D.L. Dent

A world perspective on acid sulphate soils.

Abstract Generalizations about the morphology and genesis of acid sulphate soils at a regional scale are derived from a review of extensive though patchy soil survey data. A general sequence of sedimentation, accumulation of sulphides and burial of sulphitic material by peat or alluvium of low sulphide content, and a parallel sequence of soil profile development are illustrated by examples of unripe sulphitic soils under a natural tidal regime, artificially drained raw acid sulphate soils and ripe acid sulphate soils at various stages of development. The world pattern has been driven mainly by postglacial sea level change but each regional pattern is determined by its unique sedimentary and geomorphological history. Distinctive patterns occur in deltas and estuaries, and in humid and seasonally dry climates. Very extensive areas of ripe acid sulphate soils in areas that are not known to have experienced falls in relative sea level are ascribed to human activity: forest clearance and soil erosion in catchments causing lower dry season river flows, and piecemeal clearance of peat over the floodplains permitting the dry season water table to fall. The implications for further agricultural development in other comparable areas include the absolute necessity to know the extent of potential acid sulphate soils and to maintain their water table above the sulphide datum. Most effective systems of management have been developed by farmers themselves but the sustainability of these systems is often doubtful due to their detrimental effects on the aquatic ecosystem.

Techniques to quantitatively study the microfabric of soils

Abstract Several image processing and analysis techniques, which are described individually elsewhere, are combined to provide powerful new methods for studying the mineralogy and microfabric of soils. The analysis begins with a multi-spectral classification of SEM back-scattered electron images and X-ray maps, followed by particle size routines on the segregated mineral grains. Masking these large grains then permits intensity gradient techniques to be used to study orientation patterns within the matrix. Finally, segmentation of the orientation image into domains of similar alignment shows how the orientation of the matrix relates to that of the skeleton mineral grains. Apart from a little operator assistance in the initial classification, the separate techniques may be run automatically in batch mode with no subsequent operator intervention. Thus, the problems of subjectivity, met within many other techniques, are minimized. Two contrasting soil samples are used to illustrate the technique and the results illustrate the advantage of the objective measurements over qualitative interpretation. In one sample, there is strong alignment of the matrix orthogonal to the direction of alignment of most of the mineral grains. Examples of several applications are given and other possibilities are discussed.

Modelling soil water supply to crops

Summary Process simulation models are needed to make use of static soil survey data to predict crop performance. One critical link in this chain, the supply of water by the soil to meet the transpirational demand of crops, has been modelled on the assumption that water supply is directly related to soil water potential. Field testing of this relatively simple model shows a close correlation between predicted and measured supply of water over a range of crops and of soils grouped according to their soil water release characteristics. Use of this soil water potential sub-model in turn, offers synoptic models of crop production an improved sensitivity to soil conditions.

Land evaluation for afforestation in a semi-arid environment: The montane plains of the Central Highlands of North Yemen

Summary In semi-arid environments, much land use depends on water harvesting from the upper members of soil catenas to support crops on the lower members. In the Central Highlands of North Yemen, an initial evaluation of land for forestry on the basis of a general-purpose resource inventory highlighted drought and frost as the main constraints, and identified over 150 potential forest species. Subsequent species elimination and proving trials revealed that, in the absence of data, the frost hazard had been underestimated; and under a water-harvesting regime the very stony, coarser-textured midslope soils are more suited to more species than the fine loamy soils of the footslopes because of their better infiltration and rooting characteristics. Conventional soil survey and land evaluation does not cope easily with small, scattered land units; especially where site performance is dependent upon the behaviour and management of upslope members of the catena. For field use, a site type key has been devised with a supplementary table to match suitable tree species with site type. This expert system makes use of cultural, vegetation, soil and landform features, some of which are unique to North Yemen. However, the principles involved should have general application.

Quantitative micro-mineralogy and micro-fabric of soils and sediments

Abstract Tovey, N.K., Dent, D.L., Krinsley, D.H. and Corbett, W.M., 1994. Quantitative Micro-mineralogy and microfabric of soils and sediments. In: A.J. Ringrose-Voase and G.S. Humphreys (Editors), Soil Micromorphology: Studies in Management and Genesis. Proc. IX Int. Working Meeting on Soil Micromorphology, Townsville, Australia, July 1992. Developments in Soil Science 22, Elsevier, Amsterdam, pp. 541–547.