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DIVISION S-8-NUTRIENT MANAGEMENT & SOIL & PLANT ANALYSIS

Chemical Evaluation of Nutrient Supply From Fly Ash–Biosolids Mixtures

^a Dep. of Crop and Soil Sci., Univ. of Georgia, Athens, GA 30602-7272 USA

sumnerme{at}arches.uga.edu

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Prediction of plant nutrient supply from fly ash and biosolids (sewage sludge and poultry manure) may enhance their agricultural use as crop fertilizer. Two mild extraction methods (42-d equilibration with ion-exchange resins; 2-d equilibration with pH 4.8 buffered nutrient solution) and analysis of nutrient data by the Diagnosis and Recommendation Integrated System (DRIS) were tested with 29 fly ash samples, four biosolids samples, and their mixtures. The resin method was useful for major nutrient (N, P, K, Ca, Mg, S) extraction from fly ashes and organic materials, particularly where mineralizable fractions of N and P under aerobic conditions are required. However, resins were inefficient in extracting P from high-Fe sewage sludges because organic waste samples caused premature failure of semipermeable membranes and fouling of resins. Extraction of fly ash with dilute buffered nutrient solution was more successful because micronutrient recovery was improved, major nutrients were correlated to the resin method, both addition and removal of nutrients were recorded, DRIS analysis was possible, and equilibration was rapid (2 d). The overall nutrient supply from these extremely variable fly ashes was: (high micronutrient, low major nutrient supply). For biosolids, the major nutrients ranked: P > N Ca > S > Mg > K (sewage sludges), and N > Ca K > P > Mg > S (poultry manures). In mixtures of fly ash with 26% sewage sludge the order was: Ca > S > N > Mg > P > K, while in mixtures of fly ash and 13% poultry manure, the nutrients ranked: Ca > K N S > Mg > P. Optimal plant nutrition (especially N–P–K balancing) should be possible by mixing these three waste materials.

Abbreviations: ANOVA, analysis of variance • CCE, calcium carbonate equivalence • DM, dry mass • DRIS, Diagnosis and Recommendation Integrated System • EC, electrical conductivity • EDTA, ethylenediaminetetraacetic acid • ICP-MS, inductively coupled plasma mass spectrometry • LOI, loss on ignition • NBI, nutrient balance index • PM, poultry manure • SS, sewage sludge

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
LAND APPLICATION of coal combustion residue wastes, particularly fly ash, to agricultural land may offer a sensible waste recycling alternative to current landfill disposal. Although fly ash, like the parent coal from which it was derived, contains almost every naturally occurring element, plant nutrition with this complex material is not straightforward, as demonstrated by contrasting reports in the literature. Fly ash can supply essential elements to crops growing on nutrient-deficient soils, and highly alkaline fly ashes are potential liming agents. Fly ash amendments have corrected plant nutritional deficiencies of B (Martens, 1971; Ransome and Dowdy, 1987), Mg (Hill and Lamp, 1980), Mo (Doran and Martens, 1972; Elseewi et al., 1980), S (Elseewi et al., 1978, 1980; Hill and Lamp, 1980), and Zn (Martens, 1971; Schnappinger et al., 1975).

While fly ash–amended soil may also cause phytotoxicity due to excesses of micronutrients such as B, only As, Mo, and Se have been reported to accumulate in plants at levels that could be potentially toxic to grazing animals (Doran and Martens, 1972; Elseewi et al., 1980; Tolle et al., 1983; Elseewi and Page, 1984). Fly ash application may also decrease plant uptake of elements such as Cd, Cu, Cr, Fe, Mn, and Zn (Schnappinger et al., 1975; Elseewi et al., 1980; Adriano et al., 1982; Petruzzelli et al., 1986). Phosphorus concentrations in plant foliage were often reduced by fly ash applications (Elseewi et al., 1980; Molliner and Street, 1982). These effects were attributed to an increase in soil pH by the ash and the formation of insoluble complexes. Plant availability of fly ash K was also reportedly low, despite high total concentrations in the ash (Martens et al., 1970). These conflicting results are understandable, given the high variability in fly ash samples.

Organic wastes also tend to be imbalanced sources of plant nutrients. Approximate N/P/K ratios for poultry manure are 2.5:1.0:0.9 (Edwards and Daniel, 1992), implying that P will be in excess for most field and forage crops when fertilized with poultry manures to supply the crop N requirement [e.g., N/P/K for maize, Zea mays L.: 7.5:1.0:4.4; grain sorghum, Sorghum bicolor (L.) Moench: 9.0:1.0:4.7 (Finck, 1982)]. However, as with fly ash, only a fraction of total nutrients (especially N and P) supplied by organic wastes are available to crops in a season, since they must be mineralized from organic to inorganic forms. Despite these limitations, sewage sludges and animal manures may be the most cost-effective supplement for co-utilization with fly ash in crop fertilization. Mixtures of fly ash with organic wastes already have a proven track record (Pichtel and Hayes, 1990; Belau, 1991; Schwab et al., 1991; Sims et al., 1993; Vincini et al., 1994; Sajwan et al., 1995; Wong, 1995), but the preparation of mixtures has usually proceeded by trial and error. The formulation and use of complex waste products could be greatly enhanced by improved prediction of nutrient supplies from components before they are combined.

Since plant bioassays take considerable time, and plant genetics and growth rates strongly affect the uptake of nutrient elements, an unbiased (plant-free) method of assessing nutrient availability is preferred. The various chemical solution extractants that have been used historically to predict nutrient availability in soils usually only give indices of relative availability. Results are also often unrealistic, because the disruptive acidic or basic extractants used are able to dissolve more nutrients such as P from the solid phase than would normally be extracted by plant roots. Interpretation of results is also often hampered by numerous interactions possible between the 12 essential plant nutrients. A more meaningful assessment of nutrient availability may be obtained through incubation of samples with synthetic ion-exchange resins, or equilibration with a dilute buffered nutrient solution. Anion-exchange resin beads and membrane strips have already proved useful for evaluating P availability on a wide range of soils (Sibbesen, 1977; van Raij et al., 1986; Saggar et al., 1990; Leal et al., 1994). van Raij et al. (1986) also extracted Ca, K, and Mg from soil with cation-exchange resins. Schwab et al. (1991) used a resin-free, 28-d aerobic incubation of fly ash in 0.01 M CaCl₂ (1:4 solid/solution) to obtain estimates of soluble Pb, Cd, and B. A combination extractant–nutrient solution has also been used for assessing soil nutrient supply (Baker, 1973). The DRIS method has been used in plant foliage diagnosis to obtain nutrient indices that reflect the adequacy of each nutrient in relation to all the others (Beaufils, 1973; Walworth and Sumner, 1987), thus incorporating some of the nutrient interactions. The objectives of this study were to use the above methods to predict plant nutrient availability from coal fly ash and to assess possible nutrient enhancements by mixing fly ash with sewage sludge or poultry manure.

	Materials and methods

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Twenty-nine unweathered fly ash samples were obtained in 1994 and 1995 from 22 power plants (FA1 to FA22) in the southeastern USA (Table 1) . Fly ash samples from the same power plant were given the suffix a, b, or c to differentiate coal sources (FA1a,b), ash collection without an electrostatic precipitator (FA17b), or later sampling dates (FA1c, FA2b, FA8b, FA10b, FA12b). Twenty-eight of these fly ash samples could be classified as "class-F" (silica + alumina + iron oxides constitute >700 g kg^-1 by mass), while FA11 was "class-C" (silica + alumina + iron oxides constitute 500 to 700 g kg^-1 by mass) fly ash (Mattigod et al., 1990). Dewatered, anaerobically digested sewage sludge samples (SSa,b) were collected in 1995 and 1996 from the Athens, GA municipal treatment plant, and two poultry manure samples (PMa,b) were collected from poultry farms nearby. The PMa was from an egg production facility (no additional bedding materials), while PMb was from a broiler house using wood chips as a waste absorber. A subset of 13 fly ashes was selected to represent the full range of plant performance in a separate pot experiment with these fly ashes (Schumann and Sumner, 1999a) and were each combined with fresh SSa and PMa by thorough stirring and sieving through an 8-mm nylon mesh. The 26 moist, granulated mixtures were dried for 24 h at 45 to 50°C in a forced draft oven to obtain a pelletized product with a final dry mass (DM) composition of 260 g kg^-1 SS and 130 g kg^-1 PM. These mixed proportions of fly ash and biosolids were determined experimentally to provide a product of uniform consistency which could also be pelletized.

Nutrient Extraction by Synthetic Ion-Exchange Resin
Small cation- and anion-exchange resin bags were constructed from nylon mesh stockings. These bags were filled with 10 mL of previously treated 0.226- to 0.256-mm (20–50 mesh) Dowex synthetic ion-exchange resin beads (Dow Corning, Midland, MI), so that each bag had a calculated ion-exchange capacity of 12 mmol_c. The chosen resins were strong-acid (cation) and strong-base (anion; type I) types prepared by thorough washing with deionized water, followed by excess 0.5 M HCl (cation) or 0.5 M NaHCO₃ (anion) solutions, and a final washing step with deionized water to remove excess solution. Pairs of cation- and anion-exchange bags were enclosed in 100-mm-long sections of dialysis membrane tubing (45-mm flat width, 12000–14000 molecular weight cutoff), filled with deionized water, and tied at both ends. The dialysis membrane serves to exclude waste particles and high molecular weight dissolved organic matter from fouling the exchange surfaces, and could be likened to the cell membrane at the root–soil interface. In this study, the choice of cation (H) and anion (HCO₃) on the resins for exchange with nutrient cations and anions in solution was based on similar ion exchange occurring at the surface of plant roots during nutrient uptake. An electronic timer and solenoid valve delivered intermittent streams of compressed air (8 s on, 180 s off) via activated charcoal scrubber and humidifier manifold and "spaghetti tubing" to the incubating solution and sample in 250-mL conical flasks. This air supply fulfills a dual role of aeration and gentle mixing of the solution to avoid anaerobic, stagnant areas, while avoiding excessive mechanical weathering of samples.

A 1-g topsoil sample of unfertilized soil was included in these incubations to provide microbial inoculum for the mineralization of organic wastes. A blank (soil only) and a check standard of soil and complete fertilizer solution of 12 nutrient elements (N, P, K, Ca, Mg, S, Mn, Fe, Cu, Zn, B, Mo), including both NH₄ and NO₃ (modified after Penningsfield, 1960), were included to allow the calculation of recovery for all nutrient elements with this extraction method. One-gram samples of dry fly ash or organic waste mixture, and 2- to 4-g samples of fresh organic waste (250–440 g kg^-1 DM) were incubated for 42 d at 20°C with the soil, resin bags, and 150 mL of deionized water. In a test run, it was established that the EC and pH of external solutions in different treatments converged to a stable value similar to the blank (pH 5.0 and EC <0.020 dS m^-1) in 42 d, regardless of the waste material used. This indicated that sufficient equilibration of samples with the exchange sites on the resins had occurred and that the mineralization of organic materials had stabilized.

At this point the experiment was terminated and the resin bags were extracted with 2.5 M HCl (6 x 15-mL aliquots, brought to volume in a 100-mL volumetric flask) for analysis of extractable nutrients. Pairs of resin bags were placed in an array of 30-mL disposable syringes on 100-mL volumetric flasks to facilitate simultaneous extraction in "leaching column" style. Treatments in the entire experiment were replicated twice in randomized blocks for a given 42-d run. The acid extracts were analyzed for NH₄ and NO₃ by colorimetric methods (Alpkem 3550 segmented flow autoanalyzer, Perstorp Analytical, College Station, TX); P, K, Ca, Mg, Mn, Fe, Zn, Cu, B, and Mo by ICP-MS; and SO₄ by indirect Ba absorption spectroscopy (Hue and Adams, 1979). These nutrient data were collected for the 29 fly ashes, four organic wastes, and the 26 mixtures.

Nutrient Extraction with Dilute Buffered Nutrient Solution
This simplified method was used for rapid (2-d) measurement of nutrient release from fly ashes. The procedure is analogous to one described by Baker (1973), where a combination equilibrant and buffered extractant is used for soil analysis. However, instead of a pH 7.3 buffer, a Morgan's extractant was used (Morgan, 1941) (0.72 M CH₃COONa + 0.52 M CH₃COOH, pH 4.8), modified by adding ethylenediaminetetraacetic acid (EDTA), diluting with deionized water and complete nutrient solution (modified after Penningsfield, 1960) to achieve a final composition of 100 mL L^-1 buffer, 200 mL L^-1 nutrient solution, and 0.5 mM EDTA. The final buffer pH of 5 after equilibration with fly ashes would be a good simulation of soil buffering in the southeastern USA. One-gram fly ash samples were shaken on a reciprocating shaker for 30 min with 20 mL of extractant in 50-mL centrifuge tubes, allowed to stand for 2 d, then shaken again for 30 min. The 1:20 sample/solution ratio was chosen to represent the approximate ratio obtained from a 40 000 kg ha^-1 fly ash land application (15 g kg^-1 in a 20-cm soil furrow slice and a 0.3 kg kg^-1 gravimetric soil moisture content). The resulting extract, which simulates the equilibration of fly ash with infertile acid buffered soil solution, was filtered (Whatman 42 paper) and analyzed for nutrients as described above. Nutrient concentrations in solution were compared with blanks (dilute buffered nutrient solution) in order to calculate amounts of each nutrient added or removed by fly ash. Results were converted to concentrations of nutrient (+ or - mg kg^-1) in fly ash, or were assessed by DRIS. The "optimum" norms required for this algorithm were calculated from concentrations of the 12 nutrient elements in the undiluted nutrient solution (high fertility). Replicated data from these two extraction methods were analyzed by analysis of variance (ANOVA and linear regression (Genstat 5 Committee, 1993) to test for significant treatment differences and associations.

	Results and discussion

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
The 29 fly ash samples used for these studies varied significantly (**, P < 0.01) across all measured variates (total and extractable), except DM, which was nonsignificant (NS). Typically, the variation was as high between samples obtained from the same power plant as between power plants (Table 1). Twenty-four fly ash samples were alkaline, but displayed variable buffering capacity (CCE 5.8–539.1 g kg^-1, Table 1). The main sources of CCE in fly ashes were probably Ca ( , P < 0.001) and Mg ( ) oxides, or hydroxides that are readily soluble in water and display alkaline buffering capacity. The "class C" FA11 ash, derived from western coal sources, had the highest CCE of 539.1 g kg^-1, while the remaining "class F" fly ashes had CCE < 90 g kg^-1. A smaller buffering capacity in the acid pH range, especially evident for the acidic FA2b, FA9, FA15, FA18, FA20 ashes, was probably due to Fe and Al oxides and hydroxides, but is not useful for soil liming purposes. The range of fly ash EC (1–8 dS m^-1) is sufficiently low not to pose a problem to plants, provided fly ashes are diluted with soil at moderate application rates of 10 to 20 g kg^-1 (25–50 Mg ha^-1); however, these unweathered fly ashes would all create saline conditions for plants growing directly in undiluted ash. Total N, P, K, Ca, Mg, and S in these fly ashes were lowest for N and P, and highest for K, Ca, and Mg (Table 1). Total N was highly correlated with LOI ( ), suggesting that it was present in unburned coal residue, and hence unlikely to be plant-available. Likewise, K was shown to be mostly plant-unavailable in previous studies (Martens et al., 1970) because of its inclusion in the glassy matrix of fly ash particles. Total concentrations of Mn, Fe, Cu, Zn, B, and Mo in these fly ashes are high (Table 1) relative to plant demand. Boron phytotoxicity thresholds are readily exceeded by fly ash application to crops; eleven of these fly ash samples with >200 mg B kg^-1 could cause B toxicity at moderate (20 Mg ha^-1) or higher application rates (4 kg B ha^-1 at 20 Mg FA ha^-1 x 200 mg B kg^-1). The maximum recommended fertilization rate with soluble B fertilizer is 4 kg B ha^-1 (Finck, 1982).

Total N and P from the four organic waste samples was immediately recognized as potentially useful for mixtures with fly ash to correct deficiencies of N and P (Tables 1 and 2) . Initial estimates of mineralizable N (7-d anaerobic method) showed that effectively four times more N was available from uncomposted poultry manure sources than from sewage sludge. Both organic wastes had predominantly inorganic P (730–920 g kg^-1), but the NaOH extraction method indicated that fresh poultry manures had a lower bioavailable P fraction than digested sewage sludge (Table 2). This agreed with a higher organic P fraction recorded for both poultry manures compared with the sewage sludges (Table 2). Both sewage sludge and poultry manure samples had pH 7 and low DM contents (229–438 g kg^-1), which should moderate the extremely variable pH of some fly ashes in the resulting moist mixtures. The very high EC of poultry manures is consistent with the high concentrations of soluble nutrient cations K, Ca, and Mg, and associated counter-ions (not measured), while sewage sludges, being a dewatered product, had considerably lower K concentration (Table 1).

Average capacities of the 29 fly ash samples to supply N, P, K, and Mg were extremely low (<16% of total), using either extraction method (Tables 3 and 4) . Using a moderate 20 Mg ha^-1 fly ash application rate, the mean N–P–K fertilizer values predicted from the nutrient buffer extraction were 0.045, -0.026, 0.217 and , respectively. The corresponding fertilizer values calculated from the resin extraction method were -0.4, 3.5, and 3.2 kg N, P, and K ha^-1. Correlation between these different extraction methods was good for P, K, Ca, Mg, S ( , Table 4), with slopes approaching 1.0. The more economical buffered nutrient extraction would therefore be recommended for fly ash testing. Extractable N assessed by these two methods was uncorrelated (NS) for fly ashes, and was also not correlated with total N (Tables 3 and 4). These results confirm the suspicion that there is negligible inorganic nutrient N supplied by these fly ashes and that supplies of P and K from fly ashes are also well below the requirements for sustaining crop production.

The lower correlation (R² < 0.35) between extractable and total Mn, Cu, and Zn in fly ashes was probably because these micronutrients are less soluble than B and Mo, despite a favorable solution pH of 5 (Table 4). Iron solubility was particularly low in these fly ash samples (no correlation with totals, 0.2% recovery), implying the production of Fe (III) rather than Fe (II) compounds from the coal combustion process. Supply of the micronutrients Mn, Fe, Cu, and Zn from fly ashes is nevertheless expected to be adequate due to low requirements by crops. In contrast, these fly ashes should be particularly good suppliers of B and Mo (Table 4), so that moderate fly ash applications would usually exceed plant requirements.

Mineralizable N recovered from organic wastes by the aerobic resin extraction system (Table 3) agreed very well with preliminary results obtained by anaerobic incubation (Table 2). Most of the N recovered from resins was NH₄ (97%), implying that minimal nitrification occurred during the 42-d incubation period. All major nutrients, except S, were better supplied by poultry manures than sewage sludges (Table 3). Nitrogen, P, and K supplies, which are nearly completely lacking in fly ashes, would be better augmented by poultry manure because sewage sludge is a partially composted, dewatered (loss of K), more recalcitrant product. Very low extraction of P from sewage sludge (<2%, Table 3) may be explained by the high Fe content (45 g kg^-1), which is often added as FeCl₃ during treatment in order to precipitate or adsorb dissolved P. These exchange resins were not competitive enough to extract P from the Fe–P precipitates, but it is likely that plant roots, with their ability to modify rhizosphere pH and Eh (Sharpley et al., 1994), would be able to extract this source of P. The 0.1 M NaOH extraction method used for bioavailable P is therefore suggested as an alternative for measuring P supply from sewage sludge.

The DRIS analysis of 12 essential nutrients extracted from fly ashes by buffer solution highlighted the main strengths and weaknesses of fly ash fertility (Table 5) . For simplicity, indices have been divided into high (H) and low (L) classes, according to their sign (+ or -). Column totals for the 29 samples show the status of overall nutrient supply from these fly ashes relative to the nutrient solution used as reference. The ranking of nutrients in decreasing order of supply would therefore be: . Optimum ratios of the major nutrient cations K/Ca/Mg are particularly important in plant nutrition (Fig. 1) , and since most of these fly ashes supply excess Ca, their K and Mg supplying status can be further suppressed. The nutrient balance index (NBI) was calculated as the positive sum of all indices for each fly ash, except P. Phosphorus was excluded because in this study five fly ashes had completely exhausted the supply of solution P by adsorption and precipitation (256 mg P kg^-1), which distorted the NBI. Correlation with a separately derived NBI from maize foliage grown on these 13 fly ashes (Schumann and Sumner, 1999a) was good ( ), suggesting that plant nutrient uptake from fly ash was affected by nutrient ratios in a similar way to the equilibrium reached in the buffer extraction.

View larger version (77K): [in this window] [in a new window]   Fig. 1 Ratio of major nutrients extracted for two sewage biosolids (SSa and SSb), two poultry manures (PMa and PMb), and a nutrient solution by the ion-exchange method

View larger version (119K): [in this window] [in a new window]   Fig. 2 Ratio of major nutrients extracted from 13 fly ashes (FA) and a nutrient solution by the ion-exchange method

View larger version (116K): [in this window] [in a new window]   Fig. 3 Ratio of major nutrients extracted from 13 fly ash (FA)–sewage (SS) mixtures and a nutrient solution by the ion-exchange method

View larger version (118K): [in this window] [in a new window]   Fig. 4 Ratio of major nutrients extracted from 13 fly ash (FA)–poultry manure (PM) mixtures and a nutrient solution by the ion-exchange method

	Conclusions

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

Most of these problems can be overcome by exploiting the complementary nature of fly ash, sewage sludge, and poultry manure, and additional nutritional benefits (especially N–P–K balancing) should be possible by mixing these three waste materials together. Furthermore, known contaminants such as As, which have restricted annual application limits, and phytotoxic micronutrients such as B can be efficiently constrained in triple mixtures because of extra degrees of freedom.

The buffered nutrient solution is ideal for assessing fly ashes because removal as well as addition of nutrients to the equilibrating solution is quantified and may be analyzed by DRIS. The slow (42-d) aerobic incubation with resins was a good method for estimating mineralizable nutrients such as N and P in biosolids, as these cannot be determined by the buffered nutrient solution method.Genstat 5

	ACKNOWLEDGMENTS

 
We appreciate the funding for this research from the Electrical Power Research Institute project RP-9023, and the ICP analyses done by Brian Jackson.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Contribution from the Dep. of Crop and Soil Sci., Univ. of Georgia.

Received for publication October 14, 1998.

	REFERENCES

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