Aggregate Tensile Strength Research Articles

Geographic variations of aggregate mechanical stability controlled by inorganic cementing agents across the East Asian monsoon region

Soil aggregate mechanical properties are of vital importance for plant growth, tillage and soil erosion, and strongly conditioned by the amount and type of cementing agents that differ with soils. As most research focus at the site scale, how aggregate mechanical stability vary across different types of soils at regional scales and underlying mechanisms remain poorly understood. Herein, seven typical zonal soils in heavy textures with an increased status of soil development were collected under two land uses (arable and forest) and at three pedogenic horizons along the mid-temperate to south-subtropical climatic gradient in the East Asian monsoon region. Aggregate tensile strength (Y), specific rupture energy (Esp) and friability (FI) were measured on air-dried aggregates in a compression test, as well as soil cementing agents related to particle size distribution, organic matter and its components, metal oxides, phyllosilicates and exchangeable cations. Y and Esp were most affected by soil type (F = 37.7 and 21.4, p < 0.001), and less affected by land use and soil horizon (F < 8.2). Y and Esp were overall remarkably larger for temperate moderately (257 ∼ 700 kPa and 40 ∼ 261 J kg−1) than for subtropical highly developed soils (171 ∼ 329 kPa and 22 ∼ 52 J kg−1); FI (0.35 ∼ 1.30) showed no distinct geographic trend. Among soil properties, exchangeable polyvalent cations, crystalline Fe and Al oxides, and vermiculite had much larger and significant explanatory power (R2 = 0.33 ∼ 0.54, p < 0.01) than traditional soil properties (organic matter, soil texture and bulk density) (R2 < 0.25, p > 0.01), suggesting the predominant roles of the inorganic cementing agents in aggregate mechanical stability across soil types. Aggregate mechanical stability was increased by exchangeable polyvalent cations and vermiculite (r = 0.71 ∼ 0.81, p < 0.001), and weakened by crystalline Fe and Al oxides (r = −0.77 ∼ −0.74, p < 0.001). Additionally, mean annual precipitation and temperature were negatively correlated with aggregate mechanical stability (r = −0.80 ∼ −0.76, p < 0.001). This study demonstrates the remarkable geographic variations of aggregate mechanical stability under the influence of climate and highlights the importance of clay mineralogy (mainly swelling clay and crystalline Fe and Al oxides) and exchangeable cations at the regional scale.

Conservative farming systems and their effects on soil organic carbon and structural quality

Pesticides-free integrated farming systems emerge as a viable option to promote environmental protection, due to the diversification of the agroecosystem, making the soil productive and resilient. However, there is a lack of information about the influence of integration farming systems on the relationship between soil physical quality and organic matter. The objective was to determine soil organic carbon fractions, and soil structural attributes in conservative single and integrated farming systems, to verify the potential of these farming systems to increase soil organic carbon stocks and improve soil structural quality. The experiment was carried out at a pesticide-free agricultural systems. Soil samples were collected in three layers (0–0.05, 0.05–0.10, and 0.10–0.20 m) of five farming systems (Cropland - C, Livestock - L, Forestry - F, Livestock-Forestry – LF, and Crop-Livestock-Forestry - CLF) in the randomized block design. Sampling was carried out in two transects of 25 m each, collecting five samples per transect, totaling 10 samples per layer. The following soil attributes were determined: soil organic carbon (SOC), particulate organic carbon (POC), mineral-associated organic carbon (MAOC), soil organic carbon stocks (Cst), visual evaluation of soil structure (VESS), tensile strength of aggregates (TS), soil friability, weighted mean diameter (WMD), geometric mean diameter (GMD) and the aggregate stability index (ASI). At 0–0.05 m, the SOC content was up to 12% higher in the Cropland system compared to other farming systems, while at 0.05–0.10 m and 0.10–0.20 m of depth, the difference was higher, around 27% between Cropland and other farming systems. For the VESS, Cropland had the lowest score (1.35), while the higher scores were in the Forestry, and CLF systems (2.25, 2.24, respectively). Both soil organic carbon and soil physical quality were greater in all farming systems. The particulate organic carbon and weighted mean diameter were efficiently to promote changes in soil carbon inputs and structural changes. The single and integrated farming systems promoted an increase in soil organic carbon, by a rate of 3.02, 1.81, 2.17, 1.78, and 2.01 Mg ha−1 yr−1, respectively for Cropland, Livestock, Forestry, LF, and CLF. The pesticide-free farming systems, under conservative practices, were very promising in showing the increase of SOC, POC and MAOC, supporting the soil physical quality, and cooperating with the soil health.

Investigations on Adhesion Characteristics between High-Content Rubberized Asphalt and Aggregates.

The use of waste tires to prepare rubberized asphalt has been a hot trend in recent years, and the characteristics of adhesion between rubberized asphalt and aggregates are important factors affecting the performance of asphalt pavement. However, there is a lack of uniform results on the adhesion characteristics of rubberized asphalt. Therefore, crumb-rubber-modified asphalt (CRMA) with 15%, 20%, and 25% rubber contents was prepared in this work, and the basic rheological parameters and cohesive energy of the rubberized asphalt were characterized by DSR. The adhesion properties between rubberized asphalt and aggregates were characterized based on macroscopic binder bond strength (BBS), surface free energy (SFE) theory, and nanoscale atomic force microscopy (AFM) tests. The results show that crumb rubber (CR) can improve the high-temperature elastic properties of asphalt; secondly, CR can have a negative impact on the maximum tensile strength of asphalt and aggregates. CR can improve the SFE parameter of asphalt. The work of adhesion of rubberized asphalt and limestone is the highest, followed by basalt and, finally, granite. Finally, CR can cause the catanaphase in asphalt to gradually break down and become smaller, and the adhesion of rubberized asphalt can be reduced. Overall, CR can reduce the adhesion performance of asphalt, and this work provides a reference for the application of rubberized asphalt.

Discrete element modeling of aggregate shape and internal structure effects on Weibull distribution of tensile strength

Many soil processes and functions, including carbon storage, water and element cycling, and crop productivity, are directly or indirectly affected by soil aggregates. As a key property, aggregate tensile strength is usually quantified by the Weibull distribution. This distribution was found to depend on soil texture, organic matter content, and aggregate shape; however, effects of cementing agents (i.e., clay mineral and organic compounds) on aggregate tensile strength cannot be empirically studied independent of shape. The objective was to apply Discrete Element Modeling (DEM) to simulate the rupture of soil aggregates of different shapes but the same assumed distributions of cementing agents. The hypothesis was that the Weibull modulus of aggregate tensile strength could be used to describe the heterogeneity of the particle binding forces within the aggregate. Multiple DEM simulations of aggregate rupture were carried out for sets of randomly generated binding force distributions between particles of the same diameter. Aggregate properties and shapes were obtained from those observed in arable Gleyic Cambisol topsoil from plots with and without vegetation of a soil compaction trial; additional aggregate shapes were simulated to increase the predictive range. The simulation results showed that (i) the sensitivity of Weibull modulus of aggregate tensile strength to the heterogeneity of particle binding force distribution increased with aggregate sphericity, (ii) the value of the Weibull modulus decreased with increasing heterogeneity of this distribution, and (iii) internal heterogeneity also reduced the crack surface area produced by aggregate rupture. Results corresponded to friability theory implying that small values of Weibull modulus indicated more complex soil structural organization. The DEM simulations could help explaining complex interactions between aggregate shape cementing agents and stability. Our DEM approach also uncovered the importance of Weibull modulus to assess structural features of soil aggregates and to describe the heterogeneity of the particle binding forces within the aggregate.

Experimental Investigation on Environmentally Sustainable Cement Composites Based on Wheat Straw and Perlite.

Environmentally sustainable cement mortars containing wheat straw (Southern Italy, Apulia region) of different length and dosage and perlite beads as aggregates were prepared and characterised by rheological, thermal, acoustic, mechanical, optical and microstructural tests. A complete replacement of the conventional sand was carried out. Composites with bare straw (S), perlite (P), and with a mixture of inorganic and organic aggregates (P/S), were characterised and compared with the properties of conventional sand mortar. It was observed that the straw fresh composites showed a decrease in workability with fibre length decrease and with increase in straw volume, while the conglomerates with bare perlite, and with the aggregate mixture, showed similar consistency to the control. The thermal insulation of the straw mortars was extremely high compared to the sand reference (85–90%), as was the acoustic absorption, especially in the 500–1000 Hz range. These results were attributed to the high porosity of these composites and showed enhancement of these properties with decrease in straw length and increase in straw volume. The bare perlite sample showed the lowest thermal insulation and acoustic absorption, being less porous than the former composites, while intermediate values were obtained with the P/S samples. The mechanical performance of the straw composites increased with length of the fibres and decreased with fibre dosage. The addition of expanded perlite to the mixture produced mortars with an improvement in mechanical strength and negligible modification of thermal properties. Straw mortars showed discrete cracks after failure, without separation of the two parts of the specimens, due to the aggregate tensile strength which influenced the impact compression tests. Preliminary observations of the stability of the mortars showed that, more than one year from preparation, the conglomerates did not show detectable signs of degradation.

Modeling soil aggregate fracture using the discrete element method

The Discrete Element Method (DEM) is a suitable approach for modeling and simulating arable soil and tillage processes. In this work, we focus on modeling soil fragmentation which is a vital element of many soil treatments. We present a method for computing the indirect tensile strength of soil aggregates through simulation. DEM aggregates are constructed as spherical clusters of smaller spherical particles. A number of indirect tensile tests are conducted using simulated aggregates from which the critical polar force and tensile strength are identified. We propose a method for introducing random voids such that the distribution of simulated tensile strength corresponds to a given Weibull distribution. We propose to scale the number of voids linearly with the measured porosity of the aggregates. This approach showed better performance than introducing a constant number of voids. We identified that the friction between the sub-spheres can be related to the characteristic strength and the number of voids is related to the spread of strengths in the Weibull distribution. The calibration and validation are performed on experimental data. Based on this approach, the synthetic DEM aggregates allow for deriving the aggregate strength as a function of aggregate size. The results are based on fracturing individual aggregates but can be extended to simulations involving multiple interacting aggregates.

Residual effects of phosphogypsum rates and machinery traffic on soil attributes and common-bean (Phaseolus vulgaris) yield in a no-tillage system

Phosphogypsum (PG) affects many soil attributes, but its long-term effects on soil structure and physical properties remain unclear. We evaluated the residual effects of PG and induced soil compaction on selected soil chemical and physical attributes, and on common bean (Phaseolus vulgaris) yield. The experiment was initiated on a Typic Hapludox soil using a split-plot design with four replicates. In November (2009), PG at rates of 3, 6, 9, 12 Mg ha−1 was broadcast on the soil surface. In November (2012), traffic compaction was applied to the soil and the treatments were wheel-track (WTR), between wheel tracks (BWT) and no traffic/reference (REF) zones. Compaction was applied with a four-wheel-drive tractor with 2.4 and 3.6 Mg on the front and rear axles, respectively, at field water capacity (FC). Common bean was sown in February (2013) and its yield was assessed at physiological maturity. Forty-two months after application of traffic treatments, the 0.0−0.1 and 0.1−0.2 m soil layers were sampled. Phosphogypsum decreased exchangeable acidity and Al3+ concentrations, and increased pH and Ca2+ in both soil layers, as well as SO42- in the 0.1−0.2 m layer. Despite increasing water-dispersible clay (WDC), PG decreased aggregate tensile strength (TS) at the WTR and BWT zones. Phosphogypsum rates increased organic carbon concentrations in soil aggregates. Machinery traffic increased the values of WDC and TS, but also decreased common bean yield. PG application alleviated the negative impacts of wheel traffic on TS. We conclude that the residual effect of PG can positively affect soil chemical and physical attributes and contribute to alleviate the impacts of machinery traffic.

Does biochar application alleviate soil compaction? Review and data synthesis

Biochar is considered to enhance numerous soil ecosystem services, but whether it alleviates soil compaction is not well known. This paper: 1) synthesizes the biochar impacts on soil compaction parameters including bulk density, penetration resistance, aggregate tensile strength, Proctor maximum bulk density (soil compactability), and the water content at which the Proctor maximum bulk density occurs (critical water content), 2) discusses potential factors affecting biochar performance, and 3) compares the impacts of biochar on soil compaction with those of other amendments. Literature indicates that biochar generally reduces bulk density, tensile strength, and Proctor maximum bulk density; and increases the critical water content but may or may not reduce penetration resistance. Biochar reduces bulk density from −1% to −20% (-8.8% average), tensile strength from −8 to −72% (-31% average), and Proctor maximum bulk density from −1 to −19% (-9.3% average), whereas it increases the critical water content from 1 to 64% (21% average). The increase in critical water content suggests soils with biochar can be trafficked at higher water content without causing compaction than soils without biochar. An increase in biochar application rate reduces soil compaction and explains 11% to 63% of the variability in compaction parameters, but biochar benefits can be short-lived (<2 yr). It appears that, in general, at least 10 Mg ha−1 of biochar are needed to significantly reduce compaction. Larger application rates (>20 Mg ha−1) can be needed for reducing penetration resistance and Proctor maximum bulk density than for other compaction parameters. Biochar has similar benefits to manure for reducing soil bulk density and can complement other organic amendments for reducing soil compaction. How biochar feedstock, pyrolysis temperature; and companion amendments; and soil texture influence biochar impacts on compaction is still unclear. Overall, biochar has the potential to alleviate soil compaction, but more long-term field data, particularly on penetration resistance, Proctor maximum bulk density, and critical water content for multiple biochar application rates, feedstocks, pyrolysis temperature; and amendments are needed to better elucidate biochar effects, biochar longevity, and optimum levels of biochar application.

A Study Based upon the Usage Recycled Aggregate to Enhance the Strength Aspects of Conventional Concrete: A Critical Review

Recycled aggregates are those crushed cement concrete or asphalt pavement which comes out from the construction debris which is reused in construction. They are made from the reprocessing of materials which have been used in previous constructions. This paper discusses about the study of properties of recycled aggregates from the sources which has already been published. The results are that 100% replacement of natural aggregate by recycled concrete aggregate effect on chloride ions resistance, it plays negative effects on durability of recycled concrete aggregates, and addition of fiber in recycled aggregate concrete mixture gave more effective in the performance of concrete. On experimental study of recycled aggregate, compressive, flexural and split tensile strength of the recycled aggregate were found to be lower than that of the natural aggregate. Use of recycled aggregate in a new concrete production is still limited. Recommendation of introduction of recycled aggregates standard is required for the materials to be used successfully in future. Gaps in literature reviews are also included in this paper.

Determination of Soil Properties Affecting Soil Aggregate Tensile Strength in a Semiarid Region of Iran Using a Hybrid Algorithm

ABSTRACT The present study was conducted to determine soil properties affecting soil aggregate stability in southeast Iran. For this purpose, 169 soil samples were collected at 0–20 cm depth, and some soil properties such as soil bulk density (BD), electrical conductivity (EC), calcium carbonate equivalent (CCE), and soil organic matter (OM) were determined. Subsequently, soil aggregate stability (as quantified by tensile strength (TS)) were determined using the indirect (Brazilian) method. Furthermore, soil properties affecting soil aggregate stability were determined using the Particle Swarm Optimization-Imperialist Competitive Algorithm-Support Vector Regression (PSO-ICA-SVR) hybrid algorithm, and after the sensitivity analysis, the importance of each selected property in terms of its impact on soil aggregate stability was recognized. The results indicated that OM, BD, EC, and dg (geometric mean particle diameter), in general, influence soil aggregate stability, and this selection was carefully made by the hybrid algorithm. Beside, after modeling using the features selected by the SVR method and performing the sensitivity analysis, BD and OM, among the selected properties, had the most significant effects on soil aggregate stability. In this study, the SVR method achieved greater accuracy in predicting the TS index compared to the Multiple-Linear Regression (MLR). The RMSE value (Root Mean Square Error) for the TS index using the MLR model was 30.77, while a lower RMSE value of 10.536 was observed for the SVR model.

Tensile strength values for the degrees of soil consistency using human perception and TS-Soil device

Tensile strength of aggregates, defined as the force per unit of area required to cause rupture of aggregates, is useful to quantify the effects of both use and management on soil quality. The present study considered the hypothesis that the evaluator’s perception, combined with the force applied to break the aggregate, measured by the TS-Soil device, can be used to define ranges of tensile strength for the degrees of dry soil consistency. The objective in this study was to define ranges of tensile strength for the degrees of dry soil consistency. For this purpose, aggregates (mean diameter of 22 mm after preparation) were collected in five horizons: A and Bi of a Cambissolo Háplico (Cambisol), Bt with a cohesive character of an Argissolo Vermelho Amarelo (Lixisol), Bt of a Luvissolo Crômico (Luvisol) and Bw of a Latossolo Vermelho-Amarelo (Ferralsol). One hundred aggregates of each horizon were evaluated for tensile strength, measured in the TS-Soil device, and, concomitantly, when pressed, were analyzed by an evaluator to assign the following scores to their consistency: 1- loose, 2- soft, 3- slightly hard, 4- hard, 5- very hard, and 6- extremely hard. Data were initially analyzed by descriptive statistics. Then, evaluator’s scores and tensile strength measured by the TS-Soil device were considered to define the ranges of values for the degrees of dry soil consistency. The following tensile strength (kPa) ranges were obtained: >0 to <21 for soft consistency, ≥21 to <38 for slightly hard, ≥38 to <65 for hard, ≥65 to <110 for very hard and ≥110 for extremely hard. It was concluded that the TS-Soil device makes it possible to combine the evaluator’s perception with the force applied to break the aggregate, enabling the definition of ranges of values for the degrees of dry soil consistency, and that the qualitative perception (evaluator) associated with tensile strength values (TS-soil device) gives better quality to the process of evaluating dry soil consistency.

Impact of permanent traffic lanes on the soil physical and mechanical properties in mechanized sugarcane fields with the use of automatic steering

The randomized traffic during all crop cycles in the Brazilian sugarcane production system enhances soil compaction. The use of automatic steering in controlled traffic farming represents an alternative to minimize soil compaction in the sugarcane fields. Our research aimed to evaluate the changes in the soil physical and mechanical properties under permanent traffic lanes, after three years of sugarcane mechanized harvesting, with the use of automatic steering. The experiment was carried out in the municipality of Nova Europa, state of São Paulo, Brazil, in a sugarcane area, under the following treatments: sugarcane planted in a single-row spacing (1.5 m) with conventional traffic, sugarcane planted in a single-row spacing (1.5 m) with automatic steering, and sugarcane planted with double-combined row spacing (1.50 × 0.90 m) and with automatic steering). The bulk density, soil porosity, soil penetration resistance, aggregate stability, and tensile strength of aggregates, were assessed in the inter-row center and seedbed. Soil load-bearing capacity was also included in this study by modeling of preconsolidation pressure at 0–0.1 m and 0.2–0.3 m of depth. After three years of mechanized operations, with controlled traffic using automatic steering, in a sugarcane field, the soil compaction was higher on the inter-row center (i.e. higher bulk density, tensile strength, and preconsolidation pressure and, lower macroporosity) than on the seedbed. The load-bearing capacity allowed to detect the pronounced effect of the machines’ traffic management systems on the soil compaction. The use of automatic steering reduced the load-bearing capacity in the seedbed in relation to the inter-row center, indicating that such management system preserves the soil physical quality in the ratoon region. Moreover, the lowest soil compression index values observed in the inter-row center, using management systems with automatic steering, indicates a lower susceptibility to undergo additional soil compaction.

Which evidence attests for soil aggregate rupture? A new criterion to determine aggregate tensile strength

Determining the aggregate compressive rupture force is useful to calculate aggregate tensile strength and to assess soil physical properties, such as soil friability. Here, we investigated two criteria to determine such a force, a commonly used criterion that considers it as the first force drop higher than 40 % (FD40) and a proposed one considering that the aggregate rupture force occurs simultaneously with aggregate equatorial cracks (EC). These criteria were compared based on results of aggregate compression tests, crack propagation monitoring, sound wave measurements, X-ray micro-computed tomography characterisation and analysis of numerical model based on Discrete Element Method (DEM). We demonstrated that EC is a more reliable empirical procedure to determine aggregate tensile strength than FD40, since there is a correspondence between the rupture force value and the formation of cracks in the equatorial region. Further, when using EC, the DEM model was more accurate in reproducing the empirical values of aggregate tensile strength.

DEM analysis on the role of aggregates on concrete strength

This study aims to understand the micro-mechanisms that drive fracture propagation in concrete and to assess the roles of the strength of aggregates and of the aggregate/mortar interfacial transition zone (ITZ) on concrete strength. We use the Discrete Element Method (DEM) to model concrete samples. Mortar is represented by a volume of bonded spherical elements. Bonds are governed by a new displacement-softening law. Aggregate centroids are randomly placed in the DEM sample. We use CT scan images of real aggregates to plot 3D aggregate contours. The spherical elements that are contained in 3D contours around the randomly placed centroids are replaced by clusters with aggregate properties. The number and the size of the clusters are determined from the experimental Particle Size Distribution. The DEM concrete model is calibrated against uniaxial compression tests and Brazilian tests of both mortar and concrete. It is found that: At same aggregate volume fraction, a concrete sample with randomly placed aggregates and ITZ bonds is stronger; Concrete strength is linearly related to aggregate tensile strength; A linear relationship exists between the contact ratio in the mortar/aggregate ITZ and concrete strength; The ITZ has more influence on concrete strength than aggregate tensile strength.

Critical values of physical attributes of an Ultisol under uses in South of Brazil

Changes in the soil structural quality have caused alterations in root growth and agricultural yield. With the purpose of finding critical values of physical parameters and alternative, viable and suitable ways of minimize the soil degradation, this study aimed at quantifying aggregate tensile strength (TS), friability (F), bull density (Bd), total porosity (Tp), macroporosity (Ma), penetration resistance (PR) and total organic carbon (TOC). Relations were established to estimate the structural quality of some parameters of an Ultisol under different uses. The study was carried out on a farm, in the rural area of RS, inserted in the Arroio Moreira waterbasin, Brazil. Disturbed and undisturbed samples were collected from the 0.00 to 0.10 m layer. Regarding TS and soil use, 900 samples were selected, totalling 2700 aggregates. Undisturbed samples (135) were collected for the evaluation of Bd, porosity and PR. The TOC was quantified using 45 aggregates through the dry combustion method. All physical parameters with exception of F are important in monitoring the structural quality.The lowest values of TS, PR and Bd were verified in the soil under fallow, and the highest ones in the soil under pasture. An inverse relation between TS and TOC and between Bd and TOC was verified. Critical values of Bd and TS to crop development might be estimated from the PR.

Soil Structural Degradation During Low‐Severity Burns

AbstractLow‐severity wildfires and prescribed burns have been steadily increasing for over three decades, currently accounting for more than half of total burned area in the southwestern United States. Most observations immediately after low‐severity burns report little adverse impacts on soil properties and processes. In a few studies, however, significant deterioration of soil structure has been observed several months after such fires. Here we show that rapid vaporization of pore water during low‐severity burns raises pneumatic gas pressure inside large aggregates (20–30 mm) to damaging levels, on the order of aggregate tensile strength and high enough to cause viscoplastic deformation. However, the impact on soil structure was not immediately perceptible. This suggests that other natural forces, such as wetting‐drying and thermal cycles, are required to disrupt the weakened aggregates. Thus, adverse consequences of the suggested mechanism on soil processes and services (e.g., infiltration, erodibility, and organic matter protection) are likely overlooked.

Tensile strength of mollisols of contrasting texture under influence of plant growth and crop residues addition

Tensile strength (TS) of soil aggregates is considered a sensitive and important indicator of the effects of the management practices on soil structure quality, which affects the seed germination and the initial crop growth. However, the influence of plant growth, crop residues addition and the produced aggregating agents on TS has not been widely studied. The objectives of this study were: i) to determine the specific effects of plant growth and different types, rates and location of crop residues in the aggregates tensile strength, and ii) to assess the relationship between the aggregating agents produced by plant growth and crop residues addition and the aggregate tensile strength of soils of contrasting texture. A greenhouse experiment was carried out in pots with a loamy soil (Typic Hapludoll, Santa Isabel series) and a silty-loamy soil (Typic Argiudoll, Esperanza series) under controlled conditions for 112 days. For each soil the following treatments were set up in triplicate: (i) soil type, (ii) with or without plant growth of wheat (Triticum aestivum L.), (iii) with or without residues addition, (iv) location of residues (surface vs. incorporated), (v) wheat vs. soybean (Glycine max L.) residues, and (vi) residue rates (6.3 and 15.7 g of dry matter per pot for wheat, and 6.3 and 18.8 g of dry matter per pot for soybean). Pots were exposed to wetting and drying (W/D) cycles. TS values and aggregating agent's content, i.e., total organic carbon (TOC), particulate organic carbon (POC), hot water extractable carbohydrates (HWEC), dilute acid extractable carbohydrates (DAC), total carbohydrates (TC), total glomalin-related soil protein (T-GRSP), and easily extractable glomalin-related soil protein (EE-GRSP) were measured. TS were significantly lower in the Typic Hapludoll (TS = 39.9 kPa) than in the Typic Argiudoll (TS = 61.6 kPa). TS values were significantly higher in the treatments with plants of wheat than in treatments without plants (49.5 vs. 30.3 kPa in the Hapludoll and 71.2 vs. 50.9 kPa in the Argiudoll). Plant growth increased TS through physical mechanisms, i.e. a greater number of drying-wetting cycles. Also, plant growth increased TS by producing aggregating agents. TS values were not directly affected by the addition of different types, rates and locations of crop residues. However, they increased the content of aggregating agents. Multiple regression analysis showed that TS was significantly related to soil type, TC and T-GRSP. TS increased with increasing TC and T-GRSP. These two variables explained 87% of the model variation. The obtained model provides a basis for understanding which are the most important aggregating agents and, consequently, which are the better management systems to improve o recover the structure quality of soils with different textures.

Tensile strength in horizons with and without cohesive character: Variability and relation with granulometry

Soils of Coastal Tablelands are important for the economic and social development of the coastal region of Brazil. However, these soils appear to have a number of chemical and physical limitations. One of the most expressive physical limitations is the occurrence of cohesive character, whose genesis is still not fully understood. Thus, considering that the tensile strength is a direct measure of the cohesion of particles, this research was undertaken to discover how the size of soil particles influences the genesis of the cohesive character in Bt horizons of the Coastal Tablelands of Ceará, Brazil. The study was carried out in the municipality of Aquiraz-CE, where samples of two soils were collected (Argissolo Vermelho-Amarelo Distrófico típico – PVAd; Argissolo Amarelo Eutrocoeso abrúptico – PAex) in the horizons Bt1 (cohesive) and Bt2 (non-cohesive) in the top, middle and bottom portions. The results demonstrated that the size of the particles led to sandy clay loam texture in horizons of PVAd and sandy clay texture in the horizons of PAex. Regarding sand sorting, for all investigated horizons and in the three positions (top, middle and bottom), the medium sand fraction prevailed, followed successively by fine and very fine sands together, coarse and very coarse sands. Tensile strength values were higher in cohesive horizons, with reduction in the tensile strength of aggregates from top to bottom. It was concluded that tensile strength was a useful indicator for differentiating cohesive properties in the different soil horizons. There was significant variation of tensile strength in the soil horizon defined by the pedologist – which suggests that it is necessary to include as a valuable addition to the diagnostic tools available to the working pedologist, a tensile strength scale for the identification of cohesion to avoid subjectivity in the definition of the cohesive horizon. For the studied set of horizons, tensile strength decreased from top to bottom. Sand and clay fractions, despite not being the only determinant factor, influenced the genesis of the cohesive character in soils. For the sand fraction, poor degree of sand sorting led to a denser packing of particles, significantly contributing to the cohesion of aggregates. Our experiment strongly suggests that the packing of particles allows the increase of cohesion provided there is a cementing agent even in small quantity.

Physical quality of an Oxisol under no-tillage subjected to different cropping systems

Abstract The objective of this work was to evaluate the physical quality of an Oxisol under no-tillage subjected to different crop rotations and crop sequences. The experiment was carried out in a clayey Oxisol, during six years, in a randomized complete block design, with strip plots and three replicates. The following physical indicators of soil quality were evaluated: soil density and carbon content, as well as mean weight diameter and tensile strength of aggregates. Treatments consisted of three summer crop rotations - corn/corn (Zea mays), soybean/soybean (Glycine max), and soybean/corn - and of seven second crops (crop sequences) - corn, sunflower (Helianthus annuus), oilseed radish (Raphanus sativus), pearl millet (Pennisetum americanum), pigeon pea (Cajanus cajan), grain sorghum (Sorghum bicolor), and sun hemp (Crotalaria juncea). Crop rotations and sequences did not affect soil carbon contents. Corn, as a summer crop, increased the tensile strength and mean weight diameter of aggregates, similarly to pearl millet and sorghum as second crops. Soybean/corn rotation with sun hemp as a second crop increases soil physical quality, promoting higher tensile strengths and lower soil densities.

Relationship between soil aggregate strength, shape and porosity for soils under different long-term management

Soil aggregate properties, such as strength, shape and porosity, influence a range of essential soil functions and there is a need for more detailed understanding of the effect of soil management on these aggregate properties. There is also a need for improved knowledge on the link between aggregate and bulk soil properties. The objectives of this study were to quantify the long-term effect of rotation and tillage on aggregate shape, strength and pore characteristics, to evaluate the influence of aggregate shape and pore characteristics on aggregate strength and soil friability and to correlate aggregate properties to bulk soil properties. Soil core samples were taken in spring 2010 from the long-term rotation and tillage trial (initiated in 1980) at the University of Guelph, Canada. The rotations included were continuous corn (R1) and a diverse rotation (R6), and the tillage treatments were mouldboard ploughing (MP) and no-tillage (NT). The soil cores were exposed to a drop shatter test and air-dried before separation into different size fractions. Ten aggregates from the 4–9.2mm size fraction per core sample (i.e. 320 in all) were X-ray micro-CT scanned. The size, shape and porosity of the aggregates were determined using image analysis with 40μm voxel size. Subsequently, aggregate tensile strength was determined in an indirect tension test. Rotation had a more pronounced effect than tillage treatment on the different aggregate properties. The diverse rotation resulted in higher aggregate total porosity and more rounded aggregates than the continuous corn rotation. Surprisingly, there was no treatment effect on X-ray micro-CT resolvable porosities. Aggregate strength decreased with both total and X-ray micro-CT resolvable porosity even though the correlations were weak. Significant correlation was also found to aggregate sphericity although only around 10% of the variation in tensile strength could be explained by this property. Our study highlights that caution must be taken when trying to predict aggregate strength from general aggregate characteristics. For both bulk soil and aggregates, the R6-MP had highest and R1-NT lowest porosity. Tillage had strongest effect on bulk soil porosity, whereas aggregate total porosity was only affected by rotation. Our results suggest that the scale of observation is important when evaluating the influence of soil management. A strong correlation was found between aggregate strength and pore characteristics and soil fragmentation in a drop shatter test, i.e. 55% of the variation could be explained. Our study indicates therefore that bulk soil fragmentation behaviour can be predicted from aggregate characteristics. It needs to be highlighted that our results are based on one long-term experiment on a silt loam soil. The results need to be verified for soils with different soil types, climates and management histories.