TIME-DEPENDENT SORPTION OF PHOSPHATE BY SOILS AND HYDROUS FERRIC OXIDES

The sorption of inorganic phosphate (P) by soils and hydrous ferric oxides was studied at times up to 192h. An initially rapid decrease in solution P concentration was followed by a much slower decrease between 48 and 192h with soils, Fe gel. and natural goethite, whereas synthetic goethite gave a well-defined equilibrium condition after only 48h. Resolution of the sorption isotherms showed that the increase in P sorption with time involved an appreciable shift of P from a more-physically sorbed form to a chemisorbed form. This was supported by chemical fractionation which showed that NaOH-extractable P was fairly constant with increasing sorption time, whereas the additional sorbed P was extracted by citrate-dithionite-bicarbonate from soils, and by HC1 from Fe gel and natural goethite. These sorbents contained short-range (amorphous) material, whereas synthetic goethite, from which all sorbed P was NaOH –extractable. did not. It is proposed that the time-dependent sorption of P and the associated shift of P to chemisorbed forms, involves the diffusion of P into “structurally porous”, short-range order material.     

Assessing the fixation and availability of sorbed phosphate in soil using an isotopic exchange method

An isotopic exchange method was used to characterize quantitatively the fixation and plant availability of phosphate previously sorbed by soils. In general, the exchangeability of the sorbed phosphate was much higher than its desorbability for both soils and clay minerals. Isotopic exchangeability of the sorbed phosphate increased with sorption saturation during the initial stage (15–60% saturation), but the increase was less with increasing saturation from 60–90% for all soils tested. Therefore a sorption saturation of 60% was recommended as the upper limit of P fertilization in terms of economical efficiency. For clay minerals, with increasing sorption saturation, the isotopic exchangeability of the sorbed P increased significantly for kaolinite and sesquioxides, but decreased for montmorillonite. Most of the phosphate sorbed by montmorillonite and kaolinite was found to be isotopically exchangeable, but only a small amount of the P sorbed by goethite could be exchanged. The P sorbed by Al oxide exhibited isotopic exchangeability between that of kaolinite and Fe oxide. The isotopically exchangeable phosphate pool could readily account for the P uptake of plants and the available P determined by some commonly used chemical methods, such as Olsen-P and Bray-P.     

石灰性土壤与磷酸盐的反应及吸持态磷的同位素交换性

本文研究了种石灰性土壤（Ｌｏｕ土）与磷酸盐的反应动态过程，短期反应的等温吸附研究表明，在低磷浓度下，以吸附反应机制为主，吸持态磷的同位素交换性随着吸持量的增加而增加；在高磷浓度下，以形成磷酸盐的沉淀反应机制为主，吸持态磷的同位素交换性随着反应时间的延长和吸持量磷数量的增加而了降低。认为在低施磷水平下，土壤中铁，铝氧化物对磷的吸持起重要作用。本文还探讨了在长期（２６０天）恒温恒湿培养过程中，土壤可溶     

Adsorption and Desorption of Copper in Pasture Soils

Abstract

Copper (Cu) is bound strongly to organic matter, oxides of iron (Fe) and manganese (Mn), and clay minerals in soils. To investigate the relative contribution of different soil components in the sorption of Cu, sorption was measured after the removal of various other soil components; organic matter and aluminum (Al) and Fe oxides are important in Cu adsorption. Both adsorption and desorption of Cu at various pH values were also measured by using diverse pasture soils. The differences in the sorption of Cu between the soils are attributed to the differences in the chemical characteristics of the soils. Copper sorption, as measured by the Freundlich equation sorption constants [potassium (K) and nitrogen (N)], was strongly correlated with soil properties, such as silt content, organic carbon, and soil pH. The relative importance of organic matter and oxides on Cu adsorption decreased and increased, respectively, with increasing solution Cu concentrations. In all soils, Cu sorption increased with increasing pH, but the solution Cu concentration decreased with increasing soil pH. The cumulative amounts of native and added soil Cu desorbed from two contrasting soils (Manawatu and Ngamoka) during desorption periods showed that the differences in the desorbability of Cu were a result of differences in the physico‐chemical properties of the soil matrix. This finding suggests that soil organic matter complexes of Cu added through fertilizer, resulted in decreased desorption. The proportions of added Cu desorbed during 10 desorption periods were low, ranging from 2.5% in the 24‐h to 6% in the 2‐h desorption periods. The desorption of Cu decreased with increasing soil pH. The irreversible retention of Cu might be the result of complex formation with Cu at high pH.     

On the reversibility of phosphate sorption by soils

Sorption of phosphate was induced by incubating phosphate with samples of two soils. Both desorption and further sorption of phosphate were then measured on separate subsamples of the incubated soils. The effects of varying the amount of phosphate incubated with the soil and of period of desorption, or of further sorption, were measured on one soil; the effect of period of incubation was measured on the other. Plots of desorbed phosphate versus concentration were continuous with plots of newly sorbed phosphate versus concentration. Neither of these coincided with the plots of the original additions of phosphate. These results were compatible with a model for the reaction between soil and phosphate in which phosphate is initially adsorbed and subsequently diffuses beneath the adsorbing surfaces. Sorption is reversible in the sense that a continuous curve of sorbed and desorbed phosphate is obtained when these are measured in opposite directions by increasing, or decreasing, the solution concentration of phosphate. However, because dynamic processes are involved, an earlier position of a plot of sorbed phosphate against concentration is not retraced when the concentration is changed.     

Effect of pH on Zinc Sorption–Desorption by Soils

The objective of this study was to examine the effect of soil pH on zinc (Zn) sorption and desorption for four surface soils from the Canterbury Plains region of New Zealand. Zinc sorption by the soils, adjusted to different pH values, was measured from various initial solution Zn concentrations in the presence of 0.01 M calcium nitrate [Ca(NO₃)₂]. Zinc desorption isotherms were derived from the cumulative Zn desorbed (µg g^?1 soil) after each of 10 desorption periods by sequentially suspending the same soil samples in fresh Zn‐free 0.01 M Ca(NO₃)₂. Zinc sorption and desorption varied widely with soil pH. Desorption of both native and added Zn decreased continuously with rising pH and became very low at pH values greater than 6.5. The proportion of sorbed Zn that could be desorbed back into solution decreased substantially as pH increased to more than 5.5. However, there were differences between soils regarding the extent of the hysteresis effect.     

Development and evaluation of a kinetic model to describe phosphate sorption by hydrous ferric oxide gel

Isotherms for the sorption of inorganic phosphate (P) by hydrous ferric oxide gel (Fe gel) were described by a three-equation Langmuir sorption model. Each equation described sorption within a distinct concentration range or region (I, II, and III) of the overall isotherm. Regions I and II involved chemisorption, whereas region III involved a more physical sorption type. With increasing sorption time between 0.7 and 28.7 days, the extent of sorption in region I increased by more than 30%. In contrast, the extent of sorption in regions II and III remained essentially constant. An equation was developed, based on the change in the sorption maximum of region I (b_I) with increasing sorption time, which described the change in solution P concentration with time. The increase in b_I with time, evaluated by the closeness of fit of this relationship to experimental data, was found to depend on two factors: first, the extent to which P was chemisorbed, and this was affected by pH and ionic strength; second, the batch of Fe gel used. For two different levels of P addition, the proportion of sorbed P which remained extractable in 0.1M NaOH, decreased with increasing sorption time. After 30 days only 88% of the sorbed P remained NaOH-extractable. The data obtained indicated that the increasing chemisorption of P with increasing sorption time involves the diffusion of sorbed P into the bulk of the Fe gel particles. This concept is discussed in relation to mechanisms proposed by previous workers to explain the time-dependence of P sorption.     

RATIONALIZATION OF IONIC STRENGTH AND CATION EFFECTS ON PHOSPHATE SORPTION BY SOILS

The amounts of inorganic phosphate (P) sorbed by four contrasting unfertilized soils during 40 h were influenced by the ionic strength and cation species of the contacting solution (support medium) used, as indicated by isotherms over the final P concentration range of 0 to 1 μg P/ml and 0 to 10 μg P/ml. An increase in ionic strength enhanced P sorption during 40 h but the species of cation also influenced the amount of P sorbed, as shown by the isotherms obtained in 10^?2M Ca and 3 × 10^?2M Na systems. Although pH affected the amounts of P sorbed, pH effects alone could not adequately explain the differences in P sorption. Kinetic studies indicated that within the range of P addition used for each soil, the equilibrium P concentration, at infinite time, was independent of ionic strength and cation species. Consequently, the composition of the solution affected only the rate at which equilibrium was attained. The results are attributed to the effects of ionic strength on the surface charge of retaining components and the thickness of the diffuse double layer, and the effects of specilic sorption of a divalent cation on surface charge, as they relate to the rate of P sorption.     

Cadmium Sorption and Desorption Characteristics of Tropical Alfisols from Different Land Uses

Cadmium (Cd) sorption and desorption characteristics by Alfisols from different land uses were examined, and the relationships between soil and sorption/desorption characteristics were investigated. Adsorption studies were done using Cd concentrations (0–100 mg Cd kg^?1) in 0.01 M CaCl₂. The Cd sorbed by the soils was then subjected to two desorption runs. The soils' adsorption conformed to Freundlich and Langmuir equations. The amount of Cd sorbed by the soils varied. Two desorption runs detached more than 95% of sorbed Cd, but the first accounted for more than 80% of the total. Desorption of Cd in degraded soils was more than in soils from other land uses. The amount of Cd desorbed correlated with amount applied (r = 0.90??), solution concentration (r = 0.83??), and amount sorbed (r = 0.70??). A positive relationship exists between the adsorption maxima of the soils and soil organic matter (r = 0.13, p = 0.87). The relationship between amount of Cd desorbed and sorbed is quadratic for all the soil.     

Effects of Oxalate and Humic Acid on Arsenate Sorption by and Desorption from a Chinese Red Soil

Studies on arsenate (As(V)) sorption and desorption have been mainly limited to soil minerals and sorption and desorption reactions in whole soils are poorly understood. In this study the sorption of As(V) by and phosphate-induced desorption from a Chinese red soil were studied in the presence of oxalate and humic acid (HA). Arsenate was strongly sorbed mainly through ligand exchange reactions on the soil. Arsenate sorption decreased in the presence of oxalate or HA. Oxalate and HA influenced As(V) sorption mainly by competing for sorption sites and reducing sorption sites, and oxalate could also decrease sorption through dissolving clay minerals. Oxalate and HA could also facilitate As(V) desorption from the soil. Both sorption and desorption kinetics were two stage processes. Sorption kinetics conducted from 0.2–840 h showed that As(V) sorption increased with increasing residence time. Sorption equilibrium was retarded and the maximum sorption decreased in the presence of oxalate or HA. Phosphate-induced desorption kinetics conducted on the soil with 24 h and 840 h of sorption equilibrium time showed a significant effect of equilibrium time on As(V) desorption. The presence of oxalate or HA during the sorption process resulted in more As(V) desorption. Due to the degradation of oxalate, soil treated with oxalate and with a sorption equilibrium time of 840 h showed no significant difference in desorption kinetics from untreated soil.     

Zinc sorption–desorption by two Andepts: effect of pH and support medium

Laboratory experiments were carried out to evaluate the effect of pH, ionic strength and electrolyte composition on zinc sorption–desorption by two Andepts from the Canary Islands (Spain). At the natural soil pH, the soils exhibited little net negative surface charge and small Zn sorption capacities. More than 75% of the sorbed Zn was apparently strongly bonded. The pH greatly influenced the sorption–desorption reactions. Sorption increased with increasing pH, and retention increased abruptly at pH > 6.0. Sorption also occurred at pH values below the point of zero charge (PZC) of the soils, when most of the surface sites are positively charged. Desorption decreased continuously with rising pH and became a trace at pH > 6.0. An increase in the ionic strength of the background electrolyte decreased Zn sorption and enhanced the amount of sorbed metal that could be subsequently released. In the two soils, Zn sorption diminished somewhat in the K and Ca electrolytes as compared with the Na electrolyte. However, this did not happen at small Zn loadings. Desorption was not affected by the type of electrolyte and cation used. The results are consistent with chemisorption being responsible for most of the sorption. The results also suggested a strong affinity sorption or even precipitation at high pHs.     

Sorption of chlorpyrifos to selected minerals and the effect of humic acid

Sorption of chlorpyrifos (CPF) from 2.85 microM (1 mg/L) aqueous solutions in 0.01 M NaCl to montmorillonite, kaolinite, and gibbsite was investigated at 25 degrees C. Uptake of CPF by kaolinite and gibbsite was generally <10%, with pH having at most a small effect. Sorption to montmorillonite was significantly greater, with approximately 50% of the initial CPF being removed from solution below pH 5. Above pH 5 the sorption decreased to about 30%. About 70% of CPF was sorbed to kaolinite and gibbsite after 30 min, whereas on montmorillonite only 50% sorbed in an initial rapid uptake (approximately 30 min) followed by slower sorption, with a maximum achieved by 24 h. Although CPF desorbed completely from kaolinite in methanol, only about two-thirds was desorbed from montmorillonite. CPF has only a weak affinity for the surfaces of kaolinite and gibbsite. In the case of montmorillonite, sorption is significantly stronger and may involve a combination of sorption to external surfaces and diffusion into microporous regions. At pH >6 increased negative surface charge results in a lower affinity of CPF for the external surface. In the presence of 50 mg/L humic acid (HA) the amount of CPF sorbed on gibbsite and kaolinite was 3-4 times greater than that in the binary systems. The HA forms an organic coating on the mineral surface, providing a more hydrophobic environment, leading to enhanced CPF uptake. The HA coating on montmorillonite may reduce access of CPF to microporous regions, with CPF tending to accumulate within the HA coating.     

Sorption and desorption of selenium in different soils of the mediterranean area

Abstract

The adsorption of selenium (Se) in the selenate form and its desorption by phosphate in four soils with different physiochemical properties were studied in the laboratory. To determine adsorption isotherms for selenate 25 mL of solutions containing 1 to 100 ppm of Se were added to 2.5 g of soil. Desorption isotherms were determined by resuspending the samples in phosphate solution. The selenate sorption process was adequately described by the Freundlich equation. In pine forest and woodland soils, characterized by the highest organic matter content and cation exchange capacity (CEC) values, the isotherms were classified as L type, since the amount of Se sorbed appeared to move towards saturation. The organic matter content played the most important part in the adsorption of Se, while pH appeared to have a small effect on the ability of the soil to adsorb Se. The high CaCO₃ content of the pine forest soil may have contributed in increasing the Se adsorption notwithstanding the high pH value. The cultivated and arable soils showed a reduced sorption capacity. The sorption could be described by an S type curve. At low concentrations of Se the affinity of the solid phase was less than that of the liquid phase. By increasing the concentration of Se in solution, the affinity of the solid phase increased and the sorption was favored. Selenate desorption by water was negligible, whereas the amount of Se desorbed by phosphate varied among the different soils. The desorption experiments indicated that a significant portion of the sorbed Se was irreversibly retained. This suggests the existence of linkages which allow the release of Se in the soil solution only after physico‐chemical variation such as exchange with phosphate ions.     

Sorption and desorption of cobalt by soils and soil components

The sorption of Co by individual soil components was studied at solution Co concentrations that were within the range found in natural soil solutions. Soil-derived oxide materials sorbed by far the greatest amounts of Co although substantial amounts were also sorbed by organic materials (humic and fulvic acids). Clay minerals and non-pedogenic iron and manganese oxides sorbed relatively little Co. It is considered that clay minerals are unlikely to have a significant influence on the sorption of Co by whole soils. Cobalt sorbed by soil oxide material was not readily desorbed back into solution and, in addition, rapidly became non-isotopically exchangeable with solution Co. In contrast, Co was relatively easily desorbed from humic acid and a large proportion of the Co sorbed by humic acid remained isotopically exchangeable. Cobalt sorbed by montmorillonite was more easily desorbed than that sorbed by soil oxide but less easily than that sorbed by humic acid. Cobalt sorption isotherms for whole soils at low site coverage were essentially linear and the gradients of isotherms increased with pH. A comparison of isotherm gradients for whole soils and individual soil components supported the suggestion that Co sorption in whole soils is largely controlled by soil oxide materials.     

ADSORPTION-DESORPTION CHARACTERISTICS OF HIGH LEVELS OF COPPER IN SOIL CLAY FRACTIONS

Copper adsorption and desorption under acid conditions by soil clay fractions separated from Vertisol, Planosol and Gleyic Acrisol has been studied in 0.01 M Ca(NO₃)₂. A Freundlich equation was appropriate to describe Cu adsorption. Within the range of 150 to 2600 mg of copper per kg of soil clay fraction the proportions of Cu not displaced during 5 successive 48-hour desorptions with 0.01 M Ca(NO₃)₂ decreased with increasing adsorption density and at the lower pHs. The proportions ranged from as high as 0.98 in th case of the Vertisol clay (pH 5.3) to as low as 0.12 (88% desorption) in the Planosol clay (pH 4.5). Measurement of separation factors (ga Cu/Ca) showed that the preference of the clay surface for Cu over Ca decreased in the order: Gleyic Acrisol > Planosol > Vertisol. A considerable amount of sorbed copper could be solubilized by decreasing pH values to 4 when in the Planosol clay 39% was desorbed and 45% was desorbed in the Gleyic Acrisol clay.     

Phosphate desorption from the surface of soil mineral particles by a phosphate-solubilizing fungus

High phosphate (Pi) sorption in soils is a serious limiting factor for plant productivity and Pi fertilization efficiency, particularly in highly weathered and volcanic ash soils. In these soils, the sorbed Pi is so strongly held on the surfaces of reactive minerals that it is not available for plant root uptake. The use of phosphate-solubilizing microorganisms (PSM) capable of Pi desorption seems to be a complementary alternative in the management of these soils. The aim of this study was to evaluate the effectiveness of the soil fungus Mortierella sp., a known PSM, to desorb Pi from four soil minerals differing in their Pi sorption capacity. The fungus was effective in desorbing Pi from all tested minerals except from allophane, and its desorption depended on the production of oxalic acid. The effectiveness of the fungus to desorb Pi was ranked as montmorillonite > kaolinite > goethite > allophane. The quantity of desorbed Pi increased by increasing the amount of sorbed Pi. The Pi sorption capacity expressed as P_0.2 value (amount of P required to increase a solution P concentration up to 0.2 mg L^?1) was a good indicator of the effectiveness of Mortierella sp. to desorb Pi from soil minerals.     

SORPTION OF INORGANIC PHOSPHATE BY IRON- AND ALUMINIUM- CONTAINING COMPONENTS

The amounts of inorganic P sorbed by a range of Fe- and Al- containing components varied appreciably and decreased in the order allophane > fresh Al gel > Fe gel pseudoboehmite > aged Al gel > dried Fe gel > Fe-coated kaolinite > haematite > goethite > akaganeite > gibbsite = ground kaolinite > dispersed kaolinite. Al gel sorbed 30 to 70 times more P than gibbsite, and Fe gel sorbed approximately 10 times more P than its crystalline analogues (haematite, goethite. and akaganeite). Despite large differences in the extent of P sorption, the form of the isotherm was essentially the same for each sorbent. The ability of freshly-prepared Al gel suspensions to sorb P decreased with ageing, a property not shown by Fe gel. Drying of Fe gel at 80°C, however, caused an approximately 4-fold decrease in P sorption. Precipitation of Fe gel (2% Fe) on the surface of kaolinite increased P sorption by a factor of 10. The occurrence of Fe gel as a coating apparently presents more sorption sites to solution per unit weight of Fe gel than Fe gel alone. A linear relationship (r= 0.98) was obtained between the amount OH^? sorbed per unit increase in pH value (‘hydroxyl buffering’) and the overall P sorption maximum for each sorbent. Hydroxyl buffering provided a better index of P sorption potential than specific surface area. Except for the crystalline Fe sorbents, isotherms obtained by plotting fractional sorption saturation against final solution P concentration for the sorbents were essentially coincident with those for several contrasting soils. For crystalline Fe components a lower relative amount of weaker sorption, as opposed to chemisorption, of the overall sorption maximum was obtained. Differences in the extent of P sorption. however, appear to be primarily related to the number of functional M-OH groups presented at the solid-solution interface.     

Immobilization of heavy metals in soils amended by nanoparticulate zeolitic tuff: Sorption‐desorption of cadmium

Immobilization of Cd in contaminated soil is a technique to improve soil quality. Zeolites are potentially useful additives to bind heavy metals. This study examines the influence of decreasing the grain size of raw zeolitic tuff to the nano‐range on Cd sorption‐desorption isotherms in a sandy soil. The aim was to evaluate the feasibility of using nanoparticulate zeolitic tuff as a sequestering agent for Cd in soil. Sorption and desorption of Cd on zeolitic nanotuff was investigated in batch experiments. The sandy soil was amended with zeolitic nanotuff, synthetic Al oxide nanoparticles, or raw zeolitic tuff. After a 48 h sorption step, three desorption steps were induced using the sequential dilution method. The measured sorption‐desorption data were adequately described by the Freundlich equation. A nonlinear two‐stage one‐rate model matched well with measured Cd sorption‐desorption isotherms of zeolitic nanotuff, raw zeolitic tuff, Al oxide nanoparticle, and control soil. The batch experiments demonstrated the strong influence of pH on Cd sorption. Zeta‐potential measurements of zeolitic nanotuff indicated that electrostatic interaction was important for sorption. Soil amended with zeolitic nanotuff showed the lowest hysteresis index based on the differences obtained from sorption‐desorption isotherms regarding the amount of Cd sorbed. The index decreased with increasing metal concentration. For soil amended with nanoparticulate zeolitic tuff, the estimated Freundlich coefficient was 4 orders of magnitude higher than in the control soil and 2 orders of magnitude higher than the soil amended with raw zeolitic tuff. In addition, this sorbent decreased the amount of Cd released after three desorption steps by up to 12 and 7 times compared to the control soil and the soil amended with raw zeolitic tuff, respectively. The effect on Cd sorption of grinding the raw zeolitic tuff to the nanorange was considerably more pronounced than the effect of raw zeolitic tuff.     

密集型农业下塑料膜温室中不同土地利用方式对土壤磷素吸附-解吸特性的影响

With rapid urbanization and economic growth, Chinese traditional rice-legume production is increasingly replaced by vegetable and horticultural flower production, which could affect soil properties. This study was conducted near Kunming City, Yunnan Province, Southwest China to investigate how soil phosphorus(P) sorption and desorption processes respond to land use changes and to relate P sorption and desorption parameters to soil properties. Soil samples(0–20, 20–40, 40–60, 60–80 and 80–100 cm) were collected from five sites representing four land use types: rice-legume production in a two-crop, one-year rotation(Rice), vegetable production in open fields(Vegetable), recent( 3 years) conversion from open fields to plastic-film greenhouse vegetable and flower production at two sites(VFCS1and VFCS2), and longer-term( 10 years) plastic-film greenhouse vegetable and flower production(VFCL). The changes in land use affected soil pH, electrical conductivity, available N and P and organic carbon content in topsoil and subsoil. In turn, these changes of soil properties influenced soil P sorption capacity. The P sorption maximum(Smax) was affected by land use types, soil sampling depth and their interactions(P 0.0001). For surface soil, Smax was in the order of Rice(1 380 mg kg-1) VFCL(1 154 mg kg-1) VFCS2(897 mg kg-1) VFCS1(845 mg kg-1) Vegetable(747 mg kg-1). The lowest Smax generally occurred at the surface(except for Rice at 80–100 cm) and increased with depth. The amount of P desorbed during the 8 successive extractions was in the range 23%–44% of sorbed P, and was not affected by land use types or sampling depths. The decreases in Smax suggested that soil P sorption capacity decreased when rice-legume production converted to more intensive vegetation and flower production and caution should be exercised when applying P fertilizer to minimize potential leaching and runoff P loss to the environment.     

Solubility,sorption and desorption of native and added cadmium in relation to properties of soils in New Zealand

Solution cadmium (Cd) concentrations and sorption and desorption of native and added Cd were studied in a range of New Zealand soils. The concentration of Cd in solution and the concentrations and patterns of native soil Cd desorbed and added Cd sorbed and desorbed varied greatly between the 29 soils studied. Correlation analysis revealed that pH was the most dominant soil variable affecting solution Cd concentration and sorption and desorption of native and added Cd in these soils. However, organic matter, cation exchange capacity (CEC) and total soil Cd were also found to be important. Multiple regression analysis showed that the log concentration of Cd in solution was strongly related to soil pH, organic matter and total Cd, which in combination explained 76% of the variation between soils. When data from the present study were combined into a single multiple regression with soil data from a previously published study, the equation generated could explain 81% of the variation in log Cd solution concentration. This reinforces the importance of pH, organic matter and total Cd in controlling solution Cd concentrations. Simple linear regression analysis could at best explain 53% of the total variation in Cd sorption or desorption for the soils studied. Multiple regression analysis showed that native Cd desorption was related to pH, organic matter and total Cd, which in combination explained 85% of the variation between soils. For sorption of Cd (from 2 μg Cd g^–1 soil added), pH and organic matter in combination explained 75% of the variation between soils. However, for added Cd desorption (%), pH and CEC explained 77%. It is clear that the combined effects of a range of soil properties control the concentration of Cd in solution, and of sorption and desorption of Cd in soils. The fraction of potentially desorbable added Cd in soils could also be predicted from a soil’s K_d value. This could have value for assessing both the mobility of Cd in soil and its likely availability to plants.