Zinc is a heavy metal of much interest since it is a plant micronutrient as well as a potential contaminant in soils. In soil solution, the speciation of Zn, and thus the free Zn activity, determines the plant availability of Zn as a micronutrient and its characteristics as a heavy metal contaminant. A better understanding of the mechanisms that control free Zn activity could improve soil treatments of Zn deficiency or toxicity. Possible controlling mechanisms for Zn activity include adsorption or precipitation. In our study, Zn2+ activity was measured by chelation and was related to soil properties for 18 alkaline soils from three farms in eastern Colorado. Soil organic C (OC) and pH were statistically significant parameters in a multiple regression with log Zn2+ activity. The significance of OC may suggest that adsorption onto organic matter controls Zn solubility in some of our soils. Log Zn2+ activities plotted with pH fell near the soil-Zn solubility line. However, the slope of the regression line was -1 rather than an expected - 2, which indicates that another mechanism besides precipitation and dissolution of soil Zn may occur. Another possibility is that there are two different regions of solubility, one below pH 8.4 and one above pH 8.4. It is suggested that free Zn ions may adsorb on organic matter in a region of low pH and may precipitate as franklinite or other minerals, such as a Zn-containing kerolite, at high pH.

AbstractBecause of the low solubility of Zn in alkaline soils, it is virtually impossible to measure Zn activity directly. In the present study Zn activities in 10 arid‐zone soils were obtained by chelation. The Zn activities were extremely low, ranging from 10−7.9 to 10−10.9 M and were inversely related to pH, with a correlation coefficient of 0.94. Direct identification of solid phases controlling Zn solubility in these soils has not been achieved. The Zn activities measured in these soils can be expressed by the relationship soil Zn + 2H+ ⇌ Zn2+ with log Ko = 5.7 ± 0.38, where Ko is the equilibrium constant expressed in terms of activities. This is nearly identical to that reported earlier from this laboratory. This observed solubility relationship could be explained by the formation of ZnFe2O4 (franklinite), but that possibility awaits further study.

We selected thermodynamic data for 45 fluorine minerals and solution species that may play important roles in the fluorine chemistry of soils. Equilibrium reactions and stability constants for these minerals and species were calculated and used to develop stability diagrams in soils. Aluminum fluoride is likely to precipitate in strongly acid soils of pH below 4.5. In these soils AlF3(c) in equilibrium with Al(OH)3 (gibbsite) can maintain F− below 10−4 M in solution. Fluorite appears to be stable in slightly acid and near-neutral soils. When fluorite is in equilibrium with 10−2.5 M Ca2+, it can support 10−3.96 M F− in soil solution. Fluorophlogopite mineral is probably stable in alkaline soils. However, it is a complex mineral and its solubility is also affected by the activities of H4SiO40, Al3+, K+, Mg2+, and pH. Fluorapatite is unstable in acid soil, but it is the most stable F mineral in alkaline and calcareous soils. Fluorapatite supports extremely low F− activity (<10−6.5 M) in soil solution when it is in equilibrium with hydroxyapatite. Other metal fluorides, e.g., NaF, KF, CuF2, ZnF2, PbF2, CdF2, MgF2, and FeF3 are too soluble to persist in soils. Also, fluorosilicate, fluoroaluminate, and fluoroaluminosilicate minerals, such as Na2SiF6 (malladrite), (NH4)2SiF6 (bararite), K2SiF6 (hieratite), Na3AlF6 (cryolite), and NaCa2Mg4Al3Si6O22F2 (fluorpargasite), are thermodynamically unstable in soils. Solution complexes of aluminum fluorides, e.g., AlF2+, AlF2+, AlF30, and AlF4−, and iron fluorides, e.g., FeF2+, FeF2_, and FeF30, are the major solution species in strongly acid soils below pH 6. Above this pH F− is the predominant species in soil solution. Silicon and lead fluoride complexes, along with KF0, NaF0, CuF+, ZnF20, and HF2− species, are not important in most cultivated soils.

AbstractThe solubilities of AlF3(c), CaF2 (fluorite), and KMg3AlSi3O10F2 (fluorophlogopite) were investigated. Equilibrium was approached from both undersaturated and supersaturated conditions. The average equilibrium constant (K°) determined was 10−17.37 for AlF3(c), 10−10.3 for CaF2, and 107.94 for KMg3AlSi3O10F2. Nine soil samples each of strongly acid, slightly acid, and alkaline soils were used for AlF3(c), CaF2, and KMg3AlSi3O10F2 studies, respectively. The 1:5 soil/water suspensions, with and without added mineral, were shaken for 150 d. The F‐ solubility for strongly acid and near neutral soils without added minerals was undersaturated with respect to AlF3(c) and CaF2, respectively. The F‐ solubility for alkaline soils was close to that of KMg3AlSi3O10F2.

AbstractPredictive models for the solubility of F minerals were developed and compared to the F solubility measurements made on 109 soils. The experimental findings indicate that AlF3(c) may be the solid phase controlling F solubility in strongly acid soils, CaF2 (fluorite) in slightly acid soils, and KMg3AlSi3O10F2 (fluorophlogopite) in neutral and alkaline soils. Although Ca5(PO4)3F (fluorapatite) alone or in combination with hydroxyapatite is theoretically able to lower F activity in soils below that controlled by any of the aforementioned minerals, solubility measurements on all soils indicate supersaturation with respect to these apatite minerals.

AbstractEquilibria of EDTA (ethylenediaminetetraacetic acid) chelates were used to measure the concentration of ferric ion in near‐neutral soils. Concentrations of Fe3+ and the (Fe3+)(OH‐)3 activity product were defined quantitatively by simultaneous equilibria involving EDTA4‐, Fe3+, Ca2+, and OH‐. Equilibrium between soil Fe and chelated Fe was established by varying the fraction of EDTA associated with Fe or by slightly altering the pH of the soil. Corrections were made to account for the differential sorption of various EDTA chelate species onto soils and for the formation of EDTA chelates of Zn and Cu.Concentrations of Fe3+ in the range of 10−16 to 10−18 M were measured successfully in the presence of much higher concentrations of iron present as hydrolysis species and as unknown natural complexes. In 10 experiments using three near‐neutral soils, the negative logarithm of the (Fe3+)(OH‐)3 ion product had a mean value of 39.3 with a standard deviation of approximately 0.2. The solubility of Fe3+ in equilibrium with soil was stable for at least 6 weeks.

Standard free energies of formation for various inorganic N species were selected from the literature and used to calculate equilibrium constants for reactions involving various inorganic N species. These reactions were systematized into stability diagrams and used to discuss possible N transformations that may occur in soils. The developments show that N2(g) is the only stable N species in the redox range of pe + pH of 4 to 18. At lower redox NH4+ is stable, whereas at higher redox NO3- is stable. Stability diagrams were developed on the basis that first N2, then N2O, and finally N3- do not attain equilibrium with other inorganic N species. These diagrams provide insight into possible reaction sequences that may be involved in chemical denitrification. The hypothesis proposed herein is that NO0 (nitric oxide) and possible H2N2O2 (hyponitrous acid) may be rate-limiting intermediates in chemical denitrification. These N species are metastable but reach maximum stability in the narrow redox range of pe + pH 13.5 to 14.0. The possibility that these species may be the kinetically feasible intermediates in chemical denitrification are discussed.

AbstractMetal‐chelate stability diagrams were developed as a function of pH and redox to predict the behavior of DTPA, EDTA, CDTA, EGTA, and HEDTA in soils. The metals evaluated include H+, Fe3+, Fe2+, Al3+, Mn2+, Ca2+, Mg2+, Zn3+, Cu2+, Cd2+, Pb2+, and Ni2+. In acid systems, Ni2+ at 10−5 to 10−7M was the predominant ligand species for all chelating agents. Similar results were obtained in alkaline systems when NiCO3 controlled Ni2+ solubility. In the absence of Ni2+, either Cd2+ or Pb2+ ligand species dominated at alkaline pH for all chelating agents. In acid systems, Cu, Pb, Fe, and Zn are the major chelated species.In acid soils, the chelating agents EDTA, EGTA, and HEDTA deserve further study as extractants for metals and as reagents for determining metal ion activities through competitive equilibria. The results identify ligands which may be useful in determining various metal ion activities in soils. The DTPA soil test appears to have a sound theoretical basis for evaluating plant available Pb, Cd, and Ni in soils.

AbstractCorn (Zea mays L.) ‘Pioneer‐3932A’ was grown in the greenhouse on a loamy sand amended with cadmium, cadmium‐spiked sewage sludge, and sewage sludge. Soil pH was adjusted to approximately 5.7, 6.7, and 7.8, and phosphorus was applied at 0, 200, and 400 µg/g soil. Soil treatments were applied 0, 8, and 16 weeks prior to planting, and the plants were grown for 5 weeks.Increasing the pH of the soil amended with inorganic cadmium (CdSO4) decreased the Cd concentration of corn seedlings by approximately 67%. The Cd concentration was reduced approximately 47% for a similar increase in pH on soil amended with Cd‐spiked sludge.The Cd concentration of corn seedlings was significantly reduced by the addition of P. Increasing incubation time decreased the availability of added inorganic Cd, but increased the availability of Cd applied with sewage sludge.Increasing the amount of Cd added to the soils increased the Cd concentration of the corn seedlings regardless of the form of Cd used. Cadmium concentrations in the corn seedlings were significantly correlated with DTPA‐extractable soil Cd.Solubility measurements of Cd in the acidified soils showed undersaturation with respect to all known Cd minerals, whereas in soils above pH 7.25 the Cd solubility was limited by CdCO3 at a CO2 level of 0.003 atm or higher and showed the expected hundredfold decrease in solubility for each unit increase in pH. The minerals Cd3(PO4)2 and Cd(OH)2 are much too soluble to account for the precipitation of Cd in these soils.

SummarySoil samples obtained from genetic horizons of an upland and acid sulphate soil of Sierra Leone, were equilibrated with 0.01 M CaCI using a 1:2 soil to solution ratio. Al3+ activity was estimated from total Al measured in the equilibrium extract, by accounting for hydrolysis and the formation of other complex species.The Al3+ activity was pH‐dependent, but the Al(OH)3 ion product was I'airly constant throughout the upland soil profile; Al3+ activity was near that expected for equilibrium with kaolinite and quartz.In the acid sulphate soil alunite appeared to control the activity of Al3+.

AbstractA DTPA soil test was developed to identify near‐neutral and calcareous soils with insufficient available Zn, Fe, Mn, or Cu for maximum yields of crops. The extractant consists of 0.005M DTPA (diethylenetriaminepentaacetic acid), 0.1M triethanolamine, and 0.01M CaCl2, with a pH of 7.3. The soil test consists of shaking 10 g of air‐dry soil with 20 ml of extractant for 2 hours. The leachate is filtered, and Zn, Fe, Mn, and Cu are measured in the filtrate by atomic absorption spectrophotometry.The soil test successfully separated 77 Colorado soils on the basis of crop response to Zn, Fe, and Mn fertilizers. Critical nutrient levels must be determined separately for each crop using standardized procedures for soil preparation, grinding, and extraction. The critical levels for corn using the procedures reported herein were: 0.8 ppm for Zn, 4.5 ppm for Fe, and tentatively 1.0 ppm for Mn, and 0.2 ppm for Cu.Development of the soil test was based, in part, on theoretical considerations. The extractant is buffered at pH 7.30 and contains CaCl2 so that equilibrium with CaCO3 is established at a CO2 level about 10 times that of the atmosphere. Thus, the extractant precludes dissolution of CaCO3 and the release of occluded nutrients which are normally not available to plants. DTPA was selected as the chelating agent because it can effectively extract all four micronutrient metals. Factors such as pH, concentration of chelating agent, time of shaking, and temperature of extraction affect the amount of micronutrients extracted and were adjusted for maximum overall effectiveness.

AbstractAdsorption and precipitation of Cd2+ in soil suspensions were investigated as possible factors controlling Cd2+ levels in soils. Both adsorption of Cd2+ onto soils surfaces and possibly precipitation of cadmium minerals were evident in this study. At low cadmium levels solubility relationships in soils are best described by adsorption and fit the empirical Freundlich adsorption isotherm. Based on current thermodynamic data the solid phases CdCO3 (log Ksp = −12.07) and Cd3(PO4)2 (log Ksp = −32.61) most likely limit Cd2+ activities in soils. Under alkaline conditions, Cd2+ activity decreased approximately 100‐fold for each unit increase in pH. In this study, CdCO3 precipitated in sandy soils, having low cation exchange capacity (CEC), low organic matter, and pH values >7.0.DTPA‐extractable cadmium levels in soils were highly correlated (r = 0.96) with Cd concentrations in corn (Zea mays L.) seedlings grown on soils amended with Cd solutions, sewage sludge, or Cd‐spiked sewage sludge. Increasing Cd additions to the soil increased plant Cd regardless of the form of Cd added. Addition of inorganic Cd to the soil increased the Cd concentration of corn seedlings more than equivalent Cd additions in the form of sewage sludge or Cd‐spiked sludge.

AbstractCorn (Zea mays) was grown in nutrient solutions of pH 5.2 and 7.5 with EDTA (ethylenediaminetetraacetic acid) and DTPA (diethylenetriaminepentaacetic acid) as chelating agents. At pH 5.2, corn plants were healthy and fully green, but at pH 7.5, the nutrient solutions with DTPA produced severely stunted, Zn‐deficient plants. Spraying the foliage with Zn or increasing the level of ZnSO4 added to the nutrient solution prevented Zn deficiency.A concentration of 10−10.6 M Zn2+ was sufficient to prevent Zn deficiency with corn. Even at 105 times this concentration, chelated Zn was not absorbed by the roots. Equilibrium relationships of chelating agents in hydroponic solutions were extremely helpful in interpreting the results of this study.The findings reported herein support the hypothesis that Zn2+ is the form of Zn absorbed by plant roots. Furthermore, plants can obtain adequate Zn from a nutrient media when Zn2+ is maintained at approximately 10−10.6 M. This concentration is nearly 4,000 times lower than the 10−7 M reported by Carroll and Loneragan. The hypothesis proposed herein is that chelation aids in the transport and movement of metal ions to plant roots, but only Zn2+ is absorbed.

AbstractNumerous workers have examined the weathering products of soil minerals and have proposed empirical weathering sequences. The present paper outlines the development of a thermodynamic model that predicts in a systematic way several mineral transformations that can occur in soils.According to this model, the stability of primary minerals increases in the order: Na‐glass, K‐glass, pyroxene, analcime, anorthite, low albite, muscovite, microcline, and quartz. The stability of secondary clay minerals depends on soluble silica. At pH 6 with high silica (≃ 10‐3M) the order of increasing stability is: chlorite, halloysite, gibbsite, illite, dickite, beidellite, kaolinite, and montmorillonite; at low silica (≃ 10‐5M) the order is: chlorite, halloysite, illite, beidellite, montmorillonite, dickite, kaolinite, and gibbsite. The stability of both primary and secondary minerals increases with pH.The observed weathering of volcanic ash agrees well with the predictions of this model. The model makes use of important thermodynamic data accumulated for soil minerals and helps to pinpoint deficiencies in these data. The need to examine the kinetics of mineral transformations in soils in greater detail also becomes obvious from this model.

AbstractSelf‐diffusion coefficients for Zn and ZnEDTA (zinc ethylenediaminetetraacetic acid) were determined in aqueous solution and in soil. Those in solution were estimated from diffusion measurements in agar‐agar gel and were found to be 8.8 × 10‐6 cm2 sec‐1 for Zn and 6.8 × 10‐6 cm2 sec‐1 for ZnEDTA. The apparent diffusion coefficient of Zn in soil increased from 0.36 × 10‐9 cm2 sec‐1 to 3.0 × 10‐9 cm2 sec‐1 with addition of 1.53 × 10‐8 mole of EDTA/g soil and to 9.0 × 10‐9 cm2 sec‐1 with addition of 1.53 × 10‐8 mole of ZnEDTA/g soil. The true diffusion coefficient Dp of Zn in soil was fairly constant at different levels of EDTA, averaging 1.81 × 10‐7 cm2 sec‐1.Addition of EDTA to soils increased the diffusion of Zn; this increase was explained by the transformation of the solid phase soil Zn into soluble ZnEDTA complexes, thereby increasing the concentration gradient of total diffusible Zn. Chelating agents were shown to play an important role in overcoming the rate‐limiting steps of solution and diffusion which are largely responsible for the movement of micronutrient cations in soils.

AbstractPorous ceramic tubes were embedded in soil tagged with 65Zn. Solutions were slowly passed through the tubes, allowing diffusion of the solutes into the soil and diffusion of Zn from the soil to the tubes. Solution from the tubes was analyzed for 65Zn. The solutes used were EDTA (ethylenediaminetetraacteic acid), HCl, citric acid, an amino acid mixture, and glucose. EDTA caused the greatest increase of Zn transport into the ceramic tubes (simulated roots). For example, when 10‐3M EDTA was passed through such a tube Zn diffusion increased 17‐fold over that occurring with water alone. The effectiveness of other agents decreased in the order listed above. Increasing the concentrations of solute generally increased 65Zn diffusion. This study demonstrates how complexing agents or acids from root exudates or from decomposing organic residues in soils may increase the transport and availability of insoluble nutrients.

AbstractZinc was allowed to diffuse from a ZnCO3 precipitate through an agar‐agar gel to a stream of continuously flowing water. In one treatment, part of the agar‐agar was replaced by Ca‐poly‐galacturonate to incorporate fixed negative charges in the gel in the form of carboxyl groups. In a second treatment, Cacitrate was introduced into the flowing stream of water. In every case, all phases of the system were kept in equilibrium with CaCO3.Both fixed charges and the mobile complexing agent increased the transport of Zn in the system, but the latter was considerably more effective than the former. One percent polygalacturonic acid increased the transport of Zn by 13%, whereas 2 × 10‐3 M citrate increased the transport of Zn 100‐fold.Results compare favorably with a theoretical treatment for the contribution of the mobile complexing agents to Zn transport.