Pore space characteristics as indicators of soil behaviour in gypsiferous soils

In order to study the physical properties of gypsiferous soils (Gypsic Xero chrepts and Typic Xerofluvents) from an irrigated semi-arid area in Catalon ia (NE Spain) and their relationships to their pore characteristics, twenty horizons with gypsum contents ranging between 0 and 90% were sampled for m icromorphological and physical analyses. The micromorphological study consi sted of thin section description and of image analysis of uncovered thin se ctions of samples impregnated with a W-fluorescent dye. Eight fields of eac h horizon were studied. Two images were obtained from each field (2.1 x 3.1 cm(2)): one illuminated with incident W-light and the other with transmitt ed white Light. Image processing with an IBAS-system allowed measurement of the geometric characteristics of three types of objects: actual pores (eg. diam. > 100 mu m), transparent objects infilled or not with gypsum and por es located in these transparent objects. The physical characteristics of th e same horizons considered in the study were the water-characteristic curve s and the saturated hydraulic conductivity. Although differences in total p orosity (between 6 and 16%) are not related to other properties, an increas e of gypsum content leads in the studied horizons to the replacement of lar ge pores by smaller packing pores between gypsum crystals in infillings, wh ich are interconnected and do not reduce saturated hydraulic conductivity. Available water is affected by the degree of mixing of the gypsum infilling s with the groundmass, besides the crystal size distribution. The results s tress the importance of pore clogging by gypsum in these materials as a maj or constraint for root development, as well as the need of describing the t ype of gypsum accumulation when assessing the suitability of these soils fo r plant growth. (C) 1998 Elsevier Science B.V. All rights reserved.