Field and laboratory studies indicate that utilisation of biological geotextiles constructed from palm-leaves and other selected organic materials are an effective, sustainable and economically viable soil conservation technique. The three-year plus

Preliminary investigations suggest biological geotextiles could be an effective and inexpensive soil conservation method, with enormous global potential. However, limited quantitative data are available on the erosion-reducing effects of biological geotextiles. Therefore, the objective is to evaluate the effectiveness of biological geotextiles in reducing runoff and soil loss under controlled laboratory conditions and under field conditions reflecting continental, temperate and tropical environments. In laboratory experiments, interrill runoff, interrill erosion and concentrated flow erosion were simulated using various rainfall intensities, flow shear stresses and slope gradients. Field plot data on the effects of biological geotextiles on sheet and rill erosion were collected in several countries (UK, Hungary, Lithuania, South Africa, Brazil, China and Thailand) under natural rainfall. Overall, based on the field plot data, the tested biological geotextiles reduce runoff depth and soil loss rates on average by 46 per cent and 79 per cent, respectively, compared to the values for bare soil. For the field and laboratory data of all tested geotextiles combined, no significant difference in relative runoff depth between field measurements and interrill laboratory experiments is observed. However, relative soil loss rate for the concentrated flow laboratory experiments are significantly higher compared to the interrill laboratory experiments and the field plot measurements. Although this study points to some shortcomings of conducting laboratory experiments to represent true field conditions, it can be concluded that the range and the mean relative runoff depth and soil loss rate as observed with the field measurements is similar to those as observed with the interrill laboratory experiments.

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The sequential development of kaolin by progressive alteration of smectite, involving kaolin-smectite interstratifications as a genetic link is described from a red and black complex. Mineral compositions were studied using XRD, DTA and XRF techniques. The basaltderived soils are situated along a 600 m transect and grade in colour from dark grey (10 YR 3/1 ) to red (5 YR 3/3). The kaolin proportions in the interstratification increase almost linearly with increasing reddening up to ~80%. Whole-soil chemical analyses exhibit no significant variations in the major element composition, but dithionite extractable Fe increases along the transect from 1% to 4-16%. Hematite and goethite are the only secondary iron phases. Topographic differences are slight but sub-surface bedrock contours plus appreciable variations in sand content between red and black soils could be genetically significant.

Conflicting information is available on the propensity different soil clays exhibit to dispersion. We therefore assessed the relative stability of the clay components of 12 soil samples, from various parts of South Africa, to predominantly physical disruption by a mild disruptive treatment [mechanical shaking for 5 min in distilled water (DW), and after the addition of phosphogypsum (PG; 2% by weight) and polyacrylamide (PAM; 0.04% by weight)]. The soils differed markedly in their physical and chemical properties. Clay fractions were of mixed mineralogy and dominated by kaolinite, illite, or smectite. Comparison of the clay mineral composition of the disaggregated clay with that of the original <2 �m fraction indicated that the disaggregated clay composition depended on the amendment. In DW, clay-sized quartz and feldspar were disaggregated preferentially over layer silicates. Within the phyllosilicate fraction, the 2:1 clay minerals (mica, smectite) were on average slightly more easily disaggregated than kaolinite. Goethite was the least easily detached clay component in DW. The increase in quartz and feldspar proportions relative to the other components of the clay fraction was dramatically more pronounced when the soils were mixed with PG. With PAM, however, differences in the nature of the clay fraction between original and disaggregated clay were only marginal. Disruption was not particle-size related, as the minerals of the fine-clay fraction showed no selective increase in any of the treatments. These findings indicate that the most inert members of the clay fraction are most actively involved in the process of disaggregation. PG influenced disaggregation in a manner markedly different from that of PAM. Gypsum preferentially stabilised components with a net negative charge over uncharged species. PAM, in contrast, seemed to affect all clay components equally, independent of charge characteristics.