Texture-contrast Soils Research Articles

High clay contents, dense soils, and spatial variability are the principal subsoil constraints to cropping the higher rainfall land in south-eastern Australia

Available soil information and unpublished data from soil survey indicate that high clay contents and high bulk density are the major subsoil constraints to crop growth in the high rainfall zone (HRZ) of south-eastern Australia. Seven high rainfall agroecological zones are proposed as sub-divisions of the region to focus future research and development. The HRZ is dominated by texture-contrast soils (69.9%) and soils with clay subsoil (89.4%) and high bulk density (mean 1.6 t/m3). Sodicity and acidity are also significant constraints to crop production in the HRZ. The physical limitations to root growth in the HRZ subsoils are best appreciated through the least-limiting water range concept and growth-limiting bulk densities. Management options and results of past research and intervention in soil loosening, drainage, raised beds, liming, and gypsum are reviewed. Climatic uncertainty raises questions about the future relevance of waterlogging as a constraint in the HRZ and confounds the development of reliable recommendations for engineering intervention.

Effects of subsoil amendments on soil physical properties, crop response, and soil water quality in a dry year

In southern Australia the ability of field crops to extract soil moisture and nutrients from depth depends on the physical and chemical properties of the subsoil. In texture-contrast soils accumulation of water and nutrients in the E or A2 horizon, immediately above a clay B horizon of much lower hydraulic conductivity (herein called the interface), may generate lateral flows and enhanced nutrient and solute transfer to water bodies. Evidence that deep-ripping with addition of subsoil nutrients can increase crop productivity in regions having hostile, alkaline subsoils led to experiments to test whether this response was related to an increase in the use of water and nutrients in the subsoil. Our study measured the effects of deep-ripping with and without amendments on soil physical and chemical properties of the A and upper B horizons of 2 South Australian soils. Deep-ripping and deep-placement of nutrients increased grain harvest weight even in an exceptionally dry season. The greater yield was accompanied by significantly lower field-penetration resistance to 0.35–0.50 m depth, which we hypothesise enabled the crop to better access stored soil water and deep placed nutrients in the subsoil. Residual effects from deep-ripping were minimal after 4 growing seasons; therefore, ripping will need to be practiced at regular intervals to maintain treatment effects. The ripping and nutrient amendments had no significant effect on exchangeable sodium percentage, electrical conductivity, and readily extractable phosphorus and nitrate-nitrogen, despite changes in these soil properties between spring and harvest sampling.

Wind erosion and soil carbon dynamics in south-western Australia

The incidence of wind erosion and its affect on soil organic carbon content was assessed in a dryland farming system near Jerramungup, Western Australia, using remote sensing, ground observations and soil analysis. Relationships were found between the patterns of wind erosion, past geomorphic processes and an array of soil attributes. These associations can be used to identify those soils that are most susceptible to erosion and thus soil carbon loss. Despite strong evidence of relict aeolian activity associated with a previous more arid climate, the incidence of contemporary wind erosion was only partly related to past geomorphological processes. Quartzose dune sands were particularly susceptible to erosion (47% of total area eroded), whereas texture contrast soils formed on clayey, wind-formed lunettes (16%) and deeply weathered regolith were less eroded (34%). Soils formed on stripped regolith and on loamy surfaced lunettes and swales were not eroded. Wind erosion was strongly related to soil particle size distribution and surface horizon depth, only occurring on sandy surfaced soils, with 50 cm deep. The incidence of erosion markedly increased with small decreases in clay and silt contents below these thresholds. Wind erosion resulted in the loss of ∼3% of the total stock of carbon to 1 m depth or 3.6 t C ha-1 for the eroded soils. The consideration of wind erosion induced carbon loss, both from the standpoint of sustaining farmland productivity and also in producing accurate national carbon accounts, at both the project and national scales, requires resolution.

The significance and lag-time of deep through flow: an example from a small, ephemeral catchment with contrasting soil types in the Adelaide Hills, South Australia

Abstract. The importance of deep soil-regolith through flow in a small (3.4 km2) ephemeral catchment in the Adelaide Hills of South Australia was investigated by detailed hydrochemical analysis of soil water and stream flow during autumn and early winter rains. In this Mediterranean climate with strong summer moisture deficits, several significant rainfalls are required to generate soil through flow and stream flow [in ephemeral streams]. During autumn 2007, a large (127 mm) drought-breaking rain occurred in April followed by significant May rains; most of this April and May precipitation occurred prior to the initiation of stream flow in late May. These early events, especially the 127 mm April event, had low stable water isotope values compared with later rains during June and July and average winter precipitation. Thus, this large early autumn rain event with low isotopic values (δ18O, δD) provided an excellent natural tracer. During later June and July rainfall events, daily stream and soil water samples were collected and analysed. Results from major and trace elements, water isotopes (δ18O, δD), and dissolved organic carbon analysis clearly demonstrate that a large component of this early April and May rain was stored and later pushed out of deep soil and regolith zones. This pre-event water was identified in the stream as well as identified in deep soil horizons due to its different isotopic signature which contrasted sharply with the June–July event water. Based on this data, the soil-regolith hydrologic system for this catchment has been re-thought. The catchment area consists of about 60% sandy and 40% clayey soils. Regolith flow in the sandy soil system and not the clayey soil system is now thought to dominate the deep subsurface flow in this catchment. The clayey texture contrast soils had rapid response to rain events and saturation excess overland flow. The sandy soils had delayed soil through flow and infiltration excess overland flow. A pulse of macropore through flow was observed in the sandy soils three days after the rainfall event largely ended. The macropore water was a mixture of pre-event and event water, demonstrating the lag-time and mixing of the water masses in the sandy soil system. By contrast, the clayey soil horizons were not dominated by pre-event water, demonstrating the quicker response and shallow through flow of the clayey soil system. Thus, the sandy terrain has a greater vadose zone storage and greater lag time of through flow than the clayey terrain.

Root biomass distribution and soil properties of an open woodland on a duplex soil

Data on the distribution of root biomass are critical to understanding the ecophysiology of vegetation communities. This is particularly true when models are applied to describe ecohydrology and vegetation function. However, there is a paucity of such information across continental Australia. We quantified vertical and horizontal root biomass distribution in a woodland dominated by Angophora bakeri and Eucalyptus sclerophylla on the Cumberland Plains near Richmond, New South Wales. The site was characterised by a duplex (texture contrast) soil with the A horizon (to 70 cm) consisting of loamy sand and the B horizon (to > 10 m) consisting of sandy clay. The topsoil had a smaller bulk density, a smaller water holding capacity but a larger organic component and a larger hydraulic conductivity in comparison to the subsoil. Root biomass was sampled to 1.5 m depth and declined through the soil profile. Whilst total biomass in the B horizon was relatively small, its contribution to the function of the trees was highly significant. Coarse roots accounted for approximately 82% of the root mass recovered. Lateral distribution of fine roots was generally even but coarse roots were more likely to occur closer to tree stems. Variation in tree diameter explained 75% of the variation in total below-ground biomass. The trench method suggested the belowground biomass was 6.03 ± 1.21 kg m−2 but this method created bias towards sampling close to tree stems. We found that approximately 68% of root material was within a 2 m radius of tree stems and this made up 54% of the total number of samples but in reality, only approximately 5 to 10% of the site is within a 2 m radius of tree stems. Based on these proportions, our recalculated belowground biomass was 2.93 ± 0.59 kg m−2. These measurements provide valuable data for modeling of ecosystem water use and productivity.

Determining the effect of stocking rate on the spatial distribution of cattle for the subtropical savannas

With the commercial development of the global positioning system (GPS), it is now possible to monitor the distribution of free ranging cattle and derive measures to describe landscape use. Animal GPS data can be integrated with a geographic information system (GIS) detailing topography, vegetation, soil type and other landscape features. Combining GPS and GIS information is useful for understanding how animals respond to spatial variability. This study quantified land-type preferences for Brahman cross steers over three time periods, from October 2004 to March 2006 in a replicated trial, under heavy (4 ha/AE; animal equivalent of ~450 kg steer) and light (8 ha/AE) stocking in four, ~105 ha paddocks of subtropical semi-arid savanna near Charters Towers, Queensland, Australia. The grazing trail was conducted at a scale much less than would be found in commercial situations. Consequently, the spatial pattern of cattle reported here may not represent what occurs at a commercial scale and implications are discussed. Results were analysed in terms of the spatial distribution of steers fitted with GPS devices in each of the four paddocks and for each stocking rate to provide insight into cattle distribution and land-type preferences. Steers walked in excess of 6 km per day, regardless of stocking rate, and exhibited diurnal patterns of movement, with peak activity around dawn (0500–0700 hours) and dusk (1800–2000 hours). The spatial distribution of the collared steers was not uniform and appeared to be strongly influenced by the prevailing drought conditions and location of water points within each paddock. A hierarchy of drivers for distribution was identified. With the exception of drinking water location, land subtype based on soil-vegetation associations influenced animal distribution. Preference indices (ŵi) indicated that steers selected sites associated with heavy clay and texture contrast soils dominated by Eucalyptus coolabah Blakely & Jacobs (ŵi = 5.33) and Eucalyptus brownii Maiden & Cambage (ŵi = 3.27), respectively, and avoiding Eucalyptus melanophloia F.Muell. ridges (ŵi = 0.26) and Eucalyptus cambageana Maiden (ŵi = 0.12) on sodosols. The results suggest that spatial variation in cattle distribution within a paddock may be more critical than overall stocking rate in influencing the pattern of biomass utilisation. However, to quantifying the effects of different grazing land management practices on animal distribution on a commercial scale, additional studies in extensive paddocks are required.

The relation between soil structure and solute transport under raised bed cropping and conventional cultivation in south-western Victoria

This study examined the relationship between soil structure and solute transport in a texture contrast soil under 2 different tillage treatments—raised beds and conventional cultivation—in south-western Victoria. Undisturbed soil samples were collected for resin-impregnation and image analysis. This enabled several descriptive parameters of macropore structure to be calculated. Large, undisturbed soil samples were also collected for a solute transport experiment using a KCl solution. A convective log-normal transfer function was used to model Cl– movement. The assessment of soil structure showed that the raised beds contained a better connected pore network than the conventionally cultivated soil. Solute transport was faster through the raised bed soil when close to saturation (at –5 mm tension). Under these conditions, the solute transport parameters showed a smaller ratio of transport volume to soil water volume in the raised bed than the conventionally cultivated soil. Together, these data strongly indicate that the raised beds had greater pore connectivity and were able to transmit solute faster and more efficiently than the conventionally cultivated soil. It is concluded that raised bed soils are better structured and provide less risk from waterlogging than conventionally cultivated soils. However, there is greater potential for preferential flow of pesticides and solutes in raised bed soils.

Relationships between field texture and particle-size distribution in Australia and their implications

This paper aims to establish the means and ranges of clay, silt, and sand contents from field texture classes, and to investigate the differences in the field texture classes and texture determined from particle-size analysis. The results of this paper have 2 practical applications: (1) to estimate the particle size distribution and its uncertainty from field texture as input to pedotransfer functions, and (2) to examine the criteria of texture contrast soils in the Australian Soil Classification system. Estimates of clay, silt, and sand content for each field texture class are given and this allows the field texture classes to be plotted in the texture triangle. There are considerable differences between field texture classes and particle-size classes. Based on the uncertainties in determining the clay content from field texture, we establish the probability of the occurrence of a texture contrast soil according to the Australian Soil Classification system, given the texture of the B2 horizon and its overlying A horizon. I enjoy doing the soil-texture feel test with my fingers or kneading a clay soil, which is a short step from ceramics or sculpture. Hans Jenny (1984)

Development of texture contrast soils by a combination of bioturbation and translocation

Soils and weathering profiles in a wide variety of parent materials and environmental settings exhibit coarse-over-fine vertical textural contrasts. Where these cannot be attributed to inherited texture contrasts or erosion–deposition, the most common explanations are based on translocation (eluviation–illuviation) which removes clays from surface layers and deposits them in the subsoil; or bioturbation, where preferentially fine material is delivered to the surface by organisms, from whence erosional winnowing creates a coarse surface layer. In some soils of the lower coastal plain of North Carolina, U.S.A., neither explanation is sufficient to explain the observed texture contrasts. A heuristic model based on a combination of translocation of fine material from surface to subsoil, and bioturbation-driven delivery and recycling of material to the surface can explain the observed vertical textural contrasts. The key elements in the model are coastal plain sediments which include some fine material; eluviation–illuviation by percolating water; delivery of additional fine and mixed grain size material to surface by bioturbation, making it available for translocation; concentration of fine material originally scattered throughout the parent material in a B horizon; and maintenance of vertical moisture fluxes by bioturbation. The model is supported by morphological evidence of the key mechanisms, argillic horizons that are finer than both the surface layers and underlying parent material, evidence that argillic horizon formation is not limited by the rate of clay synthesis, and the absence of texture contrasts in nearby soils formed from dune sands which lack fines.

Using Gypsum to Reduce Phosphorus in Runoff from Subcatchments in South Australia

Concentrations of phosphorus (P) in runoff from agricultural catchments in southern Australia are high and well above national and international limits. Phosphorus was found to exit two subcatchments of 3.6 and 4.2 ha in the Adelaide hills via both overland flow and interflow. The subcatchments had texture-contrast soils with high inputs of superphosphate and were openly grazed by cattle all year. Interflow at the boundary of the B and C soil horizons accounted for as much as half the total water flow that was measured (overland flow, A-B interflow, and B-C interflow). The average flow-weighted concentration of total P within overland flow was as high as 0.25 mg L(-1), and 0.05 mg L(-1) in B-C interflow. In most years P loss was in the dissolved (<0.45 microm) form. In some years, interflow was the major pathway for P loss off these catchments. The B-C interflow cannot be discounted when searching for management options to reduce P loss from texture-contrast soils to waterways. Preliminary laboratory experiments showed promise that gypsum could modify agricultural soils and reduce the concentrations of P (and dissolved organic C) in runoff before it enters public water supply reservoirs. In this study, gypsum, applied at a rate of 15 Mg ha(-1) to the 4.2-ha subcatchment, substantially modified the soil chemistry, and thereby soil structure. The size and stability of structural aggregates increased markedly and this change affected not only the A but also the upper B horizons, to a profile depth of approximately 50 cm. However, the impact of these physicochemical changes on P concentrations in runoff was not marked. Average profile P concentrations were only slightly lower in the runoff from the subcatchment following treatment. The high subsoil macroporosity of the gypsum-treated subcatchment caused an increase in the proportion of runoff by interflow.

The role of fire and nutrient loss in the genesis of the forest soils of Tasmania and southern New Zealand

The dominant soil patterns in forested or previously forested landscapes in southern New Zealand and Tasmania are described. Soil properties on adjacent sunny and shady aspects in hill country of the South Island of New Zealand are compared to soil properties under adjacent ‘dry’ and ‘wet’ eucalypt forest in Tasmania. A soil contrast index or SCI is defined for comparing soil contrasts on parent materials of different absolute nutrient contents. Three soil groups are defined using the SCI. Group 1 soil pairs are stable New Zealand soils in which exchangeable Ca + Mg + K values are higher on drier sunny aspects than on moister shady aspects. Group 2 soil pairs are New Zealand soils in which soils on sunny aspects display evidence of topsoil erosion by wind; consequently some soil pairs on dry (sunny) aspects have lower levels of exchangeable Ca + Mg + K than soils on moister (shady) aspects. Group 3 soil pairs are Tasmanian. Soils on drier sites (under dry eucalypt forest) invariably have lower exchangeable Ca + Mg + K values than soils on moister sites (under wet eucalypt forest), which is the reverse of the pattern in SCI Group 1 soils in New Zealand. Except on clay-rich parent materials, Tasmanian soils under dry forest generally have texture-contrast profiles and a mean C/N ratio in topsoils (A1 horizons) of 29. Soils under wet forest generally have uniform or gradational texture profiles and a mean topsoil C/N ratio of 15. The texture-contrast soils show strong clay eluviation with sand or sandy loam textures in upper horizons and clayey textures in lower horizons. However, in New Zealand texture-contrast soils are all but absent, and do not occur in the previously forested areas described in this paper. Topsoils (Ah horizons and soils sampled to 7.5 cm depth) in New Zealand areas sampled in this study have a mean C/N ratio of 15, regardless of whether they occur on sunny or shady aspects. We propose that the frequency and spatial occurrence of fire are the dominant processes causing: (1) the marked difference in levels of nutrients and different topsoil C/N ratios in soils of Tasmania; (2) the development of texture-contrast soils under dry forests in Tasmania; and (3) the difference between soil patterns in New Zealand and Tasmania. Fire depletes nutrients in forests by causing losses to the atmosphere, losses by runoff, and losses by leaching. Nutrient loss by fire encourages fire-tolerant vegetation adapted to lower soil nutrient status, so frequent fire is a feedback mechanism that causes progressive soil nutrient depletion. By destroying organic matter and diminishing organic matter supply to the soil surface fire inhibits clay–organic matter linkages and soil faunal mixing and promotes clay eluviation. Fire frequency is likely to have increased markedly with the arrival of humans at ca. 34 000 years B.P. in Tasmania and ca. 800 years B.P. in New Zealand. We argue that texture-contrast soils have not formed in New Zealand because of the short history of frequent fires in that country. A corollary of this conclusion is that texture-contrast soils in Tasmania are, at least in part, anthropogenic in origin.

The role of shallow drains in removing excess water from texture-contrast soils

Variability of flow in shallow drains installed in a grazing catchment in South Australia was partly caused by the enormous variability each winter in the saturated thickness of the texture-contrast soils and their hydraulic conductivity. High variability has been found in other trials of shallow drains but the causes were not well understood. This study found that perched watertables are not always found on top of the B horizon. The depth and thickness of saturation varies with landscape position (between texture-contrast soil types) and also varies seasonally at one landscape position. Because drains are on a gradient, they pass through a variety of texture-contrast soils. These soils may have different soil chemistry, physical properties and depth of saturation. The presence of sodic subsoils with low hydraulic conductivity has a major impact on drainage volumes, particularly throughflow. In some years the drains intercepted deep, perched water or groundwater, which rose into the collection zone of the shallow drain. Flow from the drains in the study catchment was compared to that from other catchments with texture-contrast soils. Drain flows are highly variable and dependent on localised soil conditions, making flow prediction difficult. Despite this, a clear relationship was found between average annual catchment drain flow and annual rainfall. Drains installed across waterlogged paddocks with texture-contrast soils in southern Australia will average 10 mm flow (2.5% of annual rainfall) when annual rainfall is 400 mm, increasing to 100 mm (14%) when rainfall is 700 mm.

Exploring pedogenesis via nuclide-based soil production rates and OSL-based bioturbation rates

New dating techniques are available for soil scientists to test fundamental pedogenic ideas. Recent developments in applications of terrestrial in situ cosmogenic nuclides (TCN) from bedrock and saprolite allow the derivation of soil production rates, at scales ranging from local (sub-hillslope) to catchment wide, generally averaged over timescales of 104–105 years. Where soil depths are relatively constant over time, soil production rates equal transport rates and are thus essential to establishing sustainable erosion rates. TCN also allow the form of the soil production function to be compared to theoretical models—a difficult task previously. Furthermore, parameterised soil production functions can now be incorporated into numerical surface process models to test landscape evolution ideas. Bedrock and saprolite conversion to soil is demonstrably dependent on the overlying soil depth, and there is general agreement that weathering declines exponentially beyond maximum soil production, consistent with theory. Whether maximum soil production occurs under a finite or non-existent soil cover at particular sites remains unresolved. We suggest that, in general, soil production from saprolite declines exponentially with increasing depth, while production from bedrock follows a humped function. Estimates of the role of flora, fauna and processes such as freeze–thaw that mix soil mantles to depth, have been limited prior to optically stimulated luminescence (OSL) dating techniques. Recently derived OSL mixing rates extend the magnitude of previous partial, short-term bioturbation rates. In fact, bioturbation appears to be the most active pedogenic process operating in many soils, with freeze–thaw environments a noted exception. Although bioturbation far outweighs soil production, it does not always lead to homogenisation as is often reported. We maintain that the above-ground component of bioturbation, i.e. mounding, may alone, or particularly when combined with particle sorting via rainwash processes, lead to horizonisation and texture contrast soils in those materials that can be sorted such as mixtures of sand and clay. Together, TCN- and OSL-based estimates of hillslope soil transport and bioturbation, suggest significant rates of downslope soil mantle movement coupled with rapid mixing, contrary to in situ soil development models.

Geogenesis, pedogenesis, and multiple causality in the formation of texture-contrast soils

Coarse-over-fine vertical texture contrasts (VTC) are common in soils and weathering profiles in a variety of environmental settings, including many where inheritance or surficial processes alone cannot account for them. A multiple causality model is presented here which shows that texture contrasts can form in response to a combination of ubiquitous phenomena. There are six key elements: downward translocation by water, erosional winnowing, soil mixing by bioturbation, the tendency for surface clay additions to be mobilized while subsurface clays are more likely to remain in place, and biological facilitation of moisture flux. No element alone is sufficient to create a VTC, but not all are necessary in any given regolith. The model is illustrated by application to case studies in the Ouachita Mountains, Arkansas; ridgetops in eastern Kentucky; the lower coastal plain of North Carolina; and the upper coastal plain of east Texas. The implications for interpretation of soils, paleosols, and weathering profiles is that the presence of a textural contrast, by itself, does not connote any particular set of geogenic or pedogenic processs or controls. Texture contrasts may be geogenic, pedogenic, or both. Given the widespread occurrence of the mechanisms of the multiple causality model, vertical texture contrasts are inevitable.

The development of columnar peds in a texture contrast soil in the Pilliga State Forests, northwestern New South Wales

The development of columnar peds in a texture contrast soil in the Pilliga State Forests, northwestern New South Wales

Soil development on dolerite and its implications for landscape history in southeastern Tasmania

Soil genesis has been examined using field description, particle-size distributions, chemical properties, mineralogy and elemental distributions of five soil profiles developed on dolerite on Mt. Nelson and Tolmans Hill near Hobart in Tasmania. The soils form a sequence ranging from a Black Vertosol (P8) to four texture contrast soils, namely, a Eutrophic Brown Chromosol (P5), two Mottled-Subnatric Grey Sodosols (MN8 and P4), and a Mottled-Mesonatric Grey Sodosol (P7). The soil stratigraphic and pedological relationships of these soils have been investigated to help understand their distribution and improve understanding of soil formation history. The knowledge of the soil stratigraphy and weathering features aided in the determination of the broader landscape history. The field observations show the local dolerite has been subjected to both deep weathering and severe erosional periods. Pockets of deeply weathered dolerite occur adjacent to thin A/C soils or hard outcropping rock. Deeper colluvial soil materials occur on lower slopes. The presence of protruding dolerite columns now buried by transported clayey slope-wash materials indicates partial landscape stripping followed by reburial. The presence of buried stone-lines separating the upper profile from the clayey subsoils supports the idea of a second major erosional–depositional cycle. A pronounced variation between the A and B horizons particle-size distribution, mineralogy and elemental distribution supports the conclusion that the modern soils are composed of several sedimentary layers which cap a variable thickness of in situ weathered dolerite (termed “mealy material”) above fresh dolerite. Bedrock jointing, veins and rock fabric extend upward from the bedrock into the mealy material, but are truncated abruptly at the contact with the clayey subsoil. Soil-forming processes have operated to modify soil colours and mottling, soil structure and cation chemistry. These findings have important implications for landscape history, slope processes and the improved understanding of the distribution of dolerite-derived soils in Tasmania.

Modelling trends in woody vegetation structure in semi-arid Australia as determined from aerial photography

Accounting of carbon stocks in woody vegetation for greenhouse purposes requires definition of medium term trends with accurate error assessment. Tree and shrub cover was sampled through time at randomly located sites over a large area of central Queensland, Australia using aerial photography from 1945 to 1999. Calibration models developed from field data for the same land types as those represented within the study area allowed for the extrapolation of overstorey and understorey cover, basal area and biomass values and these were modelled as trends over the latter half of the 20th century. These structural attributes have declined over the region because of land clearing with values for biomass changing from a mean of 58.0(±1.2) t/ha in 1953 to 41.1(±1.0) t/ha in 1991. The biomass of Acacia on clay and Eucalypt on texture contrast soils land types has declined most dramatically. Within uncleared vegetation there was an overall trend of increase from 56.1(±1.2) t/ha in 1951 to 67.6(±1.3) t/ha in 1995. The increase in structural attributes within uncleared vegetation was most pronounced for the Eucalypt on texture contrast soils and Eucalypt on clay land types. It was demonstrated that the sites sampled were representative of their land types and that spatial bias of the photography, undetected tree-killing, sampling error, inherent variability of structural attributes and measurement error should not have impacted greatly on bias or precision of trend estimates for well-sampled land types. Certainly the errors are not likely to be substantial for trends averaged over all land types and they provide an accurate assessment of the magnitude and direction of change. The technique presented here would appear to be a robust means of accounting for the above-ground woody component of woodlands and open forests and will also contribute to a broader understanding of savanna dynamics.

Seasonal changes in hydrochemistry along a toposequence of texture-contrast soils

Ameliorative strategies are urgently required in some agricultural catchments in southern Australia to reduce the loss of potential contaminants to streams. However, a better understanding of where the contaminants are generated on hillslopes, their forms, and the pathways through which they are transported were required. Thus, seasonal changes in the quantities and forms of several chemical species were measured in both vertical and lateral flow pathways at 4 sites along a toposequence in the Mt Lofty Ranges, South Australia. Instrumentation was installed to measure and quantify overland flow and throughflow, and porous-wick samplers were installed at 2 depths to study the chemistry of leachate. Neutron moisture meter access tubes were installed to measure seasonal changes in soil water content with depth as this influences chemical concentrations and mobility. In years of average to below average annual rainfall, throughflow was the most important transport pathway for contaminants. However, it was expected that overland flow will be the dominant transport pathway when annual rainfall is above about 550 mm. Changes in water content of the texture-contrast soils was caused by seasonal rainfall causing periodic saturation, by waterlogging, groundwater, or both. This affected the type and form of contaminant. For example, Na and Cl concentrations were generally only large (800 and 1500 mg/L, respectively) on the lower slopes but in the wettest seasons their concentrations increased at depth on mid-slopes due to the influence of shallow saline groundwater. These chemicals then leached when groundwater levels subsided. The results suggest that ameliorative strategies to reduce agricultural contaminants should target the transport pathways specific to each chemical species, at the point (or points) in the landscape where they are generated.

An overview of water logging and salinity in southwestern Australia as related to the ‘Ucarro’ experimental catchment

Water logging and salinity are major forms of land degradation in southwestern Australia and are predicted to become even more extensive. There are basic requirements for the development of salinisation and water logging within the specific geology and geomorphology of the region. The nature, extent and consequences of water logging and secondary salinity in the agricultural areas of southern Australia is presented in overview with a particular emphasis on the type of landscape represented by the ‘Ucarro’ experimental site, near Katanning in southwestern Australia. Three types of water logging are identified. These are associated with perched aquifers in texture-contrast (duplex) soils, inundation of terraces and valleys, and saturation in surface soil due to the hydraulic pressure being above ground level in aquifers at the base of the regolith of highly weathered granites and gneisses or within channels in broad valley sediments. A dominance of the vertical component of flow in perched aquifers is identified which contributes significantly to recharge of deep aquifers and subsequent land salinisation. Water logging has reduced the potential yield by 30–80% for many crops and pastures in the >400 mm rainfall zone. Increasing land salinisation has been accompanied by increasing trends in salinity of water resources. There is an interaction between water logging and salinity where groundwater discharge occurs at the soil-surface. Water logging may be the more serious inhibitor for plant growth than salt even at low salt levels. The interaction of water and salt is synergetic for plants. Remotely-sensed techniques using surrogates such as vegetation type and condition have been developed to provide broad spatial assessments of saline and water-logged areas since 1987. In 1990, the shires to the east and west of ‘Ucarro’ had 9.1 and 2.4% of agricultural land salt-affected, respectively. In both shires, a 0.7% increase in salt-affected land has occurred in the last 8 years.

Contingency and generalization in pedology, as exemplified by texture-contrast soils

Soils and landscapes are subject to historical and spatial contingency, leading to locally unique pedologic features. This can make broad-scale generalizations difficult, impractical, or even impossible. The development of vertical texture-contrast soils with argillic horizons, for example, is potentially subject to all general forms of spatial and historical contingency. A case study in east Texas shows evidence both supporting and refuting five general classes of explanation for the formation of vertical textural contrasts. Multiple causality is likely, and attempts to apply any single explanation to a county-size area (and sometimes to a pedon) are not likely to be successful. The implication is not that pedologists should abandon the search for generalizations, but that the context in which laws and generalizations are developed needs rethinking. Explanatory constructs should be formulated not with the notion that a single explanation is likely to be applicable to most soils, but with the idea that multiple causality and polygenesis are likely, and that location-specific characteristics cannot be ignored. The search is directed not toward a single principle to explain the majority of cases and against which exceptions can be judged, but toward a set of principles that define the possibilities (or probabilities). Two analogies that may be useful in addressing historical and spatial contingency in soils are proposed, based on demographic and synoptic metaphors.