Range Of Soil Textures Research Articles

Soil depth and precipitation moderate soil textural effects on seedling survival of a foundation shrub species

In drylands, there is a need for controlled experiments over multiple planting years to examine how woody seedlings respond to soil texture and the potentially interactive effects of soil depth and precipitation. Understanding how multiple environmental factors interactively influence plant establishment is critical to restoration ecology and in this case to broadscale restoration efforts in western U.S. drylands dominated by big sagebrush (Artemisia tridentata). We planted sagebrush seedlings across a range of soil textures and depths in the southern portion of the species' range, on the Colorado Plateau, USA. We evaluated survival of repeated plantings of caged and uncaged seedlings over 2 years across 20 plots in wet versus average precipitation years at one site, and examined broader patterns of sagebrush seedling survival during an average precipitation year in 56 plots across four sites. First‐year survival was >9× higher under wet than average precipitation. Under favorable (wet) conditions, early sagebrush seedling survival was highest on coarser soils, especially those that also had a shallower restrictive layer (e.g. 50–100 cm). Under average precipitation, soil texture and depth effects on survival of newly planted seedlings were much weaker, but older (>1 year) seedlings benefitted from growing on coarser‐textured soils. It may be possible to increase survival by sheltering seedlings with small mesh cages, which likely improve moisture availability. Our results provide new insights into environmental factors that limit woody seedling survival in drylands and illustrate that planting in wet years and incorporating detailed soil setting information could increase survival of sagebrush seedlings in restoration projects.

Field scale discharge and water quality response, to drainage water management

Subsurface (tile) drainage, while necessary for viable agricultural crop production in the humid, poorly drained regions of the world, has been linked to offsite nutrient transport, culminating in harmful algal blooms and hypoxia in downstream waterbodies. Drainage water management (DWM), also known as controlled drainage, has been promoted as a method to reduce nitrogen (N) and phosphorus (P) loss from field scale tile drainage; however, subsequent changes in surface runoff and associated water quality have not been explored. Four years of surface and subsurface discharge, N, and P loss from two sets of paired field sites in northwest Ohio, USA were evaluated using a before-after control-impact (BACI) design to quantify the impact of DWM. Mean event surface runoff was greater (significant at one site) under DWM while tile drainage discharge was less at both sites but not significant. There was no significant difference in surface nitrate-N (NO3--N) loss; however, significant reductions in tile NO3--N loss under DWM were measured at both sites. Similar to discharge, dissolved reactive P (DRP) loss measured in surface runoff was greater under DWM (significant at one site), while no significant differences were noted in mean event tile drainage DRP loss. Total P (TP) surface runoff losses were significantly greater under DWM at the site with significantly greater surface runoff, while DWM reduced tile drainage TP loss at only one site. These findings suggest that DWM reduced tile NO3--N loss, while also highlighting that any benefit from DWM with respect to tile DRP or TP loss could be negated in part by increased surface losses. Furthermore, these findings emphasize the need to understand the natural resource concern to which the practice is targeted and also call for additional research on DWM implementation that includes surface runoff across a range of soil textures, cropping management, and climates.

Infiltration under Ponded Conditions

Ponded infiltration processes occur in agricultural lands irrigated by flooding of their soil surface or under insufficient drainage conditions. The existing equations describing the phenomenon of vertical infiltration under ponded conditions have not considered the actual contribution of the pressure head gradient to the flow. In this study, simple equations are proposed to describe the horizontal and vertical infiltration under various ponding heads incorporating the actual contribution of the pressure head gradient to the flow. Six soils with known hydraulic properties, covering a wide range of soil textures, were used. Horizontal and vertical infiltration data are obtained by numerical simulation for all soils studied using the Hydrus-1D code. To validate the accuracy of the proposed equations, the solutions of horizontal and vertical infiltrations provided by the proposed equations were compared with numerically simulated ones provided by the Hydrus 1-D. The analysis of the results showed a very good agreement in all soils studied. The proposed vertical infiltration equation was also compared to a simple and accurate equation which does not incorporate the actual contribution of the pressure head gradient to the flow and differences between them were observed in all soils studied.

Spatial distribution of halophytes and environment factors in salt marshes along the eastern Yellow Sea

BackgroundSalt marshes provide a variety of ecosystem services; however, they are vulnerable to human activity, water level fluctuations, and climate change. Analyses of the relationships between plant communities and environmental conditions in salt marshes are expected to provide useful information for the prediction of changes during climate change. In this study, relationships between the current vegetation structure and environmental factors were evaluated in the tidal flat at the southern tip of Ganghwa, Korea, where salt marshes are well-developed.ResultsThe vegetation structure in Ganghwa salt marshes was divided into three groups by cluster analysis: group A, dominated by Phragmites communis; group B, dominated by Suaeda japonica; and group C, dominated by other taxa. As determined by PERMANOVA, the groups showed significant differences with respect to altitude, soil moisture, soil organic matter, salinity, sand, clay, and silt ratios. A canonical correspondence analysis based on the percent cover of each species in the quadrats showed that the proportion of sand increased as the altitude increased and S. japonica appeared in soil with a relatively high silt proportion, while P. communis was distributed in soil with low salinity.ConclusionsThe distributions of three halophyte groups differed depending on the altitude, soil moisture, salinity, and soil organic matter, sand, silt, and clay contents. Pioneer species, such as S. japonica, appeared in soil with a relatively high silt content. The P. communis community survived under a wider range of soil textures than previously reported in the literature; the species was distributed in soils with relatively low salinity, with a range expansion toward the sea in areas with freshwater influx. The observed spatial distribution patterns may provide a basis for conservation under declining salt marshes.

Impacts of residue quality and soil texture on soil aggregation pathways by using rare earth oxides as tracers

Rare earth oxides (REOs) as tracers track aggregate formation and breakdown pathways. However, investigations on the impacts of residue quality and soil texture on aggregation pathways are lacking. In this study, easily, moderately and slowly decomposable residues with different qualities (milk vetch, C/N = 16.7; maize, C/N = 35.5; and decomposed maize, C/N = 30.8) were applied to four types of red soils (Red clay, Basalt, Sandstone and Granite) with a range of soil textures. Aggregate fractions of each soil were labelled by different REOs. The soil respiration (CO2), mean weight diameter (MWD) and REOs concentrations were measured after 0, 7, 14, 28 and 56 days of incubation. The results showed that except for Basalt soil, the incorporation of organic residues increased the formation but decreased the breakdown of macroaggregates in the order of milk vetch > maize > decomposed maize in the first two weeks, thus leading to a greater standardized MWD in higher-quality residue-incorporated soils. In the mid-to-late incubation time (28–56 d), the standardized MWD and relative formation of macroaggregates in the milk vetch and maize treatments converged regardless of soil types. An exponential relation between the standardized MWD and accumulated CO2 (P < 0.05) indicated that the residue quality-dependent MWD dynamics were further driven by microbial activity. The two coarse-textured soils (Sandstone and Granite) presented a more rapid response of aggregation to microbial activity than the two fine-textured soils (Red clay and Basalt) did. The relative formation of 2–5 mm aggregates was significantly greater in Sandstone soil at a given residue incorporation, thus leading to a faster increase in the standardized MWD compared with fine-textured soils. However, a similar result was not observed in Granite soil. A positive linear relationship between the standardized MWD and 2–5 mm aggregate formation in all investigated soils (P < 0.001) suggested that MWD dynamics mainly depended on the newly formed large macroaggregate rate. Our study demonstrates that the dynamics of soil aggregation are closely related to microbial activity and newly formed macroaggregates, beyond the impacts of residue quality and soil texture on soil aggregation.

Sentinel-2 Leaf Area Index Estimation for Pine Plantations in the Southeastern United States

Leaf area index (LAI) is an important biophysical indicator of forest health that is linearly related to productivity, serving as a key criterion for potential nutrient management. A single equation was produced to model surface reflectance values captured from the Sentinel-2 Multispectral Instrument (MSI) with a robust dataset of field observations of loblolly pine (Pinus taeda L.) LAI collected with a LAI-2200C plant canopy analyzer. Support vector machine (SVM)-supervised classification was used to improve the model fit by removing plots saturated with aberrant radiometric signatures that would not be captured in the association between Sentinel-2 and LAI-2200C. The resulting equation, LAI = 0.310SR − 0.098 (where SR = the simple ratio between near-infrared (NIR) and red bands), displayed good performance ( R 2 = 0.81, RMSE = 0.36) at estimating the LAI for loblolly pine within the analyzed region at a 10 m spatial resolution. Our model incorporated a high number of validation plots (n = 292) spanning from southern Virginia to northern Florida across a range of soil textures (sandy to clayey), drainage classes (well drained to very poorly drained), and site characteristics common to pine forest plantations in the southeastern United States. The training dataset included plot-level treatment metrics—silviculture intensity, genetics, and density—on which sensitivity analysis was performed to inform model fit behavior. Plot density, particularly when there were ≤618 trees per hectare, was shown to impact model performance, causing LAI estimates to be overpredicted (to a maximum of X i + 0.16). Silviculture intensity (competition control and fertilization rates) and genetics did not markedly impact the relationship between SR and LAI. Results indicate that Sentinel-2’s improved spatial resolution and temporal revisit interval provide new opportunities for managers to detect within-stand variance and improve accuracy for LAI estimation over current industry standard models.

Modeling Distribution of Potassium in the Soil in Under Drip Irrigation

In recent years, use of drip irrigation has been increased for many products, in a wide range of soil textures and structures. So far, any model has not been provided for distribution of potassium in the soil during irrigation. The purpose of this, research is modeling distribution of potassium in the soil, under drip irrigation.

Can biochar conserve water in Oregon agricultural soils?

Few studies have evaluated the ability of biochar to improve soil water content under field conditions. Our objective was to assess the potential drought benefits of biochar across a range of soil textures by measuring indicators of hydraulic capacity and water-retention curves for field-amended soils. Two biochars, conifer wood and wheat straw, were incorporated by rotary tillage at amendment rates of 9−36 Mg ha−1 in four locations. All four soils showed a significant increase (p≤0.04) in saturated water content of 0.05–1.7 % volume per Mg ha−1 biochar added, but no increase in water content at field capacity. There were significant (p<0.01) decreases in volumetric water content at wilting point by 0.09 to 0.8 % per Mg ha−1 of biochar added in all soil textures. This translated to small, significant increases in plant-available water capacity of the same magnitude. Although the conifer wood biochar was less hydrophobic and had 14 % more internal pore space than the wheat straw biochar, both biochars had similar impacts on volumetric soil water content (θ). Simulations of soil drying and recharge using moisture retention curves and the Hydrus-1D model indicated that biochar amendment would generally decrease θ in the loam soil, and would have minimal impact on θ (± 1 %) in the more coarse textured soils. Monitoring of in situ θ in two of these soils showed similar results. Biochar sped up soil drying in the loam following irrigation, but had no impact on θ in the loamy sand. These results suggest limited utility for biochar as a drought adaptation tool in the first year following incorporation into these medium- to coarse-textured soils.

Choosing Native Species for Restoring Crested Wheatgrass Fields on the Great Plains of Northeast Montana

Large areas of the Northern Great Plains of North America were intentionally planted with the non-native crested wheatgrass 80–90 y ago. Land managers hope to restore these near-monoculture fields to more diverse native grassland. Our goal was to help guide selection of native species for restoration. We sampled vegetation at each of 24 fields planted with crested wheatgrass in the 1930's in northeast Montana across a range of soil textures. We documented composition at the edge of each crested field and in adjacent native vegetation, allowing us to rank native species as to their ability to persist with crested wheatgrass and determine how soil texture influences their ability to coexist with crested wheatgrass. We identified 29 native species that commonly re-established in crested wheatgrass fields. Sagebrush and several grasses occurred with similar abundance across all soils. Six forb species occurred preferentially on either heavy or light soils. Our results can be used to design seed mixes that will provide native species capable of coexisting with crested wheatgrass in eastern Montana. Application of our methods may be useful in other areas of the Great Plains.

Estimation of soil water retention curve in semi-arid areas using fractal dimension

The soil water retention curve (SWRC) is one of the important hydraulic functions in water flow modeling and solute transport in the porous medium. Direct measurement of SWRC is time consuming and expensive, therefore different models have been developed to describe it. In this study, a model based on fractal theory was derived to estimate water retention curve. The fractal dimension of SWRC (D SWRC ) for 130 soil samples (with a spread range of soil texture) were determined and tried to find out a simple relation between this parameter and easily available soil properties such as clay, silt and sand contents, lime percent and bulk density by applying multiple linear regression analysis. The measured D SWRC for 110 soil samples used for regression analysis and 20 soil samples was used for model validation. The regression analysis showed a linear relationship between D SWRC , with clay, silt contents and soil bulk density with the goodness of fit, R 2 = 0.909, but lime content did not show any significant effect on SWRC prediction improvement. Therefore, it can be concluded that estimating SWRC in calcareous soil using D SWRC obtained from soil easily measured properties will be a good, rapid and reliable alternative for reliable estimation of soil hydraulic properties of these areas. Keywords: Fractal model; lime percent; Regression analysis; Soil water retention curve

Soil bulk density impacts on root water potential and ABA export in drying soil

Abscisic acid (ABA) is a phytohormone known to regulate leaf gas exchange and water loss by inducing stomatal closure. ABA is synthesised in response to a variety of abiotic stressors in soil, particularly water deficit. Previous work demonstrated that across a range of soil textures, root water potential better predicted xylem ABA concentration than soil matric potential. However, the impacts of soil management practices (e.g. cultivation, compaction, organic matter addition) on the relationship between root water potential and xylem ABA concentration, when texture is held constant, has not been investigated. A loam-based growing substrate was compressed to three bulk densities (1.1, 1.3 and 1.4 g cm-3) in pots designed to fit in a Scholander-type pressure chamber, allowing the water potential of the bulk root system to be measured. After measuring root water potential, additional pressure was applied to collect root xylem sap at flow rates that matched transpirational flow. This allowed accurate determination of ABA concentrations and delivery. Low bulk density enhanced the increase in xylem ABA concentration as root water potential declined. Increasing bulk density de-sensitised the relationship between root water potential and xylem ABA concentration. Further study is required to determine whether changes in soil structure due to field management regimes will alter the relationship between root and soil water potential.

Estimation of soil water retention curve using fractal dimension

The soil water retention curve (SWRC) is a fundamental hydraulic property majorly used to study flow transport in soils and calculate plant-available water. Since, direct measurement of SWRC is time-consuming and expensive, different models have been developed to estimate SWRC. In this study, a fractal-based model was developed to predict SWRC. A wide range of soil textures (130 soil samples) was used to determine the fractal dimension of SWRC (D SWRC ). Moreover, the SWRC pedotransfer functions were established based on easily available soil properties such as particle size distribution and bulk density by applying multiple linear regression analysis. The measured D SWRC for 110 soil samples was considered for function parameterization and the remaining was used for model validation. The results illustrated that the D SWRC linearly correlates with clay and silt contents and soil bulk density (r 2 = 0.909). The SWRC can, therefore, be easily and concisely estimated by the proposed fractal-based functions. Keywords: Fractal model; Pedotransfer functions; Regression analysis; Soil water retention curve

Connecting crop models with highly resolved sensor observations to improve site-specific fertilisation

High spatial variability of soil properties restricts the benefits of process-oriented modelling for management recommendations on field scale due to rare information about the soil inventory and its distribution. Geo-electrical mapping provides with a certain spatial pattern, but results are influenced by various factors. The model HERMES was applied to 60 of altogether 80 soil sampling points of a well-documented field in North-Rhine Westphalia characterised by a wide range of soil texture. Validation of HERMES resulted in satisfactory root mean square errors (RMSE) for yield (0.76 t ha-1), water (34.70 mm) and nitrogen in soil (51.44 kg ha-1) over the whole simulation period. At the same field, values of conducted electrical conductivity (ECa) mapping ranged from 20 to 90 mS m-1 and have been assigned to the soil sampling points to proof statistical relations. Clay and sand contents of three soil layers were highly correlated with measured ECa values. Derived regression models show R2 values between 0.73 and 0.89. The subsequent calculation of soil texture at points of ECa mapping produced an improved resolution of this key value to initialize model simulation.

Soil nitrous oxide emissions in forage systems fertilized with liquid dairy manure and inorganic fertilizers

Intensive dairy production generates large amounts of manure, which is typically reused as fertilizer for the production of forage. In the Central Valley of California, application of manure often results in high inputs of N and labile organic C, a combination that can lead to large nitrous oxide (N2O) emissions. The present study included dairy forage production fields at three dairies with a range of soil textures. Liquid manure from lagoons was applied in surface irrigation water in addition to inorganic N fertilizers. Nitrogen inputs, N2O emissions, crop N removal, soil inorganic N, moisture, and temperature were assessed over two summer corn and one winter forage seasons at each dairy. Between 0.76 and 1.30% of the applied total N was emitted as N2O annually. A significant portion of the N2O emissions was explained by high soil moisture, soil inorganic N, and soil temperature, as shown by multiple regression analysis. Variability in total N2O emissions and emission patterns was likely due to differences in N management and soil texture. Large single applications of N when crop N uptake potential was low were responsible for the largest N2O emissions, whereas distributing N additions, applied with the irrigation water, over the cropping season resulted in lower emissions. The field study was complemented with laboratory incubations of soil amended with liquid manure and 15N-labeled urea-ammonium-nitrate (UAN) to determine the proportion of liquid manure and fertilizer N emitted as N2O. The incubation results showed that liquid manure stimulated the release of N2O originating from fertilizer N. Therefore, liquid manure should not be added to irrigation water following side dress fertilizer N applications. To reduce the risks of N2O emissions and other N losses to the environment, manure and fertilizer N inputs, as well as soil N mineralization, must be accounted for and N supplied in increments according to crop demand.

Changes in Relative Gas Diffusivity Explain Soil Nitrous Oxide Flux Dynamics

Nitrous oxide (N2O) is a greenhouse gas and the main anthropogenic emission contributing to stratospheric ozone depletion. Agricultural soils dominate anthropogenic N2O emissions but there is very limited information specifically relating relative soil gas diffusivity (Dp/Do) to N2O emissions. This study was conducted to determine the effects of soil bulk density (ρb) and matric potential (ψ) on Dp/Do and the associated N2O fluxes in the presence of denitrification substrates. The interaction between soil ρb and ψ on Dp/Do and N2O fluxes was investigated using 880 repacked soil cores that were saturated with a nitrate (NO3–) solution and placed on tension tables at 11 levels of ψ and 5 levels of soil ρb. After equilibration (4 d) N2O fluxes, Dp/Do, inorganic-N concentrations, and soil physical characteristics were determined. Emissions of N2O peaked at increasingly lower levels of ψ(–1.5 to –6.0 kPa) as soil ρb increased (1.1 to 1.5 Mg m–3) due to increasing microporosity. Peak N2O emissions occurred across a relatively wide range of water-filled pore space (WFPS) and volumetric water content. A Gaussian fit of N2O-N fluxes against ψ showed maximum fluxes were related to the soil's air-entry potential (r2 = 0.96). Maximum N2O emissions occurred at a Dp/Do value of 0.006 that was independent of soil ρb. Log N2O-N flux was a function of log Dp/Do (r2 = 0.82) when Dp/Do was >0.006. Soil Dp/Do is a key indicator of N2O emission potential and needs to be further explored as a predictor of N2O emissions in a range of soil textures and denitrification substrates.

Integrated Soil Fertility Management: Aggregate carbon and nitrogen stabilization in differently textured tropical soils

Soil organic matter is important to improve and sustain soil fertility in tropical agroecosystems. The combined use of organic residue and fertilizer inputs is advocated for its positive effects on short-term nutrient supply, but the effect of the integrated use on long-term stabilization of soil organic C and N is still unclear. We conducted a 1.5-y soil incubation experiment with maize (Zea mays) residue and urea fertilizer to examine the stabilization of C and N in four Sub-Saharan African soils differing in texture (sand, sandy loam, clay loam, and clay). The inputs were enriched with 13C and 15N in a mirror-labelling design to trace the fate of residue-C and N, and fertilizer-N in combination. We hypothesized that combining inputs would enhance the stabilization of C and N relative to either input alone across a range of soil textures. The treatments were destructively sampled after 0.25, 0.5, and 1.5 y to assess input-derived C and N stabilization in soil macro- and microaggregate fractions. The combination treatment had a significant but small (2% of residue-applied C) increase in residue-C stabilized in the total soil after 0.25 y, but this increase did not persist after 0.5 and 1.5 y. While combining residue and fertilizer decreased the amount of residue-N stabilized within 53- to 2000-μm sized soil aggregates (e.g., 7% less at 1.5 y), it increased the stabilization of fertilizer-N at all sampling times (e.g., 20% more at 1.5 y). The increased amount of fertilizer-N stabilized was significantly greater than the amount of residue-N lost in the combined input treatments in the three finer textured soils at 1.5 y, indicating an interactive increase in the stabilization of new N. Our results indicate that combining residue with fertilizer inputs can increase the short-term stabilization of N, which has the potential to improve soil fertility. However, benefits to N stabilization from combining organic residues and fertilizer seem to be less in coarser-textured soils.

A Physicoempirical Model for Soil Water Simulation in Crop Root Zone

To predict soil water variation in the crop root zone, a general exponential recession (GER) model was developed to depict the recession process of soil water storage. Incorporating the GER model into the mass balance model for soil water, a GER-based physicoempirical (PE-GER) model was proposed for simulating soil water variation in the crop root zone. The PE-GER model was calibrated and validated with experimental data of winter wheat in North China. Simulation results agreed well with the field experiment results, as well as were consistent with the simulation results from a more thoroughly developed soil water balance model which required more detailed parameters and inputs. Compared with a previously developed simple exponential recession (SER) based physicoempirical (PE-SER) model, PE-GER was more suitable for application in a broad range of soil texture, from light soil to heavy soil. Practical application of PE-GER showed that PE-GER could provide a convenient way to simulate and predict the variation of soil water storage in the crop root zone, especially in case of insufficient data for conceptual or hydrodynamic models.

Factors governing survival of black willow ( Salix nigra) cuttings in a streambank restoration project

A field study was conducted at Little Topashaw Creek in northern Mississippi, aimed at expanding the limited database on the survivorship of Salix nigra (black willow) cuttings planted on riparian restoration sites. We tested the hypothesis that sediment moisture availability (deficit, excess) as mediated by sediment texture and depth to the prevailing water table is a major factor governing black willow survival during the initial stage of establishment following transplanting. Replicated plots were established across elevational gradients and a range of soil texture. Each plot contained 16 planted cuttings (2.5 cm diameter × 2.5 m length). Plot depth to water table, soil texture, and soil redox potential were measured. Plant gas exchange, leaf chlorophyll content, growth, and survival were monitored periodically over two growing seasons. Survival was best at low elevation compared to cuttings planted at mid- and high elevations. Poor survival and growth were noted for cuttings that encountered sediment moisture deficits in plots with coarse texture while the best cutting survival was recorded for intermediate sand content plots. Results indicated that plot location on the bank and soil texture are two important factors that influence riverbank restoration success. Therefore, any riparian restoration plan should include careful assessment of these factors prior to undertaking such efforts.

Comparisons of different procedures of pre-compaction stress determination on weakly structured soils

Compaction is an important component of soil degradation. In this regard, the pre-compaction stress ( σ pc) concept is considered useful in mechanized agriculture nowadays. When the external forces exceed the internal strength ( σ pc) of soil, soil structure and soil physical quality will deteriorate. This concept was introduced at first for confined consolidation of non-structured, homogenized and saturated subsoils in civil engineering, though it is also suitable for agricultural conditions where the topsoil and subsoil are considered and both are often structured, heterogeneous and unsaturated. The best method for predicting σ pc is by the plate sinkage test (PST) in the field, but it is expensive and time-consuming. This study was conducted to find an alternative laboratory method besides the confined compaction test (CCT) for predicting σ pc. The CCT may not be a good method, especially at higher water contents, and for soils with low organic matter content, because of low sharpness of the critical region on the stress–strain curves. The study was performed on five soil types with a range of soil textures and organic matter content from central Iran using three loading types and three pF (i.e. Log [soil matric suction in cm]) values of 2.3, 2.7 and 2.9 with two replicates. Loading types consisted of CCT, the semi-confined compaction test (SCCT) and the kneading compaction test (KCT) at three maximum (or pre-compaction) stresses of 200, 400 and 600 kPa. The experiment was a completely randomized factorial design. The aim was to determine how accurately each loading type test could predict σ pc of soil pre-compacted by one of the other methods. The applied combinations of CCT–SCCT, SCCT–CCT and KCT–CCT mean that the soil was pre-compacted by the first loading type and evaluated by the second one. The results showed that σ pc and the sharpness of the σ pc region were significantly affected by loading types as well as the soil conditions. The sharpest σ pc region was observed in SCCT and the least sharp in CCT with the overall order being CCT–SCCT > SCCT–CCT > KCT–CCT. The sharpness of the σ pc region was reduced for the soil samples with higher water content and coarser texture. Regardless of the soil and loading conditions, the prediction by SCCT was consistently more accurate than by CCT. The prediction of σ pc by SCCT was more precise in comparison with CCT especially at higher stresses and soil water contents. However, the prediction of σ pc by SCCT was very accurate at pF values of 2.7 and 2.9, and with low σ pc values, when compared with the actual values of the σ pc. For the clay soil at a pF value of 2.3, no pre-compaction region (i.e. zero σ pc) could be determined by CCT at a maximum axial stress of 600 kPa. This was because of the incompressibility of soil water at this near-saturated soil condition at high stress. However, the sharpness of the critical region in SCCT was high enough to predict σ pc satisfactorily. There was no significant difference between the combinations of SCCT–CCT and KCT–CCT in predicting σ pc. The SCCT is a compromise method that lies between CCT and PST. SCCT has the advantage of using a limited and definite soil volume that can be modeled as a soil element. Marginal effects of disturbance caused by coring/sampling as well as pre-test sample preparation seem to have minor effects on the stress–strain curve determined by SCCT in comparison with CCT. Moreover, the soil volume needed for this test is the same as for CCT.

Water films and scaling of soil characteristic curves at low water contents

Individual contributions of capillarity and adsorptive surface forces to the matric potential are seldom differentiated in determination of soil water characteristic (SWC) curves. Typically, capillary forces dominate at the wet end, whereas adsorptive surface forces dominate at the dry end of a SWC where water is held as thin liquid films. The amount of adsorbed soil water is intimately linked to soil specific surface area (SA) and plays an important role in various biological and transport processes in arid environments. Dominated by van der Waals adsorptive forces, surface‐water interactions give rise to a nearly universal scaling relationship for SWC curves at low water contents. We demonstrate that scaling measured water content at the dry end by soil specific surface area yields remarkable similarity across a range of soil textures and is in good agreement with theoretical predictions based on van der Waals interactions. These scaling relationships are important for accurate description of SWC curves in dry soils and may provide rapid and reliable estimates of soil specific surface area from SWC measurements for matric potentials below ‒10 MPa conveniently measured with the chilled‐mirror dew point technique. Surface area estimates acquired by fitting the scaling relationship to measured SWC data were in good agreement with SA data measured by standard methods. Preliminary results suggest that the proposed method could provide reliable SA estimates for natural soils with hydratable surface areas smaller than 200 m2/g.