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Published online 9 August 2006
Published in J Environ Qual 35:1818-1824 (2006)
DOI: 10.2134/jeq2005.0404
© 2006 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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TECHNICAL REPORTS

Surface Water Quality

Change in Soluble Phosphorus in Soils following Fertilization is Dependent on Initial Mehlich-3 Phosphorus

C. Ryan Bond*, R. O. Maguire and J. L. Havlin

Department of Soil Science, North Carolina State University, Raleigh, NC 27695

* Corresponding author (crbond{at}ncsu.edu)

Received for publication October 21, 2005.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
There is a lack of information on how fertilization and initial Mehlich-3 phosphorus (M3P) interact to affect water soluble P (WSP) in soils. Our objectives were to (i) quantify the relationship between WSP and M3P for four textural diverse benchmark soils of North Carolina (NC) and (ii) quantify the change in WSP concentrations following P additions to soils over a wide range of initial M3P. Soils known to represent a wide range in M3P were collected from an Autryville loamy sand (loamy, siliceous, subactive, thermic Arenic Paleudults), Wasda muck (fine-loamy, mixed, semiactive, acid, thermic Histic Humaquepts), Georgeville silt loam (fine, kaolinitic, thermic Typic Kanhapludults), and Pacolet sandy clay loam (fine, kaolinitic, thermic Typic Kanhapludults) and analyzed for M3P, Fe, Al, and WSP. An incubation study was also conducted where four samples representing a range in M3P from each series were fertilized at rates of 150 and 300 kg P ha–1, and WSP was measured at 1, 7, and 21 d after fertilization. The Wasda muck exhibited a change point at 115 mg P kg–1 across a broad range of M3P concentrations (60–238 mg kg–1) while Autryville, Georgeville, and Pacolet series (with ranges in M3P of 32–328, 119–524, 0–1034 mg P kg–1, respectively) maintained linear relationships between WSP and M3P. For the fertilized soils, significant increases in WSP occurred regardless of P rate. Yet, WSP concentrations were greater in soils with greater initial M3P. Thus, these data suggest that shifting animal waste applications to fields of relatively lower M3P concentrations would have an immediate impact on reducing risk for P losses, if all other factors are equal.

Abbreviations: CC, container capacity • HM, humic matter • ICP–AES, inductively coupled plasma atomic emission spectroscopy • M3P, M3Al, M3Fe, Mehlich-3 phosphorus, aluminum, and iron • M3PSR, Mehlich-3 phosphorus saturation ratio • OM, organic matter • PLAT, Phosphorus Loss Assessment Tool • SMG, soil management group • WSP, water-soluble phosphorus


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
SURFACE WATER QUALITY can be threatened by elevated P concentrations in runoff from agricultural fields receiving animal wastes (USEPA, 1996; Tarkalson and Mikkelsen, 2004). Continued over-application of fertilizer and animal waste P relative to crop uptake has resulted, in some cases, in M3P concentrations greater than agronomic values identified for optimal crop yield (Sims et al., 2002; Johnson et al., 2005). In response to concerns over P losses from agricultural lands, most states have developed P indices that assess the risk for P loss from a field (Sharpley et al., 2003). These indices use P source and transport factors to identify critical source areas for P loss, and one of the criteria used to identify P sources is soil test P. If a high risk of P loss is identified, then improved P management is required on that field.

The NC P index is called the Phosphorus Loss Assessment Tool (PLAT). As in most P indices, PLAT assesses potential P loss from agricultural fields by various P loss pathways including sediment- and particulate-bound P, source (fertilizer and manure) P, and soluble P (NC PLAT Committee, 2005). Numerous researchers have shown that elevated risk of soluble P loss in runoff may be associated with increases in M3P concentrations (Pote et al., 1996; Eghball and Gilley, 1999). The PLAT uses the M3P soil test as an indicator for soluble P losses and soil dependent environmental M3P thresholds have been established above which soluble P concentrations increase rapidly and thereby increase the potential risk of P loss via runoff. This is based on the concept that soils have a finite capacity to adsorb P and as they become saturated, soluble P rises quickly with increasing soil P (Fox and Kamprath, 1970; McDowell et al., 2001; Maguire and Sims, 2002b). Variations in soil texture and Fe and Al oxide concentrations affect the relationship between M3P and WSP (Shelton and Coleman, 1968; Fox and Kamprath, 1970; Novais and Kamprath, 1978; Dobermann et al., 2002; Maguire and Sims, 2002a). Therefore, suggested soil P threshold concentrations are assumed to vary among soils (Cox and Hendricks, 2000; Maguire and Sims, 2002a). Such thresholds have been established for all NC soils organized into 27 soil management groups (SMGs) according to their respective particle size classes (50, 100, 200, and 500 mg P kg–1 for organic, sandy, loamy, and clayey SMGs, respectively). The M3P threshold concentrations in PLAT indicate potential soluble P levels in runoff equivalent to 1 mg P L–1 (NC PLAT Committee, 2005). These thresholds identify soils of concern, but PLAT uses a linear increase of soluble P in runoff per unit increase in soil M3P concentration for each soil due to lack of information for high M3P soils (NC PLAT Committee, 2005).

Our first objective was to quantify the relationship between WSP and M3P for four benchmark soils of NC over a wide range of M3P. This will help identify if linear or change point relationships are more appropriate for these soils. Where P indices rank fields high for P loss on animal farms, one option to reduce risk is to move manure P applications away from these higher ranked fields to lower ranked ones (Sharpley et al., 1996; Tarkalson, 2001). As these fields will probably have different M3P levels, it is therefore important to understand how P applications to soils varying in initial M3P affect WSP and risk for P losses. Therefore, our second objective was to quantify the change in WSP concentrations affected by P additions to soils with a broad range of initial M3P. This will assist in assessing the potential risk of soluble P loss from agricultural fields to surface waters following fertilizer P and animal waste applications.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Soil Selection and Sample Collection
From three physiographic regions (Piedmont, Upper Coastal Plain, and Lower Coastal Plain) of NC, four benchmark soil series ranging in physical and chemical properties were selected for this study. These soil series are representative of the majority of prime farm land within each region. Soil test data provided by the NC Department of Agriculture and Consumer Services were used to select soils from agricultural fields that ranged in M3P from "low" (<21 mg kg–1) to "very high" (>83 mg kg–1) in reference to current environmental M3P threshold levels in PLAT. Bulk surface soil samples were collected (0–10-cm depth) and composited from agricultural fields used for corn (Zea mays L.), wheat (Triticum spp.), soybean [Glycine max (L.) Merr.], or hay, that is, common bermudagrass [Cynodon dactylon (L.) Pers.] or tall fescue (Festuca spp.), production. The native soil data set for this study consisted of 107 topsoils (17 Autryville loamy sand, 25 Wasda muck, 22 Georgeville silt loam, and 43 Pacolet sandy clay loam soils) obtained from pasture or crop land used to produce a variety of row crops managed under no-till.

Soil Analysis
All field soil samples collected were dried at 65°C for 24 h and ground to pass a 2-mm sieve. Soil pH (1:1 soil/deionized water) and humic matter (HM) content were measured by standard methods of the North Carolina Department of Agriculture (Mehlich, 1984b). Soils were analyzed for (i) WSP (1:10 soil/deionized water, 1-h reaction time, filtration through Whatman no. 2 filter paper); (ii) M3P (1:10 soil/0.2 M CH3COOH + 0.25 M NH4NO3 + 0.015 M NH4F + 0.13 M HNO3 + 0.001 M EDTA, 5-min reaction time, filtration through Whatman no. 2 [Maidstone, UK] filter paper) (Mehlich, 1984a). The Mehlich-3 extract was analyzed for P (M3P), Al (M3Al), and Fe (M3Fe) by inductively coupled plasma atomic emission spectroscopy (ICP–AES). The Mehlich-3 P saturation ratio (M3PSR) was calculated (mmol kg–1) by (Sims et al., 2002):

Formula 1[1]
Water-soluble P extracts were analyzed colormetrically by the molybdate blue method of Murphy and Riley (1962). Our choice in analyzing P using ICP–AES and colorimetric methods was based on research by Sikora et al. (2004) who reported a strong correlation (r2 = 0.98) between ICP–AES soil P and colorimetric soil P concentrations.

Incubation Study
Soil container capacity (CC), which is an approximate measurement of field capacity, was determined for each soil series by saturating a 50 g air-dried soil sample with deionized water, freely draining it for 48 h, and reweighing it to determine water content. CC was calculated by:

Formula 2[2]
where CC (kg water kg–1 soil) represents the ratio of deionized water (Mw) held by a mass of soil (Ms) in the container draining under an atmospheric pressure gradient at room temperature (22°C) (Cassel and Nielsen, 1986).

Four soil samples per soil series were selected to represent a broad range of M3P levels and fertilized at rates equivalent to 0, 150, or 300 kg P ha–1 (assuming 2242 Mg soil ha–1), maintained at 70% CC and incubated in a dark cupboard at room temperature for 3 wk. At 1, 7, and 21 d after fertilizer additions, 2 g for WSP analysis and 2.5 cm3 for Mehlich-3 analysis subsamples were collected from each soil, dried at 35°C for 24 h, and ground to pass a 2-mm sieve. These samples were analyzed for WSP and M3P as described above.

Statistical Analysis
All correlation and regression analyses were conducted by standard procedures of SAS Version 9.1 (SAS Institute, 2002). The split-line linear regression (NLIN) procedure within SAS 9.1 was used to determine change points, as described by McDowell and Sharpley (2001) and Sims et al. (2002).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Soil Characteristics
All soils were moderately acidic and exhibited agronomic P values suitable for optimal crop performance and yield (Table 1) (Crozier et al., 2004; Fox and Kamprath, 1970). The pH of the Wasda muck (5.2) was the lowest of the four soils that ranged from 5.2 to 6.3 and can be explained by its relatively high HM content. Although not a direct measure of organic matter (OM) content, HM is well correlated with soil OM, and as a result these soils have a lower recommended soil pH for agronomic production due to less concern over Al toxicity since Al is complexed by OM (Pierzynski et al., 2000; Evans and Kamprath, 1970). The mean HM content of the Wasda muck (9.38 g 100 cm–3) was greater than the other three soils that ranged from 0.47 to 0.91 g 100 cm–3. In NC, soils are classified for agronomic nutrient and pesticide management purposes by their HM content and identified as mineral (HM < 5.5 g 100 cm–3), mineral-organic (5.5 > HM < 10 g 100 cm–3), or organic soils (HM > 10 g 100 cm–3) (Hardy et al., 2003).


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  		Table 1. Native soil characteristics for Autryville loamy sand, Wasda muck, Georgeville silt loam, and Pacolet sandy clay loam series.

 
For crop production in NC, critical agronomic soil M3P levels have been characterized for numerous crops and vary widely depending on crop and soil type and their management (Crozier et al., 2004). In NC, researchers recognize a critical agronomic M3P level of 53 mg kg–1 (Johnson et al., 2005). In some cases, geographically associated soil series with similar inherent HM contents may exhibit greater agronomic critical M3P levels, for example, Portsmouth fine sandy loam and Wasda muck (40 mg kg–1) than neighboring relatively low HM soil series, for example, Goldsboro fine loamy sand (fine-loamy, ciliceous, subactive, thermic Aquic Paleudults) and Autryville loamy sand (21 mg kg–1) (Crozier et al., 2004). This phenomenon was explained by Fox and Kamprath (1970) who illustrated that mucks of the NC Coastal Plain without appreciable clay contents exhibited no capacity to absorb P relative to NC agricultural mineral soils. Yet, the mean M3P (mg kg–1) levels for the Autryville loamy sand (132), Wasda muck (134), Georgeville silt loam (291), and Pacolet sandy clay loam (394) were greater than the NC recognized critical agronomic M3P threshold. This indicates historically excessive P applications as a sole source of P to these soils contributed to the observed accumulation of M3P (Table 1).

The mean M3Al was greatest in the Wasda muck (1705 mg kg–1) and smallest in the Autryville loamy sand (531 mg kg–1) (Table 1). The significantly (P < 0.05) greater M3Al in the Wasda muck may be due to its significantly (P < 0.05) greater HM content. Organic soils in NC with appreciable mineral matter have large amounts of Al held by organic matter (Mengel and Kamprath, 1978). Maguire and Sims (2002b) reported that OM can increase the amorphous nature and hence extractability of Al. Mean M3Al values (mg kg–1) for both the Georgeville silt loam (799) and Pacolet sandy clay loam (869) were not significantly (P < 0.05) different from one another yet were approximately half that of the Wasda muck. The mean M3Fe content of the Autryville (136 mg kg–1) soil was significantly (P < 0.05) less than the Wasda muck (Table 1). The mean M3Fe (mg kg–1) was greatest in the Wasda muck (197), however it was not significantly (P < 0.05) greater than the Georgeville silt loam (179) or Pacolet sandy clay loam (162). Relatively greater amounts of M3Fe extracted from the Wasda muck may be due to its significantly (P < 0.05) greater HM content leading to greater extractability of M3Fe as discussed above for M3Al, or due to reducing conditions that sometimes occur in these soils due to a high water table.

The M3PSR represents the ratio of labile P to the P sorption capacity of a soil, wherein numerous researchers have indicated its usefulness in serving as a potential indicator of P loss from agricultural fields (Maguire and Sims, 2002b; Nair et al., 2004). Maguire and Sims (2002b) reported a M3PSR value of 0.52 to be correlated to a DPSox [DPSox = 0.5(Al + Fe)] of 100% marking the P saturation capacity of soils based on assumed equivalent sorption capacities of Al and Fe oxides; where degree of P saturation (DPSox) on a molar basis was determined by a oxalate-P, Al, and Fe extraction procedure. Among all four soils of this study, the mean M3PSR values ranged from 0.07 to 0.36, wherein the Wasda muck exhibited the lowest degree of P saturation (0.07) due to its greater M3Al concentrations (1705 mg kg–1) than the other three soils (Table 1). A plausible explanation may lie in the relative greater efficacy by which Mehlich-3 extracts OM-complexed Al or amorphous Al in the Wasda muck vs. M3Al extracted primarily from more strongly sorbed hydrated Al oxides and silicate clays in the mineral soils (Mengel and Kamprath, 1978; Maguire and Sims, 2002b). In Delaware, the Pocomoke fine loamy sand (coarse-loamy, siliceous, active, thermic Typic Umbraquults) with a high OM content (>60 g kg–1) also exhibited a lower mean M3PSR (0.12) due to a greater mean M3Al (1718 mg kg–1) concentration (Maguire and Sims, 2002b). These data also suggest that differences lie in the reactivity of M3Al in soils high in OM content as compared to mineral soils. The mean M3PSR for all other Coastal Plain Delaware soils included in their study ranged from 0.08 to 0.24. The mean M3PSRs for the Autryville loamy sand (0.20), Georgeville silt loam (0.29), and Pacolet sandy clay loam (0.36) in this study were not significantly (P < 0.05) different from one another.

Relationship between Mehlich-3 Phosphorus and Water-Soluble Phosphorus
Across the broad range of M3P concentrations, significant (P < 0.001) positive linear relationships were observed between M3P and WSP concentrations for each soil series (Fig. 1 ). However, the mean rate of increase in WSP per unit of M3P varied by soil series and decreased in the following order for the single slope relationships (mg WSP kg–1/mg M3P kg–1): Autryville loamy sand (0.14) > Georgeville silt loam (0.09) > Pacolet sandy clay loam (0.04) (Fig. 1). Although these three soil series exhibited linear relationships across a broad range of M3P levels, nonlinear soluble P (0.01 M CaCl2 extractable) and M3P (estimated from Mehlich-1 P data) relationships for geographically associated soils with narrower M3P levels have been reported, that is, Cecil sandy clay loam (fine, kaolinitic, thermic Typic Kanhapludults) (about 55–155 mg kg–1) and Norfolk loamy sand (fine-loamy, kaolinitic, thermic Typic Kandiudults) (about 60–245 mg kg–1) (Reddy et al., 1980).


Figure 1
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  		Fig. 1. Water-soluble P as a function of Mehlich-3 P for native Autryville loamy sand, Wasda muck, Georgeville silt loam, and Pacolet sandy clay loam soils.

 
However, a change point was determined for the Wasda muck (115 mg kg–1) relationship between WSP and M3P, where the split-line linear model (NLIN) explained 90% of the overall variance and provided an accurate estimation of the change point. The change point is an estimate of the M3P concentration at which a significant change in the rate at which WSP concentrations increase with each unit increase in M3P (Kleinman et al., 2000; McDowell et al., 2001). The change point for the Wasda muck was well above an agronomic optimum M3P value (about 40 mg kg–1) for a variety of row crops grown in this soil (McCollum, 1991; Cox, 1992; Crozier et al., 2004; NC PLAT Committee, 2005). A 10-fold increase in the rate of increase in WSP per unit increase in M3P below [WSP = 0.02(M3P)] vs. above [WSP = 0.20(M3P)] the estimated change point was observed. Therefore, the risk in soluble P loss from the Wasda soil with M3P concentrations above 115 mg kg–1 was greatly elevated because P adsorption sites were saturated.

Sharpley et al. (1996) reported the regionalization of confined animal operations in many areas has resulted in elevated soil P concentrations above agronomic critical levels for optimal crop yield. In NC, researchers have reported 78% of the state's agricultural land surveyed by county has been fertilized with P to meet or exceed agronomic critical M3P levels (Johnson et al., 2005). Nearly all soils collected for this study were greater than optimal agronomic P thresholds for adequate crop yield (McCollum, 1991; Johnson et al., 2005).

Relationship between Mehlich-3 Phosphorus Saturation Ratio and Water-Soluble Phosphorus
Researchers have measured M3PSR values up to approximately 0.60 and suggested them as useful indicators in accessing the variable risk of soluble P loss (Sims et al., 2002; Maguire and Sims, 2002b). In this study, positive linear relationships of WSP as a function of M3PSR reflected corresponding increases in WSP as M3PSR increased and the soils became more P saturated (Fig. 2 ). Sims et al. (2002) reported change points for leaching and runoff at M3PSR values of 0.20 and 0.14, respectively. The Autryville loamy sand, Georgeville silt loam, and Pacolet sandy clay loam had M3PSR values that ranged well above and below their reported change points, but no change point was observed for these soils. The reason why no change points could be observed across such a wide range of M3PSR, when other researchers have reported them, is unclear. However, the variability of M3Al may be part of the explanation. For example, the Pacolet sandy clay loam had a mean M3Al concentration of 869 mg kg–1, with a standard deviation of 348 mg kg–1. Sims et al. (2002) reported regression coefficients of 0.73 and 0.87 for their change point relationships between runoff or leachate and M3PSR, but our regression coefficients were only 0.60 to 0.67 for the three linear relationships (Fig. 2). The Wasda muck had no M3PSR values above 0.20, but the relationship between WSP and M3PSR was best explained by an exponential regression. This indicates greater increases in WSP per unit increase in M3PSR at higher M3PSR values, however the NLIN split-line model did not identify or converge on a change point.


Figure 2
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  		Fig. 2. Water-soluble P as a function of Mehlich-3 P saturation ratio for native Autryville loamy sand, Wasda muck, Georgeville silt loam, and Pacolet sandy clay loam soils.

 
Influence of Time on Water-Soluble Phosphorus following Fertilization
Similar WSP trends following fertilization were exhibited for all soils, with WSP decreasing over time (data not shown). Between fertilization of the soils and 21 d, our data show that a majority (about 67%) of the initial WSP at 1 d for both fertilizer P rates (150 or 300 kg ha–1) was sorbed by the second sampling event at Day 7 (data not shown). By 21 d the regression lines were flattening out, indicating that there was little more P sorption occurring. Researchers have illustrated soils vary in their P sorption capacities as well as their P sorption rates (Fox and Kamprath, 1970; Maguire et al., 2001a). Fertilizer P is rapidly sorbed by Al and Fe oxides (Dobermann et al., 2002; Laboski and Lamb, 2003). In an incubation study of acid soils of Ireland, 55% of soluble P was sorbed by soils within 15 d (Maguire et al., 2001a). Fox and Kamprath (1970) reported that approximately 67% of soluble P applied at 500 mg P kg–1 soil was sorbed within 8 d following application.

Water-Soluble Phosphorus following Fertilization of Soils with a Wide Range of Initial Mehlich-3 Phosphorus
To isolate the effect of fertilization on WSP from the initial WSP concentration present in the soils, WSP of the unfertilized soil was subtracted from WSP of the fertilized soil to show the change that occurred. Following fertilization, WSP increased in all soils irrespective of fertilization rate or soil type (Fig. 3 and 4 ). As would be expected, WSP increased more in the soils fertilized with 300 kg P ha–1 than in equivalent soils fertilized with 150 kg P ha–1. After 1 d, there was a clear trend for WSP to increase more in soils that had greater initial M3P. For example, when the Wasda muck soil samples were fertilized with 150 kg P ha–1, the WSP increased by 3 mg kg–1 in the soil with initial M3P of 62 mg kg–1, while WSP increased by 16 mg kg–1 in the soil with initial M3P of 238 mg kg–1 (Fig. 3 and 4). These results agree with the concept of a change point, where soluble P increases more rapidly as soils become more saturated with P, even though no change points were seen in Fig. 1 and 2 for three of the soil series. Again, change points in Fig. 1 and 2 may have been obscured by scatter in the data. This agrees with Pote et al. (2003) who found that increases in soil WSP following fertilization with poultry litter or inorganic P were significantly correlated to initial soil WSP, with higher initial WSP leading to greater increases in WSP following fertilization. Fox and Kamprath (1970) also reported that increases in soluble P were directly related to initial soil test P, with greater increases in soluble P where initial soil test P was higher. Fertilizer additions also increased M3P in these soils (data not shown), but there was too much variability in the data resulting in inconsistent trends relating to the effects of fertilizer rate or initial M3P on M3P.


Figure 3
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  		Fig. 3. Change in water-soluble P (WSP) as a function of initial soil M3P (relative to an unfertilized sample), 1 d after fertilization with 150 or 300 kg P ha–1 for the (a) Autryville loamy sand, (b) Wasda muck, (c) Georgeville silt loam, and (d) Pacolet sandy clay loam.

 

Figure 4
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  		Fig. 4. Change in WSP as a function of initial soil Mehlich-3 P (M3P) (relative to an unfertilized sample), 21 d after fertilization with 150 or 300 kg P ha–1 for the (a) Autryville loamy sand, (b) Wasda muck, (c) Georgeville silt loam, and (d) Pacolet sandy clay loam soils.

 
Although the trends in changes in WSP were the same for all soils, the scale of the changes was soil dependent. For example, following fertilization with 300 kg P ha–1, WSP increased on average by 20, 27, 31, and 46 mg kg–1 for the Wasda muck, Pacolet sandy clay loam, Georgeville silt loam, and Autryville loamy sand, respectively. The M3Fe was generally similar among these soils, but the M3Al in these soils followed the same order as these increases in WSP, being 1705, 869, 799, and 531 mg kg–1 for the Wasda muck, Pacolet sandy clay loam, Georgeville silt loam, and Autryville loamy sand respectively (Table 1). Previous research has shown the links between soil Al and P sorption (Sims et al., 2002). No link between change in WSP and M3Fe could be seen, probably as Mehlich-3 is a poor extractant for reactive Fe (Novais and Kamprath, 1978; Maguire and Sims, 2002b). The increases in WSP following fertilization decreased with time from 1 to 21 d (Fig. 3 and 4). For example, for the Pacolet sandy clay loam, the increase in WSP following fertilization decreased from 27 to 15 mg kg–1 on average. Similar decreases with time have been observed over 51 d, with most of the decrease occurring in the first week (Maguire et al., 2001b). Gaston et al. (2003) also reported mean temporal (year by year) reductions in WSP levels ranging from 8 to 70% for Coastal Plain Louisiana soils amended with poultry litter dependent on their application history (1–20 yr).

Comparing the 150 and 300 kg P ha–1 applications, the 300 kg P ha–1 rate led to more than twice the increase in WSP as the 150 kg P ha–1 rate throughout the 21 d of the incubation. The slope of the regression line between change in WSP following fertilization with 300 kg P ha–1 and 150 kg P ha–1 was 2.49 after 1 d, decreased to 2.27 after 7 d and remained relatively constant to 21 d (2.26). Adding twice as much P more than doubles the increase in WSP and illustrates an increased risk in potential soluble P loss from soils receiving excessive rates of fertilizer P.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
The buildup of P in soils in areas of intensive animal production has received considerable attention due to concerns over P losses to surface waters. Despite an agronomic optimum M3P of approximately 50 mg P kg (depending on the soil and crop), we measured M3P values of up to 961 mg kg–1. Obviously, historically excessive applications of fertilizer and manure P have resulted in elevated M3P concentrations above critical agronomic P limits for optimum crop yield in many NC agricultural soils. In NC, PLAT was developed to assess risk of P loss from agricultural fields and improve P management where losses were of concern. However, PLAT does not recognize a change point in the relationship between soluble P in runoff and M3P and this relationship is based on linear extrapolations of datasets that were missing high M3P values. Our data supports the use of these linear extrapolations, as a change point was only found for one out of four soil series. Despite the lack of change points, when these soils were fertilized the WSP increased to a greater extent in soils with a higher M3P. Previous research has shown that runoff P losses are closely correlated to WSP. Therefore, these results show the importance of avoiding P applications to soils that already have elevated M3P, as this will raise the risk of P loss to a greater extent than applying P to a soil with lower M3P.


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




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