Comparison of different laboratory methods with lysimetry for soil solution composition — experimental and model results

Compositions of soil solution obtained by the following methods were compared with those obtained by lysimetry: centrifugation; 2:1 extracts of air dried (2:1_dried) and field moist (2:1_moist) samples; saturation extracts; the ‘equilibrium soil pore solution’︁ method using columns with undisturbed (ESPS) and composited soil (ESPS_comp); and a method using pressure. Two soil depths of a Spodic Dystric Cambisol at Solling, Germany, were sampled with 10 to 12 replications. A coupled equilibrium model was used to describe the effect of soil to solution ratio on the solution composition. The model included multiple cation exchange and inorganic complexation, and for the subsoil solubility products of AlOHSO₄ and Al(OH)₃. Saturation extracts gave similar results as lysimetry and thus may be useful for calculating output fluxes. However, biological transformations (N mineralisation, solubilisation of organic matter) occurred during the preparation of saturation extracts. Composition of soil solutions obtained by either 2:1_dried extracts or centrifugation differed greatly from the results of other methods, indicating that these two methods may not be the best means to investigate equilibrium soil solutions. The values of molar ion ratios depended largely on the method used to obtain soil solutions: Ca²⁺/Al³⁺ ratios for each depth ranged from less than 0.3 (which suggests that liming is required urgently) to greater than 1 (liming not necessary). Modelling described the effect of soil to solution ratio on element concentrations for the methods pressure, saturation extracts, ESPS_comp and 2:1_moist extracts qualitatively with a few exceptions. The model suggested that differences in element concentrations using these methods may be mainly due to dilution, cation exchange and solubilisation of sparingly soluble salts, depending on the soil to solution ratio used.