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Soil and Sediment Contamination: AnInternational JournalPublication details, including instructions for authors andsubscription information:http://www.tandfonline.com/loi/bssc20

Effectiveness of Triple SuperphosphateAmendments in Alleviating Soil LeadAccumulation in Missouri AlfisolsMichael Aide a , Kate Whitener a , Emily Westhoff a & Jennifer Kelleya

a Department of Agriculture , Southeast Missouri State University ,Cape Girardeau, MO, USAPublished online: 17 Oct 2008.

To cite this article: Michael Aide , Kate Whitener , Emily Westhoff & Jennifer Kelley (2008)Effectiveness of Triple Superphosphate Amendments in Alleviating Soil Lead Accumulation in MissouriAlfisols, Soil and Sediment Contamination: An International Journal, 17:6, 630-642

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Soil & Sediment Contamination, 17:630–642, 2008Copyright © Taylor & Francis Group, LLCISSN: 1532-0383 print / 1549-7887 onlineDOI: 10.1080/15320380802426533

Effectiveness of Triple Superphosphate Amendmentsin Alleviating Soil Lead Accumulation

in Missouri Alfisols

MICHAEL AIDE, KATE WHITENER, EMILY WESTHOFF,AND JENNIFER KELLEY

Department of Agriculture, Southeast Missouri State University, Cape Girardeau,MO, USA

Modern mining technologies reduce the environmental impact; however, previous miningactivities in Missouri have a legacy of soil contamination. This study focuses on resolvingPb-Cd-Zn soil contamination resulting from an abandoned Pb-Zn mine at St. Joe StatePark in Missouri by amending the soil surface with triple superphosphate (TSP) to reducethe availability of lead. Surface Pb concentrations ranged from 224 to 589 mg Pb kg−1

and the surface Cd concentrations ranged from 0.3 to 3.1 mg Cd kg−1. Two soils wereamended with three rates of TSP for each soil series. Soils were evaluated using selectiveextractions and a biological assay involving soybean (Glycine max Merr.) cultured in thegreenhouse, with the biomass and the Pb and Cd plant tissue concentrations determined.In general, soil exchangeable Pb concentrations and the Pb plant tissue concentrationsdid not support the premise that TSP treatment reduced the Pb biological availability.For the Caneyville series (Typic Hapludalfs) the TSP reacted preferentially with Ca,limiting the Pb alteration and reducing the effectiveness of TSP treatments on theseCa rich soils. For the more acidic Hildebrecht series (Oxyaquic Fragiudalfs), the TSPtreatments did not reduce plant accumulation of Pb, presumably because sufficient soilCa was present.

Keywords Cadmium, concentrated superphosphate

Terrestrial lead (Pb) abundances vary with the rock type, typically ranging from 0.1to 40 mg Pb kg−1 with an average crustal abundance is 15 mg Pb kg−1 (Kabata-Pendias,2001). Missouri Pb deposits occur primarily as galena (PbS), with sphalerite (ZnS) andpyrite (FeS2) as important auxiliary minerals (Thompson, 1995). The weathering of galenaprovides Pb2+, which may undergo hydrolysis and (1) react with carbonate materials, (2)adsorb on phyllosilicates and Fe-Mn oxyhydroxides, and (3) complex with soil organicmaterials (Essington, 2004).

The terrestrial cadmium (Cd) abundance is relatively small, ranging from undetectableto 0.55 mg Cd kg−1, with the mean Cd concentration estimated to be 0.27 mg Cd kg−1

(Kabata-Pendias, 2001). Cadmium is a relatively mobile element in soils; however, Cdmobility is reduced if the pH is greater than pH of 7.5, where precipitation reactions pre-dominate. In more acidic soils, adsorption on oxyhydroxides and bonding with soil organicmatter are the dominant reactions (Kabata-Pendias, 2001; Essington, 2004).

Address correspondence to Michael Aide, Department of Agriculture, Southeast Missouri StateUniversity, 1 University Plaza, Cape Girardeau, MO 63701, USA. E-mail: mtaide@semo.edu

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Effectiveness of Triple Superphosphate Amendments 631

Phosphate-containing materials (hydroxyapatite, rock phosphate, H3PO4, pyrophos-phate fertilizers) have been used effectively to precipitate Pb as either hydroxypyromorphite(Pb5(PO4)3OH), plumbogummite (PbAl3(PO4)2(OH)5(H2O)) or other Pb-bearing minerals(Ma et al., 1995; Ma, 1996; Ma and Rao, 1997; Ma et al., 1997; Yang et al., 2002; Brown et al.,2004; Essington, 2004). Ma et al. (1995) proposed that apatite dissolution resulted in the for-mation of fluoropyromorphite minerals (Pb10(PO4)6F2), whereas rock phosphate dissolutionresulted in a carbonate-bearing fluoropyromorphite mineral (Pb10(PO4)3(CO3)3FOH).

Scheckel and Ryan (2004) used X-ray adsorption fine structure spectroscopy to demon-strate that 45% of the soil lead in an impacted soil was precipitated as pyromorphite(Pb5(PO4)3Cl,OH,F). Fendorf et al. (2004) proposed that near-neutral soil pH conditionspromoted Cerusite-like mineral (PbCO3 or Pb3(OH)2(CO3)2) formation, whereas Kabalaand Singh (2001) and Mbila and Thompson (2004) showed that Zn and Pb resided primarilyin the residual and Fe-oxyhydroxide fractions.

Triple superphosphate (TSP as Ca(H2PO4)2H2O) should dissolve in soil to give H2PO−4 .

In the presence of calcite, TSP should act as a weak acid and form hydroxyapatite(Ca10(PO4)6(OH)2). In the presence of calcite, the aqueous HPO2−

4 activity should be suffi-ciently low that hydroxyapatite may be considered to be a substantial phosphate sink (Porteret al., 2004).

Using thermodynamic simulations involving hydrolysis and precipitation reactions,Porter et al. (2004) showed that sufficient phosphate must be supplied to convert Ca toapatite-like minerals and Pb to lead-phosphate minerals. Specifically, they demonstratedfrom a theoretical perspective that variscite (AlPO42H2O) and vivianite (Fe3(PO4)28H2O)are not able to lower the phosphate activity sufficiently, thus the conversion of galena (PbS)to pyromorphite (Pb5(PO4)3Cl,OH,F) is expected to proceed. Conversely hydroxyapatite(Ca10(PO4)6(OH)2) formation from calcite (CaCO3) or other labile Ca sources will suf-ficiently suppress the phosphate activity to inhibit pyromorphite formation. Similarly, anabundance of Mn2+ will inhibit pyromorphite formation because of MnHPO4 formation.Thus, P-induced Pb reductions in the presence of CaCO3 or large quantities of Mn2+ requiresufficient quantities of P to precipitate Ca as hydroxyapatite, Mn as MnHPO4 and Pb aspyromorphite.

The objective of this investigation is to assess the effectiveness of triple superphosphatefertilizer (0-45-0) to alter the soil Pb favorably. The effectiveness of the TSP amendmentswere evaluated using: (1) an aqua regia digestion to estimate the natural background con-centrations of Pb; (2) selective extractions to estimate the concentrations of Pb associatedwith soluble, exchangeable, organic, carbonate and residual soil environments; and (3) plantaccumulation of Pb and Cd in a greenhouse experiment to evaluate plant availability.

Materials and Methods

Study Area

The study area is located in St. Joe State Park, (St. Francois County, Missouri), approxi-mately 1 to 3 km from an abandoned Pb-Zn mine reprocessing site. The Caneyville series(Fine, mixed, active, mesic Typic Hapludalfs) consist of moderately deep, well-drained siltloam soils having A - E - Bt1 - Bt2 - dolomite sequences. The parent materials consist ofloess over clayey residuum derived from the weathering of dolomitic limestone (Brown,1981). The Hildebrecht series (Fine-silty, mixed, active, mesic Oxyaquic Fragiudalfs) con-sist of very deep, moderately well-drained soils having A - E - Bt - 2Ex - 2Btx - 3Bt horizonsequences formed in loess over dolomitic residuum (Brown, 1981).

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632 M. Aide et al.

Field Plot Experiments

One site of a Caneyville series and one site of the Hildebrecht series were selected and three3 m × 3m (9 m2) plots were established. Triple superphosphate (0 - 45 - 0), approximately20% P, was applied at 0, 0.9, 1.8 kg/plot (0, 2, 4 lbs/plot) on 20 August 2005 for theCaneyville site and December 2007 for the Hildebrecht site to give approximately 0, 200,and 400 kg P/ha. These TSP rates are higher than those typically used for agricultural usage,but small enough not to disturb the existing forest vegetation. The TSP was broadcastedwithout soil incorporation.

Plot sampling occurred on May 2007. Plot sampling consisted of the manual removalof litter having recognizable fiber structures and excavating a 1m × 1m area to a depth ofthe A1 boundary. The next deeper soil horizons (A2 horizon for the Caneyville site and Ehorizon for the Hildebrecht site) were sampled similarly. Each of these site samples werebulk homogenized after air-drying.

Laboratory Analysis

Soil pH in water, exchangeable cations, total acidity and organic matter content by losson ignition (LOI) were assessed using protocols in Carter (1993). The clay, silt and sandfractions were separated by Na-saturation of the exchange complex, washing with water-methanol mixtures, dispersion in Na2CO3(pH 9.2), followed by centrifuge fractionationand wet sieving (Carter, 1993). Two molar acetic acid extractable SO4-S and Bray-1 PO4-P concentrations were determined by the soil testing laboratory at University Missouri-Columbia Delta Center.

Exchangeable Pb was estimated from a 1 M KCl extraction. Duplicated 5 g sampleswere equilibrated with 0.025 L of 1 M KCl for 24 hours with occasional shaking. The sus-pension was filtered and analyzed for Pb using flame atomic absorption spectrophotometry(FAA). A buffered acetic acid solution (0.025 L of 0.5 M Na-acetate with acetic acid togive a pH of 5.0) was equilibrated with duplicated 5 g samples to estimate Pb associatedwith the exchangeable, organic and carbonate soil environments. Lead as determined usingFAA after filtration.

An aqua-regia digestion was performed to estimate the near total concentrations of Pb,Cd, Zn Fe, Mn, S, and other elements. In this procedure, 0.25 g of finely ground fine earthfraction was digested in 0.01 L of aqua regia (1 HCl:3HNO3) for one hour, followed by 0.45μm filtering, with a portion analyzed using inductively coupled plasma-atomic emissionspectrometry (ICP-AES).

A Na-pyrophosphate extraction was performed to estimate selected elements associatedwith soluble, exchangeable, and organically complexed soil components (Shuman, 1991).The metals recovered by a pyrophosphate extraction are considered potentially availablefor plant uptake or release into the aqueous or exchangeable soil fractions. Well homog-enized samples (0.75 g) were equilibrated with 0.01 L of 0.1 M Na-pyrophosphate (pH10) solution in a 35◦C incubator for 24 hours. Samples were shaken, centrifuged and fil-tered, with a known volume analyzed by inductively coupled plasma-mass spectrometry(ICP-MS).

A water extraction was performed to recover only the most labile or potentially labilefractions. A hot water extraction involved equilibrating 0.5 g samples in 0.02 L distilled-deionized water at 80◦C for one hour followed by 0.45μm filtering and elemental determi-nation using ICP-MS. The aqua regia digestion and the Na-pyrophosphate and the waterextraction procedures were performed by Activation Laboratories (Toronto, Canada). For

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Effectiveness of Triple Superphosphate Amendments 633

each of the three protocols, known reference materials and selected duplicates were per-formed to guarantee analytical accuracy. The detection limits, as shown subsequently inTables 1, 3, 4 and 5 are certified by Activation Laboratories. Soil site mean separationinvolved Student t confidence intervals specified at alpha = 0.05 using Excel.

Lead Activity and Simulated Solubility

The Pb activity was estimated using the solution compositions from sample suspensions,followed by use of chemical speciation software. Samples (1.5 grams) were equilibratedwith 4 × 10−2 M NaNO3 solution for ionic strength stability. The suspensions wereallowed to equilibrate for 48 hours at 27◦C, then the suspensions were centrifuged, filtered(<0.45 um) and the filtrate was analyzed for Na, K, Ca, Mg, Al, Fe, P, Zn and S usingICP-AES and Pb using ICP-MS.

MinteqA2 (version 1.50) is an EPA developed equilibrium speciation model designedto simulate element speciation, with capacities for simulating precipitation, adsorption, andother pertinent soil chemical processes (Allison et al., 1991). MinteqA2 may be obtainedfrom Allison Geoscience Consultants, Inc. and HydroGeoLogic Inc. This simulation isintended to illustrate potential relationships involving element speciation and possible pre-cipitation reactions that may control the soil solution equilibria. Minteqa2 input parametersincluded the mean solution composition (mol/L), the mean pH and the atmospheric pres-sure of CO2 (0.00034 bar). Activity coefficients were estimated using the Davis equation(Essington, 2004).

Greenhouse Experiment

To estimate Pb accumulation by plants a greenhouse experiment was implemented havinga completely randomized design. For each soil site, the main treatment consisted of thesurface horizons receiving the three rates of TSP. All treatments were replicated four times.Soybean seeds (Glycine max Merr., variety FFR-4545-RR) were planted and allowed togrow until most plants had three fully developed trifoliolate leaves. A climate controlledgreenhouse maintained the temperature at 35◦C. Watering was performed daily to maintainthe soil water content near field capacity.

Harvest consisted of clipping the aboveground portion at the cotyledon node, and dryingthe tissue at 70◦C for 24 hours. Roots were removed from the soil, distilled water washedon a 32 mesh sieve until visibly free of any adhering soil material and then washed for anadditional 2 minutes. The root tissues were also dried at 70◦C for 24 hours.

The aboveground oven-dried plant tissues were weighed in covered ceramic cruciblesand were muffle furnace ashed at 450◦C, equilibrated in reagent grade HCl and analyzedfor P, K, S, Ca, Mg, Na, Fe, Mn, Cu, Zn, B and Mo by ICP-AES and Pb and Cd by ICP-MS.The aboveground tissues samples were analyzed by Midwest Laboratories (Omaha, NE).The roots were ashed at 450◦C, equilibrated in reagent grade HCl, and analyzed for Pb andCd by air-acetylene atomic absorption spectrometry. Statistical differences were estimatedusing Analysis of Variance and Fisher’s least significant difference (alpha = 0.05) test usingExcel.

Results

Characterization of the Caneyville Pedons

The upper two soil horizons are important to the study because the likelihood of P leachingor percolation to deeper soil regions is limited. The A1 and A2 horizons have a moderately

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Effectiveness of Triple Superphosphate Amendments 635

Table 2Particle size distribution of the triple superphosphate (TSP) amended sites

TSP Treatment Sand SiltHorizon % % % Clay Texture

CaneyvilleA1 0 66 27 7 Sandy loamA1 1 61 24 15 Sandy loamA1 2 56 33 11 Sandy loamA2 0 61 32 7 Sandy loamA2 1 50 44 6 LoamA2 2 59 33 8 Sandy loam

HildebrechtA 0 39 54 7 Silt loamA 1 41 46 13 LoamA 2 41 56 3 Silt loamE 0 28 58 14 Silt loamE 1 47 48 5 LoamE 2 24 66 10 Silt loam

TSP Treatments: 0 is the control, 1 is 200 kg P/ha, and 2 is 400 kg P/ha.

alkaline pH, with Ca as the dominant exchangeable base-cation (Table 1). The texturesare generally sandy loam (Table 2). The CEC ranges from 11.6 to 14 cmolc/kg with CECdifferences most likely attributed to the natural soil variation. The loss on ignition values,as estimates for the soil organic matter contents, are substantial and consist primarily ofparticulate and fine root materials with a smaller portion considered to be humus.

Aqua regia extractable Ca and Mg concentrations are substantially greater than theexchangeable Ca and Mg concentrations. The large Ca and Mg concentrations, their nearly1-to-1 molar ratios, and the moderately alkaline pH levels infer that dolomitic dust hasimpacted the sites. The aqua regia P levels and the Bray-1 P levels reflect the TSP treat-ments, with the aqua regia P concentrations greatly exceeding the corresponding Bray-1 Pconcentrations. Aqua regia S levels are also substantial, inferring that galena (PbS) may bepresent.

Characterization of the Hildebrecht Pedon

The silt loam A and E horizons are strongly acid to medium acid, with the total acidityrepresenting a considerable portion of the exchange complex. Calcium is the dominantexchangeable base-cation, with Mg being an important secondary base-cation. The CEC islow to medium, with CEC variations attributed to the natural soil variation. The soil organicmatter contents are similar to those of the Caneyville site, consisting mostly of particulateand fine root materials.

The aqua regia Ca, Mg and S concentrations are very much smaller than the corre-sponding concentrations observed for the Caneyville site (Table 1). Given the smaller aquaregia Ca, Mg, and S concentrations, coupled with the acidic pH levels, the Hildebrechtsites appear to be considerably less impacted by carbonate mine-tailing dust. The aquaregia P and Bray-1 P concentrations generally reflect the TSP treatments, with aqua regiaP concentrations being considerably greater than the Bray1-P concentrations.

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636 M. Aide et al.

Table 3Mean Pb extraction values from various extracting agents involving triple superphosphate

(TSP) amended plots

TSP Acetic AquaHorizon Treatment Exchangeable Acid (pH5.0) Pyrophosphate Regia Water

mg/kgCaneyville

A1 0 7 40 24 638 0.5A1 1 7 28 nd 775 0.8A1 2 11 27 nd 699 0.7A2 0 8 62 14 671 0.5A2 1 6 50 nd 685 0.6A2 2 9 35 nd 743 0.4

Mean 8 40 nd 701 0.6Confidence 1.3 7.5 nd 50 0.2

(alpha = 0.05)Hildebrecht

A 0 14 19 7 118 0.3A 1 13 11 nd 94 0.2A 2 10 7 nd 135 0.3E 0 21 14 4 69 0.2E 1 17 13 nd 115 0.3E 2 16 20 nd 136 0.3

Mean 15 14 nd 111 0.3Confidence (alpha = 0.05) 2.4 4.3 nd 26 0.1

Detection Limits 4 4 0.01 0.01 0.01

TSP Treatments: 0 is the control, 1 is 200 kg P/ha, and 2 is 400 kg P/ha.Confidence is the confidence interval with alpha placed at 0.05 (95% confidence interval).nd = not determined.

Aqua Regia and Selective Extractions for Pb

Aqua regia recovered a mean Pb concentration of 701 mg/kg for the Caneyville site and 111mg/kg for the Hildebrecht site (Table 3). Concentration differences between the A1 and A2horizons in the Caneyville site and the A and E horizons in the Hildebrecht site were notsignificantly different and concentration differences because of the TSP amendments areattributed largely to the natural background variation.

Exchangeable Pb averaged 8 mg/kg in the Caneyville site and 14 mg/kg in the Hilde-brecht site, concentrations much smaller than those of the aqua regia digestion. We speculatethat the significantly greater exchangeable Pb concentrations in the Hildebrecht site are at-tributed to the more acidic pH and the smaller likelihood of hydrolysis assisted adsorption(Essington, 2004). The acetic acid buffer extraction recovered greater Pb concentrationsthan the KCl-exchangeable Pb fraction, with the Caneyville site averaging 40 mg Pb/kgand the Hildebrecht site averaging 14 mg Pb/kg. The greater acetic acid recoverable Pbconcentrations in the Caneyville site reflect greater carbonate dust deposition, correspond-ing with the greater aqua regia Pb concentrations. The acetic acid buffer extraction alsomay release Pb adsorbed on Fe-oxyhydroxides or complexed with soil organic matter. The

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magnitude of the acetic acid recoverable Pb concentrations, when contrasted with the aquaregia digestion concentrations, suggest that any Pb-bearing carbonate dust particles in theCaneyville site likely contain a substantial portion of the Pb as PbS, which is not recoverableby the acetic acid buffer. The pyrophosphate extraction (performed only on the control sites)indicated that only a small portion of the Pb exists as organic complexes. Water-extractionPb concentration values are small, suggesting that only minor quantities of Pb are availablefor interhorizon soil profile transport.

Aqua Regia Extraction of Selected Elements

Iron is the dominant metal in the near-surface horizons, with the Fe concentrations rangingfrom 22,600 to 27,800 mg Fe/kg in the Caneyville site and ranging from 10,300 to 12,600mg Fe/kg in the Hildebrecht site (Table 4). Manganese is the second most abundant metal,ranging from 2,570 to 2,990 mg Mn/kg in the Caneyville site and ranging from 210 to 484mg Mn/kg in the Hildebrecht site. The greater Fe and Mn concentrations in the Caneyvillesite may reflect some accumulation from mining activities; however, the majority of theFe and Mn concentrations in the Caneyville site more likely reflect natural accumulationbecause of pedogenic processes and parent material inheritances.

In general, the Caneyville site has greater concentrations of Co, Cu, Zn, Ni, As, Ag, In,and Cd than the Hildebrecht site, whereas the Hildebrecht site has greater Se concentrations.A portion of these element concentrations may be attributed to mining activities; however,the concentration of Co, Cu, Zn, Ni and As are somewhat typical for loess-derived soils(Kabata-Pendias, 2001) and are not considered an environmental threat. The elementalconcentrations of Ag, In and Cd in the Caneyville site are an order of magnitude greaterthan those of the less-impacted Hildebrecht site and are likely accumulations because ofmining activities.

Water-Recoverable Elements

Manganese has the greatest water-recoverable metal concentration, with the surface horizonshaving greater Mn concentrations than the underlying A2 and E horizons (Table 5). TheMn concentrations have rather large standard deviation, largely reflecting the natural sitevariation. The Co, As, Se, and Ag concentrations are similar between the Caneyville andHildebrecht sites, whereas the Caneyville site has greater concentrations of Cu, Ni, Zn, Cdand In.

United States EPA maximum contaminant levels (MCL) for Cu, As, Cd and Se weretheoretically adjusted for sample weight and solution volume to convert the MCL unitsfrom mg/L to mg/kg to provide perspective for the water-extraction concentrations. Withthe exception of Cu from the Caneyville site, these four elements have water-extractable con-centrations smaller than the adjusted MCL concentrations. The Cu values for the Caneyvillesite are equal to or slightly greater than the adjusted MCL concentrations.

Soybean Element Uptake Patterns

The aboveground soybean biomass showed significantly different P concentrations, directlycorresponding to the TSP amendment rates (Table 6). The P tissue concentrations weresmaller for the Caneyville site, presumably because of the moderately alkaline reaction of thesoil and the possibility of octacalcium phosphate precipitation and P-carbonate adsorptionreactions. Sufficiency levels indicate that the P concentrations in the aboveground biomass

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640 M. Aide et al.

Table 6Mean soybean top growth tissue concentrations and Pb root concentration

TSPTreatment P K Mg Ca S Na Fe Mn B Cu Zn Pb Cd Root-Pb

g/kg mg/kgCaneyville

0 1.3a 18.8 6.3 10.9 1.6 58 121 54 36 6 69 4.1a 0.6 157a1 2.8a 17.7 7.4 13.4 2.2 33 116 49 50 7 87 6.4b 0.7 174a2 4.0b 17.1 7.2 12.1 2.1 40 96 41 42 6 74 7.4b 0.6 287b

Hildebrecht0 2.0a 22.6 4.9 7.6 2.8 65 123 184 54 6 85 15.4 0.6 1171 5.1a 25.9 4.6 8.1 3.3 35 124 263 50 8 67 13.7 0.5 802 8.3b 28.1 3.9 7.0 3.0 56 142 333 51 8 52 34.6 0.9 93

Sufficiency levelsUpper 6 25 6 22 6 — 300 100 60 30 80Lower 3 17 0.3 11 2.5 — 25 17 20 4 21

TSP Treatments: 0 is the control, 1 is 200 kg P/ha, and 2 is 400 kg P/ha.Within a location site, different letters in a column indicate significant differences attributed to

LSD having alpha of 0.05.Columns lacking letters are not significantly different within a location site.Sufficiency levels are from Tisdale et al., 1985. (Na, Pb and Cd are not plant essential elements).

for control plant tissues (0 rate of TSP) are deficient, corresponding with the low Bray-1P values. Sufficiency levels further indicate that the P concentrations from plant tissuesreceiving the highest rate of TSP show elevated P accumulation. The K, S, Fe, Mn, B, andCu concentrations do not reflect differences attributed to the TSP amendments and theirconcentrations indicate an optimal potential for plant growth and development (Tisdaleet al., 1985). Zinc concentrations are similar to the sufficiency levels; however, the Znconcentrations from the Hildebrecht site are inversely proportional to the P concentrations.The P-Zn plant uptake antagonism is a well known phenomenon (Tisdale et al., 1985).

The aboveground Pb tissue concentrations indicate greater Pb concentrations from theHildebrecht site than the Caneyville site, consistent with the greater KCl-exchangeablePb concentrations in the more acidic Hildebrecht site. Plant Pb concentrations were notsignificantly different among the TSP treatments for the Caneyville site. Interestingly, theaboveground Pb tissue concentrations from the Hildebrecht site were significantly greaterfor the highest TSP treatment than for the other treatments. Cadmium aboveground tissueconcentrations were not significantly different with site location or TSP treatment.

Root tissue Pb concentrations were generally greater in the Caneyville site, averaging209 mg Pb/kg. The root tissue Pb concentrations from the Hildebrecht site averaged 97mg Pb/kg. Root tissue Pb concentrations were significantly greater for the highest TSPamendment rate in the Caneyville site when compared with the other treatments, whereasthe Hildebrecht site showed no significant differences attributed to the TSP treatments.

The Pb Activity and Saturation Indices for Pb-Bearing Minerals

Surface horizon (A1 horizon) suspensions, having NaNO3 as a background electrolyte,showed very low Pb concentrations: (1) Caneyville (TSP control = 0.06 mg Pb/L, TSP

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Effectiveness of Triple Superphosphate Amendments 641

Rate 1 = 0.29 mg Pb/L, TSP Rate 2 = 0.09 mg Pb/L), and (2) Hildebrecht (TSP control= 0.02 mg Pb/L, TSP Rate 1 = 0.14 mg Pb/L, TSP Rate 2 = 0.14 mg Pb/L). For theCaneyville site (pH 8.1), MinteqA2 simulation estimated the Pb speciation to be 11% Pb2+,20% PbOH+, and 60% PbCO3 and other minor species, whereas the Hildebrecht site (pH5.5) estimated the Pb speciation as 77% Pb2+, 22% PbNO+

3 , and 1% other species. Saturationindices for the Caneyville site suggest that the Pb-bearing minerals plumbogummite, hy-droxylpyromorphite, and hydrocerrusite are favored thermodynamically. Hydroxyapatitealso is favored thermodynamically. Saturation indices for the Hildebrecht site show thatplumbogummite is thermodynamically favored and carbonate minerals are undersaturated.We acknowledge that saturation indices will not predict which reaction has precedence orif the reaction will occur. The dissolved organic carbon content was not determined, thusthe Al activity is likely smaller than that simulated by MinteqA2. Plumogummite is anAl-bearing mineral.

Porter et al. (2004) demonstrated that Ca and Mn may sufficiently react with P tolimit hydroxylpyromorphite or plumbogummite formation. Given the uncertainty involvedin artificially constructed soil suspensions, the SI values for the Caneyville site suggestthat either hydroxyapatite or octacalcium phosphate may be especially competitive in theirreactivity with phosphate, thereby limiting the desired reduction in the availability of Pb.For the Hildebrecht site, plumogummite is the only Pb-bearing mineral phase having asolution composition that would support its formation.

Conclusions

This study demonstrates that aeolian deposition of PbS-bearing carbonate materials hasaccumulated Pb in the near-surface horizons. The Caneyville and Hildebrecht sites havebeen impacted as shown by the aqua regia extractable Pb concentrations. The Hildebrechtsite has smaller aqua regia Pb concentrations; however, the acidic reaction of this site haspromoted a greater concentration of exchangeable Pb.

Water-soluble Pb concentrations of Cd and Pb are small, suggesting that the biologicalavailabilities are correspondingly small. Plant uptake rates of Pb and Cd are small, indicatingthat the overall Pb plant availability is low. The TSP treatments did not reduce the plantavailability of Pb, presumably because sufficient Ca exists to preferential react with the TSP.The use of TSP treatment to alleviate lead may need to be restricted to soil environmentsthat have low Ca activities.

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