An evaluation of 2D-image analysis techniques for measuring soil microporosity

To compare soil porosity measurements, manual, semiautomatic and automatic 2D-image analyses were performed on three sets of images of the same fields. The first and second image sets were obtained, by a fluorescence microscope, on the polished surfaces of soil blocks impregnated with a fluorescent resin and on the thin sections made from them, respectively. A scanning electron microscope in backscattered mode was used to acquire the third set of images on the thin sections. In the manual image analysis procedure, image segmentation was based on the best visual impression and carried out individually for each image with the UTHSCSA Image Tool software. For the semiautomatic method, the software mu CTanalySIS was used; the images were segmented by double hysteresis, after interactive selection of the thresholding values. Automatic thresholding, based on an analysis of the image intensity histogram, was performed using the image analysis software Image-Pro((R)) Plus. Average microporosity measurements were compared between image types and image analysis methods using marginal regression models for continuous outcomes. The mean area covered by pores was significantly different depending on the type of image and the method of image analysis. Results after the automatic procedure were significantly larger than after manual or semiautomatic thresholding. On the other hand, average porosities were larger for backscattered electron images than for fluorescent images, regardless of the image analysis procedure used. These results stress the need for standardization of image analysis protocols and warn of the dangers of comparing soil porosity measurements performed on different types of images.