The application of lime in order to correct soil pH has been shown to be agronomically feasible in many cropping situations. Lime application, however, is only desired in low pH soils, and may even be detrimental if applied to high pH soils. Consequently, it is desirable to accurately assess the spatial variability of soil pH and lime requirements.
Buffer methods are typically used by soil testing laboratories in the U.S. for the prediction of the LR of acid soils. For example, the AE buffer procedure (Adams F., and C. E. Evans, 1962. A rapid method for measuring the lime requirement of Red-Yellow Podzolic soils. Soil Sci. Am. Proc. 26:355-357) is used widely in the southeastern and mid-Atlantic regions of the U.S., and the SMP buffer (Shoemaker H. E., E. O. McLean, and P. F. Pratt, 1961. Buffer methods for determination of lime requirements of soils with appreciable amount of exchangeable aluminum. Soil Sci. Soc. Am. Proc. 25:274-277) is widely used in the Midwest regions (Sims, J. T., 1996. Lime requirement. p. 491-515. In D. L. Sparks (ed.) Methods of soil analysis. Part 3. SSSA Book Series Book Series No. 5 SSSA, Madison, Wis.). The accuracy of these buffers has been widely studied (Follett, R. H. and R. F. Follett, 1980, Strengths and weaknesses of soil testing in determining lime requirements for soils. p. 40-51. In Proc. of the Natl. Conf. on Agric. Limestone 16-18 Oct. 1980. Tennessee Valley Authority, National Fertilizer Development Center, Muscle Shoals, Ala.; Tran, T. S., and W. van Lierop, 1981. Evaluation and improvement of buffer-pH lime requirement methods. Soil Scci. 131: 178-188; Alabi K. E., R. C. Sorensen, D. Knudsen, and G. W. Rehm. 1986. Comparison of several lime requirement methods on coarse-textured soils of Northeastern Nebraska. Soil Sci. Soc. Am. J. 50:937-941). A recent concern about buffers is the potential toxicity of their components. Both the AE and SMP buffers contain p-nitrophenol, a potentially toxic compound.
A possible alternative method for determining the LR of acid soils is by direct titration with a base, although these methods are time-consuming and would need to be simplified for routine use. Dunn studied direct titration to predict the LR of acid soils and focused on the time to reach equilibrium for the reaction between the added base and soil acids. Dunn, L. E. 1943. Lime requirement determination of soils by means of titration curves. Soil Sci. 56:341-351. Dunn found that 4 days were needed for pH values to reach equilibrium when a 0.022 M Ca(OH)2 solution was added to acid soils. Dunn also discovered that shaking affected the time required for soil pH values to reach equilibrium. Dunn reported that a constant pH was reached within 8 hours when the soil suspensions were mixed by shaking, whereas 4 days were required for the suspensions to obtain a stable pH without shaking. Finally, Dunn suggested a direct titration method for LR by incorporating acid soils with different rates of 0.022 M Ca(OH)2 for 4 days. The Ca(OH)2 titration method suggested by Dunn for measuring the LR was widely accepted as a reliable method for evaluating buffer methods that were developed for estimating the LR (Follett and Follett, 1980; Alabi et al., 1986; McConnell, J. S., J. T. Gilmour, R. E. Baser, and B. S. Frizzell. 1991. Lime requirement of acid soils of Arkansas. Arkansas Exp. Stn. Spec. Rep. 150. Arkansas Agric. Exp. Stn., Fayetteville; Owusu-Bennoah, E. Acquaye, D. K. Mahamah, T. 1995. Comparative study of selected lime requirement methods for some acid Ghanaian soils. Commun. Soil Sci. Plant Analy. 26:937-950). However, Dunn's method was also considered to be a time-consuming procedure and not suitable for routine use in soil testing laboratories.
Many studies focused on the titration curve itself. Magdoff et al. concluded that the relationship between pH and OH− added is nearly linear within the pH range of most agricultural soils (4.5-6.5). Magdoff, F. R., and R. J. Bartlett. 1985. Soil pH buffering revisited. Soil Sci. Soc. Am. Proc. 49:145-148. Weaver et al. also reported a linear relationship between pH and base added for a series of Georgia soils. Weaver, A. R. D. E. Kissel, F. Chen, L. T. West, W. Adkins, D. Rickman, and J. C. Luvall. 2004. Mapping soil pH buffering capacity of selected fields in the coastal plain. Soil Sci. Soc. Am. J. 68:662-668. The slope of the linear relationship of pH versus OH− added is also a fundamental property of the soil, which could possibly be determined by other methodologies.