Scanning electron microscopy applied to thin sections and polished blocks by using backscattered electrons produces images which can be submitted to image analysis (Bisdom and Thiel, 1981; Chrétien and Bisdom, 1983; Fies and Bruand, 1990). The backscattered electron Scanning image (BESI) technique has not been used frequently in soil science since the first works because of the lower resolution of BESI mode when it is compared to the secondary electron mode. The aim of study is to show that the BESI associated with image analysis could be used much more frequently hen an accurate description of soil porosity is required. Samples were collected from "crusting soils" in Zimbabwe. The "crusting soils", which are present within the area of intensive commercial agriculture, are generally degraded by tillage practices Nyampfene, 1985). The degradation processes are mainly crusting of the soil Surface and hardening of the tilled layer (Ferrari, 1991). Samples varying in their hardness were collected from tilled horizons and from the upper part of B horizons. Mercury porosimetry and water retention properties revealed the presence of three pore size classes A, B and C. The increase in the hardening is mainly related to a decrease in the pore volume due to the class A. The class C is attributed to the fabric of clay particles. As for the classes A and B, they required observations in microscopy to be related to the fabric of soil samples. The BESI technique was applied to thin sections obtained after impregnation of clods of 8-10 cm3 in volume. The thin sections were polished in order to minimize the topographic contrast. Image analysis as made witih a Quantimet on pictures taken at different magnifications. The pictures obtained at high magnification (4 103 - 8 103) allow pores up to 0,1 um in diameter to be distinguished. Image analysis was made on pictures taken at lower magnifications in order to analyse representative areas. The observations Show two types of pores: i) pores resulting from the fabric of clay and skeleton (silt and sand) particles. Image analysis, which reveals that their volume does not vary with hardening intensity, indicates that they correspond to the class B; ii) pores larger which do not correspond to a well defined level of organisation. They are partly related to biological activity and aggregate packing. Their volume decreases when hardening increases. That evolution and the pore volume deduced from image analysis show that this second type of pore correspond to the class A. BESI gives pictures with a high contrast between the voids occupied by resin and the minerals. It appears to be particularly well adapted to the study of the groundmass porosity.
Contribution of backscattered electron scanning image (BESI) and image analysis to the study of soil porosity. Application to the study of "crusting soils" under intensive commercial agriculture in Zimbabwe
D'Acqui L P;
1992
Abstract
Scanning electron microscopy applied to thin sections and polished blocks by using backscattered electrons produces images which can be submitted to image analysis (Bisdom and Thiel, 1981; Chrétien and Bisdom, 1983; Fies and Bruand, 1990). The backscattered electron Scanning image (BESI) technique has not been used frequently in soil science since the first works because of the lower resolution of BESI mode when it is compared to the secondary electron mode. The aim of study is to show that the BESI associated with image analysis could be used much more frequently hen an accurate description of soil porosity is required. Samples were collected from "crusting soils" in Zimbabwe. The "crusting soils", which are present within the area of intensive commercial agriculture, are generally degraded by tillage practices Nyampfene, 1985). The degradation processes are mainly crusting of the soil Surface and hardening of the tilled layer (Ferrari, 1991). Samples varying in their hardness were collected from tilled horizons and from the upper part of B horizons. Mercury porosimetry and water retention properties revealed the presence of three pore size classes A, B and C. The increase in the hardening is mainly related to a decrease in the pore volume due to the class A. The class C is attributed to the fabric of clay particles. As for the classes A and B, they required observations in microscopy to be related to the fabric of soil samples. The BESI technique was applied to thin sections obtained after impregnation of clods of 8-10 cm3 in volume. The thin sections were polished in order to minimize the topographic contrast. Image analysis as made witih a Quantimet on pictures taken at different magnifications. The pictures obtained at high magnification (4 103 - 8 103) allow pores up to 0,1 um in diameter to be distinguished. Image analysis was made on pictures taken at lower magnifications in order to analyse representative areas. The observations Show two types of pores: i) pores resulting from the fabric of clay and skeleton (silt and sand) particles. Image analysis, which reveals that their volume does not vary with hardening intensity, indicates that they correspond to the class B; ii) pores larger which do not correspond to a well defined level of organisation. They are partly related to biological activity and aggregate packing. Their volume decreases when hardening increases. That evolution and the pore volume deduced from image analysis show that this second type of pore correspond to the class A. BESI gives pictures with a high contrast between the voids occupied by resin and the minerals. It appears to be particularly well adapted to the study of the groundmass porosity.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
