Hand-feel soil texture and particle-size distribution in central France. Relationships and implications

Due to cost constraints, field texture classes estimated by hand-feel by soil surveyors are more abundant than laboratory measurements of particle-size distribution. Thus, there is a considerable potential to use field estimated soil textures for mapping on the condition that they are reliable and can be characterized by a probability distribution function similar to values obtained by laboratory measurements. This study aimed to investigate and elucidate the differences between the field texture classes estimated by hand-feel and soil texture determined from particle-size analysis under laboratory conditions in a region of Central France. We tested several hypotheses to explain the discrepancies between field estimates and laboratory measurements (organic C content, pH, more detailed particle-size analyses, and CEC). Finally, we simulated the consequences of using particle-size distribution estimated from field texture on a pedotransfer function (PTF) for water retention. Laboratory measurements of clay, silt, and sand content for each field texture class were available for about 17,400 samples. Considering laboratory measurements and the French texture triangle as the reference, the overall accuracy of field texture class allocation was 73%, which was better than most of the results previously reported in the literature. When looking at each field texture class, most predictions were consistent; however, there were noticeable differences between a few field texture classes and particle-size classes. The extreme texture classes located at the corners of the texture triangle were better predicted than those located at the centre of the triangle. We found the discrepancy of field texture classes can be explained by the very fine sand (50-100 mu m) and very coarse sand (1000-2000 mu m) contents. Based on the particle-size distribution from each field texture class, we calculated their joint probability distribution function of their corresponding laboratory measurements of clay, silt, and sand content. Results showed that PTF values predicted using hand-feel texture were consistent with those obtained with the measured particle-size distribution. Overall, we demonstrated the value of hand-feel texture in expanding the soil texture database and supporting the expansion of the national database to inform soil water retention properties.