THE COVARIANCE OF PHOSPHATE SORPTION WITH OTHER SOIL PROPERTIES IN SOME BRITISH AND TROPICAL SOILS

Phosphate sorption capacity estimated by Piper's (1942) ‘anion exchange capacity’ and Bache and Williams's (1971) phosphate sorption index were correlated with soil pH, clay, organic matter, ‘free iron oxides’ and ‘extractable aluminium’ (McLean et al., 1958) for topsoil and subsoil samples from twenty tropical and twenty British acidic soil profiles. These two groups of soils did not differ significantly in phosphate sorption. Extractable aluminium and free iron oxide were well correlated with phosphate sorption, free iron oxide being superior to aluminium in freely drained British soils but not in poorly drained ones. Organic matter content correlated well with phosphate sorption for the poorly drained British soils, and for the tropical soils when sorption capacitywas measured using a high phosphate concentration.     

The effects of lime on adsorption and desorption of phosphate in five Colombian soils

The effects of lime (applied in the field) on the amounts of total and isotopically-exchange-able phosphate adsorbed from solutions were measured in five soils. The total amount of phosphate adsorbed without lime was in the range 200 to 1700 μg P per g of soil at 0.05 μg P cm⁻³ of solution. Lime diminished the amount of phosphate adsorbed at all concentrations of solution in an oxisol and a dystropept; in an ultisol and another dystropept, lime tended to increase sorption at small concentrations and diminish it at large concentrations; in a dystrandept that contained spheroidal allophane and a great deal of organic matter, lime increased adsorption at all concentrations up to 1 μg P cm⁻³. Lime increased the proportion of added phosphate that was isotopically exchangeable in the oxisol and one dystropept, had no effect in the other dystropept, and diminished the proportion in the ultisol and dystrandept.
Adsorbed phosphate was subsequently desorbed by suspending the soils in solutions without phosphate. After desorption the quantity of exchangeable phosphate in all soils was closely correlated with aluminium extracted by ammonium oxalate; buffer power was correlated in all except the dystrandept, in which it was larger per unit of aluminium than in the other soils; possibly the cause was aluminium associated with organic matter. In all soils lime diminished buffer power allowing a specific amount of exchangeable phosphate to maintain a larger concentration in solution. The beneficial effects of lime on exchangeable phosphate after desorption were consistent among soils, despite inconsistent results when the phosphate was adsorbed.     

Lead and zinc levels and chemical fractionation in road-side soils of Caracas,Venezuela

Twenty five soil samples located nearby highways and streets of Caracas, Venezuela were collected and chemical, physical and mineralogical analysis were done to characterize them. The soils have light textures, neutral or slightly alkaline pH, medium to high organic matter content, and all of them have carbonate. Kaolinite and mica were the dominant clay minerals in all soil samples. Lead and Zn were extracted with 1N-₃ to investigate the levels of these polluting heavy metals in these roadside soils. A very high level of Pb was found in the soils (average enrichment factor of 151.4), while Zn levels were much lower (average enrichment factor of 5.25), but still higher than normal soils' levels. These results are indication of strong metal pollution (especially by Pb) of Caracas' roadside soils, due to heavy transit of motor vehicles as well as to the exclusive use of highly leaded gasoline in Venezuela. Nevertheless both metals accumulate only on the surface layers of those soils located within 5 m from the roadside. These facts were taken as evidence of the low vertical and horizontal mobility of the metals, which was mainly attributed to the high pH of the studied soils (between 7.5 and 7.8). Three surface soil samples having high Pb and Zn levels were selected for chemical fractionation by McLaren and Crawford's (1973) methodology. It was found that less than 1% Pb and below 5.5% Zn were in exchangeable form in these soils. Therefore, Pb and Zn are predominantly present in non-exchangeable forms in the studied soils. These non-exchangeable metals tend to become associated with different soils materials. Lead is mainly associated with the organic fraction, as well as to the inorganic and residual fractions, and Zn is mainly associated to the inorganic and residual fractions, and also the iron oxides, being practically absent in the organic fraction.     

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

The 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 b_e, 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.     

Comparison of different models for phosphate sorption as a function of the iron and aluminium oxides of soils

Phosphate sorption on topsoil and subsoil samples from different soils located in the eastern part of Germany was studied. Two models were fitted to sorption data obtained after 4 and 40 d of gentle shaking. The models differ with respect to the fractions of iron and aluminium (hydr)oxides that are considered and whether the phosphate initially sorbed in the soil is taken into zccount. Oxalate-extractable P, (P_ox), appears to be a major part of the total soil P. The total P sorption measured, F, was predominantly related to the amounts of amorphous iron (Fe_ox) and aluminium (Al_ox). A significant relation between crystalline iron (Fe_d– Fe_ox) and total P sorption was not found. Reversibly adsorbed phosphate (P_i), measured after 40 d reaction time, was a function of clay content and content of amorphous iron and aluminium (hydr)oxides.     

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.     

Sorption of dimethylselenide by soils

Air-dry and moist soils were shown to possess the capacity to sorb substantial amounts of (⁷⁵Se)dimethylselenide produced by the yeast Candida humicola in culture, or by soil supplied with (⁷⁵Se)selenite, depending largely upon the organic matter content and selenium concentration of the soils. The sorption capacities of individual soil constituents followed the order; organic matter > clay minerals > manganese oxides > iron oxides > acid-washed sand.A chemical fractionation procedure applied to soils fumigated with (⁷⁵Se)dimethylselenide revealed that the majority of the selenium sorbed was converted after 1 month to other forms, extractable mainly with strong acid solutions. Experiments with sterilized (autoclaved and γ-irradiated) soils indicated that soil microorganisms played little, if any, part in the sorption process.The work reported here indicates that soil is an important natural “sink” for atmospheric dimethylselenide.     

茶园—土壤系统铝和氟的生物地球化学循环及其对土壤酸化的影响

黄棕壤植茶以后,土壤pH下降,土壤酸度随植茶年限的增长而增大,且上层土壤pH的减幅大于下层土壤。茶园土壤的酸化是与茶树对铝和氟的生物积聚、土壤交换性铝与铝络合物的增加以及土壤盐基的淋溶有关。茶树落叶中铝和氟的含量分别高达5836—6136 ppm和469—520ppm;茶树透冠水和土壤渗漏液中均有相当多的Al和F,茶园土壤系统中铝和氟的循环,不仅导致土壤Al³⁺及F^-的增多,还使表土的有机络合态铝以及土壤交换性复合体和土壤溶液中的氟铝络合物积聚。因此,土壤中铝和氟的积累、转化及其生物地球化学循环是茶园土壤酸化的主要原因。     

Short-term release and fixation of K in calcareous clay soils. Consequence for K buffer power prediction

Non-exchangeable K always contributes to some degree to plant nutrition. To understand this contribution and to deduce a predictive buffer power model the release and fixation of K was studied on a range of 44 calcareous clay soils. Short-term K sorption and desorption experiments (16 h) were followed by the measurement of soil exchangeable K (ammonium acetate extraction). Soil K-Ca exchange properties and the contributions of exchangeable K and non-exchangeable K to K dynamics of the soil-solution system were estimated. The change in the amount of non-exchangeable K during the experiment was generally proportional to the initial constraint imposed to the soil-solution system (ø), i.e. the solution: soil ratio multiplied by the difference between the solution K concentration imposed at the beginning of the experiment and the solution K concentration for which neither sorption nor desorption of K would occur. The proportionality coefficient (β) called the 'soil ability for K release and fixation’ was identical for release and fixation for 36 soils, whereas eight soils showed some difference between their ability for fixation and their ability for release of K. When β was considered identical in the release range and in the fixation range for all soils it was inversely proportional to the initial K saturation ratio of the CEC corrected for the amount of ammonium extractable K which was not in exchange equilibrium with Ca. This quantity of ammonium extractable K which is not in equilibrium with Ca probably contributes only partially to plant nutrition. For the studied soils this quantity contributed a large proportion of exchangeable K (26–65%) especially in soils with a small K content. Due to the contribution of non-exchangeable K to soil-solution K dynamics the buffer power of the system does not only depend on exchange properties but also on soil release and fixation properties. When β was taken into account buffer power was better estimated than when it was deduced from ion exchange alone.     

Fractionation of zinc in some New Zealand soils

Abstract

The amounts and forms of zinc in twenty surface soils from Canterbury and Southland, New Zealand were determined using a sequential fractionation scheme. Total soil zinc concentrations ranged from 38.1 mg#lbkg^‐1 to 113.8 mg#lbkg^‐1. Although the proportions of zinc found in individual fractions varied between soils, on average approximately 3% occurred as exchangeable zinc, 5% as organic‐bound zinc, 9%, 18%, 24% was associated with manganese, amorphous iron and crystalline iron oxides, respectively, and 40% was in the residual fraction. In a group of soils formed in greywacke alluvium or loess, exchangeable zinc was inversely related to soil pH. Within the same group of soils, those of similar age with greater concentrations of total and organic‐bound zinc were present in imperfectly‐ and poorly‐drained soils compared with well‐drained soils. Zinc extracted from the soils with a range of reagents used to assess ‘plant available’ zinc was correlated strongly with the concentrations of zinc present in the exchangeable and organic‐bound zinc fractions.     

Isolating the influence of pH on the amounts and forms of soil organic phosphorus

Soil pH influences the chemistry, dynamics and biological availability of phosphorus (P), but few studies have isolated the effect of pH from other soil properties. We studied phosphorus chemistry in soils along the Hoosfield acid strip (Rothamsted, UK), where a pH gradient from 3.7 to 7.8 occurs in a single soil with little variation in total phosphorus (mean ± standard deviation 399 ± 27 mg P kg^?1). Soil organic phosphorus represented a consistent proportion of the total soil phosphorus (36 ± 2%) irrespective of soil pH. However, organic phosphorus concentrations increased by about 20% in the most acidic soils (pH < 4.0), through an accumulation of inositol hexakisphosphate, DNA and phosphonates. The increase in organic phosphorus in the most acidic soils was not related to organic carbon, because organic carbon concentrations declined at pH < 4.0. Thus, the organic carbon to organic phosphorus ratio declined from about 70 in neutral soils to about 50 in strongly acidic soils. In contrast to organic phosphorus, inorganic phosphorus was affected strongly by soil pH, because readily‐exchangeable phosphate extracted with anion‐exchange membranes and a more stable inorganic phosphorus pool extracted in NaOH–EDTA both increased markedly as soil pH declined. Inorganic orthophosphate concentrations were correlated negatively with amorphous manganese and positively with amorphous aluminium oxides, suggesting that soil pH influences orthophosphate stabilization via metal oxides. We conclude that pH has a relatively minor influence on the amount of organic phosphorus in soil, although some forms of organic phosphorus accumulate preferentially under strongly acidic conditions.     

Effect of heating on phosphate sorption and availability in some north-east Nigerian soils

Studies on phosphate sorption and availability in some north-east Nigerian soils showed that phosphate adsorption and inorganic-P concentrations increased considerably after the soil was heated either in a furnace or during the field-burning of straw. The increase in phosphate adsorption after heating was thought to be caused by an increase in free Fe and A1 oxides, whereas the high contents of exchangeable Ca and possibly carbonates and hydroxyl ions in the ash were probably responsible for the increase in phosphate sorption after field-burning of straw. Investigations into the availability of P to maize over a 7-d period of growth showed that there was no significant nutritional benefit from the P released after soil heating. The effect of heating was to increase P sorption and so reduce P in solution and P availability.     

Specific sorption of trace amounts of cadmium by soils

Abstract

Sorption of trace quantities of Cd in four soils of different chemical and mineralogical properties, was studied. Initial Cd concentrations were between 15 to 150 μg. 1^?1. The sorption isotherms were linear and had a positive intercept in three of the soils, indicating a constant partition‐high affinity sorption isotherm (Giles et. al⁶). The data also followed the Freundlich sorption isotherm, and the Freundlich K parameter was taken as a measure of the relative affinity of the different soils for the Cd metal sorbed. Cadmium sorbed was extracted by IN‐NH₄C1 followed by 0.1N HC1, and the fraction remaining in the soils was considered specifically sorbed Cd. This fraction also followed a linear sorption isotherm, and was around 30% for the four soils studied. The sorption order for the amount of specifically sorbed Cd showed that the Boomer soil (kaolinite‐iron oxides) had the lowest affinity for specific sorption of this metal. This was taken as evidence that kaolinite and iron oxides have a lower capacity for retaining cadmium through specific sorption mechanism(s) than the materials present on the other soils (2:1 layer silicates and humic substances). The existence of specific mecha‐nism(s) responsible by the sorption of trace quantities of Cd in soil solutions has important implications on soil‐plant relationships, Cd mobility in soil profiles and control of Cd activity in soil solutions.     

Availability of exchangeable and non-exchangeable K in Argentine soils with different mineralogy

There were two objectives in the study: 1) To determine exchangeable K and non-exchangeable K in soils with different potassium depletion levels and mineralogy as plant sources 2) To establish a relationship between the mineralogy vs exchangeable K (Ke) and non-exchangeable K (Kne) mobilization. An extraction experiment of soils was carried out in a greenhouse, with a total of 6 consecutive crops of ryegrass. Different supply rates for plant K were determined by Ke and Kne mobilization according to the soil intensity of use. The contribution of the Ke was greater and generated higher amount of K uptake during maximum availability period (from 0 to the 1st harvest) than in the later period when soil K was already depleted. For this the initial exchangeable K and the illite concentration of soils accounted for almost 100% (R² = 0.981 P=0.01) of the K taken up by ryegrass. For the following period (from the 2nd harvest to the 6th), Kne forms became more important. Plant K supply was not only a result of initial exchangeable K and illite concentration but presumably also of primary K-bearing minerals.     

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.     

Influence of soil composition on adsorption of glyphosate and phosphate by contrasting Danish surface soils

The herbicide glyphosate and inorganic phosphate are strongly adsorbed by inorganic soil components, especially aluminium and iron oxides, where they seem to compete for the same adsorption sites. Consequently, heavy phosphate application may exhaust soil's capacity to bind glyphosate, which may lead to pollution of drain‐ and groundwater. Adsorption of phosphate and glyphosate to five contrasting Danish surface soils was investigated by batch adsorption experiments. The different soils adsorbed different amounts of glyphosate and phosphate, and there was some competition between glyphosate and phosphate for adsorption sites, but the adsorption of glyphosate and phosphate seemed to be both competitive and additive. The competition was, however, less pronounced than found for goethite and gibbsite in an earlier study. The soil's pH seemed to be the only important factor in determining the amount of glyphosate and phosphate that could be adsorbed by the soils; consequently, glyphosate and phosphate adsorption by the soils was well predicted by pH, though predictions were somewhat improved by incorporation of oxalate‐extractable iron. Other soil factors such as organic carbon, the clay content and the mineralogy of the clay fraction had no effect on glyphosate and phosphate adsorption. The effect of pH on the adsorption of glyphosate and phosphate in one of the soils was further investigated by batch experiments with pH adjusted to 6, 7 and 8. These experiments showed that pH strongly influenced the adsorption of glyphosate. A decrease in pH resulted in increasing glyphosate adsorption, while pH had only a small effect on phosphate adsorption.     

Forms of Al in soil and soil solution in a long‐term fertilizer application experiment

Under the conditions of a long‐term fertilizer experiment, this study aimed to determine the contents of total and exchangeable aluminium in soil as well as the Al concentration in the soil solution. Additionally, Al speciation was evaluated with the use of the MINTEQA2 software. The results obtained indicated that under the conditions of long‐term application of different mineral fertilizers or farmyard manure, the soil reaction changed to a great extent (pH 3.58–6.78). At the same time, the content of total Al in soil fluctuated from 18.85 to 22.13 g/kg and that of exchangeable Al ranged from 1.42 to 102.66 mg/kg. The concentration of Al in the soil solution was highly differentiated (5.19–124.07 μmol/L) as well as that of free aluminium ions (Al³⁺) (0–16.9 μmol/L). In acidic soils, aluminium complexes with organic matter are the predominant forms of Al in the soil solution. In soils with neutral soil reaction, there were no free aluminium ions. Soil liming and addition of organic amendment were the treatments that restricted the presence of toxic aluminium forms in soil.     

Influence of organic matter on phosphate adsorption by aluminium and iron oxides in sandy soils

The phosphate adsorption capacity (P_max) of samples from various horizons of five Danish podzolized soils were investigated before and after organic matter removal. Removal of organic matter had no direct influence on P_max suggesting that organic matter did not compete with phosphate for adsorption sites. In the soils investigated aluminium and iron oxides were the main phosphate adsorbents. Thus, more than 96% of the variation in P_max could be accounted for by poorly crystalline aluminium and iron oxides (extractable by oxalate) and by well-crystallized iron oxides (taken as the difference between dithionite-citrate-bicarbonate-extractable iron and oxalate-extractable iron). Organic matter affected phosphate adsorption indirectly by inhibiting aluminium oxide crystallization. The resulting poorly crystalline oxides had high P_max. In contrast, the influence of organic matter on the crystallinity of the iron oxides, and therefore on their capacity to adsorb phosphate, seemed limited.     

Phosphate sorption characteristics of major soils in Okinawa,Japan

Abstract

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.     

Modelling the effects of ageing on Cd,Zn, Ni and Cu solubility in soils using an assemblage model

Ageing reactions can reduce trace metal solubility and can explain natural attenuation of contaminated soils. We modelled ageing reactions in soil with an assemblage model that considers slow reactions in Fe‐oxyhydroxides and reversible sorption on organic matter and clay minerals. Metal adsorption kinetics on Fe‐oxyhydroxides was obtained from data with synthetic oxyhydroxides. Metal solubility and isotopic exchangeability data were obtained from 28 soils amended with Ni, Zn, Cu and Cd metal salts and monitored for 850 days. The assemblage model was constructed in WHAM 6.0 and used soil properties and dissolved organic matter as input data. The model was first validated to predict dissolved metal concentrations, based on the concentration of isotopic exchangeable metals. The model overestimated metal solubility without parameter adjustment by mean factors of 4–7, and successful fits were obtained by increasing the specific surface area of Fe‐oxyhydroxides from measured values of synthetic systems to a value of 600 m² g^?1 recommended by other authors. The effect of ageing on the isotopic exchangeable metal fraction was subsequently modelled starting from the predicted fraction of metals present on Fe‐oxyhydroxides immediately after soil spiking. The observed isotopic exchangeable metal fractions of Ni, Zn and Cd agreed reasonably well with predicted values. The model predicts that ageing reactions are more pronounced at higher pH because metal sorption is increasingly directed to oxyhydroxide surfaces with increasing soil pH. Modelling fixation of Cu requires more information on fixation of that metal in organic matter.