| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
a USDA, ARS, ANRI, AMBL, B-163F Rm. 5, BARC-East, 10300 Baltimore Ave., Beltsville, MD 20705
b Dep. of Plant and Soil Sciences, 152 Townsend Hall, University of Delaware, Newark, DE 19717
* Corresponding author (pvadas{at}anri.barc.usda.gov)
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
|---|
Wß, has been proposed to predict such P desorption, but equations originally proposed to predict values for the constants K, , and ß from the ratio of soil clay content/soil organic C content may not be accurate for Delaware soils. Therefore, we measured P desorption for 23 sandy Delaware soils for times of 5 to 180 min, water/soil ratios of 10 to 1000 L kg-1, and three initial levels of soil desorbable P. Values for the constants K, , and ß were calculated and related to soil properties. We found that K, , and ß values were not well related to clay/OC, but were better related to the ratio of oxalate-extractable Fe/OC content () or the sum of oxalate extractable Fe and Al (ß and K). These results can be used to help refine the FHANTM 2.0 model in predicting P loss from agricultural areas in Delaware and similar landscapes in the Mid-Atlantic Coastal Plain.
Abbreviations: Alox, acid ammonium oxalate-extractable Al • AWS, amount of rainfall needed to saturate the topsoil layer in the FHANTM 2.0 model • B, extraction coefficient used in the FHANTM 2.0 model • (Cav)p, quantity of P in the topsoil available for runoff used in the FHANTM 2.0 model • CPLAB, quantity of desorbable P in the topsoil used in the FHANTM 2.0 model • (Cw)p, concentration of P in runoff used in the FHANTM 2.0 model • Feox, acid ammonium oxalate-extractable Fe • FHANTM, Field Hydrologic and Nutrient Transport Model • OC, organic C • OM, organic matter • UDSTP, University of Delaware Soil Testing Program • K, empirical constant in soil P desorption equation • Kd, partitioning coefficient used in the FHANTM 2.0 model • Pd, amount of P desorbed from the soil • Po, initial concentration of desorbable P in soil • POR, porosity used in the FHANTM 2.0 model • Pr, value for precipitation used in the FHANTM 2.0 model • Q, value for runoff used in the FHANTM 2.0 model • SSG, soil specific gravity used in the FHANTM 2.0 model • t, time of P desorption • TMDL, total maximum daily load • W, water/soil ratio during P desorption • , empirical constant in soil P desorption equation • ß, empirical constant in soil P desorption equation
|
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
|---|
Field-scale nutrient transport models have been proposed as a means to characterize the environmental risk of agricultural P to water quality. In Florida, FHANTM 2.0 (Field Hydrologic and Nutrient Transport Model, version 2.0; Fraisse and Campbell, 1997) was developed to simulate water and P movement from individual fields as part of an effort to reduce P loads to Lake Okeechobee. The hydrology of FHANTM 2.0 is based on DRAINMOD (Skaggs, 1980), and the nutrient components are based on GLEAMS (Leonard, 1987). Because Florida's physical and hydrologic conditions of flat fields, high water tables, and high P sandy soils are similar to those in Delaware, FHANTM 2.0 could potentially be used in Delaware to simulate field-scale P export. However, several of FHANTM 2.0's mathematical representations of soil P processes were designed either for pesticide transformations in soils for GLEAMS or for specific chemical and physical properties of Florida soils. Therefore, to use FHANTM 2.0 in Delaware, its P components must be modified to more accurately represent the chemical and physical processes in Delaware soils. One such modification is the representation of P desorption to runoff waters. Currently in FHANTM 2.0, the quantity of P in the topsoil available for runoff, (Cav)p (mg kg-1), is calculated with the equation
 | [2] |
The partitioning coefficient, Kd, assumes that the equilibrium relationship between (Cav)p and (Cw)p is linear and is a function of the Mg and OC content of the topsoil. The extraction coefficient, B, accounts for the fact that the concentration of P in runoff is typically less than the P concentration in the soil solution and is calculated based on the value of the soil's Kd value. Because Eq. [1] and [2] were originally designed to represent pesticide transformations in soils (Leonard et al., 1987), and were calibrated in FHANTM 2.0 for Florida soils, they may not accurately represent desorption of P from soils to runoff for conditions in Delaware.
In many studies of P desorption, it is commonly observed that desorption reactions occur rapidly at first and then decrease as equilibrium is approached. It is also observed that the quantity of P desorbed is largely a function of the time allowed for desorption and the water/soil ratio during desorption (Barrow, 1979). Such desorption data are typically best described by exponential or logarithmic equations, such as Elovich or Freundlich equations (Chien and Clayton, 1980; Kuo and Lotse, 1974). Sharpley et al. (1981) proposed a simplified P desorption equation:
 | [3] |
, and ß are empirical constants for a given soil. Equation [3] represents a significant improvement from the current P desorption equations in FHANTM 2.0 (Eq. [1] and [2]) because it accounts for the nonlinear characteristics of P desorption and the strong influence of time and water/soil ratio. For any given runoff event, either simulated or observed, the time, water/soil ratio, and desorbable P parameters are either known, as is the case with FHANTM 2.0, or can be easily measured or calculated. Sharpley (1983) pointed out that the application of Eq. [3] is limited if the values of the constants K, , and ß must be experimentally determined for a given soil before P desorption can be predicted. Application of Eq. [3] is much broader if the values for the constants can be predicted from known or easily estimated soil physical and chemical properties. Therefore, Sharpley (1983) related K, , and ß to soil properties and found statistically significant relationships with the ratio of soil Fe/OC and clay to OC for 43 soils collected from throughout the USA. However, the clay and Fe contents of these soils were much greater than those typically found in the sandy soils of Delaware's Inland Bays watershed and of the Mid-Atlantic Coastal Plain in general. Because the clay and Fe contents of soils have a strong influence on P desorption phenomena, the relationships provided by Sharpley to predict K, , and ß and to subsequently predict P desorption may not accurately represent Delaware soils. Given these considerations, the objectives of this research were to determine if Sharpley's relationships for predicting K, , and ß are applicable to Delaware soils; and, if not, to develop relationships for predicting K, , and ß from known or easily measured properties of Delaware soils. Although the research presented here was conducted for only Delaware soils, it should be applicable to Mid-Atlantic Coastal Plain soils with similar physical and chemical characteristics. |
MATERIALS AND METHODS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
|---|
All samples had been previously air-dried and ground to pass a 2.0-mm sieve. We characterized the soils for pH (1:1 soil/water ratio), soil test P (STP; Mehlich-1 extraction; 1:4 ratio of soil/0.05 M HCl + 0.0125 M H2SO4; 5-min reaction time [Sims and Heckendorn, 1991]), particle size by the hydrometer method (Bouyoucos, 1962), and acid ammonium oxalate-extractable Al and Fe (Alox and Feox; 1:40 ratio of soil/0.2 M [NH4]2C2O4, 2-h reaction time in darkness [McKeague and Day, 1996]). Soil OC was measured by the Walkley-Black wet oxidation procedure (Nelson and Sommers, 1982). The initial amount of desorbable P (P0, Eq. [3]) was measured with Fe-oxide impregnated filter strips (1:40 ratio of soil/0.01 M CaCl2 + Fe-oxide coated filter paper strip; 16-h reaction time, followed by desorption of P with 1 M H2SO4 [Chardon et al., 1996]). The P in the Mehlich-1 extraction and the Fe and Al in the oxalate extraction were measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES). The P in the filter strip procedure was measured by the molybdate blue method of Murphy and Riley (1962) with absorbance measured at 882 nm.
Phosphorus Desorption Experiments
The desorbable P status of each soil was varied by adding 0, 95, and 190 mg P kg-1 soil as a solution of KH2PO4 (equivalent to fertilizer application rates of 0, 50, and 100 kg P ha-1 to a 4-cm soil depth). Both the P-amended and unamended soils were incubated at 25°C for 3 d prior to the P desorption study. This incubation time was chosen to duplicate the experiments of Sharpley et al. (1981), and because Sharpley and Ahuja (1982) showed that longer incubation times did not have a significant effect on the values of K, , and ß. The desorption of P from soils was investigated by shaking duplicate samples with distilled water at water/soil ratios of 10:1, 100:1, and 1000:1 (mL g-1) on an end-to-end shaker at 25°C for times of 5, 30, 60, and 180 min. After shaking, each solution was filtered through Gelman 0.45-µm millipore filters(Gelman Sciences, Ann Arbor, MI). The P in the filtered solutions was measured colorimetrically on a Sequoia-Turner model 340 spectrophotometer (Sequoia Turner Corp., Mountain View, CA) by the molybdate blue method of Murphy and Riley (1962) with absorbance measured at 882 nm.
Calculation of K, , and ß
Given that Eq. [3] is valid and is a power equation, at any given combination of W and Po, the logarithm of Pd should be linearly related to the logarithm of t for each soil. The slope of this line is the value of for that soil at that combination of W and Po. Similarly, at any given combination of t and Po, the logarithm of Pd should be linearly related to the logarithm of W for each soil. The slope of this line is the value of ß for that soil at that combination of t and Po. In the final case, at any given combination of t and W, Pd should be linearly related to Po. The slope of this line is the value of K for that soil at that combination of t and W. Throughout the desorption experiments for each soil, nine values of from all combinations of W and Po, 12 values of ß from all combinations of t and Po, and 12 values of K from all combinations of t and W were calculated. Average values of , ß, and K were then calculated from these nine or 12 values. These average values of , ß, and K were then related to soil properties using a least-squares regession. The parameters of the resulting regression equations and their correlation coefficients were analyzed for significance by an ANOVA procedure. These statistical analyses were performed within Microsoft EXCEL spreadsheets (Microsoft, Inc., Redmond, WA). Correlation coefficients of the regression equations were compared for statistically significant differences as described by Snedecor and Cochran (1971). To do this, r values were converted to z values using statistical tables. The differences between z values were then tested for significance using a two-tailed t test. These comparisons were made only among equations for each constant , ß, and K, and not between constants.
|
RESULTS AND DISCUSSION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
|---|
|
were also much less than the extractable Fe and Al contents of Sharpley's soils 
. Sharpley used 1 M NH4Oac, pH 4.8, to extract Fe and Al, whereas we used 0.2 M (NH4)2C2O4. Therefore, some of the difference in the amount of Fe and Al content extracted from Sharpley's and our Delaware soils is likely because of the difference in the inherent ability of acetate and oxalate to extract Fe and Al from soil. However, the literature is sparse with research directly comparing the ability of acetate and oxalate to extract soil Fe and Al. Myers et al. (1988) extracted Al from seven acid Ohio soils with both 1 M NH4Oac, pH 4.8, and 0.2 M (NH4)2C2O4 and found that the oxalate always extracted more Al than the acetate, by an average greater amount of nearly nine times. The same is likely true for soil Fe, but we could not find any sources in the literature that directly compare the ability of oxalate and acetate to extract Fe from soil. Indirectly, Kraske et al. (1989) extracted Fe from six New England forest soils with both 1 M NH4Oac, pH 4.8, and Mehlich 3 (0.2 M CH3COOH + 0.25 M NH4NO3 + 0.015 M NH4F + 0.01 M HNO3 + 0.001 M EDTA) and found that the Mehlich 3 always extracted more Fe than the acetate, by an average greater amount of 22 times. Comparatively, research from the University of Delaware (J.T. Sims, unpublished data, 2000) that extracted Fe from 96 Delaware soil samples with both oxalate and Mehlich 3 shows that oxalate always extracted more Fe than Mehlich 3, by an average greater amount of six times. Therefore, if oxalate extracts more Fe from soils than Mehlich 3, and Mehlich 3 extracts more Fe from soils than acetate, than oxalate extracts more Fe from soils than acetate. All this research provides evidence that Sharpley extracted more Fe and Al with acetate from his soils than we extracted with oxalate from our Delaware soils because his soils actually contained more Fe and Al and not because acetate is generally capable of extracting more Fe and Al from soils than oxalate. The point of the above discussion is to show that the properties of Sharpley's soils that he used to predict K, , and ß values, specifically clay and Fe content, were significantly different from those in our Delaware soils This is a key consideration in regard to the objectives of our research. Because Sharpley's soils contained much more clay and Fe than our soils, we hypothesized that the relationships he developed between soil clay or Fe content and values for K, , ß would not be accurate for Delaware soils.
Phosphorus Desorption Characteristics
The term Po in Eq. [3] represents the amount of soil P that can potentially be desorbed to water for the time periods and water/soil ratios used during our experiments. Sharpley and Ahuja (1982) stated that Po represents the amount of soil P that is potentially readily desorbable with water, especially under conditions that would occur during a runoff event. The cutoff point between readily desorbable P and more slowly desorbable P is based on where the kinetic mechanism of desorption shows a change from one to the other. The cutoff point may be somewhat arbitrary, but the applicability of Eq. [3] is not affected as long as the method to estimate Po is consistently used. In Sharpley's (Sharpley et al., 1981, 1985; Sharpley and Ahuja, 1982; Sharpley, 1983; Sharpley and Smith, 1989) use of Eq. [3] to predict P in runoff, Po was measured by a variety of methods, including extraction by water, NaHCO3, and the Bray-1 solution, and isotopic exchange. In our research, Fe-oxide strips were used to measure Po. It should be emphasized that the Fe-oxide method is not an absolute measure of desorbable P. Chardon et al. (1996) describe that the Fe-oxide strip method was initially developed for soil chemical studies by Sissingh (1983) as an alternative to anion exchange resins for estimating the amount of P already sorbed on a soil when determining a P adsorption isotherm. Later, its use was applied to estimate P availability to plants (Menon et al., 1990) and during water quality studies (Sharpley, 1995). The test is appropriate for these uses because the Fe-oxide strip does not react with the soil but rather adsorbs P from the solution, keeps the solution P concentration relatively low, and promotes the desorption of P from the soil to replace the solution P that adsorbed to the strip. Therefore, Fe-oxide strips can be considered to extract that portion of soil P that will easily desorb to water during a relatively short-time period, which in our research was 16 h. We used Fe-oxide strips to estimate P0 to be consistent with the method used to quantify labile P (desorbable P) in other research conducted in Delaware (Maguire et al., 2000; Pautler and Sims, 2000). Maguire et al. (2000) measured P in 16 Delaware, Maryland, and Virginia Coastal Plain soils with five sequential extractions with Fe-oxide strips. They found that the most P was extracted with the first strip, and the amount of P extracted with the remaining four strips was less than the first strip and was about the same for each strip. As well, Lookman (1995) stated that desorbable P measured with four Fe-oxides strips was consistently greater (as much as four times) than the amount of P that represented a fast desorbing pool in five sandy Dutch soils. Both these research studies suggest that one Fe-oxide strip will extract easily desorbable P, while additional strips will extract consistent amounts of P that represent the P buffering capacity of a soil beyond the fraction of easily desorbable P. Furthermore, Fig. 1c demonstrates that for one of the Sussex County soils used in our research, Pd was well related to Po. This trend was true for all our soils. At the widest water/soil ratio (1000:1) and the longest desorption time (180 min), Pd desorbed for all soils was on average 140% of desorbable P measured with Fe-oxide strips. At all other lesser desorption times and water/soil ratios, measured P desorbed was less than desorbable P measured with Fe-oxide strips. This entire discussion above provides evidence that Po determined by a single Fe-oxide strip in the Delaware soils represents a reasonable estimate of the amount of P that can be released to water in the desorption times and water/soil ratios used in our research.
|
ranged from 0.073 to 0.244, with an average of 0.133. Our range and average for values are very similar to those of Sharpley (1983), which ranged from 0.045 to 0.319, with an average of 0.177. The K values obtained in our study were also similar to those of Sharpley (1983). In the 23 Delaware soils, K ranged from 0.034 to 0.267, with an average of 0.150. In Sharpley's soils, K ranged from 0.021 to 0.302, with an average of 0.142. An example of the linear relationship between Pd and Po used to calculate K for various combinations of t and W is shown for the same Sussex County soil in Fig. 1c. Figure 1b shows the linear relationship between the logarithm of Pd and the logarithm of W for various combinations of t and Po for the same Sussex County soil. For all Delaware soils, the values for ß ranged from 0.194 to 0.378, with an average of 0.266. These ß values were generally less than those of Sharpley (1983), which ranged from 0.204 to 0.850, with an average of 0.520. However, our ß values did fall into the lower end of the range of ß values measured by Sharpley. This lower end corresponded to his soils that had a low clay/OC ratio. Because the OC values for Sharpley's soils were generally low (range of 7–490 g kg-1, average of 130 g kg-1) and the range in clay content (8–530 g kg-1, average of 220 g kg-1) was broader than that for OC content, his low clay/OC ratios were probably more a result of low clay content than high OC content. Therefore, the range of ß values measured in our study, which were similar to those measured for Sharpley's apparently low clay soils, are most likely representative of those that would be obtained for low clay soils. Our 23 Delaware soils can all be considered low clay soils.
Relation of Phosphorus Desorption to Soil Properties
Sharpley (1983) found that K, , and ß were highly correlated with the ratio of extractable Fe/OC and clay/OC (Table 2). He used these two ratios to represent the interactive specific surface area involved in soil P adsorption and desorption. He postulated that adsorption and desorption of P by soils is dominated by sesquioxides or mineral surfaces, and that OC competes with P for adsorption sites, thus decreasing a soil's P adsorption capacity and possibly altering P desorption characteristics. Since soil Fe contents are not usually available from soil survey information, and soil clay content has historically been related with P adsorption parameters, Sharpley used the clay/OC ratio to predict K, , and ß.
|
represents a P desorption rate term; and, mathematically, an increase in results in an increase in the rate of P desorption. Therefore, should be related to those soil properties that affect how quickly P desorbs from soil. In our study, values were best correlated, although not significantly, to both Feox/OC and clay/OC (Table 2), which is similar to what Sharpley observed. Therefore, either ratio could be used to predict values. However, Feox was also well related to the K and ß values, as discussed below, while clay content was not. So for consistency sake and to minimize the soil analysis required to use Eq. [3], we recommend the Feox/OC regression equation (Fig. 2a)
to predict in Delaware and similar Mid-Atlantic Coastal Plain soils.
|
than the ratio of Alox/OC, although its r value was numerically greater. However, desorption of P from Fe hydroxides may be more affected by desorption time than desorption of P from Al hydroxides. Lookman (1995) investigated P desorption from synthetic amorphous coprecipitates of Fe and Al with phosphate. He found that in desorption experiments of times up to 1004 h, the rate of P desorption from Fe hydroxides consistently decreased with desorption time, but the rate of P desorption from Al hydroxides was more or less constant for all times of desorption. Also, more P was desorbed from Fe hydroxides than from Al hydroxides in the first 400 h. This trend was reversed at desorption times >400 h. Lookman speculated that P associated with the Fe was present as a surface adsorbate and not a precipitate, thus providing more P in contact with the solution that could desorb quickly and less P that would slowly diffuse from interior sites of precipitates. Comparatively, P desorption from Al hydroxides may have been a process of continuous slow dissolution of the Al phosphate phase. Lookman's research provides evidence for why the value for P desorption in Delaware soils was better correlated to the ratio of Feox/OC than Alox/OC.
Including the OC parameter in the regression equations generally improved the ability to predict , although not always significantly. This suggests that the association of OC with Fe or Al hydroxides in soil can increase the rate at which P is desorbed from soil. This could be because of the ability of OC to maintain Fe and Al hydroxides in poorly crystalline forms that are more susceptible to P desorption (Toor and Bahl, 1999), the replacement of P on soil surfaces with organic anions, or the formation of soluble complexes between Al and organic anions that can prevent reprecipitation of desorbed P (Fox et al., 1990; Bhatti et al., 1998).
During a P desorption event, if the water/soil ratio increases, for example because of an increase in soil water content or in the amount of runoff, P will desorb from the soil to maintain an equilibrium between the soil and solution. A soil that has a relatively large capacity to supply P to the soil solution can be considered well buffered against changes in solution P concentrations. This soil may therefore exhibit fairly uniform P desorption under various water/soil ratios. Phosphorus desorption in a poorly buffered soil is more influenced by changing water/soil ratios. The value of ß for a given soil reflects this buffering phenomenon because it is the exponent for the variable W in Eq. [3]. As ß increases, dilution of the interacting soil and water has a greater influence on P desorption. Therefore, ß should be related to those soil properties that determine a soil's P buffering capacity. Unlike Sharpley (1983), we found that ß values in our Delaware soils were not well correlated to the ratios of Feox/OC or clay/OC (Table 2). The best relationship observed between ß and soil properties was with the sum of Feox and Alox, although not statistically better than the relationship with Alox or Feox alone. In our primarily sandy, low clay Delaware soils, Fe and Al hydroxides dominate P sorption and therefore represent the interactive soil surfaces from which P will desorb (Pautler and Sims, 2000). An increase in [Feox + Alox] represents an increase in P buffering capacity, and thus an increase in ß. This is consistent with the data in Fig. 2b, which shows how ß increases with increasing content of [Feox + Alox]. Including OC in the regression equations generally did not improve, and sometimes significantly decreased, the ability to predict ß. This suggests that while OC may affect the rate at which P desorbs from these Delaware soils, as seen with the data, it may not affect their overall P buffering capacity.
As with the ß results, K values seemed to be best related to soil [Feox + Alox], although not statistically better than with Alox alone or the ratios of Feox/OC or clay/OC (Table 2). The constant K represents a P desorbability or capacity term. It expresses the proportion of available soil P that can be desorbed from a soil for a given time and water/soil ratio. An increase in K means that the proportion of Po desorbed from a soil will also increase. This may be attributed to low soil sorption capacity, which in Delaware soils is represented by a low content of [Feox + Alox]. This is consistent with data in Fig. 2c, which shows that as [Feox + Alox] increases, the P sorption capacity of the soil increases, and the K value subsequently decreases. As with the ß results, including OC in the regression equations changed the exponent from negative to positive and significantly decreased the ability to predict K when Al was included as a factor. When Fe or clay was included as a factor, including the OC parameter in the regression equations also changed the exponent from negative to positive, but did not statistically change the ability to predict K, although the r values for the equations including OC were numerically greater. This difference in the effect of OC on Al and Fe and clay may be because Al is more typically complexed with soil OM than Fe or clay.
In summary, the following equations are proposed for predicting , ß, and K values for Delaware and similar Mid-Atlantic Coastal Plain soils:
 | [4] |
 | [5] |
 | [6] |
The relationships between soil properties and the values of , ß, and K as described by Eq. [4] through [6] were all statistically significant at the 0.1% probability level; and all coefficients in Eq. [4] through [6] were statistically significant at the 5.0% probability level. Also, even though Eq. [4] through [6] were determined using data from only Delaware soils, they should be applicable to similar soil types in the Mid-Atlantic Coastal Plain. It is important to emphasize that those relationships developed by Sharpley (1983) to predict , ß, and K should still be used for soils that are chemically and physically similar to the ones he used. For soils that are similar to our 23 Delaware soils, Eq. [4] through [6] may provide a better prediction of , ß, and K and thus of P desorption.
Predicting Phosphorus Desorption
To determine if the relationships to predict K, , and ß as proposed by Sharpley (Table 2) were accurate for Delaware soils, Eq. [3] was tested for its ability to predict P desorption. First, K, , and ß were calculated for the 23 Delaware soils using Sharpley's recommended clay/OC equations. Values for t and W in Eq. [3] were taken from the specific methods of our desorption experiments, and values for P0 in Eq. [3] were taken from the quantities of desorbable P as measured in all soils before the desorption experiments. Then, Eq. [3] was used to predict P desorption for all soils and all combinations of t, W, and P0. The predicted amounts of P desorbed were then compared with the amounts of P desorbed as measured during the desorption experiments. Figure 3
shows that using Sharpley's equations for Delaware soils resulted in a very weak relationship between measured and predicted P desorption. These results suggest that the equations recommended by Sharpley to predict values for K, , and ß may not provide a good prediction of P desorption for our Delaware or similar Mid-Atlantic Coastal Plain soils. Therefore, Eq. [4] through [6] will likely provide a better prediction of P desorption for Delaware and Mid-Atlantic Coastal Plain soils. These equations are currently being tested using several independent data sets collected during simulated rainfall experiments using Delaware soils to better assess their potential to predict P desorption and to justify their incorporation into the FHANTM 2.0 model for use in Delaware and the Mid-Atlantic Coastal Plain.
|
|
REFERENCES |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
|---|
This article has been cited by other articles:
|
|
 |
|
  C. J. Penn, G. L. Mullins, L. W. Zelazny, and A. N. Sharpley Estimating Dissolved Phosphorus Concentrations in Runoff from Three Physiographic Regions of Virginia Soil Sci. Soc. Am. J., September 20, 2006; 70(6): 1967 - 1974. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
P. A. Vadas, T. Krogstad, and A. N. Sharpley Modeling Phosphorus Transfer between Labile and Nonlabile Soil Pools: Updating the EPIC Model Soil Sci. Soc. Am. J., March 29, 2006; 70(3): 736 - 743. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
Y. Arai, K. J. T. Livi, and D. L. Sparks Phosphate Reactivity in Long-Term Poultry Litter-Amended Southern Delaware Sandy Soils Soil Sci. Soc. Am. J., April 11, 2005; 69(3): 616 - 629. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 K. E. Staats, Y. Arai, and D. L. Sparks Alum Amendment Effects on Phosphorus Release and Distribution in Poultry Litter-Amended Sandy Soils J. Environ. Qual., September 1, 2004; 33(5): 1904 - 1911. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. T. Siddique and J. S. Robinson Differences in Phosphorus Retention and Release in Soils Amended with Animal Manures and Sewage Sludge Soil Sci. Soc. Am. J., July 1, 2004; 68(4): 1421 - 1428. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
P. A. Vadas, J. T. Sims, A. B. Leytem, and C. J. Penn Modifying FHANTM 2.0 to Estimate Phosphorus Concentrations in Runoff from Mid-Atlantic Coastal Plain Soils Soil Sci. Soc. Am. J., November 1, 2002; 66(6): 1974 - 1980. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| The SCI Journals | Agronomy Journal | Crop Science | |||
| Journal of Natural Resources and Life Sciences Education |
Vadose Zone Journal | ||||
| Journal of Plant Registrations | Journal of Environmental Quality |
The Plant Genome | |||
