Comparison of soil test phosphorus methods in neutral to calcareous Manitoba soils

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1 Comparison of soil test phosphorus methods in neutral to calcareous Manitoba soils D. V. Ige, O. O. Akinremi 1 D. Flaten, and M. A. Kashem Department of Soil Science, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2. Received 14 June 2005, accepted 24 February Ige, D. V., Akinremi, O. O., Flaten, D. and Kashem, M. A Comparison of soil test phosphorus methods in neutral to calcareous Manitoba soils. Can. J. Soil Sci. 86: Increasing concern for the amount of P entering lakes in Manitoba may lead to regulation of P concentration in agricultural soils. A possible means for this regulation is the use of soil test P. This may require a means of comparing soil test P analyses as various laboratories in Manitoba employ different methods of soil test P determination. Thus, the objectives of this study were to (i) compare the methods of P determination in Manitoba soils, and (ii) develop equations for converting different soil test P methods from one to another. One hundred and fifteen archived surface soil samples representing major soils of Manitoba were used for the study. Soil test P was determined in these soils using the original Kelowna (K1) method and the two modified Kelowna methods (K2 and K3). Mehlich-3, Olsen- and water extractable-p were also determined for all soils. The results were analyzed statistically and were related using a simple regression analysis model. The amount of P extracted by the different extracting agents varied widely. Mehlich-3 extracted the largest amount of P (a range of mg kg 1 ) while water extracted the smallest amount (a range of mg kg 1 ). The P extracted by the Olsen and the three Kelowna methods were intermediate between Mehlich-3 and water- P. Our results also showed that the three Kelowna methods were not significantly different from one another. The Olsen method compared well with those of the modified Kelowna methods, but extracted less P than the original Kelowna and Mehlich-3 methods. Overall, the different agronomic soil test P methods were well correlated to each other with correlation coefficients (r) ranging between 0.95 and However, the correlations between these soil test methods and water extractable P was not as high. Water extractable P and the agronomic soil test P methods were better related by a non-linear relationship than by a linear relationship. The coefficient of determination (R 2 ) for all the regression equations relating the different agronomic soil test P methods ranged from 0.91 to As such, the equations generated in this study can be used to convert the result from one soil test P method to another. Key words: Kelowna extractable phosphorus, Mehlich-3 extractable phosphorus, Olsen extractable phosphorus, water extractable phosphorus Ige, D. V., Akinremi, O. O., Flaten, D. et Kashem, M. A Comparaison des méthodes de dosage du phosphore dans les sols neutres à calcaires du Manitoba. Can. J. Soil Sci. 86: Les craintes grandissantes concernant la quantité de P susceptible de pénétrer dans les lacs du Manitoba pourraient aboutir à une régulation de la concentration de P sur les terres agricoles. Un moyen de régulation éventuel serait de doser le P dans le sol. Pour cela cependant, on devra peut-être comparer les méthodes d analyse du P, car elles varient d un laboratoire à l autre. La présente étude devait i) comparer les méthodes d analyse du P appliquées aux sols manitobains et ii) élaborer des équations permettant de convertir les résultats de l une à l autre. Pour mener à bien leur projet, les auteurs ont recouru à 115 échantillons de sol de surface en archive représentant les principaux sols du Manitoba. La concentration de P dans les échantillons a été déterminée par la méthode de Kelowna (K1) classique et ses deux variantes (K2 et K3). On a aussi calculé la quantité de P par les méthodes Mehlich-3, Olsen et du P extractible dans l eau. Les résultats ont ensuite fait l objet d une analyse statistique et puis ont été reliés au moyen d un simple modèle d analyse par régression. La quantité de P recueillie grâce aux différents réactifs varie considérablement. La méthode Mehlich-3 est celle qui permet l extraction de la plus grande quantité de P (de 5,4 à 200 mg par kg) tandis que l extraction à l eau en récupère la quantité la plus faible (de 0,2 à 70 mg par kg). La quantité de P extraite par les méthodes Olsen et les trois variantes de la technique Kelowna se situe entre les deux. Les résultats indiquent aussi que les trois variantes Kelowna ne diffèrent pas sensiblement entre elles. La méthode Olsen se compare bien aux deux méthodes Kelowna modifiées, mais la quantité de P récupérée est inférieure à celle extraite avec la méthode classique et avec la méthode Mehlich-3. Dans l ensemble, les diverses méthodes agronomiques de dosage du P du sol sont bien corrélées, le coefficient de corrélation (r) se situant entre 0,95 et 0,98. Il n existe toutefois pas de grande corrélation entre ces méthodes et celle du dosage du P extractible dans l eau. Le lien entre le dosage du P extractible dans l eau et les méthodes agronomiques d analyse du P du sol s explique mieux par une relation non linéaire que linéaire. Le coefficient de détermination (R 2 ) des équations de régression se rapportant aux diverses méthodes agronomiques d analyse du P du sol varie de 0,91 à 0,97. Les équations élaborées dans le cadre de cette étude peuvent donc servir à convertir les résultats d une méthode de dosage du P à l autre. Mots clés: Phosphore extractible par la méthode Kelowna, phosphore extractible par la méthode Mehlich-3, phosphore extractible par la méthode Olsen, phosphore extractible dans l eau 1 To whom correspondence should be addressed: (akinremi@ms.umanitoba.ca). 691 Abbreviations: P K3, Kelowna-3 extractable P; P K2, Kelowna-2 extractable P; P K1, Kelowna-1 extractable P; P Ols, Olsen extractable P; P M3, Mehlich-3 extractable P; P W, water extractable P

2 692 CANADIAN JOURNAL OF SOIL SCIENCE There is an increasing environmental concern over the amount of P entering lakes in Manitoba. This concern may lead to the regulation of P concentration in agricultural soils, since soil P levels influence the amount of P in runoff and subsurface drainage (Sharpley et al. 1985; Heckrath et al. 1995; Pote et al. 1996; Sims et al. 1998a; Ige et al. 2005). One of the ways by which P can be regulated is by setting a threshold for soil test P. However, the development and implementation of such a regulation requires a means of comparing the soil test P values from various laboratories that analyze soils in Manitoba. Agronomic soil testing methods measure plant nutrients that are expected to become available from soil during the growing season and, consequently, serve as a guide in making fertilizer recommendations for optimum crop yield. Thus, soil testing is a valuable tool for identifying the economically optimum rates of nutrient addition required by most crops. The primary requirement of an extractant for agronomic testing is that the amount of nutrient extracted must be closely related to the nutrient availability to the crop. However, nutrient availability to plants is dependent on soil properties such as ph, organic matter content, soil texture and the cation exchange capacity. Hence, these properties must be taken into consideration when selecting an extracting agent (Sims et al. 1998b). As such, a wide range of extracting solutions have been employed for soil test P, ranging from dilute, to strong acid solutions, to buffered alkaline solutions. Olsen extracting solution, 0.5 N sodium bicarbonate solution with ph adjusted to 8.5, was developed for mildly acidic to alkaline ph soils and has been widely used for calcareous soils (Olsen et al. 1954). For the highly weathered acid to neutral soils, with low cation exchange capacity, Mehlich-1 extracting solution (0.05 N hydrochloric acid and N sulphuric acid) has been developed (Mehlich 1953). Mehlich-3 extractant is a combination of acids (acetic and nitric acids), salts (ammonium fluoride and ammonium nitrate), and a chelating agent (ethylene diamine tetraacetic acid) and is appropriate for extracting P and other elements from a wide range of soils, from acid to calcareous soils (Mehlich 1984). Kelowna extracting reagent (Kelowna-1) was developed in British Columbia for multi-element extraction. It was intended for use in calcareous and non-calcareous soils of neutral to alkaline ph. The reagent is a mixture of 0.25 M acetic acid and M ammonium fluoride (Van Leirop 1988). This was later modified (Kelowna-2) by the inclusion of 0.25 M ammonium acetate (Qian et al. 1994) and further modification was made by Ashworth and Mrazek (1995) who increased the ammonium acetate and acetic acid concentrations to 1.0 M and 0.5 M, respectively (Kelowna-3). Since extracting agents have different ability to extract different forms and amounts of P, the choice of an ideal method has been based on regional understanding of soil properties and the solubility of the forms of P prevalent in the region. The Olsen method was the traditional method for P analysis in Manitoba soils due to their calcareous origin; and Mehlich-3 is used rarely, if at all, for making fertilization recommendation. Most western Canadian soil testing laboratories use either the Kelowna method or one of the modified Kelowna methods. Although the results obtained from the different soil tests are related to plant available P, the actual quantities of extractable P can differ widely. To effectively manage P both for agricultural and environmental sustainability, there should be a means of comparing the soil test P from various laboratories in Manitoba. Studies in Alberta showed that soil test P methods were generally well correlated for Alberta soils (Zhang et al. 2004; McKenzie et al. 2003) but no such studies exist for neutral to calcareous soils that are typical of Manitoba. Thus, the objectives of this study were to (i) compare the different methods of soil test P in Manitoba s predominantly neutral to calcareous soils; and, (ii) develop equations for converting the analysis from one soil test P method to another. MATERIAL AND METHODS One hundred and fifteen (115) archived surface (0 15 cm) soil samples from distinct soil groups across the province of Manitoba were obtained from the Manitoba Soil Survey Department. The soils were archived from different Soil Survey research studies between 1974 and 1997 and were used for P sorption studies by Ige et al. (2005). The soils are representative of major Manitoba soils covering a wide range of soil properties including texture, organic matter and carbonate content. The selected soils include the wet sands in the southeast corner (24 samples), the dry sands in western Manitoba (12 samples), the high lime tills in the Interlake region (22 samples), the clay soils along the Red River valley (30 samples) and the regional till loams (27 samples). The description of the soil is provided in Table 1. The physical and chemical properties of the soils such as soil texture, ph, organic matter and carbonate content were obtained from the Agricultural Resources Section, Manitoba Agriculture, Food and Rural Initiatives. Exchangeable cations and the cation exchange capacity of the soils were determined using the ammonium acetate method (McKeague 1978). Extractable P was determined by five methods used for P analysis in Manitoba, namely Olsen P, Mehlich-3 P, Kelowna-1 P (original Kelowna method), Kelowna-2 P (modified Kelowna method adopted by Enviro-Test laboratory) and Kelowna-3 P (modified Kelowna method adopted by Norwest laboratory). Mehlich-3 extractable P (P M3 ) was determined by shaking 2.5 g of air dried soil samples with 25 ml of Mehlich-3 extracting reagent for 5 min and filtering the suspension through Whatman No. 40 filter paper (Mehlich 1984). Olsen extractable P (P Ols ) was obtained by shaking 1.0 g of soil with 20 ml of 0.5 N sodium bicarbonate solution (ph of 8.5) in the presence of 0.25 g of charcoal, for 30 min (Olsen and Sommers 1982). Kelowna-1 P (P K1 ) was extracted by equilibrating 2.5 g soil with 25 ml of Kelowna extracting solution (0.25 M acetic acid and M ammonium fluoride). This was shaken for 15 min and the suspension was filtered to obtain a clear solution for P determination (Van Lierop 1988). For Kelowna-2 P (P K2 ), 2.5 g of soil was shaken with 25 ml of Kelowna-2 reagent (0.015 M ammonium fluoride, 0.25 M ammonium acetate, and 0.25 M acetic acid) for 15 min on a reciprocal shaker. The suspension was filtered, and the clear solution stored for P deter-

3 IGE ET AL. PHOSPHORUS SOIL TEST METHODS 693 Table 1. The description of the soils used in the study and the year of sampling Year of Year Soil no. Soil classification sampling Soil no. Soil classification of sampling Soil 1 Rego Humic Gleysol 1986 Soil 61 Orthic Dark Gray soil 1984 Soil 2 Orthic Black soil 1987 Soil 62 Gleyed Rego Black carbonated soil 1988 Soil 3 Orthic Black soil 1987 Soil 63 Gleyed Rego Black carbonated soil 1994 Soil 4 Gleyed Regosol 1995 Soil 64 Gleyed Rego Black carbonated soil 1988 Soil 5 Gleyed Regosol 1995 Soil 65 Gleyed Rego Black carbonated soil 1989 Soil 6 Gleyed Regosol 1990 Soil 66 Gleyed Dark Gray Soil 1994 Soil 7 Peaty Rego Humic Gleysol 1989 Soil 67 Gleyed Dark Gray Soil 1987 Soil 8 Gleyed Black soil 1995 Soil 68 Rego Humic Gleysol 1990 Soil 9 Gleyed Rego Black soil 1987 Soil 69 Rego Humic Gleysol 1990 Soil 10 Gleyed Rego Black soil 1989 Soil 70 Gleyed Rego Black soil 1989 Soil 11 Gleyed Black soil 1987 Soil 71 Gleyed Rego Black soil 1996 Soil 12 Gleyed Black soil 1989 Soil 72 Orthic Black soil 1989 Soil 13 Gleyed Black soil 1987 Soil 73 Orthic Black soil 1989 Soil 14 Gleyed Rego Black soil 1987 Soil 74 Orthic Black soil 1989 Soil 15 Gleyed Rego Black soil 1990 Soil 75 Orthic Black soil 1989 Soil 16 Gleyed Rego Black soil 1990 Soil 76 Orthic Black soil 1989 Soil 17 Gleyed Rego Black soil 1990 Soil 77 Gleyed Carbonated Rego Black Soil 1988 Soil 18 Rego Humic Gleysol 1995 Soil 78 Gleyed Carbonated Rego Black Soil 1988 Soil 19 Gleyed Solonetzic Dark Gray soil 1989 Soil 79 Orthic Dark Gray soil 1990 Soil 20 Rego Humic Gleysol carbonated soil 1994 Soil 80 Gleyed Rego Black soil 1982 Soil 21 Gleyed Dark Gray Soil 1989 Soil 81 Rego Humic Gleysol 1987 Soil 22 Gleyed Dark Gray Soil 1989 Soil 82 Rego Humic Gleysol 1994 Soil 23 Rego Humic Gleysol 1990 Soil 83 Gleyed Regosol 1989 Soil 24 Gleyed Black Solonetz soil 1996 Soil 84 Gleyed Regosol 1990 Soil 25 Rego Humic Gleysol 1989 Soil 85 Gleyed Dark Gray Soil 1989 Soil 26 Gleyed Rego Black soil 1987 Soil 86 Gleyed Dark Gray Soil 1989 Soil 27 Gleyed Rego Black soil 1992 Soil 87 Gleyed Dark Gray Soil 1974 Soil 28 Gleyed Rego Black soil 1992 Soil 88 Gleyed Rego Black soil 1990 Soil 29 Rego Humic Gleysol 1989 Soil 89 Gleyed Rego Black soil 1986 Soil 30 Rego Humic Gleysol 1989 Soil 90 Gleyed Rego Black soil 1995 Soil 31 Orthic Dark Gray soil 1990 Soil 91 Gleyed Rego Black carbonated soil 1995 Soil 32 Orthic Dark Gray soil 1988 Soil 92 Gleyed Black soil 1992 Soil 33 Orthic Gray Luvisol 1990 Soil 93 Gleyed Black soil 1993 Soil 34 Gleyed Rego Black carbonated soil 1992 Soil 94 Gleyed Rego Black soil 1992 Soil 35 Rego Black soil 1991 Soil 95 Gleyed Rego Black soil 1991 Soil 36 Orthic Black soil 1988 Soil 96 Gleyed Rego Black soil 1993 Soil 37 Orthic Black soil 1990 Soil 97 Gleyed Regosol 1993 Soil 38 Orthic Black soil 1992 Soil 98 Gleyed Gray Luvisol 1980 Soil 39 Orthic Dark Gray soil 1988 Soil 99 Gleyed Black soil 1990 Soil 40 Orthic Dark Gray soil 1988 Soil 100 Gleyed Dark Gray Soil 1984 Soil 41 Orthic Black soil 1988 Soil 101 Gleyed Rego Black soil 1985 Soil 42 Orthic Black soil 1990 Soil 102 Gleyed Rego Black carbonated soil 1978 Soil 43 Orthic Black soil 1988 Soil 103 Gleyed Dark Gray Soil 1985 Soil 44 Rego Humic Gleysol 1996 Soil 104 Orthic Black soil 1988 Soil 45 Rego Black soil 1986 Soil 105 Orthic Black soil 1988 Soil 46 Rego Black soil 1986 Soil 106 Orthic Regosol 1994 Soil 47 Gleyed Rego Black carbonated soil 1986 Soil 107 Orthic Regosol 1994 Soil 48 Orthic Gray Luvisol 1983 Soil 108 Gleyed Dark Gray Soil 1989 Soil 49 Dark Gray Luvisol 1987 Soil 109 Eluviated Dystric Brunisol 1989 Soil 50 Gleyed Rego Black soil 1989 Soil 110 Eluviated Dystric Brunisol 1989 Soil 51 Calcareous Black soil 1990 Soil 111 Eluviated Eutric Brunisol 1988 Soil 52 Rego Humic Gleysol 1987 Soil 112 Orthic Dark Gray soil 1987 Soil 53 Gleyed Black soil 1983 Soil 113 Orthic Black soil 1995 Soil 54 Humic Luvic Gleysol 1996 Soil 114 Orthic Regosol 1987 Soil 55 Gleyed Rego Black soil 1995 Soil 115 Gleyed Gray Luvisol 1997 Soil 56 Gleyed Dark Gray Soil 1983 Soil 57 Gleyed Rego Black soil 1984 Soil 58 Rego Black soil 1988 Soil 59 Rego Black soil 1988 Soil 60 Rego Black soil 1994 mination (Qian et al. 1994). Kelowna-3 P (P K3 ) was obtained by extracting 2.5 g of soil with 25 ml of Kelowna-3 extracting reagent (0.015 M ammonium fluoride, 1.0 M ammonium acetate and 0.5 M acetic acid). The suspension was shaken for 15 min and filtered (Ashworth and Mrazek 1995). Water soluble P (P W ) was determined by extracting 2 g of soil with 20 ml of deionised water, shaking the suspension for 1 h on a reciprocating shaker (Self-Davis et al. 2000). The suspen-

4 694 CANADIAN JOURNAL OF SOIL SCIENCE Table 2. Selected physical and chemical properties of the soils used in the study Soil properties z Minimum Maximum Mean Median ph (0.01 M CaCl 2 ) Carbonate (g kg 1 ) OC (g kg 1 ) Sand (g kg 1 ) Silt (g kg 1 ) Clay (g kg 1 ) Ex-Ca(cmol c kg 1 ) Ex-Mg(cmol c kg 1 ) Ex-Na(cmol c kg 1 ) Ex-K(cmol c kg 1 ) Ex-cat(cmol c kg 1 ) CEC(cmol c kg 1 ) z Ex-Ca, Ex-Mg, Ex-K and Ex-Na are exchangeable Ca, Mg, K and Na, respectively; Ex-cat is the sum of exchangeable cations; and CEC is the cation exchange capacity. Mean extractable P (mg kg -1 ) d c bc sion was centrifuged and filtered and the extract was stored in a refrigerator at 4 C for P determination. The P in the various soil extracts was determined colorimetrically using the molybdate blue method as described by Murphy and Riley (1962). The amount of P extracted by the various extracting agents was subjected to multiple mean comparison for mean differences using Jandel SigmaStat (1995), and the various methods were related using the regression analysis of SAS Institute, Inc. (2001). RESULT AND DISCUSSIONS Soil Extractable P A wide variation was observed in the soil test P values obtained by all the methods used in this study (Table 2). The soils were mainly alkaline with about 74% of all soils having a ph above 7. The values for P Ols ranged between 4.7 and mg P kg 1 while those for Mehlich-3 ranged bc ab P W P Ols P K3 P K2 P K1 P M3 P Extraction methods Fig. 1. Mean of P extracted by the different soil test P methods. (Means indicated with the different letters were significantly different (P < 0.05); P W = water extractable P; P Ols = Olsen extractable P; P M3 = Mehlich-3 extractable P; P K1 = Kelowna-1 extractable P, P K2 = Kelowna-2 extractable P and P K3 = Kelowna- 3 extractable P; n = 115). a between 5.4 and 200 mg P kg 1. These values ranged from very low to very high agronomic soil test P values. Considering the mean extractable P by the various methods, water extracted the least amount of P while Mehlich-3 reagent extracted the highest amount of P; however, of all the agronomic P extractants, Olsen extracted the least amount of P (Fig. 1). Sims (2000a) noted that Olsen extractant has less ability to remove P from the soil than the acidic extractants. The amount of P extracted by the various extractants increased with increasing acidity of the extracting solution. Mehlich-3 being the most acidic, extracted the greatest amount of P while Olsen, the most alkaline, extracted the least amount of P of all the agronomic soil test methods. This is probably due to the fact that as the ph of the extracting solution decreases, more of the phosphate associated with Ca is dissolved. The ability of Mehlich-3 to extract more P than the other extractants could account for the relatively high value of extractable P M3 set for optimum crop production. Sims (2000b) reported that the value of P M3 generally set for optimum crop growth and yield (45 50 mg P kg 1 ) is higher than the critical values used for other standard methods such as Olsen P (critical value set for Olsen P is 10 mg P kg 1 ; Sims 2000a). Comparison of the P Extracting Ability of Different Extractants Tukey s test for multiple means comparison showed that the means of P Ols and P M3 ; P M3 and P K2 ; P M3 and P K3 and P Ols and P K1 are significantly different at the 95% probability level, while no significant difference (P 0.10) was observed in the means of P K2 and P Ols ; P K2 and P K3 ; and P K3 and P Ols (Fig. 1). The lack of significant differences in the amount of P extracted by the three Kelowna methods is indicative of the similar mechanisms of extraction due to the similarities in their chemical composition. The differences among the P extraction methods probably arose from the fact that plant available P in the soil is not from a discreet fraction but from a continuum of fractions; extracting agents preferentially extract from different fractions depending on their reaction with soil components involved in P sorption (Council for Agricultural Science and Technology 2000). In addition, each extracting solution has a different ability to extract varying portions of soil P

5 IGE ET AL. PHOSPHORUS SOIL TEST METHODS 695 Table 3. Pearson correlation coefficient (r) z between the different methods of P extraction for the whole soil collection, soils with ph 7 and soils with ph > 7 P extraction methods y P W P Ols P M3 P K1 P K2 P K3 All soils (n = 115) x P W 1 P Ols P M P K P K P K Soils with ph 7 (n = 33) x P W 1 P Ols P M P K P K P K Soils with ph>7 (n = 82) x P W 1 P Ols P M P K P K P K z The correlations are all significant at P y P W = water extractable P; P Ols = Olsen extractable P; P M3 = Mehlich-3 extractable P; P K1 = Kelowna-1 extractable P, P K2 = Kelowna-2 extractable P and P K3 = Kelowna-3 extractable P. x Numbers in parentheses indicate the number of samples in each group. Table 4. Regression parameters for the relationships between the different soil test P methods z (n = 115) Y y X y Intercept Slope R 2 RMSE P Ols P M P Ols P K P Ols P K P Ols P K P Ols P W P M3 P K P M3 P K P M3 P K P M3 P W P K1 P K P K1 P K P K1 P W P K2 P K P K2 P W P K3 P W RMSE 1 2 Extractable P- Predicted P. n z All the regression equations are significant at P y P W = water extractable P; P Ols = Olsen extractable P; P M3 = Mehlich-3 extractable P; P K1 = Kelowna-1 extractable P, P K2 = Kelowna-2 extractable P and P K3 = Kelowna-3 extractable P. = ( ) because they were targeted at different pools of soil P (Zhang et al. 2004). The mean of P W is significantly smaller than the means of P from other extracting reagents. This is an indication that other extracting agents extracted some forms of labile P that are not immediately soluble in water but are potentially available to the plant during the entire growing season (Kuo 1996; Houba et al. 1997). Although water extractable P has been used for determining the P requirement of plants and Luscombe et al. (1979) reported a good correlation between P W and dry matter yield of crops, its use as an environmental indicator for estimating the risk of P loss to runoff is more prominent (Sharpley 1995; Pote et al. 1996; Ige et al. 2005). Correlation between the Different Soil Test P Methods The correlation coefficient (r) between the agronomic soil test methods ranged from 0.95 to 0.99 (Table 3). A correla-

6 696 CANADIAN JOURNAL OF SOIL SCIENCE Fig. 2. Linear and non-linear plot of agronomic soil test P against water extractable P. tion coefficient of 0.99 was obtained between P K2 and P K3, while the correlation coefficient between P Ols and P M3 was Wolf and Baker (1985) reported a similar relationship between Mehlich-3 P and Olsen P. The high correlation between the various soil test methods indicates that, even though each of the extracting agents extracted different proportion of available P, they are all capable of estimating P availability to plants. This is, however, not surprising since the soil system is in a state of dynamic equilibrium where the different forms of P are in equilibrium with soil solution P. Thus, even though Olsen was the traditional extractant used for P determination in Manitoba soils, the use of any of the three Kelowna methods was found to be adequate; however, the differences in the range of values obtained called

7 IGE ET AL. PHOSPHORUS SOIL TEST METHODS 697 Table 5. Regression parameters relating Olsen extractable P (P Ols ) to other extractable P for soils with P Ols level within the agronomic ( 30 mg kg 1 ; n = 82) and environmental ( 60 mg kg 1 ; n = 108) range z X y Intercept Slope R 2 RMSE Soils with P Ols 30 mg kg 1 P M P K P K P K P W Soils with P Ols 60 mg kg 1 P M P K P K P K P W RMSE 1 2 Extractable P- Predicted P. n z All the regression equations are significant at P y P W = water extractable P; P M3 = Mehlich-3 extractable P; P K1 = Kelowna-1 extractable P, P K2 = Kelowna-2 extractable P and P K3 = Kelowna-3 extractable P. = ( ) for caution in making decisions relating to soil P management. The correlations between P W and the other agronomic P extraction methods used in this study were lower than the correlations observed among the various agronomic extraction methods. The correlation coefficient between P W and the other extractable P methods ranged between 0.69 and 0.73 (compared with the range of 0.95 and 0.98 among the other extractants). Atia and Mallarino (2002) and Zhang et al. (2004) also reported a better correlation between different agronomic P test methods than between environmental P tests, such as water extractable P, and other agronomic methods for P. However, when the soils were partitioned based on soil ph, the correlation between agronomic soil test P methods and water extractable P for soils with ph less than 7 was greater than for the entire soil collection (Table 3). Such improvement was not observed among the agronomic soil test methods after ph partitioning. It may be that water extractable P, which is adopted more for environmental assessment of P, is more sensitive to soil variability than the other agronomic soil test P methods, especially for soils with ph less than 7. This may also be due to the presence of less soluble Ca phosphate in the high ph soils which was dissolved by the agronomic soil test methods but not extracted by water. Relationships among Soil Test P Methods Highly significant linear relationships (P 0.01) were observed among the different soil test P methods with coefficient of determination, R 2, values ranging between 0.91 and 0.97 (Table 4). Similar observations have been reported by other authors. For example, McKenzie et al. (1995) reported a coefficient of determination, R 2, of 0.95 between the Kelowna 1 method and the modified Kelowna (Kelowna 2) method. Zhang et al. (2004) working with a set of Alberta soils, also reported a high regression coefficient between the various P extraction methods. The significant linear relationship between the soil test methods indicates that the results from one soil test P method can be converted to the other using the equations generated in this study. The coefficient of determination for the linear relationship between agronomic soil test P and water extractable P was smaller than that obtained among the agronomic soil test P methods (Table 4). The coefficient of determination, R 2, for agronomic soil test P with water extractable P was improved by adopting a binomial model. For example, the linear equation relating P K1 with P W produced an R 2 value of 0.53 while the binomial equation gave an R 2 value of 0.70 (Fig. 2). A similar curvilinear relationship was reported by Atia and Mallarino (2002) and was attributed to improved relative efficiency of P extraction by water as soil P concentration increased. This may also account for the increasing risk of P loss to runoff as soil P concentration increases. When soils with Olsen extractable P within the agronomic (P Ols 30 mg kg 1 ) and environmental (P Ols 60 mg kg 1 ) ranges were considered, the coefficient of determination was lower than that obtained for the whole soil collection; however, the error associated with the prediction was improved as the root mean square error (RMSE) was lower (Table 5). The agronomic threshold of P Ols 30 mg kg 1 was selected as the level of P beyond which further P addition to agricultural soils will not be recommended (Johnston and Roberts, 2001), while the environmental threshold of P Ols 60 mg kg 1 is the level of P beyond which significant loss of P to runoff is likely to occur (Heckrath et al. 1995; McDowell et al. 2001). Table 6 showed the measured P Ols and the predicted P Ols for 26 randomly selected soils using the conversion equation for the whole soil collection as well as the conversion equations for soil with P Ols within the agronomic and environmental ranges. No statistical difference (P = 0.46) was observed between the measured P Ols and the P Ols predicted from P M3 using the equation for the whole soil collection. While no significant difference (P 0.15) was observed in the measured and predicted P Ols (from P M3 ) using any of the conversion equations, the RMSE associated with the prediction increased when the equations were

8 698 CANADIAN JOURNAL OF SOIL SCIENCE Table 6. Measured and predicted P Ols for 26 selected soil subset z Lab no. Measured P Ols Predicted P Ols from P y M3 Predicted P Ols from P x M3 Predicted P Ols from P w M3 Soil Soil Soil Soil Soil Soil Soil Soil Soil Soil Soil Soil Soil Soil Soil Soil Soil Soil Soil Soil Soil Soil Soil Soil Soil Soil RMSE z P M3 = Mehlich-3 extractable P; P Ols = Olsen extractable P. Using conversion equation for all soil collection. x Using conversion equation for soils with P Ols 30 mg kg 1. w Using conversion equation for soils with P Ols 60 mg kg 1. applied to soils with P Ols outside the range for which the equations were developed (Table 6). Correlation between Soil Test P and Soil Properties Correlation analysis between the extractable P and soil properties was conducted to evaluate the influence of soil properties on P extractability and, consequently, P availability to plants. Of all the soil properties evaluated, exchangeable K was the only soil property that was significantly related to extractable P for all the 115 soils with r values ranging between 0.50 and 0.54 (P 0.05). The reason for the relationship between exchangeable K and extractable P is unknown. It may be that P and K in these soils have a common source such as the presence of soil minerals or addition of manure/fertilizer containing both elements, which account for their relationships. However, there is the need for further investigation to fully understand the relationship between the two soil parameters. CONCLUSION The results obtained from the various soil test P methods varied due to varying ability of the extracting solutions to extract P as a result of differences in their reactions with soil components controlling P availability in the soil. While sodium bicarbonate extracted the least amount of P, Mehlich-3 extracted the greatest amount of P. Significant relationships were found among the various extractants which makes it possible to convert the results from one soil test P method to another using equations generated in this study. ACKNOWLEDGMENT The funding for this study was provided by the Manitoba Phosphorus Expert Committee (MPEC). The contributions of Bob Eilers of Agriculture and Agri-Food Canada and Peter Haluschak of Manitoba Agriculture and Rural Initiatives in the selection and provision of archived soil samples is hereby gratefully acknowledged. Ashworth, J. and Mrazek, K Modified Kelowna test for available phosphorus and potassium in soil. Commun. Soil Sci. Plant Anal. 26: Atia, A. M. and Mallarino, A. P Agronomic and environmental soil phosphorus testing in soils receiving liquid swine manure. Soil Sci. Soc. Am. J. 66: Council for Agricultural Science and Technology Relevance of soil testing to agriculture and the environment. Issue Paper No. 15. CAST, Ames, IA. pp Heckrath, G., Brookes, P. C., Poulton, P. R. and Goulding, K. W. T Phosphorus leaching from soils containing different phosphorus concentrations in the Broadbalk experiment. J. Environ. Qual. 24: Houba, V. J. G., Van der Lee, J. J. and Novozamski, I Soil analysis procedures. Dept of Soil Sci. Plant Nutr., Landbouwuniversiteit, Wageningen Agricultural University, Wageningen, the Netherlands.

9 Ige, D. V., Akinremi, O. O. and Flaten, D. N Environmental index for estimating the risk of P loss in calcareous soils of Manitoba. J. Environ. Qual. 34: Jandel SigmaStat Jandel sigmastat statistical software. Version 2 (demonstration version). Jandel Corporation, San Rafael, CA. Johnston, A. M. and Roberts, T. L High soil phosphorus is it a problem in Manitoba? Proceedings of the 2001 Manitoba Agronomists Conference, Faculty of Agriculture and Food Sciences, University of Manitoba, Winnipeg, MB. pp Kuo, S Phosphorus. Pages in D. L. Sparks, ed. Methods of soil analysis. Part 3. Chemical methods. SSSA, Madison, WI. Luscombe, P. C., Syers, J. K. and Gregg, P. E. H Water extraction as a soil testing procedure for phosphate. Commun. Soil Sci. Plant Anal. 10: McDowell, R., Sharpley, A., Brookes, P. and Poulton, P Relationship between soil test phosphorus and phosphorus release to solution. Soil Sci. 166: McKeague, J. A Manual on soil sampling and methods of analysis. 2nd ed. Canada Soil Survey Committee, Ottawa, ON. McKenzie, R. H., Kryzanowski, L., Cannon, K., Solberg, E., Penney, D., Coy, G., Heaney, D., Harapiuk, J. and Flore, N Field evaluation of laboratory tests for soil phosphorus. Final Report. Alberta Agricultural Research Institute Project No. 90- M230. Alberta Agriculture, Food and Rural Development, Edmonton AB. Mehlich, A Determination of P, Cu, Mg, K, Na, and NH 4. Mimeo. North Carolina Soil Testing Division, Raleigh. Mehlich, A Mehlich 3 soil test extractant: A modification of Mehlich 2 extractant. Commun. Soil Sci. Plant Anal. 15: Murphy, J. and Riley, J. R A modified single solution method for the determination of phosphate in natural water. Anal. Chim. Acta 27: Olsen, S. R. and Sommers, L. E Phosphorus. Pages in A. L. Page et al., eds. Methods of soil snalysis. Part 2. 2nd ed. Agronomy Monograph no. 9. ASA, SSSA, Madison, WI. Olsen, S. R., Cole, C. V., Watanabe, F. S. and Dean, L. A Estimation of available phosphorus in soils by extraction with sodium bicarbonate. USDA circular 939. U.S. Government Printing Office, Washington, DC. Pote, D. H., Daniel, T. C., Sharpley, A. N., Moore, P. A., Jr., Edwards, D. R. and Nichols, D. J Relating extractable soil phosphorus to phosphorus losses in runoff. Soil Sci. Soc. Am. J. 60: IGE ET AL. PHOSPHORUS SOIL TEST METHODS 699 Qian, P., Schoenau, J. J. and Karamanos, R. E Simultaneous extraction of available phosphorus and potassium with a new soil test: a modification of the Kelowna extraction. Commun. Soil Sci. Plant Anal. 25: SAS Institute, Inc SAS user s guide. Version 8.2 ed. SAS Institute, Inc., Cary, NC. Self-Davis, M. L., Moore, P. A., Jr. and Joern, B. C Determination of water or dilute salt extractable phosphorus in soils. Pages in G. M. Pierzynski, ed. Methods of phosphorus analysis for soils, sediments, residuals and waters. Southern Cooperative Series Bull. No 396. Kansas State University, Manhttan, KS. Sharpley, A. N Dependence of runoff phosphorus on extractable soil phosphorus. J. Environ. Qual. 24: Sharpley, A. N., Smith, S. J., Berg, W. A. and Williams, J. R Nutrient runoff losses as predicted by annual and monthly soil sampling. J. Environ. Qual. 14: Sims, J. T. 2000a. Soil test phosphorus: Olsen P. Pages in G. M. Pierzynski, ed. Methods of phosphorus analysis for soils, sediments, residuals and waters. Southern Cooperative Series Bull. No. 396 Kansas State University, Manhttan, KS. Sims, J. T. 2000b. Soil test phosphorus: Mehlich-3 P. Pages in G. M. Pierzynski, ed. Methods of phosphorus analysis for soils, sediments, residuals and waters. Southern Cooperative Series Bull. No Kansas State University, Manhttan, KS. Sims, J. T., Simard, R. R. and Joern, B. C. 1998a. Phosphorus losses in agricultural drainage: Historical perspective and current research. J. Environ. Qual. 27: Sims, J. T., Hodges, S. C. and Davis, J. 1998b. Soil testing for phosphorus: Current status and uses in nutrient management programs. Pages in J. T. Sims, ed. Soil testing for phosphorus: Environmental uses and implications. Southern Cooperative Series Bull. No University of Delaware, Newark, DE. Van Leirop, W Determination of available phosphorus in acid and calcareous soils with Kelowna multiple-element extractant. Soil Sci. 146: Wolf, A. M. and Baker, D. E Comparison of soil test phosphorus by the Olsen, Bray P1, Mehlich 1 and Mehlich 3 methods. Commun. Soil Sci. Plant Anal. 16: Zhang, M., Wright, R., Heaney, D. and Vanderwel, D Comparison of different phosphorus extraction and determination methods using manured soils. Can. J. Soil Sci. 84:

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