Verhalten von Schwermetallen in Böden. 2. Extraktion mobiler Schwermetalle mittels CaCl2 und NH4NO3

Behaviour of heavy metals in soils. 2. Extraction of mobile heavy metals with CaCl₂ and NH₄NO₃ 156 soil samples from arable fields, grassland and forest stands were analysed for the CaCl₂^? and NH₄NO₃^? extractable contents of Cd, Zn, Mn, Cu and Pb. The average amounts of Cd, Zn, Cu and Pb extracted with CaCl₂ are higher compared with NH₄NO₃ whereas the relation for Mn is vice versa. The proportion of the NH₄NO₃^? extractable contents in percent of the CaCl₂^? extractable contents of Cd, Zn and Pb decrease with increasing pH, whereas the contents of Mn and Cu increase. Inspite of a differing extraction behaviour of the two salt solutions the CaCl₂^? and NH₄NO₃^? extractable amounts of Cd, Mn, Zn und Pb are highly correlated and can be converted one into another. The mobile (CaCl₂, NH₄NO₃) proportion of the corresponding total, EDTA and DTPA heavy metal contents is in close relation to the pH of the soils. Using CaCl₂ solution the threshold pH values for an increasing mobility decrease in the order Cd > Mn > Zn > Cu > Pb, using NH₄NO₃ as extractant the order is Mn > Cd > Zn > Cu > Pb. In the case of CaCl₂ as extractant soluble chloro-Cd-complexes will be formed so that the Cd mobility in soils will be overestimated in most cases.     

Soil micronutrient contents and relation to other soil properties in Ethiopia

Abstract

Alfisols, Vertisols, Inceptisols, Aridisols, Mollisols, and Entisols were sampled (0–30 cm) from 32 locations across Ethiopia. The soils were analyzed for copper (Cu), zinc (Zn), manganese (Mn), and iron (Fe) contents using 0.005 M diethylene triamine pentaacetic acid (DTPA), 0.05 M hydrochloric acid (HC1), and 0.02 M ethylene diamine tetraacetic acid (EDTA) extractants. EDTA extracted more of each micronutrient than DTPA, which extracted greater amounts than HC1. The quantities of EDTA and DTPA‐extractable micronutrients were significantly correlated, and were in the order: Mn>Fe>Cu>Zn. The order of HCl‐extractable micronutrients was Mn>Fe>Zn>Cu. Micronutrient contents of Mollisols, Vertisols, and Alfisols were usually greater than those of the other soils, and Entisols usually had the lowest micronutrient contents. The contents were mostly positively correlated with clay and Fe₂O₃ contents, but negatively correlated with soil pH and A1₂O₃contents. While comparison of DTPA‐ and EDTA‐extractable micronutrients with critical levels showed that most soils had adequate amounts of the micronutrients for crops, the amounts extracted by HC1 were below critical levels in most soils. Since the critical levels that were used in the comparisons were not established in Ethiopia, calibration of the soil contents of these micronutrients with crops grown in Ethiopia is required to identify the most suitable extractant(s).     

Plant Availability of Cadmium in Soils: I. Extractable Cadmium in Newly and Long-Term Cultivated Soils

Abstract

Effects of long-term use of phosphate fertilizers on extractable soil Cd in relation to its concentrations in plants were investigated. “Paired” soil samples were collected from newly and long-term cultivated fields and analyzed for Cd by extraction with NH₄OAc, DTPA, NH₄OAc-EDTA, NH₄NO₃, HCl and CaCl₂. Plant samples were also collected and analyzed for Cd. Significant differences in extractable Cd by all the extractants except NH₄NO₃ were observed between the newly and long-term cultivated soils. The Cd concentrations in plants were not increased by the elevated extractable Cd. Although significant relationships were observed between plant Cd and extractable soil Cd, none of the extractants used alone gave a good assessment of plant-available Cd for all the samples used in this study.     

Appraisal of Some Soil Tests for Zinc Availability to Late‐Sown Wheat Grown in Mollisols

Abstract

Five soil extractants, namely, 0.005 M diethylene triamine pentaacetic acid (DTPA) (pH 7.3), 0.005 M DTPA+1 M ammonium bicarbonate (pH 7.6), Mehlich 3, 0.01 M ethylene diamine tetraacetic acid (EDTA)+0.05 M ammonium carbonate (pH 8.6), and 1 M magnesium chloride (MgCl₂) (pH 6.0), were evaluated to predict the response of wheat to zinc (Zn) application in Mollisols. These extractants could be arranged in the following decreasing order of their Zn extracting power: Mehlich 3>0.005 M DTPA+1 M ammonium bicarbonate>0.01 M EDTA+0.05 M ammonium carbonate>0.005 M DTPA>1 M MgCl₂. The critical limits of Zn in soil, below which the yield response to late sown wheat (var. UP‐2338) to Zn application could be expected, were 0.57 mg 0.005 M DTPA (pH 7.3) extractable and 1.72 mg Mehlich 3–extractable Zn kg^?1 soil. The critical limit of Zn in whole shoot at 60 days after emergence was found to be 26.1 mg Zn kg^?1 plant tissue. The DTPA and Mehlich 3–extractable soil Zn also correlated significantly and positively with Zn concentration in whole shoot at 60 days after emergence and total Zn uptake by wheat at harvest.     

Plant Availability of Cadmium in Soils: II. Factors Related to the Extractability and Plant Uptake of Cadmium in Cultivated Soils

Abstract

Eighty four soil samples collected from southeastern Norway were analyzed for Cd by extraction with NH₄OAc, DTPA, NH₄OAc-EDTA, NH₄NO₃, HCl and CaCl₂. The total Cd, pH, exchangeable K and Ca, dithionite-extractable Mn, available P and fine sand (0.2–0.02 mm) contents were the principal factors related to the extractable Cd, with some inter-extractant variations. Cadmium extracted by NH₄NO₃, NH₄OAc, HCl and CaCl₂ decreased with increasing soil pH, but the Cd extracted by all the extractants increased with increasing total Cd, exchangeable K and Ca, available P, and Mn-oxide contents in the soils. The Cd concentrations in plants were significantly related to the extractable Cd, exchangeable Ca and Mg, pH, Mn-oxides and organic matter content.     

Verhalten von Schwermetallen in Böden 1. Untersuchungen zur Schwermetallmobilität

Behaviour of heavy metals in soils. 1. Heavy metal mobility 158 soil samples with widely varying composition were analysed for their total, EDTA, DTPA and CaCl₂ extractable contents of Cd, Zn, Mn, Cu and Pb. By means of single and multiple regressions the relations between the different heavy metal fractions and the pH, organic carbon and clay content were considered. The correlations between the total, EDTA and DTPA extractable contents are very close, whereas the CaCl₂ extractable contents are not or only weakly correlated with these fractions. According to these statistical results the former fractions are considered to be the total quantity (total content) and the reactive quantity (EDTA and DTPA extractable contents) of the heavy metals, whereas the CaCl₂ extractable fraction represents the mobile fraction of the heavy metals in soils. The multiple regressions show that the mobile content of heavy metals is closely correlated with each of the quantity fractions and with soil pH. In the same way the proportion of the mobile fraction (in %) of the total, EDTA and DTPA extractable heavy metal content of the soil samples is closely related to the soil pH. Hereby the proportion of the mobile content of the various elements increases in the pH range 6,5 - 3 below element-specific threshold pH values (in brackets) in the order Cd (6,5) > Mn (5,7) > Zn (5,3) > Cu (4,5) > Pb (3,5). In the pH range 6,5 - 7,5 mainly Cu and to a lesser degree also Pb show an increasing mobility due to the influence of soluble organic substances.     

Chemical Fractions of Mn in Acidic Soils and Selection of Suitable Soil Extractants for Assessing Mn Availability to Maize (Zea Mays L.)

Twenty surface (0–15 cm) samples of acidic soils were analyzed for water soluble (WS), exchangeable (EX), lead displaceable (Pb-disp.), acid soluble (AS), manganese (Mn) oxide occluded (MnOX), organically bound (OB), amorphous Fe oxide occluded (AFeOX), crystalline iron (Fe) oxide occluded (CFeOX) and residual (RES) fractions of Mn, and also for extractable Mn in some common soil extractants: (diethylenetriaminepentaacetic acid (DTPA) (pH 7.3), DTPA (pH 5.3), AB-DTPA (pH 7.6), Mehlich-3 (pH 2.0), Modified Olsen, 0.005 M calcium chloride (CaCl₂), 1 M magnesium chloride (MgCl₂) and ion exchange resins. The WS-Mn fraction showed a significant and positive correlation with Mn extractable in DTPA (pH 5.3) and AB-DTPA (pH 7.6), while both WS-Mn and EX-Mn fractions correlated significantly and positively with Mn concentration and uptake by maize plants grown in these soils. The AB-DTPA (pH 7.6) and DTPA (pH 5.3) appeared suitable to assess the availability of Mn in acidic soils.     

Selection of Suitable Extractant for Predicting the Response of Chickpea to Zinc Application in Vertisols

Twenty-four surface soils (0–15 cm) were collected from Tal land soils (vertisols) in the southern part of Bihar state in India. Six extractants were used to predict the extractability and their suitability for measuring available zinc (Zn) in these soils. Pot experiment with chickpea (Cicer arietinum cv. C-235) as test crop was conducted with five levels of Zn (0, 2.5, 5.0, 7.5, and 10.0 mg kg^?1) to determine critical levels of Zn in soils and chickpea. The efficiency rating of different extractants in extracting available Zn from soils followed the order ethylenediaminetetraacetic acid (EDTA)–ammonium carbonate [(NH₄)₂CO₃] > diethylenetriaminepentaacetic acid (DTPA)–ammonium bicarbonate (NH₄HCO₃) > DTPA– calcium chloride (CaCl₂) > DTPA–sodium bicarbonate (NaHCO₃) > magnesium nitrate [Mg (NO₃)₂] > magnesium chloride (MgCl₂). The DTPA-CaCl₂-extractable Zn was significantly and positively correlated with clay, organic carbon, cation exchange capacity, dry-matter yield, and plant Zn concentration and uptake but significantly and negatively correlated with soil pH. These properties attributed 87% variability in DTPA-CaCl₂-extractable Zn.     

Soil Test to Predict the Copper Availability in Pasture Soils

Abstract

The proportion of copper (Cu) that can be extracted by soil test extractants varied with the soil matrix. The plant‐available forms of Cu and the efficiency of various soil test extractants [(0.01 M Ca(NO₃)₂, 0.1 M NaNO₃, 0.01 M CaCl₂, 1.0 M NH₄NO₃, 0.1 M HCl, 0.02 M SrCl₂, Mehlich‐1 (M1), Mehlich‐3 (M3), and TEA‐DTPA.)] to predict the availability of Cu for two contrasting pasture soils were treated with two sources of Cu fertilizers (CuSO₄ and CuO). The efficiency of various chemical reagents in extracting the Cu from the soil followed this order: TEA‐DTPA>Mehlich‐3>Mehlich‐1>0.02 M SrCl₂>0.1 M HCl>1.0 M NH₄NO₃>0.01 M CaCl₂>0.1 M NaNO₃>0.01 M Ca(NO₃)₂. The ratios of exchangeable: organic: oxide bound: residual forms of Cu in M1, M3, and TEA‐DTPA for the Manawatu soil are 1:20:25:4, 1:14:8:2, and 1:56:35:8, respectively, and for the Ngamoka soil are 1:14:6:4, 1:9:5:2, and 1:55:26:17, respectively. The ratios of different forms of Cu suggest that the Cu is residing mainly in the organic form, and it decreases in the order: organic>oxide>residual>exchangeable. There was a highly significant relationship between the concentrations of Cu extracted by the three soil test extractants. The determination of the coefficients obtained from the regression relationship between the amounts of Cu extracted by M1, M3, and TEA‐DTPA reagents suggests that the behavior of extractants was similar. But M3 demonstrated a greater increase of Cu from the exchangeable form and organic complexes due to the dual activity of EDTA and acids for the different fractions and is best suited for predicting the available Cu in pasture soils.     

Extractable Concentrations of Cobalt from Serpentine Soils with Several Single‐Extraction Procedures

In this study, we selected three soil pedons on the shoulder, backslope, and footslope along a serpentine toposequence to measure cobalt (Co) extractability using six single‐extraction procedures. These extraction procedures are distilled water, 0.11 M acetic acid in the first step of the BCR sequential extraction (BCR1), 1 M ammonium acetate (NH₄OAc; pH 7.0), 0.01 M calcium chloride (CaCl₂), diethylenetriamine pentaacetic acid (DTPA), and 0.1 M hydrochloric acid (HCl). Although the Co concentrations in the water extracts of the study soils ranged from 0.15 to 0.93 mg kg^?1, those with HCl extraction can be up to 22.1 mg kg^?1. The extractable Co concentrations in the study soils demonstrate that the extraction capacity is in the order HCl > DTPA > CaCl₂ ? NH₄OAc > BCR1 > H₂O. The percentages of extractable Co after applying the six single‐extraction procedures reveal that Co mobility is greatest in the soils on the backslope, moderate on the footslope, and least mobile on the shoulder.     

Evaluation of Some Chelates in Estimating Available Zn for Soybean in Calcareous Soils

Six fertilizer trials on calcareous soils in Saudi Arabia were conducted for the prediction of Zn deficiency in soybean (Glycine max L., var Merr). Zinc level before planting was tested by using 3 different extractants, i.e. DTPA, AB-DTPA and EDTA. Zinc was applied in the form of ZnSO₄ · 7H₂O at 0, 5, 10, 15, 20 and 40 kg Zn ha^?1. Plant samples were taken at early bloom and tissue was analysed for Zn. Two methods were used to judge the critical deficiency level of Zn: Cate-Nelson and chisquare models. The critical level estimated according to the Cate-Nelson method for DTPA extractable soil Zn was 0.43 mg kg^?1 in the growing season 1991. EDTA gave a much higher level (1.80 mg kg^?1) and AB-DTPA gave an intermediate level (0.68 mg kg^?1). Chi-square statistical procedure gave a very similar critical level of 0.66 mg kg^?1 for AB-DTPA but lower for either DTPA (0.38 mg kg^?1) or EDTA (1.32 mg kg^?1). The critical level based for three growing seasons ranged from 0.25 to 0.68, 0.32 to 0.82 and 1.12 to 3.4 mg Zn kg^?1 for DTPA, AB-DTPA and EDTA extractants, respectively. The values obtained by the linear regression equation with soybean leaf concentration were 0.45 and 0.70 mg Zn kg^?1 for DTPA and AB-DTPA, respectively. Such values are very close to those determined by using the Cate-Nelson method. On the other hand, the value obtained for EDTA (1.15 mg Zn kg^?1) was comparatively lower than that calculated by applying the Cate-Nelson method.     

Selection of a soil extraction and a multiple regression model to predict plant available manganese

Abstract

The ability of seven extractants to predict Mn uptake by barley and soybeans was compared in the greenhouse. Predictive capacities were substantially improved by inclusion of other variables, particularly soil pH. There was no advantage to extraction of moist soil samples and thus air drying proved to be a suitable sample handling procedure.

Little difference in predictive capacity occurred between the equations based on the seven extractants but the extractants were ranked as DTPA + NaHCO₃ > DTPA + NH₄HCO₃ > H₃PO₄ > hydroquinone > NH₄PO₄ > CH‐COONH₄ > DTPA + CaCl₂. All of the equations developed were useful for soil testing procedures.     

Use of Single Extraction Methods to Predict Bioavailability of Heavy Metals in Polluted Soils to Rice

Human exposure to toxic heavy metals via dietary intake is of increasing concern. Heavy-metal pollution of a rice production system can pose a threat to human health. Thus, it was necessary to develop a suitable extraction procedure that would represent the content of metal available to rice plants (Oryza sativa L.). The aim of this study was to predict, on the basis of single extraction procedures of soil heavy metals, the accumulation of heavy metals (cadium, lead, copper, and zinc) in rice plants. Six extracting agents [Mehlich 1, Mehlich 3, EDTA (ethylenediaminetetraacetic acid), DTPA–TEA (diethylenetriaminepentaacetic acid–triethanolamine), ammonium acetate (NH₄OAc), and calcium chloride (CaCl₂)] were tested to evaluate the bioavailability of heavy metals from paddy soils contaminated with lead–zinc mine tailings to rice. The extraction capacity of the metals was found to be of the order EDTA > Mehlich 3 > Mehlich 1 > DTPA–TEA > NH₄OAc > CaCl₂. The correlation analysis between metals extracted with different extractants and concentrations of the metals in the grain and stalk of the plant showed positive correlations with all metals. The greatest values of correlation coefficients were determined between the NH₄OAc- and CaCl₂-soluble fractions of soil and contents in plants in all four metals studied. Therefore, NH₄OAc and CaCl₂ were the most suitable extractants for predicting bioavailability of heavy metals in the polluted soils to rice. The results suggested that uptake of heavy metals by rice was mostly from exchangeable and water-soluble fractions of the metals in the soils. Soil-extractable metals were more significantly correlated with metal accumulation in the stalk than in the grain. The pH had more significant influence on availability of heavy metals in the soils than total content of metals and other soil properties. The bioavailability of metals for rice plants would be high in acidic soils.     

Evaluation of universal extractants for determining plant available potassium in intensively cultivated soils

Abstract

Eighteen soils from northwestern Switzerland were used to study the value of seven universal extractants (CaCl₂; DB‐DTPA; Mehlich 1, 2, and 3; Morgan‐Wolf; and NH₄OAc‐EDTA) for predicting plant available potassium (K) as compared to a bioassay (a modified Neubauer test with winter rye). These extractants were evaluated on the basis of K uptake by the bioassay test and the soil K status. In order to create the sufficiency level of exchangeable K for plant growth, soils were treated with 0, 20, 40, 80, and 160 mg K/kg of soil. The range of K uptake by the bioassay tests was between 89.2 and 403.0 mg/kg of soil for the control pots, and 136.6 to 495.8 for the K treatments with optimal conditions for plant growth. The average amounts of K extracted by the seven universal extractants, in ascending order, were: CaCl₂ < Morgan‐Wolf < Mehlich 1 < Mehlich 2 < NH₄OAc‐EDTA < Mehlich 3 < DB‐DTPA. The highest simple correlation with K uptake versus the bioassay test was obtained with the DB‐DTPA (r = 0.89) extractant and the lowest with the Mehlich 1 (r = 0.53) extractant. The DP‐DTPA, NH₄OAc‐EDTA and Mehlich 3‐K procedures showed an advantage over K procedures based on water soluble and exchangeable K pools in the investigated soils in order to predict the amount of plant‐available K. A simple regression and the Cate‐Nelson graphic method offer the possibility of assessing the soil‐K status using K values obtained by these universal extractants and to calibrate them against K forms as follows: exchangeable, water soluble, and non‐exchangeable.     

An evaluation of chemical methods for extracting copper from rice soils

Abstract

Rice (Oryza sativaL. CV. Lemont) was grown on 19 soils, and eight extractants were evaluated for determining the availability of Cu to rice plants. Correlation analyses were employed as criteria for evaluating methods that would provide the best index of Cu availability. The order of removal of Cu from soils was: 0.5NHC1 + 0.05NA1C1₃> 0.5NHNO₃> 0.5 N HC1 > EDTA + NH₄OAc > 0.1NHC1 > EDTA + (NH₄)₂CO₃? DTPA‐TEA, pH 7.3 >>> 1 N NH₄0Ac, pH 4.8.

Uptake of Cu by rice plants was significantly correlated with soil Cu. Among the eight extractants evaluated, Cu extracted with DTPA‐TEA, pH 7.3 was better related to the concentration (r = 0.563 ) and uptake (r = 0.673 ) of Cu by rice plants grown on the soils with different chemical and physical properties.

A significant negative correlation was found between the concentration of Cu in rice plants and the organic matter content of the soils. Each one percent increase in the organic matter of the soils resulted in a corresponding decrease of approximately one mg/kg in the concentration of Cu in the rice‐plant tissue. Multiple regressions of extractable Cu by eight methods with soil organic matter content accounted for from 53.4 to 70.0% of the variations in the prediction of the concentration of Cu in the rice plants. Combinations of other soil chemical properties measured with extractable Cu did not significantly improve the predictability     

Crop uptake and extractability of cadmium in soils naturally high in metals at different pH levels

Abstract

A greenhouse experiment was conducted for three years to study the effect of different pH levels on metal concentrations in plants and the cadmium (Cd) extractability by DTPA and NH₄NO₃. The soils used were an alum shale (clay loam) and a moraine (loam), which were adjusted to pH levels of 5.5, 6.5, 7.0, and 7.5. Wheat (Triticum aestivum), carrot (Daucus carota L.), and lettuce (Lactuca sativa) were grown as test crops. Crop yields were not consistently affected at increasing soil pH levels. The concentration of Cd in plant species decreased with increasing soil pH in both soils and in all three years. Significant concentration differences between soil pH levels were only seen in wheat and carrot crops. Increasing soil pH also decreased the nickel (Ni) and zinc (Zn) concentrations in plants in the first year crop but the copper (Cu) concentration was not consistently affected by soil pH. The effect of pH was more pronounced in the moraine then the alum shale soil. The DTPA‐and NH₄NO₃‐extractable Cd was decreased with the increasing soil pH and the pH effect was more pronounced with NH₄NO₃ extractable Cd. Both extractants were found equally effective in relation to the Cd concentration in plants in this study.     

Monocot‐dicot variability in response to an iron oxide‐metallic iron source

Abstract

Iron from a mixture of Fe oxide and metallic Fe was more available to corn (Zea mays L.) than it was to soybeans when the plants wore grown in calcareous soil or in nutrient solution. All this Fe, however, was DTPA (diethylene triamine pentaacetic acid) extractable. In solution culture the Fe was available to the soybean (Glycine max L.) plants unless CaCO₃ was included in the nutrient solution.     

Evaluation of Universal Extractants for Determination of Some Macronutrients from Soil

Evaluation of nutrient status in soil is important for nutritional, environmental, and economical aspects. The objective of this work was to find out the most suitable universal extractant for determination of available phosphorus (P) and nitrate (NO₃-) and exchangeable potassium (K), calcium (Ca), and magnesium (Mg) from soils using 0.01 M calcium chloride (CaCl₂), 0.01 M barium chloride (BaCl₂), 0.1 M BaCl₂, 0.02 M strontium chloride (SrCl₂), Mehlich 3, and ammonium bicarbonate diethylene triamine penta acetic acid (AB-DTPA) extractants. Composite surface soil samples (0–20 cm) were collected from the Eastern Harage Zone (Babile and Haramaya Districts), Wolaita Zone (Damot Sore, Boloso Bombe, Damot Pulasa, and Humbo Districts), and Dire Dawa Administrative Council by purposive sampling. The experiment was carried out in a completely randomized design (CRD) with three replications. Results indicated that the greatest correlations were found between Mehlich 3 and Olsen method and also between 0.02 M SrCl₂ and Olsen method for available P. The amount of NO₃^– extracted by 0.02 M SrCl₂ was significantly correlated to the amount determined by 0.5 M potassium sulfate (K₂SO₄). The amounts of exchangeable K, Ca, and Mg determined by ammonium acetate (NH₄OAc) method were significantly correlated to the amount determined by universal extractants tested. In general, both 0.02 M SrCl₂ and Mehlich 3 can serve as universal extractants for the macronutrients considered in this study with the former being more economical when NO₃^– is included.     

Development of a New Soil Extractant for Simultaneous Phosphorus,Ammonium, and Nitrate Analysis

Abstract

A new soil extractant (H³A) with the ability to extract NH₄, NO₃, and P from soil was developed and tested against 32 soils, which varied greatly in clay content, organic carbon (C), and soil pH. The extractant (H³A) eliminates the need for separate phosphorus (P) extractants for acid and calcareous soils and maintains the extract pH, on average, within one unit of the soil pH. The extractant is composed of organic root exudates, lithium citrate, and two synthetic chelators (DTPA, EDTA). The new soil extractant was tested against Mehlich 3, Olsen, and water for extractable P, and 1 M KCl and water‐extractable NH₄ and NO₂/NO₃. The pH of the extractant after adding soil, shaking, and filtration was measured for each soil sample (5 extractants×2 reps×32 soils=320 samples) and was shown to be highly influential on extractable P but has no effect on extractable NH₄ or NO₂/NO₃. H³A was highly correlated with soil‐extractable inorganic N (NH₄, NO₂/NO₃) from both water (r=0.98) and 1 M KCl (r=0.97), as well as being significantly correlated with water (r=0.71), Mehlich 3 (r=0.83), and Olsen (r=0.84) for extractable P.     

Evaluation of Soil Tests for Plant‐available Mercury in a Soil–Crop Rotation System

A reliable soil test is needed for estimating mercury (Hg) availability to crop plants. In this study, four extraction procedures including 0.1 M hydrochloric acid (HCl), 1 M ammonium acetate (NH₄OAc) (pH 7.0), 0.005 M diethylenetriaminepentaacetic acid (DTPA), and 0.1 M calcium chloride (CaCl₂) (pH5.0) were compared for their adequacy in predicting soil Hg availability to crop plants of a rice–cabbage–radish rotation system. The amounts of Hg extracted by each of the four procedures increased with increasing equilibrium time. The optimal time required for extraction of soil Hg was approximately 30 min, though it varied slightly among the four extractants. The amounts of Hg extracted decreased with increasing soil/solution ratio, and a soil/solution ratio of 1:5 appeared to be adequate for soil Hg availability tests. The amounts of Hg extracted increased in the order of NH₄OAc < CaCl₂ < DTPA < HCl in silty loam soil (SLS) soil, and the order was NH₄OAc < CaCl₂ ≈ DTPA < HCl in yellowish red soil (YRS) soil. Significant positive correlations among the four extractants were obtained in SLS soil. In contrast, the correlations were poor in YRS soil, especially for HCl. There were significant correlations between concentrations of Hg in edible tissue of three plants and the amounts of soil Hg extractable to the four extractants for soil–rice system and soil–radish system, but not for soil–Chinese cabbage system. The 0.1M HCl extraction overall provided the best estimation of soil‐available Hg and could be used to predict phytoavailability of Hg in soil–crop systems.