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Journal of Environmental Quality 31:1279-1285 (2002)
© 2002 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

TECHNICAL REPORTS
Surface Water Quality

Phosphorus Saturation in Spodosols Impacted by Manure

V.D. Nair* and D.A. Graetz

Soil and Water Science Dep., 106 Newell Hall, P.O. Box 110510, Univ. of Florida, Institute of Food and Agricultural Sciences, Gainesville, FL 32611-0510

* Corresponding author (vdna{at}mail.ifas.ufl.edu)

Received for publication August 15, 2001.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Significant amounts of phosphorus (P) accumulate in soils receiving animal manures that could eventually result in unacceptable concentrations of dissolved P loss through surface runoff or subsurface leaching. The degree of phosphorus saturation (DPS) relates a soil's extractable P to its P sorbing capacity, and is reportedly a predictor of the P likely to be mobilized from a system. A DPS value (DPS-1) was derived that expressed the percentage of Mehlich 1–extractable P to the sorbing capacity of a Spodosol (expressed as the sum of oxalate-extractable Fe and Al). Values of DPS-1 were determined in various horizons of soil in current and abandoned dairy systems in South Florida's Lake Okeechobee watershed to assess P release potential. Land use within the dairies was classified as highly impacted by cattle (intensive and holding), and minimally impacted by cattle (pasture, forage, or native) areas. The A and E horizon of soils in heavily manure-impacted intensive and holding areas for both active and abandoned dairies generally had higher DPS-1 values than the pasture, forage, and native area soils, which were minimally impacted by manure. Degree of P saturation was also calculated as a percentage of Mehlich 1–extractable P to the sum of Mehlich 1–extractable Fe and Al (DPS-2). Both DPS-1 and DPS-2 were shown to be significantly (P = 0.0001) related to water-extractable P for all soil horizons, suggesting that either index can be used as an indicator for P loss potential from a soil.

Abbreviations: DAAl, DAFe, and DAP, double acid–extractable aluminum, iron, and phosphorus, respectively • DPS, degree of phosphorus saturation • DPS-1, double acid–extractable P/oxalate-extractable Fe and Al • DPS-2, double acid–extractable P/double acid–extractable Fe and Al • HIM, soils highly impacted by manure • MIM, soils minimally impacted by manure • OxAl, OxFe, and OxP, oxalate-extractable aluminum, iron, and phosphorus, respectively • Smax, Langmuir sorption maximum • WSP, water-soluble phosphorus


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
PHOSPHORUS becomes a pollutant if it moves from the site of its intended use to surface waters either via surface runoff or subsurface drainage. The Spodosols of Florida receiving significant loadings of animal manure are especially prone to subsurface leaching of P. Spodosols account for more than 64% of the soils in Okeechobee County of Florida where Lake Okeechobee, one of the largest freshwater lakes in the USA, is located (Herdendorf, 1982). The A and E horizons of Spodosols are sandy and have poor sorption capacities while the Bh (spodic) and the Bw horizons have much greater affinities for P retention (Mansell et al., 1991). The Spodosols are characterized by a high water table located between the Bh and the A horizons during the summer rainy season, which drops to 125 cm during drier months (Soil Survey Staff, 1996). Rainfall that infiltrates the soil during high water table conditions may move laterally, transporting P to surface drainage ditches (Burgoa et al., 1991; Mansell et al., 1991).

Phosphorus losses are common concerns on pasture systems subjected to fertilizer or manure applications. The Okeechobee Basin in Florida is home to several large dairies and contains both soils minimally impacted by manure (MIM), such as pasture or forage soils, and some soils highly impacted by manure (HIM). Though concentrated in relatively small areas, the potential for HIM soils to alter water bodies via subsurface flow can be great (Nair et al., 1998). Total P concentrations in the A horizon of manure-impacted Spodosols range from 2300 mg P kg-1 under high cattle density to about 25 mg P kg-1 under low cattle density (Nair et al., 1998). Nair et al. (1999) used water-soluble P and Mehlich 1–extractable P or double acid–extractable phosphorus (DAP) as indicators of the potential for P to be released from manure-impacted soils. Other indicators of the P status in soils include soil tests such as Bray 1, Olsen P (Haygarth and Jarvis, 1999), and Mehlich 3–extractable P (Sharpley and Tunney, 2000). The use of iron oxide–impregnated paper (Haygarth and Jarvis, 1999) and P sorption saturation methodology, also referred to as degree of P saturation, DPS (Sharpley and Tunney, 2000), may also be used as P status indicators. Sharpley and Tunney (2000) assembled examples from various states of the USA where soil test P values (Mehlich 3, Mehlich 1, Olsen, Bray, or Texas A&M test) are used as guides for management recommendation for water quality protection.

The concept of the Phosphorus Index for predicting P losses is currently being developed in several parts of the USA, though subsurface P losses have not been incorporated in the Phosphorus Index to the extent of surface runoff losses (Heathwaite et al., 2000). The Phosphorus Index was developed as a field-based index that could be used to assess site conditions and potential vulnerability to P loss. The index uses transport factors such as soil erosion, soil runoff class, and distance from a watercourse and source factors such as soil test P, P application method, and application rate.

Attempts to incorporate subsurface runoff potential in the Phosphorus Index are currently being tested in Florida (The Florida Phosphorus Index, http://www.fl.nrcs.usda.gov/flgeneral/pifinal.pdf, verified 7 Mar. 2002), based primarily on visual observation of the location of the Bt horizon within the soil profile. Sims et al. (2000) presented an overview of the Phosphorus Index and the use of soil P testing as a parameter in formulating the Phosphorus Index in parts of the USA. However, soil tests were not originally designed or calibrated for environmental purposes, that is, the difference between critical soil test P (STP) values for agronomic yields and critical STP values for P loss are generally not known. Thus, many agronomists are reluctant to use STP values for environmental purposes (Sharpley et al., 1999).

Studies in the Netherlands (Breeuwsma and Silva, 1992) showed that P concentrations in the soil solution can approach a critical concentration well before the soil is completely saturated with P. Dutch scientists have developed a test referred to as the "degree of P saturation" (DPS), which related an extractable P concentration to the soil P sorption capacity (PSC) as follows:

An overview of the Dutch approach was presented by Shoumans and Groenendijk (2000). Van der Zee et al. (1988) defined the saturation factor, {alpha}, as the ratio of the amount of P sorbed in laboratory experiments and the P already present as oxalate-extractable P (OxP), to oxalate-extractable Fe and Al (OxFe + OxAl). Thus, {alpha} allows comparison of different soils with respect to P saturation, because the results are normalized with respect to the reactive soil constituents. The oxalate-extractable P (OxP) was determined by extraction with 0.2 M ammonium oxalate buffered to pH 3.0.

Sharpley (1995) suggested calculating P sorption saturation as the percentage of P sorption maximum (calculated using Langmuir isotherms) extractable as Mehlich 3–extractable P (Mehlich, 1984). In the southeastern and mid-Atlantic USA, Mehlich 1 or double acid–extractable P (Mehlich, 1953) is used routinely as the soil test P. Oxalate-extractable P is seldom determined in any of the soil testing laboratories. Thus we hypothesized that it might be more practical to express the DPS as the percentage of DAP (in moles) to the P sorption capacity of the soil. Due to the time involved in determining adsorption isotherms, the PSC in the Dutch test was estimated as the sum of oxalate-extractable Fe and Al (OxFe + OxAl), expressed in moles.

Beauchemin et al. (1996) cautioned that P leaching potential should not be predicted from the DPS value alone. Rather, they recommended a measure of P desorbability (e.g., water-soluble P or soil test P) to "fully assess the risk of contamination of drainage waters by P leaching." Thus, management recommendations intended to minimize loss of P via subsurface drainage must consider the relationship between degree of P saturation (DPS) and water-soluble P (or soil test P).

The USDA Natural Resources Conservation Service (NRCS) policy for nutrient management in Florida uses the Phosphorus Index to determine whether manure utilization may be nitrogen or phosphorus based. In addition, the policy stresses the use of the Phosphorus Index in designated P-limited areas as well, which will include all manure-impacted areas, including the heavy manure-impacted areas. While the first of the policies will primarily involve MIM soils, the latter policy will be applicable to HIM soils as well.

Our objectives were to (i) determine the degree of P saturation in differentially manure-impacted Spodosols using a combination of DAP, OxFe and OxAl concentrations (DPS-1) or DAP, double acid–extractable iron (DAFe), and double acid–extractable aluminum (DAAl) concentrations (DPS-2); (ii) determine the relation between water-soluble P (WSP) and DPS-1 and DPS-2 for MIM soils; (iii) examine the possibility of using DPS-1 to track P movement through a HIM soil profile; and (iv) determine {alpha} (the maximum saturation factor for total sorption) for Spodosols.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Soil Sampling
Soil profiles were selected from three active dairies, three abandoned dairies, and two native areas (forested areas not yet significantly impacted by cattle activities). Two beef pastures were also included in this study. Detailed information on these sites are provided in Graetz et al. (1999). All sites were on Spodosols, that is, Myakka fine sand (sandy, siliceous, hyperthermic Aeric Alaquods) or Immokalee fine sand (sandy, siliceous, hyperthermic Arenic Alaquods).

Four components of each active dairy were sampled: intensive areas (areas next to the barn where cattle are held immediately prior to milking), holding areas (area where cattle are fed and held overnight), pasture (areas used for grazing), and forage areas (used for forage production). Only the intensive and holding components, the soils highly impacted by manure (HIM), were sampled for the abandoned dairies. Four randomly located profile samples were collected for each component within each location. These profile samples were divided into horizons A, E, Bh (spodic), and Bw.

Soil Characterization
Oxalate-extractable Al (OxAl) and Fe (OxFe) were determined by extraction with 0.1 M oxalic acid + 0.175 M ammonium oxalate (pH = 3.0) solution (McKeague and Day, 1966). The suspension was equilibrated for 4 h with continuous shaking, centrifuged, filtered through a 0.45-µm filter, and analyzed for Al and Fe. Mehlich 1 or double acid–extractable P (DAP), Fe (DAFe), and Al (DAAl) were obtained using a 1:4 soil to double acid ratio (Mehlich, 1953). Water-soluble P was determined by extracting the soil with water at 1:4 soil to water ratio, and determining P on the filtrate collected after passing through a 0.45-µm filter. All metals were determined using atomic adsorption spectroscopy. Soil pH was determined on a 1:2 soil to water ratio, and the C content of the air dried samples was determined by an automated combustion procedure using a Carlo Erba (Milan, Italy) CNS Analyzer.

Phosphorus Sorption
Phosphate sorption was measured using 2 g of air-dried, homogenized soil treated with 20 mL of 0.01 M KCl solution containing various levels of P (0, 0.01, 0.1, 5, 10, 25, 50, 100 mg P L-1) in 50-mL centrifuge tubes. The tubes were shaken on a mechanical shaker for a 24-h equilibration period. The suspensions were allowed to settle for an hour, the supernatant was filtered through a 45-µm filter, and the filtrate was analyzed for soluble reactive P (SRP) on a UV visible recording spectrophotometer at a wavelength of 880 nm. All extractions and determinations were at room temperature (298 ± 3 K).

Calculations
The linear plot of the Langmuir equation, C/S = (1/kSmax) + (C/Smax), was used to calculate the adsorption maximum, Smax, and a constant, k, related to P bonding characteristics (Nair et al., 1998). The term C is the concentration of P remaining in solution after a 24-h equilibration, and S is the actual P sorbed.

The formula for DPS-1 is:

[1]
where {alpha} is the saturation factor that allows comparison of different soils with respect to P saturation. The value of {alpha} was taken as 1, since DPS-1 in this case is used for comparison purposes for the same soil type (Spodosols).

The formula for DPS-2 is:

[2]
where P, Fe, and Al are determined in the same double-acid extract. The value of {alpha} was taken as 1. The terms DPS-1 and DPS-2 differ only in the solution used in the extraction of Fe and Al.

Statistical Analyses
Statistical analyses were performed using software of the Statistical Analysis System (SAS Institute, 1989a,b).


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Degree of Phosphorus Saturation in Differentially Manure-Impacted Spodosols
Values of the degree of phosphorus saturation (DPS-1), as calculated using Eq. [1], are presented in Table 1 . The HIM soils for both active and abandoned dairies generally had higher DPS-1 values than the pasture (dairy or beef), forage, and native areas (MIM soils) for the A and E horizons. A low DPS value does not necessarily mean that a soil is suitable for further P loading; due consideration must be made of the capacity of a soil to retain P, especially when considering a site for locating a new dairy. Sorption capacity of a soil will depend on soil physical properties such as clay content, and also chemical properties such as Al and Fe concentrations. Mean DPS-1 for the Bh horizon of HIM soil was 75% compared with 2% for the Bh horizon of MIM soils, suggesting that the Bh horizon of HIM soil are likely to release P laterally above the horizon in spite of the large Al (and Fe) concentrations that are typical of the horizon. Water-soluble P concentrations in the Bh horizons of HIM soil were also much higher than in the Bh horizons of MIM soils.


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  		Table 1. Phosphorus concentrations and degree of P saturation (DPS-1 = double acid–extractable P/oxalate-extractable Fe and Al x 100) for the various land use components, by horizon.

 
Although WSP and DAP concentrations were higher in the Bh horizons than in the E horizons for both the HIM and the MIM soils, large DPS-1 values suggest that the E horizons will likely release P to the water bodies via lateral transport due to the poor sorbing capacity of these soils. Values of DPS (DPS-1) for the Bh and the Bw horizons were not different (p < 0.05) among the differentially manure-impacted soils. This is because of the large concentrations of Al in these horizons (Table 2) , which serve as good sorbing sites for P.


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  		Table 2. Comparison of pH, C content, and double acid–extractable Ca (DACa), Mg (DAMg), Fe (DAFe), and Al (DAAl) for the various land use components, by horizon.

 
Relationship of DPS-1 and DPS-2 to Water-Soluble Phosphorus for Soils Minimally Impacted by Manure
Simple regression equations indicate that DPS-1 is a useful measure of WSP, not only in the A horizon, but also for subsurface horizons. For MIM soils, where WSP was <6 mg P L-1, the relationships between WSP and DPS-1 (Fig. 1) for the A horizon and the Bh horizon are:



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  		Fig. 1. Relationships between water-soluble P and DPS-1 (double acid–extractable P/oxalate-extractable Fe and Al) and DPS-2 (double acid–extractable P/double acid–extractable Fe and Al) for soils of the A and Bh horizons of Spodosols.

 
A relationship between WSP and DPS-1 for the E horizon was not established since this horizon has poor retention capacity and may be considered a temporary medium for P movement.

The use of the above relationship requires the measure of DAP (in a double acid extract) as well as OxFe and OxAl (in an oxalate extract). While DAP is a routine measurement in many soil laboratories, OxFe and OxAl determinations are not made often. The use of (OxFe + OxAl) as a predictor of the sorbing capacity of a soil is well documented (Breeuwsma and Silva, 1992; Nair et al., 1998) whereas the use of DAFe + DAAl as a measure of the sorbing capacity of a soil is not well established. Therefore, the relationships between WSP and DPS-2 were examined for the same MIM soils, and the following relationships determined (Fig. 1):


The highly significant relationships suggest that DPS can be calculated using P, Fe, and Al concentrations from the same Mehlich 1 extract, and that DPS-2 can be used as an indicator of soluble P in both surface soils and deeper horizons.

The relationship between DPS-2 and DPS-1 can be expected to differ for the different horizons of Spodosols because P-retention mechanisms in the various horizons can vary (Nair et al., 1998). Oxalate and double acid solutions probably extract different forms of Fe and Al (and associated P), but may, nevertheless, be correlated. The polynomial relationships between DPS-1 and DPS-2 for the two most important horizons, the surface (A) and the spodic (Bh), are shown in Fig. 2 . The corresponding significant (P < 0.0001) linear relationships are:



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  		Fig. 2. Relationships between DPS-1 (double acid–extractable P/oxalate-extractable Fe and Al) and DPS-2 (double acid–extractable P/double acid–extractable Fe and Al) for soils of the A and Bh horizon of Spodosols.

 
These significant relationships (P < 0.0001) suggest that it is possible to convert DPS-1 to DPS-2 for a given Spodosol horizon, and will allow DPS data conversion from those determined in most analytical laboratories to information available in research laboratories.

Determination of {alpha} for Spodosols
The slope of the relationship between Smax and (OxFe + OxAl) is a measure of the maximum saturation factor for reversible adsorption (van der Zee et al., 1987). This slope is approximately one-third of the value of {alpha}, the maximum saturation factor for total sorption, as determined for coarse-textured soils in the Netherlands (van der Zee et al., 1987). Khiari et al. (2000) determined the slope for Quebec acid coarse-textured soils to be 0.22. The slope of the relationship between Smax and (OxFe + OxAl) was 0.18 (Fig. 3) , and {alpha} calculated as 0.55 for Spodosols. This value agrees well within ranges between 0.4 and 0.6 reported for Dutch soils by Breeuwsma and Silva (1992), 0.68 for Delaware soils (Paulter and Sims, 1998), and 0.66 for Quebec soils (Khiari et al., 2000). Our {alpha} values were derived from Smax values for all horizons and for all manure-impacted areas. However, the majority of the observations in Fig. 3 are for Bh and Bw horizons, as it is difficult to obtain Langmuir isotherms for sandy surface horizons of Spodosols (Ballard and Fiskell, 1974; Yuan and Lucas, 1982; Nair et al., 1998). Consequently, the calculated {alpha} value of 0.55 should be considered as a good estimate for Spodosols.



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  		Fig. 3. The Langmuir P sorption maximum (Smax) as a function of oxalate-extractable Fe and Al (OxFe + OxAl).

 
Introducing an {alpha} value of 0.55 into the equations (Eq. [1] and [2]) used in DPS calculations in our current studies increases all values by approximately 80%. Soil profiles with weighted DPS values > 25% in the soil profile were identified as contributing to ground water pollution with P in the Netherlands (Breeuwsma et al., 1995). Our data suggest that DPS-1 values above 45% (calculated by introducing an {alpha} value of 0.55 instead of 1.0 in DPS calculations corresponding to the Netherlands value of 25%) represent soils with environmentally unacceptable P levels. However, DPS values are strictly operationally defined and thus will depend on the calculation method. Sallade and Sims (1997) showed that for the same set of sediments, DPS values gave a mean of 4.5% (calculated using DAP as the extractable P and Langmuir P sorption capacity) and a mean of 43% (calculated using bioavailable P and Langmuir P sorption capacity). Therefore, a DPS value of 25% that is a critical value in the Netherlands may not be the critical value for the sandy soils of Florida and other parts of southeastern USA. Our ongoing research on sandy soils including Spodosols focuses on arriving at a critical DPS value for all Florida soils based on a suitable protocol for DPS calculations.

Phosphorus Movement in the Soil Profile of Soils Highly Impacted by Manure
Water-soluble P, DAP, and total P concentrations were higher in the A and E horizons (Table 1), but the HIM soils, particularly the abandoned dairies, did show higher WSP and DAP values in the Bh horizons compared with the less manure-impacted pasture, forage, and native area soils. Although the Bw horizon is at a considerable depth from the surface, WSP and DAP values were higher in this horizon, especially for the intensive component of both active and abandoned dairies. This suggests that there is some P that has moved through the Bh horizon, probably as Ca-P or Mg-P (Table 2).

There was a general trend for DPS-1 values to be higher for the abandoned dairies than for active dairies throughout the soil profile (Fig. 4) . The higher values in the subsurface may be attributed to movement of P with time for the abandoned dairies though such conclusions may be biased based on the tremendous variability in P concentrations, within the same dairy component. The Bh and the Bw horizons of the abandoned dairies show high Ca and Mg concentrations (Table 2), which are among the more important manure constituents (Nair et al., 1995), and together with pH values could also be used as an indicator of P movement through a soil profile.



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  		Fig. 4. The DPS-1 (double acid–extractable P/oxalate-extractable Fe and Al) values within a soil profile (A, E, Bh, and Bw horizons) of intensive and holding area (highly impacted by manure) soils for active and abandoned dairies.

 

   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
The use of double acid extractant for all parameters in the DPS equation is shown to be possible for Spodosols. In Florida, where DAP is the predominant soil test P analysis, this method of DPS calculation might be preferable to using information from multiple extractions. Determination of Fe and Al in the same double acid solution and subsequently calculating DPS shows that WSP can be predicted from these DPS-2 values. Since P, Al, and Fe determined in a single extract (double acid) can be related to WSP, this method for the calculation of DPS will be a valuable tool in places where double acid extracts are used routinely for soil test purposes.

Relationships between DPS-1 or DPS-2 and WSP were highly significant for the A horizon as well as for the Bh horizon, suggesting that DPS could be a good indicator of the potential of a soil for P loss, via surface or subsurface leaching. For Spodosols with varying P retention capacities for the different horizons, DPS values for the A horizon will be a good indicator of the potential of the soil to release P to runoff. For soils with less variability in P sorption characteristics within a soil profile, it might be possible to obtain better WSP–DPS relationships throughout the soil profile. The incorporation of {alpha} values in DPS calculations will be necessary if surface DPS values (calculated using the same methodology) are to be compared across a variety of soil types.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
This research was supported by the Florida Agricultural Experiment Station, and approved for publication as Journal Series no. R-08304.


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




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