Published in J. Environ. Qual. 33:373-379 (2004).
© ASA, CSSA, SSSA
677 S. Segoe Rd., Madison, WI 53711 USA
TECHNICAL REPORTS
Waste Management
Solubility of Phosphorus and Heavy Metals in Potting Media Amended with Yard Waste–Biosolids Compost
M. K. Zhanga,
Z. L. He*,a,b,
P. J. Stoffellab,
D. V. Calvertb,
X. E. Yanga,
Y. P. Xiaa and
S. B. Wilsonb
a College of Agriculture and Life Sciences, Zhejiang University, Huajiachi Campus, Hangzhou 310029, China
b University of Florida, Institute of Food and Agricultural Sciences, Indian River Research and Education Center, Fort Pierce, FL 34945-3138
* Corresponding author (zhe{at}mail.ifas.ufl.edu).
Received for publication February 26, 2003.
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ABSTRACT
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The potential risk of surface and ground water contamination by phosphorus (P) and heavy metals leached from compost-based containerized media has become an environmental concern. Solubility and fractionation of P and heavy metals were evaluated in media containing 0, 25, 50, 75, or 100% compost derived from biosolids and yard trimmings for potential impacts on the environment. As compost proportion in peat-based media increased from 0 to 100%, concentrations of total P, Cd, Cu, Ni, Pb, Zn, and Mn in the media increased whereas concentrations of total Co and Cr decreased. Except for Cu, all heavy metals in the water-soluble fraction decreased with increasing compost proportion in the media, because of higher Fe, Al, and Ca concentrations and pH values of the composts than the peat. When the media pH is controlled and maintained at normal range of plant growth (5.5–6.5), leaching of the heavy metals is minimal. Incorporation of compost to the peat-based media also decreased the proportion of total P that was water-soluble. However, concentrations of bioavailable inorganic phosphorus (NaHCO3–IP), readily mineralizable organic phosphorus (NaHCO3–OP), potentially bioavailable inorganic phosphorus (NaOH-IP), and potentially bioavailable organic phosphorus (NaOH-OP) were still higher in the media amended with compost because of higher total P concentration in the compost. Further study is needed to verify if less or no topdressing of chemical P fertilizer should be applied to the compost-amended media to minimize P effect on the environment when compost-amended potting media are used for nursery or greenhouse crop production systems.
Abbreviations: ICP–AES, inductively coupled plasma–atomic emission spectroscopy • IP, inorganic phosphorus • OP, organic phosphorus
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INTRODUCTION
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THE USE OF PEAT as a major constituent of potting media in the nursery industry is challenged by economic and environmental pressures. High-quality peat is becoming less available and high transportation costs have become a major component for containerized media. Composts derived from municipal wastes have become increasingly available for use in containerized media for plant production in greenhouses and nurseries (Bugbee and Frink, 1989; Fitzpatrick et al., 1998; Lamanna et al., 1991). A wide variety of composts have been reported to improve media used in the production of nursery and greenhouse crops (Bugbee et al., 1991; Goldstein and Steuteville, 1996; Sanderson, 1980; Wilson et al., 2001a, 2001b). Composts contain substantial amounts of nutrients that may improve plant growth and reduce the need for fertilizer, and thus may be economical partial substitutes for conventional media components such as peat. Fitzpatrick (2001) has reviewed and cited numerous investigations illustrating the beneficial effect of compost utilization in greenhouse and nursery crop production systems. Compost-based media had higher media stability, nitrogen mobilization, pH, electrical conductivity, bulk density, particle density, air-filled porosity, container capacity, and total porosity and lower carbon to nitrogen ratios (Wilson et al., 2001b, 2001c). The effects of media composition on plant growth and development varied with each species tested. Wilson et al. (2001c) found that golden shrimp plant (Pachystachys lutea Nees) grown in media with high volumes of compost (75–100%) had less leaf area and lower shoot and root dry weight compared with the controls (no compost), but the plant was considered marketable after eight weeks, regardless of percentage of compost composition (0–100%) in either peat or coir-based media. Guerin et al. (2001) observed the growth of laurustinus (Viburnum tinus L.) in peat-based and peat-substitute growing media, and found that substrates with yard compost or raw coir produced plants of similar size to those in the control substrate (peat and pine bark). Hartz et al. (1996) also observed that composted green yard and landscape waste could contribute to crop macronutrients and was equivalent or superior to peat as plant growth media. Hicklenton et al. (2001) studied effectiveness and consistency of source-separated municipal solid waste (MSW) and bark compost as components of container growing media, and found that heavy metal uptake was no greater in the MSW than in the bark media and there were no signs of nutrient deficiency or toxicity at any time during the study.
Movement of N, P, and heavy metals from containerized media can contaminate surface or ground water (Fytianos et al., 1998; Marconi and Nelson, 1984; Rathier and Frink, 1989; Sawhney et al., 1995; Yeager and Barrett, 1984), which may restrict the use of compost as a potting material. The leachability of P and heavy metals from containerized media containing compost should be determined to ensure that composts, or mixtures of compost and peat, can be safely used as growing media in a container nursery production system. The leachability of P and heavy metals from solids depends on chemical and physical associations and solubilities of the elements. A fundamental knowledge of P and heavy metal species is essential for predicting their leaching behavior in the media. Minimal information is available on the solubilities and chemical forms of P and heavy metals in media containing yard waste–biosolids compost. The objectives of this study were to (i) characterize P and heavy metal fractions in the peat media amended with various amounts of compost and (ii) determine the solubility of P and heavy metals in the media mixture with different pH values.
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MATERIALS AND METHODS
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The compost used for this study was obtained from a commercial facility that composts a 1:1 ratio (w/w) of biosolids (sewage sludge) and yard trimmings (screened to 1.3 cm) for 18 d in an agitated bed system. The compost is stockpiled and then rescreened to 1.3 cm. Commercial peat-based medium consisted of 70% peat, 20% perlite, and 10% vermiculite. The peat medium was amended with 0, 25, 50, 75, and 100% compost by dry volume and incubated for 14 d at room temperature. Mixed medium samples were air-dried and passed through a 2-mm sieve before analysis. The pH was measured in water at the solid to water ratio of 1:2 (w/w) using a pH/ion/conductivity meter (Accumet Model 50; Fisher Scientific, Pittsburgh, PA). The total P, Ca, Al, Fe, Cd, Co, Cr, Cu, Ni, Pb, Mn, and Zn concentrations in the media were determined using inductively coupled plasma–atomic emission spectroscopy (ICP–AES) (Ultima; J-Y Emission Divisional Instruments SA, Edison, NJ) following digestion in a mixture of concentrated H2SO4, HF, and HClO4 (Hossner, 1996). The detection limits of the ICP–AES are 11, 26, 21, 125, 30, 98, and 30 µg kg–1, respectively, for Cd, Co, Cr, Cu, Ni, Pb, and Zn. Total C was determined using a Vario MAX CN Macro Elemental Analyzer (Elemental Analysensystem GmbH, Hanau, Germany).
Selected chemical characteristics of the compost and peat-based media used in this study are shown in Table 1. Concentrations of Cd, Zn, Cu, Ni, and Pb in both compost and peat-based medium are below the USEPA Part 503 pollutant limits for land-applied biosolids (Walker, 2001). Wilson et al. (2001b)(2001c, 2002) evaluated growth of several plants grown on compost–peat mixture media with the same composition and source of compost and peat as this study, and found that plants grown in media with high volumes of compost (75 or 100%) were slightly smaller (stem weight, leaf weight, and stem length) compared with the controls (no compost). However, plants were still acceptable in terms of visual color and quality, regardless of media composition. The media amended with or without the compost amendment did not pose significant adverse effect on growth of the plants.
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Table 1. Chemical characteristics (mean ± standard deviation) of the compost and peat-based media used in this study.
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Phosphorus Fractionation
A modification of the method of Hedley et al. (1982) was adopted to determine P fractions in the media. Triplicate samples (each 0.5 g, oven-dry basis) were placed into 50-mL centrifuge tubes and sequentially extracted with 30 mL of deionized water, 0.5 M NaHCO3 (pH = 8.2), 0.1 M NaOH, and 1 M HCl. Each extraction lasted 16 h on an end-to-end shaker (180 cycles min–1). After each extraction, the tubes were centrifuged at 5000 x g (rcf) for 30 min and the supernatant was then passed through a Whatman (Maidstone, UK) 42 filter paper. The P concentrations in the filtrates from deionized water and HCl extracts and the inorganic phosphorus (IP) in the NaHCO3 and NaOH extracts were colorimetrically determined by the molybdenum-blue method (Kuo, 1996). Total P in the NaHCO3 and NaOH extracts was determined by the molybdenum-blue method after digestion with acidified ammonium persulfate (Greenberg, 1992). The organic phosphorus (OP) concentrations in the NaHCO3 and NaOH extracts were calculated from the difference between the total P and the IP contents in the extracts. Residual P was calculated by subtracting the sum of the above four types of extractable P from the total P content in the media. Based on the sequence of extractions, medium P fractions were referred to as water-soluble P (H2O-P), bioavailable inorganic P (NaHCO3–IP), readily mineralizable organic P (NaHCO3–OP), potentially bioavailable inorganic P (NaOH-IP), potentially bioavailable organic P (NaOH-OP), acid-soluble P (Ca-bound P and HCl-P), and residual P (Hedley et al., 1982).
Fractionation of Heavy Metals
A modification of the procedure of Amacher (1996) was employed to fractionate heavy metals in the media. A deionized water extraction step (shaking 1 h) characterized heavy metal water solubility. The sequential fractionation consists of (i) water-soluble (solid to water = 20); (ii) exchangeable by 0.1 M Mg(NO3)2 (solid to solution = 20); (iii) carbonate-associated (1 M sodium acetate at pH 5, solid to solution = 20); (iv) iron- and iron oxide–associated (0.2 M ammonium oxalate + 0.2 M oxalic acid + 0.1 M ascorbic acid at pH 3.3 in a boiling water bath for 30 min, solid to solution = 50); (v) organic matter–associated (30% H2O2 at pH 2, solid to solution = 50); and (vi) residual (digestion with nitric acid and perchloric acid). After each successive extraction, the supernatant solution was separated by centrifuging the suspension at 5000 rpm for 30 min and filtering through Whatman 42 filter paper. The concentrations of Cd, Co, Cr, Co, Ni, Pb, Mn, and Zn in the filtrate were determined using ICP–AES.
Effect of pH on Water Solubility of Phosphorus and Metals from the Media
Portions of 2-g (dry basis) medium samples were placed into 50-mL centrifuge tubes with 20 mL deionized water. The pH of the suspensions was adjusted to values ranging from 2 to 10 using 1 M HNO3 or NaOH for decrease or increase of pH, respectively. The volume of HNO3 and NaOH used for pH adjustment was recorded for each tube. Deionized water was added to yield a total of 40 mL solution (liquid to solid ratio of 20:1). After shaking for 24 h, the suspensions were centrifuged and filtered. The filtrates were used for measuring P, Cd, Co, Cr, Co, Ni, Pb, and Zn by ICP–AES and pH.
Successive Extraction
The water solubilities of P and heavy metals in the media were also evaluated by successive water extractions. Portions of the media, each containing 2.0 g of soil (oven dry basis), were placed into 50-mL centrifuge tubes and extracted with 40 mL of deionized water. This procedure was repeated for a total of five successive sequential extractions. Each extraction lasted for 16 h on an end-to-end shaker (180 cycles min–1). After each extraction, the tubes were centrifuged at 5000 x g (rcf) for 30 min, and the supernatant was filtered through a Whatman 42 filter paper. The first 10 mL of filtrate were discarded. The P and metal concentrations in the remaining filtrates were determined using ICP–AES. All the above measurements, including the studies of fractionation of P and heavy metals and pH effect, were run on triplicate samples.
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RESULTS AND DISCUSSION
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Phosphorus Distribution in Media
Sequential extractions of the media amended with varying rates of compost using deionized water, 0.5 M NaHCO3, M NaOH, and 1 M HCl indicated that a large proportion of P in the media was nonresidual (e.g., extractable in water, NaHCO3, and mild alkaline and acid extractants; Table 2). Concentrations of all the P fractions in the media increased with increasing compost rate. Total P, H2O-P, NaHCO3–IP, NaHCO3–OP, NaOH-IP, NaOH-OP, HCl-P, and residual P in the 100% compost medium were 17.6-, 1.8-, 10.8-, 25.3-, 81.1-, 13.6-, 37.6-, and 4.2-fold greater, respectively, than in the peat media (0% compost). Compost application significantly increased P concentration in the media. Of the total P, the percentages were: nonresidual, 94%; extractable as H2O-P, 2.1%; NaHCO3–IP, 12.6%; NaHCO3–OP, 1.8%; NaOH-IP, 30.6%; NaOH-OP, 5.3%; and HCl-P, 4.6% for 100% compost medium (Table 2). For the 0% compost medium, the percentages were: nonresidual extractable as H2O-P, 20.3%; NaHCO3–IP, 20.4%; NaHCO3–OP, 1.2%; NaOH-IP, 6.6%; NaOH-OP, 6.9%; and HCl-P, 19.4%. Compared with peat-based medium (0% compost), application of compost decreased percentages of total P in H2O-P and NaHCO3–IP fractions, and increased percentages of total P in the NaOH-IP and HCl-P fractions in the media. Absolute concentrations of H2O-P and NaHCO3–IP were still nevertheless greater in the media amended with compost because of higher total P concentration in the compost.
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Table 2. Mean pH and concentrations of various fractions of P in the media containing varying proportions of compost.
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Heavy Metal Distribution in Media
Potential release of heavy metals from the media was evaluated using a six-step sequential extraction procedure. The results indicate wide variations of heavy metals in different fractions (Table 3). As compost proportion in the media increased from 0 to 100%, concentrations of total Cd, Cu, Ni, Pb, Zn, and Mn increased, and concentrations of total Co and Cr tended to decrease, since the compost contained higher Cd, Cu, Ni, Zn, and Mn and lower Co and Cr than the peat (Table 1). Cadmium concentrations in CaCO3–, oxide-, organic matter–, and residual-associated fractions significantly increased with increasing compost, whereas Cd concentration in the water-soluble fraction decreased slightly from 0.03 mg kg–1 for the 0% compost to below detection limit (0.004 mg kg–1) for the 100% compost (Table 3). Cadmium concentration in the exchangeable fraction ranged from 0.07 to 0.09 mg kg–1 for all compost-containing media.
Cobalt concentrations in all fractions, except residual, were unaffected by compost added. The Co concentration in the residual fraction decreased with increasing compost rate. Chromium concentrations in water-soluble and exchangeable fractions were similar for all media, ranging from 0.02 to 0.03 mg kg–1 for water-soluble fraction and from 0.12 to 0.13 mg kg–1 for the exchangeable fraction. However, Cr concentrations in CaCO3–, oxide-, and organic matter–associated fractions significantly increased, and the Cr concentration in the residual-associated fraction decreased with increasing compost ratio.
Copper concentrations in all fractions, except exchangeable, increased with increasing compost. Nickel concentrations in water-soluble, exchangeable, and residual-associated fractions decreased, and those in CaCO3–, oxide-, and organic matter–associated fractions increased with increasing compost rate. Lead concentrations in the water-soluble fraction decreased, and those in other fractions increased, with increasing compost rate. Zinc concentrations decreased in water-soluble and exchangeable fractions, and increased in other fractions with increasing compost. Manganese concentrations decreased in water-soluble and exchangeable fractions, and increased in CaCO3–, oxide-, and organic matter–associated fractions with increasing compost. Except for Cu, all heavy metals in the water-soluble fraction decreased with increasing compost, suggesting that the application of compost tends to shift metals from mobile fractions to immobile fractions, thus decreasing potential leaching of heavy metals from the media.
Percentages of total metals that are water-soluble for the peat-based media were 5.0 (Cd), 0.5 (Co), 0.03 (Cr), 5.8 (Zn), 1.9 (Cu), 3.0 (Ni), 4.7 (Pb), and 7.1% (Mn). Percentages of total metals that are water soluble in media amended with varying rates of compost were 0.2 to 0.6 (Cd), 0.4 to 0.6 (Co), 0.04 to 0.1 (Cr), 0.4 to 1.7 (Zn), 0.7 to 0.8 (Cu), 1.5 to 2.0 (Ni), 0.3 to 0.5 (Pb), and 0.8 to 2.0% (Mn). Most of the heavy metals in all the media were in more stable fractions including CaCO3–, oxide-, organic matter–, and residual-associated fractions (Fig. 1)
. As compost addition increased, the proportions of water-soluble and exchangeable fractions for Cd, Cu, Ni, Pb, Zn, and Mn decreased considerably. Pichtel and Anderson (1997) studied land application of composted municipal solid waste (CMSW) and composted sewage sludge (CSS) and observed that the more labile fractions of Cr, Cu, Pb, and Zn typically decreased with application of CMSW and CSS.
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Fig. 1. Heavy metal distributions in different fractions (H2O, water-soluble; Exch, exchangeable; CaCO3, CaCO3–associated; Oxide, oxide-associated; OM, organic matter–associated; Res, residual-associated) for the media amended with varying proportions of compost.
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The reduced proportions of the water-soluble and exchangeable heavy metals in the media amended with composts can be attributed to the increased Fe, Al, and Ca concentrations, and pH value of the compost-amended media that results in the increased fixation of heavy metals and decreased solubilities of the metals. Basallote and Carrillo (2003) reported that application of composted wastes with high levels of oxides and organic matter could increase Cu and Zn retention. In a study of using municipal sewage sludge for the stabilization of soil contaminated by mining activities, Theodoratos et al. (2000) concluded that the decrease of Pb, Zn, and Cd solubilities in soil was attributed to metal retention in sewage sludge by adsorption and organic complexation mechanisms.
The sum of the water-soluble and exchangeable fraction can be used to reflect the maximum availability of metals in contaminated soils or sediments (Zhang et al., 2003). The proportions of available metals, including water-soluble and extractable fractions, were 3.9 to 8.9 (Cd), 4.4 to 6.5 (Co), 0.2 to 0.5 (Cr), 0.5 to 2.6 (Zn), 0.8 to 1.0 (Cu), 3.2 to 4.8 (Ni), 5.4 to 12.1 (Pb), and 5.0 to 10.5% (Mn) in the media amended with compost, and 17.7 (Cd), 4.2 (Co), 0.2 (Cr), 8.7 (Zn), 3.9 (Cu), 5.4 (Ni), 13.6 (Pb), and 25.5% (Mn) in the peat-based medium. Incorporation of compost into the peat-based media significantly decreased the proportions of available Cd, Zn, Cu, Ni, Pb, and Mn in the media (Fig. 1), suggesting that addition of compost to peat-based medium can decrease availability of the metals.
pH Effects on Water Solubility of Phosphorus and Heavy Metals
The effects of varying pH on P and heavy metal release in water from the media with varying application rates of compost are presented in Fig. 2
. For all elements, the amounts extracted depended highly on pH, and water solubility of all elements increased sharply with decreasing pH. The pH values at which a sharp increase in element concentration occurred were 4 to 5 for Cd, Cu, Co, Cr, Ni, Zn, and Pb, and 6.5 for P. At pH 5.0 to 8.0, solubilities of heavy metals, including Cd, Co, Cr, Pb, Zn, and Ni, were generally low, and the released percentages of Cd, Co, Cr, Cu, Ni, Pb, and Zn at pH 5 to 8 were <3.2, 1.4, 0.3, 2.9, 2.8, 1.4, and 2.5%, respectively. A possible explanation is that adsorption of the metal ions on hydrous oxide of Fe or Al, or coprecipitation with CaCO3 at this pH range, was high (Burns et al., 1999; Chuan et al., 1996; Martinez and McBride, 2001). The pH values of media amended with varying rates of compost ranged from 6.52 to 6.86 (Table 2), thus falling within this pH range. Media amended with compost have a relatively high pH value and, thus, minimize environmental pollution by soluble heavy metals. However, when pH is greater than 8.0, the solubility of Cu increased sharply, probably because of an increase in dissolution of organic complex Cu at high pH (Tack et al., 1996). The variation in the solubilities of heavy metals at different pH values was larger than the variation due to a difference in compost proportion. Thus, pH seems to be the dominating factor controlling water solubility of the metals.
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Fig. 2. Solubility of heavy metals and P in the media amended with varying proportions of compost in relation to pH.
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The solubility of P was generally high at all pH values. At pH 6.5 to 8.0, the percentage of P released ranged from 1.2 to 5.5% for the media amended with compost and 16.7 to 18.0% for the medium without the compost amendment. The percentages of P released at pH of <6.5 increased steadily with decreasing pH. Therefore, the difference in pH of media with (pH 6.52–6.86) and without compost (pH 5.44) does not affect significantly the solubilities of heavy metals, but the difference probably poses a significant influence on P solubility in the media. These results suggest that lower percentages of water-soluble P and NaCO3–IP in the media amended with compost may be partially due to increased pH in the media.
Ideal medium pH for growth of good-quality foliage plants was 5.5 to 6.5 (Conover and Poole, 1990). Leaching of heavy metals is minimal when the media pH is controlled and maintained at normal range of plant growth (5.5–6.5). However, high medium pH (>6.5) is a common cause of micronutrient deficiency for container-grown plants (Bishko et al., 2003). Therefore, when a high proportion of compost is applied, some acidic materials such as aluminum sulfate, elemental sulfur, and ferrous sulfate are required to correct an excessively high medium pH. Otherwise, the proportion of compost addition should be limited to less than 25%.
Leaching Potential of Phosphorus and Heavy Metals from Media
The concentrations of P and heavy metals in the media recovered by five successive water extractions can be used to indicate leaching potential of these elements. Leaching potential of the elements varied with compost rates and water extraction sequence. In the first extraction, soluble Cd, Co, Ni, Pb, Zn, and Mn decreased (52.3 to 3.1 µg kg–1 for Cd, 36.5 to 21.0 µg kg–1 for Co, 518.8 to 306.5 µg kg–1 for Ni, 417.7 to 83.4 µg kg–1 for Pb, 4.75 to 1.08 mg kg–1 for Zn, and 6.98 to 0.68 mg kg–1 for Mn), whereas Cr, Cu, and P increased (17.4 to 41.5 µg kg–1 for Cr, 0.68 to 1.86 mg kg–1 for Cu, and 189.7 to 241.8 mg kg–1 for P) with increasing compost (Fig. 3) . The various changes corresponded to levels of the compost's water-soluble fractions (Table 3). For the media amended with compost, extractable Cd and Pb were below detection limits (4.4 and 39.2 mg kg–1) in the second extraction, whereas extracted Co and Cr were below their detection limits in the third and fourth extractions. Extractable Ni, Zn, and Cu decreased steadily with extraction. These results suggest that application of compost to the peat-based media could decrease solubilities of Cd, Co, Ni, Pb, and Zn by increasing pH. The quantities of water-soluble heavy metals in the compost-amended media were very low, in spite of the increased total concentrations. There is an extremely low possibility for these metals to be leached. Therefore, the risk of heavy metal leaching from compost application is low. However, in the media that contained 
25% compost, the amount of extractable P was greater than that of the peat-based medium. The extractable P amounts were almost the same for the media amended with compost at the 50, 75, or 100% levels. Once the readily soluble P was removed from the compost-amended media, the extractable P amounts declined to a relatively constant level after the fourth extraction. In the fifth extraction, the P amounts extracted from the media amended with compost were 64 to 231 mg kg–1, which were still 419 to 656% higher than those of the peat-based medium.
The total amount of P released from the media amended with compost during the five extractions ranged from 610 to 805 mg kg–1, which accounted for 4.7 to 11.4% of the total P in the media. Five successive extractions released only 32.6% of the total P from the peat-based medium. The decrease in P release with extraction was small, even up to the last extraction, during which considerable amounts of P were released from most of the media. Phosphorus release from the media could last with extraction. The results suggest that all the media had a high solubility of P regardless of compost amendment. The P in the leachate can be potential nonpoint sources of contaminant if the leachate is allowed to directly drain to surface waters. In traditional ornamental crop production systems, additional P fertilizer is often top-dressed. The compost contains substantial amounts of bioavailable inorganic P (NaHCO3–IP), readily mineralizable organic P (NaHCO3–OP), potentially bioavailable inorganic P (NaOH-IP), potentially bioavailable organic P (NaOH-OP) that may meet P requirements for plant growth. When compost is used as growing media, addition of fertilizer P should be avoided or reduced, depending on the crop type. The reduction in P leaching in the growing media containing compost can also be achieved either by limiting compost addition to a level of <25% or using controlled drip irrigation to regulate the volume of water delivered. Further study is needed to verify if less or no top-dressing of chemical P fertilizer should be applied to the compost-amended media to minimize P effect on the environment when the compost-amended potting media is used for nursery or greenhouse crop production systems.
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CONCLUSIONS
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Phosphorus and heavy metal solubility from the peat-based media amended with yard waste–compost determines potential contamination of these elements to surface and ground water. Both water solubility and sequential extraction studies revealed that concentrations of water-soluble heavy metals were very low. Organic matter and oxides appeared to be important carriers for the heavy metals in the media containing the compost. These results suggest a low risk of heavy metal contamination for surface and ground water from media amended with the compost. Application of compost to the media also decreased percentage of water-soluble P fraction through its effect on pH. However, concentrations of bioavailable inorganic P (NaHCO3–IP), readily mineralizable organic P (NaHCO3–OP), potentially bioavailable inorganic P (NaOH-IP), and potentially bioavailable organic P (NaOH-OP) in the media amended with the compost were higher than the peat-based medium because of higher total P concentration in the compost. These results suggest that less, or no, top-dressing of chemical P fertilizer should be performed, or drip irrigation should be controlled to regulate the volume of water delivered for minimizing P effects on the environment if compost-amended media are used.
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ACKNOWLEDGMENTS
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This study was, in part, supported by a grant (Approval no. 2002CB410804) from the Science and Technology Ministry of China and by Florida Department of Environmental Protection (Contract #WM746). Florida Agricultural Experiment Station Journal Ser. no. R-09208.
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ABSTRACT
INTRODUCTION
