Published in Soil Sci. Soc. Am. J. 67:1928-1934 (2003).
© 2003 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
DIVISION S-8—NUTRIENT MANAGEMENT & SOIL & PLANT ANALYSIS
Field Calibration for Corn of the Mehlich-3 Soil Phosphorus Test with Colorimetric and Inductively Coupled Plasma Emission Spectroscopy Determination Methods
Antonio P. Mallarino*
Dep. of Agronomy, Iowa State Univ., Ames, IA 50011
* Corresponding author (apmallar{at}iastate.edu).
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ABSTRACT
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Use of the Mehlich-3 soil extractant with an inductively coupled plasma emission spectroscopy (ICP) determination method (M3-ICP) is displacing the original colorimetry-based test (M3-COL). Current interpretations do not distinguish between these two versions, although the M3-ICP test often measures more P. This study correlated these tests and the Bray-P1 (BP) test with corn (Zea mays L.) yield response at 59 Iowa locations (78 site-yr). The mean P measured by the M3-ICP, M3-COL, and BP tests was 31, 19, and 17 mg P kg-1, respectively. The M3-ICP/M3-COL ratio decreased exponentially with increasing soil P (P 
0.01) but their difference was not correlated with soil P. Relative or absolute differences tended to decrease linearly (P 0.01) with increasing soil pH or organic C, but the strength of the relationship was poor (R2 = 0.14–0.32). The BP test measured significantly less P in a CaCO3–affected soil (pH 8.1). The R2 of the relationship between M3-ICP and M3-COL was 0.84, and was 0.89 between the M3-COL and BP (0.97 excluding the site with pH 8.1). Critical concentrations defined by Cate-Nelson and linear-plateau models for the M3-ICP, M3-COL, and BP tests were 20 to 32, 16 to 21, and 13 to 20 mg kg-1, respectively. The M3-COL and M3-ICP are equally effective for Iowa soils but interpretations differ. The M3-ICP test should be considered a different test and its interpretations should be based on field calibrations rather than conversions based on M3-COL data. A range of 25 to 35 mg kg-1 for the M3-ICP test would correspond to the optimum class (16–20 mg kg-1) used in Iowa for the M3-COL and BP tests.
Abbreviations: BP, Bray-P1 • M3-COL, Mehlich-3 with colorimetric determination of extracted P • M3-ICP, Mehlich-3 with inductively coupled plasma emission spectroscopy determination of extracted P • STP, soil-test P • Q-P, quadratic-plateau
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INTRODUCTION
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SEVERAL SOIL P TESTS are used to estimate plant-available P in production agriculture. The BP (Bray and Kurtz, 1945; Frank et al., 1998) and Mehlich-3 (Mehlich, 1984; Frank et al., 1998) tests are widely used in the USA Midwest. Early research (Kamprath and Watson, 1980; Fixen and Grove, 1990) showed that chemical reactions between various soil constituents and the extracting solutions could explain differences in the suitability of soil-test P (STP) extractants for different soils. Laboratory and field studies (Eik et al., 1961; Smith and Pesek, 1962; Sen Tran et al., 1990; Mallarino and Blackmer, 1992; Mallarino, 1997) showed that the BP test is reliable on neutral or acid soils but underestimates plant-available P on some high-pH CaCO3–affected soils. The underestimation of plant-available P by the BP test is usually attributed to neutralization of the acid extracting solution by CaCO3 and/or precipitation of the F by dissolved Ca (Smith et al., 1957; Fixen and Grove, 1990).
The Mehlich-3 extractant was developed for routine soil testing of P, K, and other nutrients (Mehlich, 1984), and the determination of extracted orthophosphate P was based on a colorimetric method. The North Central Region Soil Testing and Plant Analysis Committee (NCR-13) recommends an ascorbic acid-ammonium molybdate colorimetric method (Frank et al., 1998) that is based on the Murphy and Riley (1962) method. Research comparing the Mehlich-3 test and other P tests (Beegle and Oravec, 1990; Sen Tran et al., 1990; Mallarino and Blackmer, 1992; Mallarino, 1997) showed that P measured with the Mehlich-3 test was similar to or only slightly higher than P measured with the BP test in neutral or acidic soils, and suggested similar interpretations for both tests. However, the Mehlich-3 test often measures more P than the BP test on high-pH CaCO3–affected soils (Sen Tran et al., 1990; Mallarino and Blackmer, 1992; Mallarino, 1997), which has been attributed to a significant buffer capacity of the Mehlich-3 extracting solution (Sen Tran et al., 1990; Mallarino and Blackmer, 1992). Iowa field calibrations for corn (Mallarino and Blackmer, 1992; Mallarino, 1997) showed that the Mehlich-3 test was more effective than the BP test and almost equally effective to the Olsen test (Olsen et al., 1954) for predicting corn response to P across many Iowa soils with pH values ranging from 5.2 to 8.2.
Use of ICP in routine soil testing laboratories has expanded rapidly since the early 1990s (Munter, 1990; Jones, 1998). The ICP is based on characteristic optical emission of atoms excited in a high-temperature (5000–8000 K) Ar plasma and provides simultaneous analysis of many elements. This method is displacing the original Mehlich-3 test colorimetric P determination because ICP equipment costs have decreased and the same extractant is used for other nutrients. Moreover, ICP is also being used to determine P for other soil P extractants (Soltanpour et al., 1979; Khiari et al., 2000; Masson et al., 2001). Because sample molecules injected into the plasma undergo instantaneous vaporization, dissociation, and ionization, ICP measures other P forms in addition to orthophosphate P. Thus, the P measured with ICP sometimes is up to 50% higher than P measured with the colorimetric methods and research has suggested that the additional P is mainly organic P (Hylander et al., 1995; Eckert and Watson, 1996; Eliason et al., 2001; Nathan et al., 2002). However, no consistent relationships have been reported between the additional P measured with ICP and manure applications or soil organic matter (Nathan and Sun, 1998b; Nathan et al., 2002).
Most soil-test interpretations in the USA do not specify the method used to determine P extracted with the Mehlich-3 extractant. In Iowa, however, interpretations apply only to the M3-COL test (Voss et al., 1999). Yet, 66% of routine soil testing labs enrolled in the North American Proficiency Testing Program for the Mehlich-3 test were using the M3-ICP version by March 2002 (Miller, 2002). Field response research is needed to compare the efficacy of the M3-COL and M3-ICP tests for predicting crop response to P and to establish critical concentration ranges for the M3-ICP test. The objectives of this study were to correlate the M3-ICP test with the grain yield response of corn across several Iowa soils, to compare these correlations with those for the M3-COL and BP tests, and to establish preliminary agronomic interpretations for the M3-ICP test for Iowa soils.
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MATERIALS AND METHODS
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Grain yields and soil samples for this study were collected from P response trials conducted at 59 Iowa locations from 1989 to 1997. There were 78 site-yr of data because 13 trials were evaluated 2 yr and three trials were evaluated 3 yr (treatments were reapplied each year). The soils represented 17 soil series in which row-crop production predominates, and corresponded to soil survey subgroups Aquic Argiudoll, Aquic Hapludoll, Mollic Hapludalf, Typic Argiudoll, Typic Endoaquoll, Typic Hapludalf, Typic Hapludoll, and Udollic Endoaqualf. Details of most trials such as tillage system, fertilization rates, placement methods, yield responses, and optimum P rates were partly summarized before (Mallarino et al., 1991; Mallarino and Blackmer, 1992; Webb et al., 1992; Bordoli and Mallarino, 1998; Borges and Mallarino, 2001). Thirty-one trials evaluated four P fertilization rates for corn managed with plow and/or disk tillage, 13 trials evaluated three P fertilization rates applied broadcast or banded for no-tilled corn, and 15 trials evaluated three various P fertilization rates applied either broadcast or banded for ridge-tilled corn. The P treatments always included a nonfertilized control, and the largest P rate at each trial ranged from 35 to 75 kg P ha-1 (granulated triple superphosphate). There were three or four replications at each site, and randomized-plot or completely randomized-block designs were used.
The P treatments were applied in October or November (fall) or in March or April (spring) before planting. Broadcast P was incorporated by chisel plowing and/or disking at sites managed with chisel-plow tillage, was applied in the fall and not incorporated at sites managed with no-tillage, and was incorporated with the sweep of the planters at ridge-till sites. The banded treatments were deep-banded (15 cm) in the fall using a 76- to 97-cm lateral spacing depending on the row spacing used or were banded with the planter (5 cm beside and below the seed level). Other crop management practices were those normally used by the farm operators, and N and K were applied were uniformly applied to all plots at each sites following Iowa State University recommendations.
Composite soil samples were collected before applying the treatments. In sites managed with chisel-plow or no-tillage, 12 cores were collected from a 15-cm depth. In fields managed with ridge-tillage, 45 cores were collected from a 15-cm depth in equal proportions from the ridge top, ridge shoulders, and valley positions. The samples were stored at 5°C from 4 to 12 wk after sampling, dried at 30 to 40°C (depending on the trial and year), crushed to pass through a 2-mm sieve, and stored in plastic-lined bags in a room with temperature maintained at approximately 23°C and 65% relative humidity. For this study, all soil samples were dried again at 35°C and re-analyzed. Soil pH (based on a 1:1 soil/water ratio) ranged from 5.3 to 8.1 across sites. The sum of soil CaCO3 and MgCO3 was measured with the pressure-calcimeter method (Sherrod et al., 2002) in soils with pH >7.2, and ranged from 0 to 58 g kg-1. These high-pH soils were classified as Canisteo (fine-loamy, mixed, superactive, calcareous, mesic Typic Endoaquolls). Soil organic C was measured with a high-temperature induction furnace combustion method by measuring total C with a LECO CHN-2000 analyzer (LECO Corp., St. Joseph, MI) and correcting the result for samples with pH >7.2 by subtracting inorganic C measured with the pressure-calcimeter method.
Soil P was analyzed with the BP and M3-COL tests using procedures recommended for the North Central Region (Frank et al., 1998). For the BP test, 1 g of soil was extracted with 10 mL of the extractant solution (0.03 M NH4F and 0.025M HCl) by shaking during 5 min. For the M3-COL test, 1 g of soil was extracted with 10 mL of the extractant solution (0.2M CH3COOH, 0.25M NH4NO3, 0.015M NH4F, 0.013M HNO3, and 0.001M EDTA) by shaking during 5 min. Both extractions were performed on duplicate samples. Extracts were filtered through Whatman No. 42 paper, and P was determined by an ascorbic acid-ammonium molybdate colorimetric method based on the Murphy and Riley (1962) method. Comparisons of recent and older analyses for soil pH, BP, and M3-COL tests showed no consistent differences. Thus, means of all results were used in this study. For the ICP P determination, two aliquots of the Mehlich-3 extracts used for the most recent colorimetric determination were analyzed for P with a Thermo Jarrell-Ash ICP atomic emission spectrometer (Thermo Elemental, Franklin, MA) equipped with a charged injection device (Epperson et al., 1988; Jones, 1997).
The grain yield data used in this study is expressed as relative responses to P. Relative response was calculated for each site-year by dividing the mean yield of the control plots by the mean of the highest P fertilization treatment, and multiplying the result by 100. In sites that evaluated P placement methods, means of the highest P rate were calculated across all methods because yield response was seldom affected by the P placement method, including sites managed with no-tillage or ridge-tillage (Bordoli and Mallarino, 1998; Borges and Mallarino, 2001). For 1-yr trials, one pair of data (relative yield and soil-test P) is represented in figures by one point because initial soil P was measured on one initial composite soil sample. For trials evaluated 2 or 3 yr, one pair of data for each year is represented by one point, and the soil-test data represent the mean value for the control plots.
Soil test critical concentrations were calculated with the statistical Cate-Nelson (C-N) method (Cate and Nelson, 1971), and with the linear-plateau (L-P) and quadratic-plateau (Q-P) segmented models (Waugh et al., 1973). The critical concentration defined by the C-N method was determined with the General Linear Models (GLM) procedure of SAS (SAS Institute, 2000) as the value that split the yield response data into the two groups that accounted for the largest proportion of the total variability (R2). Critical concentrations defined by the segmented models were determined with the Nonlinear Model (NLIN) procedure of SAS, and represent the soil-test values at which the two portions of each model joined.
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RESULTS
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Amounts of Phosphorus Extracted by the Soil Tests
The average soil P measured by the M3-ICP, M3-COL, and BP tests across all sites was 31, 19, and 17 mg P kg-1, respectively. Coefficients of determination (R2) of relationships between tests across sites were 0.84 for the M3-ICP and M3-COL tests and 0.89 for the M3-COL and BP tests. Excluding a site with CaCO3–affected soil (pH 8.1 and 58 g kg-1 CaCO3, the highest values in the study) from the regression analyses did not affect the relationship between the M3-ICP and M3-COL tests (the R2 increased only to 0.85). However, excluding this site significantly improved the relationship between the M3-COL and BP tests and the R2 increased to 0.97 (Fig. 1)
. Excluding four other soils with pH ranging from 7.4 to 7.7 did not change the R2 of the relationships, probably because CaCO3 plus MgCO3 was lower (<3.6%) and the BP test was not affected as much. The higher amount of P measured with ICP compared with the colorimetric determination method across the soils of this study agree with other results (Hylander et al., 1995; Eckert and Watson, 1996; Nathan and Sun, 1998a, 1998b; Eliason et al., 2001; Nathan et al., 2002). The lower P measured with the BP test in some high-pH (CaCO3–affected) soils compared with the M3-COL test was also observed by others (Sen Tran et al., 1990; Mallarino and Blackmer, 1992; Mallarino, 1997). Mallarino (1997) evaluated differences in amount of P extracted by the BP, M3-COL, and Olsen P tests from samples collected from a wider range of Iowa soil series and management practices, and showed that the M3-COL test measured more P than the BP test in CaCO3–affected soils and that it correlated better with the Olsen test than with the BP test.
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Fig. 1. Correlations between amounts of soil P measured with three soil P tests. M3-COL = Mehlich-3 with P determination based on colorimetry, and M3-ICP = Mehlich-3 with P determination based on inductively coupled plasma emission spectroscopy. The determination of P extracted with the Bray-P1 test was also based on colorimetry. Arrows indicate data for a site with the highest pH (8.1) and CaCO3 concentration (58 g kg-1).
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Data in Fig. 2a
show that the relative difference between the M3-ICP and M3-COL (their ratio) decreased exponentially with increasing soil P (M3-COL). However, there was no correlation between the absolute difference between the two tests and the soil P level (Fig. 2b). Similar trends were observed for relationships between the M3-ICP and BP tests (not shown), and the absolute or relative differences between the M3-COL and BP tests were not correlated with the soil P level. These results indicate that the additional P measured with ICP was proportionally higher at low values of extractable orthophosphate P. These relationships must be interpreted with caution, however, because they may have been affected by variation in soil pH and organic C. The additional P measured with the M3-ICP test compared with the M3-COL test was negatively and linearly related with soil pH and soil organic C when it was expressed either in relative or absolute terms (Fig. 3 and 4)
. The strength of the relationships was weak, however. The negative relationships between the additional P measured with the M3-ICP test compared with the M3-COL test and organic C, although weak, suggests that the additional measured P is not extracted organic P or is organic P not related to the measured organic C. Increasing soil organic C could have increased the additional P measured with the M3-ICP test because previous research (Eliason et al., 2001; Nathan et al., 2002) showed that at least part of the additional P measured with the ICP determination method derived from organic P. However, other research showed no relationship between additional P measured with an ICP determination method and soil organic matter or previous manure applications (Nathan and Sun, 1998b, Nathan et al., 2002). Interpretations of cause and effect are difficult and speculation is risky, however, because the high pH soils tended to have higher organic C (r = 0.38, P 0.01).
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Fig. 2. Relationships between soil P measured with the Mehlich-3 extractant with determination of extracted P based on colorimetry (M3-COL) and the relative (a) or absolute (b) difference from soil P measured by the same extractant but with determination of extracted P based on inductively coupled plasma emission spectroscopy (M3-ICP).
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Fig. 3. Relationships between soil pH and the (a) relative or (b) absolute difference in soil P measured by the Mehlich-3 extractant with determination of extracted P based on colorimetry (M3-COL) or inductively coupled emission spectroscopy (M3-ICP).
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Fig. 4. Relationships between soil organic C and the (a) relative or (b) absolute difference in soil P measured by the Mehlich-3 extractant with determination of extracted P based on colorimetry (M3-COL) or inductively coupled emission spectroscopy (M3-ICP).
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Field Correlation of Yield Response to Phosphorus
Field response trials that consider crop production conditions and encompass wide STP ranges provide the most appropriate basis for soil test interpretations. The variety of growing conditions resulted in grain yields across sites that ranged from 5.5 to 13.2 Mg ha-1 (means of the treatment that received the largest P rate at each site). Analysis of variance of P effects on yield indicated a yield response (P 0.05) in approximately 40% of the site-years (not shown). Relationships between relative corn yield response and soil P measured with the three tests are shown in Fig. 5
. When the result for the soil with high CaCO3 concentration is excluded, the shapes of relationships for the three tests suggest no major differences in the capacity of the tests to estimate plant-available P across the sites included in the study. The location of the data point for the high-CaCO3 soil demonstrates that the BP test grossly underestimated plant-available P in this CaCO3–affected soil compared with the M3-ICP or M3-COL tests. This result was shown before when comparing the M3-COL, BP, and Olsen tests (Mallarino and Blackmer, 1992, Mallarino, 1997), and this study confirms that the M3-ICP also is better than the BP test for these soils.
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Fig. 5. Relationships between relative yield response of corn and soil-test P measured with three soil P tests. M3-COL = Mehlich-3 with P determination based on colorimetry, and M3-ICP = Mehlich-3 with P determination based on inductively coupled plasma emission spectroscopy. The determination of P extracted with the Bray-P1 test was also based on colorimetry. Arrows indicate data for a site with the highest pH (8.1).
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Critical soil-test P concentrations calculated by the C-N, L-P, and Q-P models are shown in Table 1. Critical concentrations varied markedly depending on the model used, a result that was shown before using more models (Dahnke and Olson, 1990; Mallarino and Blackmer, 1992). The critical concentrations determined for the M3-ICP test was higher than those determined for the M3-COL and BP tests. The minor difference between the M3-COL and BP tests coincide with results from Pennsylvania soils (Beegle and Ovarec, 1990) and with previous results from Iowa (Mallarino and Blackmer, 1992; Mallarino, 1997). Plots of distribution of residuals did not add more meaningful information concerning model fit than data presented in Fig. 5 or Table 1 and are not shown. The results show that the M3-ICP has a similar capacity to predict response to P as the M3-COL, although the critical concentration is higher independently of the model used to estimate it and a different calibration is required. Previous research (Mallarino and Blackmer, 1992) showed that short-term economically optimum soil P critical concentrations usually are within a range of critical concentrations defined by the C-N and L-P models. They showed that critical concentrations defined by the quadratic or Q-P models usually are much higher compared with those defined by the C-N and L-P models, and its use in simulated scenarios across many fields resulted in smaller returns to fertilization. Critical concentration ranges defined by the C-N and L-P models in this study were 20 to 32 mg kg-1 for the M3-ICP test, 16 to 21 mg kg-1 for the M3-COL test, and 13 to 20 mg kg-1 for the BP test.
The critical concentration ranges defined by the C-N and L-P models for the M3-COL and BP tests match well with the optimum category of current Iowa interpretations for corn and soybean [Glycine max (L.) Merr.] for most soils of the state (16–20 mg P kg-1 for both tests, Voss et al., 1999), which are based on previously described relationships between soil-test values and yield response. Only P fertilization to maintain STP based on expected crop P removal is recommended within this category. These interpretations approximately coincide with interpretations in neighboring states, although the names of the classes used and the resulting fertilizer recommendations vary (Gerwing and Gelderman, 1998; Hoeft and Peck, 2001; Rehm et al., 2001; Shapiro et al., 2001).
There is no clearly superior or widely accepted method for defining agronomically optimum critical concentration ranges (Dahnke and Olson, 1990; Mallarino and Blackmer, 1992). Applying the current optimum interpretation range (16–20 mg P kg-1) used in Iowa for the M3-COL and BP tests to corn responses in this study corresponds to mean relative response values of 95 to 96%. However, the 20 to 32 mg kg-1 range identified by the C-N and L-P models for the M3-ICP test corresponds to a slightly lower mean yield response (92%). The range is also wider, probably because of less well-defined distinction between responsive and not responsive soils by the M3-ICP test and also because of higher measured P. Calculations based on moving averages from response data in Fig. 5 for the M3-ICP test based on a range width of 12 mg P kg-1 (the range defined by the C-N and L-P models) indicated that STP ranges varying from 22 to 34 to 24 to 36 mg kg-1 correspond to a 95% mean relative yield response. Thus, an interpretation category for the M3-ICP test encompassing 25 to 35 mg P kg-1 would approximately correspond to the Optimum interpretation category currently used for the M3-COL and BP tests. The data do not support a narrower range.
Because the difference between the M3-ICP and M3-COL tests is caused by the use of a different method for measuring extracted P, it is reasonable to assume that interpretations for the M3-ICP derived for corn in this study would also apply to other Iowa crops for which the same interpretation class and P fertilization criterion are used. The same optimum category for both the M3-COL and BP tests and a P recommendation for only maintenance fertilization (based on expected P removal in harvested products) are used for corn grown for grain or silage, soybean, sorghum [Sorghum bicolor (L.) Moench], oat (Avena sativa), sunflower (Helianthus annus), and several forages used for hay or pasture.
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CONCLUSIONS
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The results showed that M3-COL and M3-ICP tests had similar capacity to predict corn response to P fertilization but the M3-ICP test measured more soil P. The M3-COL test measured only slightly higher (2 mg kg-1 on average) P than the BP test, which confirms previous Iowa research. The absolute difference between the M3-ICP and M3-COL tests was not correlated with the soil P level, ranged from 0 to 28 mg kg-1 across sites and the mean difference was 12 mg kg-1, and was negatively but poorly correlated with soil pH or organic C. These results indicate that the M3-ICP test should be considered as a different test from the traditional M3-COL test, and its interpretations should be based on separate field correlations with yield response. Critical concentration ranges across all soils defined by the C-N and L-P models were 20 to 32 mg kg-1 for the M3-ICP test, 16 to 21 mg kg-1 for the M3-COL test, and 13 to 20 mg kg-1 for the BP test. The M3-ICP and M3-COL tests were better predictors of crop response to P than the BP test in one high-pH, CaCO3–affected soil. A range of 25 to 35 mg kg-1 for the M3-ICP test would approximately correspond to the current optimum class of 16 to 20 mg kg-1 used in Iowa for the M3-COL and BP tests and several crops.
Received for publication August 27, 2002.
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ABSTRACT
INTRODUCTION
