Texturbestämning genom fält-, pipett- och hydrometermetoder

Sammanfattning: Texture is an important soil property and accurate particle size analysis of soil texture is therefore crucial. There are a number of well-established and newly developed methods available for determining soil texture. In order to compare the results of these methods, the differences between them must be determined. The difficulties, problems and differences regarding texture class determination were examined through a study performed on forest soil samples. The study compared different methods (pipette, hydrometer and field methods) and two types of dispersing agents (sodium pyrophosphate and sodium hexametaphosphate (calgon)) commonly used for particle size analysis. Several field methods are summarised in this paper under the term 'finger testing'. A total of 35 samples were analysed. Additional data were collected from 'BioSoil', an EU project. All samples were analysed by the hydrometer method, performed separately with each dispersing agent. The samples had already been analysed within BioSoil using the pipette method and by finger testing. Texture class was also determined by finger testing of the full set of samples in both field and laboratory. The results from the particle size analyses were used to identify differences between the pipette and hydrometer methods, between the two dispersing agents and between the hydrometer method and finger testing. Field and laboratory classifications obtained by finger testing were also compared. The pipette and hydrometer methods gave comparable results in cases where high precision was not required. The pipette method gave a slightly lower clay content compared with the hydrometer method, but the difference was not significant. Most of the differences in clay content could be explained by errors in the results of the pipette method. The dispersing agents showed a greater correlation than the pipette and hydrometer methods. In conditions such as those in this study, sodium pyrophosphate and calgon can therefore give similar results. No satisfactory correlation was found between the hydrometer method and finger testing. There was a similar lack of correlation between field and laboratory classifications obtained by finger testing. Finger testing is a subjective method that requires calibration and practice, and therefore some uncertainties can be expected. However these uncertainties were larger than expected in this study and could be explained by conditions differing between field and laboratory. Other possible explanations are the narrow classes used in this classification method and a lack of experience amongst staff performing the analyses. However, despite the variation in results, finger testing can be regarded as a useful method for soil texture class determination.