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SOIL FERTILITY

Agronomic Value of Alkaline-Stabilized Sewage Biosolids for Spring Barley

^a Agric. and Environ. Sci. Division, Dep. of Agric. and Rural Dev. for N. Ireland, Newforge Lane, Belfast, United Kingdom BT9 5PX
^b N. Ireland Agric. Res. Inst., Large Park, Hillsborough, United Kingdom BT26 6DR
^c Greenmount College of Agric. and Hortic., Antrim, United Kingdom BT41 4PU
^d N. Ireland Water Serv., 39 Slaght Road, Ballymena, United Kingdom BT42 2JE. Research supported by the Dep. of Agric. and Rural Dev. for N. Ireland and N. Ireland Water Serv

Corresponding author (peter.christie{at}dardni.gov.uk)

	ABSTRACT

TOP ABSTRACT INTRODUCTION Materials and methods Results Discussion Conclusions REFERENCES

 
Land application of sewage biosolids is a cheap disposal method that permits recycling of plant nutrients, but there are concerns about its long-term agronomic value and environmental effects. This study investigated the fertilizer value of alkaline-stabilized biosolids applied annually to spring barley (Hordeum vulgare L.). Dewatered biosolids [320–350 g kg^-1 dry matter (DM)] were alkaline stabilized by mixing them with cement kiln dust and composting aerobically. The product had some liming value (300 g kg^-1 DM CaCO₃ equivalent on average) and contained an average of 7.2, 2.3, and 19.5 g kg^-1 DM of N, P, and K. Two field experiments compared the P or K value of the biosolids with inorganic fertilizer P or K for seven consecutive annual spring barley crops on two contrasting soils. All biosolid and fertilizer treatments gave higher yields than the controls. Biosolids gave higher grain and straw yields than fertilizer P, similar grain and straw yields to fertilizer K, and higher grain weights and more grains per ear than fertilizer P or K. These effects may have been due to, inter alia, higher soil pH and S inputs. An increasing soil pH from biosolid application was associated with lower shoot Mn concentrations, but no Mn deficiency symptoms were observed. Alkaline biosolids acted as a slow-release P fertilizer, and biosolid P was at least as available to the crops as inorganic fertilizer P. Biosolid K was also as available as fertilizer K. A calculation of nutrient balances indicated that current fertilizer P recommendations could be lowered.

Abbreviations: ANOVA, analysis of variance • DM, dry matter • OM, organic matter

	INTRODUCTION

TOP ABSTRACT INTRODUCTION Materials and methods Results Discussion Conclusions REFERENCES

 
THE USE OF ALKALINE-STABILIZED BIOSOLIDS as organic fertilizers or soil conditioners has become widespread in the USA. The N-viro soil process (Burnham et al., 1992) involves mixing biosolids with quicklime and cement kiln dust (an inorganic waste material from precipitators in cement factories) and quicklime followed by accelerated drying. Besides cement kiln dust, other alkaline wastes can be used such as lime kiln dust, limestone, and coal fly ash (Burnham et al., 1992; Wong et al., 1995). The resulting products can be used as substitutes for agricultural limestone, organic fertilizers in land reclamation, soil amendments for landscaping, ingredients in the manufacture of synthetic topsoils, and substitutes for soil landfill cover (Logan and Burnham, 1995; Logan and Harrison, 1995; Pierzynski and Schwab, 1993; Sloan and Basta, 1995; Stehouwer et al., 1999; Wong, 1995).

Much of the published research on alkaline-stabilized biosolids has largely been concerned with the application of potentially toxic trace metals to soils. Logan and Harrison (1995) found that the physical properties of a silt loam of pH 6.5 could be improved with high application rates of N-viro soil. In a glasshouse study, Pierzynski and Schwab (1993) found that N-viro soil applied at the rate of 5 Mg DM ha^-1 decreased Zn in the soil labile fraction and Zn, Cd, and Pb concentrations in soybean [Glycine max (L.) Merr.] plants. Sloan and Basta (1995) reported that N-viro soil effectively remediated soil acidity and Al toxicity in three highly acidic soils. Wong (1995) showed that mixtures of alkaline fly ash and biosolids mixed with loam soil decreased the availability of Zn, Cu, and Cd to tall wheatgrass [Elytrigia elongata (Host) Nevski] plants in a pot experiment and increased plant yield. In contrast, Sajwan et al. (1995) found increased concentrations of Cu and Zn in sorghum/sudangrass hybrid plants in a loamy sand amended with a mixture of coal fly ash and sewage biosolids. One problem with using coal fly ash is its relatively high concentration of B. Relatively low concentrations of B can be phytotoxic, especially to cereals, and B may affect plant growth when high application rates of biosolids stabilized with fly ash are used.

The Water Service of the Department of the Environment for Northern Ireland has developed the agri-soil process for the alkaline stabilization and aerobic composting of sewage sludge solids before land spreading as a means of sludge disposal. The initial pH of the biosolid mixture and kiln dust rises above 11.0 to kill pathogens and suppress odors and declines to about 7.8 after composting. Heat generated during the composting stage further contributes to pathogen kill and increases the DM content of the final product. The composted biosolids contain less N and P than raw sludge biosolids, but they have a relatively high K content that is derived from the kiln dust. They also have a neutralizing value (CaCO₃ equivalent) of 300 g kg^-1 on average (DM basis). The process is relatively inexpensive and can easily be automated.

Although alkaline products can remediate heavy metal phytotoxicity by raising soil pH, there is still concern about the long-term accumulation of trace metals in agricultural soils. The European Union banned sea dumping of sewage biosolids at the end of 1998, and all member states have legal regulations imposing maximum limits on the total metal concentrations in agricultural soils (e.g., U.K., 1989). Agri-soil was developed for agricultural use by using rural batches of sewage biosolids with lower metal loadings than urban or industrial sources. Manganese deficiencies have been reported in some crops on coarse-textured soils receiving alkaline biosolids in the USA, and foliar application of Mn to soybeans and wheat (Triticum aestivum L.) has led to crop yield responses (Brown et al., 1997).

There is little published information on the agronomic value of alkaline biosolids, especially under U.K. conditions. This paper outlines the agri-soil process and describes two long-term field experiments on contrasting soils in which the agronomic value of the alkaline biosolids for spring barley was investigated in seven consecutive annual crops. One experiment was conducted on a basaltic clay soil with low available P status and was designed to evaluate the P value of different application rates of alkaline biosolids. The other experiment investigated the K value of the biosolids applied to a shale clay loam with low exchangeable K. Some of the preliminary results from the first 5 yr of the K experiment have been reported briefly (Christie and Easson, 1997). Nutrient balances were calculated for P and K to determine the optimum reserves in the two soils. The chemical speciation and bioavailability of trace metals from alkaline biosolids that were applied to the two soils in glasshouse studies have been reported (Luo, 1997; Luo and Christie, 1997, 1998). Soil samples collected from the plots of the field experiments in February 1999 are currently being analyzed for trace metal concentrations and speciation, and those results will be reported in a separate paper dealing with the uptake of trace metals by the crops.

	Materials and methods

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The Agri-Soil Process
The agri-soil process was developed by Love (1990) and is shown schematically in Fig. 1 . Rural batches of sewage sludge that are screened and picket-fence thickened are mixed with an anionic or cationic polyacrylamide polyelectrolyte solution to act as a flocculent and then passed through a modified belt press. Each batch of sludge must be tested to determine whether it should be treated with an anionic or cationic flocculent. The solids (300–350 g kg^-1 DM) are then mixed in a ratio of 65:35 w/w fresh weight with cement kiln dust. The mixture is composted by turning daily in windrows under cover for 5 d to produce a short-term sanitized but organically unstabilized material with a DM content of 500 to 550 g kg^-1. This can then be turned regularly in the open for another 45 d to achieve organic stability and a DM content of 750 to 800 g kg^-1. This process differs from the N-viro soil process (Burnham et al., 1992) developed in the USA in its use of composting rather than accelerated drying. It is essential to dewater the sludge solids to a relatively high DM content (>300 g kg^-1) to achieve a successful composting stage. Once organic stability has been achieved, the alkaline biosolids can be stored outdoors indefinitely without any deterioration in chemical or physical properties.

View larger version (33K): [in this window] [in a new window]   Fig. 1 Agri-soil process flow diagram

The K experiment had a similar design and was established in 1992 at Corcreeny near Hillsborough in Down, Northern Ireland (Irish Grid Reference J208588) on a sandy loam derived from Silurian shale and Triassic sandstone (Typic Dystrochrept). Alkaline biosolids were applied to plots (4 by 20 m) at rates of 4, 8, 12, and 16 Mg DM ha^-1. Potassium chloride was applied to other plots at application rates of 42, 83, 124, and 166 kg K ha^-1, which were calculated to give approximately the same range of K application rates as the organic amendment. Fertilizer K application rates were based on an analysis of biosolid batches made in early 1992; however, subsequent batches were higher in K and the actual application rates of K in the sludge product averaged 80, 160, 240, and 320 kg ha^-1 from 1992 to 1997. Controls received no K applications, but supplementary N and P were applied where necessary to bring the total application of N and P up to recommended rates for barley in the United Kingdom (MAFF, 1994) so that any yield responses could not be attributed to N or P. Ground limestone was applied (2 Mg ha^-1) to all plots to raise the soil pH to 6.7, the target value for the mineral soil (4 g kg^-1 OM) (MAFF, 1994).

The plots were plowed each year to incorporate the fertilizer or biosolids into the soil. Plowing was done in opposite directions in alternate years to limit the mixing of the soil in adjacent plots to the edges of the plots. All plant and soil samples were collected from the central part of each plot to avoid edge effects. Spring barley (cv. Forrester in 1992 and 1993 and cv. Chariot from 1994–1996) was grown at both sites. The crops were grown using all recommended inputs of herbicides, pesticides, and growth regulators for optimum yield. There were four replicates of nine treatments in a fully randomized block, giving a factorial design of 2 x 4 + control and a total of 36 plots at each site.

Plant and Soil Analysis
Shoot samples were collected at the tillering stage for nutrient analysis. At harvest, the grain and straw from the center of each plot were collected and weighed, and the subsamples were retained for oven drying and chemical analysis. Subsamples of grain from the K experiment were used to determine the proportion of dirt present (avg. <20 g kg^-1), the hectoliter weight, and the 1000-grain weight. Plant, tiller, and head densities were counted.

Soil properties were determined on composite samples collected to a 15-cm depth every February using standard methods (MAFF, 1986). Plant N was determined by standard dry combustion using a CHN Analyzer. Other plant nutrients were determined by inductively coupled plasma–atomic emission spectrometry (ICP-AES) following digestion in a mixture of nitric and perchloric acids using standard methods (MAFF, 1986). Quality control of all analytical methods was monitored using standard reference materials and by participation in the International Plant and Soil Analytical Exchange Programs.

Statistical Analysis
The mean yield, grain quality, grain and straw nutrient concentrations and offtakes at harvest, and shoot nutrient concentrations at the tillering stage were tested for seven consecutive annual crops by analysis of variance (ANOVA) in a 2 x 4 + control factorial design. In addition, the effects of time (growth year) were tested by repeated-measures ANOVA; the data for all seven crops were combined, and the variance ratios in the time stratum were multiplied by the calculated Greenhouse–Geisser epsilon values before determining the significance levels (Genstat Committee, 1993). The rate degrees of freedom were broken out into orthogonal contrasts, and some of the linear contrasts of the rate factor were found to be significant.

	Results

TOP ABSTRACT INTRODUCTION Materials and methods Results Discussion Conclusions REFERENCES

 
The DM and major nutrient concentrations in the batches of alkaline biosolids that were applied from 1992 to 1998 are presented in Table 1. The product varied widely in DM content (378–775 g kg^-1), reflecting differences in composting time before delivery to the sites. The batches applied in 1995 and 1996 were fully composted and organically stabilized, but the material used in other years was only partially stabilized and would have continued to decompose after incorporation into the soil. The concentrations of N, P, and K, expressed on a DM basis, also varied by two to threefold among batches. The average annual application rates of N and K at Antrim were high because of the high application rates of biosolids required to cover the desired range of P application rates. Sulfur was not analyzed in the alkaline biosolids. If we assume a similar S content in the freshly dewatered biosolids to the average for farmyard manure (10 g S kg^-1 DM; Simpson, 1986, p. 90–99) and take into account the ratio of biosolids/cement kiln dust (containing 50 g S kg^-1) and the mean DM content of the alkaline biosolids used, we can calculate that the biosolids contained up to 35 g S kg^-1 DM on average. This also assumes no loss of S by volatilization during composting. This gives an estimated range of S application rates up to about 600 to 2400 kg ha^-1 year^-1 at Antrim and 140 to 560 kg ha^-1 year^-1 at Hillsborough.

View larger version (13K): [in this window] [in a new window]   Fig. 2 Mean annual grain-response curves to increasing application rate of fertilizer and biosolid P at the Antrim site

View larger version (14K): [in this window] [in a new window]   Fig. 3 Mean annual grain-response curves to increasing application rate of fertilizer and biosolid K at the Hillsborough site

View larger version (12K): [in this window] [in a new window]   Fig. 4 Calculated mean annual P balance (inputs from fertilizer or biosolids - outputs in grain and straw) plotted against mean soil Olsen P at Antrim. . Diamond, control; squares, fertilizer; triangles, biosolids

View larger version (13K): [in this window] [in a new window]   Fig. 5 Calculated mean annual K balance (inputs from fertilizer or biosolids - outputs in grain and straw) plotted against mean soil exchangeable K at Hillsborough. . Diamond, control; squares, fertilizer; triangles, biosolids

	Discussion

TOP ABSTRACT INTRODUCTION Materials and methods Results Discussion Conclusions REFERENCES

The grain and straw yields and the grain-quality results show that the alkaline biosolids performed well compared with the inorganic fertilizers. It might be suggested that this was due to an improvement in the soil condition by the supply of additional C from the biosolids. However, this is unlikely to be an important factor because the Hillsborough soil has a relatively high organic C content for a temperate mineral soil (24 g kg^-1 in the top 15 cm), and the Antrim clay would be classified by the Ministry of Agriculture, Fisheries, and Food (1994) as an organic soil. A more likely explanation is that the high soil pH maintained by the sludge product may have contributed to the yield effect, possibly together with the alleviation of S deficiency. These two factors may have interacted with plant growth. Murphy (1990) described the low atmospheric inputs of S to soils in Ireland and reported responsiveness of crops to S in Irish sandy soils with less than 30 g kg^-1 organic C. Sulfur in the alkaline biosolids is derived not only from the sewage sludge, but also from the cement kiln dust, which typically contains 50 g kg^-1 S (Love, 1990). The possibility of S deficiency was not considered when the experiments were designed; therefore, S was not routinely analyzed in the batches of biosolids used each year, and N, P, and, K were the only major plant nutrients whose inputs were experimentally controlled. The higher shoot S concentrations at the tillering stage in plots receiving biosolids and the marked increase in soil-available S after six consecutive crops indicate that improved S nutrition may have contributed to the performance of the biosolids. However, S may not be the only nutrient involved in the yield response to biosolids. The mean offtakes of P and K in grain and straw were all higher in the biosolid treatments than in the fertilized plots. Although alkaline biosolids do not have high N concentrations, partly due to NH₃ volatilization during the mixing of the biosolids with the cement kiln dust and during the exothermic stage of composting, the liming effect may have stimulated the mineralization of soil organic N, especially in the organic basaltic clay. This effect has been reported in the forest soils to which alkaline biosolids were applied (Luo and Christie, 1995).

The fate of trace metals derived from the biosolids will be the subject of a separate paper, but it is interesting to note that both Zn and Cu were well below their toxic thresholds of 200 and 20 mg kg^-1, respectively (Bailey, 1993), in the shoots at the tillering stage. Concentrations in the grain and straw were similar to those in the control and fertilizer treatments. These results were obtained despite the use of excessive biosolid application rates, especially at the Antrim site. There was some evidence of a decline in Mn availability, particularly in the later years of the study, but few of the shoot concentrations fell below the deficiency threshold of 10 mg L^-1 (Bailey, 1993). Manganese deficiency is the most widespread nutrient deficiency in Northern Ireland cereals, but it is usually associated with adverse weather conditions and coarse sandy soils (Dickson and Christie, 1985). Thus, care would need to be taken to avoid Mn deficiency when applying alkaline biosolids to these soil types for cereal production. The deficiency could be corrected at the tillering stage by foliar application of Mn (Brown et al., 1997).

The P and K balances help explain the absence of grain yield-response curves to increasing P or K inputs in these experiments. Although the Antrim soil would be classed as deficient in P and the Hillsborough soil as low in K, the initial Olsen P at Antrim was only 1 mg L^-1 below the indicated soil level for P balance, and the exchangeable K at Hillsborough was actually 5 mg kg^-1 above the calculated soil K level for K balance. This is especially important for P because agronomically small annual losses of P from agricultural soils can eventually lead to environmentally damaging enrichment and eutrophication of rivers and lakes. The U.K. fertilizer recommendation system categorizes soil Olsen P into index values as follows:

• Index 0 0–9 mg P l^-1 Deficient
  
• Index 1 10–15 mg P l^-1 Low
  
• Index 2 16–25 mg P l^-1 Adequate
  

Samples of farmers' soils are analyzed for Olsen P, and the result of each soil test is placed in the appropriate index. A recommendation is then made that is designed to adjust the soil-available P to the target index, which is Index 2. It would not be possible to make fine adjustments to the system based on two field experiments on two soil types. However, the present study indicates that the system could be adjusted by changing the target range of Olsen P to Index 1, instead of Index 2. This could make a convenient and useful contribution to reducing nonpoint-source P pollution from agricultural soils. Although P in organic wastes is usually regarded as having an average of 50% availability to the next crop after application (Simpson, 1986, p. 90–99), our P balance study indicates that availability is similar to that of P in inorganic fertilizer over a 7-yr period (Fig. 4). This agrees with the study of McLaughlin and Champion (1987) who conducted a pot-experiment study on the plant availability of sewage sludge P applied to two sesquioxide soils using monocalcium phosphate as a standard. They found that the relative efficiency of sludge P reached 90% in one of the soils and over 100% in the other with time, and the sludge appeared to act as a slow-release P fertilizer. In the present study, it was very difficult to find a field with a soil Olsen P value as low as Index 0 for the P experiment, and the Olsen test may have also underestimated the available P in the basaltic soil as discussed above. This illustrates the widespread accumulation of P in intensively managed agricultural soils over the last 50 yr and indicates that many soils may have substantial P reserves for crop growth. Such soils may require very modest, routine P application rates to maintain optimum crop yields while minimizing losses of P to surface waters.

	Conclusions

TOP ABSTRACT INTRODUCTION Materials and methods Results Discussion Conclusions REFERENCES

 
The results of this study indicate that P and K in alkaline biosolids have similar plant availability to P and K in inorganic fertilizer. Relatively low application rates (up to about 5 Mg ha^-1) of alkaline biosolids could be used for several years to maintain crop production, even on soils with small reserves of available P. This might supply up to about 150 kg S ha^-1. Nitrogen and K could be supplemented using inorganic fertilizers to achieve optimum yields. The alkaline biosolids could replace ground limestone for control of soil pH in acidic soils when used regularly at low application rates. Each batch of biosolids would need to be analyzed before use to determine the correct application rate, and soil analysis would enable the monitoring of trace metal accumulation in the soil. Barley grows well at relatively high soil pH, but other cereal crops may be more sensitive to increases in soil pH from repeated use of alkaline biosolids. Our results indicate that alkaline biosolids could be useful as a combined seedbed fertilizer and liming material for cereal crops grown in acidic soils under carefully controlled conditions.

	ACKNOWLEDGMENTS

 
We thank the numerous laboratory and field staff who contributed to the field experiments and chemical analyses. We are grateful to Dr. D.J. Kilpatrick of the Biometrics Division of the Department of Agriculture and Rural Development for Northern Ireland, for advice on the design of the experiments and statistical analysis and interpretation of the data. Finally, we thank the three anonymous reviewers whose comments have greatly improved the paper.

Received for publication June 10, 1999.

	REFERENCES

TOP ABSTRACT INTRODUCTION Materials and methods Results Discussion Conclusions REFERENCES

 

• Bailey, J.S. 1993. Soil and crop analysis—a pocket manual. Department of Agriculture for N. Ireland, Belfast, United Kingdom.
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• Cruickshank, J.G. 1997. Soil and environment: Northern Ireland. The Queen's Univ. of Belfast Dep. of Agric. and Environ. Sci., United Kingdom.
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This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (10) Citing Articles via Google Scholar Google Scholar Articles by Christie, P. Articles by Love, S. C.P. Search for Related Content PubMed Articles by Christie, P. Articles by Love, S. C.P. Agricola Articles by Christie, P. Articles by Love, S. C.P. Related Collections Other Grain Crops Nutrient Cycling Nutrient Management Soil Fertility and Productivity Municipal Waste