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1 DETERMINATION OF THE LIME REQUIREMENT FOR ACID SOILS IN ONTARIO USING THE SMP BUFFER METHODS The accuracy of the Shoemaker-Mclean-Pratt single buffer method and its double buffer variation was compared for determining the lime requirement of 41 acid mineral soils in Ontario. The single buffer method gave good prediction of the lime requirement when calibrated against Ca(OH)r-titrated soil acidity using a quadratic equation. Key words: Lime requirement, acid soils, SMP buffer [Etablissement des besoins en calcaire des sols acides de l'ontario par les mdthodes tampons SMP.I Titre abrdg6: Besoins en calcaire selon les m6thodes tampons SMP. On a compar6 I'exactitude de la m6thode du tampon simple Shoemaker-Mclean- Pratt et de sa variante au tampon double pour tablir les besoins en calcaire de 4l sols min6raux acides de I'Ontario. La mdthode du tampon simple fournit une bonne pr6diction des besoins en calcaire lorsqu'elle est 6talonn6e par rapport d I'acidit6 titrde au Ca(OH), au moyen d'une 6quation quadratique. Mots cl6s: Besoin en calcaire, sols acides, tampon SMP Of the several buffer methods for measur- mean of 5.4 and a median value of 5.5. The ing lime requirement of acid soils, the one samples chosen represented different agriproposed by Shoemaker et al. (1961) is cultural regions in Ontario and ranged in probably the most widely adopted by soil soil texture from loamy sand to clay. testing laboratories (Mcl.ean et al. 1966). According to Mclean et al. (1978), ti- Webber et al. (1977) recommended it for tration of soils to ph7.2 with Ca(OH), in Canadian acid soils. Tran and van Lierop l:5 (wvvol) soil:water suspensions yielded (1982) and van Lierop (1983) also found limerequirementsthatweresimilartothose the method to be suitable for acid mineral obtained by incubation with CaCO, to ph and organic soils in Quebec. Mclean et al. 6.5 over a 20-mo period. There is no gen- (197'l, 1978) subsequently modified the eral agreement in the literature as to what original SMP single buffer method by in- constitutes an appropriate incubation pecluding a second buffer ph of 6.0 (therefore riod. Webber et al. (1977) apparently conequivalent to a double buffer) and reported sidered a 30-d incubation period adequate, an improvement in results obtained for soils whereas Tran and van Lierop (1982) with low lime requirement. Of the 36 soils thought that a 3-mo incubation was retested by Webber et al. (19'17), only one quired. The uncertainty over attainment of was from Ontario. Therefore, we decided equilibrium with the incubation method is to test the suitability of the SMP single not a problem when a titration method is buffer (SMP-SB) and double buffer (SMP- used. However, the wide soil to solution ra- DB) methods for predicting the lime re- tios inherent with a titration method entails quirements to achieve ph 5.5, 6.0 and 6.5 a limitation due to dilution effects, alin acid mineral soils in Ontario. though these effects can be obviated to The 4l soils used in this study were se- some extent by using 0.01 M CaCl2 as the lected from samples submitted by farmers equilibrating solution (Black 1968). Therefor soil testing. The ph measured in satu- fore, it was decided to titrate l0 g of soil rated paste ranged from 4.5 to 6.3, with a suspended in 5 ml of 0.1 M CaClr solution Can. J. Soit Sci. 66l (May r9e6) 3'73 against 0. 2 M Ca(OH)r. Distilled water was added to bring the final solution volume to

2 374 CANADIAN JOURNAL OF SOIL SCIENCE CE J ld^ F(6 Z.c, uj> fio, E8 Cte ujo (re lj = ml, i.e., the final concentration of CaCl, was 0.01 M. Two drops of chloroform were added to inhibit microbial activity, and the suspension was shaken mechanically and intermittently for a total of 8 h over a 72-h period. Lime requirements to attain ph 5.5, 6.0 and 6.5 were read off plots of the final ph of the supernatant solution vs. the amount of Ca(OH), added. Eight of the experimental soils were incubated at field capacity for 30 d with various amounts of Ca(OH)r. Linear regression showed that the lime requirements to attain ph 6.5, obtained by incubation and by titration in 0.01 M CaCl, solutions, were quite similar (P:0.874, b:0.945 and, sr.:0.565). This indicated that the titration method was a suitable choice for a reference method. The SMP-SB (single buffer at ph 7.5) method (Shoemaker et al. 1961) and a variation (single buffer at ph7.3) were tested first. The latter buffer ph had been suggested to improve the lime requirement prediction of south-eastern U.S. soils (Mcl-ean et al. 1978). The ph of the soil-smp buffer suspension measured following l0 min of shaking gave an index of the lime requirement. (We found that standing the soilbuffer suspension for 2O-l2O min after shaking had no measurable effect on the final suspension ph.) The actual lime requirement was derived from a calibration regression equation, obtained using up to 4l soils, expressing the relationship between soil-buffer ph and the reference lime requirement indicated by titration with Ca(OH)r. There was no difference between the SMP buffers at initial ph values of 7.5 and7.3 for predicting the lime requirement of Ontario soils. Linear regression analysis of the test results using the SMP buffer at an initial ph of 7.5 produced coefficients of determination (r2) ranging from to for the three objective ph values (Table 1). LR = DHg phg2 (r2 = o.9ol) :t':"-t:1ry8 o oo (l=o.s56) SolL- SMP BUFFER PH (PHe) Fig. l. Relationship between soil-smp-sb buffer ph and titration lime requirement for achieving ph 6.5 in 4l acid soils.

3 SOON AND BATES-LIME REQUIREMENTBY SMP BUFFER METHODS 375 Table l..lime requirement (LR)f for attaining soil ph values of 5.5, 6.0 and 6.5 as predicted by regression equations incorporating soil-smp-sb buffer ph (ph") LR(6.5)= pgs LR (6.0) = PHa LR (5.5) = PHg LR (6.5) = ph" + LR (6.0) : phs + LR (5.5) = PH" + fmeasured in Meq of CaCO3 100 g-' soil. ** Significant at99% probability level. Linear regression Cumilinear regression 4.23 phs ph"'? 3.66 phs2 Quite large errors were apparent at the low and high ends of the lime requirement scale using linear regression (Fig. l). This suggested (as does Fig. 2. of Webber et al. (1977)) that the calibration equation could a J F z uj s1 Eo r,u E5 o io 6P lll L tr ut LR Y + o.1o7y p2 = 0.910) ** 0.750** 0.765** 0.901** 0.827** 0.828** s"." t.l No. of Samples 4l l be improved by curvilinear regression analysis. We analyzed our data using either a squared or square root term to generate the curvilinearity. The resulting regression equations with the squared term gave 1.62Y (r2 = 0.896) smp -TNDTCATED SO L ACTDTTY (Y) meq/100 g Fig. 2. Relationship between titration lime requirement (LR) for attaining ph 6.5 and SMP-DB indicated soil acidity (Y) for 4l acid soils.

4 376 CANADIAN JOURNAL OF SOIL SCIENCE slightly higher 12 values than equations with the square root term (not shown). The accuracy of the lime requirement predicted was considerably improved by the quadratic equations (Table 1; Fig. l). These quadratic equations form the basis of our subsequent work. The SMP-SB (ph 7.5) method was next compared with the double buffer variation (Mclean et al. 1977). The results of a linear and a curvilinear regression of lime requirement to ph 6.5 estimated by titration on SMP-DB indicated soil acidity are shown in Fig. 2. The curvilinear regression analysis slightly improved the r2 value (both linear and curvilinear f were significant at 99Vo level). The standard deviation (sr.) which was for linear regression improved to with the curvilinear regression. However, in soil testing laboratories which must handle a large number of soil samples expeditiously, the slight gain in correlation and accuracy with the SMP-DB method over the SMP-SB method (with curvilinear calibration) may not justify the extra work required in re-equilibrating the soil for a second soil-buffer ph measurement (compare Figs. I and 2). Our data and those oftran and van Lierop (1982) showed that the SMP-SB method was calibrated most advantageously with a curvilinear regression equation. Black, C. A. 196E. Soil-plantrelationships. 2nd ed. Wiley, New York. Mclean, E. O., Dumford, S. W. and Coronel, F A comparison of several methods of determining lime requirements of soils. Soil Sci. Soc. Am. Proc. 30: Mclean, E. O., Trieweiler, J. F. and Eckert, D. J Improved SMP buffer method for determining lime requirements of acid soils. Commun. Soil Sci. Plant Anal. 8: Mclean, E. O., Eckert, D. J., Reddy, G. Y. and Trierweiler, J. F An improved SMP soil lime requirement method for incorporating double buffer and quick-test features. Soil Sci. Soc. Am. I. 42:3ll-316. Shoemaker, H. E., Mclean, E. O. and Pratt, P. F Buffer methods for determining lime requirement of soils with appreciable amounts of extractable aluminum. Soil Sci. Soc. Am. Proc. 25: Tran, T. S. and van Lierop, W Lime requirement determination for attaining ph 5.5 and 6.0 ofcoarse-textured soils using buffer-ph methods. Soil Sci. Soc. Am. J. 46: van Lierop, W. f9m. Lime requirement determination of acid organic soils using buffer-ph methods. Can. J. Soil Sci. 63: Webber, M. D., Hoyt, P. B., Nyborg, M. and Corneau, D A comparison of lime requirement methods for acid Canadian soils. Can. J. Soil Sci. 57:361-37O. Y. K. SOON' and T. E. BATES'? tresearch Stqtion, Agriculture Canadu, Beaverlodge, Alberta TOH 0C0; and 2Department of Land Resource Science, University of Guelph, Guelph, Ontario NIG 2WI. Received li July accepted 28 Jan

5 This article has been cited by: 1. Elias M. Gichangi, Pearson N. S. Mnkeni, Philip C. Brookes Effects of goat manure and inorganic phosphate addition on soil inorganic and microbial biomass phosphorus fractions under laboratory incubation conditions. Soil Science and Plant Nutrition 55:6, [Crossref]