SEM image analysis for characterization of sand grains in Thai paddy soils

This study presents a detailed characterization of sand grains in 34 samples of Thai paddy soils (mostly Alfisols and Ultisols) that have formed from diverse parent materials. Computer analysis of SEM images of thin sections provided a novel and reliable method to analyse the size and shape of quartz grains using the software ImageJ 1.34S. Sand grain size expressed as the feret value showed a log normal distribution for 16 samples whereas for the other samples the distribution was positively skewed. A logit transformation of grain circularity values generated a normal distribution for 26 samples and eight samples had bimodal distributions. These granulometric data indicate the diversity of the soil parent materials and the observed soil layers represent discrete sedimentary events with materials of different origins or different depositional energies. Pedogenesis including pedoturbation has not disrupted these sedimentary layers. Some sand grains consist of gold, brass, copper, ilmenite, zircon, magnetite, barite or monazite. The diversity of these grains reflects the complex geology of the catchments providing the sediments and some anthropogenic materials may have been incorporated into the soils.