Texture-contrast Soils Research Articles

Soil arthropod responses to subsoil manuring in irrigated vegetable and rainfed grain production

Diverse soil biology is being increasingly recognised as important for productive soils. Subsoil manuring – the process of placing organic materials underground using a modified deep ripper – has been used to improve the structure and productivity of texture contrast soils. Using soil arthropods as indicator species, this study investigated the soil biology responses of the subsoil manuring approach. Different organic materials were placed in the upper zone of a clay subsoil at field sites used for irrigated vegetable and rainfed grain production. Soil samples were taken from the topsoil and from the depth of organic material placement and processed to extract soil arthropods. Of three suborders of soil arthropods recorded, mesostigmatid mites showed the most notable increases in abundance in response to subsoil manuring. The placement of organic materials at depth resulted in elevated abundance of soil arthropods at depths that are not the normal habitat of these organisms. Abundance and richness were both suppressed under irrigated production compared to rainfed, although it is acknowledged that a number of management practice differences between the two production systems could contribute to the differences observed.

Subsoil manuring produces no measurable change in soil or yield in irrigated vegetable production in the first year after treatment

The expansion of irrigation infrastructure in Tasmania, Australia, has led to intensified irrigated crop production, including for vegetables. This has resulted in increased irrigation of texture contrast soils, and other soils with high clay content in the subsoil. Widely used for rain-fed grain production in other parts of Australia, these soils have been subject to a range of soil amelioration approaches in an effort to improve their soil physical characteristics and capacity to produce crops. Subsoil manuring – the process of placing organic soil ameliorants at a target depth using a specially modified deep ripper – has proven to be successful in increasing yields of rain-fed grain crops. Locally available organic materials (chicken manure and poppy seed meal) were placed in the upper zone of a clay subsoil at three sites used for irrigated vegetable production. The crops grown were green peas for processing and carrots for seed production. Normal grower management practices were the reference against which treatments were compared. Subsequent measurements showed no consistent or significant benefit from subsoil manuring one year after treatment under the soil and crop management conditions used in this research.

Texture contrast soils (TCS) as indicators of eolian dust inputs in the coastal area of west central Senegal, West Africa

Different hypotheses have been addressed to explain the origin of texture contrast soils (TCS) in coastal regions. Our study investigates the TCS in the coastal region of west central Senegal, West Africa, in order to appreciate the influence of eolian dust inputs on their formation and characteristics. Nine soil profiles, from a levee to backswamp toposequence, three on each landscape position; floodplain, low terrace and middle terrace, were investigated. Soil profiles were described according to the World Reference Base. Particle-size analysis was performed by wet-sieving for the sand fraction and pipette analysis for the silt and clay fractions. Elements were determined in finely powdered soil mixed with cellulose in a ratio of 2:1. In this work, we focused principally on elemental zircon (Zr) and Titan (Ti). Our results show the presence of TCS in the coastal area of west central Senegal. We attribute the formation of these TCS to mainly two different sources of soil material: marine and continental. Marine sediments build the floodplain profiles and the subsoil of low terrace profiles, while the colluvial material derived from continental uplands form the middle terrace profiles and the topsoil of low terrace profiles. The main finding remains the influence of eolian dust inputs on the formation of TCS in this coastal area. Evidence of dust addition was confirmed by the uniformity of soil parent material tested through mS/(cU+mU) and Ti/Zr ratios. Such knowledge enhances understanding of local pedogenesis and may help develop sustainable soil management strategies.

Elucidating the geogenic and pedogenic pathways of formation of soils of Peninsular India – Signatures of past landscape modifications

The soils of modified landforms of Peninsular India present an opportunity to unearth soil and landscape evolution. We explore the potential of vertical texture contrast (VTC) soils as signatures of past landscape modifications and associated soil formation in Peninsular India. We studied six VTC soils occurring on different geological formations for their morphology, geochemistry, and mineralogy in order to interpret the geogenic and pedogenic pathways of their formation. The presence of a lithological discontinuity with sharp contrast in particle size in pedons P1 and P3 suggested that fluvial sedimentary layering led to their formation. Pedons P2 and P5 are examples of buried soils where basaltic alluvium of varying thickness is overlying the soils of granitic and gneissic complex origin. These soils are potential proxies of landform changes attributable to paleo-alluvial mass deposition, possibly during the late Pleistocene. Pedon P4 is a relict paleosol in which the presence of a Ck horizon between the argillic (Bt) and BC horizons indicates sedimentation of stratified rocks (geogenic) followed by clay illuviation (pedogenic) during the past wet climate. The occurrence of smectite-dominant shrink-swell (S-W) soil (pedon P6) over the shale parent material suggests that the S-W soil is from a transported alluvium of weathering Deccan Basalt. Our results demonstrate the usefulness of VTC soils in interpreting the climate-shift-driven soil-landscape evolution in Peninsular India and elsewhere.

Beneficial soil profile differences associated with tropical grass pastures on sodic texture contrast soils in Northern New South Wales

Volunteer native pastures on widespread sodic texture contrast soils in northern New South Wales slopes and plains are known for their limited agricultural production. Fertilised tropical grass pastures on these soils are reported to have much increased pasture production, deeper, more abundant root mass and greater soil profile moisture storage. The subsoil physical differences between native and tropical grass pastures are not well understood. This observational study compared root abundance, soil structure and soil physical parameters (dispersion, bulk density, porosity and pore distribution) in sodic texture contrast soils under native and adjacent, well established and fertilised tropical pastures in a 14-year chronosequence. The physical differences observed may have contributed to improved soil water storage reported by other authors. Fourteen years after establishment, mean root abundance was significantly lower in soils under native pasture and greater in the tropical grass pasture system with 4600 and 8400 m of roots m–3 respectively. Dispersion values were high in native pastures but soils under tropical pastures had to be physically worked to cause dispersion. Bulk density under native pasture was significantly higher than in tropical grass pastures by 0.08 g cm–3 at 0–10 cm and by 0.2 g cm–3 in the upper B horizons. Total soil porosity of topsoils and upper B horizons was consequently lower in native than in tropical grass pasture. Tropical grass pasture upper B horizons had a three-fold greater macroporosity (pores > 30 µm), than under native pastures. This is equivalent to significantly greater potential water flow through stable macropores in dense sodic B horizons in tropical pastures. These findings indicate that pasture system selection and management positively affects deep soil structural properties which promote pasture productivity. The study contributes to a better understanding of mechanisms of published deeper water storage in tropical grass pasture systems on these normally low production soils.

Carbon stability in a texture contrast soil in response to depth and long-term phosphorus fertilisation of grazed pasture

It is important to understand the stability of soil organic matter (SOM) sequestered through land management changes. In this study we assessed differences in carbon (C) stability of pasture soils that had high and low C content (2.35% vs 1.73% whole soil C in the 0–10 cm layer) resulting from long-term phosphorus fertilisation. We used soil size fractionation (fine fraction, coarse fraction and winnowing) to assess the amount of stable C and indicators of microbial decomposition capacity (catabolic profiles, metabolic quotient) to assess C stability. As a main effect throughout the 60-cm profile, C concentrations were higher in the fine fraction soil in the high (excess P fertiliser; P2) than low (no P fertiliser; P0) treatments, demonstrating a larger stable C fraction. For both P2 and P0, there was a strong correlation between C measured in the fine fraction and winnowed fraction in the 0–30 cm layer (R = 0.985, P < 0.001), but no correlation was observed for the 30–60 cm layer (R = 0.121, P > 0.05). In addition, we conducted two incubation experiments to assess C stability in the treatments with depth and to assess C stability in the physical soil fractions. For the surface soils (0–10 cm), the highest respiration occurred in fractions containing plant material, including roots (coarse fraction, 0.65 g CO2-C kg–1 soil; whole soil, 1.48 g CO2-C kg–1 soil), which shows that the plant material was less stable than the fine and winnowed soil fractions (0.43 and 0.40 g CO2-C kg–1 soil respectively). Soil respiration, microbial metabolic quotient and substrate utilisation were similar in P0 and P2. Collectively, the data show that the increased C in P2 was associated with increased C concentrations in the more stable fine soil fraction, but with no change in the stability of the C within the fractions.

Can highly saline irrigation water improve sodicity and alkalinity in sodic clayey subsoils?

The concept of irrigating crops with saline irrigation water is not new, but impacts of this practice on soil properties remain debatable, particularly the use of highly saline water. In this work, key soil chemical properties were assessed to a depth of 300 cm following 2.5 years of application of highly saline irrigation to a sodic texture-contrast soil (Brown Sodosol) in south-eastern Tasmania, Australia. Control plots (rainfall only) were compared to irrigation treatments of low (0.8 dS/m) and high salinity (16 dS/m) waters at application rates of both 200 and 800 mm/year. Whilst significant increases in both electrical conductivity and chloride concentration occurred throughout the soil profile in the high salinity treatment, these values were well below those of the irrigation water, indicating effective deep leaching. In the upper soil profile, 0–50 cm, of the high salinity treatments both the exchangeable Na+ and its ratio to total base cations (ESP) were significantly increased whilst the lower soil profile between 50 and 200 cm, was improved via both reduced alkalinity and sodicity. Leaching of the exchangeable base cations Ca2+, Mg2+ and K+ was significant in the upper soil profile (0–50 cm). As expected, the low salinity treatment (0.8 dS/m) had minimal impacts on soil chemical properties. The upper topsoil (0–10 cm) total organic carbon was significantly reduced in the high salinity plots and was negatively correlated with Cl− concentration. The data confirms the general concerns about application of saline irrigation, namely increased whole profile salinisation and upper soil profile (0–50 cm) sodicity, but they also show unexpected and desirable reductions in the lower soil profile (> 50 cm) alkalinity and sodicity. It appears the Na+ ions present in the saline waters led to differential leaching of base cations from the rooting zone, especially Ca2+ which then ameliorate the alkalinity and sodicity deeper in the soil profile (> 50 cm). Thus, surface application of gypsum may help sustain the application of highly saline waters; alternatively, subsurface irrigation above the sodic clayey subsoils could be trailed.

Drip irrigation wetting patterns and nitrate distribution: comparison between electrical resistivity (ERI), dye tracer, and 2D soil–water modelling approaches

Despite the widespread use of drip irrigation and fertigation, there is limited research on soil wetting patterns and nutrient distribution from drip emitters in agricultural soils. We compared the use of electrical resistivity imaging (ERI) with dye tracer patterns, measured soil nitrate distribution, and modelled changes in soil moisture and nitrate distribution, in a commercial apple orchard in southern Tasmania. Time-lapse two-dimensional ERI revealed wetting plumes beneath the emitters ranged from nil infiltration, to infiltration beyond 1.0 m depth. Lack of infiltration beneath some emitters was attributed to runoff caused by either water repellence or the surface leaf mulch, whilst infiltration to 1.0 m depth was attributed to preferential flow processes. The uncalibrated ERT was unable to discern the separate contributions from of solute concentration, moisture content and temperature to the change in electrical resistivity. The dye tracer study revealed the majority of the fertigated nitrate was retained in the A1 horizon, whilst the fertigated water infiltrated much deeper into the A2 and B2 horizons. The 2D modelling was not able to replicate the variations in infiltration demonstrated by the ERT, principally due to the lumped nature of the soil parameterisation, and inability of the model to simulate preferential flow. As such, use of modelling tools and texture-based guidelines for design and management of drip emitters is not recommended for the texture contrast soils.

The usefulness of soil-agricultural maps to identify classes of soil truncation

Abstract Soil erosion led to the severe transformations of the soil cover of young morainic areas of northern Poland. Main alterations are connected with soil truncation on summits and in upper part of slopes, whereas at foot slopes and within depressions colluvial material is accumulated. Information and knowledge about the extent or intensity of erosion are mainly derived from sophisticated geospatial models or laborious field works. To reduce the effort associated with development of studies on erosion the use of easily available cartographic sources is required. The main aim of the paper is an elaboration of key to reinterpret information taken from soil-agricultural maps in the context of determining the degree of pedons truncation. The study is based on a comparison of the properties of soils representing various classes of erosional alterations with the data on existing maps. The correlation between descriptions recorded in the form of cartographic symbols with properties of pedons divided into several classes of vertical texturecontrast soil truncation and results from potential erosion maps was elaborated. The application of developed interpretative principles allows calculating the share of soil truncation classes within investigated area. The five test plots (each - 1 km2) were located along the north slopes of Noteć Middle Valley and Toruń Basin. The proposed interpretation of soil-agricultural maps reveals their significant value in studies on extent and degree of erosional alterations recorded in soil cover.

Spatial Pedological Mapping Using a Portable X-Ray Fluorescence Spectrometer at the Tallavera Grove Vineyard, Hunter Valley

Wine consumers desire to drink a high quality wine. For producing high quality wine, high quality soil is required. Conventionally, soil quality is assessed qualitatively. Using traditional laboratory methods, quantitative data can be obtained for management purpose, but it is time consuming and expensive. Therefore, new technology aims to address these limitations, namely portable X-Ray fluorescence spectrometers (pXRF). This instrument can be used directly in the field, requires no soil sample preparations, and can simultaneously measure a wide range of elements qualitatively that are useful for pedological studies. The chemical composition (Ca, Fe, Ti and Zr) of soils at Tallavera Grove vineyard in New South Wales, Australia, was studied using a pXRF. The analysis of the soil’s elemental concentration (i.e. Ca and Fe) using pXRF supports management decisions. Measuring the soil’s Ca concentration can be used to identify Ca-rich parent materials (limestone). The limestone indicates good soil conditions for vine production. Fe content was used to identify areas of texture-contrast soils or soil with accumulation of clays in the B horizon. In addition, a soil weathering index was calculated using elemental concentrations (i.e. Ti and Zr) to explore the history of soil formation for making decision of management. This index showed that the soil in the vineyard was affected by two processes: the deposition of materials from elsewhere (Aeolian transport or soil erosion) and mixing of materials from upslope.Existence of marl (limestone) with Ca (A), duplex soil with Fe (B) and three soil profile development patterns derived using the Ti/Zr index (C).

Blade loosening creates a deeper and near-stable rooting zone that raises the productivity of a structurally unstable texture contrast soil

Improving the workability and raising the productivity of structurally weak and/or dispersive texture contrast soils has been the objective of many research projects. These have used applications of gypsum, with and without ripping the top 300–400mm depth of soil, and responses have been moderate and short lived. The approach taken in the present study was to ameliorate the soil by a combination of subtle soil disturbance to a depth of approximately 300mm using a specially designed blade loosener, with controlled traffic and no-tillage crop establishment practices. The aim was to use the roots of the stimulated plant growth to stabilise a loosened and deepened root zone. Comparative conditions in the 0–500mm depth of soil in blade-loosened and normal seedbeds were monitored over three very different growing seasons (2001, 2002 and 2003) using chemical analyses, bulk density (BD), penetration resistance (PR) and soil moisture content measurements. Productivity was monitored by dry matter and grain yield, and profitability by gross margin analyses. Structural stability of the rooting zone soil, or the lack of it, was shown to be a consequence of how the seasonal dynamics of the profile moisture content affected the probability of waterlogged surface soil conditions. In the normal seedbed (control) the surface soil quickly reconsolidated (BD ≥1500kgm–3), and subsoil BD (BD ≈ 1800kgm–3), PR (≥1.8MPa) and percentage saturation (≥95%) remained at levels restrictive of root growth. Conversely, the same properties in the surface and subsoil of the blade-loosened seedbed remained at levels conducive to unrestricted root growth (BD ≤1400kgm–3, PR ≤1.1MPa, saturation ≤70%). The blade-loosened treatment was substantially more productive (average grain yield increase 35%; P<0.05) and profitable (average gross margin increase 56%).

Size of subsoil clods affects soil-water availability in sand–clay mixtures

Clay delving in strongly texture-contrast soils brings up subsoil clay in clumps ranging from large clods to tiny aggregates depending on the equipment used and the extent of secondary cultivation. Clay delving usually increases crop yields but not universally; this has generated questions about best management practices. It was postulated that the size distribution of the subsoil clumps created by delving might influence soil-water availability (and hence crop yield) because, although the clay increases water retention in the root-zone, it can also cause poor soil aeration, high soil strength and greatly reduced hydraulic conductivity. We prepared laboratory mixtures of sand and clay-rich subsoil in amounts considered practical (10% and 20% by weight) and excessive (40% and 60% by weight) with different subsoil clod sizes (<2, 6, 20 and 45 mm), for which we measured water retention, soil resistance, and saturated hydraulic conductivity. We calculated soil water availability by traditional means (plant-available water, PAW) and by the integral water capacity (IWC). We found that PAW increased with subsoil clay, particularly when smaller aggregates were used (≤6 mm). However, when the potential restrictions on PAW were taken into account, the benefits of adding clay reached a peak at ~40%, beyond which IWC declined towards that of pure subsoil clay. Furthermore, the smaller the aggregates the less effective they were at increasing IWC, particularly in the practical range of application rates (<20% by weight). We conclude that excessive post-delving cultivation may not be warranted and may explain some of the variability found in crop yields after delving.

A trench study to assess transfer of pesticides in subsurface lateral flow for a soil with contrasting texture on a sloping vineyard in Beaujolais.

Subsurface lateral flow in both texture-contrast soils and catchments with shallow bedrock is suspected to be a non-point source of contamination of watercourses by pesticides used in agriculture. As a case study, the north of the Beaujolais region (eastern France) provides a favorable environment for such contamination due to its agro-pedo-climatic conditions. Environments seen in the Beaujolais region include intense viticulture, permeable and shallow soils, steep hillslopes, and storms that occur during the periods of pesticide application. Watercourse contamination by pesticides has been widely observed in this region, and offsite pesticide transport by subsurface lateral flow is suspected to be involved in diffuse and chronic presence of pesticides in surface water. In order to confirm and quantify the potential role of such processes in pesticide transfer, an automated trench system has been designed. The trench was set up on a steep farmed hillslope in a texture-contrast soil. It was equipped with a tipping bucket flow meter and an automatic sampler to monitor pesticide concentrations in lateral flow at fine resolution, by means of a flow-dependent sampling strategy. Four pesticides currently used in vine growing were studied to provide a range of mobility properties: one insecticide (chlorpyrifos-methyl) and three fungicides (spiroxamine, tebuconazole, and dimethomorph). With this system, it was possible to study pesticide concentration dynamics in the subsurface lateral flow, generated by substantial rainfall events following pesticide applications. The experimental design ascertained to be a suitable method in which to monitor subsurface lateral flow and related transfer of pesticides.

Increased profile wettability in texture-contrast soils from clay delving: case studies in South Australia

Clay delving is becoming a popular practice to increase productivity of texture-contrast soils in southern Australia. The practice brings subsoil clay to the surface to be mixed with the sandy topsoil, and unlike clay spreading, it combines the addition of hydrophilic material with a ripping effect that disrupts the sharp boundary between the sandy topsoil and clayey subsoil. Our objective was to evaluate the magnitude of effects caused by delving on the spatial distribution of water through the profile for three Sodosols (Stagnic Solonetz soils) in the south-east of South Australia. We also wished to evaluate the extent to which clay delving might reduce water ponding at the A–B horizon interface. We wetted both delved and undelved texture-contrast soils with a Brilliant Blue dye solution under initially dry and wet conditions (to evaluate the effect of antecedent water content), and then took digital images of the stained profiles for quantitative comparison of the wetted areas. The stained soil profiles indicated that clay delving reduced preferential water flow (finger flow) and resulted in deeper and more uniform wetting of the A horizon, particularly under initially dry conditions. Under wet conditions (where water repellence was largely overcome), finger flow was significantly reduced regardless of delving but it still occurred to varying degrees depending on site characteristics. Delving significantly reduced ponding of water at the A–B horizon boundary and allowed greater penetration into the B horizon. At all sites, greater effects occurred directly on the delving lines and diminished with distance, implying that closer spacing of delving tines would increase uniformity of wetting throughout the profile. The effectiveness of delving on profile wetting was highly variable across the three sites, indicating that the outcome depends inter alia on the intrinsic soil characteristics and the delving equipment used in the field.

Hydropedology and Preferential Flow in the Tasmanian Texture‐Contrast Soils

The two‐way interaction between soil morphology and the processes governing soil water movement were investigated for a range of texture‐contrast soil profiles. The texture‐contrast soils consisted of a seasonally water‐repellent sandy loam A1 horizon over a bleached silica‐cemented A2e horizon and a mottled vertic clay subsoil. Differences in soil morphology and structure among sites had little influence on the proportion of soil that participated in infiltration or the maximum depth of infiltration; however, differences in subsoil structure influenced the processes by which water infiltrated and was stored within the B2 horizons. The occurrence of preferential flow was largely controlled by the effects of antecedent soil moisture content on water repellence in the A1 horizon, silica bridging in the A2e horizon, and clay shrinkage in the B2 horizons. Under dry soil conditions, infiltration resulted from up to five different forms of preferential flow. When soils were near field capacity, most forms of preferential flow ceased; however, wetting front instability and lateral flow developed in the A1 horizon. Preferential flows are not thought to have contributed to the pedogenesis of the texture‐contrast soils. Development of the contrasting soil texture horizons, sand infills, and bleached A2e horizons developed before and independently of the observed preferential flow processes in which reworked aeolian sands buried previously developed clay columns.

Land use and management influences on surface soil organic carbon in Tasmania

The effects of environmental parameters, land-use history, and management practices on soil organic carbon (SOC) concentrations, nitrogen, and bulk density were determined in agricultural soils of four soil types in Tasmania. The sites sampled were Dermosols, Vertosols, Ferrosols, and a group of texture-contrast soils (Chromosol and Sodosol) each with a 10-year management history ranging from permanent perennial pasture to continuous cropping. Rainfall, Soil Order, and land use were all strong explanatory variables for differences in SOC, soil carbon stock, total nitrogen, and bulk density. Cropping sites had 29–35% less SOC in surface soils (0–0.1 m) than pasture sites as well as greater bulk densities. Clay-rich soils contained the greatest carbon stocks to 0.3 m depth under pasture, with Ferrosols containing a mean of 158 Mg C ha–1, Vertosols 112 Mg C ha–1, and Dermosols 107 Mg C ha–1. Texture-contrast soils with sandier textured topsoils under pasture had a mean of 69 Mg C ha–1. The range of values in soil carbon stocks indicates considerable uncertainty in baseline values for use in soil carbon accounting. Farmers can influence SOC more by their choice of land use than their day-to-day soil management. Although the influence of management is not as great as other inherent site variables, farmers can still select practices for their ability to retain more SOC.

Spatial distribution of diuron sorption affinity as affected by soil, terrain and management practices in an intensively managed apple orchard

We investigated how the sorption affinity of diuron (3′-(3,4-dichlorophenyl)-1,1-dimenthyl-urea), a moderately hydrophobic herbicide, is affected by soil properties, topography and management practices in an intensively managed orchard system. Soil-landscape analysis was carried out in an apple orchard which had a strong texture contrast soil and a landform with relief difference of 50m. Diuron sorption (Kd) affinity was successfully predicted (R2=0.79; p<0.001) using a mid-infrared – partial least squares model and calibrated against measured data using a conventional batch sorption technique.Soil and terrain properties explained 75% of the variance of diuron Kd with TOC, pHw, slope and WI as key variables. Mean diuron Kd values were also significantly different (p<0.05) between alley and tree line and between the different management zones. Soil in the tree line generally had lower sorption capacity for diuron than soil in the alleys. Younger stands, which were found to have lower TOC than in the older stands, also had lower diuron Kd values. In intensively managed orchards, sorption affinity of pesticides to soils was not only affected by soil properties and terrain attributes but also by management regime.

Subsurface Lateral Flow in Texture-Contrast (Duplex) Soils and Catchments with Shallow Bedrock

Development-perched watertables and subsurface lateral flows in texture-contrast soils (duplex) are commonly believed to occur as a consequence of the hydraulic discontinuity between the A and B soil horizons. However, in catchments containing shallow bedrock, subsurface lateral flows result from a combination of preferential flow from the soil surface to the soil—bedrock interface, undulations in the bedrock topography, lateral flow through macropore networks at the soil—bedrock interface, and the influence of antecedent soil moisture on macropore connectivity. Review of literature indicates that some of these processes may also be involved in the development of subsurface lateral flow in texture contrast soils. However, the extent to which these mechanisms can be applied to texture contrast soils requires further field studies. Improved process understanding is required for modelling subsurface lateral flows in order to improve the management of waterlogging, drainage, salinity, and offsite agrochemicals movement.

Effect of antecedent soil moisture on preferential flow in a texture-contrast soil

The effect of soil moisture status on preferential flow in a texture-contrast soil was investigated by applying 25 mm Brilliant Blue dye tracer to soil profiles at high and low antecedent soil moisture. Differences in soil morphology and chemistry between soil profiles had little effect on the depth of dye infiltration and dye distribution down the profile. Antecedent soil moisture strongly influenced the type, depth and rate of dye tracer movement. In the wet treatment, the dye tracer infiltrated to depths between 0.24 and 0.40 m, at an average rate of 120 mm h−1. Whilst in the dry treatment, the same volume of dye tracer infiltrated to between 0.85 and 1.19 m depth at an average rate of 1160 mm h−1. In dry antecedent conditions, finger flow developed in the A1 horizon as a result of water repellency. In the wet treatment, the wetting front developed permutations but did not break into fingers. Despite similar particle size distributions, flow in the A2e was slower than the A1 horizon, due to the absence of macropores. In the dry treatment, the dye tracer ponded on the upper surface of the B21 horizon, which then spilled down the sides of the large clay columns as rivulets, at rates of between 2000 and 3000 mm h−1. The dye tracer accumulated at the base of the columns resulting in backfilling of the inter column shrinkage cracks, at an estimated rate of 750 mm h−1. In the subsoil, water movement occurred via shrinkage cracks which resulted in flow by-passing 99% of the soil matrix in the B21 horizon and 94% of the soil matrix in the B22 horizon. Evidence of rapid and deep infiltration in ‘dry’ texture-contrast soils has implications for water and solute management. This knowledge could be used to: (i) improve irrigation and fertilizer efficiency (ii) explain variations in crop yield (iii) reduce salinity through improved leaching practices, (iv) reduce the risk of agrochemicals contaminating shallow groundwater.

Inheritance and formation of smectite in a texture contrast soil in the Pilliga State Forests, New South Wales

Smectite genesis is generally considered to require an alkaline environment, for in acid environments it is reportedly unstable. This study shows that smectite is forming in an acid, texture-contrast soil in the Pilliga State Forests in north-western New South Wales. Three modes of smectite genesis in the study soil are presented. The first mode involves direct inheritance from the underlying parent rock. The second and third modes involve precipitation of smectite from solution and its deposition from suspension, respectively. While the bulk of the smectite in the study soil is inherited from the transformation of labile primary minerals and rock fragments in the parent rock, restricted drainage coupled with a parent material capable of supplying the elemental constituents of smectite are also important factors in its genesis.