Soil testing for phosphorus: comparing the Mehlich 3 and Colwell procedures for soils of south-western Australia.
When first cleared for agriculture, most soils of south-western Australia were acutely deficient in phosphorus (Wild 1958; McArthur 1991). Regular applications of superphosphate and ammonium phosphate fertilisers have been required first to increase and then to maintain the P status of the soils for profitable agricultural production (Bolland 1998). The Colwell (1963) procedure, a modified version of the original Olsen et al. (1954) sodium bicarbonate soil test for E is used in Western Australia to estimate the current P status of soils when providing P fertiliser advice.
Many different procedures are used to test for different elements in soils. The procedures use different reagents, soil:solution ratios, extraction times, and different procedures to measure the concentration of the element in the extract. The Colwell procedure only considers P, it has a 16-h extraction procedure, and concentration of P in the extract is measured colourimetrically. By contrast, the Mehlich 3 procedure (Mehlich 1984; Wolf and Beegle 1995) can be used to test for multiple elements, it has a 5-min extraction time, and the concentration of all the elements in the extract solution are measured at the same time using either UV-visible spectrometry or inductively coupled plasma-atomic emission spectrometry (ICP-AES). The ICP-AES procedure is widely used by commercial laboratories. Consequently, the Mehlich 3 procedure is potentially a cheap, multi-element soil testing procedure presently being evaluated in south-western Australia. As part of the evaluation, the study reported here was done to assess if the Mehlich 3 procedure could be used instead of the Colwell procedure for soil P testing in the region.
Materials and methods
Soil samples for this study were collected from plots of various field experiments carried out in southwestern Australia (for details see Table 1). After the soils had been used to measure soil test P in the original experiments, subsamples were stored in the air-dry state at 10-30[degrees]C for 5-23 years. The <2-ram fraction of the top 10 cm of soil was collected, stored, and used for this study.
The original long-term field experiments measured the residual value of superphosphate and phosphate rocks as determined using plant yield and Colwell soil test P, or compared how different crop and pasture species used P from superphosphate for plant production. In the field experiments, different amounts of superphosphate and phosphate rock were applied once only to plots that were 1.44-4.2 m wide and 30-70 m long (Table 1). The fertiliser was spread over the soil surface (topdressed) and either left there in some pasture experiments, or incorporated into the top 10 cm of soil with tines or a rotary hoe before sowing the first crop or pasture. Seed was sown 2-5 cm deep in rows 18 cm apart. When sown to crops in the years after P application, the top 10 cm of soil was cultivated with tines down the length of each plot to control weeds before cropping, so incorporating the original P treatments further into the top 10 cm of soil.
The soil samples used in this study were originally collected each January-February from the field experiments in the years after each experiment started. The samples were collected using 2.5-cm-diameter metal tubes that were pushed into the top 10 cm of soil. Samples (30 50) of soil were collected from random locations within each plot and bulked. The <2-ram fraction of soil was used for the original studies, and subsamples were stored in the air-dry state until used for this study.
The soil test P values measured in this study were related to plant yields measured in the original field experiments. Soil test P values were related to plant yields measured later in the year during which the original soil samples were collected. For example, Expt I started in 1976. Soil samples were collected from Expt 1 during January February 1978, 1979, and 1980 and were stored in the air-dry state until 2001 before being used for this study. The soil test P values measured in 2001 on subsamples of soil collected in January February 1978 were related to yields of pasture measured in Expt 1 during August 1978. Likewise. soil test P measured in 2001 on subsamples of soil collected from Expt 1 during January February 1979 and 1980 were related to pasture yields measured in the field experiment during August 1979 and 1980.
Measuring soil test P in this study
Colwell soil test P was measured as outlined by Colwell (19633. Soil (1 g) was mixed with 100 mL of 0.5 mol/L of sodium bicarbonate (w/v) at pH 8.5 for 16 h at 23[degrees]C on an end-over-end shaker (10 r.p.m.). The soil and solution were separated by centrifugation and the concentration of P in the extract solution was measured colourimetrically using the automated procedure described by Colwell (1965).
Soil test P was measured by the Mehlich 3 procedure as described by Mehlich (1984). Soil (2.5 g) was mixed with 25 mL of solution containing (0.2 mol/L of acetic acid + 0.25 mol/L of ammonium nitrate + 0.015 mol/L of ammonium fluoride + 0.13 mol/L of nitric acid + 0.001 mol/L of EDTA) at pH 2 at 23[degrees]C for 5 min. The soil and solution were separated by centrifugation, and the concentration of P in the extract solution was measured in 2 ways:
(1) By UV-visible spectrophotometry (882 nm), hereafter called colourimetric Mehlich 3, which measured inorganic forms of P extracted from the soil.
(2) By ICP-AES, hereafter called ICP-AES Mehlich 3, which measured both inorganic and organic (total) extracted forms of P.
Results and discussion
Soil test values for the colourimetric and ICP-AES Mehlich 3 procedures were very similar (Fig. 1). This was so for soil treated with single or triple superphosphate and phosphate rock. Differences between the 2 values were not significantly different (P > 0.05), with the ICP-AES values usually tending to be slightly larger. For soil treated with either superphosphate or phosphate rock, linear regression equations fitted separately to either the ICP-AES or colourimetric Mehlich 3 values, or when data for both the ICP-AES and colourimetric Mehlich 3 values were fitted to the same equation, were very similar (see legend to Fig. 1). There was no significant decrease in the residual sum of squares if data were fitted to separate equations or to the same equation, confirming the data were very similar. The ICP-AES procedure is widely used by commercial soil and tissue testing laboratories and so the procedure is likely to be used to measure soil test P by the Mehlich 3 procedure in Western Australia.
[FIGURE 1 OMITTED]
The relationship between Mehlich 3 and Colwell P values was different for soil treated with superphosphate and phosphate rock (Fig. 2). The Mehlich 3 reagent is acidic (pH 2), and so in addition to extracting some P that was sorbed by the soil following dissolution of P from phosphate rock, the reagent is also likely to have dissolved unreacted phosphate rock present in the soil samples. Consequently, the Mehlich 3 procedure probably overestimated the supply of P to plants from soil treated with phosphate rock in a previous year. By contrast, the Colwell reagent is alkaline (pH 8.5) and so extracted much less P from soil than the Mehlich procedure. Previous studies have shown that bicarbonate soil test procedures (Olsen, Colwell) extract less P from soil treated with apatite phosphate rocks than from soil treated with superphosphate (Bolland and Gilkes 1992; Roberts et al. 1994). Previous studies, using Expts 8, 9, and 10 reported here, have shown that for up to 5 years after application most phosphate rock is present in the soils as undissolved fertiliser (Kumar et al. 1993, 1994a)
[FIGURE 2 OMITTED]
Previous studies have shown that, for diverse soil test procedures, different calibrations relating plant yields to soil test P are usually required for each procedure for soil treated with superphosphate and phosphate rock in Western Australia (Bolland and Allen 1987; Bolland et al. 1987, 1988a, 1989; Kumar et al. 1992, 1994b; Bolland 1993) and elsewhere (Chien 1979; Roberts et al. 1994; Menon and Chien 1995). This has been attributed to the different fertiliser types (superphosphate, apatite phosphate rock) leaving different residues in the soil and different amounts of P being extracted by different soil test procedures from the residues (Kumar et al. 1992, 1994b).
For each sampling time of each experiment, for soil treated with the same fertiliser (superphosphate, apatite phosphate rock, Calciphos phosphate rock), Mehlich 3 values were well correlated with the corresponding Colwell values. Examples are shown for some experiments in Fig. 3 for soil treated with superphosphate. The relationship between Mehlich 3 and corresponding Colwell values were well described by a linear equation, and these are listed in the legend to Fig. 3.
[FIGURE 3 OMITTED]
For each sampling time of each experiment, soil test P values were related to plant yields measured later on in the year during which the soil samples were collected. For soil treated with superphosphate, the relationships were very similar for the 3 soil test procedures (Colwell, ICP-AES Mehlich 3, colourimetric Mehlich 3), as is shown for some of the data in Fig. 4 when one crop or pasture species was grown at each site in each year. The same result was obtained for each plant species when more than one plant species was grown at each site in each year (Fig. 5). By contrast, for soil treated with phosphate rock, the relationship differed for the Colwell and Mehlich 3 procedures (Fig. 6), for possible reasons provided above.
[FIGURE 4-6 OMITTED]
Research in south-western Australia has shown that all types of phosphate rock fertilisers (crandalite millisite phosphate rocks, low and highly reactive apatite phosphate rocks) are poorly effective relative to superphosphate for most soils, environments, and crop and pasture species in south-western Australia (Bolland et al. 1997). By contrast, the manufactured, solid, granulated superphosphate and ammonium phosphate fertilisers are very effective, convenient, and profitable to use in the region (Bolland and Gilkes 1998). Field studies in the region indicate that superphosphate and the ammonium phosphate fertilisers are equally effective per unit of applied P for crops and pastures (Bolland and Gilkes 1998). Superphosphate and ammonium phosphate fertilisers have similar soil test P calibrations (J. W. Bowden and M. G. Mason, unpublished data, Department of Agriculture, Western Australia). We therefore conclude that the Mehlich 3 procedure can be used instead of the Colwell procedure for soil P testing in the region.
Some of the long-term field experiments used fur this study compared the effectiveness of superphosphate and different phosphate rocks as P fertilisers for crop and pasture production in south-western Australia. In the experiments, fertiliser P was applied once only at the start of the experiments. Soil test P measured on soil samples collected in each January--February of each experiment were related to plant yields measured later on in the year in which the soil samples were collected, to provide soil P test calibrations. For the same site, fertiliser (superphosphate, apatite phosphate rock, Calciphos phosphate rock), soil test procedure (Colwell, Olsen, Bray 1, calcium acetate lactate, Truog, [P.sub.I]), and plant species (same cultivar of the same crop species, subterranean clover-based pasture), the calibration was often different in different years (Bolland and Allen 1987; Bolland and Gilkes 1992; Bolland et al. 1987, 1988a, 1989). This was attributed to the effect of different seasonal conditions on the following:
(1) soil test P;
(2) plant growth;
(3) the ability of plant roots to access P already present in the soil (no native P, so from fertiliser P applied at the start of the experiment);
(4) how the plants used the P taken from the soil to produce yield.
Soil test P (mg P/kg soil)
The same result was obtained in this study for the Colwell and Mehlich 3 procedures, as is shown for some of the data in Fig. 7. Our data therefore extend the previous findings to the Mehlich 3 procedure, not used in the previous research, and further confirm the findings for the Colwell procedure.
[FIGURE 7 OMITTED]
Critical soil test P is the soil test value that is related to 90% of the maximum yield and below which P deficiency is likely to reduce plant production (Moody and Bolland 1999). We estimated critical soil test P from the relationship between percentage of the maximum (relative) yield, as the dependent (y) axis, and soil test E as the independent (x) axis. Relative yield was calculated by dividing absolute yield by the maximum yield and expressing the result as a percentage. In most cases maximum yields were on well-defined maximum yield plateaus for the relationship (Fig. 4). The Cate and Nelson (1965) procedure was used to estimate critical soil test E For each harvest of each experiment, critical soil test P was similar for the 3 soil test procedures (Colwell, ICP-AES Mehlich 3, colourimetric Mehlich 3). However, for all harvests of the various experiments, critical soil test P varied, from-20 to 120 mg P/kg soil, with most being in the range 30-60 mg P/kg soil, depending on plant species, soil type, and season, for reasons already discussed, as is shown for some of the data in Fig. 8.
[FIGURE 8 OMITTED]
In our study, samples of soil collected from plots of field experiments were stored in the air-dry state for 5-23 years. In this study, subsamples taken from the stored samples were used to measure soil test P by the Colwell and Mehlich 3 procedures. In laboratory studies, Bramley el al. (19921 have shown that, when P is added to soil and the soil is air-dried, reaction between P and the soil continues. Bramley et al. (1992) suggested that the continuing reaction between P and air-dry soil could affect soil test P subsequently measured on the soil samples. To reduce this effect, Bramley et al. (19921) suggested that air-dry soil samples should be stored at as cold a temperature as possible, preferably <0[degrees]C. A previous study indicated no systematic changes in Colwell soil test P when soil samples collected from field experiments in south-western Australia were stored in the air-dry state at 10-30[degrees]C for up to 17 years (Bolland et al. 19941. In the study reported here, soil samples collected from field experiments in 1978, 1979, and 1980 were used to provide subsamples to measure Colwell soil test P in: (1) The years the soil samples were collected (1978, 1979, and 1980); (2) 2001, after storage for 21-23 years of the subsamples of soil in the air-dry state at 10 30[degrees]C.
Within the errors typically associated with soil test P field studies, the results showed no systematic decrease in Colwell P measured on subsamples of soil that had been stored for 21-23 years (Table 2), supporting the results of our previous research (Bolland et al. 1994). Continued reaction of P with air-dried soil during storage is not an issue for the study reported here. This is because we compared soil test P measured at about the same time in 2001 by the Colwell and Mehlich 3 procedures. The soil test values were measured on different subsamples of soil collected from the same soil sample, with Colwell P being measured on some subsamples, and Mehlich 3 P on the other subsamples. That is, soil test P was for subsamples of soil taken from soil samples that had been treated identically up to that time. Table 2 indicates the Colwell P values were similar whether measured in 2001 or 21-23 years earlier.
We used existing plant yields and stored soil samples from experiments done in previous years to compare the Mehlich 3 and Colwell procedures as soil P tests in Western Australia. Similar studies need to be undertaken for different soils and environments in the region, such as alkaline soils common in the low rainfall cropping areas. Other elements can be tested in a similar way to assess the multi-element Mehlich 3 procedure with standard test presently used in the region for the different elements.
As measured on subsamples of soil collected from field experiments and stored in the air-dry state at 10-30[degrees]C for 5-23 years, for soil treated with superphosphate: (1) Mehlich 3 soil test P values were closely correlated with Colwell soil test P values.
(2) When the soil test values were related to plant yields measured in the field later on during the year in which the soil samples were collected (the soil P test calibration), the relationship was similar for the Mehlich 3 and Colwell soil test P values.
For soil treated with phosphate rock, the Colwell procedure provided much smaller soil test values than the more acidic Mehlich 3 procedure, indicating separate soil test P calibrations would be required for the Colwell and Mehlich 3 procedures.
We conclude the Mehlich 3 procedure can be used instead of the Colwell soil test procedure for soils of south-western Australia fertilised with superphosphate and ammonium phosphate fertilisers. This is a fortunate result because research using 172 soils from south-western Australia has shown that the Mehlich 3 procedure is a promising multi-element soil test procedure for the region (D. G. Allen and K. S. Walton, unpublished data).
Table 1. Location, soil type and profile form, some soil properties measured on soil samples of the <2-mm fraction of the top 10 cm of soil collected before the experiments began, some experimental details, and publications presenting further experimental details and results of the original experiments Experiment 1 2 Location Mt Barker Newdegate Soil type Loamy sand Sandy gravel Soil Profile Form A KS-Uc5.22 KS-Uc4.11 p[H.sub.Ca.sup.B] 5.5 4.9 P buffer capacity (mg/kg) C 30 9 Colwell P (mg/kg) D 2 2 Total P (mg/kg) E 80 52 %Clay F 10 6 %Organic carbon G 2.60 2.10 Total nitrogen H 0.100 0.070 P fertiliser J SSP SSP Year P applied K 1976 1976 Method of P application L TD TD INC Further incorporation M No Yes Year soil samples collected N 1978 1979 1979 1980 1980 Plot size 2.1 by 50 m 2.1 by 50m Publication O 1 2 Experiment 3 4 Location WonganHill WonganHills s Soil type Sandy loam Sandy loam Soil Profile Form A Gn2.21 Gn2.21 p[H.sub.Ca.sup.B] 5.4 5.3 P buffer capacity (mg/kg) C 2 2 Colwell P (mg/kg) D 2 6 Total P (mg/kg) E 38 146 %Clay F 5 4 %Organic carbon G 0.53 0.75 Total nitrogen H 0.028 0.031 P fertiliser J SSP SSP Year P applied K 1976 1976 Method of P application L TD INC TD INC Further incorporation Yes Yes Year soil samples collected N 1978 1980 1978 1980 Plot size 2.1 by 70 m 2.1 by 50 m Publication O 2 3 Experiment 5 6 Location Newdegate Mt Barker Soil type Sandy Loamy sand gravel Soil Profile Form A KS-Uc4.11 KS-Uc5.22 p[H.sub.Ca.sup.B] 5.0 5.4 P buffer capacity (mg/kg) C 9 30 Colwell P (mg/kg) D 4 2 Total P (mg/kg) E 130 85 %Clay F 5 8 %Organic carbon G 2.71 2.60 Total nitrogen H 0.076 0.100 P fertiliser J SSP SSP Year P applied K 1976 1977 Method of P application L TD INC TD Further incorporation Yes No Year soil samples collected N 1978 1980 1982 Plot size 2.1 by 50 m 2.1 by 30 m Publication O 3 4 Experiment 7 8 Location New Norcia West Dale Soil type Sandy Sandy gravel gravel Soil Profile Form A KS-Uc4.21 KS Uc4.21 p[H.sub.Ca.sup.B] 5.3 5.1 P buffer capacity (mg/kg) C 10 8 Colwell P (mg/kg) D 2 2 Total P (mg/kg) E 90 67 %Clay F 6 7 %Organic carbon G 2.10 3.70 Total nitrogen H 0.073 0.090 P fertiliser J SSP Cal TSP QPR NCPR Cal Year P applied K 1977 1984 Method of P application L TD INC TD RH Further incorporation Yes Yes Year soil samples collected N 1979 1980 1996 Plot size 2.1 by 60 m 2 by 30 m Publication O 5 6,7,8 Experiment 9 10 Location South Gibson Carrabin Soil type Sandy loam Sandy gravel Soil Profile Form A Uc4.21 Dy5.82 p[H.sub.Ca.sup.B] 5.0 4.9 P buffer capacity (mg/kg) C 4 3 Colwell P (mg/kg) D 2 2 Total P (mg/kg) E 21 15 %Clay F 12 6 %Organic carbon G 0.47 1.00 Total nitrogen H 0.026 0.032 P fertiliser J TSP NCPR TSP CPR QPR Year P applied K 1984 1984 Method of P application L TD RH TD RH Further incorporation Yes Yes Year soil samples collected N 1996 1996 Plot size 2 by 30 m 2 by 30 m Publication O 6,7,8 6,7,8 Experiment 11 12 Location Badgingarra North Bannister Soil type Sand Sandy gravel Soil Profile Form A Uc2.21 KS-Uc4.11 p[H.sub.Ca.sup.B] 5.3 6.0 P buffer capacity (mg/kg) C 1 18 Colwell P (mg/kg) D 2 2 Total P (mg/kg) E 60 88 %Clay F 1 8 %Organic carbon G 0.24 3.20 Total nitrogen H 0.015 0.094 P fertiliser J SSP SSP Year P applied K 1985 1985 Method of P application L TD INC TD INC Further incorporation Yes Yes Year soil samples collected N 1986 1986 Plot size 4.2 by 40 m 2.1 by 30 m Publication O 9 10 Experiment 13 14 Location North West Dale Cunderdin Soil type Loamy clay Sandy gravel Soil Profile Form A Db3.12 KS-Uc4.21 p[H.sub.Ca.sup.B] 6.0 6.3 P buffer capacity (mg/kg) C 4 27 Colwell P (mg/kg) D 17 2 Total P (mg/kg) E 70 70 %Clay F 20 5 %Organic carbon G 1.60 2.33 Total nitrogen H 0.070 0.085 P fertiliser J SSP SSP Year P applied K 1986 1989 Method of P application L TD INC TD Further incorporation Yes No Year soil samples collected N 1987 1993 Plot size 1.44 by 40 m 2 by 50 m Publication O 11 12 A Northcote (1979). B 1:5 soil: 0.01 M CaCl2, w/v (Rayment and Higginson 1992). C P buffer capacity of Ozanne and Shaw (1968), which is P sorbed by soil between 0.25 and 0.35 g L/L. D Colwell (1963). E Measured by digesting soil in concentrated sulfuric acid. F Plummet method as modified by Loveday (1974). G Walkley and Black (1934). H Kjeldahl method. J SSP, single superphosphate; TSP, triple superphosphate; Cal, Calciphos; NCPR, North Carolina phosphate rock; QPR, Queensland (Duchess) phosphate rock; for experiment 7, soil treated with TSP used in 1979, and with Cal in 1980. K Fertiliser applied April-June in years listed. L TD, fertiliser spread over the soil surface (topdressed, TD) and left there; TD INC, fertiliser topdressed and then incorporated into the top 10 cm of soil with tines; TD RH, fertiliser topdressed and then incorporated into the top 10 cm of soil with a rotary hoe. M after the first year of the experiment, when the plots were sown to crops, the top 10 cm of soil was cultivated with tines to control weeds before sowing crops in May which would have incorporated fertiliser further into the top 10 cm of soil. N Soil samples collected January-February in the years listed. O1, Bolland et al. 1984; 2, Bolland et al.1988a; 3, Bolland and Bowden 1984, 4, Bolland and Bowden 1984; 5, Bolland 1985; 6, Bolland et al 1986; 7, Bolland et al 1988b; 8, Bolland and Gikes 1995; 9, Bolland 1992a; 10, Boland 1992b; 11, Bolland and Paynter 1992; 12, Barrow et al. 1998. Table 2. Colwell soil test P (mg P/kg soil) values measured twice on soil samples collected from the field during 1978, 1979, and 1980, either in the year the samples were collected, or in 2001, 21-23 years after the samples had been stored in the air-dry state at 10-30[degrees]C Data are mean of 3 replications Year soil sample collected: 1978 1979 Year Colwell P measured: 1978 2001 1979 Expt 1, superphosphate-treated soil 4 5 3 9 9 11 14 16 15 24 20 25 78 62 73 l.s.d. (P = 0.05) 22 17 25 Expt 2, superphosphate-treated soil 3 10 15 52 133 157 l.s.d. (P = 0.05) 38 Expt 3, superphosphate-treated soil 2 2 58 73 89 110 109 120 159 160 l.s.d.(P = 0.05) 41 37 Expt 4, superphosphate-treated soil 2 10 9 13 20 36 1.s.d. (P = 0.05) 8 Expt 7, superphosphate-treated soil in 1979, Calciphos-treated soil in 1980 3 8 12 20 55 86 l.s.d. (P = 0.05) 21 Year soil sample collected: 1979 1980 Year Colwell P measured: 2001 1980 2001 2 5 7 10 9 13 15 10 22 21 23 32 57 46 65 l.s.d. (P = 0.05) 19 17 15 4 2 2 12 5 8 19 14 19 53 30 36 135 130 120 200 140 140 l.s.d. (P = 0.05) 50 30 24 3 5 11 12 23 16 50 33 62 63 l.s.d.(P = 0.05) 18 15 2 2 2 8 6 7 9 7 9 12 9 11 21 16 17 43 33 33 1.s.d. (P = 0.05) 6 7 4 Expt 7, superphosphate-treated soil in 1979, Calciphos-treated soil in 1980 5 3 5 8 8 6 12 12 10 18 17 13 36 18 16 87 21 21 l.s.d. (P = 0.05) 17 5 4
Funds were provided by the Government of Western Australia and the Grains Research and Development Corporation (project CHC20). Comments of anonymous referees helped to improve this paper.
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Manuscript received 13 December 2002, accepted 24 March 2003
M.D.A. Bolland (A, C), D. G. Allen (B), and K. S. Walton (B)
(A) Department of Agriculture, PO Box 1231, Bunbury, WA 6231, Australia; and School of Plant Biology, The University of Western Australia, 35 Stirling Highway. Crawley, WA 6009, Australia.
(B) Chemistry Centre (WA), 125 flay Street, East Perth. WA 6004, Australia.
(C) Corresponding author; email: firstname.lastname@example.org
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|Author:||Bolland, M.D.A.; Allen, D.G.; Walton, K.S.|
|Publication:||Australian Journal of Soil Research|
|Date:||Nov 1, 2003|
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