Search

This paper presents the details of experimental testing of block masonry triplets using the direct shear test to investigate the shear behaviors of block unit-mortar interfaces. Hollow blocks of 100 and 150 mm (4 and 6 in.) thickness and solid blocks of 100 mm (4 in.) thickness were included in the testing program. These were combined with mortars of three grades to cast a total of 84 triplets. In addition to testing the triplets in an unconfined state, three increasing levels of precompression stresses were used separately to test the confined specimens. The shear behaviors of the tested triplets were not influenced by block strength, while shear strength increased (almost) linearly with mortar strength. The mean peak shear stress for the unconfined triplets was 0.4 MPa (58 psi), whereas the average shear modulus of the joint for these triplets was 6.20 times the mortar compressive strength. The Mode II fracture energy of the masonry joints increased at higher precompression levels. The methods of determining shear strength, shear modulus, and shear strength parameters for the mortar joint in block masonry are proposed using the observed data. [ABSTRACT FROM AUTHOR]

Soil compaction is one of the main causes of soil degradation, and some parameters have been used to characterize it, like those related to compressibility and the degree of compactness. To evaluate the rotational grazing time during winter and the presence and absence of grazing, on the compressibility and degree of compactness of a Cambisol, an experiment was installed and consisted of corn planting for silage, a fallow and pasture planting period, with two treatments evaluating the amount of rotational grazing (two and three times in a period of, respectively, 2 and 3 months), subdivided into the presence and absence of dairy cattle grazing. The mean bulk density value of 1.47 Mg/m3 separates the occurrence (plastic deformation) or not (elastic deformation) of additional soil compaction, while the traffic of machinery and animal trampling should occur with soil moisture lower than 0.23 kg/kg, when the soil has a larger load-bearing capacity. For the conservation of soil structure, our study recommends the permanence of cattle in the plots for 30 to 40 min/day and an exit when the pasture height is 0.07 to 0.10 m, with two or three grazing in a period of, respectively, 2 and 3 months.

Soil compaction is one of the main causes of soil degradation, and some parameters have been used to characterize it, like those related to compressibility and the degree of compactness. To evaluate the rotational grazing time during winter and the presence and absence of grazing, on the compressibility and degree of compactness of a Cambisol, an experiment was installed and consisted of corn planting for silage, a fallow and pasture planting period, with two treatments evaluating the amount of rotational grazing (two and three times in a period of, respectively, 2 and 3 months), subdivided into the presence and absence of dairy cattle grazing. The mean bulk density value of 1.47 Mg/m3 separates the occurrence (plastic deformation) or not (elastic deformation) of additional soil compaction, while the traffic of machinery and animal trampling should occur with soil moisture lower than 0.23 kg/kg, when the soil has a larger load-bearing capacity. For the conservation of soil structure, our study recommends the permanence of cattle in the plots for 30 to 40 min/day and an exit when the pasture height is 0.07 to 0.10 m, with two or three grazing in a period of, respectively, 2 and 3 months. [ABSTRACT FROM AUTHOR]

Society calls for protection of agricultural soils in order to sustain the production of foods for a growing population. Compaction of subsoil layers is an increasing problem in modern agriculture and a cause of serious concern because of the poor resilience in natural amelioration. The concept of soil precompression stress has been adapted from civil engineering, although in soil science it is applied to unsaturated soils that have developed a secondary structure from the action of weather, biota and tillage. It assumes strain is elastic at loads up to the precompression stress, while plastic deformation is expected at higher stresses. To determine this threshold we performed uniaxial, confined compression tests for a total of 584 minimally disturbed soil cores sampled at three subsoil layers on nine Danish soils ranging in clay content from 0.02 to 0.38 kg kg-1. The cores were drained to either of three matric potentials (-50, -100 or - 300 hPa) prior to loading. Stress was applied by a constant-strain rate method. We estimated the point of maximum curvature of the strain-log10(normal stress) relation by a numerical procedure. This point is considered here as a compactive stress threshold, typically labeled the soil precompression stress, σpc. The preload suction stress (PSS) was calculated as the product of initial (i.e., before loading) water suction and initial degree of pore water saturation. Multiple regressions were performed to evaluate the effect of soil properties (textural classes, volumetric water content, bulk density (BD), soil organic matter (SOM), and PSS) on σpc. The best model explained 39% of the variation in σpc, and indicated that σpc increases with increasing PSS, BD and SOM. For a given combination of clay, BD and SOM, PSS affected σpc negatively. We recommend our regression model for use in risk assessment tools for estimating sustainable traffic on agricultural soils. The model was validated by five independent data sets from the literature. Our study shows that caution should be applied when regarding σpc as a fixed threshold for compressive strength. We hypothesize that plastic deformation is initiated over a range of stress rather than at a distinctive single value. Further studies are needed to better understand--and potentially quantify--to what extent the predicted σpc can be regarded a central estimate of allowable stress for a given soil. [ABSTRACT FROM AUTHOR]

Machinery traffic and animal trampling can deform the soil and, consequently, impair soil pore functioning. This study aimed to evaluate how soil structure affects the compressibility, physical properties and air permeability of a Typic Paleudalf under forest, pasture and eucalyptus. Soil samples with preserved structure were used to determine soil physical (bulk density, porosity, degree of water saturation at 33 kPa-tension, air permeability) and mechanical properties (soil deformation, precompression stress, compressibility index). After these evaluations, each soil sample was fragmented, sieved, and the metal rings filled with structureless soil, and underwent the same determinations as the samples with preserved structure. For loads greater than the precompression stress (load greater than 200 kPa), soil with non-preserved structure had the largest deformation. An increase in bulk density decreased macropores linearly (R2 = 0.77 and 0.87, respectively, to preserved and non-preserved soil structure) and air flow exponentially. The soil with preserved structure was less susceptible to further compaction. Air flow was greatest in soils with lower bulk density, microporosity and water saturation degree, and a high volume of macropores. Soil structure (preserved and non-preserved) had more significative differences in microporosity, compressibility index, soil deformation, and bulk density at the end of the compression test. [ABSTRACT FROM AUTHOR]

Understanding the soil-plant relationship is important to determine critical limits of soil properties that influence crop growth and yield. The objectives were to quantify the influence of soil compaction levels on physical properties and crop growth and yield in a clayey Oxisol. An experiment was performed having in the main plots, levels of soil compaction (NT: no-tillage during six years, NTC: NT + four passes of a 10 Mg machine in the agricultural year 2003/2004, and Chisel: NT + chiseling and harrowing in the agricultural year 2003/2004), and in the sub-plots, the summer crops soybean and corn, and wheat in the winter season. We measured soil physical and mechanical properties, along with yield of corn, soybean and wheat, and root growth of that last two crops. After four passes of a 10 Mg machine, the soil resistance to penetration increased to a 0.12 m depth, while Chisel disrupted the 0–0.20 m soil layer, with the effects persisting for at least nine months. Soil compaction in no-tillage concentrated in the 0.05–0.15 m layer. Corn yield was similar among the treatments, soybean yield was highest in NT, and the highest yield of wheat was in the sequence with soybean under NT. [ABSTRACT FROM AUTHOR]

Soil structure development can be described with tensile and shear processes as well as the further stabilization of interparticle bonds by hydraulic, chemical, biological, and physicochemical processes. The related shrink, swell or stress strain processes, as well as organic bindings and biological glueing processes, however, define the rigidity limits of soil structure and soil functions, which also coincide with defined boundaries that can be applied in modelling approaches. Aggregate formation due to volume separation occurs in soils depending on these interactions and undergo further strengthening or weakening processes with consequences for their rigidity. The goal of this review is to document these processes with corresponding results and to discuss some consequences for global change impacts on, e.g., plant growth and yield or mechanical strength. It is obvious that the hydraulic and mechanical processes have become neglected to some extent in the study of soil structure formation and aggregation, which caused remaining research gaps identified in this review. Consequently, there is an urgent need for a more precise determination of the rigidity limits of soils under various land use and climatic conditions to better predict or model climatic impacts but also the effect of soil management changes or amelioration impacts. • Aggregate formation undergoes intense changes during pedo- and anthropogenesis. • Hydraulic, mechanical, and thermal processes affect internal soil strength. • Structural shrinkage and precompression stress define the rigidity of the pore system. • The rigidity limit is the boundary for the predictability of soil functions • Tensorial hydraulic, stress strain, and thermal functions vary beyond the rigidity limit. [ABSTRACT FROM AUTHOR]

The mechanical strength of agricultural soils depends on many soil properties and functions. The database, “soil strength and consequences for sustainable land use and soil management SOILMECHDAT-Kiel”, originates from the “Horn Research Group” includes analyses of undisturbed soil samples taken from more than 460 profiles in and is developed in collaboration with BONARES, a funding initiative of the German Federal Ministry for Education and Research that focuses on the sustainable use of soils. For over 40 years, over 42 different authors recorded 59 physical and 29 chemical parameters for complete soil profiles. In order to the aim of the initial analyses of this data is to determine the influence of bulk density (BD) organic matter (OM) and time (year) on precompression stress (Pc) and saturated hydraulic conductivity (ks) as a function of Pc. Three main textural groups sand, loam, and silt for both topsoils and subsoils (SS) were studied. In loamy and silty subsoils BD and OM are not related to Pc (R2 = 0.17 and R2 = 0.25). OM and bulk density are more related to Pc in sandy soils (R2 0.55–0.59). The link between ks and Pc showed that sandy soils have a significantly higher Pc (>150 kPa) and conductivities did not change much. In loamy soils, with a Pc > 90 kPa, 50% of the ks fell below the critical value of 10 cm d−1. For silty soils, at a Pc of 60 kPa, 50% of the data fall below the critical value of ks. These findings suggest that the stability of loamy and silty soils not only depends on OM and BD, but requires further data to explain the variation in the measurements. With respect to ks, the results show that fertile silty soils are more sensitive than formerly defined.

Machinery traffic and animal trampling can deform the soil and, consequently, impair soil pore functioning. This study aimed to evaluate how soil structure affects the compressibility, physical properties and air permeability of a Typic Paleudalf under forest, pasture and eucalyptus. Soil samples with preserved structure were used to determine soil physical (bulk density, porosity, degree of water saturation at 33 kPa-tension, air permeability) and mechanical properties (soil deformation, precompression stress, compressibility index). After these evaluations, each soil sample was fragmented, sieved, and the metal rings filled with structureless soil, and underwent the same determinations as the samples with preserved structure. For loads greater than the precompression stress (load greater than 200 kPa), soil with non-preserved structure had the largest deformation. An increase in bulk density decreased macropores linearly (R2 = 0.77 and 0.87, respectively, to preserved and non-preserved soil structure) and air flow exponentially. The soil with preserved structure was less susceptible to further compaction. Air flow was greatest in soils with lower bulk density, microporosity and water saturation degree, and a high volume of macropores. Soil structure (preserved and non-preserved) had more significative differences in microporosity, compressibility index, soil deformation, and bulk density at the end of the compression test.

In this work, we aimed to investigate changes in compressive properties and least limiting water range (LLWR) of the plough layer and plough pan induced by successive years of sugarcane cultivation. Undisturbed soil samples were collected in a sugarcane field at the plough layer and below tillage depth (plough pan). Uniaxial compression tests were performed for a water tension (h) range of 30 to 15,000 hPa for calculations of the compressive parameters N (the specific volume at 1 kPa), λ (compression index) and κ (recompression index), besides the precompression stress (σp). The LLWR was examined through the water retention and penetration resistance curves. σp linearly increased with the increase in the compaction state (plough pan) and h, whereas κ decreased with the increase in the h with no significant effect of compaction level. N and λ presented peaks at intermediate h values (100–2,000 hPa). Considerable reductions in the LLWR were observed from the plough to plough pan layer, which was induced by the increase in the soil penetration resistance. These results suggest that the residual plough pan induced by the successive years of intensive use of machinery for field operations in sugarcane areas originates a soil layer of lower compressibility and higher strength that further increases as the soil dries in response to h. Moreover, the consequent increase in the soil penetration resistance from the plough to plough pan is the main factor negatively affecting the LLWR. [ABSTRACT FROM AUTHOR]

Soils are the most critical life-supporting compartments of the biosphere. They provide numerous ecosystem services such as habitat for biodiversity, water and nutrients, as well as producing food, feed, fiber and energy. Soils undergo intense and irreversible changes due to a non-site adjusted land management and improper application of machinery and techniques in its broadest sense. In combination with the growing population (until 2050 we will have approx. 9 Billion people) the urgent need for a more reliable dataset of soil properties and soil functions gains in importance in order to even prepare more reliable models for various requests. The mechanical strength – the precompression stress - as the result of geo-, pedo and anthropogenic long-term processes - can be defined as the basis for quantifying the rigidity boundary. It distinguishes between the recompression stress (i.e. elastic, rigid properties) and the virgin compression stress range where plastic deformation including irreversible changes of properties and functions occur. The changes in the hydraulic or pneumatic functions like hydraulic or air conductivity, the pore size distribution primarily all occur in the virgin compression stress range, The same is also true for redox reactions and the biological activity (respiration) in soils but also carbon sequestration potential is also linked with the precompression stress value. Thus, a more precise definition and following of site specific functionality differences, which may exclude or concentrate certain land use or management forms are needed, in order to optimize yield, soil protection and a sustainable land use management considering the limited site specific resilience at the same moment.

The objective of this study was to evaluate the effect of wheeling with two different wheel loads (1.7 and 2.8 Mg) and contrasting wheeling intensities (1x and 10x) on the bearing capacity of a Stagnosol derived from silty alluvial deposits. Soil strength was assessed by laboratory measurements of the precompression stress in topsoil (20 cm) and subsoil (40 and 60 cm) samples. Stress propagation, as well as elastic and plastic deformation during wheeling were measured in the field with combined stress state (SST) and displacement transducers (DTS). We also present results from soil physical analyses (bulk density, air capacity, saturated hydraulic conductivity) and barley yields from the first two years after the compaction. Although the wheel loads used were comparatively small, typical for the machinery used in Norway, the results show that both increased wheel load and wheeling intensity had negative effects on soil physical parameters especially in the topsoil but with similar tendencies also in the subsoil. Stress propagation was detected down to 60 cm depth (SST). The first wheeling was most harmful, but all wheelings led to accumulative plastic soil deformation (DTS). Under the workable conditions in this trial, increased wheeling with a small machine was more harmful to soil structure than a single wheeling with a heavier machine. However, the yields in the first two years after the compaction did not show any negative effect of the compaction.

ABSTRACT Soil compression curve (CC) provides parameters to identify soil load-bearing capacity and susceptibility to compaction. An Excel add-in (ACC) incorporating graphical procedures for mathematical models for soil CC description and calculation of parameters was developed. By using the ACC, soil CC can be described by means of the Casagrande method, mathematically operationalized with the van Genuchten equation, with or without restrictions on its parameters, and by Dias Junior and Pierce method in its original form and also modified using the void ratio rather than soil bulk density. The ACC uses a single Excel spreadsheet for input and output data, in addition to a graphical interface and a tool for exporting editable charts. Compared to SAS statistical software, the ACC minimized the sum of squared residuals and estimated parameters of mathematical models with the same efficiency for 347 compression curves. The ACC programming script is available and can be modified or used as a framework for other programming projects.

Eucalyptus forests play a major role in the world economy, providing raw materials for different purposes. In planted forests, harvest operations performed by heavy machinery may cause severe soil compaction. This study aimed to evaluate the impact of a full‐tree harvesting system in planted eucalyptus forests from Northeastern Brazil. Different soils were evaluated (two Hapludults and one Haplorthod) in two horizons (BA and Bt). We tested different equipment, namely feller buncher, skidder (with traffic intensities of 3, 6, 12 and 16 passes), flail (at different ground‐contact points), grapple saw and loader. The soil physical attributes reflected not only the impact of equipment traffic but also the intrinsic differences between the soils. Bulk density (ranging from 1.36 to 1.80 t m−3 after trafficking) related well to soil class and horizon. Precompression stress (ranging from 203 to 430 kPa) and degree of compaction (76%–94%) following trafficking were well correlated, while increase in bulk density (reaching a maximum of 20%) related more strongly to soil moisture. A contingency table was constructed with the number of compacted samples and further examined by correspondence analysis. Compaction varied according to soil, horizon and equipment, indicating that machine–soil interactions are very specific and demand detailed research under different conditions. The Haplorthod experienced the greatest amount of compaction, whereas the Hapludult‐2 was more resistant (60% and 25% of compacted samples, respectively). The grapple saw and the skidder at higher traffic intensities (12 and 16 passes) exerted the highest mechanical impacts (81% and 67% of compacted samples, respectively). [ABSTRACT FROM AUTHOR]

Some soil physical properties can easily be measured using classical laboratory methods. However, explicit valuable information on the real morphology of the pore structure as well as soil physical properties cannot be obtained at the same time with classical methods. This requires non-destructive measurements such as X-ray computed tomography (CT). However, explicit valuable information on the real morphology of the pore structure as well as soil physical properties cannot be obtained at the same time with classical methods. This paper combines parameters obtained from CT analysis (mean macropore diameter, macroporosity, pore connectivity, anisotropy) and classical laboratory methods (dry bulk and aggregate density, saturated hydraulic conductivity, mechanical precompression stress) to analyse soil compaction, exemplified on samples from two tillage treatments (cultivator and plough) and at two moisture states (6 and 1000 kPa matric potential) on a Chernozem collected at a soil depth of 16–22 cm (texture 0–30 cm: silty clay loam). The study shows that the matric potential can have a decisive impact on the mechanical stability of soil. In the loose but less stable plough treatment a more negative matric potential was clearly beneficial to the mechanical stability. In already dense soil structures, as in the cultivator treatment, a reduction of water content was less effective in increasing soil stability. The CT parameters were all closely and uniquely related to each other. The shown CT parameters can be used for a standardized characterization of the soil. Ploughing has a positive effect on soil structure which persists only as long as macroporosity and mean macropore diameter remain high. Plough maintains higher pore connectivity when compacted under dry conditions. • The CT parameters were all closely and uniquely related to each other. • The CT parameters can be used for a standardized characterization of the soil. • For cultivator water content was less effective in increasing already high soil stability. • Positive effect of plough as long as macroporosity and pore connectivity remains high. • Plough maintains higher pore connectivity when compacted under dry conditions. [ABSTRACT FROM AUTHOR]

The objective of this study was to evaluate the effect of wheeling with two different wheel loads (1.7 and 2.8 Mg) and contrasting wheeling intensities (1x and 10x) on the bearing capacity of a Stagnosol derived from silty alluvial deposits. Soil strength was assessed by laboratory measurements of the precompression stress in topsoil (20 cm) and subsoil (40 and 60 cm) samples. Stress propagation, as well as elastic and plastic deformation during wheeling were measured in the field with combined stress state (SST) and displacement transducers (DTS). We also present results from soil physical analyses (bulk density, air capacity, saturated hydraulic conductivity) and barley yields from the first two years after the compaction. Although the wheel loads used were comparatively small, typical for the machinery used in Norway, the results show that both increased wheel load and wheeling intensity had negative effects on soil physical parameters especially in the topsoil but with similar tendencies also in the subsoil. Stress propagation was detected down to 60 cm depth (SST). The first wheeling was most harmful, but all wheelings led to accumulative plastic soil deformation (DTS). Under the workable conditions in this trial, increased wheeling with a small machine was more harmful to soil structure than a single wheeling with a heavier machine. However, the yields in the first two years after the compaction did not show any negative effect of the compaction. [ABSTRACT FROM AUTHOR]

Soil compressibility and elasticity are fundamental physical properties related to soil strength and resistance to compaction. The objective of this study was to evaluate the effect of soil structural condition, granulometric composition, organic matter, and moisture on compressive and elastic properties of subtropical soils under no-tillage system. Undisturbed soil samples (a total of 536) from 0–0.075 and 0.075–0.15 m layers were capillary-saturated for 24 h and subjected to 6, 10, 30, and 100 kPa water tension, to determine the composition soil physical (bulk density BD and soil volumetric moisture θv) properties, and the functional soil properties: compressibility (precompression stress σ p and compressibility coefficient Cc) and elasticity (decompressibility Dc and recovery Rc coefficients). Soil clay and organic matter (SOM) contents were evaluated using disturbed soil samples. Soil BD ranged from 1.15 Mg m −3 up to 1.67 Mg m −3 , whereas soil σ p ranged from 52.9 to 398.7 kPa, and increased with BD increase and decreased with an increase in SOM, clay and θv. Soil Cc ranged from 0.06 to 0.64, increased with rise in water tension, and had a significant inverse relationship with soil BD, and significant direct with SOM, clay and θv content for all studied water tension. These results suggest soils with high soil organic matter are less resistant to compaction, but may recover more from compaction. Soil Dc ranged from 0.0224 to 0.1416, and had an inverse relationship with BD and direct with SOM, clay content and θv. The mean Dc value of 0.0666 implies a rebound of about 0.17 in void ratio e , 14% in total porosity, and 0.12 Mg m −3 in soil bulk density, when assuming stress unloading from 300 (the load applied by a harvester, for instance) to 0 kPa. Soil elasticity is influenced by soil properties also controlled by soil moisture, such as soil bulk density and organic matter. Elastic behavior of the soil may be described primarily based on Dc, due its significant relationships with BD, θv, clay and SOM in different water tensions. These results show elasticity is influenced by soil structure, where soil management that improves structure is vital in the partial recovery after soil loading. Soil Rc (from 2.4 to 22.6%; with an overall mean of 12.1%) had most significant relationships between soil properties, but there was no discernible trend of relationships with soil properties; thus, extra care is needed when this parameter is used as indicator of soil elasticity. Soil elasticity and compressibility is increased by soil moisture and clay and organic matter contents, and decreased by soil compactness as suggested from the simple regression analysis. Further, when the integrated effect of soil physical properties is evaluated by multiple regressions, decompression and recovery coefficients are most influenced by soil bulk density and soil organic matter analysis, while compressibility coefficient is also influenced by volumetric moisture. [ABSTRACT FROM AUTHOR]

Divergent processes in soils affect the pore systems and alter hydraulic, gaseous and thermal fluxes as soon as the internal soil strength (= the precompression stress) is exceeded. Especially the intense changes of the hydraulic flux properties during soil deformation result in retarded soil settlement due to their dependency on time and stress dependent changes in pore water pressure as well as the water saturation degree. These processes depend besides on texture and bulk density especially on aggregation level, but the linkage between changes in effective stress and soil structure in various soil horizons defined by the precompression stress is still not analyzed and is therefore subject of this investigation. In this study, stress strain and pore water pressure data from almost 900 (predrained to −60 hPa) samples from arable top- and subsoil horizons subjected to oedometer compaction tests are evaluated. Among others, hydraulic stress parameters like the effective stress and χ-factor are calculated and linked to the precompression stress values. Results are presented for 6 differently aggregated soil horizons. Ap horizons with a very weak soil structure document no defined soil rigidity due to a reduced functionality of the pore systems and a stress dependent weakening, while especially the soil deformation behavior of sandy Bw horizons is less controlled by the precompression stress and the calculated effective stresses. The E and Bg horizons indicate anthropogenic impacts on structure properties and the related stress-strain behavior. While it could be documented that the enhanced strain coincides with an increase in the calculated χ-factor, it does not coincide in correspondingly enhanced effective stresses due to the non- equilibrated pore water pressure even after 45 h of increasing stress application. We conclude that further investigations are needed to quantify the highly dynamic interactions between the stress dependent hydraulic conductivity / pore water pressure ratio and the impact on the stress affected pore water pressure reequilibration. Furthermore, the contribution of the sorptive potential in soils with more negative initial matric potential on these changes would elucidate also the physico chemical impacts on soil strength more clearly especially under drier soil conditions. • The changes in pore water pressure due to stress application are time dependent. • The χ-factor increases in the virgin compression stress range. • Changes of the effective stress are the more pronounced the weaker the soil. • The hydraulic conductivity affects strain and causes strength decrease or increase. [ABSTRACT FROM AUTHOR]

The objective of this work was to evaluate the effect of tractor wheeling with a light tractor on the root growth and soil properties of smooth brome in South Estonia. Field experiment was conducted on sandy loam Haplic Luvisol in 2007. Data were collected during September 2008 in both an uncompacted and compacted area. Because of the compaction, the precompression stress increased in the upper soil layer (0-10 cm) by 12.6% and 15.2% at a depth of 10-20 cm. Compaction had only a minor effect on the bulk density, values increased 4.7% in the upper soil layer and 1.8% in the deeper layer. Differences in the saturated hydraulic conductivity (ks) were not significant; however, the decrease in the ks was 26.6% in the upper level and 12.5% in the deeper (10-20 cm) layer. At a depth of 0 to 30 cm compaction decreased the root length by 44.7% and the root mass by 60.5% compared with the uncompacted soil. Altogether, this study confirms the unfavourable effect of wheeling on grasslands even when the wheeling is performed with a light tractor on dry soil.

Considering conventional tillage with a mouldboard plough and conservation tillage with a cultivator or disc harrow, this study analysed whether structural differences in the soil of the lower topsoil led to any difference in this layer's susceptibility to compaction, and also how density changed – in the whole soil and also in the individual aggregates – during the compaction process in both tillage variants. To this end, soil samples were taken from the lower topsoil of seven medium-term and long-term soil tillage trials conducted in Central Europe. Compression tests were performed on these samples and they were also used to determine dry bulk density, aggregate density, air capacity and saturated hydraulic conductivity. The stress/bulk density functions as well as the stress/strain functions from the compression tests were analysed and the precompression stress determined. At two test sites, compaction behaviour was analysed for whole soil and for aggregates separately. In the case of conservation tillage, the soil structure demonstrated higher dry bulk density as well as lower air capacity and saturated hydraulic conductivity. Aggregate density was mostly similar. It increased relatively slowly during compaction, and often not before high loading steps. This is why higher precompression stress values in the variants under conservation tillage were mostly the result of a dense compaction of aggregates, and indicated higher stability against mechanical loads. However, for both variants the virgin compression section of the stress/bulk density functions displayed similar compression behaviour; and generally higher settlement for conventional tillage in the compression test did not result in higher dry bulk densities than with conservation tillage. Stability against mechanical loads in the conservation tillage variants should therefore not be overestimated. [ABSTRACT FROM AUTHOR]

The idea of the study was to combine the saturation-dependent hydraulic conductivity, K(Se), as intensity parameter with the volume fraction related to the fractional capillary potential for each of the characteristic pore size classes that based on the matric flow potential, φ, and the soil water diffusivity, D(θ). The analysis is focused on the soil hydraulic properties of differently textured soils with differences in the pore size distribution, PSD. The soil-moisture retention, θ(h), and K(Se) as function of the effective saturation were determined with undisturbed samples for each soil profile and horizon and fitted by Mualem–van Genuchten type analytical function. The results indicate a strong linear relationship between φ values and the soil water diffusivity, D(θ), of soils with its volume fractions of macropores (r2: 0.65–0.91), wide coarse pores (r2: 0.55–0.77), and medium pores (r2: 0.83–0.85). The precompression stress, Pc, as mechanical soil strength parameter affects the soil water diffusivity of the chosen datasets especially in the wide coarse pore range (r2: 0.85). The results suggest that the PSD's-related φ and D(θ) values are affected by both textural and structural effects. The soil water diffusivity itself strongly depends on the PSD and is an effective link between soil's intensity and capacity parameter. • Soil water diffusivity is affected by textural and structural effects. • The more humous, the more important is the medium pores range for the soil water diffusivity. • Precompression stress positively affects soil water diffusivity in wide coarse pores range. [ABSTRACT FROM AUTHOR]

Precompression stress is an important property for assessment of tropical soil structure sustainability and is often determined in laboratory tests. The objective of this study was to compare the load bearing capacity models obtained with controlled moisture in laboratory and those obtained with natural field moistures determined a long one year. The evaluation of soil structural sustainability follows four distinct steps: soil sampling in the field, uniaxial compression test of the samples in the laboratory, determination of precompression stress and estimation of the load bearing capacity models. Laboratory estimates of precompression stress were obtained from moisture controlled in laboratory and from natural moisture determined in a field a long one year. In this process, the soil samples were saturated by capillarity with distilled water in laboratory, and after 48 hours, the samples were air dried to obtain the different moisture contents. Then, the precompression stress was determined for this both conditions. To verify if the load bearing capacity models obtained with controlled moisture in laboratory may represent the load bearing models obtained with natural field moisture, these models were compared using the homogeneity test procedure. It was observed that 75% of field models analyzed were similar to the laboratory models. Thus, due to the similarity on the load-bearing capacity models obtained using natural (field) or controlled (laboratory) moisture contents, the assessment of the soil structure sustainability can be done using both methods.Pressão de pré-consolidação é uma importante propriedade na avaliação da sustentabilidade estrutural do solo e é determinada em testes de laboratório. Neste estudo, objetivou-se comparar modelos de capacidade de suporte de carga obtidos com umidade controlada em laboratório e com umidade natural de campo ao longo de um ano. A avaliação da sustentabilidade estrutural do solo ocorreu em quatro etapas: coleta de amostras de solo indeformado, ensaio de compressão uniaxial das amostras em laboratório, estimativa da pressão de pré-consolidação e obtenção dos modelos de capacidade de suporte de carga. As estimativas da pressão de pré-consolidação de laboratório e de campo foram obtidas com umidades controladas em laboratório e com umidade de campo, respectivamente, ao longo de um ano. No laboratório, a saturaçaõ das amostras se deu por capilaridade, com água destilada e depois de 48 horas, essas amostras foram secas ao ar para obter diferentes valores de umidade. Em seguida, realizou-se o ensaio de compressão uniaxial dessas amostras e a pressão de pré-consolidação foi determinada para as duas condições aqui proposta. Para verificar se os modelos de capacidade de suporte de carga com umidade controlada em laboratório podem representar os modelos de capacidade de suporte de carga de umidade natural, esses modelos foram comparados utilizando o teste de homogeneidade de dados. Observou-se semelhança em 75% dos modelos de umidade de campo com modelos de umidade controlada em laboratório. Portanto, em razão da similaridade, a análise da sustentabilidade estrutural do solo pode ser feita por ambos os modelos de capacidade de suporte carga, ou seja, modelos obtidos com amostras de solo com umidade natural (campo) ou com umidade controlada (laboratório).

Incongruous management techniques have been associated with some significant loss of agricultural land to degradation in many parts of the world. Land degradation results in the alteration of physical, chemical and biological properties of the soil, thereby posing a serious threat to sustainable agricultural development. In this study, our objective is to evaluate the changes in a Cambisol structure under six land use systems using the load bearing capacity model. Sampling was conducted in Amazonas Region, Brazil, in the following land use: a) young secondary forest; b) old secondary forest; c) forest; d) pasture; e) cropping, and f) agroforestry. To obtain the load bearing capacity models the undisturbed soil samples were collected in those land use systems and subjected to the uniaxial compression test. These models were used to evaluate which land use system preserved or degraded the Cambisol structure. The results of the bulk density and total porosity of the soil samples were not adequate to quantify structural degradation in Cambisol. Using the forest topsoil level (0-0.03 m) as a reference, it was observed that pasture land use system was most severe in the degradation of the soil structure while the structure were most preserved under old secondary forest, cropping system and forest. At the subsoil level (0.10-0.13 m depth), the soil structure was most degraded in the cropping land use system while it was most preserved in young secondary forest and pasture. At the 0.20-0.23 m depth, soil structure degradation was most severe in the old secondary forest system and well preserved in young secondary forest, cropping and agroforestry.Técnicas inadequadas de manejo têm sido associadas com a degradação de terras agricultáveis em muitas partes do mundo. A degradação do solo resulta em alterações das propriedades físicas, químicas e biológicas do solo, o que representa séria ameaça ao desenvolvimento agrícola sustentável. O objetivo deste estudo foi avaliar as alterações da estrutura de um Cambissolo sob seis sistemas de uso da terra por meio dos modelos de capacidade de suporte de carga. A amostragem foi realizada na região Amazônica, Brasil, nos seguintes sistemas de uso: a) floresta secundária nova; b) floresta secundária velha; c) floresta; d) pastagem; e) roça; e f) agrofloresta. Para obter os modelos de capacidade de suporte de carga, as amostras indeformadas foram coletadas nesses sistemas de uso da terra e submetidas ao ensaio de compressão uniaxial. Esses modelos foram usados para avaliar qual sistema de uso da terra preserva ou degrada a estrutura do Cambissolo. Os resultados da densidade do solo e porosidade total do solo não foram adequados para quantificar a degradação estrutural do Cambissolo. Utilizando a profundidade de 0-0,03 m da floresta como referência, observou-se que a pastagem foi o sistema de uso da terra que mais promoveu degradação da estrutura do solo, ao passo que a estrutura foi mais preservada na floresta secundária velha, roça e floresta. Na profundidade de 0,10-0,13 m, a estrutura do solo foi mais degradada no sistema de cultivo roça e mais preservada na floresta secundária jovem e pastagem. Na profundidade de 0,20-0,23 m, a degradação da estrutura do solo foi mais intensa na floresta secundária velha e mais preservada na floresta secundária nova, roça e agrofloresta.

The main source of soil structure degradation in coffee plantation is the machinery traffic because these operations may cause soil compaction affecting the crop development. This study aimed to generate the load-bearing capacity models for a Red-Yellow Latosol and to determine through the use of these models the soil susceptibility to compaction of the coffee plantation due to the implantation time and the compaction caused by the machinery traffic on the traffic lines located at the top and bottom of the ground. This study was carried out in the EPAMIG Experimental Farm, located at Três Pontas, MG, in coffee plantations (Coffee arabica L.) with 2, 7, 18 and 33 years of establishment. To obtain the load-bearing capacity models, 12 undisturbed soil samples were randomly collected in the 0-3 cm and 15-18 cm layers in the position between the rows for each establishment time of the coffee plantation. It was also randomly collected 10 undisturbed soil samples for each establishment time of the coffee plantations along the tractor traffic lines located at the top and bottom of the ground. These undisturbed soil samples were used in the uniaxial compression tests. The use of the load-bearing capacity models allow to identify the soil susceptibility to compaction due to the implementation time of the coffee plantation and the compaction caused by the machinery traffic on the traffic lines located at the top and bottom of the ground. The percentage of compacted soil samples increases with the establishment time in the layer of 15-18 cm.A principal fonte de degradação da estrutura do solo na exploração cafeeira é o tráfego de máquinas, pois essas operações podem causar compactação do solo, afetando o desenvolvimento da cultura. Neste estudo, objetivou-se gerar os modelos de capacidade de suporte de carga para um Latossolo Vermelho-Amarelo e determinar por meio do uso desses modelos a suscetibilidade à compactação do solo, pelo tempo de implantação da lavoura cafeeira e os impactos causados pelo tráfego das operações mecanizadas nas linhas de tráfego localizadas na parte de cima e de baixo do terreno. Este estudo foi realizado na Fazenda Experimental da EPAMIG, situada em Três Pontas, MG, em lavouras cafeeiras (Coffee arabica L.) com 2, 7, 18 e 33 anos de implantação. Para a obtenção dos modelos de capacidade de suporte de carga, 12 amostras indeformadas de solo foram coletadas aleatoriamente nas camadas 0-3 e 15-18 cm no centro das entrelinhas, para cada tempo de implantação da lavoura cafeeira. Foram também coletadas aleatoriamente 10 amostras indeformadas para cada tempo de implantação da lavoura cafeeira ao longo das linhas de tráfego do trator situadas nas partes de cima e de baixo do terreno. Essas amostras foram usadas no ensaio de compressão uniaxial. O uso dos modelos de capacidade de suporte de carga permitiu identificar a suscetibilidade do solo à compactação, em razão do tempo de implantação da lavoura cafeeira e a compactação causada pelo tráfego do maquinário nas linhas de tráfego situadas na parte de cima e de baixo do terreno. A porcentagem de amostras compactadas aumentou com a idade da lavoura na camada de 15-18 cm.

O conhecimento das relações entre propriedades físicas e mecânicas do solo pode contribuir no desenvolvimento de funções de pedotransferência, que permitam estimar outras propriedades do solo de difícil mensuração. Neste trabalho, objetivou-se avaliar a relação entre a susceptibilidade à compactação e o suporte de carga com propriedades físicas de solos do sul do Brasil. Foram avaliadas a resistência à penetração, a umidade, a densidade e a compressibilidade de seis solos. A resistência à penetração pode ser estimada pelo modelo que considera a umidade e densidade do solo. Solos com maior densidade inicial apresentaram menor susceptibilidade à compactação e menor deformação, quando submetidos a pressões externas. Quanto maior a resistência do solo à penetração, menor é a deformação e maior é a capacidade de suporte de carga, embora isso não indique solos com qualidade física adequada para as culturas; quanto maior a deformação do solo, maior a susceptibilidade à compactação e menor a capacidade de suporte de carga. A susceptibilidade de um solo à compactação e sua capacidade de suporte de carga podem ser estimadas, respectivamente, pela densidade inicial e pela resistência do solo à penetração.Quantifying the relationship between physical and mechanical soil properties can contribute to the development of pedotransfer functions that allow estimating hard-to-measure soil properties. The objective of this study was to evaluate the interrelations between susceptibility to compaction and load support with some physical properties of soils from Southern Brazil. Penetration resistance, moisture, bulk density and compressibility of six soils were evaluated. In a model including soil moisture and bulk density as independent variables, the relation with penetration resistance values obtained in the field was high. Soils with higher initial bulk density were less susceptible to compaction and exhibited less deformation under external loads. With increasing soil penetration resistance, less deformation and greater load support were observed, which does not necessarily indicate a satisfactory soil physical quality for crop cultivation. The greater the soil deformation, the higher is the susceptibility to compaction and the lower is its load support. The compaction susceptibility and load support of a soil can be estimated, respectively, by its initial bulk density and resistance to penetration.

In Central Europe, various plant species including large-grain legumes and their mixtures are grown as catch crops, particularly between grains harvested early and subsequent summer crops. This article investigates the question of how soil structure in the topsoil is influenced when catch cropping with large-grain legumes (experimental factor A: without catch crop, with catch crop) under different ploughless tillage conditions during catch crop seeding (experimental factor B: deep tillage/25–30 cm, shallow tillage/8–10 cm). Five one-year trials were performed using standard machinery at various sites in Germany. Soil core samples extracted from the topsoil in the spring after catch crop cultivation served to identify air capacity, saturated hydraulic conductivity and precompression stress. The above-ground and below-ground biomass yields of the catch crops were also determined at most of the sites. In addition, the soil compaction risk for the working steps in the experiments was calculated using the REPRO model. The dry matter yield of the catch crops varied considerably between the individual trial sites and years. In particular, high levels of dry matter were able to form in the case of early seeding and a sufficient supply of precipitation. The soil structure was only rarely affected positively by catch crop cultivation, and catch crops did not contribute in the short term to loosening already compacted topsoils. In contrast, mechanical soil stresses caused by driving over the ground and additional working steps used in cultivating catch crops often led to lower air capacity in these treatments. This is consistent with the soil compaction risks calculated using the REPRO model, which were higher in the treatments with catch cropping. Catch crop cultivation also only resulted in improved mechanical stability at one location. The positive effect of deep ploughless tillage on air capacity and saturated hydraulic conductivity, however, became more clearly evident regardless of catch crop cultivation. In order for catch crop cultivation with large-grain legumes to be able to have a favourable impact on soil structure, it is therefore important that cultivating them does not result in any new soil compaction. In the conditions evaluated, deep tillage was more effective at loosening compacted topsoil than growing catch crops. [ABSTRACT FROM AUTHOR]

The effects of machine traffic were assessed on a Luvisol in a temperate climate area in Belgium. Soil samples were taken from topsoil (0.07–0.25 m) and subsoil (0.35–0.50 m), on plots under long-term reduced tillage (RT) and conventional tillage (CT). Cone index (CI), bulk density (BD) and precompression stress (Pc) were chosen as indicators of mechanical strength. Mercury intrusion porosimetry was used to characterise the soil microporosity structure. It was presented in two forms: (i) cumulative pore volume vs. equivalent pore radius r , from which four classes of porosity were defined: r < 0.2 μm, 0.2 ≤ r < 9 μm, 9 ≤ r < 73 μm and r ≥ 73 μm and (ii) pore-size distribution (PSD). In the reference situation where there had been no recent passage of machines, the voids with 0.2 ≤ r < 9 μm were the most important class in RT topsoil. The voids with r ≥ 73 μm represented the main porosity class in the topsoil of CT. In the subsoil, for both tillage systems, the porosity was almost equally distributed between voids with 0.2 ≤ r < 9 μm and voids with r greater than 9 μm. Machine traffic was carried out when the soil water content was close to the optimum Proctor. Although unfavourable, these wet conditions often occur during the beet harvesting period in Belgium. The highest modifications in soil structure (increase in BD and Pc, reduction of macroporosity r ≥ 73 μm) were observed in the topsoil of CT. More limited modifications were noticed in the soil structure of RT topsoil and subsoil layers but these latter are problematic in that the soil would no longer be loosened by subsequent tillage. These modifications could lead to soil consolidation as a result of wheel traffic year after year. [ABSTRACT FROM AUTHOR]

It is well known that soil compaction affects root growth and disrupts the activity of soil microfauna and microorganisms, resulting in yield loss. With the more intensive use of heavy machines in agriculture and forestry, the risk of soil compaction is increasing. In this study, precompression stress (Pc) was chosen as an indicator of the susceptibility of soils to compaction and was calculated using pedotransfer functions (PTFs). PTFs involve eight variables related to the hydraulic and mechanical behaviour of soils: organic matter content, bulk density, air capacity, available water capacity, non-plant available water capacity, saturated hydraulic conductivity, cohesion, and angle of internal friction. Combining these PTFs with geostatistics and Monte Carlo simulations, Pc maps were produced at the regional scale for Wallonia in Belgium, accompanied by uncertainty quantification maps. These maps were then used to produce compaction risk maps based on common scenarios. The results showed that the modal Pc map was coherent with the spatial distribution of the main variables, namely soil texture and organic matter content. The risk maps enabled areas with a compaction risk in both agricultural and forestry contexts to be identified. These maps could be useful in drawing up soil protection measures and policies. [ABSTRACT FROM AUTHOR]

Soil compaction is harmful to agricultural and forestry areas, and strategies to avoid soil deformation are of great interest. The objective of this study was to evaluate the compressibility of a soil submitted to different land uses in an area located in southern Brazil. The land uses assessed were a) Anthropized Forest: constituted by tree and shrub species; b) Pasture: five years old; c) Eucalyptus 20: twenty years old; and d) Eucalyptus 4.5: conducted in 2nd rotation, and 4.5 years old, with Eucalyptus harvesting in the first cut made manually with a chainsaw and wood extraction with a Forwarder. Undisturbed soil samples were taken in 2006, and they were submitted to the uniaxial compression test. Bulk density and volumetric moisture were calculated before the test, as well as soil deformation, compression index and precompression stress at the end of the test. Anthropized Forest and Pasture soils presented the highest volumetric soil moisture, a mean of, respectively, 0.301 and 0.304 m3 m−3. Eucalyptus 20 had the lowest bulk density (mean of 1.15 Mg m−3), and Eucalyptus 4.5 the highest bulk density (mean of 1.47 Mg m−3). Precompression stress is low for all soil land uses, ranging from 31.24 to 50.92 kPa, and care should be taken to avoid further compaction. Load-bearing capacity of the soil presented positive and negative relationship with, respectively, bulk density and volumetric moisture, but the low coefficient of correlation (r) and determination (r2) shows the need of further studies for a better adjustment of the data. From the knowledge of volumetric soil moisture, it is possible to monitor and define the most suitable soil condition for traffic of machinery and animals trampling into the areas to avoid additional compaction, while the load-bearing capacity of the soil can be estimated by bulk density, being useful to monitor the occurrence or not of additional compaction. • The traffic of machinery to extract wood in Eucalyptus causes soil compaction. • Load-bearing capacity of the soil can be estimated from bulk density. • Monitoring volumetric soil moisture may be used to avoid additional compaction. • Bulk density larger than 1.32 Mg m−3 can compromise soil aeration for the plants. [ABSTRACT FROM AUTHOR]

Most of Brazilian agricultural land is occupied by pastures, which often show some degree of degradation. Grazing is usually associated to damages on soil structure because of soil compaction. The aim of this work was to evaluate short term changes on soil physical quality following different pasture renovation methods, including fertilization (FERT), conventional soil tillage (TILL) and two crop-livestock system (no-till and conventional tillage, respectively COTI and CONT), on a very clayey Red Latosol (Ustox), also comparing these to the original pasture and a nearby forest fragment. Bulk density and macroporosity were significantly different on the pasture sites (averaging 1.12 g cm-3 and 0.16 cm3 cm-3 respectively) from that under the forest fragment (respectively 0.87 g cm-3 and 0.34 cm3 cm-3); however, significant differences between the renovation methods and the original pasture were not observed. The load bearing capacity model (LBCM), which relates decrease on soil precompression stress (σp) with increase on water content (U), obtained for the original pasture, ORIG (σp = 10(2.82-1.21*U)), was considered different from that obtained for the soil under natural vegetation (σp = 10(2.69-1.25*U)), with the former being more resistant to compaction. When the ordinate pairs (U, σp) obtained from soil cores collected on the different renovation methods experimental plots were plotted on the LBCM from ORIG, an increase on soil strength could be noticed, but with small differences between renovation methods. This was because of higher stocking rates on FERT and TILL and corn harvest for silage production on COTI and CONT. Tillage failed to improve soil physical quality even on the short turn (only one growing season) and should be disregarded as a renovation method for pastures on initial stages of degradation, while soil fertility is the main limitation. [ABSTRACT FROM AUTHOR]

Soil compaction impairs all essential soil functions, which are crucial for the lives of humans, animals, plants and soil organisms. In order to secure the various soil functions, soil compaction must be avoided. One successful method of preventing soil compaction could be based on the precautionary principle, and mathematical modelling might be used to support farmers or consultants when making decisions about husbandry operations. This paper presents a model which calculates an indicator and assesses the risk of soil compaction on arable land based on site-specific data including information on soil, weather and specific husbandry. The first step is to estimate the soil strength in response to soil stress for a topsoil (20 cm) and a subsoil (35 cm) layer. The estimations of these parameters take into account changes in soil moisture throughout the year. Soil strength compared with soil stress is used to calculate the indicator Soil Compaction Index (SCI) for each time the machinery passes over the soil. The results from the separate passes are then integrated for a comprehensive assessment of the risk of soil compaction at farm level. The model was validated in numerous trials. It was found that the calculated SCI was a good reflection of the actual change in soil structure. The model is already being applied on arable farms in Germany. As an example presented in this paper, the calculations for the subsoil at these farms result in low to medium compaction risks. [ABSTRACT FROM AUTHOR]

Subsoil compaction is a major problem in modern agriculture caused by the intensification of agricultural production and the increase in weight of agricultural machinery. Compaction in the subsoil is highly persistent and leads to deterioration of soil functions. Wheel load-carrying capacity ( WLCC) is defined as the maximum wheel load for a specific tyre and inflation pressure that does not result in soil stress in excess of soil strength. The soil strength and hence WLCC is strongly influenced by soil matric potential ( h). The aim of this study was to estimate the seasonal dynamics in WLCC based on in situ measurements of h, measurements of precompression stress at various h and simulations of soil stress. In this work, we concentrated on prevention of subsoil compaction. Calculations were made for different tyres (standard and low-pressure top tyres) and for soil under different tillage and cropping systems (mouldboard ploughing, direct drilling, permanent grassland), and the computed WLCC was compared with real wheel loads to obtain the number of trafficable days ( NTD) for various agricultural machines. Wheel load-carrying capacity was higher for the top than the standard tyres, demonstrating the potential of tyre equipment in reducing compaction risks. The NTD varied between years and generally decreased with increasing wheel load of the machinery. The WLCC simulations presented here provide a useful and easily interpreted tool to guide the avoidance of soil compaction. [ABSTRACT FROM AUTHOR]

Abstract: Many arable soils have significant horizon-specific gravel content levels. Just how these influence compaction behaviour, and in particular precompression stress as an important criterion of a soil's susceptibility to compaction, has yet to be sufficiently clarified. This article is intended to contribute towards answering this question. Firstly, three different fine earths, from the “Clay”, “Silt Loam” and “Sandy Loam” soil texture classes were mixed with staggered proportions (0, 10, 20, 30, 40% by volume) of a quartz gravel (the shape of which was subrounded to rounded, average weighted diameter 6mm). Soil core samplers were filled with the mixtures at a typical density for a natural site. In the case of the 30% by volume variant only, in addition to the quartz gravel an angular to subangular limestone gravel with the same size graduation was also used. The tests were supplemented by 20 samples from a natural site; the gravel content of these varied between 0.1 and 23.5% by volume. All of the disturbed and natural samples were adjusted to a water content at a matric potential of −6kPa. Subsequently, an oedometer test was used to apply loads to them in stages (5–550kPa). Precompression stress was calculated using the resulting stress–bulk density functions. While fine earth bulk density remained constant, the staggered addition of quartz gravel led to an increase in the whole soil density after packing, and thus also to a vertical shift in overall stress–bulk density functions. However, the stress–density functions of the fine earth do show that the overall compaction of fine earth decreased as gravel content increased. In the case of low gravel content levels of no more than 10% by volume, the increase in precompression stress (log) in the disturbed samples was, on the whole, very low. In the disturbed samples, however, as gravel content increased precompression stress (log) increased exponentially. Contrary to this, a continuous linear increase in precompression stress (log) could be observed with increasing gravel content in the natural samples. The angular to subangular shape of the gravel only resulted in greater precompression stress (log) in the “Silt Loam”. At gravel-rich sites, gravel content influences soil compaction behaviour and precompression stress very strongly. For this reason, it is essential that it be considered when assessing such sites' risk of compaction damage. [Copyright &y& Elsevier]

This study aimed to investigate the mineralogy, moisture retention, and the compressive response of two agricultural soils from South West Nigeria. Undisturbed soil cores at the A and B horizons were collected and used in chemical and hydrophysical characterization and confined compression test. X-ray diffractograms of oriented fine clay fractions were also obtained. Our results indicate the prevalence of kaolinite minerals relating to the weathering process in these tropical soils. Moisture retention by the core samples was typically low with pre-compression stress values ranging from50 to 300 kPa at both sites. Analyses of the shape of the compression curves highlight the influence of soil moisture in shifts from the bi-linear to S-shaped models. Statistical homogeneity test of the load bearing capacity parameters showed that the soil mineralogy influences the response to loading by these soils. These observations provide a physical basis for the previous classification series of the soils in the studied area. We showed that the internal strength attributes of the soil could be inferred from the mineralogical properties and stress history. This could assist in decisions on sustainable mechanization in a datapoor environment. [ABSTRACT FROM AUTHOR]

Re-cultivated soils (previously piled soils used as the final surface cover in renovation of open cast mine sites) are particularly susceptible to compaction, which is why a simple estimate of mechanical strength is necessary for land management. In this study, therefore, precompression stress (−6 kPa matric potential) was determined for a total of 20 soil layers from 9 repeatedly cultivated areas of arable land in North Rhine-Westphalia (Germany), along with the aggregate density/dry bulk density ratio (as a measure of density heterogeneity) and air capacity (as a soil ecological parameter). These results are contrasted with the determination of packing density. Packing density (PD) is an integrated parameter that combines various properties (aggregate size, cohesion of the soil structure, root distribution, biogenic macropores and aggregate arrangement) and is assessed visually in the field. Packing density levels range between 1 (very loose soil) and 5 (very highly compacted). There is a strongly negative relationship between packing density and both the aggregate density/dry bulk density ratio and air capacity. Conversely, mechanical precompression stress increases with packing density. Ranges of the individual parameters can be assigned to each of the packing density levels. Packing density level 3 represents an optimization with regard to mechanical soil stability whilst maintaining minimum air capacity requirements (5-8 Vol.-%). [ABSTRACT FROM AUTHOR]

Abstract: Soil physical properties continuously change as a result of wetting and drying cycles (WD), soil tillage and grazing events. In order to define sustainable grazing management, it is necessary to determine how the soil responds during and after applied external stresses. This is relevant in southern Chile where pastures are the main livestock feed throughout the course of the year and Andosols are exposed to great mechanical stresses by a wide range of soil water contents. Therefore, the purpose of this paper was to illustrate the effect of grazing, considering two grazing intensities in the winter: 50 and 200cowsha−1, on the temporal changes of structure-dependent properties of an Andosol in southern Chile. In order to characterize the impact of grazing, 15 soil samplings (5–10cm depths) were conducted over a 2 year period. The penetration resistance (PR) and soil water content were determined in the field. Air capacity (AC), air (k l ) and saturated hydraulic (k s ) conductivity, precompression stress (Pc), and aggregate strength (AS) were measured. Changes in the soil mechanical stability and pore functions were observed as a consequence of grazing events and WD cycles, indicating that the structure-dependent properties are dynamic. Animal trampling induced an increase in PR; however, the increase in soil mechanical strength may not affect root growth, considering the changes in time of the PR and the specific properties of Andosol (e.g. bulk density <0.9Mgm−3). Temporal variations in Pc did not exceed critical values for root growth even under high grazing intensities, indicating that the soil recuperates after the impact of trampling during grazing. The air capacity/conductivity values measured in our study reached critical values for gas exchange as a consequence of animal trampling; however, the Andosol showed its ability to recuperate its integral pore functionality. Finally, in spite of the low bulk density of the Andosol, the increasing grazing intensity during the winter did not affect their structural properties. This, however, does not mean that the soil can support large stresses indefinitely. Therefore, long-term investigations on the effects of grazing management on Andosol physical properties in southern Chile are strongly needed. [Copyright &y& Elsevier]

Abstract: Precompression stress is an important criterion in soil mechanics and is often determined at a water content equivalent to a matric potential of −6kPa. In German-speaking countries, this matric potential corresponds to field capacity. Yet in order to assess the risk of compaction in arable soils, it needs to be known for a wide range of soil water content levels. The site-specific determination of relationships between precompression stress and matric potential or water content is, however, highly labour intensive. Furthermore, previous regression models can only deduce changes in precompression stress depending on water content to a limited extent and not for all values. Alternatively, these models do not directly include precompression stress at a matric potential of −6kPa as the basis of calculation. Thus the derivation and validation of a simple model are to be presented, which can be used to predict any precompression stress for decreasing soil water content levels. This requires only an initial precompression stress for a matric potential of −6kPa and the respective soil water content as a percentage of field capacity. The model is based primarily on an analysis of numerous studies in which precompression stress was determined for various matric potentials. Relationships between precompression stress at a matric potential of −6kPa and the relative water content as a percentage of field capacity at a matric potential of −30kPa were also derived in the laboratory. These data were used to develop a mathematical model for four soil texture classes, as well as “All texture classes” collectively. This model was tested by way of soil compression tests and the determining of precompression stress at 25 sites. All soil compression tests were initially carried out with a matric potential of −6kPa. Tests were carried out in parallel to this with greater matric potentials (−10 to −1500kPa). The accuracy of the modelling approach presented here is good, both in terms of the use of systems of equations for “All texture classes” and for differentiated soil texture classes. In comparison to the regression model for all texture classes, calculation according to soil texture class causes a reduction of the mean absolute errors from 0.15 to 0.11 and of the RMSE from 0.19 to 0.14. Simultaneously, the coefficient of determination and the index of agreement (IA) increase, from 0.54 to 0.67 and 0.92 to 0.95 respectively. Calculation according to different soil texture classes is therefore particularly recommended in the case of applications with high accuracy requirements. [Copyright &y& Elsevier]

Abstract: The compressive behaviour of soil, the compression curve, is typically expressed in a semi-logarithmic diagram in the relationship between the logarithm of applied stress, σ, and the void ratio, e. The compression curve yields three important soil compression properties: the swelling index, C s, the compression index, C c, and the precompression stress, σ pc. The compression index is considered an indicator of soil resistance to compaction, C s is used as a measure of soil mechanical resilience, and σ pc is considered the soil load support capacity. The objective of this paper was to investigate the impact of soil particle and aggregate size distribution on the characteristics of the soil compression curve and on C s, C c, and σ pc for a range of Swedish soils. We found no or only weak impacts of particle size and aggregate size distribution on soil compression properties. Instead, soil compression properties were largely controlled by initial void ratio, e 0, in such a way that C c and C s decreases, and σ pc increases with decreasing e 0, although the correlation between σ pc and e 0 was not statistically significant for subsoil samples. We show that the soil compression properties (σ PC, C c and C s) and their dependency upon e 0 are partly a consequence of the compression curve being analyzed in a semi-logarithmic (log σ − e) diagram. The use of log σ (instead of σ) forces the compression curve to bend at a certain value of log σ, and this bend is associated with σ pc. We showed that a precompression stress, as commonly defined, can be obtained for an ideal linear-elastic material. Therefore, the use of a logarithmic stress scale may lead to misinterpretation of soil mechanical behaviour. Our empirical results and theoretical exercise emphasize the need for further studies of physically-based expressions for soil strength for use in compaction modelling. [Copyright &y& Elsevier]

Over the last few decades, due to increase in grazing intensity, animal trampling has led to soil structure deterioration in Inner Mongolia, China. We investigated two different steppe ecosystems: Leymus chinensis (LCh, characterized by relatively higher precipitation) and Stipa grandis (SG) and two grazing intensities: ungrazed since 1979 (UG79) and grazed (continuously grazed, CG, at the Stipa grandis site and winter grazed, WG, at Leymus chinensis). Soil mechanical and hydraulic properties of semiarid steppe soils from each site and treatment were determined for soil aggregates and disturbed and bulk soil samples from different depths (4-8, 18-22, 30-34 and 56-60 cm for disturbed and bulk samples and 0-15 cm for the aggregates). Grazing causes a significant increase in tensile strength of aggregates and in the precompression stress of the bulk soil as well as a decrease in air and saturated hydraulic conductivity, irrespective of the vegetation type. Furthermore, exclusion from grazing led to more pronounced recovery of soil strength and pore continuity and hydraulic conductivity at the LCh site but it also depended on the moisture conditions of the sites. Under wetter conditions as well as after repeated freezing and thawing the soil strength declined. [ABSTRACT FROM AUTHOR]

Abstract: Compaction caused by traffic, results in degradation of soil structure with possible agronomic and environmental consequences. On the other hand, no-till systems have a great impact on soil structure and can improve their mechanical resistance. This property is described by compression curves using two parameters: the precompression stress P c and the compression index C c . Both mainly depend on bulk density, water content and soil texture. We aim to study the effect of farming practices on bulk density and mechanical resistance and also on the relation between those two properties in order to define diagnostic tools on the lasting effect of practices. Unsaturated soils were used, since cohesion is mainly attributed to capillary forces, themselves structure-dependent. “Equilibrium states” of soil structure corresponding to a fixed matric potential value, are then compared at various bulk densities. Pedo-tranfer functions (PTF) were defined by the relationship between P c , C c and the initial void ratio (e 0). Soils originated from two long-term experimental sites: Versailles and Boigneville (France), where till and no-till systems are being compared. PTF have been established from remolded samples taken in till systems using a large range of e 0 values. The effect of practices has been measured through mechanical properties of undisturbed soil cores. Uniaxial compression tests were performed in laboratory in drained conditions. In till systems, the global shape of compression curves varies for remolded cores from an S-shaped model to a rounded shape when e 0 decreases, followed by a bi-linear one. C c appeared simply proportional to e 0 in all soils due to the progressive water saturation of soil core at inflexion point, when e 0 decreases. For intact cores coming from till systems, this PTF should reveal heterogeneities and variability in soil structure. According to this PTF, there is no effect of practices in field. On remolded samples and till system, P c was mostly constant in Versailles while P c did strongly increase when e 0 decreased in Boigneville, due to progressive saturation of porosity. Effects of no-till were contrasted among soils and shape of curves. In Boigneville, mechanical resistance deduced from bi-linear curves increased as it decreased with S-shape curves and lead to a global increase in e 0 values. The opposite was observed in Versailles soils probably due to the opposite change induced by no-till in soil–water–air interaction among soils. The original approach developed is therefore relevant to study other soil, climates, land uses and plants. [Copyright &y& Elsevier]