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Journal of Environmental Quality 32:1114-1121 (2003)
© 2003 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

TECHNICAL REPORTS
Waste Management

Phosphorus Sorption and Availability in Soils Amended with Animal Manures and Sewage Sludge

Muhammad Tariq Siddique and J. Stephen Robinson*

Department of Soil Science, The Univ. of Reading, Whiteknights, P.O. Box 233, Reading RG6 6DW, UK

* Corresponding author (j.s.robinson{at}reading.ac.uk)

Received for publication March 7, 2002.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Soils that receive large applications of animal wastes and sewage sludge are vulnerable to releasing environmentally significant concentrations of dissolved P available to subsurface flow owing to the gradual saturation of the soil's P sorption capacity. This study evaluated P sorption (calculated from Langmuir isotherms) and availability of P (as CaCl2–P and resin P) in soils incubated for 20 d with poultry litter, poultry manure, cattle slurry, municipal sewage sludge, or KH2PO4, added on a P-equivalent basis (100 mg P kg-1). All the P sources had a marked negative effect on P sorption and a positive effect on P availability in all soils. In the cattle slurry– and KH2PO4–treated soils, the decreases in P sorption maximum (19–66%) and binding energy (25–89%) were consistently larger than the corresponding decreases (7–41% and 11–30%) in poultry litter–, poultry manure–, and sewage sludge–treated soils. The effects of cattle slurry and KH2PO4 on P availability were, in most cases, larger than those of the other P sources. In the poultry litter, poultry manure, and sewage sludge treatments, the increase in soil solution P was inversely related (R2 = 0.75) to the input of Ca from these relatively high Ca (13.5–42 g kg-1) sources. Correlation analyses implied that the magnitude of the changes in P sorption and availability was not related to the water-extractable P content of the P sources. Future research on the sustainable application of organic wastes to agricultural soils needs to consider the non-P- as well as P-containing components of the waste.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
MANY AGRICULTURAL SYSTEMS in Europe and the USA currently operate with an annual phosphorus (P) surplus after accounting for crop uptake. The cumulative long-term effect of this P surplus has been an increase in the total amount and availability of P in many soils (Tunney et al., 1997). Excessively high-P soils are most common in areas that support intensive, housed livestock production. Numerous studies have shown that the overapplication of P in the form of animal manures is largely responsible for soil P accumulation in these areas, often to values well in excess of crop requirements (Sharpley et al., 1984; Simard et al., 1995; Nair et al., 1998). These soils are vulnerable to releasing environmentally significant levels of dissolved P to subsurface flow as well as to surface runoff, and have been linked to accelerated eutrophication of fresh water bodies. In areas that support dense livestock operations, it has been shown that P can move from the topsoil to depths of at least 70 cm (Campbell et al., 1986; James et al., 1996; Leinweber et al., 1999). In addition to animal wastes, sewage sludge also provides a readily available alternative to inorganic N and P fertilizers in agriculture. As sewage P removal technologies improve, the P content of sludges increases further. As a consequence, there is increasing concern about soil enrichment with P and subsequent potential losses following repeated and long-term application of sewage sludge to agricultural land (Rydin and Otabbong, 1997; Siddique et al., 2000).

The P content and composition of manures and sewage sludges (collectively termed organic wastes in subsequent text) is highly variable. In the case of animal manures, P composition is influenced largely by the type and age of the animal, diet, amount and type of bedding material, and method of manure or effluent storage (Simons and Jongbloed, 1981; Prins and Snijders, 1987). With regard to sewage sludge, the variability in P composition is explained largely by the combination of sewage treatment processes (Gestring and Jarrell, 1982; Frossard et al., 1996). Although P in organic wastes is present in inorganic and organic forms that display a complex continuum of solubilities, several researchers have suggested that the water-extractable inorganic P provides a simple indicator of the potential release of dissolved P into agricultural runoff (Dou et al., 2000; Sharpley and Moyer, 2000). This relationship is probably the strongest where the wastes are broadcast at the land surface. Without incorporation into the soil, P released to rainwater can contribute readily to surface runoff, but if the waste comes into contact with the soil, a large proportion of the dissolved P becomes adsorbed, preventing it from directly entering the leachate. However, as the degree of P sorption saturation increases, more of the dissolved P remains in the soil solution, where it may be vulnerable to subsurface flow (Holford et al., 1997).

Much of our current understanding of the potential for topsoils receiving organic wastes to release dissolved P to subsurface flow relies on data generated from studying the reactions and availability of P in soils that receive inorganic P fertilizers. Few studies have provided a quantitative evaluation of the differential P sorption and availability characteristics of manure and sewage sludge sources of P in soil. In most of the previous studies, amendments were added on the basis of N content and/or at excessive rates (Reddy et al., 1980).

In a comparison between sewage sludge– and inorganic P fertilizer–amended soils, Siddique et al. (2000) correlated the relatively low rate of P leaching from sludge-amended soils to a low rate of P saturation of sorption sites in the soil, which in turn was attributed to the relatively low water solubility (8%) of P in the sludge. Some researchers have reported that organic sources can modify the P sorption characteristics of soils in and around the zone of application, with implications for the movement of P to greater soil depths (Reddy et al., 1980; Gerritse, 1981; Holford et al., 1997). The high Ca contents of some manures may be responsible for modifying the P sorption characteristics of manured soils (Robinson and Sharpley, 1996). The addition of Ca can lead to the formation of Ca-P precipitates and complexes in the soil. Also, the addition of soluble low molecular weight (<50 000) organic compounds may form complexes with Ca, Fe, and Al that increase the sorptive area of soil material (Gerritse, 1981; Frossard et al., 1995). Conversely, increased availability of P following additions of organic manures has been attributed to the production of organic acids during the decomposition of the manure. These acids can form stable complexes with Fe and Al and consequently block available P retention sites (Nagarajah et al., 1970; Reddy et al., 1980; Holford et al., 1997).

The aim of this study is to evaluate and explain the P sorption and availability characteristics of soils that are mixed with different organic wastes on a total P–equivalent basis and incubated, and to discuss the implications for P loss via subsurface pathways. Previous studies have evaluated the reactions in soil of P released in the leachate from poultry litter (Robinson and Sharpley, 1996; Sharpley and Sisak, 1997). This study aims to extend and develop these evaluations by investigating a wider range of organic sources of P.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Soil Characteristics
Samples were taken from the 0- to 20-cm layer of five agricultural soils in Berkshire, UK. The soils were selected to represent typical, continuously cultivated (for wheat [Triticum aestivum L.] and maize [Zea mays L.]), limed, loam topsoils with a range of Olsen-extractable (plant-available) P values (Olsen et al., 1954). Soils were air-dried and sieved (<2 mm). Classification and selected properties of the soils are shown in Table 1 and also presented in Siddique at al. (2000). Soils were analyzed for sand, silt, and clay contents by pipette analysis following dispersion by sodium hexametaphosphate (Day, 1965). Organic matter was estimated as loss-on-ignition, and pH was measured in CaCl2 at a soil to solution ratio of 1:2.5 (Rowell, 1994). Olsen P was extracted from 1 g soil by 20 mL 0.5 M NaHCO3 (pH 8.5) for 30 min (Olsen et al., 1954). For the determination of labile inorganic P, air-dried soil was extracted with a mixed cation–anion exchange resin in the NaHCO3 form (Saggar et al., 1990). Total P was determined after digestion of oven-dried samples in a nitric acid–perchloric acid mixture (Olsen and Sommers, 1982). Values for P sorption maximum and binding energy were determined from Langmuir P sorption isotherms; these parameters, and the P sorption index, are described later for the soil–P source incubations.


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  		Table 1. General and phosphorus-related properties of the five soils.

 
Organic Waste Characteristics
The four organic wastes used in the study were poultry litter, poultry manure, cattle slurry, and urban sewage sludge. They were chosen to represent common alternatives to inorganic P fertilizers in agriculture, while varying widely in their physical and chemical characteristics. Poultry litter and manure samples were obtained from broiler houses in southeastern England. The poultry litter was a mixture of manure and wood shavings (bedding material). The cattle slurry was sampled from an aboveground storage tank at Reading University farm, southeastern England. The slurry was produced from dairy cattle housing. In the storage tank, the slurry had been subjected to regular mixing to maintain aeration and to prevent sedimentation or surface crusting. The sewage sludge was a biological, sludge cake product, obtained from Thames Water Utilities, UK. The sewage sludge had been produced by the activated sludge process followed by mesophilic anaerobic digestion and then processed for the agricultural and horticultural markets by dewatering, centrifugation, and compression. The sludge did not receive any chemical treatment. Potassium orthophosphate was the inorganic P fertilizer source used in this experiment.

The organic waste samples were thoroughly mixed. Subsamples of the poultry litter and sewage sludge were lightly ground to pass through a 2-mm sieve. Poultry manure and cattle slurry subsamples were also passed through a 2-mm sieve to remove any discernible fibrous material. All materials were stored in polythene containers at 4°C. These subsamples were used for the determination of the general properties in duplicate (Table 2). The dry matter content was determined on a 105°C basis. The pH of the sludge and litter was measured in water at a solid to solution ratio of 1:2.5, after allowing the suspension to equilibrate for 30 min (Ministry of Agriculture, Fisheries and Food, 1986). The pH of the slurry was determined in a continually stirred, undiluted suspension. The loss-on-ignition of the materials was determined by combustion (Sommers et al., 1976). For determination of water-extractable P, 1 g of fresh material (dry weight) was extracted by end-over-end shaking with 200 mL of distilled deionized water for 1 h, centrifuging, and filtering (0.45 µm). Total P, Ca, and Fe contents were determined after digestion of oven-dried samples in a nitric acid–perchloric acid mixture. All extracts were filtered through a Whatman (Maidstone, UK) no. 42 filter paper before analysis for P, Ca, and Fe. The Ca and Fe were determined by inductively coupled plasma atomic emission spectroscopy (ICP–AES) analysis. The total N content was determined by a semimicro-Kjeldahl procedure (Bremner and Mulvaney, 1982).


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  		Table 2. Properties of the animal manures and sewage sludge.

 
Soil–Phosphorus Source Incubations
Air-dried and sieved (<2 mm) samples (100 g) of the Yattendon, Swanwick, Wickham, Sonning I, and Sonning II soils were mixed thoroughly with poultry litter, poultry manure, cattle slurry, sewage sludge, or KH2PO4 at rates equivalent to 100 mg P kg-1 soil. Incorporation of the P sources into the soil was achieved by mixing for a period of 5 min with a spatula. Distilled water was added to bring the incubation mixtures to the field capacity of the soil. The mixtures were incubated in duplicate at this moisture level at 25°C for a period of 20 d. After the incubation period, P sorption and availability parameters were determined on duplicate samples.

Phosphorus Sorption and Availability
The P sorption method was adapted from Nair et al. (1984). Duplicate samples (5 g air-dry equivalent) of the incubation mixtures were equilibrated under continuous shaking for 24 h with 50 mL 0.01 M CaCl2 solution containing from 0 to 150 mg P L-1 plus three drops of toluene (microbial biocide). For each P addition, sorbed P (X) was calculated as the difference between P added and P in the filtered (Whatman no. 42 filter paper) supernatant (C) following equilibration (Nair et al., 1984). Following correction of the sorption data for the amount of adsorbed P after incubation, measured as Olsen P, Langmuir P sorption isotherms were plotted in the linearized form: C/X = C/Smax + (KSmax)-1, where Smax is sorption maximum (mg kg-1), and K is binding energy (L mg-1). The Smax was calculated as the reciprocal of the slope, and K as slope/intercept (Syers et al., 1973). The phosphorus sorption index (PSI) was calculated with the quotient X/log C following the addition of 1.5 g P kg-1 (Bache and Williams, 1971).

For measuring P availability, duplicate samples (5 g air-dry equivalent) of the incubation mixtures were shaken in 25 mL 0.01 M CaCl2 for 30 min. The extracts were filtered through Whatman no. 42 filter paper. Extraction with 0.01 M CaCl2 evaluates the intensity factor, or the concentration of P in the soil solution (Asyling, 1964). For the determination of labile inorganic P, duplicate samples (1 g air-dry equivalent) were extracted with a mixed cation–anion exchange resin in the NaHCO3 form (Saggar et al., 1990).

Release of Calcium in the Soil–Phosphorus Source Incubations
Soil-exchangeable Ca was determined in duplicate samples of the incubated soils. The exchangeable Ca was determined in extracts obtained by shaking with 0.1 M BaCl2 for 2 h at a soil to solution ratio of 1:20 (Hendershot and Duquette, 1986). The Ca in the filtered (Whatman no. 42 filter paper) extract was measured by inductively coupled plasma atomic emission spectroscopy analysis.

Phosphorus Analysis
Inorganic P in all extracts was measured as dissolved reactive P by the colorimetric method of Murphy and Riley (1962). Organic P in the CaCl2 and resin extracts was calculated as the difference between total and inorganic P contents where total P was measured as inorganic P following acid digestion of the filtrate in ammonium persulfate (Bowman, 1989).

In all cases, the results are presented as means of duplicates. Statistical differences and significance of treatment effect were evaluated by analysis of variance.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Characteristics of the Soils and Organic Wastes
The wide range in P status among the soils (Table 1) is attributable to different histories of P fertilizer and manure applications and cropping (Siddique, 2000). The organic wastes displayed marked variation in all properties with the exception of pH (Table 2). For example, all wastes differed significantly (P < 0.05) in terms of their contents of dry matter (101–720 g kg-1), water-soluble P (1.7–6.6 g kg-1), and Ca (0.8–42 g kg-1).

Effect of the Phosphorus Additions on Phosphorus Sorption
The addition of all P sources significantly (P < 0.05) decreased the P sorption maximum and binding energy in the five soils (Table 3). In all soils, the negative effects of the poultry litter, poultry manure, and sewage sludge on P sorption maximum and binding energy were very similar and, in the case of the poultry litter and manure, were statistically the same (Table 3). In the cattle slurry– and KH2PO4–treated soils, the decreases in P sorption maximum (19–66%) and binding energy (25–89%) were consistently larger than the corresponding decreases (7–41% and 11–30%) following treatment with poultry litter, poultry manure, and sewage sludge. In the cattle slurry and KH2PO4 incubations, the decrease in P sorption maximum relative to the control differed among the five soils (Table 3). For example, in the cattle slurry–treated Yattendon soil, the P sorption maximum decreased by as much as 181 mg kg-1 compared with much lower decreases in the cattle slurry–treated Swanwick (56 mg kg-1), Wickham (91 mg kg-1), Sonning I (95 mg kg-1), and Sonning II (73 mg kg-1) soils. The size of the decrease in P sorption maximum following incubation with cattle slurry and KH2PO4 was inversely related to the degree of phosphorus saturation (DPS) in the control soil, by a logarithmic function (r = 0.73, significant at the 0.01 probability level), where the DPS was expressed as the ratio of Olsen P to P sorption maximum. The fact that the decrease in P sorption maximum in the Yattendon soil (181 mg kg-1) was much larger than the total amount of applied P also indicates the potential impact of organic acids in blocking sorption sites. Other researchers also have reported decreases in soil P sorption characteristics following the application of animal manures and effluents (Sharpley et al., 1984; Mozaffari and Sims, 1994, 1996; Holford et al., 1997). In incubation studies, Mozaffari and Sims (1996) determined the influence of poultry litter applications on the phosphorus sorption index (PSI) in three coarse-textured soils from Delaware. They reported that the litter treatment decreased the PSI by 3 to 19 and 12 to 24% at application rates of 227 and 454 mg P kg-1 soil, respectively. In the current experiment, at an application rate of 100 mg P kg-1, the poultry litter addition decreased PSI values by 9 to 34% (data not presented).


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  		Table 3. Soil P sorption maximum (Smax) and binding energy (K), calculated from Langmuir isotherms, following incubation with the different P sources at rates equivalent to 100 mg P kg-1.

 
Effect of the Phosphorus Additions on Solution and Labile Phosphorus
The addition of all P sources increased the CaCl2–P and resin P in the five soils, and the extent of these changes was dependent on the P source (Fig. 1) . In all soils except Wickham, the positive effect of P source on CaCl2–P increased significantly (P < 0.05) in the order: poultry manure < litter < sludge < KH2PO4 < slurry (Fig. 1a). In the Wickham soil there was no significant difference (P > 0.05) between the poultry manure, litter, and sludge treatments. In three of the soils (Yattendon and Sonning I and II), the positive effect of P source on resin P increased significantly (P < 0.05) in the order: poultry manure < litter < sludge < KH2PO4 < slurry (Fig. 1b). In the Swanwick and Wickham soils, there was no significant difference (P > 0.05) between the effects of poultry manure, litter, and sewage sludge (Fig. 1b). Many previous researchers have reported increases in solution and labile pools of P in soils following animal manure and sewage sludge applications but did not evaluate these effects as a function of P source (Sharpley et al., 1984; Mozaffari and Sims, 1996; Nair et al., 1998).



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  		Fig. 1. (a) Calcium chloride P and (b) resin P in five soils following incubation with no P addition (control) or with poultry litter, poultry manure, cattle slurry, sewage sludge, or KH2PO4 applied at 100 mg P kg-1 soil.

 
It is acknowledged that soil solutions often contain significant proportions of organic P, particularly those soils treated with organic wastes (Chapman et al., 1997). However, in the current study, as much as 95 ± (standard deviation) 2.1% of total P in the CaCl2 and resin extracts was in the dissolved, reactive form (data not presented). The dominance of dissolved inorganic P may be attributed to the prevalence of inorganic forms of P in the labile fractions of the soil and P sources (Sharpley and Moyer, 2000; Siddique et al., 2000). Ion chromatography would be needed for a more accurate analysis of the true content of inorganic ortho-P.

It is clear that in all soils, cattle slurry and KH2PO4 consistently had a much larger influence on P sorption characteristics, CaCl2–P, and resin P than did sludge and litter (Table 3; Fig. 1). Similar to the current experiment, Reddy et al. (1980) conducted a laboratory incubation study to determine the effect of animal waste loading on the P adsorption–desorption characteristics of two sandy soils. Unlike the current work, Reddy et al. (1980) applied the wastes at very high and variable P rates (up to 345 mg P kg-1 soil). Nevertheless, they calculated decreases in P sorption capacity and increases in several measurements of P solubility and availability per unit of P (100 mg kg-1 soil) applied. The extent of these changes, per unit of P applied, was dependent on the waste, and followed the order: beef = poultry < swine. Reddy et al. (1980) generally attributed these effects of the manures to saturation of P sorption sites and to the possibility of competition for P adsorption sites from organic acids that were released during the decomposition of organic matter in the wastes. However, no explanation was given for the different effects on soil P solubility of the three waste materials studied. Holford et al. (1997) studied the P sorption characteristics of a range of loam soils that had received chicken litter, poultry manure, dairy cattle and/or pig effluent, and superphosphate, in various combinations. In most cases, waste applications decreased both the P sorption capacity and sorption strength in the soils. It was suggested that the P-containing wastes were not only blocking P sorption sites but may also be lowering the P sorption strength by organic anion interactions. The low sorption capacity and strength of the waste-treated soils indicated that these new sites adsorb P very weakly and perhaps allow leaching to occur. Similarly, Weaver and Ritchie (1994) found that pig effluent increased the leaching of P and that this was related to the capacity of the soil to adsorb P.

Factors Controlling Phosphorus Sorption and Availability in the Amended Soils
Although the P sources were incubated with the soils on a total P equivalent basis, the water-extractable P content of the P sources was highly variable (Table 2). As a consequence, the addition of 100 mg P kg-1 soil provided a range of applications of water-extractable P, from 8 (sewage sludge) to 100 mg kg-1 (KH2PO4). The wide range of P solubility reflects the diverse forms of inorganic and organic P forms in the different wastes. The inorganic P forms display a continuum of solubility while the P-containing organic forms (nucleic acids, inositol polyphosphates, and lipids) mineralize at different rates during extraction with water. In the vast majority of cases, the additions of water-extractable P had no significant effect (P > 0.05) on the increases in CaCl2–P and resin P or on the decreases in P sorption maximum and binding energy (Table 4). Therefore, the differential reactions of the various sources of P in the soil were not explained by the water-extractable P contents of the sources. On the basis of the current data, it is argued, therefore, that the simple relationship between water-extractable P and the potential for land-applied wastes to enrich leachate and surface runoff P (Dou et al., 2000; Sharpley and Moyer, 2000) is complicated by reactions involving the soil matrix and non-P-containing waste constituents.


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  		Table 4. Values for the correlation coefficient, r, for the linear relationship between water-soluble P added in the different P sources and the change ({Delta}) in CaCl2–P, resin P, P sorption maximum, and binding energy following incubation.

 
The Ca contents of the different P sources were highly variable and decreased in the order: sewage sludge (42 g kg-1) > poultry manure (32.4 g kg-1) > poultry litter (13.5 g kg-1) > cattle slurry (0.8 g kg-1) (Table 2). It has been reported that the Ca released from some organic wastes may have a significant influence on the sorption and availability of P in soils (Robinson and Sharpley, 1996; Siddique et al., 2000). In this study, the organic waste treatments provided a range of Ca additions to soil, from 11.3 mg kg-1 soil (cattle slurry) to 200 mg kg-1 soil (sewage sludge). However, in the vast majority of cases (85%), there was no significant relationship (P > 0.05) between Ca added and the changes in P availability and sorption (data not presented). It is likely that the amounts of Ca released were determined by the availability of the Ca in the wastes as well as the size of the total pool added. The availability of the organically bound Ca in organic materials depends partly on the nature of the organic molecules to which the Ca is bound (Fletcher and Beckett, 1987; Wen et al., 1999). Generally, fresh manure and biological sewage sludges contain a high proportion of aliphatic C associated with polysaccharides and proteinaceous compounds, which are of low molecular weight and are shorter in length. Composts, on the other hand, contain a higher percentage of humic acids that are similar to those in soils (Deiana et al., 1990). Therefore, Ca is readily available in fresh manures and sewage sludges and can contribute significantly to the pool of soil-exchangeable Ca. The increase in the content of exchangeable Ca in the soil, relative to the initial content, provides a reasonable estimate of the amount of Ca released from the P source. Previous workers have used the increase in the amount of exchangeable Ca ({Delta}Ca) to estimate the release of Ca from rock phosphate in soils (Hughes and Gilkes, 1984). Following the application of poultry litter, manure, and sewage sludge, the exchangeable Ca contents in all soils increased markedly relative to the control (Table 5). Exchangeable Ca did not differ significantly from the control following the slurry and KH2PO4 treatments, both of which contain negligible Ca (0.8 and 0 g kg-1, respectively).


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  		Table 5. Exchangeable Ca in the five soils following incubation with no P addition (control) or with different P sources at rates equivalent to 100 mg P kg-1.

 
It is hypothesized that the relatively small changes in P sorption characteristics and CaCl2–P following poultry litter, manure, and sludge additions, compared with the control (Table 3, Fig. 1a) were partly due to the high contents and ready availability of Ca in these materials. The increase in CaCl2–P following the addition of the poultry litter, manure, and sewage sludge was inversely related (R2 = 0.75; P < 0.001) to the increase in soil-exchangeable Ca ({Delta}Ca) and supports this hypothesis (Fig. 2a) . The data imply that the large inputs of Ca in the litter-, manure-, and sludge-amended soils (100, 137, and 200 mg Ca kg-1 soil, respectively) had a dominant role in regulating the readily available pool of P. As stated earlier, water-soluble P alone had no significant influence (P > 0.05) on the changes in P availability or sorption. However, multiple regression analysis showed that a combination of water-soluble P and {Delta}Ca explained as much as 86% of the variation in the increase in CaCl2–P in the litter-, manure-, and sludge-amended soils. A possible mechanism for the buffering of the water-soluble P inputs in these soils is through the adsorption of the P by metastable Ca-P precipitates (e.g., dicalcium phosphate) that may have formed close to the P source, where the Ca and P concentrations are at their highest. Increases in resin P were less sensitive to the release of Ca (Fig. 2b). The relatively poor inverse relationship (R2 = 0.37; P < 0.05) between the increase in resin P and {Delta}Ca may have been due to the resin extraction releasing poorly defined, variable amounts of P from metastable Ca-P precipitates through dissolution reactions and/or the dissolution of non Ca-P materials; for example, Al- and Fe-phosphates.



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  		Fig. 2. Relationship between the increase in (a) CaCl2–P and (b) resin P and change in exchangeable Ca ({Delta}Ca) for soils following incubation with poultry litter, poultry manure, and sewage sludge applied at 100 mg P kg-1 soil.

 
There were no significant correlations between {Delta}Ca and Smax or binding energy. The apparent lack of any significant influence of the Ca inputs on the P sorption characteristics may have been due to P precipitation acting as the dominant P retention mechanism in the soil matrix during the method to determine the Smax and binding energy. The method involves the addition of large concentrations of P (up to 150 mg L-1) to soil that already has an elevated P and Ca status. In these conditions, precipitates of Ca-P can form. Other workers have interpreted discontinuities in P adsorption measurements at high concentrations (near mM) of P in the soil solution as evidence for precipitation of P (Sample et al., 1980).

In the slurry- and KH2PO4–amended soils, the relatively low and negligible inputs of Ca (11 and 0 mg Ca kg-1 soil) contributed very little to the P sorption characteristics of the soils. The values for CaCl2–P and resin P in the cattle slurry–treated soils were consistently higher than those in the KH2PO4 treatments (Fig. 1). These data imply that soils receiving cattle slurry applications are more susceptible to P leaching than are the same soils receiving the same amount of P as an inorganic source, possibly through the input of soluble organic compounds that may form metal complexes, which block available P retention sites (Nagarajah et al., 1970).


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Application of poultry litter, poultry manure, cattle slurry, sewage sludge, and KH2PO4 decreased the P sorption maximum and binding energy and increased CaCl2–P and resin P in five loam soils with variable initial P status. In most cases, the influence of P source on the extent of these changes increased in the order: poultry manure < litter < sewage sludge < KH2PO4 < cattle slurry. However, the magnitude of the changes were not related to the content of water-soluble P in the different sources. In the poultry manure, litter, and sludge treatments, correlation analysis implied that soil solution P concentration (CaCl2–P) was regulated by large inputs of available Ca, possibly through the formation or presence of Ca-P precipitates. High concentrations of both Ca and P were likely to exist in the zone of application of these two high Ca-status materials. With regard to the higher solubility of P in the slurry- than KH2PO4–treated soils, it was speculated that the organic acids released in the slurry treatments effectively blocked P sorption sites, thereby reducing the effective P sorption capacity.

Future work on the agricultural and environmental sustainability of organic waste applications to soils needs to investigate the non-P- as well as P-containing components of the waste. Focus should be on the speciation of the organic acid component of organic wastes to further increase our understanding of their Ca binding capacities and their ability to block P sorption sites in the soil. Generally, the current findings support the argument that future, P-based applications to soils that are susceptible to P leaching should not rely solely on data obtained from inorganic P fertilizer studies.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 


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C. H. Bolster and G. M. Hornberger
On the Use of Linearized Langmuir Equations
Soil Sci. Soc. Am. J., September 28, 2007; 71(6): 1796 - 1806.
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K. J. Parsons, V. D. Zheljazkov, J. MacLeod, and C. D. Caldwell
Soil and Tissue Phosphorus, Potassium, Calcium, and Sulfur as Affected by Dairy Manure Application in a No-Till Corn, Wheat, and Soybean Rotation
Agron. J., September 10, 2007; 99(5): 1306 - 1316.
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V. R. Haden, Q. M. Ketterings, and J. E. Kahabka
Factors Affecting Change in Soil Test Phosphorus Following Manure and Fertilizer Application
Soil Sci. Soc. Am. J., June 8, 2007; 71(4): 1225 - 1232.
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J. P. Casson, D. R. Bennett, S. C. Nolan, B. M. Olson, and G. R. Ontkean
Degree of Phosphorus Saturation Thresholds in Manure-Amended Soils of Alberta
J. Environ. Qual., October 27, 2006; 35(6): 2212 - 2221.
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J. T. Spargo, G. K. Evanylo, and M. M. Alley
Repeated Compost Application Effects on Phosphorus Runoff in the Virginia Piedmont
J. Environ. Qual., October 27, 2006; 35(6): 2342 - 2351.
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S. K. Marshall and C. A. M. Laboski
Sorption of Inorganic and Total Phosphorus from Dairy and Swine Slurries to Soil
J. Environ. Qual., August 9, 2006; 35(5): 1836 - 1843.
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R. C. Schwartz and T. H. Dao
Phosphorus Extractability of Soils Amended with Stockpiled and Composted Cattle Manure
J. Environ. Qual., April 20, 2005; 34(3): 970 - 978.
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G. M. Pierzynski and K. A. Gehl
Plant Nutrient Issues for Sustainable Land Application
J. Environ. Qual., January 1, 2005; 34(1): 18 - 28.
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A. N. Sharpley, R. W. McDowell, and P. J. A. Kleinman
Amounts, Forms, and Solubility of Phosphorus in Soils Receiving Manure
Soil Sci. Soc. Am. J., November 1, 2004; 68(6): 2048 - 2057.
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