A comparison of methods for measuring extractable Ca, Mg, K, Na, Mn, Al, Fe, and P from New England forest soils

Abstract

An experiment was designed to evaluate several of the commonly used extractants and methods for determining "available" elements in soils. The purpose of the study was to evaluate the suitability of these extraction procedures for use on forest soils typical for New England commercial forests. The extraction procedure selected included NH4OAc pH 4.8, NH4OAc pH 7.0, NH4Cl, Double Acid, Bray, and Mehlich methods. The elements measured varied somewhat by procedure but included the base cations, Al, Fe, Mn, and P. As a bioassay of element availability, a greenhouse study was conducted using six forest soil materials from different horizon types (i.e. O, Ap B) and three conifer seedling species (red spruce, balsam fir, and white pine). Relatively small differences among extraction procedure were found among the methods used for exchangeable Ca, Mg, K, and Na. Large differences, however, were found among the different horizon types in the amount of exchangeable base cations present. In contrast, significant differences were found among extraction procedures for Al, Fe, Mn, and P depending on the degree of buffering and acidity of the extracting solution. Of the elements measured in this study, only P appeared to be growth limiting with the NH4OAc pH 4.8 being best correlated with P uptake by seedlings. Further work under field conditions over longer time periods is required to evaluate these methods for measuring P availability in forest soils.