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Dep. of Plant and Soil Sci., Univ. of Delaware, Newark, DE 19717-1303 USA
mpautler{at}udel.edu
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMaterials and methodsResults and discussionConclusionsREFERENCES |
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, soluble
, strip
, and oxalate-extractable
. Strip P was a better predictor of soluble P than
. The ratio of strip P/Pox (the percentage of reversibly sorbed P in the fast desorbing pool) increased as P sorption capacity, estimated from oxalate-extractable Al and Fe (Alox + Feox), decreased. We also determined the degree of P saturation (DPS) using three methods: Langmuir P sorption isotherms; oxalate extractions of P, Al, and Fe; and STP plus a single-point P sorption index (PSI). Soluble P, STP, and desorbable P increased for DPS values >30%, similar to upper DPS limits in the Netherlands and Belgium. Soils rated agronomically excessive in STP (>50 mg kg-1) had higher ratios of soluble P, strip P, and Pox to total P than those in agronomically optimum or lower categories.
Abbreviations: Alox, Feox, Pox, aluminum, iron, and phosphorus extracted by acid ammonium oxalate • DPS, degree of soil P saturation • ICP–AES, inductively coupled plasma atomic emission spectroscopy • OM, organic matter • Pw, water soluble P • PSCr, remaining P sorption capacity • PSCt, total P sorption capacity • PSI, single-point P sorption index • STP, soil test P • UDSTP, University of Delaware Soil Testing Program
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResults and discussionConclusionsREFERENCES |
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Delaware is the site of one of the most concentrated poultry industries in the USA, producing about 260 million broiler chickens each year in a state with 
225000 ha of cropland (DDA, 1998). Most (>90%) poultry are produced in Sussex County, Delaware, which has 100000 ha of cropland. It has been estimated that based on fertilizer consumption and poultry litter production alone, annual P surpluses of 16 and 49 kg P ha-1 exist statewide and in Sussex County, respectively (Cabrera and Sims, 1999). Furthermore, applying poultry litter based on crop N requirements adds 135 kg P ha-1, relative to crop P removal of 25 kg P ha-1 (Mozaffari and Sims, 1994). Because of these P surpluses and the historical use of N-based manure management practices, the amount of P in manure applications has often exceeded the amount of P removed by crop harvest. This has resulted in the widespread buildup of soil P to values well above those needed for optimum crop production. For example, recent soil test summaries from Delaware (14000 samples from 1992–1997) showed that 92% of the agricultural soils tested from Sussex County are now optimum or excessive in STP (Sims, 1998). The median STP value (Mehlich 1) was 70 mg P kg-1 and 30% of the soils tested were >100 mg P kg-1. In comparison, the Mid-Atlantic Soil Test and Plant Analysis Work Group (a regional consortium of public and private soil test laboratories) stated in 1997 that for corn (Zea mays L.), a Mehlich 1 value of 20 mg P kg-1 was the "soil test P level at 100% yield," and 35 mg P kg-1 was the "soil test P level where no fertilizer is recommended" (Sims, 1998).
Soil P testing is now used in some countries and U.S. states and is under consideration in others as a means to identify areas where P applications in fertilizers and manures should be prohibited to protect water quality (Sibbesen and Sharpley, 1997; Sims et al., 1999). The rationale for these actions is three-fold: First, building soil test P above agronomic optimum values is perceived as an inefficient use of a limited natural resource (phosphate). Second, soils with higher STP concentrations are considered more likely to lose P to surface waters by erosion and surface or subsurface runoff. And, third, geographic summaries of agronomic STP results are readily available in most states. These summaries provide a large, historical database on soil P status, which can be related to soil management practices, cropping systems, and soil properties. If STP is related to the potential for P loss to ground or surface waters, this database could be used in state or regional planning efforts to reduce nonpoint-source pollution.
Consequently, our objectives were (i) to determine if quantitative relationships exist between agronomic STP values and P measurements that are proposed as indicative of the potential for P loss to waters, such as soluble P, easily desorbable P, and the degree of P saturation, and (ii) to assess which soil P testing methods would be most easily adopted by routine soil testing laboratories that may soon be asked to provide environmental, as well as agronomic, P recommendations.
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Materials and methods |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResults and discussionConclusionsREFERENCES |
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Phosphorus Sorption Parameters
A representative subset (41 soils) of the 127 Delaware and Dutch soils was selected for characterization of P sorption capacity (Table 1). Phosphorus sorption isotherms were conducted by a modification of the standard batch technique of Nair et al. (1984). Thirty milliliters of six P solutions ranging in initial concentrations from 0 to 25 mg P L-1 (as KH2PO4) were equilibrated with 2.0 g of each soil in 50-mL test tubes in an end-over-end shaker. The soil suspension was then centrifuged, filtered through 0.45-µm Gelman millipore filters, and analyzed for P by ICP–AES. The difference between P added in the initial solutions and P remaining in the filtered solutions was considered to have been sorbed.
The P sorption index (PSI), a rapid means to estimate soil P sorption capacity, was also determined for these 41 soils (Bache and Williams, 1971; Mozaffari and Sims, 1994). The PSI was calculated as the amount of P sorbed (mg kg-1) after equilibration of 0.4 g soil with 40 mL of a 15 mg P L-1 solution (as KH2PO4, equivalent to 1.5 g P kg-1 soil) in an end-over-end shaker for 24 h, followed by centrifugation, filtration, and analysis for P by ICP–AES.
Soil Phosphorus Saturation
We determined the degree of soil P saturation (DPS) by three methods:
(i) DPSLangmuir: In this approach we calculated DPS for the 41 subset soils only, as DPSLangmuir (%) = Pox / (PSCt) x 100 PSCt = (Pox + PSCr)
where PSCt is the total P sorption capacity of a soil, Pox is the amount of P already sorbed, and PSCr is the remaining P sorption capacity. We estimated PSCr from P sorption maxima calculated from the linearized version of the Langmuir equation using experimental data from P sorption isotherms conducted with the 41 soils (Olsen and Watanabe, 1957).
(ii) DPSSTP: We also estimated DPS for the 41 soils using the ratio of STP to (PSI + STP):
DPSSTP (%) = {STP (mg kg-1) / [PSI + STP (mg kg-1)]} x 100
(iii) DPSox: We also used the approach of van der Zee and van Riemsdijk (1988) and calculated DPS in all 127 soils based on Pox, Feox, and Alox.
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is an empirical parameter calculated as the ratio of PSCt (determined experimentally for a defined group of soils, in this case our 41 subset soils) to [Alox + Feox]. |
Results and discussion |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResults and discussionConclusionsREFERENCES |
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) and Belgian topsoils investigated by Lookman et al. (1995b)
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Relationships Between Soil Test Phosphorus and Other Forms of Soil Phosphorus
The 122 Delaware soils were selected to be typical of the agricultural soils in the state and to provide a range and distribution in STP (Mehlich 1) values similar to that in each Delaware county today. The current STP rating scale used in Delaware is as follows: low (<13 mg P kg-1), medium (13–24 mg P kg-1), optimum (25–50 mg P kg-1), and excessive (>50 mg P kg-1) (Sims and Gartley, 1996). The mean STP value for all Delaware soils in our study was 86 mg P kg-1 and mean STP values were greatest in Sussex county (104 mg P kg-1), followed by Kent county (63 mg P kg-1), and then New Castle county (28 mg P kg-1) (Table 1). Sims (1998) reported mean STP values, based on analysis of 12000 agricultural soils of 79, 89, 67, and 40 mg P kg-1 for the state of Delaware, Sussex, Kent, and New Castle counties, respectively. Soil test P values reflect the differing land uses in these counties. Sussex county is dominated by an intensive poultry industry and grain farming (230 million broiler chickens per year and 100000 ha of cropland), Kent county by mixed agriculture (grain, vegetable, poultry), and New Castle county by grain crops with little intensive animal production. Soil test P values in all five Dutch soils
would be rated as excessive and likely reflect a history of long-term manuring and fertilization.
One of the goals of this research was to quantify the relationships between STP and other tests for soil P. We found, based on all 127 soils (Delaware and Dutch), that STP was significantly correlated with total
(Fig. 1)
, oxalate-extractable
, Fe oxide–strip
, and dilute salt (0.01 M CaCl2)–extractable
. Statistically significant linear or curvilinear regression equations were developed to predict Pox, strip P, and 0.01 M CaCl2–extractable P from
; however, the r2 value for the STP–total P relationship was too low
for predictive purposes (Fig. 1). These results are consistent with past research. Beauchemin et al. (1996) reported that average values for water soluble P and Pox increased with STP and were higher in soils from Canadian watersheds with surplus P from animal manures than in forested soils. Sharpley (1996) showed consistent increases in Fe oxide–strip P as STP (Mehlich 3) increased because of long-term applications of beef, poultry, and swine manures. Barberis et al. (1996) reported significant, positive correlations between several soil P tests and both Fe oxide–strip
and water soluble
for 12 over-fertilized European soils.
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. Our results differ from those of van der Zee and van Riemsdijk (1988), who reported a higher correlation coefficient
and a near 1:1 relationship between total P and
for 84 topsoils of the Netherlands. There were different relationships between Pox and total P in soils from the three Delaware counties and the Dutch soils. Oxalate-extracted P made up 65% of the total P from the coarse-textured soils of Sussex county, only 20% of total P in the fine-textured Piedmont soils of New Castle county, and 93% of the total P in the five Dutch soils (Table 1). For Sussex county soils, Pox was better correlated with total
than when all 127 soils were considered together
. In general, our data suggest that STP could be used to predict the concentrations of Pox and desorbable P (strip P) with reasonable accuracy but would not be as reliable an indicator of total or soluble P (Fig. 1). Multiple-regression equations between STP and other forms of P that included parameters such as pH, OM, and oxalate Al and Fe gave only slight improvements in predictive accuracy relative to regressions with STP alone (data not shown). The most significant improvement obtained was with total P, where the R2 value for the relationship between total P and [STP and Feox] was 0.56***, compared with 0.36** based only on STP.
Improved prediction of soluble P could be achieved by use of Fe oxide–strip P (
, Fig. 2a)
. Other research has shown that strip P may provide a more accurate way to estimate P desorption into runoff waters than STP (Pote et al., 1996; Sharpley et al., 1996), as well as the biological availability of P in sediments in agricultural runoff (Sharpley et al., 1994). We also observed that the ratio of Fe oxide–strip P to Pox decreased in a curvilinear manner as the concentration of [Alox + Feox] increased (Fig. 2b). This suggests that P-enriched soils with low concentrations of [Alox + Feox] will have a higher percentage of easily desorbed P. Lookman et al. (1995a) found an almost identical trend in studies of the long-term kinetics of P desorption with 44 Belgian and German soils. They reported that most Pox was reversibly sorbed, that Fe oxide–P represented a "fast desorbing pool" of soil P with a maximum value of 22% of Pox, and that the ratio of Fe-oxide P to Pox increased in a near linear manner as soils became increasingly saturated with P.
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We determined DPSLangmuir for the subset of 41 Delaware and Dutch soils (Table 1) and confirmed that concentrations of soluble P and the release of P into solution through desorption and dissolution reactions increased as soils become more saturated with P. Significant linear relationships existed between DPSLangmuir and
,
,
, and strip
. However, further regression analysis indicated that curvilinear (exponential) equations better fit the data for these 41 soils in all cases
(Fig. 3a and b)
. Soil DPS values of 25 to 40% are commonly associated with greater risks of P loss in leaching or runoff and thus nonpoint-source pollution. Breeuswma et al. (1995) stated that DPS values of >25%, in the soil profile to the depth of the mean high water table, would contribute to shallow ground water pollution in the Netherlands. De Smet et al. (1996) reported mean DPS values of 57% for depths of 0 to 30 cm, 22% for depths of 30 to 60 cm, and 11% for depths of 60 to 90 cm in the profiles of 296 Belgian soils; a DPS value of 30% for depths of 0 to 90 cm in the soil profile was proposed as an upper limit to protect shallow ground waters. Pote et al. (1999) showed that dissolved reactive P in runoff from three Ultisols in Arkansas increased at DPS values >20 to 30%.
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, an experimentally determined parameter that relates PSCt (the sum of Pox + PSCr) to [Alox + Feox]. Reported values of for acid, sandy soils have usually ranged between 0.3 and 0.7 and an value of 0.5 is commonly used, often without experimental justification (De Smet et al., 1996; Lookman et al., 1995b; Lookman et al., 1996; van der Zee et al., 1987). Past research has also shown that not only soil properties but the reaction time between added P and the soil will affect estimation of PSCr and thus values of PSCt and . For example, van der Zee and van Riemsdijk (1988) reported an value of
for 84 Dutch topsoils based on 40-h sorption experiments but an value of
for the same soils based on a 249-d reaction time. Extending the length of the sorption experiment allows for slow P sorption to occur and thus increases the ratio of PSCt to [Alox + Feox]. Consequently, van der Zee and van Riemsdijk (1988) suggested multiplying short-term estimates of by 1.8 to adjust for slow P sorption kinetics.
As with past research, we found for the 41 subset soils that PSCt was significantly correlated with [Alox + Feox] (
; Fig. 4)
, providing further evidence that the P sorption capacity of acid, low OM soils is controlled by amorphous Al and Fe. The average value for these 41 soils was
. This is within the range of short-term estimates of reported for Belgian, Dutch, and German soils (Lookman et al., 1995a; van der Zee et al., 1987). If multiplied by 1.8 to adjust for slow P sorption kinetics, an value of 0.68 is obtained for our soils, very similar to the 0.61 value determined by van der Zee and van Riemsdijk (1988) for acid, sandy, low organic-matter topsoils in the Netherlands. We next calculated DPSox for all 127 soils, based on Pox, Alox, and Feox and using
. As with DPSLangmuir, soluble and desorbable–extractable P increased linearly with DPSox (Fig. 3c and d).
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, which suggests that PSI groupings could be developed for a watershed based on textural class. We also found that PSCr and PSI were highly correlated for the 41 subset soils (
, Fig. 5a)
and that soluble P (0.01 M CaCl2–P and Pw) and P in the fast desorbing pool (Fe oxide–strip P) increased linearly with DPSSTP (Fig. 5b).
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1.5 to 2 x Mehlich 1 P; Sims, 1989).
We grouped our data for total P, soluble P, desorbable P, and DPSox for all 127 soils into the current soil test categories used by the University of Delaware. As shown in Table 2
, 24% of the samples were in the low–medium categories, 18% were in the optimum category, and 58% were excessive in STP. By way of comparison, the most recent summary of STP values in Delaware showed that 17% were low–medium, 28% were optimum, and 55% were excessive in P (Sims and Vadas, 1997). In Sussex County, site of Delaware's geographically dense poultry industry, 65% of the soils tested were rated as excessive in P. Note that the categories used in Delaware are consistent with those used in the Mid-Atlantic and Northeast states (Beegle et al., 1998; Sims, 1998). In our study, as would be expected, soils rated as excessive in P had higher mean values for soluble P, desorbable P, total sorbed P (Pox), DPSox, and total P than soils in the optimum, medium, and low STP categories (Table 2). However, we also observed that as STP values increased from low to excessive, the more labile forms of P increased relatively more than total P. For example, total P values in the excessive category were 1.4 times as great as total P in the agronomically optimum STP range. In contrast, soluble P, strip P, STP, and DPSox were 10.7, 2.7, 3.8, and 2.7 times higher, respectively, in the excessive STP range than in the optimum range. This means that the percentage of total P in soluble, desorbable, and total sorbed (Pox) forms is increasing as soils become excessive in STP due to over-application of P in fertilizers or manures (Table 2). For example, the percentage of total P that could be desorbed by Fe oxide–strips increased from 1% in the low STP to 13% in the excessive STP category, and the percentage of total P extracted by oxalate increased from 21 to 70% (Table 2). This suggests that not only will soils rated as excessive by an agronomic soil test or as highly saturated by a DPS measurement have higher concentrations of soluble and desorbable P, but they will have a proportionately larger pool of labile P than soils in the optimum or lower STP categories. Soils with higher percentages of total P in soluble or easily released forms will also presumably have a greater potential to release P into runoff waters or into surface waters following erosion of P-rich particles. Based on long-term desorption studies, Lookman et al. (1995a) reported that "all oxalate extractable P is assumed to be reversibly fixed," and several studies have shown strong correlations between Fe oxide–strip P and biologically available or algal-available P in soils and runoff (Sharpley et al., 1994).
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Conclusions |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResults and discussionConclusionsREFERENCES |
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One of our objectives was to determine if current soil testing practices can be used to identify fields, farms, and watersheds where nonpoint-source pollution by P is a significant environmental issue. Clearly, soil P testing alone will be inadequate to predict P loss from soil to water because it does not provide direct information on other factors that affect the transport of P to surface waters (e.g., hydrology, topography, nutrient management practices). However, it seems equally apparent that agronomic soil P tests will be useful predictors of other parameters related to the potential for P loss to water, such as soluble P, desorbable P, and the degree of P saturation. For example, the current STP value (Mehlich 1 soil test) used in Delaware to rate soils as agronomically excessive is 50 mg P kg-1, which, based on our data (Fig. 3d) corresponds to a DPSox value of 25%, in the range of DPSox values now used for water quality protection in the Netherlands and Belgium (25–40%). Comparison with Dutch standards seems reasonable, given the similar trends we observed in our study between Delaware and Dutch soils. Therefore, the use of existing agronomic STP databases to assist in the initial prioritization of management efforts to reduce nonpoint-source pollution of ground and surface waters by P seems justified. Soil testing laboratories should also consider adopting P saturation tests to improve their ability to identify soils with greater potential to release P into runoff or leaching waters.Cabrera Sims 2000; Heathwaite Sharpley Gburek 2000; Irish Environmental Protection Agency 1997; Sims Edwards Schoumans Simard 2000
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ACKNOWLEDGMENTS |
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Received for publication May 10, 1999.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResults and discussionConclusionsREFERENCES |
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