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Soil Science Society of America Journal 66:1696-1705 (2002)
© 2002 Soil Science Society of America

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

Agronomic and Environmental Soil Phosphorus Testing in Soils Receiving Liquid Swine Manure

A. M. Atiab and A. P. Mallarino*,a

a Dep. of Agronomy, Iowa State Univ., Ames, IA 50011
b Alberta Agriculture Food and Rural Development, 6903 116 Street, Edmonton, AB, Canada T6H 4P2 (formerly Graduate Research Assistant, Dep. of Agronomy, Iowa State Univ.)

* Corresponding author (apmallar{at}iastate.edu)


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
There is uncertainty concerning evaluation of bioavailable P in manured soils. This study assessed P availability in manured Iowa soils by measuring soil P with agronomic and environmental P tests and P uptake by corn (Zea mays L.) and soybean [Glycine max (L.) Merr.]. Soil and plant samples were collected at the V5-V6 growth stage from trials established at nine locations that received various rates of liquid swine (Sus scrofa) manure, and from farmers' fields that received other animal manures. Soil P was analyzed by the Bray-P1 (BP), Olsen (OP), and Mehlich-3 (M3P) agronomic tests, and by the Fe-oxide impregnated filter paper (FeP), anion-exchange resin membrane (RP), and water (WP) environmental tests. Soil P at a 15-cm depth ranged from deficient to 15 times optimum levels for crops. Extracted P was highest for BP, M3P, and RP, intermediate for OP and FeP, and lowest for WP. Relationships between soil P extracted by the tests were linear, trends were similar for manured and unmanured plots, and correlation coefficients were >=0.70 (correlations were poorest for WP). There was no conclusive evidence for differences between tests in detecting manure-derived soil P at most sites. However, in some conditions BP, M3P, OP, and RP may extract proportionally more manured-derived P than FeP and WP. Only the agronomic tests were significantly correlated (0.42 for M3P, 0.44 for BP, and 0.47 for OP) with plant P uptake across sites. Agronomic soil tests would predict plant P availability for crops better than environmental P tests in soils receiving liquid swine manure.

Abbreviations: BP, Bray-P1 • FeP, Fe-oxide impregnated filter paper strip • OP, Olsen • M3P, Mehlich-3 • RP, resin membrane • and WP, water extractable P


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
ADVANCED EUTROPHICATION of surface water resources of many regions has promoted renewed interest in P aspects of manure management and soil P testing in manured soils. Animal manures supply N, P, and other nutrients for crops. Because of the relative N and P content of swine and poultry manure and often significant N losses through leaching or volatilization, rates of application that would supply the N needs of cereal crops often result in P accumulation in soils (Mikkelsen and Gilliam, 1995). Application of manure P at rates higher than crop needs may result in increased potential for P loss to water resources. Dissolved P in runoff is readily available for algal growth and particulate P associated with sediment and organic material may also constitute a source of P in surface water resources (Dorich et al., 1984; Sharpley, 1993). Thus, an appropriate assessment of soil P levels in soils is important for both crop production and environmental purposes.

Soil testing is a diagnostic tool that can be used to assess the plant availability of nutrients in soils. Numerous soil P tests have been developed with this objective, which adapt to different soil chemical and mineralogical properties. The BP, OP, and M3P agronomic soil P tests are recommended for soils of the central and western Corn Belt of the USA by the North Central Regional Committee for Soil Testing and Plant Analysis (Brown, 1998). Iowa soil-test interpretations for these tests (Voss et al., 1999) are based on numerous field response trials (Mallarino et al., 1991; Webb et al., 1992; Mallarino, 1997). These P tests are being used in Iowa and other states for soils receiving either fertilizer or animal manures, but information is needed concerning their performance in manured soils. Relationships between amounts of P extracted by various soil tests and interpretations may be different for soils receiving fertilizers or manure (Abbott and Tucker, 1973; Reddy et al., 1978; Pratt and Laag, 1981; Sharpley, 1996). The forms and content of P in animal manure vary widely, mostly because of variations in P nutritional physiology, P content of feedstuffs, and varying mineral P supplements. As a result, the proportion of inorganic and organic P in manures also varies widely (van Faassen and van Dijk, 1987), and this variation (in addition to factors such as preapplication treatment of manure, climate, and soil factors) can influence estimates of availability of P for crops based on soil testing.

Agronomic soil P tests are also being used as indices of the potential impact of soil P levels on P delivered to water bodies (Mallarino et al., 2001). However, other tests have been proposed that do not emphasize estimating plant-available P as routine soil tests do. These tests, usually referred to as environmental or bioavailable P tests (hereafter referred to as environmental tests) are thought to better estimate P that stimulates algal growth if it were transported to surface water resources (Sharpley, 1991; Pote et al., 1996). Three environmental tests are receiving increased attention. One is based on the P extracted by Fe-oxide impregnated filter paper strips or discs (FeP). Several minor modifications to the basic concept of the method have been published (i.e., Menon et al., 1989; Sharpley, 1991, 1993; Chardon et al., 1996). Another test extracts P from soils with anion-exchange resins. Initially based on resin beads (Amer et al., 1955; Olsen et al., 1983), recent technology has allowed for impregnating a resin onto a plastic membrane (Abrams and Jarrell; 1992; Fernandes and Warren, 1995), which facilitates the lab procedures and produces similar results. These two tests are sink-based methods that rely on sorption-desorption reactions, and could provide different estimates of soil or sediment P available for either plants or algae compared with tests based on extractive solutions. A third environmental test is based on weak desorption reactions and extracts soil P with deionized water. The basic aspects and theory of this method were developed long ago (van Der Paauw, 1971), but interest in P losses to water resources have promoted recent research on this test (Pote et al., 1996).

Research has been conducted in different regions to study correlations between amounts of P extracted by some environmental tests and P extracted by agronomic soil tests or plant growth under laboratory or greenhouse conditions (Sorn-Srivichai et al., 1988; Menon et al., 1989; Sharpley, 1991; Raven and Hossner, 1994; Korndorfer et al., 1995; Pote et al., 1996; Fernandes and Coutinho, 1997; Nuernberg et al., 1998; Magdoff et al., 1999; Pautler and Sims, 2000). An excellent review of the applications of the FeP test for soils receiving soluble P fertilizers or rock phosphate was published by Menon et al. (1997). As expected, these studies showed that the relationships among amounts of soil P extracted, and between soil P extracted and indices of plant P availability are highly affected by soil properties and few generalizations can be made across contrasting soils and regions. These and other studies suggested that soil properties known to influence P sorption such as soil pH, particle-size composition, or mineralogy would explain variations across soils. Previous Iowa research showed that soil pH and CaCO3 concentration explained a comparatively lower P extraction by the BP test compared with the OP and M3P tests in high pH soils (Mallarino, 1997).

Scarce reports suggest that relationships among P extracted by agronomic and environmental soil tests for a particular soil may, or may not, change when the soil is manured. Lucero et al. (1998) showed that the BP and M3P tests were similar in evaluating soil P after poultry litter applications. Rubaek and Sibbesen (1995) showed similar P levels and variation when a resin-based P extraction and the OP test were used to assess soil P across plots that received fertilizers or liquid manure. Results of P extraction and fractionation laboratory studies of soils that received various kinds of animal manures (Sharpley and Smith, 1995; Sharpley, 1996) suggested that acid-based extractants such as the BP and M3P tests may overestimate P availability for crops. Although plant availability was not directly measured, these authors based their conclusions on high correlation between the NaHCO3-extractable soil P fraction and P extracted by the FeP test and sharp increases of the Ca-bound P fraction after manure applications. If this result were confirmed for all soils and conditions, the BP and M3P tests, which are widely used in central and eastern regions of the USA, could be underestimating plant-available P in manured soils.

The objectives of this research were to study relationships between soil P extracted by various agronomic and environmental soil P tests, test sensitivity in evaluating the impact of field-applied liquid swine manure on extractable P, and relationships between soil P extracted by the tests and plant P uptake by corn and soybean in soils of a soil association typical of Iowa and Southern Minnesota that supports a high concentration of both swine and row crop production.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
Soil and plant samples were collected in Iowa from 1996 through 1998 from plots of nine long-term and 1-yr field experiments with liquid swine manure treatments, and from farmers' fields with varied histories of manure application. Five experiments were located at a research center near Ames in central Iowa, three were located at a research center near Kanawha in north-central Iowa, and one was on a farmer's field near Boone. The experimental areas had received P fertilization in previous years but no manure application during the previous 10 yr (older records were unavailable or were unreliable). Information about soil series and selected physical and chemical properties of the top 15-cm layer of the soils measured before manure was applied are shown in Table 1. Particle-size distribution was measured by the standard pipette method (Day, 1965). Procedures for pH, extractable cations, and organic matter were among those recommended and described for the North Central region (Brown, 1998). Briefly, pH was measured using a 1:1 soil/water ratio, extractable K, Ca, and Mg were measured by the NH4C2H3O2 test, and organic matter was measured by the Walkley-Black test.


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  		Table 1. Classification, initial physical, and chemical characteristics (0–15 cm depth) of the soils.

 
Information about crop planting dates, corn hybrids, soybean cultivars, and manure treatments are shown in Table 2. In this table and hereafter, the suffixes a, b, or c were added to the site numeric code to denote the first, second, or third year of sampling of long-term experiments (Sites 1, 2, and 3). All sites were managed with chisel-plow/disk tillage. The sites did not receive P fertilization during the period of study. The liquid swine manure for the treatments was collected from covered storage pits at each farm. The manure was incorporated into the soil by chisel plowing and disking or by injecting it using 67-cm spacing to a depth of 10 to 15 cm. The experiment at Site 1 was a 2-yr experiment established in 1996. Corn was planted the first year and soybean was planted in the second year. The treatments (seven) included a check that received no P from fertilizer or manure, three rates of manure applied in the spring 1 to 2 wk before planting corn, and three preplant N fertilization rates for corn. The experiment at Site 2 was another 2-yr experiment that was established in 1997. Crop management and treatments were similar to those for Site 1, except that two rates of manure were applied instead of three rates. The experiment at Site 3 was established in 1994, and corn and soybean were rotated until 1998 (corn was planted in 1994). The treatments included a check that received no manure or P fertilizer, manure applied 1 to 2 wk before planting corn (once every 2 yr), and manure applied every year 1 to 2 wk before planting corn or soybean. All plots at this site received an additional uniform N fertilizer rate of 162 kg N ha-1. The experiments at Sites 4 through 9 had similar treatments, which were a check that received no manure or P fertilizer, and one rate of liquid swine manure. The manure was applied in November (fall) before planting corn at Site 4 and in the spring 1 to 2 wk before planting corn at Sites 5 through 9. The field experimental layout of all experiments was a randomized complete-block design, and the number of replications ranged from three to eight depending on the experiment.


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  		Table 2. Manure application dates and rates, manure analysis, soil sampling dates and depth, and crop variety and planting dates.

 
Soil samples for the soil P analyses were collected when corn plants had five to six leaves with visible collars and soybean plants had five to six open trifoliolate leaves (V5 to V6 growth stages). Twelve to 15 cores were collected from a 15-cm soil depth and combined into one composite sample for each plot. The samples were dried at 40°C and crushed to pass through 2-mm sieve before the analyses. All analyses were performed on duplicate samples. Procedures followed for the agronomic P soil tests were those recommended and described for the north-central region (Frank et al., 1998). For the BP test, 1 g of soil was extracted with 10 mL of 0.03 M NH4F and 0.025 M HCl by shaking during 5 min. For the OP test, 1 g of soil was extracted with 20 mL of 0.5 M NaHCO3 (pH 8.5) by shaking for 30 min. For the M3P, 1 g of soil was extracted with 10 mL of 0.2 M CH3COOH, 0.25 M NH4NO3, 0.015 M NH4F, 0.013 M HNO3, and 0.001 M EDTA by shaking for 5 min. All extracts were filtered through a Whatman No. 42 filter paper and P was determined colorimetrically by the Murphy and Riley (1962) method.

The procedure followed for the FeP method was described by Menon et al. (1989) and Sharpley (1993). Iron-oxide impregnated filter paper was prepared by immersing paper discs (15-cm diam., Whatman No. 50) in a solution prepared by dissolving 10 g of FeCl3·6H2O in 100 mL of deionized water. The paper discs were removed from the solution, dried, immersed in a 2.7 M NH4OH solution to convert FeCl3 to FeP, air-dried, cut into 10 by 2 cm strips, and stored for subsequent use. Phosphorus was extracted from soil by shaking 1 g of soil, one strip, and 40 mL of 0.01 M CaCl2 for 16 h. The strips were removed, rinsed free of adhering soil particles with deionized water, and air dried. Phosphorus retained on the strip was removed by shaking with 40 mL of 0.1 M H2SO4 for 1 h. Procedures followed for the RP test were those described by Tiessen and Moir (1993). Sheets of a resin-impregnated plastic material (BH15 1TD, BDH Laboratory Supplies, Poole, England) were cut into strips measuring 4.6 by 1.0 cm. The strips were shaken in 0.5 M NaHCO3 for 30 min and were allowed to dry. Phosphorus was extracted from the soil by shaking one strip with 1 g of soil and 20 mL of deionized water in 200-mL bottles for 16 h. The strips were removed from the bottles, rinsed with deionized water, and were shaken for 1 h in 0.5 M HCl to desorb the retained P. The procedure followed for the WP test was described by Pote et al. (1996). One gram of soil was shaken with 25 mL of distilled water for 1 h, centrifuged, and filtered through Whatman No. 42 filter paper. The P extracted by all methods was measured colorimetrically by the Murphy and Riley (1962) method.

Composite samples of 10 plant shoots were collected by cutting plants at ground level at the V5 to V6 growth stage from plots of four selected treatments of Site 1, three treatments of Site 2, and from all plots at other sites. At Site 1, treatments sampled were the check and the three rates of manure. At Site 2, treatments sampled were the check and the two rates of manure. Plants were weighed before drying them at 65°C in a forced-air oven, weighed again to calculate dry weight, ground to pass a 1-mm sieve, and digested using a H2SO4–H2O2 method (Digesdahl Analysis System, Hach Inc., Boulder, CO). Plant P concentration in the digests was determined colorimetrically by the Murphy and Riley (1962) method. Phosphorus uptake per plant was calculated from the P concentrations and dry matter weights. Relative plant P uptake was calculated by dividing the plant P uptake for each plot by the mean P uptake of the treatment or treatments that resulted in the statistically greatest P uptake at each site, and then multiplying by 100.

In addition to samples collected from the field trials, soil samples were collected from 25 farmers' cornfields that had varied histories of manure application (including higher accumulated manure application rates over time than those of the trials) to supplement the study of relationships between amounts of P extracted by the soil tests. Detailed information provided by the farmers about field management histories, type of manure used, and methods of application not presented in detail here. Briefly, the fields had histories of swine, poultry, dairy, or beef manure applications for the corn–soybean rotation, and no P fertilizer had been applied for several years previous to the sampling dates. At these sites, soil was sampled to a depth of 30 cm by collecting two composite samples made up of 32 cores from a seemingly uniform area approximately 0.1 ha in size when corn was at the V5 to V6 growth stage. The sampling depth was different from the sampling depth at the trials because an additional major objective of sampling these fields was to assess soil NO3 by the presidedress soil NO3 test (which requires a 30-cm sampling depth). The areas sampled represent 16 Iowa agricultural soil series, and included Argiudolls, Endoaquolls, Haplaquolls, Hapludalfs, Hapludolls, and Udorthents. Five sampling areas included calcareous soil series (Canisteo [Fine-loamy, mixed, superactive, calcareous, mesic Typic Endoaquoll] and Harps [fine-loamy, mixed, superactive, mesic Typic Calciaquoll]). The samples were analyzed for soil P, pH, and organic matter as already described. Organic matter ranged from 23 to 60 g kg-1 and pH from 5.1 to 8.2 (five soils had pH greater than 7.2 and only two had pH below 5.5).

Correlation and regression analyses were used to study the relationships between P extracted by the soil tests. Analyses were conducted separately for the trials and farmers' fields data sets because the soil sampling depth was different. When analyzing the data set for the trials, regression analyses were conducted across all plots of all trials and also separately for the unmanured plots and the manured plots. A t test of differences between each pair of linear regression coefficients (for unmanured and manured plots) for each soil test was conducted by using the pooled residual variances as an error term. Curvilinear trends are presented only when addition of a quadratic term significantly reduced (F test, P <= 0.1) the residual sums of squares of the model compared with that of a linear model. Analysis of variance for a randomized complete-block design was used to assess the effect of the manure treatments on soil P for each test and trial. When more than two treatments were applied, the treatment sums of squares were partitioned into orthogonal comparisons of linear and quadratic effects. Because different soil tests extract varying amounts of P, standardized soil test values were used to compare slopes of statistically significant linear trends within each site. The standardized values were calculated for each site by subtracting the mean of each soil test (across all treatments) from each soil test value and dividing by the standard deviation. A t-test of differences between pairs of linear regression coefficients was conducted by using a pooled residual variance within each site as an error term. Correlation analyses were used to study the relationship between each soil test and plant P uptake across all trials.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
SUMMARY AND CONCLUSIONS
 REFERENCES
 
Correlations Between Amount of Phosphorus Extracted by Agronomic and Environmental Soil Tests
Data in Fig. 1 show relationships between P extracted by the three agronomic P tests from samples collected from the top 15 cm of soil at the V5 to V6 crop growth stage across all field trials. The relationships were always linear (P <= 0.1). The points of the graphs, fitted lines, and t-tests of differences between linear coefficients (all paired comparisons were not significant at P <= 0.1) indicate that the linear trends were similar for check plots and manured plots for all relationships. The check plots received no manure or P fertilizer during the evaluation period, had received P fertilization in the past, and had received no manure during the previous 10 yr. The comparisons of linear coefficients for check and manured plots also showed no significant differences when only the range of soil P values common to both data sets was included (data not shown).



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  		Fig. 1. Relationships between soil P extracted by three agronomic tests for manured and unmanured plots of field trials.

 
The correlation between amounts of P extracted by the BP and M3P tests was higher than the correlation between P extracted by the OP and BP tests or the OP and M3P tests. Two reasons may partly explain this result. One reason is the high soil pH of some plots. Although the average soil pH of each experimental area was always below 7.2, the pH of some individual plots was as high as pH 8.2. This small-scale soil pH variation is typical of Iowa and Southern Minnesota, and high pH indicates high concentrations of CaCO3. All correlations, but especially those involving the BP test, were slightly higher when samples with pH greater than 7.3 were not considered (not shown). Previous Iowa research with unmanured soils (Mallarino, 1997) showed that correlations between the BP test and the OP or M3P tests were low when soil pH was higher than 7.3 and when the soil CaCO3 concentration was >5 g kg-1. The other possible reason is that the BP and M3P test tended to extract relatively more P than the OP test from some high-testing manured plots. This difference was not detected by the test of linear coefficients for check and manured plots, perhaps because it occurred only for a few plots. Previous research in other regions (Sharpley and Smith, 1995; Sharpley, 1996) suggested that the acid-based tests (such as the BP and M3P tests) could extract proportionally more P from manured soils compared with other agronomic P tests.

Figure 2 shows relationships between the amount of soil P extracted by the three environmental tests, and Fig. 3 shows the relationships between the environmental tests and each agronomic test. Relationships between amounts of P extracted were poorer for the environmental tests, especially for the WP test, than for the agronomic tests. This result was not due to higher analytical error. Other authors (van der Zee et al., 1987) found higher correlations between the FeP and WP tests. Thus, the poorer strength of relationships for our study suggests that the environmental tests may be sensitive to variability in soil properties within and across sites. Relationships (regression coefficients and trends) between P extracted by the environmental tests were similar (P <= 0.1) for manured and unmanured plots. Amounts of P extracted by the FeP and RP environmental tests were linearly correlated, as was the P extracted by any of these tests with the P extracted by the three routine tests. Although data for a few high-testing plots (all of them manured plots) suggest that the P extracted by these two environmental tests was proportionally higher than P extracted by the agronomic tests, a quadratic term was not statistically significant (P <= 0.1) after the linear term in any case. A higher correlation between the FeP and RP tests was expected because of their sink-based extracting mechanisms. The correlations shown in published reports between P extracted by the FeP test and some of the agronomic tests evaluated in this study varied markedly. In general, the FeP test has been better correlated with the OP test across many soils, and Sharpley (1991) found that the FeP, BP, and M3P were closely correlated for noncalcareous soils.



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  		Fig. 2. Relationships between soil P extracted by three environmental tests for manured and unmanured plots of field trials.

 


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  		Fig. 3. Relationships between soil P extracted by three environmental tests and three agronomic tests for manured and unmanured plots of field trials.

 
In contrast to results for the FeP and RP tests, relationships between the WP test and the other tests (Fig. 2 and Fig. 3g–i) show obvious cluster of points at low P levels and curvilinear trends. A clustering at low P levels is explained by the known lower P extraction with water than with other extractants, which includes P in solution and the most weakly adsorbed P. Curvilinear relationships between P extracted with water and P extracted by all other tests were statistically significant, and would indicate that the WP test extracted relatively more P than the other tests at high soil P levels. However, relationships between the WP test and the other tests observed for the set of samples collected from the farmers' fields (Fig. 4) did not corroborate the curvilinear relationships observed for the trials. Data for the BP test are not shown because results were similar to results for the M3P test. The results for the farmers' fields samples showed linear relationships up to the highest amount of P extracted with water (58 mg kg-1), which was almost twice the highest value observed for the trials. The different sampling depth used for these samples likely affected the amount of P extracted but not the relationships between tests. Correlations and trends for the other two environmental tests are not shown in Fig. 4 because trends for these farmers' fields samples were similar to relationships shown previously for samples collected from the field trials.



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  		Fig. 4. Relationships between soil P extracted from high-testing farmers' fields samples by the water environmental test and P extracted by two other environmental tests and two agronomic tests.

 
Theoretical considerations suggest that availability estimates based on desorption-based tests such as the FeP, RP, and WP tests could be better correlated with P loss because the extraction mechanisms do not involve an arbitrary chemical extraction. Although results of this study cannot confirm or disprove this consideration because P loss to water was not measured, the high correlations between soil P extracted by most tests suggest that large differences in correlations with P loss are not likely for these soils at these soil P levels. The slightly lower correlations for the WP test suggest, however, that this test could provide different estimates of potential dissolved P loss through leaching or surface runoff because it could estimate better the saturation of the P adsorption complex.

Effect of the Manure Application Rate on Phosphorus Extracted by the Soil Tests
The soil P data corroborated the well-known fact that some tests (such as the WP and OP test) extract less P than other tests. A more relevant aspect of this study was to investigate if the P tests differed in assessing soil P increases because of manure application. Table 3 shows summary statistics of effects of manure application on soil P extracted by each test. Low manure application rates and high soil-test variability (which is typical of manured soils) contributed to small and statistically not significant (P < 0.1) increases in soil P extracted by any soil test at four sites (at Site 2b, which evaluated residual effects to manure applied for the previous year crop, and at Sites 5, 6, and 7). Most tests, with the only exception of WP, increased (P < 0.1) soil P at Site 1a. Significant increases were detected for the BP, M3P, and RP tests at Site 1b, and only for the OP test at Site 2a. Soil P for all tests increased at Sites 3, 4, 8, and 9. The absence of a statistically significant response to increasing manure rate for a particular test or site could be due to higher error, smaller P extraction, or low rates of applied P. When the manure treatments increased extracted P significantly, trends were always linear (P < 0.1). The linear trends are shown in Fig. 5 and 6 , which show data for sites where the manure treatments increased values of at least one soil test. Data for Site 3 are averages across the three sampling dates to simplify the presentation of results and because trends were similar for the 3 yr. The lines for BP, M3, and RP tended to be steeper than lines for other tests because these tests extracted more P and a common scale was used for all tests within a graph. However, a test of differences between slopes (not shown) indicated no significant differences (P < 0.1) existed when standardized soil test values were regressed on the manure rates.


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  		Table 3. Summary of analyses of variance of the effect of manure application on soil P extracted by six soil tests (main effect of manure treatments).{dagger}

 


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  		Fig. 5. Effect of manure application rates on soil P extracted by six P tests from plots of field trials having two or more manure treatments (BP, Bray-P1; OP, Olsen; M3P, Mehlich-3, FeP, Fe-oxide impregnated paper strip; RP, resin membrane; and WP, water P).

 


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  		Fig. 6. Effect of manure application rates on soil P extracted by six P tests from plots of field trials having one manure treatment (BP, Bray-P1; OP, Olsen; M3P, Mehlich-3, FeP, Fe-oxide impregnated filter paper strip; RP, resin membrane; and WP, water P).

 
Calculations of mean relative soil P increases across all sites and soil tests because of application of the highest rate of manure provide an overall estimate of the capacity of the tests to assess manure-derived P. The mean soil P increases were 115, 115, 133, 79, 86, and 77% for the BP, M3P, OP, RP, FeP, and WP tests, respectively. The high mean relative increase for the OP test compared with all other tests is explained by results from Site 2a (OP was the only test for which a soil P increase was detected) and Site 9 (where all tests increased soil P). When data for these two sites were not considered, mean relative increases were larger for the BP, M3P, and OP tests (131, 121, and 122%, respectively) than for the RP, FeP, and WP tests (86, 100, and 84%, respectively). The chemical and physical analyses used to characterize the manure sources and the soils of the study cannot explain the contrasting results for these two sites.

A reasonable overall conclusion from the sometimes conflicting results of these analyses is that the evidence was not conclusive for supporting the hypothesis that there are differences among tests in assessing manure-derived P; but in some conditions (which we could not satisfactorily identify), the BP, M3P, OP, and RP tests extracted relatively more manure-derived P than the FeP and WP tests. Our results for the BP and M3P tests confirm, only partially, suggestions by Sharpley and Smith (1995) that these acid-based tests extract proportionally more P from manured soils compared with these other commonly used soil P tests.

Effect of Manure Application Rates on Early Plant Dry Weight and Phosphorus Uptake
Data in Table 4 show the effect of the manure treatments on plant dry weight, plant P concentration, and aboveground plant P uptake measured at the V5 to V6 growth stages. While the lower rates of manure increased plant growth considerably, the highest rate did not increase growth further. Results of N fertilizer treatments not shown here and other soil tests (mainly K) suggest that the early plant growth response at this stage was not due only to N. Thus, although other factors cannot be ignored, responses in plant growth and P uptake likely were due to the P supplied by the manure. The magnitude of responses of plant P uptake followed closely the trends described for plant growth. Plant P concentration was increased by the manure treatment only at four sites and was decreased at one site. These results can be explained by effects of manure applications on P supply, plant growth, and by interaction effects of these two factors with plant P uptake and tissue P concentration. When fertilizer or manure application increases growth compared with a check treatment, the P concentration may decrease, remain the same, or may increase depending on the relative impacts on growth and P uptake. The results suggest that in low testing soils manure applications would increase P supply, early plant growth, P uptake, and tissue P concentration. In high-testing soils, manure applications may, or may not, increase plant growth (because of growth factors other than manure P because soil P is high) and if growth is increased, this increase may not correspond to a proportional increase in P uptake and, thus, the tissue P concentration may decrease. Previous work on high-testing unmanured Iowa soils (Mallarino et al., 1999) showed that vegetative corn and soybean tissue had a lower capacity for luxury accumulation of P compared with the capacity of grain.


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  		Table 4. Plant tissue P concentration, dry weight, and P uptake at the V5 to V6 crop growth stage as affected by manure application.{dagger}

 
Relationship Between Soil-Test Phosphorus and Plant Phosphorus Uptake
The relationship between P extracted by different P tests and relative P uptake by corn and soybean crops were calculated to evaluate the capacity of the tests to predict soil plant P availability across sites. Relative P uptake was used to eliminate the possibility of variations across sites that likely occur because of differences in cultivars or hybrids, plant sampling stage, and other factors. Table 5 shows the linear correlation coefficients of the relationships. Curvilinear trends provided no statistically significant better fit than the linear model for any soil test (not shown). Only P extracted by the BP, M3P, and OP tests was significantly (P < 0.1) correlated with plant P uptake, which suggests that they are better predictors of plant P availability when these soils are manured. Because P extracted by the FeP and WP tests was correlated with P extracted by the BP, M3P, or OP (Fig. 3), these two environmental tests could also be used to estimate plant P availability even though the predictive capacity will likely be lower (as results shown in Table 5 suggest). Our results agree with some published reports but disagree with others. Several authors (Schoenau and Huang, 1991; Fernandes and Coutinho, 1997) found high correlations between P extracted by a resin test and plant P uptake in controlled in-house experiments, although they did not compare the RP test with the tests we compared in our study. Sorn-Srivichai et al. (1988) found that the amounts of P extracted by WP and OP tests across many soils was equally and highly correlated with plant P uptake, and the correlation with the BP test was lower.


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  		Table 5. Correlations between soil P extracted by six soil tests and relative early plant P uptake across all trials.

 

   SUMMARY AND CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
REFERENCES
 
Correlations of soil P extracted by agronomic and environmental tests usually were high and similar for manured and unmanured soils with two exceptions. One exception was that the BP test extracted less P from a few plots with high pH, which presumably had higher CaCO3 content. The other exception was that correlations involving the WP test usually were lower than correlations between the other tests. Curvilinear relationships for the WP test (proportionally higher WP values at high soil P values) for data from the trials were not corroborated by data from manured farmers' fields that included higher soil P values. There was no conclusive evidence for differences between tests in detecting manure-derived soil P at most sites. However, the results suggest that in some conditions the BP, M3P, OP, and RP tests extract proportionally more manured-derived P than the FeP and WP tests. The three agronomic tests had higher correlations with P uptake across sites than any environmental test, which suggests they are better predictors of manure-derived P availability for crops when soils similar to those included in this study receive swine manure. The OP test had the highest correlation with plant P uptake among all tests. The lack of correlation between early plant P uptake and P extracted by the three environmental P tests found in this study may, or may not, suggest lower effectiveness for predicting P loss to surface water supplies. This assessment should be derived from relationships between extracted soil P and P loss with water, an aspect that was beyond the scope and objectives of this study.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
Iowa Agric. Home Econ. Exp. Stn. Journal Paper no. J-19495. Project 4062.

Received for publication August 13, 2001.


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




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