Journal of Environmental Quality 32:1082-1088 (2003)
© 2003 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
TECHNICAL REPORTS
Surface Water Quality
Estimating Soil Phosphorus Requirements and Limits from Oxalate Extract Data
E. M. D'Angelo*,
M. V. Vandiviere,
W. O. Thom and
F. Sikora
University of Kentucky, Soil & Water Biogeochemistry Laboratory, Dep. of Agronomy, N-122 Agricultural Science Building North, Lexington, KY 40546-0091
* Corresponding author (edangelo{at}uky.edu)
Received for publication June 21, 2002.
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ABSTRACT
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Excessive fertilizer and manure phosphorus (P) inputs to soils elevates P in soil solution and surface runoff, which can lead to freshwater eutrophication. Runoff P can be related to soil test P and P sorption saturation, but these approaches are restricted to a limited range of soil types or are difficult to determine on a routine basis. The purpose of this study was to determine whether easily measurable soil characteristics were related to the soil phosphorus requirements (Preq, the amount of P sorbed at a particular solution P level). The Preq was determined for 18 chemically diverse soils from sorption isotherm data (corrected for native sorbed P) and was found to be highly correlated to the sum of oxalate-extractable Al and Fe (R2 > 0.90). Native sorbed P, also determined from oxalate extraction, was subtracted from the Preq to determine soil phosphorus limits (PL, the amount of P that can be added to soil to reach Preq). Using this approach, the PL to reach 0.2 mg P L-1 in solution ranged between -92 and 253 mg P kg-1. Negative values identified soils with surplus P, while positive values showed soils with P deficiency. The results showed that P, Al, and Fe in oxalate extracts of soils held promise for determining PL to reach up to 10 mg P L-1 in solution (leading to potential runoff from many soils). The soil oxalate extraction test could be integrated into existing best management practices for improving soil fertility and protecting water quality.
Abbreviations: PL, phosphorus limit • Preq, phosphorus requirement • S, total sorbed phosphorus • S0, native phosphorus sorbed • S1, added phosphorus sorbed
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INTRODUCTION
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DURING THE LAST DECADE, two main concerns have refreshed interest in phosphorus (P) concentrations of soils: (i) a critical amount of P is required for economic crop production, and (ii) soil P can contribute to eutrophication of aquatic environments (Sibbesen and Sharpley, 1997). While it is clear that P limits should be set relative to both concerns, it has been difficult to devise a suitable method or agree on a value by the various stakeholders (i.e., soil scientists, producers, regulators, and fertilizer companies).
Most P in soils is not immediately available for crop uptake, because P has a very low solubility and is strongly retained by several constituents in soils. Although a very dynamic equilibrium exists between P in solid and dissolved phases, P in the solid phase is favored by several-fold, so many soils do not have sufficient P in solution to meet plant needs. Over the last century, many soil P tests have been developed and calibrated for making fertility recommendations. However, when recommendations are not followed or are inaccurate, deficient or excessive amounts of P can accumulate in soils (Gartley and Sims, 1994; Potash and Phosphate Institute, 1994).
Losses of dissolved and particulate P from agricultural soils are a major concern in the USA and Europe. According to the 1998 National Water Quality Inventory of the USA, nonpoint-source pollution from agricultural activities impairs about three times more river miles, and two times more lake acres, than other sources (USEPA, 1998). The loss of particulate P mainly occurs via erosion, which can be controlled by several best management practices including conservation tillage and installing buffer strips near water bodies (Litke, 1999). In contrast, losses of dissolved P are more difficult to reduce, and control measures are mostly limited to preventing soil P accumulation.
Relatively few studies have been conducted to ascertain when a soil has accumulated enough P to pose an environmental threat (examples include Romkens and Nelson, 1974; Sharpley, 1985, 1995; Daniel et al., 1993; Pote et al., 1996, 1999). Most studies have shown that runoff P depends on the amount of P in a particular soil (see also Sibbesen and Sharpley, 1997). However, across a wide range of soils, soil tests are often poor predictors of runoff P. To a large degree, the problem lies in the variability of the P buffering capacities, amounts of Fe and Al hydroxyoxides, carbonates, clay, and organic matter that are not accounted for by the tests. While many states have established critical P levels in the range 75 to 200 mg Mehlich-III P kg-1 (Sharpley et al., 1996), it seems unlikely that these criteria will be adequate for making accurate P management recommendations.
A different approach is used in the Netherlands for identifying when soils pose a risk to surface and ground water quality (Van der Zee et al., 1987; Breeuwsma and Silva, 1992). Referred to as the P sorption saturation approach, this method takes into account the amount of P a soil contains relative to the maximum amount of P that the soil can sorb, as shown by the equation:
 | [1] |
where extractable P and P sorption capacity are in units of mmol kg-1 and are determined from the amounts of oxalate-extractable P, Al, and Fe in noncalcareous soils (Breeuwsma and Silva, 1992). This approach was based on the premise that soils have a finite capacity to sorb P, and when a critical P sorption saturation level was reached there was a greater potential for P to be released to surface or ground water. Dutch regulators have set a P sorption saturation limit of 25% for protecting ground water quality, which was roughly equivalent to about 0.1 mg P L-1 in soil solution. Adapting this approach for USA soils, Sharpley (1995) observed that runoff P from 10 somewhat diverse soils was best described by a single P sorption saturation value, as opposed to variable runoff P levels predicted from soil Mehlich-III P. So the P sorption saturation approach may prove useful for two purposes: (i) assessing the potential for a soil to contribute to runoff P and (ii) estimating the amount of added P that can be sorbed by soil before an unacceptable level is reached. The main disadvantage of the approach is the complexity of obtaining a reliable estimate of soil P sorption capacity, as compared with routine soil tests (Sibbesen and Sharpley, 1997).
This study was conducted to determine relationships between easily measurable soil characteristics and P requirements and limits of soils. Results may be helpful to resource managers for identifying high or low P status soils, quantifying soil P requirements, and estimating P amendment levels to satisfy crop needs and protect water quality.
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MATERIALS AND METHODS
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Soils
Surface samples (A horizon) were obtained from 16 different soil series that represented four major Kentucky physiographic regions and are found in 13 other states in the eastern USA (Table 1). Samples differed widely in chemical properties, containing between 139 and 3861 mg total P kg-1, 15 and 115 mmol oxalate Al kg-1, 625 and 2700 mg Mehlich-III Ca kg-1, and soil water pH values between 4 and 7.
Contrasting soil characteristics were attributed to parent material and weathering differences, and were expected to elicit a broad spectrum of P sorption behaviors. Bluegrass soils, found in north central Kentucky, are mostly Udalfs derived from phosphatic limestones (e.g., apatite), shale, and siltstone. Many of these soils have naturally high P fertility owing to the weathering of P bearing minerals. This was borne out from results of total P, oxalate P, and Mehlich-III P analysis (Table 1). Cumberland Plateau soils, found in eastern Kentucky, are mostly Udults formed of shale and sandstone. Highland Rim soils, found in south central and western Kentucky, are mostly Udults and Udalfs. Shawnee Hills soils, found in northwestern Kentucky, consist primarily of Udalfs developed from sandstone, shales, and loess with limestone. The crystalline clay fractions of the soils are mixtures of illite, hydroxyinterlayered minerals, kaolinite, and smectite (A.D. Karathanasis, personal communication, 2002). Soils were air-dried and sieved to pass 2 mm before being used in chemical characterization and sorption studies.
Chemical Analyses
Mehlich III– and oxalate-extractable elements were analyzed at the University of Kentucky Regulatory Services in Lexington. Mehlich III–extractable P, K, Ca, Mg, Fe, and Al were determined by equilibrating soil (2.0 g) with Mehlich-III solution (20 mL for 5 min) and filtering through #2 filter paper (Whatman, Maidstone, UK) (Mehlich, 1984). Oxalate-extractable P, Fe, and Al were determined by equilibrating soil (0.5 g) with oxalate solution (20 mL 0.175 M ammonium oxalate + 0.1 M oxalic acid, pH 3.5 for 2 h in the dark) and filtering through Whatman #2 filter paper (Loeppert and Inskeep, 1996). Analytes were determined by inductively coupled argon plasma (ICP) (Thermo Jarrell Ash Model 61; Thermo Elemental, Franklin, MA). A phosphorus saturation index (PSI) was calculated by dividing the molar concentration of oxalate-extractable P by the sum of the molar concentrations of oxalate-extractable Fe and Al, and multiplying by 100 to convert to percent. Soil pH was determined in a 1:1 (soil to water ratio) mixture after a 30-min equilibration, with an Orion combination semi-microelectrode (Thermo Orion, Beverly, MA) calibrated with pH 4 and 7 buffer solutions. Phosphate obtained from the sorption experiments was determined by a colorimetric procedure with a microplate reader as described by D'Angelo et al. (2001). Total organic carbon was determined with a total organic carbon analyzer (Shimadzu, Kyoto, Japan). Total P was determined by colorimetric determination of phosphate after H2SO4–potassium persulfate digestion (Nelson, 1987).
Sorption Isotherms
Batch sorption incubations were conducted according to the procedure outlined by Nair et al. (1984). Briefly, triplicate samples (0.4 g) were equilibrated on a horizontal shaker with 10 mL 0.01 M CaCl2 solution containing 0, 1, 2, 6, 12, and 30 mg P L-1 as KH2PO4–P. After a 48-h equilibration period, mixtures were centrifuged, filtered (0.45 µm), and analyzed for phosphate. Although laboratory batch experiments cannot duplicate the P retention characteristics of field samples, the protocol was found to be highly reproducible and was efficient at characterizing P retention at close to equilibrium conditions by the largest and most reactive soil fractions.
The P not recovered from the solution was used to determine the amount of added P retained by the soil (S1). Because the samples had different P fertilization histories and different amounts of native sorbed P (S0), the total amount of P sorbed (S) was determined by adding S1 and S0. Several methods have been used to estimate S0, including isotopic exchange, extractant solutions, anion exchange resins, iron hydroxide filter strips, and curve fitting techniques (Chardon and Blaauw, 1998, and references therein). In the present study, we chose to estimate S0 from the amount of P that was extractable with oxalate solution, on the basis that oxalate solution removes P bound with the dominant and most reactive Fe and Al oxide surfaces of acidic soils (Van der Zee and Van Riemsdijk, 1988).
The sorption data were fitted to three common sorption models (Tempkin, Freundlich, and Langmuir) to determine their influence on interpretation of results and to facilitate comparisons with other studies:
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 | [4] |
where S is the total sorbed P (mg kg-1), C is the equilibrium concentration (mg L-1), and a, b, Kf, n, Smax, and k are fitting parameters.
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RESULTS AND DISCUSSION
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Phosphorus Concentrations
Total P in soils ranged between 139 and 3861 mg P kg-1, which consisted of about 5% Mehlich-III P (R2 = 0.78) and 52% oxalate P (R2 = 0.99) (Table 1). There was a curvilinear relationship between Mehlich-III P and oxalate P (Mehlich-III P = 1.5 x ln oxalate P - 1.2; R2 = 0.87). As a reference point, at least two states in the USA (Kentucky and Arkansas) have experimentally determined 30 to 40 mg Mehlich-III P kg-1 (1 to 1.3 mmol kg-1) to be about optimal for crop production. Based on this criterion, four soils had almost optimal P, eight soils were P deficient, and six soils had excess P.
Phosphorus Sorption
Sorption isotherms for four representative soils, Maury, Lowell 2, Mountview, and Newark, illustrated that the soils differed considerably in sorption characteristics (Fig. 1)
, which was also evident from sorption equations listed in Table 2. Sorption behavior was adequately described by each of the sorption models, with R2 values ranging between 0.94 and 1.00. It should be noted that the Smax values are more empirical curve-fitting parameters than true sorption maxima, since input concentrations were not sufficient to saturate the soils. In this study, reduced P levels were chosen to reflect most soil environments, and were useful in determining the soil P requirement in the lower concentration range.
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Fig. 1. Phosphate sorption isotherms for four soils fitted by the Tempkin equation adjusted for native sorbed P. The arrow shows the location of the soil phosphorus requirement (Preq) to reach 1 mg P L-1 in equilibrium solution.
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Table 2. Regression equations for phosphate sorption using the Tempkin, Freundlich, and Langmuir equations (corrected for native sorbed P, S0 = oxalate P).
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Sorption equations were used to determine the soil phosphorus requirement (Preq), that is, the P necessary to maintain a predetermined P concentration in solution (Fox and Kamprath, 1970; Juo and Fox, 1977; Dodor and Oya, 2000). Soil P requirements were determined by substituting the desired P concentration into the fitted sorption equations. The value of choice for the P concentration will depend on intended land use. For crop production purposes, as an example, a value of 0.2 mg P L-1 may be considered as an appropriate value (Barber, 1995). For the purposes of water quality protection, however, it may be necessary to consider other levels. For example, the USEPA has recommended a maximum level of 1 mg P L-1 in surface runoff (USEPA, 1996).
Soil P requirements at 0.2 and 1.0 mg P L-1 were determined with the Tempkin, Freundlich, and Langmuir equations (Tables 2 and 3). The models generated slightly different Preq values, with the average relative standard deviation between models ranging between 11 and 13% depending on the P level. This was probably attributed to slight differences in curvature generated by the models in the 0.2 to 1.0 mg L-1 range. Since the differences were generally small, average values were used to estimate soil P requirements.
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Table 3. Soil phosphorus requirements (Preq) to reach 0.2 and 1.0 mg P L-1 in equilibrium solution, as determined from three isotherm equations. The phosphorus limit (PL) was determined from the difference between the average Preq and native sorbed P (S0 = oxalate P).
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To reach 0.2 mg P L-1 in soil solution, soil P requirements ranged between 154 and 1901 mg P kg-1, with a mean of 416 mg P kg-1. This was in the range found for other soils (Juo and Fox, 1977; Geodert, 1983; Dodor and Oya, 2000). In many studies, however, the true Preq value was probably underestimated because native sorbed P was not considered.
Soil P requirements to reach 1 mg P L-1 ranged between 222 and 2221 mg P kg-1, with a mean of 556 mg P kg-1. At both the 0.2 and 1.0 mg P L-1 levels, Bluegrass soils had greater than average P requirements, while soils from other regions generally had below average requirements. Clearly, the soils had considerably different P requirements; however, besides Bluegrass soils, it was difficult to explain differences based on placement into soil orders or physiographic regions.
It was expected that soil P requirements would be related to the amounts of P-retaining agents in soils. Regression analysis was performed to relate selected soil properties in Table 1 and sorption behavior parameters in Table 2. The strongest predictor of Preq was oxalate-extractable Al (R2 = 0.76), which was considerably improved by including oxalate Fe (R2 = 0.87). Plots of Preq verses the sum of oxalate Fe and Al at the two levels revealed highly significant (R2 > 0.90) exponential relationships between the soil properties, as shown in Fig. 2
and by the general equation:
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where oxalate Fe + Al are in mmol kg-1, and q and r are constants whose values depend on the selected P solution concentration.
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Fig. 2. Relationships between soil phosphorus requirements (Preq) to reach (a) 0.2 mg L-1 and (b) 1 mg L-1 in soil solution and the sum of oxalate-extractable Fe and Al (mmol kg-1).
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These results suggested that amorphous Al and Fe oxides were primarily responsible for P sorption. Other factors (pH and organic matter) were not significantly related to Preq. Several studies have likewise implicated reactive amorphous Fe and Al oxide surfaces in governing P sorption by acid soils from North America and several other continents (Saini and McLean, 1965; Yuan and Breland, 1969; Lopez-Hernandez and Burnham, 1974; Kleinman and Sharpley, 2002). Phosphorus retention by amorphous Fe and Al oxides is primarily through the formation of relatively strong monodentate, bidentate, and polynuclear bonds (Parfitt, 1977). Together, these results suggest that oxalate (Fe + Al) may be a good estimator of Preq in a wide range of noncalcareous soils.
One benefit of determining Preq is to help determine the soil phosphorus limit (PL), defined as the amount of P that can be added to soil to reach an acceptable level in soil solution. The PL was calculated from the difference between Preq and S0. Therefore, soils with the highest Preq and lowest S0 will have the greatest PL. The PL to reach 0.2 mg P L-1 for the soils in this study ranged between -92 and 253 mg P kg-1 (Table 3). Negative values identified soils with surplus P levels, while positive values showed soils with deficient P. Accordingly, all but three soils should be amended with P to satisfy plant requirements. These three soils had the highest Mehlich-III P values (Table 2). The PL to reach the USEPA limit of 1 mg L-1 in solution ranged between 82 and 403 mg kg-1, indicating that significant amounts of P could be added to these soils before they would pose a significant threat to water quality based on this criterion.
In many instances it may be necessary to determine Preq (and PL) at other P concentrations besides 0.2 or 1.0 mg L-1. For example, the soil solution P limit is set at 0.1 mg P L-1 to protect ground water in the Netherlands (Breeuwsma and Silva, 1992) and is the recommended limit for streams in the USA (Litke, 1999). Therefore, estimates of Preq were made at several values 0.05, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0, 5.0, and 10 mg L-1 using the average of three models as described previously. When the resulting Preq values were plotted as a function of the sum of oxalate Fe and Al, similar relationships to those in Fig. 2 were obtained, except for the values of the q and r constants. It was found that the q and r terms, however, were a function of the desired standard P concentration (R2 > 0.98) (Fig. 3)
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Fig. 3. Relationships between (a) the base term q and (b) the exponential term r of the equation relating the sum of oxalate Fe and Al (mmol kg-1) with soil P requirements, and desired P levels in soil solution (mg L-1).
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Many studies have shown links between soil test P levels and concentrations in surface runoff; however, relationships have been very site specific. Few studies have integrated soil test P results with P retention characteristics for making P recommendations. However, this is necessary because it would not be practical to evaluate all sites for P in surface runoff. Moreover, this approach is not very helpful for estimating P limits, because a large fraction of added P may be transferred to insoluble (immobile) P forms in soils. By combining soil test P and retention characteristics, it may be possible to better evaluate P fertility, pollution potential, requirements, and amendments for optimizing plant growth and protecting water quality.
A procedure illustrating the approach is provided in the following example. If it is deemed acceptable to have 0.5 mg P L-1 in solution for a soil that has 350 mg oxalate P kg-1, 3000 mg oxalate Fe kg-1 (53.7 mmol kg-1), and 1250 mg oxalate Al kg-1 (46.3 mmol kg-1), then the Preq for this soil would be 128 exp[0.0137 x (53.7 + 46.3)] = 504 mg P kg-1 (from Fig. 3). Since this soil already has an S0 amount of 350 mg P kg-1, the PL for this soil is 504 - 350 = 154 mg P kg-1. This translates into 277 kg P ha-1 or 14 Mg manure ha-1, assuming the manure has 2% P and is incorporated into the top 0.15 m of soil with a bulk density of 1.2 Mg m-3. The approach described in this study is most valid for situations when P is well mixed with soil and allowed to react for a sufficient amount of time (>2 d). The approach is probably not suitable when soil–phosphate interactions are restricted (e.g., when manure is surface-applied or washed away during a heavy rainfall event). Studies are needed to validate the model under these and other field conditions, such as different P incorporation depths and amendment levels, and with a wider range of soils with different Preq and S0 levels.
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CONCLUSIONS
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Results from sorption studies showed that soils varied considerably in P requirements, that is, the amount of P that a soil contains when a particular P level exists in solution. Regression analysis showed that P requirements were closely related to amounts of amorphous Al and Fe oxides. Subtracting the initial sorbed P from the P requirements provided the P limit for that soil. Using this approach, a single soil extraction with oxalate solution and analysis for P, Fe, and Al showed considerable promise for identifying high P requiring soils and recommending P amendment levels. However, relationships determined in this study need to be verified across a wider range of soils (e.g., alkaline soils). In combination with proper timing and other site assessments, these tests could be readily integrated into existing best management practices for improving soil fertility and protecting water quality.
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ACKNOWLEDGMENTS
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Kentucky Agricultural Experimental Station Journal Series no. 03-06-002. Appreciation goes to Stephen Rogers (undergraduate student) and Danny Reid (UK Soil Testing Laboratory, Division of Regulatory Services) for technical assistance in this project. This research was supported in part by funds provided by the Commonwealth of Kentucky, SB-271 Competitive Research Grant to E.M. D'Angelo.
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ABSTRACT
INTRODUCTION
