Determination of soil texture: Comparison of the sedimentation method and the laser-diffraction analysis

Laser-diffraction analysis (LDA) is a rapid automated method achieving highly resolved fre-quency distributions of particle sizes. Recently, LDA has come into use in environmental sciences. However, in the size range of silt and clay deviations from the particle-size analysis with the standard pipette method, which is regarded as the reference method for soil-texture classification, have been reported. Therefore, this study concentrates (1) on the verification of systematic relations between both methods using a series of soils of Lower Saxony (Germany) and (2) on the general applicability of the laser-diffraction method to soil-texture classification as well as (3) texture-based estimates of air capacity, available field capacity, and permanent wilting point. The comparison of LDA with the pipette method demonstrated highly significant linear correla-tions in each of the particle-size fractions from clay to coarse silt. The slope of regressions ranged from 0.4 with fine silt to 3.1 with clay. If the clay content derived from LDA was applied to texture classification, the resulting textural classes differed from the standard textural classes, except for purely sandy samples with a clay content of <5%. However, the linear-regression model enabled an approach of the LDA-based clay content to values produced with the standard pipette method. Using this transformation, a texture classification became practicable in many cases, but, despite of a high significance level between LDA and pipette method, still led to wrong textural classes in several cases. A comparison with regression models from other regions in Europe showed both similarities and discrepancies, even for similar substrates. Hence, the laser-diffraction analysis cannot be used for the texture classification of soil samples without verification by the standard pipette method