Several factors examined herein which control opal dissolution are: specific surface area, Al content, hydration state, age, and rate of organic matter biodegradation of the encasing vegetative tissues. These factors are covariable with opal of different origin. Recent (6 months old) opal phytoliths of deciduous origin are most hydrated (11%), have lower A1 content (2%) and highest dissolution (9 mg Si/liter, cold water; 50 mg Si/liter, hot water; and 3 mg Si/liter under natural environments). In contrast, opal of coniferous origin is older (30 months), more rigid, has higher Al content (3 to 4%), is encased within litter that is more slowly biodegradable and yields lower dissolution (2 to 3 mg Si/liter, cold water; 20 mg Si/liter, hot water; and 0.5 mg Si/liter under natural environments). Gramineous phytoliths associated with understory forest vegetation generally are intermediate in the above properties and dissolution. Biogenic opal has solubilities that approximate geologic opal‐A. It is relatively stable and not sufficiently labile under most soil environments to support observed soluble Si levels.

SUMMARY Soil structure was studied using the concept of fractals and related to soil texture and aggregate properties such as surface charges and aggregate stability. The mass and porosity fractal dimensions (Dm and Dp) of silty and sandy soils were determined on in situ soils using a variety of soil sections (thin, very‐thin and ultra‐thin), by image analysis on a continuous scale from m to 10−9 to 10−1m. Surface fractal dimensions (Ds) of these soils were determined on < 2 mm air‐dried samples using mercury porosimetry and the fractal cube generator model. The results suggest that soils are not pore fractals but mass and surface fractals with Dm= 1.1 Ds when the dimension of the embedding Euclidean space d is 3. The soil structures could possibly be described by fractal diffusion‐limited aggregation with complex interconnected aggregates or by fractal cluster–cluster aggregation models. As a preliminary conclusion, the fractal approach appears to be a potentially useful tool for understanding the underlying mechanisms in the creation or destruction of soil structure.

Summary Mercury porosimetry data can be interpreted in terms of soil structure using ideas drawn from (i) network modelling and percolation theory and (ii) fractal geometry. We linked mercury intrusion to soil structure quantified by image analysis within a relevant common pore radius scale. We compared (i) three independent methods for computing fractal dimensions of the matrix and of the solid–pore interface, namely fitted square boxes method and pore chord distribution on scanning electron microscope images of soil thin sections, and mercury porosimetry, and (ii) two independent methods for characterizing pore connectivity (image analysis) and percolation process (pressure threshold from mercury porosimetry). The results from analyses of the pore size distribution by mercury porosimetry differed from those from the image analysis. Mercury intrusion is controlled by both the connectivity of the pore space network and locally by pore throats leading to larger pore bodies. By contrast, image analysis is unaffected by pore connectivity and measures pore bodies. On the other hand, the chord length method might not adequately capture the scaling properties of the solid–pore interface, whereas the mercury porosimetry data were also difficult to interpret in terms of fractal geometry because of the effects of pore connectivity. However, fractal dimension values of both the solid phase and the solid–pore interface increased as a function of clay content, whereas both percolation probability values and throat radius values at the mercury percolation threshold decreased. The results show the merit of applying both fractals and percolation theory for determining structural parameters relevant to mercury and water transport in soil.

The effect of bacterial secretion of an exopolysaccharide (EPS) on rhizosphere soil physical properties was investigated by inoculating strain NAS206, which was isolated from the rhizosphere of wheat (Triticum durum L.) growing in a Moroccan vertisol and was identified as Pantoea aglomerans. Phenotypic identification of this strain with the Biotype-100 system was confirmed by amplified ribosomal DNA restriction analysis. After inoculation of wheat seedlings with strain NAS206, colonization increased at the rhizoplane and in root-adhering soil (RAS) but not in bulk soil. Colonization further increased under relatively dry conditions (20% soil water content; matric potential, −0.55 MPa). By means of genetic fingerprinting using enterobacterial repetitive intergenic consensus PCR, we were able to verify that colonies counted as strain NAS206 on agar plates descended from inoculated strain NAS206. The intense colonization of the wheat rhizosphere by these EPS-producing bacteria was associated with significant soil aggregation, as shown by increased ratios of RAS dry mass to root tissue (RT) dry mass (RAS/RT) and the improved water stability of adhering soil aggregates. The maximum effect of strain NAS206 on both the RAS/RT ratio and aggregate stability was measured at 24% average soil water content (matric potential, −0.20 MPa). Inoculated strain NAS206 improved RAS macroporosity (pore diameter, 10 to 30 μm) compared to the noninoculated control, particularly when the soil was nearly water saturated (matric potential, −0.05 MPa). Our results suggest that P. agglomerans NAS206 can play an important role in the regulation of the water content (excess or deficit) of the rhizosphere of wheat by improving soil aggregation.

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