P. M. Huang

Low-molecular-weight organic acids in rhizosphere soils of durum wheat and their effect on cadmium bioaccumulation

Cadmium (Cd) accumulation has been found to vary between cultivars of durum wheat (Triticum turgidum var. durum), and it is hypothesized that low-molecular-weight organic acids (LMWOAs) produced at the soil-root interface (rhizosphere) may play an important role in the availability and uptake of Cd by these plants. The objective of this study, therefore, was to (1) investigate the nature and quantity of LMWOAs present in the rhizosphere of durum wheat cultivars Arcola (low Cd accumulator) and Kyle (high Cd accumulator) grown in three different soils: Yorkton, Sutherland and Waitville, and (2) determine the relationship between Cd accumulation in these plants and LMWOAs present in the rhizosphere. Plants were grown for two weeks in pot-cultures under growth chamber conditions. Oxalic, fumaric, succinic, L-malic, tartaric, citric, acetic, propionic and butyric acids were found and quantified in the water extracts of rhizosphere soil, with acetic and succinic acids being predominant. No water extractable LMWOAs were identified in the bulk soil. Total amount of LMWOAs in the rhizosphere soil of the high Cd accumulator (Kyle) was significantly higher than that for the low Cd accumulator (Arcola) in all three soils. Furthermore, large differences in amounts of LMWOAs were found in the rhizosphere soil for the same cultivars grown in different soils and followed the pattern: Sutherland > Waitville > Yorkton. Extractable soil Cd (M NH4Cl) and Cd accumulation in the plants also followed the same soil sequence as LMWOA production. Cadmium accumulation by the high and low Cd accumulating cultivars was proportional to the levels of LMWOAs found in the rhizosphere soil of each cultivar. These results suggest that the differing levels of LMWOAs present in the rhizosphere soil played an important role in the solubilization of particulate-bound Cd into soil solution and its subsequent phytoaccumulation by the high and low Cd accumulating cultivars.

RETENTION OF PHENOLIC ACIDS BY NONCRYSTALLINE HYDROXY-ALUMINUM AND -IRON COMPOUNDS AND CLAY MINERALS OF SOILS

A study was made on the adsorption of phenolic acids, which are known to be plant growth inhibitors, by kaolinite, illite, vermiculite, noncrystalline hydroxy-Al and -Fe compounds, and four Taiwan soils. The adsorption sequence of phenolic acids by kaolinite, illite, and vermiculite followed the general decreasing order: p-hydroxyben-zoic acid > p-coumaric acid > ferulic acid > syringic acid > vanillic acid. The internal surfaces of vermiculitic minerals were not of great significance in the retention of phenolic acids. This is attributed to steric hindrance and negative repulsion by the interlayers. The removal of the noncrystalline sesquioxidic components from the soils significantly decreased their adsorption of the phenolic acids. The rate and capacity of the noncrystalline hydroxy-Al and -Fe components for the retention of the phenolic acids were by far higher than kaolinite, illite, and vermiculite. The high rate and capacity of these noncrystalline components for phenolic acids are essentially attributable to the great reactivity of their positively charged Al-OH20.5+ and Fe-OH20.5+ functional groups towards the negatively charged carboxyl and phenolic hydroxyl groups.

Speciation of particulate-bound Cadmium of soils and its bioavailability

A modified sequential chemical extraction procedure was developed for partitioning particulate Cd into eight fractions: exchangeable, carbonate-bound, metal–organic complex-bound, easily reducible metal oxide-bound, organic-bound, amorphous mineral colloid-bound, crystalline Fe oxide-bound, and residual. Results of experimental data on 16 surface soils of Saskatchewan, widely varying in physico-chemical properties, indicate the presence of little exchangeable Cd. Cadmium in these soils was predominantly in the form metal–organic complex-bound, accounting for 31–55%, with an average of 40%, of the total Cd present in the soils. The average relative abundance of the different forms of Cd present in these soils is in the order: metal–organic complex-bound (0.107 mg kg–1) > carbonate-bound (0.052 mg kg–1) > residual (0.042 mg kg–1) > organic-bound (0.035 mg kg–1) > crystalline Fe oxide-bound (0.016 mg kg–1) > easily reducible metal oxide-bound (0.010 mg kg–1) > amorphous mineral colloid-bound (0.002 mg kg–1). Statistical treatment of the Cd availability index, measured as ammonium hydrogencarbonate–diethylenetriaminepentaacetic acid (ABDTPA)-extractable Cd, with different particulate-bound Cd species showed high correlation (r= 0.916, p= 6 × 10–7) of the Cd availability index with the metal–organic complex-bound Cd. The beta coefficients obtained from the multiple regression analyses have given an insight into the importance of Al–organic complex-bound Cd species in estimating the bioavailability of Cd in these soils. The relationship of the metal–organic complex-bound Cd and the mobility and bioavailability of soil Cd merits in-depth research in explaining the toxicity and food chain contamination of Cd in the environment.

Role of manganese in the oxidation of arsenite by freshwater lake sediments

The importance of various sediment components in the oxidation of As(III) (arsenite) to As(V) (arsenate) by freshwater lake sediments in southern Saskatchewan was examined. Treating the sediments with hydroxylamine hydrochloride or sodium acetate to remove Mn greatly decreased the oxidation of As(III). Furthermore, synthetic Mn(IV) oxide was a very effective oxidant with respect to As(III): 216 µg As(V)/ml was formed in solution when 1000 µg As(III)/ml was added to suspensions of 0.1 g of the oxide. These results indicate that Mn in the sediment was probably the primary electron acceptor in the oxidation of As(III). The conversion of As(III) to As(V) by naturally occurring carbonate and silicate minerals common in sediments was not evident in the system studied. Sediment particles >20 µm in size are the least effective in oxidizing As(III); the oxidizing ability of the 5–20-, 2–5-, and <2-µm particle size fractions varies depending on the sediment. The concentration of As(V) in equilibrated solutions after adding increasing amounts of As(III) (as much as 100 µg/ml) to 1 g of the three sediments ranged from approximately 3.5 to 19 µg/ml. Because As(III) is more toxic and soluble than As(V), Mn-bearing components of both the colloidal and non-colloidal fractions of the sediments may potentially detoxify any As(III) that may enter aquatic environments by converting it to As(V). This is very important in reducing the As contamination and in maintaining the ecological balance in aquatic environments.РезюмеИсследовалось значение различных компонентов осадка при окислении As(III) (мышьяковистокислой соли) в As(V) (мышьяковокислую соль) при помощи осадков из свежей озерной воды в нижном Саскачеване. Воздействие гидроксиламинового гидрохлорида или натриевого ацетата на осадки, чтобы удалить Mn, значительно уменьшает окисление As(HI). Кроме этого, синтетические окиси Mn(IV) были очень эффективными окислителями по отношению к As(IIl): 216 µг AS(V)/MJI было образовано в растворе при добавке 1000 µг As(V)/мл в суспензиях 0,1 г окиси. Эти результаты указывают, что Mn в осадке был, вероятно, акцептором первичных электронов при окислении As(III). Переход As(III) в As(V) при помощи натурально выступающих карбонатовых и кремниевых минералов, часто находящихся в осадках, не быд очевиден в изучаемой системе. Частицы осадка размером >20 µM были наименее эффективны при окислении As(III); окислительная способность частиц размером 5–20, 2–5, и 2 изменялась в зависимости от осадка. Концентрация As(V) в равновесных растворах после добавки увеличивающегося количества As(III) (до 100 µг/Mл) к одному грамму трёх осадков находилась в диапазоне приблизительно от 3,5 до 19 µг/Mл. Так как As(III) более токсический и растворяемый, чем As(V), Mn содержащие компоненты коллоидальных и неколлоидальных фракций осадков могут потенциально обезвредить любой As(III), который может поступать в водную среду, путём превращения её в As(V). Это очень важно при уменьшении загрязнения As и при сохранении экологического баланса водной среды. [Е.С.]ResümeeDie Bedeutung der verschiedenen sedimentären Komponenten bei der Oxidation von As(III), Arsenit, zu As(V), Arsenat, durch Süßwassersedimente wurde im südlichen Saskatchewan untersucht. Die Behandlung der Sedimente mit Hydroxylaminhydrochlorid oder Natriumacetat—um Mangan weitgehend zu entfernen—führte zu einer Abnahme der Oxidation von As(III). Darüberhinaus war synthetisches Mn(IV)-Oxid ein sehr wirksames Oxidationsmittel im Hinblick auf As(III): 216 µg As(V)/ml bildete sich in der Lösung bei einer Zugabe von 1000 µg As(III)/ml zu Suspensionen von 0,1 g Oxid. Diese Ergebnisse deuten darauf hin, daß Mangan in den Sedimenten wahrscheinlich der wichtigste Elektronenakzeptor bei der Oxidation von As(III) war. Die Umwandlung von As(III) in As(V) durch natürlich auftretende Karbonate und Silikatminerale, die gewöhnlich in Sedimenten auftreten, wurde in den untersuchten System nicht beobachtet. Die Kornfraktionen größer als 20 µm spielen bei der Oxidation von As(III) die kleinste Rolle. Die Oxidationskraft der Fraktionen 5–20, 2–5, und <2 µm variiert je nach Sediment. Die Konzentration von As(V) in den Gleichgewichtslösungen, die sich einstellten, wenn zunehmende Mengen As(III) (bis zu 100 µg/ml) zu jeweils 1 g der 3 Sedimente hinzugefügt wurde, reichte von etwa 3,5 bis 19 µg/ml. Da As(III) giftiger und leichter löslich als As(V) ist, können Mangan-haltige Komponenten der kolloidalen und nichtkolloidalen Fraktionen der Sedimente als mögliches Entgiftungsmittel für As(III) gelten, das in aquatisches Milieu gelangt, indem sie es in As(V) umwandeln. Dies ist von großer Bedeutung für die Verringerung der As-Kontamination und für die Erhaltung des ökologischen Gleichgewichtes im aquatischen Milieu. [U.W.]RésuméL’importance de composants sédimentaires variés dans l’oxidation d’As(III) (arsenite) à As(V) (arsenate) par des sédiments de lac d’eau douce dans le Saskatchewan du sud a été examinée. Le traitement des sédiments à l’hydrochloride hydroxylamine ou à l’acétate de sodium pour retirer Mn a fortement décru l’oxidation d’As(III). De plus, l’oxide Mn(IV) synthétique s’est montré un oxidant très éfficace en ce qui concerne As(III): 216 µg As(V)/ml a été formé en solution après que 1000 µg As(III)/ml avait été ajouté à des suspensions de 0,1 g de l’oxide. Ces résultats indiquent que Mn dans le sédiment était probablement l’accepteur d’électrons primaire dans l’oxidation d’As(III). La conversion d’As(III) à As(V) par des minéraux carbonates et silicés courants dans les sédiments n’était pas évidente dans le système étudié. Les particules sédimentaìres de taille >20 µm sont les moins éfficaces pour oxyder As(III); l’abilité d’oxydation des fractions de particules de taille 5–20, 2–5-, et <2-µm varie selon le sédiment. La concentration d’As(V) dans des solutions équilibrées, après avoir ajouté des quantités croissantes d’As(III) (jusqu’ à 100 µg/ml) à 1 g des trois sédiments s’étageait d’approximativement 3,5 à 19 µg/ml. Des composants portant Mn des fractions colloïdales et noncolloïdales des sédiments peuvent partiellement détoxifier tout As(III) qui pénètre les environements aquatiques en le convertissant en As(V), parcequ’ As(III) est plus toxique et plus soluble qu’As(V). Ceci est très important pour la réduction de la contamination par As et pour le maintient de l’équilibre écologique dans les environements aquatiques. [D.J.]

Influence of inorganic and organic ligands on the formation of aluminum hydroxides and oxyhydroxides

Hydroxide and oxyhydroxide products of aluminum were formed at room temperature at an initial Al concentration of 2 × 10-3 M, pH 8.2, and at varying concentrations of organic and inorganic ligands commonly found in nature. The effectiveness of the ligands in promoting the formation of noncrystalline products over crystalline Al(OH)3 polymorphs was found to be in the following order: phthalate ≅ succinate < glutamate < aspartate < oxalate < silicate ≅ fluoride < phosphate < salicylate ≅ malate < tannate < citrate < tartrate. The lowest ligand/Al molar ratio at which the production of Al hydroxides or oxyhydroxides was inhibited ranged from 0.02 to 15. Above critical ligand/Al ratios, crystalline products were inhibited and ligands coprecipitated with noncrystalline products which remained unchanged for at least 5 months. Polydentate and large ligands generally were more inhibitive than those with fewer functional groups or of smaller size.The perturbing ligands promoted and stabilized the formation of pseudoboehmite over crystalline Al(OH)3 polymorphs in the following sequence: chloride < sulfate < phthalate ≅ succinate < glutamate < silicate < aspartate < phosphate < salicylate ≅ malate < tannate < citrate < tartrate. The optimal range of the ligand/Al molar ratios for the formation of pseudoboehmite varied, for example, from 0.005–0.015 for tartrate to 600–1000 for chloride. Pseudoboehmite was not formed in the presence of fluoride.РезюмеГидроокисные и оксигидроокисные продукты алюминия формировались при комнатной температуре, начальной концентрации Al равной 2 × 10-3 М, pH равным 8,2 и различных концентрациях органических и неорганических лигандов, обычно находящихся в природе. Эффективность лигандов в способствовании образованию некристаллических продуктов предпочтительно полиморфом А1(ОН)3 находилась в следующем порядке: фталат ≅ сукцинат < глутамат < аспартат < оксалат < силикат ≅ фторид < фосфат < салицилат ≅ малат < таннат < цитрат < тартрат. Наиболее низкая величина молярного отнощения лиганд/Аl, при которой задерживалось образование гидроокисей или оксигидроокисей Al, находилась в диапазоне от 0,02 до 15. Выше критических величин отношений лиганд/Аl, кристаллические продукты не образовывались, а лиганды осаждались вместе с некристаллическими продуктами, которые оставались неизмененными в течение не менее, чем 5 месяцев. Полидентат и большие лиганды имели большую способность задерживания, чем лиганды, содержащие меньшее количества функциональных групп или лиганды меньших размеров. Возмущающие лиганды способствовали и стабилизировали формирование скорее псевдобемита, чем кристаллических полиморфов Аl(ОН)3 в следующем порядке: хлорид < сульфат < фталат ≅ сукцинат < глутамат < силикат < аспартат < фасфат < салицилат ≅ малат < таннат < цитрат < тартрат.Оптимальный диапазон величин молярных отнощений лиганд/Аl для образования псевдобемита изменялся, например, от 0,005–0,015 для тартрата до 600–1000 для хлорида. Псевдобемит не образовывался в присутствии фторида. [E.G.]ResümeeHydroxid- und Oxyhydroxid-Verbindungen von Aluminium wurden bei Raumtemperatur und mit einer ursprünglichen Al-Konzentration von 2 × 10-3 m, bei pH 8,2 und mit verschiedenen Konzentrationen von organischen und anorganischen Liganden, die in der Natur üblich sind, gebildet. Die Wirksamkeit der Liganden bei der Förderung der Bildung von nichtkristallinen Produkten gegenüber kristallinen polymorphen A1(OH)3-Modifikationen geht in folgender Reihenfolge: Phtalat ≅ Succinat < Glutamat < Asparat < Oxalat < Silikat ≅ Fluorid < Phosphat < Salicylat ≅ Malat < Tannat < Citrat < Tartrat. Das niedrigste Ligand/Al-Molverhältnis, bei dem die Entstehung von Al-Hydroxiden oder -Oxyhydroxiden verhindert wurde, liegt bei 0,02–15. Oberhalb der kritischen Ligand/Al-Verhältnisse wurde die Bildung kristalliner Verbindungen verhindert und die Liganden fielen mit nichtkristallinen Produkten zusammen aus, die über mindestens 5 Monate unverändert blieben. Polydentat und große Liganden wirkten sich im allgemeinen mehr verhindernd aus als solche, mit weniger funktionellen Gruppen oder mit geringer Größe.Die störenden Liganden förderten und stabilisierten die Bildung von Pseudoboehmit gegenüber kristallinen polymorphen Al(OH)3-Modifikationen in der folgenden Reihenfolge: Chlorid < Sulfat < Phtalat ≅ Succinat < Glutamat < Silikat < Asperat < Phosphat < Salicylat ≅ Malat < Tannat < Citrat < Tartrat. Der optimale Bereich der Ligand/Al-Molverhältnisse für die Bildung von Pseudoboehmit variierte, z.B. von 0,005–0,015 für Tartrat bis 600–1000 für Chlorid. Pseudoboehmit wurde in Gegenwart von Fluorit nicht gebildet. [U.W.]RésuméDes produits d’aluminium hydroxide et oxyhydroxide ont été formés à une concentration initiale d’Al de 2 × 10-3 M, au pH 8,2 et à des concentrations variées de ligands organiques et inorganiques trouvés communément dans la nature. On a trouvé que l’efficacité des ligands à promouvoir la formation de produits non-cristallins plutôt que des polymorphes Al(OH)3 cristallins était dans l’ordre suivant: phthalate ≅ succinate < glutamate < aspartate < oxalate < silicate ≅ fluoride < phosphate < salicylate ≅ malate < tannate < citrate < tartrate. La proportion molaire ligand/Al la plus basse à laquelle la production d’hydroxides Al ou d’hydroxides Al a été inhibée s’étendait de 0,02 à 15. Au dessus des proportions ligand/Al critiques, les produits cristallins étaient inhibés et les ligands ont coprécipite avec des produits non-cristallins qui sont restés inchangés pendant au moins 5 mois. Les ligands polydentates ou larges étaient généralement plus inhibants que ceux avec moins de groupes fonctionnels ou de plus petite taille.Les ligands perturbants ont promu et stabilisé la formation de pseudoboéhmite relativement aux polymorphes cristallins Al(OH)3 selon la séquence suivante: chloride < sulphate < phthalate ≅ succinate < glutamate < silicate < aspartate < phosphate < salicylate ≅ malate < tannate < citrate < tartrate. L’étendue optimale des proportions molaires ligand/Al pour la formation de pseudoboéhmite a varié, par exemple, de 0,005–0,015 pour la tartrate à 600–1000 pour la chloride. La pseudoboéhmite n’a pas été formée en la présence de fluoride. [D.J.]

pH effect on kinetics of adsorption of organic and inorganic phosphates by short-range ordered aluminum and iron precipitates

Abstract pH is an important factor affecting the adsorption of inorganic and organic phosphate compounds by soils and their components. The effects of solution pH on the kinetics of adsorption of orthophosphate (Pi), inositol hexaphosphate (IHP) and glucose 6-phosphate (G6P) by short-range ordered Al and Fe precipitates were investigated under pH-stat conditions. The adsorption reaction was monitored from 0.083 to 6 h at pH 4.5, 5.5 and 6.5 for the Al precipitate system, and 4.0, 5.0 and 6.0 for the Fe precipitate system, respectively. The adsorption reaction was described by first-order kinetics and divided into two stages, namely, fast adsorption before 0.5 h and slow adsorption after 0.5 h. Although the rate of adsorption of both organic and inorganic phosphates by Al and Fe precipitates generally decreased as pH increased, the individual reaction systems responded differently with pH changes. The pH effect was evident in the adsorption of Pi by both precipitates and IHP by Al precipitate. There was little or null pH effect on the adsorption of G6P by both precipitates. The proportion of P adsorbed over 6 h that was adsorbed at the end of 0.083 h decreased with increasing pH, suggesting that pH had more influence on the adsorption reaction at the beginning than at the later stage. The relatively low exchange ratio of OH to P for the Pi adsorption compared with the adsorption of organic phosphates was attributed to the dissociation of more protons from Pi ions than from IHP and G6P. The information on release kinetics of the surface hydroxyl groups strongly indicated that aquo groups on the surfaces were more reactive than hydroxo groups to phosphates and dominated the fast adsorption of phosphates.

Cadmium uptake and bioaccumulation in selected cultivars of durum wheat and flax as affected by soil type

Accumulation of cadmium (Cd) in crop plants is of great concern due to the potential for food chain contamination through the soil-root interface. Although Cd uptake varies considerably with plant species, the processes which determine the accumulation of Cd in plant tissues are affected by soil factors. The influence of soil type on Cd uptake by durum wheat (Triticum turgidum var. durum L.) and flax (Linum usitatissimum L.) was studied in a pot experiment under environmentally controlled growth chamber conditions. Four cultivars/lines of durum wheat (Kyle, Sceptre, DT 627, and DT 637) and three cultivars/lines of flax (Flanders, AC Emerson, and YSED 2) were grown in two Saskatchewan soils: an Orthic Gray Luvisol (low background Cd concentration; total/ABDTPA extractable Cd: 0.12/0.03 mg kg-1, respectively) and a Dark Brown Chernozem (relatively high background Cd concentration; total/ABDTPA Cd: 0.34/0.17 mg kg-1 respectively). Plant roots, stems, newly developed heads, and grain/seeds were analyzed for Cd concentration at three stages of plant growth: two and seven weeks after germination, and at plant maturity. The results showed that Cd bioaccumulation and distribution within the plants were strongly affected by both soil type and plant cultivar/line. The Cd concentration in roots leaves and stems varied at different stages of plant growth. However, all cultivars of both plant species grown in the Chernozemic soil accumulated more Cd in grain/seeds than plants grown in the Orthic Gray Luvisol soil. The different Cd accumulation pattern also corresponded to the levels of ABDTPA extractable and metal-organic complex bound soil Cd found in both soils. Large differences were found in grain Cd among the durum wheat cultivars grown in the same soil type, suggesting the importance of rhizosphere processes in Cd bioaccumulation and/or Cd transport processes within the plant. Distribution of Cd in parts of mature plants showed that durum grain contained up to 21 and 36% of the total amount of Cd taken up by the plants for the Orthic Gray Luvisol and Chernozemic soils, respectively. These results indicate the importance of studying Cd speciation, bioaccumulation and cycling in the environment for the management of agricultural soils and crops.

Oxidation and sorption of arsenite by manganese dioxide as influenced by surface coatings of iron and aluminum oxides and calcium carbonate

Manganese dioxide (birnessite) was coated with two levels of Fe and Al oxides and CaCO3, and the influence of these coatings on the surface features and the reactivity of MnO2 with respect to the oxidation and sorption of As(III) (arsenite) was examined.For all untreated and coated MnO2 samples, the depletion (oxidation plus sorption) of As(III) by the samples follows first-order kinetics. The rate constants are smaller for the samples with the high levels of coating of Fe and Al oxides and CaCO3 on MnO2 than they are for the untreated MnO2 and the MnO2 with the low levels of coating. The extent of masking of the electron-accepting sites on the MnO2 for converting the toxic As(III) to the less toxic As(V) significantly varies with the kinds and levels of coatings.Coatings of Fe and A1 oxides and CaCO3, on MnO2 distinctively affect the sorption of As. Manganese oxide evidently catalyzes the sorption of As by Al oxide through oxidation of As(III) to As(V). The relative affinities of the oxides of Mn, Fe, and Al and CaCO3, toward As(III) and As(V) account for the coating effects.

Soil Mineral–Organic Matter--Microorganism Interactions: Fundamentals and Impacts

Abstract Minerals, organic matter, and microorganisms are integral parts of the pedosphere and related environments. These three components are not separate entities but rather a united system constantly in association and interactions with each other in the terrestrial environment. Interactions of these components mediated by soil solution and atmosphere govern mechanisms of mineral weathering reactions, formation of short-range ordered (SRO) metal oxides, abiotic catalysis of the formation of humic substances, formation of organo-mineral complexes, microbial ecology, enzymatic activity, soil structure stability, dynamics of aggregate turnover, biogeochemical cycling of C, N, P, and S, and transformations and dynamics of metals and organic pollutants in the terrestrial environment. Foreseeable impacts of these interactions include global ion cycling and climate change, biodiversity, biological productivity and human nutrition, geomedicine, ecotoxicology and human health, biotechnology in relation to food production and security, risk assessment, and ecosystem restoration. Therefore, soil mineral–organic matter–microorganism interactions play a key role in influencing agricultural sustainability and ecosystem health. Fundamental understanding of these interactions at the molecular level is essential for developing innovative management strategies for land and water resources.

A new soil test method for the determination of plant-available cadmium in soils

Abstract A new soil test procedure using 1M NH4Cl was developed for the extraction of plant‐available cadmium (Cd) from soils. Five grams of soil is weighed into a 50‐mL polyethylene vial to which 30 mL of 1M NH4Cl solution is added. The soil suspension is then shaken on a horizontal shaker for 16 h at 25°C at 180 cycles per min. The suspension is then centrifuged at 2,500g for 5 min and the supernatant filtered through a 0.45 μm nitrocellulose filter under vacuum. Cadmium in the extract is then determined at 228.8 nm on a graphite furnace equipped atomic absorption spectrophotometer. A highly significant correlation was observed between the natural logarithm (In) of 1M NH4Cl‐extractable Cd in soils and the Cd content in the grain of durum wheat (Triticum turgidum var. durutn L.) grown on the same soils (r = 0.974, p = 3.8 x 10‐7). In comparison with several commonly used extradants, such as ABDTPA, CaCl2, NH4OAc, and NH4NO3, the 1M NH4Cl‐extracted Cd from soils was found to be a better index of Cd availa...