. The phosphate sorption kinetics, isotherms, and matrix effect were studied. The maximum sorption capacity at initial pH 7 indicates the possible applicability M-La2(MoO4)3 (2:1) in removing phosphate from the aquatic environment. Phosphate removal by M-La2(MoO4)3 (2:1) with high selectivity was achieved in the presence of other co-existing anions, while calcium and magnesium ions were found to inhibit the sorption process. The sorption isotherm study showed that Freundlich and Sips models fit better the Langmuir model, indicating that heterogeneous multilayer sorption was dominant during the phosphate sorption process. Sorption kinetic results showed that the pseudo-first-order kinetic model can describe well the phosphate sorption process by M-La2(MoO4)3 (2:1). Consecutive sorption-desorption runs showed that M-La2(MoO4)3 (2:1) could be reused for a few cycles. Simultaneous removal of phosphate and organic matter was achieved in real wastewater by using M-La2(MoO4)3 (2:1). The sorption mechanism was inner-sphere complexation.

Synthesis process and hydrodynamic behavior of a new filtration material for passive wastewater dephosphatation

Abstract The preparation optimization of a filter material intended to be used as a phosphate sorbent in flow-through conditions is investigated. The mixing of ferrihydrite (Fh) and pozzolana (Pz) using a “dry contact method” is found to be the most efficient and leads to the formation of a micrometric thick Fh coating deposited into the honeycombed structure of Pz. The maximal Fh content of ~ 8.5 wt.% is significantly higher than the quantity deposited on other classical substrates such as sand. The phosphate sorption kinetics and isotherms, under dynamic conditions in batch experiments, are best described by pseudo-second-order and Freundlich models respectively. Moreover, under static conditions, sorption kinetics reveals intra-aggregate diffusion process. Phosphate ion retention in packed columns, and especially the breakthrough point, can be adequately predicted when coupling the classical convection dispersion equation and the surface complexation model. Breakthrough curves of phosphate ions are strongly dependent on flow rate; which is important for future industrial applications. Finally, and unexpectedly, a high value of phosphate adsorption capacity (80 mg PO 4 g − 1 ) is observed when phosphate-containing wastewater is used as feed solution. This is mostly explained by soluble calcium ions, which favors the phosphate fixation onto the filter material surface.

Phosphate Requirement of Acidic Soils in Northeast India: A Reappraisal Based on Phosphate Sorption Isotherms

Batch adsorption studies were conducted to assess the P sorption characteristics and standard P requirement (SPR) of two representative acidic soils of northeast India: (1) Alfisol with pH 4.5 (S4.5), and (2) Inceptisol with pH 5.0 (S5.0). It is found that the actual P requirement of the acidic soils in northeast India may be higher than its present recommendation; and also, there exists considerable variation in P requirement of soils within the same region, which underlines the need of site-specific nutrient recommendation rather than a single blanket recommendation for the whole region.

Carbonated ferric green rust as a new material for efficient phosphate removal

Phosphate uptake from aqueous solutions by a recently discovered ferric oxyhydroxide is investigated. Carbonated ferric green rust {GR(CO32-)∗} is prepared by varying two synthesis parameters, which are (1) the aging period after the ferrous–ferric green rust {GR(CO32-)} synthesis step and (2) the rate of the hydrogen peroxide addition to oxidize GR(CO32-) into GR(CO32-)∗. These two parameters permit the control of the size, morphology and cristallinity of the synthesized particles. As prepared GR* samples are then evaluated, in batch experiments, as possible low-cost efficient phosphate removal materials. Firstly, kinetic experiments reveal that a fast sorption step initially occurs and equilibrium is reached at ∼500min. The adsorption kinetics data at pH=7 can be adequately fitted to a pseudo-second order model. Secondly, the Freundlich model provides the best correlation and effectively describes phosphate sorption isotherms for all GR(CO32-)∗ samples synthesized. Finally, the phosphate adsorption capacity decreases when pH increases. The highest adsorption capacity is 64.8mgg−1 at pH=4 and corresponds to the GR(CO32-)∗ sample displaying the smallest and least crystallized particles thus reflecting the importance of the synthesis conditions. Overall, all sorption capacities are higher than the main iron oxide minerals, making GR(CO32-)∗ a potentially attractive phosphate adsorbent.

Iron(III) Coordination and Phosphate Sorption in Peat Reacted with Ferric or Ferrous Iron

The molecular bonding configuration of Fe(III) associated with organic matter (OM) in soils potentially affects its binding affinity, reducibility, mobility, and interactions with other cations and anions. Given that Fe(III) can be incorporated into soil OM either directly as Fe(III) or through oxidation of Fe(II), the coordination and reactivity of OM‐associated Fe(III) may depend on soil redox conditions. Our objective was to determine if the molecular coordination environment and phosphate sorption characteristics of OM‐associated Fe(III) depend on the pathway of Fe(III) incorporation. Pahokee peat soil material was reacted at pH 2.5 with either Fe(III) or Fe(II) at 300, 900, or 1200 mmol Fe kg−1 peat, equilibrated at pH 6.8 and oxidized. Extended x‐ray absorption fine structure and wavelet transform analyses indicated the presence of mononuclear Fe(III) coordinated with organic ligands in all samples along with polynuclear Fe‐hydroxide species at higher concentrations. The pathway of Fe incorporation did not significantly affect its local coordination environment or phosphate sorption isotherms. Our results suggest that OM‐associated Fe(III) formed under oxidized or reduced soil conditions would have similar molecular‐level coordination and be equally accessible for phosphate sorption.

Phosphate sorption capacity of European volcanic soils

The aim of the presented study was to prepare the phosphate sorption isotherms of 20 European volcanic soil profiles and some other Hungarian and German volcanic soils (n = 114) used in the experiment and to establish the soil characteristics determining the phosphate sorption capacity of these soils. The Langmuir isotherm well describes the phosphate sorption of European volcanic soils at bright concentration interval 0–600 mg·dm -3 P. The calculated phosphate adsorption maximum (P max ) is an excellent soil property for characterizing the surface activity of soils developed on volcanic parent material. The calculated phosphate sorption maxima of soils included in the experiment ranged from 0 to 10.000 mg P·kg -1 . Some of the volcanic soils sorbed a high ratio of the added phosphate at low concentrations, while others sorbed somewhat less. The difference in the phosphate binding affinity of soils caused the differences in the shape of the Langmuir adsorption isotherms. P retention % is a WRB diagnostic requirement of andic soil horizon. It was supposed that the phosphate sorption maximum (P max ) gives a better characterization of the surface reactivity of volcanic soils. As it was predicted, oxalate soluble Al is the most important soil property, which dominantly (in 73%) explained the phosphate sorption ability of European volcanic soils.

Assessment of phosphorus leaching losses from arable land

Phosphorus (P) losses from soil to water by erosion and surface runoff have been much studied and quantified. However, P losses by leaching have received much less attention, mainly because, until recently, the quantities involved were not considered to be of environmental significance. Furthermore, P leaching losses, unlike P losses from erosion or surface runoff were not believed to be related to rates of P addition, as inorganic fertilizer or manures. Here we report results from a number of field and laboratory experiments, designed to assess the significance of P leaching losses from soil to water. Annual cumulative total P losses in drainage waters from four UK field sites ranged from about 0.03 to 5 kg P ha−1 during 2001–2002. Molybdate reactive P ranged from 45–57%, soluble organic P from 10–13% and particulate P from 29–45% of total P on the two sites (Broadbalk and Woburn) where they could be accurately measured. The proportions of these different P forms were comparable in all treatments, including drainage waters from the unfertilised soils and soils receiving long-term applications of farmyard manure or inorganic fertilizer. In all soils, there was indication of an Olsen- (0.5 M NaHC03-, pH 8.5) extractable P concentration, (termed the Change-Point), where P measured in field drainage waters or in laboratory soil extracts of 0.01 M CaCl2 began to increase linearly as Olsen-P increased. There was also some agreement between drainage water-P or CaCl2-P and the Olsen-P concentration where the Change-Point occurred. This suggests that CaCl2-extractable P may provide an approximate indicator of soil P concentrations above which significant quantities of P may be lost by leaching under field conditions. There were positive linear relationships between soil dithionate-extractable Al and soil organic C with the Change-Point: [Change Point = [(0.049)[Al3+] minus (9.2)(% organic C)] accounting for 93% of the variance in the data. If this relationship holds under further testing it could well be a useful predictor of Change-Points in different soils. Phosphate sorption isotherms were used to study the soil P concentrations above which P was at risk of moving from soil to water. They showed that soil solution P concentrations were significantly lower between pH 6.9–7.2 than between pH 7.7–8.1, with implications for P loss from soil to water.

Phosphorus removal from aqueous solutions by sorption on two volcanic soils

The use of low-cost materials for P removal is of interest for developing cost-effective techniques for preventing P pollution. This paper reports a study on phosphate removal from aqueous solutions by sorption on two volcanic soils. The raw and HCl-treated soils were characterized with respect to oxalate-extractable and dithionite-extractable Al and Fe contents, surface area, and P sorption capacities. The phosphate sorption isotherms, kinetics, pH effects, and desorbability were evaluated in batch tests. The measured isotherm data were well fitted by the Freundlich and Temkin models. Phosphate sorption on these soils was relatively fast and the kinetics could be satisfactorily described by the simple Elovich and power function equations. The two soils had maximum phosphate sorption capacities of approximately 0.85 and 1.35 mg g-1 gram of soil at pH 6.0–6.5. The pH had different effects on phosphate sorption on these soils, likely due to either calcium phosphate precipitation or surface repulsion of the negatively charged phosphate species at a higher pH. Column flow-through tests using both synthetic phosphate solution and liquid swine manure confirmed the phosphate removal ability of the volcanic soils. It was concluded that volcanic soils could be potential low-cost materials for controlling P pollution from agricultural sources. Key words: Phosphate removal, volcanic soil, sorption, isotherm, kinetics, desorption

Mechanisms of Nutrient Attenuation in a Subsurface Flow Riparian Wetland

Riparian wetlands are transition zones between terrestrial and aquatic environments that have the potential to serve as nutrient filters for surface and ground water due to their topographic location. We investigated a riparian wetland that had been receiving intermittent inputs of NO3- and PO4(3-) during storm runoff events to determine the mechanisms of nutrient attenuation in the wetland soils. Few studies have shown whether infrequent pulses of NO3- are sufficient to maintain substantial denitrifying communities. Denitrification rates were highest at the upstream side of the wetland where nutrient-rich runoff first enters the wetland (17-58 microg N2O-N kg soil(-1) h(-1)) and decreased further into the wetland. Carbon limitation for denitrification was minor in the wetland soils. Samples not amended with dextrose had 75% of the denitrification rate of samples with excess dextrose C. Phosphate sorption isotherms suggested that the wetland soils had a high capacity for P retention. The calculated soil PO4(3-) concentration that would yield an equilibrium aqueous P04(3-) concentration of 0.05 mg P L(-1) was found to be 100 times greater than the soil PO4(3-) concentration at the time of sampling. This indicated that the wetland could retain a large additional mass of PO4(3-) without increasing the dissolved P04(3-) concentrations above USEPA recommended levels for lentic waters. These results demonstrated that denitrification can be substantial in systems receiving pulsed NO3- inputs and that sorption could account for extensive PO4(3-) attenuation observed at this site.

Phosphate sorption characteristics of major soils in Okinawa, Japan

Phosphate sorption isotherms were determined for 16 representative major soils developed from different parent materials on Okinawa. Phosphate sorption characteristics were satisfactorily described by the Langmuir equation, which was used to determine phosphorus (P) sorption maxima of the soils. Phosphate sorption maxima ranged from 630 to 2208 mg P kg‐1 soil (mean 1,362 mg P kg‐1). The standard P requirement (i.e., the amount of P required to attain 0.2 mg P L‐1 equilibrium solution) followed the same trend as sorption maximum (r =0.94***), with values ranging from 132 to 1,020 mg P kg‐1 soil (mean 615 mg P kg‐1). This mean value corresponds to fertilizer addition of 923 kg P ha‐1 indicating that the soils have high P fertilizer requirements. Results of simple linear regression analysis indicated that sorption maximum was significantly correlated with clay content, organic matter, oxalate iron (Fe), pyrophosphate Fe, DCB aluminum (Al), oxalate Al, and pyrophosphate Al, but not with DCB Fe, pH, or available P content. The best regression model for predicting sorption maximum was the combination of clay, organic matter, pyrophosphate Fe, and DCB Al which altogether explained 79% of the variance in sorption maximum. The equation obtained could offer a rapid estimation of P sorption in Okinawan soils.

Phosphate sorption to matrix and fracture wall materials in a Glossaqualf

Phosphate retaining properties of macropore wall materials may influence the extent of phosphate leaching via preferential flow. Phosphate sorption to soil matrix material has been compared with sorption to fracture wall materials consisting of inner iron-oxide depleted albic coatings rimmed by reddish iron-oxide enriched quasicoatings. The latter contains 15 times as much Fe oxide, 2 times as much Al (hydr)oxide and 5–6 times as much total P than the albic material, which represent the most P-depleted material in the profile. Oxalate extractable P (P o) amounts to 1 4 − 1 2 of the total P contents of the samples. Smectite predominates in the fracture walls and acid subsoils, but has partly transformed into hydroxy-interlayered smectite (HIS) in the upper limed soil horizons. Langmuir parameters derived from phosphate sorption isotherms show no correlation for sorption to whole soil samples and the corresponding clay fractions. The phosphate sorption capacity at a threshold equilibrium concentration of 10 μM (P ads t (10 μM)) is measured as the content of P o plus the amount of phosphate sorbed during 7 days, and can be related to Al o+Fe o: P ads t (10 μM)=(0.0404±0.0046)·(Al o+Fe o)***+3.569. The P ads t (10 μM) values vary between 4 and 14 mmol P kg −1, lowest in the albic fracture walls and highest in the iron enriched quasi-coatings. The sorption capacity of the metal oxide free clay does not appear to correlate with smectite or HIS contents. In total, the clay fraction contributes with about 50% of the whole soil sorption capacity in agreement with 40–60% of the total soil Al o+Fe o being present in the clay fraction. At a solution phosphate concentration of 10 μM, the Ap horizon is found to be almost phosphate saturated whereas subsoil horizons including the fractures have phosphate saturations ≤50% and, hence, can strongly sorb P. The low phosphate saturation of fracture walls indicates that they do not act as major sinks of phosphate-rich solutions from topsoil horizons, either due to kinetic constraints, or due to a flow pattern not allowing P-rich solution to percolate fractures.

Phosphorus availability and sorption under alternating waterlogged and drying conditions

The effect of alternating waterlogged and drying conditions on phosphorus (P) availability and sorption was studied in three soils of contrasting chemical and physical properties. Soils were treated with two levels of P (0 and 50 mg kg‐1; P0 and P50), waterlogged for 21 days, then allowed to dry at room temperature for 14 days. The availability of P, iron (Fe), and manganese (Mn) over the waterlogged and drying periods was determined by shaking samples of each soil with 1M NaOAc (pH 3). Increasing concentrations of 1M NaOAc (pH 3) extractable P (Pac) over the waterlogged period was attributed to solubilization of Fe(OH)3 materials under reducing conditions with the release of sorbed and occluded P. The released P appeared to be resorbed by freshly precipitated amorphous Fe(OH)2 material since earlier studies showed that water soluble P concentrations in these soils were reduced to negligible levels under waterlogged conditions. The Fe(OH)2 material remained readily extractable with 1M NaOAc (pH 3) since Feac increased dramatically with waterlogging. Drying the waterlogged soils caused a rapid decrease in Pac, Feac and Mnac suggesting the Fe(OH)2 may have been transformed into more stable forms [e.g., Fe(OH)3]. Much of the changes in Pac appeared to be due to changes in Feac, with limited influence from Mnac. and mineralization of organic P. Phosphate sorption isotherms were determined using the standard batch technique for air‐dry, waterlogged (with and without ponded water), and waterlogged/dried conditions. Sorption isotherms were not affected by waterlogging and subsequent drying. Most soils sorbed all of the added phosphate irrespective of moisture treatment.

Phosphate sorption isotherms in relation to P nutrition of wheat (Triticum aestivum)

The phosphate sorption isotherms are needed to explain differential plant responses to P fertilization in soils. Laboratory and greenhouse experiments investigated the use of phosphorus sorption isotherms in relation to P fertilizer requirement of wheat in ten benchmark soils of Punjab, India. The modified Mitscherlich Equation (3) was used to describe plant response observed in different soils. Maximum obtainable yield (MOY) ranged from 11.6 g pot−1 in Gurdaspur (I) sandy clay loam to 7.0 g pot−1 in Nabha sandy clay loam. Response to P applied @ 25 mg P kg−1 soil was maximum (77%) in Bathinda sand and minimum in Chuharpur clay loam (33%). The response curvature varied from 3.74 × 10−2 in Nabha sandy clay loam to 4.43 × 10−2 in Kanjli sandy loam. The soil solution P required to produce optimum yield (90% MOY) varied from 1.61 µg ml−1 in Bathinda sand to 0.10 µg ml−1 in Sadhugarh clay. Dry matter yield obtained at 0.2 µg ml−1 solution P concentration ranged from 55% in Bathinda sand to 85% of MOY in Gurdaspur (II) clay loam. At the same solution P concentration (0.1 µg P ml−1), dry matter yield was 91% in Sadhugarh clay, 80% in Gurdaspur (II) clay loam and, 43% of MOY in Bathinda sand and eventually coincided with the decreasing maximum buffer capacity (MBC) in these soils. At the same level of sorbed P (100 mg P kg−1 soil) the yield was observed to be inversely proportional to MBC. The study, therefore, concludes that, soils should be grouped according to their P sorption characteristics and MBC before using critical soil solution P as a criterion for obtaining optimum yields.

Phosphorus sorption isotherm characteristics and availability parameters of Appalachian acidic soils

Abstract Phosphate (P) sorption isotherms have been widely used to characterize the P status and to establish fertilizer requirements of soils. Recently, mechanistic models have successfully described the nutrient uptake by plants under changing soil and plant parameters. Phosphorus sorption characteristics of eleven representative soils of the Appalachian Region of the United States were evaluated, and experimental P adsorption data were fitted to Temkin, Freundlich, and Langmuir equations to determine the relative importance of the soil parameters in P retention and supply to plant roots. The Barber and Cushman model was used to predict the effect of P fertilization on P uptake by plant. The soils of Appalachia differ considerably with respect to the retention of added P. All three isotherm equations gave good fit with the experimental data and were reliable to describe the P quantity/ intensity relationships of these soils. The following sequence of P adsorption capacity in various soils was establishe...

Influence of soil properties on the response to phosphorus in some tropical soils: I. Initial response to fertilizer

SummaryThe availability of fertilizer P in six P‐deficient tropical soils from Brazil, Kenya, Malaysia and Indonesia was assessed by grass in a pot experiment. Grass dry matter yield (D) and fertilizer P(F) were fitted to a Mitscherlich equation: D= a−b exp(−cF), and P uptake (U) and F to the linear equation: U=α+βF. Fitted parameter β equals the proportion of P recovered in one crop and it varied widely between soils, ranging from 12 to 51%. Quantitative assessments of fertilizer‐P availability could also be made using dry‐matter data alone if the rates of fertilizer used were well distributed along the response curve, when Mitscherlich parameter c was correlated well with β.Chemical measurements were made on uncropped soil. Phosphate sorption isotherms were measured, using 32P to assess exchangeable and non‐exchangeable phosphate. The availability parameters c and β were correlated best with parameter be, the phosphate buffer capacity derived from the fitted Freundlich isotherm for exchangeable phosphate, suggesting that the mobility of exchangeable phosphate is a major influence on P availability. Al and Fe were extracted with acid oxalate, citrate‐dithionite and pyrophosphate reagents, and parameters c and β correlated best with Al extracted by acid oxalate. These relationships were inverse, showing that Al in disordered mineral forms lowers the availability of fertilizer P.

Phosphate Sorption Isotherms Revisited

AbstractA criterion is proposed for assessing ranges of validity of equilibrium isotherms for phosphate sorption by soils. This criterion is based on the geometrical features commonly exhibited by such isotherms and is analogous to the one that was first formulated by Aharoni and Ungarish in connection with the kinetics of chemisorption reactions. Its use with equilibrium isotherms is illustrated with three widely used expressions for the representation of phosphate sorption by soils but it would also be expected to be useful in connection with sorption of other solutes by soils as well as by other adsorbents.

Effects of decalcification on the phosphate sorption characteristics of eight calcareous soils

Phosphate sorption isotherms and parameter values were determined on eight calcareous soils which were carefully decalcified using a procedure which minimized changes in cation saturation. Calcite content of the original soils varied from 0.8 to 24 2% and calcite surface areas from 4 . 0 to 8.5 m2 g-1. Sorption parameters were derived from the Langmuir 'two-surface' equation. Decalcification increased phosphate sorption at low residual P concentrations (<0.8 mg L-1) but decreased it at higher concentrations. The higher P sorption was associated with an increase in affinity because the calculated sorption capacities of high-affinity surfaces were not increased. These sorption capacities were well correlated with iron oxide contents of the soils, so the increase in phosphate affinity of these surfaces was consistent with the decrease in pH (0.5 to 1.5 units) of the decalcified soils. The lower P sorption at higher concentrations was associated with a substantial decrease in sorption capacity of the postulated low-affinity surfaces. These latter decreases were quantitatively correlated with the calcite surface areas of the original soils. These and other changes in phosphate sorption characteristics support the utility of the Langmuir 'two-surface' equation in providing information, compatible with what would be expected from more complex mechanistic models, and which exceeds what one would expect from other simpler models such as the Freundlich equation. They also support an hypothesis that an important component of low-affinity surfaces of these calcareous soils is calcite on which organic anions are co-adsorbed.

Phosphate sorption isotherms for evaluating phosphorus requirement of wetland rice soils

Phosphate sorption isotherms were developed for five Philippine wetland rice soils using the conventional technique and a modified one. In the conventional method, P requirements of soils varied between 280 and 810 μg P/g soil. In the modified method, they varied from 160 to 540 μg P/g soil at 0.2 ppm P in solution. Soils with high P-sorption capacities had vermiculite and halloysite as the dominant clay minerals. Soil reduction by flooding decreased P-sorption by 28–70 percent at 0.2 ppm P in solution. The decrease in P-sorption due to soil reduction was greatest in a crystalline soil with vermiculite and halloysite as the dominant clay minerals and least in a soil with dominant X-ray amorphous silicates in the clay fraction. Desorption of freshly adsorbed P under reduction was greater in HCO 3 − solution than in CaCl2 and it increased with level of applied P. Desorption patterns of freshly adsorbed P were similar to adsorption patterns but values of P in solution were lower at desorption. Soils varied with respect to desorption of freshly sorbed P. Desorption studies indicate that soils vary in intensity factor with respect to P and thus influence P availability to plants. Use of P-sorption and P-desorption data obtained under reduced soil condition was proposed for detecting P needs of submerged rice soils. Results of a pot study with IR36 at different levels of solution P (reduced) in one soil indicated a high degree of correlation between adjusted P levels and the measured growth parameters. About 0.12 ppm P in the soil solution or 0.46 ppm P desorbed in HCO 3 − solution (equivalent to 100 mg P/kg soil) was adequate for near-maximum plant height, tiller production, total dry matter yield, plant P content, and total P uptake.

Relationships between phosphate sorption and iron oxides in Alfisols from a river terrace sequence of Mediterranean Spain

Phosphate sorption isotherms for samples of the A- and upper B-horizons of Alfisols situated on a sequence of terraces of the River Guadalquivir, southern Spain, showed that phosphate sorbed at an equilibrium concentration of 0.3 μg P ml −1 was correlated with several soil properties. Crystalline Fe-oxides (goethite and hematite) appeared to be the most important P-sorbing components of the soil samples. Sorption was highly correlated with percent clay and with dithionite soluble Fe. This last property was as good a property to predict sorption as the specific surface of crystalline Fe-oxides (which was estimated by line profile analysis of the X-ray diffractograms), probably because the range of surface areas for goethite and hematite was relatively narrow in the samples and because of the limited accuracy of the method of line profile analysis. Well drained and imperfectly drained soils differed little in sorption properties. In imperfectly drained, hematite-free soils, chroma was correlated with sorption. This is promising for it would allow rapid field estimation of sorption. The sorption maximum, calculated from the fitted Langmuir isotherm, was about 1.7 μmol P m −2 of Fe-oxides, a value similar to those reported in the literature for natural and synthetic oxides.