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Journal of Environmental Quality 31:1601-1609 (2002)
© 2002 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

TECHNICAL REPORTS
Landscape and Watershed Processes

Soil Testing to Predict Phosphorus Leaching

Rory O. Maguire* and J. Thomas Sims

Department of Plant and Soil Sci., Univ. of Delaware, Newark, DE 19717-1303

* Corresponding author (rmaguire{at}udel.edu)

Received for publication October 3, 2001.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Subsurface pathways can play an important role in agricultural phosphorus (P) losses that can decrease surface water quality. This study evaluated agronomic and environmental soil tests for predicting P losses in water leaching from undisturbed soils. Intact soil columns were collected for five soil types that had a wide range in soil test P. The columns were leached with deionized water, the leachate analyzed for dissolved reactive phosphorus (DRP), and the soils analyzed for water-soluble phosphorus (WSP), 0.01 M CaCl2 P (CaCl2–P), iron-strip phosphorus (FeO-P), and Mehlich-1 and Mehlich-3 extractable P, Al, and Fe. The Mehlich-3 P saturation ratio (M3-PSR) was calculated as the molar ratio of Mehlich-3 extractable P/[Al + Fe]. Leachate DRP was frequently above concentrations associated with eutrophication. For the relationship between DRP in leachate and all of the soil tests used, a change point was determined, below which leachate DRP increased slowly per unit increase in soil test P, and above which leachate DRP increased rapidly. Environmental soil tests (WSP, CaCl2–P, and FeO-P) were slightly better at predicting leachate DRP than agronomic soil tests (Mehlich-1 P, Mehlich-3 P, and the M3-PSR), although the M3-PSR was as good as the environmental soil tests if two outliers were omitted. Our results support the development of Mehlich-3 P and M3-PSR categories for profitable agriculture and environmental protection; however, to most accurately characterize the risk of P loss from soil to water by leaching, soil P testing must be fully integrated with other site properties and P management practices.

Abbreviations: CaCl2–P, phosphorus extractable with 0.01 M CaCl2 • DPS, degree of phosphorus saturation, calculated from a single extraction with acid ammonium oxalate as the molar ratio P/(0.5[Al + Fe]) x 100 • DRP, dissolved reactive phosphorus, measured with the molybdate blue method following 0.45-µm filtration • FeO-P, phosphorus extractable with an iron-oxide-impregnated strip • M3-PSR, Mehlich-3 phosphorus saturation ratio calculated as the molar ratio of Mehlich-3 P/[Al + Fe] • OM, organic matter • WSP, water-soluble phosphorus


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
LOSSES OF NUTRIENTS, including P, from agricultural land have been identified as one of the main causative factors in reducing water quality in the USA (Boesch et al., 2001; USEPA, 2000). There has been much study of the movement of agricultural P via surface processes; however, processes that enhance P losses via subsurface pathways have received relatively little attention. Recent research has shown that in some circumstances subsurface pathways can play an important role in P loss from agriculture (Heckrath et al., 1995; Sims et al., 1998). For example, Turner and Haygarth (2000) studied P leaching from monolith lysimeters and concluded that "subsurface P transfer from soil to surface water can occur at concentrations that could cause eutrophication."

The most commonly used approach today to identify soils sufficiently high in P to be of concern to water quality is some form of soil P testing. This is primarily because soil testing is already widely conducted, is inexpensive, and has been shown to be well correlated with soluble and bioavailable P (Sims et al., 2000). However, soil testing alone will not answer all questions about subsurface or surface losses of P, because it cannot characterize transport processes. For example, with respect to subsurface losses, preferential flow pathways, such as cracks and earthworm burrows, can enhance P movement through the soil profile, resulting in greater P losses by leaching than would be expected based solely on soil test P concentrations (Heathwaite and Dils, 2000; Simard et al., 2000). Cox et al. (2000) agreed and stated that "a P adsorption index based only on the chemical properties of a soil did not accurately predict the mobility of P through soils with macroporosity."

Despite the possible, somewhat unpredictable roles of preferential flow pathways and subsoil factors affecting P sorption–desorption, several researchers have found good relationships between extractable soil P and subsurface P losses. Heckrath et al. (1995) found that the concentration of P in tile drainflow was low when agronomic soil test P was less than 60 mg Olsen P kg-1, but the concentration of P in tile drains increased rapidly above this soil test value, which they called the "change point." They suggested that below 60 mg Olsen P kg-1, inorganic P is "sorbed on high energy sites but that above this value it is held on low energy sites." Research such as this has encouraged regulators in many areas to consider setting environmental limits for soil test P as a relatively cheap method to target limited resources for reducing agricultural P losses. For example, soil test P limits are now being considered in areas with intensive animal operations where manure surpluses exist and P losses from agriculture have been identified as having a negative effect on surface waters, such as the Delmarva Peninsula in the USA (Sims, 1999; Sims and Coale, 2002).

One soil test that has been suggested as suitable to set environmental limits for soil P, especially where subsurface losses are of concern, is the degree of phosphorus saturation (DPS). The DPS is usually calculated as the ratio of acid ammonium oxalate P to [Al + Fe] (van der Zee and van Riemsdijk, 1988) and has been shown to be closely correlated to P concentrations in leachate waters (Leinweber et al., 1999; Maguire and Sims, 2002). Schoumans and Groenendijk (2000) recognized the importance of P losses in subsurface flow in noncalcareous sandy Dutch soils. They modeled inorganic P release to solution and showed an exponential increase in soil solution P concentration with increasing DPS. Other researchers have shown a low potential for P losses via leaching when DPS is <20 to 25% and sharp increases in P leaching above this DPS value (Maguire et al., 1998, 2001; McDowell and Sharpley, 2001b). Hooda et al. (2000) studied the relationship between DPS and P release to solution and found that for a range of soils in the United Kingdom, little desorption of P occurred below values equivalent to a DPS of 20%, as calculated with the Dutch method, while above this value the amount of P desorbed increased linearly. These results imply that there is a rapid increase in P losses in leachate as soils become more saturated with P.

Due to the current concerns about nutrient losses from agriculture and the growing recognition of subsurface P losses, there is a need to develop and evaluate methods for predicting losses of P in subsurface pathways from agricultural soils. Despite past research that identifies links between some soil tests and possible subsurface P losses, a comprehensive evaluation of a wide range of soil tests for predicting P leaching from undisturbed soils has yet to be conducted. Therefore, this study evaluated the ability of agronomic and environmental soil tests to predict P losses in leachate from major agricultural soils of the Delmarva Peninsula, ranging from "low" to "excessive" in soil test P. The results were also used to develop preliminary soil test P criteria to identify soils with a greater potential to lose P to shallow ground waters via leaching.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Soil Selection and Sample Collection
Five soil series from the Delmarva Peninsula, USA, with a range of chemical and physical properties, were selected for this study (Table 1). For each soil series, soil test data provided by farmers were used to select soils that ranged in soil test P from "low" to "excessive." Intact soil columns were collected in the spring of 2000, based on the method of McDowell and Sharpley (2001b). For each soil type, 21 columns were obtained by driving 15-cm-diameter PVC pipes (coated on the inside with paraffin wax to seal between the soil and the pipe) to a depth of 20 cm into the soil. Before the columns were removed from the soil, six soil cores were collected to a depth of 20 cm from immediately outside the columns and composited. For leachate collection from the columns, one hole was drilled into a 15-cm-diameter PVC endcap and a short tube filled with glass wool was glued into this hole. The columns were inserted into the endcaps, with sand placed in the space between the soil and the endcap. The capped columns were placed in racks in a greenhouse, pre-wetted by adding excess water (400 mL), and left to drain to field capacity for two days. Four leaching events were performed on successive days with each column. For each leaching event, the equivalent of 5 mm of rainfall (as deionized water) was added to the top of the columns and leachate was collected for the following 24-h period. The first leaching event produced unequal amounts of leachate from some columns, probably due to incomplete soil saturation, but leachate volume became stable after this first leaching event. Therefore, the concentration of P in leachate water from only the second leaching event was used in this paper where only a single leaching event is shown, as it represented the first stable leaching event after the soils were sampled. Although 21 columns were initially collected for each soil series, the number of columns used varied because (i) some columns deteriorated during the leaching process and leachate could not be collected and (ii) additional columns were collected from soil series where there was not a good range in soil test P (Table 1).


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  		Table 1. Classification and selected properties of the five Delmarva Peninsula soils used in the column leaching study.{dagger}

 
Soil and Leachate Analysis
All soil samples collected adjacent to the column sampling locations were air-dried and ground to pass a 2-mm sieve. Soil texture was obtained from soil survey manuals. Soil pH and organic matter (OM) were measured by standard methods of the University of Delaware Soil Testing Laboratory (Sims and Heckendorn, 1991). Soils were analyzed for (i) iron-oxide-strip P (FeO-P) (1:40 soil to 0.01 M CaCl2 + iron-oxide-coated filter paper strip, 16-h reaction time, followed by dissolving Fe and P from the filter paper strip for 1 h in 1 M H2SO4; Chardon et al., 1996); (ii) water-soluble phosphorus (WSP) (1:10 soil to deionized water, 1-h reaction time, filtration through 0.45-µm Millipore [Bedford, MA] membrane); (iii) calcium-chloride-extractable P (CaCl2–P) (1:10 soil to 0.01 M CaCl2, 1-h reaction time, filtration through Whatman [Maidstone, UK] #2 filter paper); (iv) Mehlich-1 P (1:4 soil to 0.05 M HCl + 0.0125 M H2SO4, 5-min reaction time, and filtration through Whatman #2 filter paper); and (v) Mehlich-3 P (1:10 soil to 0.2 M CH3COOH + 0.25 M NH4NO3 + 0.015 M NH4F + 0.13 M HNO3 + 0.001 M EDTA, filtration through Whatman #2 filter paper). The Mehlich-3 extract was analyzed for P (PM3), Al (AlM3), and Fe (FeM3) by inductively coupled plasma atomic emission spectroscopy (ICP–AES). All other extracts were analyzed for P colorimetrically by the molybdate blue method of Murphy and Riley (1962).

Leachate from the columns was filtered through 0.45-µm Millipore membranes and analyzed for DRP colorimetrically by the molybdate blue method (Murphy and Riley, 1962).

Phosphorus Saturation Calculations
The Mehlich-3 P saturation ratio (M3-PSR) was calculated as follows (where values for PM3, AlM3, and FeM3 are expressed in mmol kg-1):

[1]

The agronomic optimum Mehlich-3 P value for most crops in Delaware is 50 mg P kg-1 (1.61 mmol Mehlich-3 P kg-1; Sims et al., 2001). Agronomic optimum M3-PSR values were calculated with mean [AlM3 + FeM3] for each soil series (all values expressed as mmol kg-1) as follows:

[2]

Statistical Analysis
Linear regression analysis was performed with the Data Analysis tool pack in Excel 2000 (Microsoft, 2000). For the relationships between leachate P and extractable soil P, a split line model was used to determine change points, as described by McDowell and Sharpley (2001a), with the Statistical Analysis System, Version 8 (SAS Institute, 1998). The split line model describes separate linear relationships on either side of a change point.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Soil Classification, pH, and Organic Matter Content
Of the five soils used in this study, four were Ultisols and one was an Entisol, while the soil texture ranged from silt loam to sandy loam (Table 1). The mean pH of the Pocomoke soil was 5.2, while the mean pH values for the other four soils ranged from 5.7 to 6.3. The lower mean pH of the Pocomoke soil was almost certainly due to less frequent lime applications and its high organic matter (OM) content (mean = 61 g OM kg-1) compared with the other four soils (means range from 12 to 25 g OM kg-1). In Delaware, so called "black" soils with an OM content > 60 g OM kg-1 have a lower target pH value for most crops than soils with a lower OM content (Sims and Gartley, 1996). For example, the target pH for corn (Zea mays L.) grown on most Delaware soils is 6.0, but for black soils such as the Pocomoke series, the target pH is 5.6 (Sims and Gartley, 1996).

Extractable Soil Phosphorus
A wide range of extractable soil P was observed for each of the soil tests used (Table 2). Mean FeO-P, which measures P in the "fast desorbing pool" (Pautler and Sims, 2000), was more than twice the magnitude of mean WSP for all soil series. Water-soluble P ranged more than 10-fold for each of the five soil series and median WSP ranged from 1.4 mg P kg-1 for the Butlerstown series to 9.6 mg P kg-1 for the Sassafras series. The Matapeake series had the greatest mean WSP (27.6 mg P kg-1) and a skewed distribution of WSP, with several very high values. The CaCl2–P was always less than WSP, with the mean CaCl2–P being less than half the mean WSP for all soil series (Table 2).


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  		Table 2. Iron-strip phosphorus (FeO-P), water-soluble phosphorus (WSP), 0.01 M CaCl2–extractable P (CaCl2–P), Mehlich-1 P, Meh-lich-3 P, and the Mehlich-3 P saturation ratio for each of the five soil series.

 
Mehlich-1 P is an agronomic soil test used to rank soils according to their ability to supply P for crop production, with the optimum range being 25 to 50 mg Mehlich-1 P kg-1 (Sims et al., 2001). Each soil series had soils that ranged from below to above optimum Mehlich-1 P, except for the Sassafras series, which had no soil below optimum Mehlich-1 P. The Mehlich-3 P soil test has an optimum range of 50 to 100 mg Mehlich-3 P kg-1 (Sims et al., 2001) and, as for Mehlich-1 P, all soil series had Mehlich-3 P values that ranged from below to above agronomic optimum values, except the Sassafras series. The P contents of these soil series generally reflect the history of agricultural nutrient management practices, rather than native soil properties. In their native state (mixed woodland) these soils would be expected to have Mehlich-1 or Mehlich-3 P contents well below the agronomically optimum ranges.

Similar to extractable soil P, the M3-PSR, calculated from Eq. [1], ranged widely for each soil series. The M3-PSR has been shown to be well correlated to the DPS based on an acid ammonium oxalate extraction, which has been suggested as an indicator of the potential for P loss from soils to surface and ground waters (Breeuwsma et al., 1995; Leinweber et al., 1999; Hooda et al., 2000; Maguire and Sims, 2002). Sims et al. (2002) suggested a range of 0.10 to 0.15 M3-PSR to identify soils sufficiently saturated with P to represent a risk of excessive P loss to surface and ground waters. All soil series had soils above and below this M3-PSR range.

Relationship between the Agronomic and Environmental Soil Tests
Agronomic soil P tests such as Mehlich-1 and Mehlich-3 were developed to assess the fertility status of soils; they generally function by dissolution and desorption reactions that use mixtures of strong acids, bases, or complexing agents (Sims, 2000; Mehlich, 1984). However, soil P tests such as FeO-P, WSP, and 0.01 M CaCl2, which use distilled water or salt solutions at a similar ionic strength to the soil solution, were developed more as environmental soil tests. These tests were designed to predict P losses to surface waters by measuring soluble and easily desorbable P in soils (Sharpley et al., 1996; Sims et al., 2000, 2002). Despite the different aims behind the development of the soil tests used in this study, they were all significantly correlated (P < 0.001) with each other (Table 3). The correlations with FeO-P were similar for the Mehlich-1 and Mehlich-3 soil tests and WSP and CaCl2–P. Mehlich-3 is a stronger chemical extractant than Mehlich-1, extracting approximately twice as much P as the Mehlich-1 extract (Sims et al., 2001). Therefore, it is not surprising that the correlation coefficients for Mehlich-3 P vs. the milder environmental soil tests were not as good as those for Mehlich-1, being 0.93, 0.82, and 0.85, compared with 0.95, 0.93, and 0.94 for FeO-P, WSP, and CaCl2–P, respectively (Table 3). The M3-PSR is a combination of the agronomic Mehlich-3 P soil test and the environmental aspects of a soil P saturation test (Khiari et al., 2000; Sims et al., 2002; Maguire and Sims, 2002). Although the M3-PSR is usually considered as an environmental soil test and is reportedly better at identifying soils more susceptible to soluble P losses by leaching than Mehlich-1 and Mehlich-3 P, the correlation coefficients for M3-PSR vs. FeO-P, WSP, and CaCl2–P were no better than those noted for Mehlich-1 and Mehlich-3 P.


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  		Table 3. Correlation matrix representing the correlation coefficients for P extracted by agronomic and environmental soil tests.{dagger}

 
Evaluation of the Environmental Soil Tests for Predicting Phosphorus Leaching Losses
Leachate DRP from the five soil series evaluated ranged from below the detection limit to almost 12 mg P L-1 (Fig. 1) . Similar trends were noted in the relationships between leachate DRP and soil FeO-P, WSP, and CaCl2–P, in which leachate DRP remained very low until soil P concentrations approached a certain value, above which DRP concentrations increased much more rapidly with soil P. For example, based on split line model analysis, the rate of increase in leachate DRP was seven times as great, per unit increase in soil FeO-P, above a "change point" (at 42.6 mg FeO-P kg-1) compared with below it. Heckrath et al. (1995) first suggested the concept of a change point for P losses in tile drains, with P losses in drainage increasing rapidly above 60 mg Olsen P kg-1. For soil WSP, leachate DRP increased slowly up to a change point at 8.6 mg WSP kg-1 and then increased almost five times as rapidly, per unit increase in WSP, above this change point (Fig. 1b). Extractable CaCl2–P has previously been shown to be a useful indicator for P leaching (McDowell and Sharpley, 2001a), including identification of change points (Hesketh and Brookes, 2000). For soil CaCl2–P vs. leachate DRP, the change point was at 1.59 mg CaCl2–P kg-1, and leachate DRP increased seven times as rapidly, per unit increase in CaCl2–P, above this change point (Fig. 1c).



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  		Fig. 1. Comparison of the concentration of dissolved reactive phosphorus (DRP) in the second leaching event from the intact soil columns and the environmental soil tests (a) iron-strip phosphorus (FeO-P), (b) water-soluble phosphorus (WSP), and (c) 0.01 M CaCl2–extractable P (CaCl2–P).

 
The coefficients of determination for the overall split line models for environmental soil test P vs. leachate DRP decreased in the order WSP (r2 = 0.85) > CaCl2–P (r2 = 0.84) > FeO-P (r2 = 0.80). Leachate DRP concentrations at the change points for the environmental soil tests were 0.14 mg P L-1 (FeO-P), 0.22 mg P L-1 (WSP), and 0.10 mg P L-1 (CaCl2–P), which were at or above the 0.1 and 0.05 mg P L-1 guidelines established by the USEPA for streams and lakes (USEPA, 1986) and exceeded the upper limit of 0.10 mg P L-1 suggested for protection of shallow ground waters in the Netherlands (Breeuwsma et al., 1995).

Evaluation of the Agronomic Soil Tests for Predicting Phosphorus Leaching Losses
As with the environmental soil tests, a change point was observed in the relationship between leachate DRP and each of the agronomic soil tests, with leachate DRP increasing rapidly above the change points (Fig. 2) . The change points were 81 mg P kg-1 for Mehlich-1 P, 181 mg P kg-1 for Mehlich-3 P, and 0.20 for the M3-PSR. The leachate DRP at the change points for the agronomic soil tests were -0.11 mg P L-1 for Mehlich-1, 0.14 mg P L-1 for Mehlich-3 P, and 0.24 mg P L-1 for the M3-PSR. The negative value for Mehlich-1 indicates negligible increase in leachate P below the change point and is negative due to two points that had low Mehlich-1 P, but leached much more DRP than would be expected based on soil test P concentrations, possibly due to preferential flow. McDowell and Sharpley (2001b) showed a similar change point for Mehlich-3 P vs. P in leachate, in the range 105 to 245 mg P kg-1, depending on the soil type. The agronomic optimum value for Mehlich-3 P (50 mg P kg-1; Sims et al., 2001) was well below the change point.



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  		Fig. 2. Comparison of the concentration of dissolved reactive phosphorus (DRP) in the second leaching event from the intact soil columns and the agronomic soil tests (a) Mehlich-1 P, (b) Mehlich-3 P (with the agronomic optimum range of 50 to 100 mg P kg-1 indicated by vertical lines; Sims et al., 2001), and (c) the Mehlich-3 P saturation ratio.

 
Using the agronomic optimum value of 50 mg Mehlich-3 P kg-1 for Delaware and Eq. [2], Maguire and Sims (2002) identified the agronomic optimum M3-PSR values for the soil series, from Eq. [2], as 0.048 (Butlerstown), 0.058 (Evesboro), 0.036 (Matapeake), 0.024 (Pocomoke), and 0.057 (Sassafras). These agronomic optimum M3-PSR values were all well below the change point of 0.20 for all soils, indicating that it is possible to optimize agricultural production at low P saturation ratios, without exceeding the change point and greatly increasing the risk of P leaching.

The coefficients of determination for leachate DRP vs. the agronomic soil tests decreased in the order M3-PSR (r2 = 0.78) > Mehlich-1 P (r2 = 0.73) > Mehlich-3 P (r2 = 0.58). Although the M3-PSR was the best agronomic soil test for predicting leachate DRP, the coefficient of determination was lower than for any of the environmental soil tests. However, if two outliers (out of 111 samples) were omitted (Fig. 2c), the coefficient of determination for M3-PSR increased from 0.78 to 0.87, equaling that for WSP, which was the best of the environmental soil tests.

Setting Soil Test Phosphorus Limits to Control Phosphorus Leaching
Sims et al. (2001)(2002) proposed four agri-environmental Mehlich-3 soil test P categories to optimize crop production and minimize nonpoint P pollution of surface waters and shallow ground waters: (i) <50 mg P kg-1, where profitable crop response to applied P would usually be expected; (ii) 50 to 100 mg P kg-1, where P is considered adequate and will probably not limit crop growth; (iii) 101 to 150 mg P kg-1, where P is considered more than adequate and will not limit crop yield, so no P applications are recommended; and (iv) >150 mg P kg-1, where no P should be applied because of the high potential for P losses by erosion, runoff, or leaching.

Leachate DRP from the columns for all four leaching events (except the Pocomoke series) were grouped according to these categories, to evaluate their suitability to identify soils with greater potential for P losses by P leaching (Table 4). The Pocomoke series was evaluated separately, as it behaved differently than the other soil series. Two outliers were omitted from the optimum category, the same two outliers that resulted in a negative value for the Mehlich-1 change point, as discussed earlier (Fig. 2a).


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  		Table 4. Relationship between soil test P (Mehlich-3 P) and P concentrations and loads in leachate from all leaching events. All soils (except Pocomoke series) were separated into the four categories, according to agronomic or environmental criteria. The Pocomoke soils in the "environmental" category are represented separately.

 
Leachate DRP concentrations varied widely within each Mehlich-3 P category. These variations were probably due to both the range in Mehlich-3 P concentrations, and thus soluble and easily desorbable P, between upper and lower category limits and to differences in the soil chemical and physical characteristics that affected P sorption–desorption and water flow in the different intact soil columns. However, we did observe that maximum and mean leachate DRP concentrations increased with increasing M3-P category limits. Therefore, as soil test P values increase from the below optimum to the environmental category, there is no guarantee that more P will be leached, but the probability increases. For all of the four leaching events, the lowest mean DRP was for the agronomic category (M3-P < 50 mg kg-1) and mean leachate DRP in the "environmental" category (M3-P > 200 mg kg-1) was more than 10 times the mean of any other category (Table 4). Values for leachate DRP in the 50 to 100 mg kg-1 and 101 to 150 mg kg-1 categories were intermediate between the "below optimum" and "environmental" categories. We also observed that the columns within each category continued to leach P throughout the four leaching events, without any trend for decreasing P concentration with successive leaching events. This indicates that P may continue to leach for a long time (Table 4). The mean total P load for the four leaching events, calculated by multiplying leachate DRP concentrations by the volume of leachate, increased in the order 0.3 < 5.3 = 5.3 < 99.9 g ha-1 for the <50 mg kg-1, 50 to 100 mg kg-1, 101 to 150 mg kg-1, and >150 mg kg-1 categories, respectively (Table 4).

Leachate DRP from the Pocomoke series was never great, in comparison with the other soil series. If we compare the Pocomoke soils to the other soils that lie within the environmental category, we see that the highest mean leachate DRP for the Pocomoke series was 0.045 mg L-1, compared with 2.877 mg L-1 for the other soils (Table 4). This was almost certainly due to the Pocomoke soil having the highest Al content and lowest pH (Table 1), which increased the P retention capacity of the soil (Adams et al., 1987; Bolan and Barrow, 1984; Stevenson, 1986). These results suggest caution when applying M3-P categories to the potential for P leaching from soil with properties similar to the Pocomoke series.

Setting Soil Phosphorus Saturation Limits to Control Phosphorus Leaching
Soil P categories can be set according to soil P saturation instead of soil test P, and Sims et al. (2002) suggested that values of 0.10 to 0.15 M3-PSR, equivalent to oxalate DPS values of 25 to 40%, could be used to identify soils where increased efforts at P management are needed to protect surface water quality. When the soils in this study were grouped according to these P saturation categories, as for the Mehlich-3 P categories, there was a wide range in leachate DRP within each P saturation category (Table 5). Mean leachate DRP from the <0.10 and 0.10 to 0.15 categories were similar (0.049 and 0.071 mg P L-1, respectively), but the mean DRP concentration in the >0.15 category was much greater (2.4 mg P L-1). As with the Mehlich-3 P categories, results for the Pocomoke soil did not fit well with the other soil series in the M3-PSR categories, despite the M3-PSR taking the high Al content of the Pocomoke soil into account. This is probably due to the low pH of this soil, which could have increased P sorption by Al oxides (Stevenson, 1986). In the >0.15 category, mean leachate DRP was 0.010 mg P L-1 for the Pocomoke series, compared with mean leachate DRP of 2.4 mg P L-1 for the other soil series. As with the Mehlich-3 P categories, the M3-PSR categories give a reasonable idea of which soils will have the greatest risk of P losses by leaching. However, the M3-PSR categories should be more accurate for assessing environmental risk, as shown by the greater r2 value for M3-PSR (r2 = 0.78) compared with Mehlich-3 P (r2 = 0.58) vs. leachate DRP (Fig. 2b,c).


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  		Table 5. Relationship between the Mehlich-3 P saturation ratio and P concentrations in the second leaching event, separated into the three categories according to environmental criteria, for all soils except the Pocomoke series. The Pocomoke soils in the high category are shown separately for comparison.

 
Comparison of USEPA Water Quality Guidelines with Soil Phosphorus Values
Given the current interest in using some form of soil P testing to identify the relative risk of soils to contribute to nonpoint-source pollution, there is a need to begin to establish numerical criteria for the various soil P tests that can be related to water quality guidelines. We grouped leachate DRP concentrations according to USEPA surface water quality guidelines for streams (0.10 mg P L-1; note that this value is also used in the Netherlands as an environmental upper limit for P in shallow ground waters) and lakes (0.05 mg P L-1; Breeuwsma et al., 1995; USEPA, 1986) and determined soil P concentrations in each water quality category. A hypothetical category of 0.10 to 1.00 mg P L-1 was also used, not because this level of loss to surface waters is acceptable, but as a realization that some P leaving the top 20 cm will be intercepted before reaching surface waters. We recognize that the fact that leachate DRP from intact topsoil columns exceeds water quality guidelines does not mean that similar DRP concentrations will be found in edge-of-field discharge into surface waters or in water percolating into shallow ground waters. Other factors, such as P sorption by subsoils, can decrease P concentrations during subsurface transport. However, the USEPA guidelines can serve as a starting point to develop numerical criteria for comparison of agronomic and environmental soil P tests. Once established, numerical criteria for soil P can then be combined with site hydrology and P management practices for a more comprehensive risk assessment, such as is done with the phosphorus site index (Gburek et al., 2000). Note that the Pocomoke soil was omitted from the comparisons shown in Table 6, for reasons discussed earlier.


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  		Table 6. Soil extractable P values categorized according to leachate dissolved reactive phosphorus (DRP) concentration, for all soils except the Pocomoke series.

 
There was a wide range in the concentration of all forms of soil P within each leachate DRP category, but there were clear trends for changes in mean soil P between categories (Table 6). From the <0.05 mg P L-1 to the 0.05 to 0.10 mg P L-1 category, all of the mean soil P values increased substantially, for example mean WSP increased from 4.5 to 8.5 mg P kg-1. However, each mean soil P value was similar in the 0.05 to 0.10 mg P L-1 and 0.10 to 1.00 mg P L-1 categories, before more than doubling in the >1.00 category. While the ranges within each category indicate the level of uncertainty involved with predicting P leaching by soil testing, the mean values of Mehlich-3 and M3-PSR support the categories suggested by Sims et al. (2001)(2002). The mean Mehlich-3 P concentration in the <0.05 mg P L-1 category was 109 mg P kg-1, close to the upper limit of 100 mg P kg-1 for the optimum range, above which fertilization is not recommended. Corresponding values for FeO-P, WSP, CaCl2–P, Mehlich-1P, and M3-PSR were 21, 4.5, 1.1, and 53 mg kg-1, and 0.10 respectively. The mean Mehlich-3 P values in the 0.05 to 0.10 mg P L-1 and 0.10 to 1.00 mg P L-1 categories were close to 150 mg P kg-1, which was suggested by Sims et al. (2002) as a possible environmental threshold. The M3-PSR averaged 0.16 to 0.17 in this range, slightly above the suggested environmental limit of 0.15. All soil P values were markedly higher in the >1.00 mg P L-1 category, where Mehlich-3 P averaged 401 mg kg-1, and mean values for FeO-P, WSP, CaCl2–P, Mehlich-1P, and M3-PSR were 90, 34.1, 14.5, and 263 mg kg-1, and 0.38, respectively.


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Soil testing alone cannot be expected to answer all questions about the potential for subsurface P losses, as every agricultural field will have variable chemical (e.g., pH, content of clay, Al, Fe) and hydrological properties (e.g., soil permeability, absence or presence of field drains) that will affect P retention and transport. However, quantifying the ability of various soil tests to predict P leaching from topsoils is very important, as the best soil P tests can then be combined with site hydrology to identify soils where P losses by subsurface flow are most likely to be of concern.

Our results showed that DRP in leachate from undisturbed topsoil columns can frequently be above concentrations required for eutrophication of surface waters. We also observed rapid increases in leachate DRP above certain soil P concentrations (e.g., the change points in the relationship between DRP and soil P). The change points for the soil P tests evaluated were 181, 81, 42.6, 8.6, and 1.59 mg kg-1 for Mehlich-3, Mehlich-1, FeO-P, WSP, and CaCl2–P, respectively, and 0.20 for M3-PSR. Maintaining soil P concentrations below these change points is thus important to minimize the risk of P losses from topsoils by leaching. This, in turn, will decrease the likelihood for both short-term losses of P by preferential flow and long-term accumulations of P in subsoil horizons, which will gradually enrich field drainage waters with P.

The environmental soil P tests (FeO-P, WSP, and CaCl2–P) were slightly better at predicting DRP concentrations in leachate than the agronomic soil P tests (Mehlich-1 P, Mehlich-3 P, M3-PSR). However, if two outliers were omitted, the M3-PSR was as good as any of the environmental soil tests at predicting P concentrations in leachate. This supports the suggestions by Khiari et al. (2000) and Sims et al. (2002) that a single soil extraction with Mehlich-3 can be useful for both agronomic (Mehlich-3 P) and environmental (M3-PSR) risk assessments for P. Indeed, the close correlations we found between some of the environmental P tests (e.g., FeO-P, WSP, CaCl2–P) and agronomic P tests raise the question of when there will be value in using them relative to either M3-P or M3-PSR.

The agronomic optimum values for Mehlich-1 P, Mehlich-3 P, and M3-PSR were always below the change point, confirming that maintaining (or lowering) soil P at optimal levels for crop growth should be recommended to reduce P losses by leaching. It is also important to note that the acidic, high OM Pocomoke soil did not behave in the same manner as the other soil series, indicating the need for caution when applying soil testing standards to soils with a wide range of properties.

Finally, while our results support the development of Mehlich-3 P and M3-PSR categories for profitable agriculture and environmental protection, we reemphasize the need to integrate any form of soil P testing with other site properties and P management practices to be most effective in any risk assessment effort designed to protect or improve water quality. While environmental soil limits may be useful in initial efforts to identify potential problems or target limited resources to these ends, a more comprehensive approach, such as the phosphorus site index, will be more accurate at identifying the relative risk of P loss from differing fields or watersheds than soil P testing alone.


   ACKNOWLEDGMENTS
 
We acknowledge and thank Dr. P.J.A. Kleinman and Dr. R.W. McDowell for help and advice with developing the intact soil column leaching methodology.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Published as Paper no. 1710 in the journal series of the Delaware Agricultural Experiment Station.


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




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