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Soil Science Society of America Journal 67:645-653 (2003)
© 2003 Soil Science Society of America

DIVISION S-8—NUTRIENT MANAGEMENT & SOIL & PLANT ANALYSIS

Changes in Soil Phosphorus from Manure Application

T. S. Griffin*, C. W. Honeycutt and Z. He

USDA-ARS, New England Plant Soil and Water Laboratory, University of Maine, Orono, ME 04469-5753

* Corresponding author (tgriffin{at}maine.edu)


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Manure and mineral fertilizer P sources vary in their contributions to soil P pools. We conducted incubation experiments to (i) assess temporal changes in soil P concentration for 84 d following application of KH2PO4 or manure (beef, dairy, poultry, or swine), and (ii) to evaluate interactive effects of P rate, P source, and background P level on soil P pools 90 d after P application. Changes in soil P over time were evaluated following the amendment of a coarse-loamy, mixed, frigid Typic Haplorthod soil (pH 5.8) at a rate of 100 mg total P kg-1. Water soluble P (WSP), 0.01 M CaCl2–extractable P (CaCl2–P), and modified Morgan P (MMP) declined to <3 mg P kg-1 soil within 21 d of application, following an exponential decay function; P extracted by anion-exchange membrane (AEMP) and Mehlich-III (M3-P) also declined rapidly and are attributed to the high level of exchangeable Al and Fe in this soil. Ninety days after application, all soil P pools exhibited linear increases in concentration for application rates up to 800 mg P kg-1 soil, regardless of P source. Phosphorus applied as KH2PO4 was more efficient at increasing CaCl2–P and M-3P. Manure P sources generally had a greater effect on MMP, and poultry manure was more efficient than all other sources. Efficiency of P sources at increasing soil P concentration (b, slope of linear regression) varied from <1% for rapidly available P pools (CaCl2–P) to nearly 50% for more recalcitrant P (M3-P). Efficiency also increased as background M3-P increased from 150 to 750 mg kg-1 soil. The amount of CaCl2–extractable P increased rapidly when soil P saturation ([Mehlich III-P]/[Mehlich-III Al + Fe]) exceeded 0.25 mol mol-1. Manure and KH2PO4 contribute to different soil P pools, and these differences are magnified at high application rates and high background soil P levels.

Abbreviations: AEMP, anion-exchange membrane-extractable P • CaCl2–P, CaCl2–extractable P • DSSPox, degree of soil saturation with P using pH 3 ammonium oxalate extraction • DSSPM3, degree of soil saturation with P using Mehlich-III extraction • ICP, inductively coupled plasma emission spectroscopy • M3, Mehlich-III extraction • M3-P, Mehlich-III extractable P • MMP, modified Morgan extractable P • STP, soil test P • WSP, water soluble P


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
SUSTAINABLE AGRICULTURAL SYSTEMS depend on maintaining adequate amounts of plant nutrients, including P, without unduly increasing either environmental nutrient load or loss. The availability of both applied and indigenous soil P is influenced by a number of soil characteristics. A strong inverse relationship between soluble P concentration and extractable Al and Fe in acid soils, indicating adsorption of P on Al- and Fe-oxides, was demonstrated by Sharpley (1983) and Agbenin and Tiessen (1995). Soil P may also form phosphate precipitates with soil Ca, Al, or Fe (Sharpley et al., 1984; Barber, 1995). The availability of soil P is further influenced by soil texture, primarily because of differences in clay content and soil organic C (Sharpley, 1983; Sharpley and Sisak, 1997). Initial soil P levels also influences the availability of added P, as shown by Indiati et al. (1995) and Indiati and Sharpley (1997).

All of these factors interact to establish equilibrium between soil P pools that vary in plant availability. Mattingly (1975) defined P pools across soil types as (i) soluble P in solution, (ii) labile P in the solid phase that is easily exchangeable from the mineral surface, and (iii) nonlabile P that is slowly exchangeable or nonexchangeable from the mineral surface. Because these P pools are differentially available and are in equilibrium with each other, added P is not completely available for plant uptake. The efficiency of added mineral fertilizer P for increasing soil test P (STP) levels is typically <20%. Furthermore, the availability of manure-P and its impact on soil P pools is clearly different from mineral fertilizer P. Lucero et al. (1995) found that 3 to 4.5 kg P ha-1 from poultry manure was required to increase M3-P by 1 mg kg-1, compared with 16.5 kg P ha-1 to achieve the same result with mineral fertilizer. Reddy et al. (1999) showed very similar results, with 5.6 kg P ha-1 from poultry manure and 17.9 kg P ha-1 from mineral fertilizer needed to increase Olsen-extractable P by 1 mg kg-1. Conversely, Sharpley and Sisak (1997) found the availability of P from poultry manure leachate was somewhat lower than from KH2PO4, using Fe-oxide impregnated paper extraction. These results indicate that manure P availability is variable and not well understood. The objectives of this research were to (i) compare the effects of KH2PO4 and animal manure P application on soil P concentrations over time and (ii) to determine the effects of mineral fertilizer and manure P application rate and indigenous soil P level on soil P concentration.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 REFERENCES
 
Soil and Manure Materials
Soil used for these laboratory experiments was collected from the USDA-ARS research site in Newport, ME. The sandy loam soil (unnamed series; coarse-loamy, mixed, frigid Typic Haplorthod) had a particle-size distribution of 61% sand, 29% silt, and 10% clay, as measured by the pipette method (Gee and Bauder, 1986). Soil was collected from the Ap horizon (0–20 cm) of a field previously cropped to potato (Solanum tuberosum L.), sieved (2 mm), air-dried, and stored at 4°C until experiments were initiated (approximately 120 d). Selected soil properties include: soil pH = 5.8 (1:1, soil/water); cation-exchange capacity (CEC) = 3.4 cmol kg-1; P = 16.5 kg ha-1; K = 303 kg ha-1; Mg = 169 kg ha-1; and Ca = 1130 kg ha-1, as determined using modified Morgan extraction (McIntosh, 1969; 2 g of dry soil in 10 mL of pH 4.8, 0.62 M NH4OH + 1.25 M CH3COOH, shaken for 15 min.) and inductively coupled plasma emission spectroscopy (ICP). Effective CEC was estimated by summing cation concentrations and exchangeable acidity in the modified Morgan extraction, without altering soil pH, as discussed by Sumner and Miller (1996). Mehlich-III (Mehlich, 1984; 1.5 g soil in 15 mL of 0.2 M CH3COOH + 0.25 M NH4NO3 + 0.015 M NH4F + 0.013 M HNO3 + 0.001 M EDTA, shaken for 5 min.) extractable Fe and Al were 87 and 1435 mg kg-1, respectively.

Beef, dairy, swine, and poultry manure were collected directly from storage structures on local commercial farms and are the same as the manures used by Griffin and Honeycutt (2000), except that they were dried and ground (1 mm) for the experiments described here. The beef and swine manures contained sawdust-bedding material, and the dairy manure included sand bedding. There was no bedding material in the poultry manure. Manure analyses at the beginning of incubation (t = 0) are shown in Table 1. Total C was determined by thermal conductivity detection following combustion at 1650°C, on a CE Instruments NA2500 Elemental Analyzer (ThermaQuest Italia S.p.A., Rodano, Italy1). Total N was measured by total Kjeldahl digestion. Manure total P and K content were measured by dry combustion, followed by digestion in 0.5 M H2SO4 and ICP. Water-soluble P was determined by shaking 1.0 g of soil in 25 mL of H2O for 1 h. Sample was centrifuged (2200 x g for 10 min), and filtered through a 53-µm filter. Water soluble inorganic P was assayed by a molybdate blue method as modified by Dick and Tabatabai (1977). Total WSP was determined in the same way following H2SO4–H2O2 digestion and adjustment to pH 5. Water-soluble organic P was the difference between total soluble P and soluble inorganic P.


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  		Table 1. Chemical composition of beef, dairy, poultry, and swine manures used in incubation experiments.

 
Experiment I: Changes in Soil Phosphorus over Time
Ten grams of air-dry soil was weighed into 20-mL scintillation vials. Four replications of the following treatments were prepared: beef, dairy, poultry, and swine manures, and KH2PO4, all applied at 100 mg total P kg-1 dry soil, and an unamended control. Sufficient samples were prepared for each treatment to allow for destructive sampling at 0, 7, 14, 28, 56, and 84 d after amendment. Six P sources, six sample dates, and four replications resulted in a total of 144 experimental units. Dry soil and manure were thoroughly mixed, while KH2PO4 was dissolved in water and added to soil at the same rate as the manures, using 3.0 mL vial-1. Soil water was adjusted to 0.30 kg kg-1 (approximately field capacity), and capped vials were incubated in the dark at 24°C; soil water was not adjusted during the incubation period. An additional set of 24 amended vials was used to evaluate the impact of wetting and drying cycles, compared with constant moisture content. Every 15 d, these samples were dried at 40°C for 24 h in a forced-air oven, and then rewet. Extraction took place only at 84 d.

At each sampling time, subsamples were extracted in water (1 g soil in 25 mL, shaken for 1 h), 0.01 M CaCl2 (1 g soil in 10 mL, shaken for 1 h) and by anion-exchange membrane (AEM; Type AR204, Ionics Inc., Watertown, MA). The membrane had been cut into strips (1.2 by 2.5 cm) and gently shaken in four volumes (1 L) of 0.5 M NaHCO3 solution, to saturate with HCO-3. A single strip was placed in a 50-mL centrifuge tube with 1 g of soil and 25 mL of deionized water and shaken for 16 h. The strip was removed, adhering soil particles were rinsed off, and the strip was eluted for 4 h in 25 mL of 0.5 M HCl. In addition, two soil test extractions were used, M3-P and MMP. Samples of water, 0.01 M CaCl2, M3-P, and MMP extractions were centrifuged (2200 x g for 10 min.); the resulting supernatant was clear and samples were not filtered, except by pouring through 53-µm screen to remove floating particulate material. Total P concentration in all extraction solutions was measured by ICP. Concentration of Al and Fe in M3 extractions was also measured by ICP. Extracted P available from manure or KH2PO4 at each sampling time was corrected for the unamended soil as:

[1]

The efficiency of an added P source in altering a soil P pool was determined as:

[2]
where Padded = 100 mg P kg-1 soil.

Analysis of variance (ANOVA) was used to identify significant effects of time, P source, and their interaction on extractable soil P. Changes in soil P over time were evaluated using regression analysis. Both linear and nonlinear models were evaluated, in the following forms, respectively:

[3]

[4]

Equations were calculated using nonlinear curve fitting via Marquardt iterations (SYSTAT, Version 10; SPSS Corp., Chicago, IL). Linear and nonlinear regressions were compared based on reductions in root mean square error (RMSE) of the regression. Regression equations from different treatments were deemed significantly different if the 95% confidence intervals around the parameters (the rate constant, b, for example) did not overlap, or if the best-fit form was different (i.e., linear vs. nonlinear). Otherwise, regressions were recalculated using all manure treatments. Where regression did not adequately describe changes in soil P pools over time, fertility sources were compared at each sampling date using Fisher's Protected least significant difference (LSD), which was not calculated unless analysis of variance indicated significant treatment effects at P = 0.05.

Experiment II: Soil Phosphorus Level and Phosphorus Application Rate
To evaluate the effects of both indigenous soil P level and P application rate from different sources, soils were augmented with KH2PO4 to achieve varying levels of STP. Phosphorus was added to 1-kg portions of dry soil, assuming 8 mg P kg-1 soil was required to increase STP by 1 mg P kg-1 soil (Barber, 1995; Lucero et al., 1995). Dry soil was placed in a thin layer and misted with KH2PO4 solution. The amount of water applied was calculated to bring soil to 80% field capacity, and contained a preweighed amount of KH2PO4 to increase soil P to the desired level. Soil, kept in unsealed 4-L plastic freezer bags at 17°C, was allowed to dry on a greenhouse bench every 20 d and was then rewet to 80% field capacity. This wet-dry cycle was repeated five times over a 100-d period, and the soil was then stored dry for an additional 150 d before use. The resulting three soils had M3-P concentrations of 150, 471, and 732 mg P kg-1 dry soil, and are hereafter referred to as Low, Medium, and High. The P status of these soils was further characterized by calculating the degree of soil saturation with P (DSSPox), as estimated by Hooda et al. (2000). A 0.50-g subsample of soil was shaken in 30 mL of 0.2 M ammonium oxalate (pH 3.0) in the dark for 2 h, centrifuged (2200 x g), and P, Al, and Fe concentration were determined by ICP. The DSSPox was calculated as

[5]
where elemental concentrations are in mole per kilogram (mol kg-1) of soil. Phosphorus sorption and desorption of each soil was evaluated as described by Nair et al. (1998). Subsamples of soil (2 g) were equilibrated in 20 mL 0.01 M CaCl2 (0, 0.01, 0.10, 1, 5, 10, 25, 50, and 100 mg P L-1) for 24 h, followed by centrifugation and determination of P concentration by ICP. The amount of P sorbed was calculated as

[6]

The experiment was designed using a complete factorial arrangement, examining P source (five sources, plus control), P application rate (5), and soil P level (3). Air-dried soil (10 g) was placed into scintillation vials. For each soil P level, dried manure (beef, dairy, poultry, and swine as above) was added to quadruplicate vials at rates of 0, 100, 200, 400, and 800 mg total P kg-1 dry soil, and the soil and manure were mixed thoroughly. Mineral fertilizer P (as KH2PO4) was dissolved in water and added to soil at rates equivalent to those applied in manure, using 3.0 mL vial-1. Water, sufficient to increase soil moisture to field capacity, was added to each vial. Vials were capped and incubated at 24°C, without drying. After 90 d, soil was air dried, and the following extractions were performed: 0.01 M CaCl2, AEMP, M3-P, and MMP, all as described in Exp. I. Total P concentration in all extractions was measured by ICP; Al and Fe concentration in M3 were also determined by ICP. Response of soil P pools to P application rate was evaluated by linear regression, in the form

[7]

Thus, the coefficient b is an estimate of the efficiency of added P in changing a given soil P pool. Comparison of regressions between treatments was accomplished by conducting ANOVA on the regression parameters a (y-intercept) and b (slope or efficiency). Aluminum and Fe extractable by M3 were also used to calculate the degree of P saturation (DSSPM3), similar to Khiari et al. (2000) but using molar concentrations as in Eq. [4]:

[8]

The relationship between DSSPM3 and CaCl2–P was evaluated using the statistical procedure followed by McDowell and Sharpley (2001), for their analysis of CaCl2–P versus STP. The analysis identifies a value for the independent variable beyond which CaCl2–P increases more rapidly. This value is referred to as the "change point." The estimation of the change point was accomplished using SYSTAT Version 10, by iteratively solving for the parameters a (intercept), b1 (slope of relationship when DSSPM3 is less than change point), b2 (slope when DSSPM3 is greater than the change point), and Change Point in the following equations: for DSSPM3 < Change Point:

[9]
for DSSPM3 > Change Point:

[10]


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
REFERENCES
 
Experiment I: Changes in Soil Phosphorus over Time
Analysis of variance indicated that all extractable soil P pools changed over time, and most exhibited significant differences because of P source, time by P source interaction, or both (Table 2). All P fractions declined rapidly after manure or KH2PO4 was added to the soil. Water-soluble P and CaCl2–P, two P fractions that are essentially immediately available to plants, exhibited similar changes over time (Fig. 1A and 1B , respectively). The reduction in the P concentration of both fractions could be described by a three-parameter exponential decay function (as in Eq. [4]), with P concentration stabilizing within 14 to 21 d of amendment. This very rapid decline in soluble P, coupled with the fact that both fractions stabilized at <3 mg P kg-1 soil (<3% of applied P), are indicative of rapid sorption by soil Al and Fe. Changes in these two soluble P pools over time also indicate that effects of KH2PO4 and manure were described by different regression equations. The primary difference between KH2PO4 and manure P (regardless of which manure was added) being that KH2PO4 had a higher initial solubility in both extractants. This was expected because this mineral P fertilizer source is completely soluble in water, while the average WSP concentration in the manures was between 18.6 (dairy) and 33.3% (swine; Table 1) of total P. The changes in MMP over time (Fig. 1C) were very similar to WSP and CaCl2–P, with a rapid exponential decline to levels as low as 2.0 mg kg-1 soil. These results demonstrate that all P sources were <5% efficient in altering these soluble P pools. These are much lower than previous estimates of 5 to 30% (e.g., Lucero et al., 1995 for M3-P; Reddy et al., 1999 for Olsen-P). However, as pointed out above, the soil used here has a high level of extractable Al (e.g., M3-Al was 1435 mg kg-1 soil, nearly ten-fold greater than M3-P concentration of 150 mg kg-1 soil), indicating the P sorption capacity is high.


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  		Table 2. Summary of Analysis of Variance results for Exp. 1, evaluating mineral fertilizer (KH2PO4) and manure-induced changes in soil P extractable in water (WSP), 0.01 M CaCl2, anion exchange membrane (AEMP), modified Morgan (MMP), and Mehlich-III (M3-P).***

 


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  		Fig. 1. Changes in (A) water-soluble P (WSP), (B) CaCl2–P, and (C) modified Morgan-P (MMP) over time after amendment with KH2PO4 or manure (beef, dairy, poultry, or swine) applied at 100 mg total P kg-1 soil. Data are corrected for extractable P in unamended soil, and data points are means of four observations; t is time (in days) after application of P sources.

 
The remaining two P fractions, AEMP and M3-P, also declined rapidly after addition to soil (Fig. 2A and 2B , respectively), with very little change after 14 d. Agbenin and Tiessen (1995) found that the amount of AEMP, from an initial application of 400 mg fertilizer P kg-1, declined steadily over 40 to 50 d period. Van der Zee and van Riemsdijk (1988) noted much more rapid sorption, finding that the amount of P sorbed in 40 h could be used to predict long-term sorption with a multiplicative factor of 1.8, that is, more than 50% of the long-term P sorption occurred within 2 d of application. Although the decline in AEMP and M3-P concentration were very similar in appearance to that in Fig. 1, these data could not be modeled with Eq. [4]. Additionally, these two extraction methods were able to distinguish between P sources on most sampling dates, with fertilizer P resulting in higher P concentration than manure. Poultry manure increased M3-P more than dairy manure at most sampling dates, while other manure P sources were similar. The effectiveness of KH2PO4 or manure P in increasing these P pools was greater than for those extractants described above. The efficiency of added KH2PO4 was 31 to 39% (for AEMP and M3-P, respectively), while manure P efficiency ranged from near zero (dairy manure) to 33% (poultry manure, M3-P), averaging 12.8% for AEMP and 14.7% for M3-P. This is similar to the results of Cooperband and Logan (1994), who showed that anion-exchange resin-extractable P fell 75% between Day 2 and Day 8 in a short-term incubation of fertilizer P added to soil. Conversely, Agbenin and Tiessen (1995) found that 30 to 50% of applied P was still AEM-extractable after 200 d, across different soil types.



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  		Fig. 2. Changes in (A) anion-exchange membrane extractable P (AEMP) and (B) Mehlich-3 P over time after amendment with fertilizer or manure (beef, dairy, poultry, or swine) applied at 100 mg total P kg-1 soil. Data are corrected for extractable P in unamended soil, and data points are means of four observations; t is time (in days) after application of P sources. Numbers provided at each sampling are least significant difference (LSD) at P = 0.05 level.

 
Periodic drying and rewetting of P-amended soils through the 84-d incubation had some impact on certain extractable P pools, as shown in Table 3. Water-soluble P increased in the control soil but not in any of the P-amended soils. In contrast, wet-dry cycles increased CaCl2–P two- to threefold for all P sources in the incubation, and also in the unamended control soil, although all soils still contained <1 mg CaCl2–P kg-1 dry soil. Presumably, this represents a flush of microbial P resulting from periodic drying, as shown by Grierson et al. (1998). Modified Morgan P and AEMP from both KH2PO4 and poultry manure were reduced by wet-dry cycles, as was M3-P from poultry manure (Table 3). Pote et al. (1999) evaluated the effects of wet-dry cycles and also reported that they increased soluble P pools without greatly affecting M3-P, while Walbridge and Vitousek (1987) noted that wetting-drying-mixing reduced acid-fluoride extractable P in short-term incubations.


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  		Table 3. Effects of constant versus wet–dry cycle soil water status on soil P extractable in water (WSP), 0.01 M CaCl2, anion exchange membrane (AEMP), modified Morgan (MMP), and Mehlich-III (M3-P), 84 d after addition of KH2PO4 or manure.

 
Experiment II: Soil Phosphorus Level and Phosphorus Application Rate
A second experiment was conducted to further investigate the efficiency of applied P in altering soil P pools. The primary objective was to simultaneously evaluate the effects of three factors on efficiency: P source, P application rate, and baseline soil P level.

The three augmented soils with Low, Medium, and High levels of soil P clearly differed in P saturation. The DSSPox (Eq. [5]) of these soils were 0.126, 0.216, and 0.310 mol mol-1 for Low, Medium, and High levels, respectively, using the calculation of Hooda et al. (2000). They also showed distinct differences in their ability to adsorb P (Fig. 3) , which is a function of P saturation. As shown in Fig. 3, the soils with Low and Medium P levels had the capacity to adsorb P as solution P concentration increased, with the Medium soil having somewhat less capacity to do so. The soil with the High P level, on the other hand, exhibited a net desorption at all solution P concentrations evaluated (0–100 mg P L-1). Beauchemin and Simard (1999) reviewed the use of DSSP (and other parameters) to identify "critical thresholds," at which P sorption capacity would be compromised. The difference we found between the Medium and High soils is informative in this regard, as Beauchemin and Simard (1999) pointed out that soils with DSSP >25% have increased levels of P in solution.



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  		Fig. 3. Phosphorus sorption-desorption characteristics of unamended Low, Medium, and High soil-P level pretreatments, equilibrated for 24 h in 0.01 M CaCl2.

 
All P fractions were affected by P application rate, P source, and soil P level, and interactions were common (Table 4). In all cases, extractable soil P was a linear function of P application rate. Within each soil P level, there were no significant differences in the parameter a (intercept); this was expected since all regression equations were calculated using the same unamended control values. As expected, the linear regression intercept, a, increased as soil P level increased for all extractions.


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  		Table 4. Efficiency (b) of fertilizer and manure P source in altering soil P pools extractable in 0.01 M CaCl3, anion exchange membrane (AEMP), modified Morgan (MMP), and Mehlich-III (M3-P), resulting from linear regression analyses.*

 
The results of CaCl2 extraction across these factors are presented in Fig. 4 , to illustrate the interactive effects of the treatment factors. Fertilizer P increased this P fraction more than manure P. The b values for KH2PO4 ranged from 0.015 to 0.105 (i.e., efficiency of 1.5 to 10.5%; see also Table 4) under Low and High soil P, respectively. The manure P sources generally had slopes between 0.00 and 0.02, although the dairy manure had a negative slope at Medium and High soil P level. This particular dairy manure originated at a farm that uses sand as bedding material. The sand may have contributed enough soluble Al, Fe, or both, to adsorb manure-derived P and indigenous soil P, a theory supported by soil Fe analysis. Mehlich-III extractable Fe was not affected by application rate when fertilizer, beef, poultry, or swine manures were applied (data not shown). Increasing dairy manure application rate, however, led to a linear increase in extractable Fe, generally by 100 mg Fe kg dry soil-1 as application rate increased from 0 to 800 mg P kg-1 dry soil.



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  		Fig. 4. Effect of P application rate, P source, and background soil P level on CaCal2–extractable P.

 
Differences in solubility of fertilizer versus manure P, via direct extraction in water or a dilute salt solution like 0.01 M CaCl2, were demonstrated by Dou et al. (2000). They reported that 53 and 64% of P in poultry and dairy manure, respectively, was soluble by repeated water extraction. The manures used in our research contained on the average only about 25% WSP, as a proportion of total P (Table 1). Our results indicate that these differences in solubility are evident even after a 90-d incubation in soil, with significantly more of the P from KH2PO4 remaining in this soluble P fraction.

An additional trend, best illustrated using KH2PO4, is also shown in Fig. 4; as soil P level increases, the slope (b) also increases, presumably because the capacity of the soil to adsorb added P is reduced with higher initial soil P level. The amount of fertilizer P needed to increase CaCl2–P by 1 mg kg-1 dry soil was 66, 16, and 9.5 mg kg-1 soil, for soil with Low, Medium, and High soil P level, respectively.

The effects of treatment factors for all extractants are summarized in Table 4. Comparison of b values was accomplished by ANOVA on estimates of efficiency. Modified Morgan P follows a trend similar to CaCl2–P, in two regards; (i) for all P sources, b increases with soil P level, generally doubling or tripling from Low to High soil P level (data not shown); and (ii) there is a clear delineation between P sources. There are, however, important differences between these two extractants. Specifically, within each soil P level, poultry manure (not KH2PO4, as with CaCl2 extraction) is more efficient at increasing MMP than any other source, with b ranging from 0.133 (Low) to 0.277 (High; Table 4). The other manures (beef, dairy, and swine) tended to be similar to each other, and generally had greater b values than fertilizer P, as also reported by Lucero et al. (1995), Reddy et al. (1999), and others.

Mehlich-III P was also capable of distinguishing between P sources. At each soil P level, there were no significant differences in efficiency between fertilizer P and poultry and swine manure, ranging from 0.273 to 0.487. These results are considerably higher than the findings of Barrow (1974), Barber (1995), Lucero et al. (1995), and Reddy et al. (1999). The two ruminant manures used here, beef and dairy, were similar within each soil P level and were less efficient at increasing M3-P than the other P sources. The contrasting results of the MMP and M3-P extractions indicates that, although both of these soil tests are useful for predicting crop response to P, they are extracting different P pools in the soil. Clearly, the F-based Mehlich-III extractant is capable of cleaving Al-bound P, while the modified Morgan extractant is not. Our results suggest, however, that different sources of P contribute to different pools of soil P.

Anion-exchange resins and AEMs (as used here) have been widely used to estimate plant availability of applied and soil P (Cooperband and Logan, 1994; Cooperband et al., 1999; Sibbeson, 1977), with extractable P being well correlated with plant P uptake (Schoenau and Huang, 1991; Tran and N'dayegamiye, 1995). Our experience, including that described earlier in this paper, has found AEM to be an easily standardized method for assessing P availability. In this second experiment, however, both soil P and applied P levels were outside the ranges normally encountered. At the Low soil P level, we found the primary effect to be a difference between KH2PO4 and manure P sources; fertilizer P was more effective at increasing AEMP levels (b = 0.359), compared with manures, which had a mean slope of 0.186. As soil P level increased, however, two effects were noted: (i) b values decreased, and were generally not different from zero in soil with High soil P; and (ii) the relationship between applied P and extractable P consistently declined, as indicated by R2 values near zero in the High P soil. We do not believe that P levels alone were high enough to saturate exchange sites on the resin, as Cooperband and Logan (1994) calculated that strips of this size could theoretically adsorb nearly 70 mg P. Rather, there was likely considerable interference from NO-3, which can reduce P adsorption by up to 75%, as shown by Cooperband and Logan (1994). The amount of organic N applied in the manures was generally two- to 2.5-fold greater than the amount of P applied. If 25 to 50% of this N were mineralized during the incubation period, within the range observed by Klausner et al. (1994) and Cabrera et al. (1994) for beef and poultry manure, respectively, mineral N concentration would be greater than P concentration.

Estimating Soil Phosphorus Saturation using Mehlich-3 Extraction
The calculation of soil P saturation has been suggested as a way to identify critical soil P levels above which soluble P levels increase substantially (van der Zee and van Riemsdijk, 1988). A number of different extractants have been used to estimate P saturation, usually using not only extractable P concentration but also extractable Al, Fe, or both, because of the important role they play in immobilizing P under acid conditions. A number of methods were reviewed by Beauchemin and Simard (1999), including ammonium oxalate and M3 extractions. The calculations based on M3 have the advantage of using a common soil test extraction, and Khiari et al. (2000) used the ratio of [P]M3 to [Al]M3 concentration to estimate P saturation, as we showed in Eq. [8]. We combined the data from Exp. I and II to assess the relationships between CaCl2–P and P saturation using molar concentrations from M3, in the form [P]M3/[Al + Fe]M3.

McDowell and Sharpley (2001) successfully used M3-P concentration to identify what they termed change points in the relationship between soil P level and CaCl2–P. The rate of increase in CaCl2–P per unit of M3-P was greater above the change point than below the change point. Fig. 5 demonstrates that the change point concept can also be used to identify critical P saturation levels, based on the M3. The change point [P]M3/[Al + Fe]M3 is 0.207 mol mol-1. Below this level, CaCl2–P concentration is very low (approximately 1.5 mg P kg-1 soil) and there is no relationship between P saturation and CaCl2–P; that is, the slope is zero. This suggests that our earlier interpretation (Exp. I), that applied P is rapidly sorbed by Al in this soil, is correct. Above the change points, the slopes increase to 1.42, indicating that above this level, applied P remains in the more soluble CaCl2–P pool, rather than being sorbed by either Al or Fe in the soil.



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  		Fig. 5. Relationship between degree of soil saturation with P from Mehlich-III extraction (DSSPM3) and CaCl2–P, using combined data from two incubation experiments.

 

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ABSTRACT
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 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Partial Funding from USDA-SARE, Project ANE94-20.

1 Mention of trade names or commercial products in this article is solely for the purpose of providing specific information, and does not imply recommendation or endorsement by the USDA. Back

Received for publication March 11, 2002.


   REFERENCES
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 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 




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