Aggregate Tensile Strength Research Articles

Evaluation of soil aggregate microstructure and stability under wetting and drying cycles in two Ultisols using synchrotron-based X-ray micro-computed tomography

Soil structure and the soil pore system are important for many soil environmental processes. However, little is known about the influence of intra-aggregate microstructure on aggregate stability. The objective of this study was to evaluate the effect of soil pore characteristics on wet aggregate stability and aggregate tensile strength (TS) using synchrotron-based X-ray micro-computed tomography (SR-μCT) under wetting and drying cycles. Undisturbed topsoils (200cm3) derived from shale (S) and Quaternary red clay (Q) were submitted to three wetting and drying treatments under controlled laboratory conditions. 3–5mm aggregates were collected from different treatments and scanned at 3.7μm voxel-resolution for the reconstruction of 3D micro tomography images. The wet aggregate stability and TS were measured after each treatment. The relationships among pore characteristics, wet aggregate stability, and TS were analyzed using partial least squares regression (PLSR). The results indicated that porosity (P), percentage of pores >100μm (Pd>100), and fraction of elongated pores (FEP) all significantly increased with an increasing number of wetting and drying cycles, while the opposite trends were observed for the total number of pores (TNP), the percentage of pores with a diameter of 75–100, 30–75, and <30μm. Decrease in wet aggregate stability and TS of both soils was mainly due to higher P, a more extensive and complex pore network, which caused increased air pressure, increased rate of water entry, and high probability of crack propagation and interaction. The TNP, Pd75–100, P, and Pd>100, were identified as the primary factors controlling the wet aggregate stability and TS according to PLSR. The pore characteristics and soil clay content accounted for as much as 99% of the variation in wet aggregate stability and TS. This study provides insights for improved understanding of the change in topsoil microstructure; however, the effect of pore characteristics on the aggregate breakdown mechanism requires further investigation.

Triaxial Testing to Determine the Effect of Soil Type and Organic Carbon Content on Soil Consolidation and Shear Deformation Characteristics

The purpose of this paper is to determine the effect of soil type and organic matter on the deformation characteristics of soil. We used triaxial testing to measure both the consolidation and shear deformation of soil. The novel application of this method was applied to soils from two experiments: on a clay loam and a sandy loam. The clay loam soil was from the long-term Broadbalk Experiment at Rothamsted Research, UK, where contrasting treatments had been applied since 1843. The sandy loam soil was from a straw incorporation experiment started in 1986. The clay loam soil from selected treatments from the Broadbalk Experiment had large difference in soil organic C (SOC) content due to additions of farmyard manure (FYM) or inorganic fertilizers for over 170 yr. There were no detectable differences in SOC or microbial biomass C in the sandy loam soil where straw had been incorporated. In addition to triaxial testing, we assessed soil physical condition with measurements of aggregate stability and aggregate tensile strength. From triaxial tests of the repacked soil, we found that soil type and SOC affected the compression characteristics. We also observed that deformation characteristics are more sensitive to small changes in soil management practices designed to increase the SOC content, than measurements of aggregate tensile strength, friability, or stability. We suggest that measurement of deformation characteristics is a potentially powerful approach for detecting or predicting changes in soil physical conditions as impacted by small changes in SOC content resulting from management practices.

Influence of mechanical loading on static and dynamic CO2 efflux on differently textured and managed Luvisols

Mechanical disturbance of soil structure is commonly related to altered physical changes in pore systems, which control CO2 effluxes e.g. by changes in gas transport properties and in microbial activity. Soil compaction mostly leads to reduced CO2 fluxes. In contrast, structured soils can also release physically entrapped CO2 or give access to protected carbon sources inside aggregates due to aggregate breakdown by disruptive forces. In this study it was investigated how far arable soil management affects structure- and compaction-related CO2-releases using incubation experiments and CO2 gas analysis under standard matric potentials (−6kPa). CO2 efflux was analyzed before, during and after mechanical loading using the alkali trap method (static efflux) and a gas flow compaction device (GaFloCoD, dynamic efflux). Intact soil cores (236 and 471cm3) were collected from a Stagnic Luvisol with loamy sand (conservation and conventional tillage systems) and a Haplic Luvisol with clayey silt (under different fodder crops) from the topsoil (10–15cm) and subsoil (35–45cm). Mechanical stability was reflected by the pre-compression stress value (Pc) and by the tensile strength of aggregates (12–20mm). Changes in pore systems were described by air conductivity as well as air capacity and total porosity. While CO2-releases varied highly during the compaction process (GaFloCoD) for different stress magnitudes, soil depths and management systems, basal respiration rates were generally reduced after mechanical loading by almost half of the initial rates irrespective of soil management. For both methods (dynamic and static efflux) restriction in gas transport functionality was proved to have major influence on inhibition of CO2 efflux due to mechanical loading. GaFloCoD experiments demonstrated that decreases in CO2 efflux were linked to structural degradation of pore systems by exceeding internal soil strength (Pc). Otherwise, re-equilibrating matric potentials to −6kPa and re-incubating offset inhibition of soil respiration suggest a re-enhancement of microbial activity. At this state, physical influences were apparently overlapped by biological effects due to higher energy supply to microbes, which could be offered by spatial distribution changes of microorganisms and organic substrates within a given soil structure. This implies the susceptibility of physical protection mechanism for carbon by disruption of soil structure. In future, special focus should be given on a clear distinction between physical and microbiological effects controlling CO2 fluxes in structured soils.

Effect of time of autumn primary tillage on soil structure, grain yield and risk of nitrogen leaching in two Swedish clay soils

Delaying tillage and incorporation of arable crop residues in autumn may limit accumulation of soil mineral nitrogen (SMN) and reduce nitrogen (N) leaching. However, on clay soils, high water content during late tillage can impair soil structure and fertility. This study investigated the effects of tillage timing on SMN (0–90 cm depth), soil physical properties in the topsoil and yield in two Swedish clay soils differing in clay content. SMN generally increased during autumn and also with earlier tillage. On a heavy clay soil (Ultuna; ∼60% clay), SMN increased throughout winter until spring and the amount of N leached was probably small. In contrast, on a clay loam soil in southernmost Sweden (Rydsgård; ∼35% clay), SMN decreased during winter by up to 26 kg ha−1. In general, measured soil water content (w) was lower than 0.9 × plastic limit (PL) at the time of early tillage (mouldboard ploughing or stubble cultivation), while it exceeded 0.9 PL at the time of normal and late tillage. The proportion of small aggregates produced by tillage was negatively correlated to relative soil water content at the time of tillage (w/PL). Conventional methods to determine soil physical qualities (bulk density, saturated hydraulic conductivity, aggregate tensile strength) did not show any clear differences between treatments. However, plant emergence and yield were significantly reduced after late compared with early tillage at Ultuna, a trend that has persisted from 2000 until the present. These results suggest that the suitability of delayed autumn tillage as a means to reduce SMN accumulation and N leaching varies between Swedish clay soils. A reduction in N leaching can be achieved on lighter clay soils in warmer and moister areas, whereas the effects may be small on heavier clay soils in central Sweden, where repeated late tillage carries a risk of yield reduction.

Tillage System and Cover Crop Effects on Soil Quality: I. Chemical, Mechanical, and Biological Properties

Optimal use of management systems including tillage and winter cover crops is recommended to improve soil quality and sustain agricultural production. The effects on soil properties of three tillage systems (as main plot) including direct drilling (D), harrowing to a depth of 8 to 10 cm (H), and moldboard plowing (MP) with and without a cover crop were evaluated in a long-term experiment on a sandy loam soil in Denmark. Chemical, physical, and biological soil properties were measured in the spring of 2012. The field measurements included mean weight diameter (MWD) after the drop-shatter test, penetration resistance, and visual evaluation of soil structure (VESS). In the laboratory, aggregate strength, water-stable aggregates (WSA), and clay dispersibility were measured. The analyzed chemical and biological properties included soil organic C (SOC), total N, microbial biomass C, labile P and K, and pH. Reduced tillage (D and H) resulted in a stratification of the chemical properties within the 0- to 20-cm topsoil layer but a uniform distribution for MP. There was an accumulation of SOC, total N, and labile P and K and reduced pH in the 0- to 10-cm layer. Reduced tillage increased soil strength in terms of greater MWD, VESS, WSA, aggregate tensile strength, and rupture energy. Five years of using a cover crop alleviated plow pan compaction at the 20- to 40-cm depth by reducing penetration resistance. A significant interaction between tillage and cover crop treatments indicated the potential benefit of using a combination of cover crops and direct drilling to produce a better soil friability. The usefulness of the VESS method for soil structural evaluation was supported by the high positive correlation of MWD with VESS scores.

Tensile and shear strength of porous dust agglomerates

Context.Within the sequential accretion scenario of planet formation, planets are build up through a sequence sticking collisions. The outcome of collisions between porous dust aggregates is very important for the growth from very small dust particles to planetesimals. In this work we determine the necessary material properties of dust aggregates as a function the porosity. Aims: Continuum models such as SPH that are capable of simulating collisions of macroscopic dust aggregates require a set of material parameters. Some of them such as the tensile and shear strength are difficult to obtain from laboratory experiments. The aim of this work is to determine these parameters from ab-initio molecular dynamics simulations. Methods: We simulate the behavior of porous dust aggregates using a detailed micro-physical model of the interaction of spherical grains that includes adhesion forces, rolling, twisting, and sliding. Using different methods of preparing the samples we study the strength behavior of our samples with varying porosity and coordination number of the material. Results: For the tensile strength, we can reproduce data from laboratory experiments very well. For the shear strength, there are no experimental data available. The results from our simulations differ significantly from previous theoretical models, which indicates that the latter might not be sufficient to describe porous dust aggregates. Conclusions: We have provided functional behavior of tensile and shear strength of porous dust aggregates as a function of the porosity that can be directly applied in continuum simulations of these objects in planet formation scenarios.

The effects of organic matter application and intensive tillage and traffic on soil structure formation and stability

Management strategies like organic matter (OM) amendment and mechanical energy inputs are known to influence the soil system. A long-term (13–14-year) field experiment was used to evaluate the effects of these management strategies on soil structural formation, structural stabilization and soil tilth of a sandy loam soil in Denmark. OM was applied as manure and by retention of plant residues (ORG) to be compared with plots dressed only with mineral fertilizer (MIN). The soils were rotovated (ROT), compacted (PAC) or left undisturbed (REF) as split-plot treatments in the main plots with OM management over two years prior to sampling. In two consecutive years, undisturbed soil samples were collected from the 6 to 13cm soil layer in the field grown with winter wheat to assess soil organic carbon (C) fractions (total organic C, polysaccharide C and microbial biomass C), total organic C and polysaccharide C of 1–2mm macro-aggregates, bulk density, hyphal length, aggregate stability, clay dispersibility, aggregate tensile strength, direct tensile strength and shear strength. The ease of fragmentation and the torsional shear strength of soil were measured in the field as well. OM application increased all soil organic C fractions. Response patterns of organic C fractions in aggregates were the same patterns as for whole-soil. Polysaccharide C appeared to be an important agent in the aggregation process. The effect of microbial C and fungal hyphae on the aggregation process was not clear. Extensive tillage and traffic produced unfavourable tilth conditions in terms of a greater degree of clay dispersion, lower aggregate stability, higher soil tensile strength and poorer soil fragmentation. OM affected soil reaction to compressive and tensile stresses applied at differing initial bulk densities. The results also indicated the profitability of supplementing the classical laboratory analysis with in situ measurements to better evaluate management effects on soil structure.

Aggregate Shape and Tensile Strength Measurement

Tensile strength (TS) of soil aggregates is an important indicator of soil quality. However, TS varies with aggregate shape. Thus, the objective of this study was to quantify the influence of aggregate shape on TS and propose a shape standardization protocol to increase accuracy in the measurement of TS. The latter was determined on 7,560 aggregates divided into three shapes, (i) irregular shape (IS), (ii) spherical shape (SS), and (iii) flat surface (FS), while preserving the inherent structure of the aggregate. The aggregates with IS had a larger range in the TS (306 kPa) because of the shape variability when compared with SS (238 kPa) and FS (129 kPa). The TS determined in aggregates with FS had smaller coefficient of variation (46%) in comparison with those of IS (70%) and SS (66%), indicating that the aggregate uniformity reduced the influence of shape on the TS. A smaller force (42.12 kPa) was needed to rupture aggregates with FS than IS (58.43 kPa) and SS (56.89 kPa) because of better force distribution in causing the tensile stress. The use of aggregates with the FS enables an accurate assessment of TS in relation to a wide range of management treatments.

Physical Properties of an Alfisol Under Biofuel Crops in Ohio

There is an increasing need to develop renewable energy sources from biofuel crops to replace fossil fuels. Biofuel crops may also enhance ecosystem functions such as soil quality, water availability, and nutrient reserves. Therefore, the effects of four biofuel crops (corn (Zea mays), switchgrass (Panicum virgatum), indiangrass (Sorghastrum nutans) and willow (Salix spp.) were evaluated on soil quality at three sites in Ohio to assess the effects of crop species on soil bulk density (ρb), soil moisture characteristics (SMC), water stable aggregate distribution (WSA), and aggregate tensile strength (TS) to 40 cm depth. Overall, results were site-specific, with most differences occurring for the clayey soil at the Northwest site. At the Jackson site, soil in the 0-10 cm layer under switchgrass had a higher moisture content (θ) between 0 and 100 kPa than that under indiangrass. At the Western site, θ under corn at 1500 kPa was higher at 30-40 cm depth. At the Northwest site, soils under corn in the 0-10 cm depth tended to have the lowest θ at 0 and 3 kPa, while soils under switchgrass and willow had 50% more large macroaggregates and fewer small microaggregates than that under corn. Soil TS in the 0-10 cm depth under corn was nearly 160% more than that under other perennial crops. These results suggest that management of perennial biofuel crops can improve soil physical quality. Changes over seven years occur first in the surface soil layers, but further differences may evolve in subsoil layers with increase in time.

Long‐Term No‐Till Impacts on Organic Carbon and Properties of Two Contrasting Soils and Corn Yields in Ohio

Tillage influence on soil properties and crop productivity depends on soil, crop, climate, and duration. Two long-term experimental sites with contrasting soils were selected to assess the influence of no-till (NT), minimum tillage (MT), and plow tillage (PT) under continuous corn (Zea mays L.) and corn–soybean [Glycine max (L.) Merr.] (CS) rotations on soil organic C (SOC) stock, bulk density (ρb), water-stable aggregation (WSA), aggregate tensile strength, penetration resistance, available water capacity (AWC), and corn yield. Experiments began in 1962 in northeast Ohio on a well-drained silt loam soil and in 1964 in northwest Ohio on a poorly drained clay loam soil. Results were compared with soil under an adjacent undisturbed woodlot (WL). The WL soils had the highest SOC content and stock. In the cultivated silt loam soil, stock was higher under NT (20.7 Mg ha−1) followed by MT (17.3 Mg ha−1) and PT (16.8 Mg ha−1) for the 0- to 10-cm depth. Soil ρb for this depth was lower under NT by 8 and 3% than PT and MT, respectively. The percentage of total WSA >2000 μm in soil under NT (47%) was significantly higher than under MT (38%) or PT (34%). A similar trend was observed for the clay loam soil. Rotation also influenced soil properties. Corn yields were higher (3 yr) and lower (1 yr) during 5 yr for the silt loam soil under PT than NT and unaffected or slightly higher under NT for the clay loam soil. Long-term (47–49 yr) use of NT practices are highly sustainable and result in higher SOC and WSA, lower ρb, and greater AWC content than MT or PT.

Tensile strength and organic carbon of soil aggregates under long-term no tillage in semiarid Aragon (NE Spain)

Tensile strength (TS) of soil aggregates is recognized as a useful parameter to detect structural changes associated with soil management. Although conservation tillage has been encouraged as a management alternative to preserve soil and water resources in semiarid Aragon (NE Spain), little information concerning soils on which these techniques are applied is available for this region. The main objective of this study was to assess the effect of long-term no tillage (NT) on TS, rupture energy (E) and organic carbon (OC) of soil surface aggregates (0–5cm depth) in five different cereal production areas of Aragon. In four of the sites, the study was conducted under on-farm conditions where pairs of adjacent fields under NT and conventional tillage (CT) were compared. In the last site, the study was carried out in research plots from a long-term tillage experiment where a third tillage treatment was also considered (reduced tillage). In all cases, a nearby undisturbed soil under native vegetation was included. Results indicate that NT reduced aggregate TS and E with respect to CT systems through lower soil disturbance and higher OC content. Aggregate size (16–8, 8–4, 4–2 and 2–1mm in diameter) and aggregate-associated OC explained 70–80% of the variation in TS and E. Strength properties varied with OC in quadratic way with minimum values at about 20gkg−1 of OC. This behavior can be explained by the significant interactions found between clay and OC in such a way that in the soils with the highest values of clay and OC, aggregate strength increased considerably. This study shows that, under the rainfed conditions of semiarid Aragon, NT improves soil physical conditions by reducing aggregate strength at the soil surface. This means, for example, that NT could provide a more favorable environment for seedling emergence and root growth than the traditional practices in the area.

Surfactant effects on soil aggregate tensile strength

Little is known regarding a soil aggregate's tensile strength response to surfactants that may be applied to alleviate soil water repellency. Two laboratory investigations were performed to determine surfactant effects on the tensile strength of 1) Ap horizons of nine wettable, agricultural soils collected from across the continental U.S., and 2) two of the nine soils (Latahco and Rad silt loams from the Pacific Northwest) that were sampled at two depths (5 and 15mm) after being sprinkler irrigated. Along with an untreated control, three surfactants (an alkyl polyglycoside, an ethylene oxide/propylene oxide block copolymer, and a blend of the two) were spray applied by hand at rates of 0, 1, 1.63, 3.35, 4.79, or 8.14kg active ingredient ha−1 to 1) air-dry, loose soil in Study 1 and 2) field-moist, tamped soil in Study 2 before being irrigated with surfactant-free water at 88mmh−1 twice, once for 0.33h, then about 8d later for 0.25h. Tensile strength was measured on oven-dry, 4- to 6.35-mm-diameter aggregates (18≤n≤37) of known mass for each treatment using a load cell with an attached flat-tip probe moving at a constant 0.27-mms−1 rate that applied continuous strain to each aggregate until it failed. In Study 1, tensile strength ranged widely, from 27kPa for Adkins loamy sand to 486kPa for Bolfar loam, averaged across surfactant treatments. Tensile strength for all nine surfactant-treated soils averaged 164kPa, 7% greater (P=0.099) than the control. In Study 2, surfactants significantly affected the tensile strength of Latahco but not Rad aggregates, when averaged across irrigations and sampling depths. After irrigation, aggregate tensile strength averaged 26% less (P<0.001) at the 5- than 15-mm depth, likely due to droplet kinetic energy fracturing near-surface, intra-aggregate bonds or surfactant leaching. All told, tensile strength varied more by soil series and depth than by surfactants.

Structural quality of a no-tillage red latosol 50 months after gypsum aplication

Gypsum application may enhance the soil quality for plants in terms of soil chemical and physical properties. The objective of this study was to evaluate the effects of gypsum application on the structural quality of a no-tillage Red Latosol. The experiment was initiated in September 2005 in Guarapuava-PR, with gypsum applications of 0; 4; 8; and 12 Mg ha-1 on the soil surface. In November 2009, two soil blocks were sampled from the 0-0.3 m layer for visual evaluation of the soil structure quality (Sq) and to determine the aggregate-tensile strength (ATS). Soil penetration resistance (PR) and gravimetric moisture (H%) of the 0-0.300 m layer were evaluated, and soil cores were collected (layers 0.000-0.075 and 0.075-0.150 m), to determine soil bulk density (BD), total soil porosity (TP), microporosity (Mi), and macroporosity (Ma). Data were subjected to analysis of regression at 5 %. No significant effects of gypsum application on ATS and H % of aggregates were observed, but for Sq, a quadratic effect (0.000-0.075 m) and linear increase (0.075-0.150 and 0.150-0.300 m) were stated, indicating soil quality decrease, although Sq remained mostly below 3.0, with good to intermediate soil quality. Soil PR increased with gypsum, but also remained below critical levels. No effect was observed for soil H % at the moment of PR determination on the field. The gypsum applications decreased BD in the 0.075-0.150 m layer, and increased PT and Ma, while in 0.000-0.075 m some Ma was converted to Mi, without affecting PT and BD. These last results indicate a gain in soil structural quality by gypsum applications, but the higher scores of soil structure and values of soil penetration resistance, though still below thresholds, should be monitored to prevent limitations to soil use in the future.

Tensile Strength and Friability of Ultisols in Sub-Tropical China and Effects on Aggregate Breakdown Under Simulated Rainfall

Many studies have shown the effects of mechanical properties on soil erosion. However, most have focused on the strength of bulk soil, with little attention paid to the strength of the aggregates. The objective of this study was to assess the effects of aggregate tensile strength and friability on aggregate breakdown under simulated rainfall. Different size aggregates of Ultisols, derived from Shale (S) and Quaternary red clay (Q), were exposed to simulated rainfall of 60 mm h−1. Fragment size distribution and splash loss were compared. The results showed that tensile strength decreased when aggregate size increased. For 1- to 2-mm aggregates, tensile strength of Q was greater than S, but the opposite was true for 10- to 20-mm aggregates. Estimates of friability from the coefficient of variation method were different with aggregate sizes tested and always gave larger numerical values than the volume dependence method. Aggregate breakdown was closely related to tensile strength because both depend on aggregate microstructure. Good correlations were observed between tensile strength and normalized mean weight diameters under different amounts of cumulative rainfall. As aggregate size increased, mean weight diameter of fragments increased, but normalized mean weight diameter and splash loss decreased. The tendency that aggregate splash decreased with increasing aggregate size was more obvious for the soil with low friability. For all size classes, the aggregates of soil with low friability produced more splash materials. It might be possible, with further experimentation, to develop empirically based criteria for stability of soil to erosion using tensile strength and friability.

Colloid Release From Differently Managed Loess Soil

The content of water-dispersible colloids (WDC) in a soil can have a major impact on soil functions, such as permeability to water and air, and on soil strength, which can impair soil fertility and workability. In addition, the content of WDC in the soil may increase the risk of nutrient loss and of colloid-facilitated transport of strongly sorbing compounds. In the present study, soils from the Bad Lauchstädt long-term static fertilizer experiment with different management histories were investigated to relate basic soil properties to the content of WDC, the content of water-stable aggregates (WSA), and aggregate tensile strength. Our studies were carried out on soils on identical parent material under controlled management conditions, enabling us to study the long-term effects on soil physical properties with few explanatory variables in play. The content of WDC and the amount of WSA were measured at a series of time steps giving a colloid release and aggregate disaggregation rate and a quantification of the content of WDC and WSA at a given time for each of the six investigated experimental field plots. The content of WDC in the moist soil was linearly correlated (r = 0.82* [P < 0.05]) to the part of the total clay not associated with organic matter. No significant difference in release rate was found for air-dry aggregates. The low-carbon soils initially had a higher content of WSA but were more susceptible to disaggregation than the high-carbon soils. Furthermore, the application of NPK fertilizer had a destabilizing effect on the WSA and also caused a decrease in the cation exchange capacity of the soils. The mean tensile strength was positively correlated to the colloid release rate and the content of WDC after 2 min of shaking and therefore to the amount of clay not associated with organic carbon.

Soil resistance and resilience to mechanical stresses for three differently managed sandy loam soils

Abstract To improve our understanding of how clay-organic carbon dynamics affect soil aggregate strength and physical resilience, we selected three nearby soils (MFC, Mixed Forage Cropping; MCC, Mixed Cash Cropping; CCC, Cereal Cash Cropping) with identical clay content and increasing contents of organic carbon (CCC k a ), void ratio ( e ) and air-filled porosity ( e ) as functional indicators and to characterise aggregate stability, strength and friability. Aggregate tensile strength measurements were done at − 1000 hPa and in the air dried state. Soil cores were subjected to uniaxial confined compression (200 kPa) followed by a period of natural recovery and wet–dry or freeze–thaw cycles. The MFC soil displayed greater tensile strength, specific rupture energy and friability at both soil-water potentials than the MCC and CCC soils possibly due to higher biotic binding of soil particles by the greater organic carbon content. The water dispersible clay was negatively correlated with the level of clay saturation by organic carbon. The resistance of the soils to compaction, quantified by both the compression index and a proposed functional index, was significantly greater for the MFC soil compared to the other two soils. The change in compression index with initial void ratio was significantly less for the MFC than the other soils. Plastic reorganisation of the soil particles immediately after compaction, prior to removal of the load, was greatest for the high organic soil. Physical resilience after natural recovery followed the organic carbon gradient (MFC > MCC > CCC). After wet–dry cycles, the MFC soil showed a significantly greater resilience for all three indicators ( k a , e and e ). Resilience ( k a , and e ) after freeze–thaw cycles was significantly lower for the CCC soil than the MFC and MCC soils. A significant positive correlation between resistance and resilience was observed for k a and e but not for e . Resilience to compaction was found to be independent of the soil properties monitored.

Soil-profile distribution of carbon and associated properties in no-till along a precipitation gradient in the central Great Plains

No-till (NT) farming is considered as a potential strategy for sequestering C in the soil. Data on soil-profile distribution of C and related soil properties are, however, limited, particularly for semiarid regions. We assessed soil C pool and soil structural properties such as aggregate stability and strength to 1 m soil depth across three long-term (≥21 year) NT and conventional till (CT) experiments along a precipitation gradient in the central Great Plains of the USA. Tillage systems were in continuous winter wheat ( Triticum aestivum L.) on a loam at Hutchinson and winter wheat–sorghum [ Sorghum bicolor (L.) Moench]-fallow on silt loams at Hays and Tribune, Kansas. Mean annual precipitation was 889 mm for Hutchinson, 580 mm for Hays, and 440 mm for Tribune. Changes in profile distribution of soil properties were affected by differences in precipitations input among the three sites. At Hutchinson, NT had 1.8 times greater SOC pool than CT in the 0–2.5-cm depth, but CT had 1.5 times greater SOC pool in the 5–20-cm. At Hays, NT had 1.4 times greater SOC pool than CT in the 0–2.5-cm depth. Differences in summed SOC pool for the whole soil profile (0–1 m depth) between NT and CT were not significant at any site. The summed SOC pool with depth between NT and CT were only significant above the 5 cm depth at Hutchinson and 2.5 cm depth at Hays. At Hutchinson, NT stored 3.4 Mg ha −1 more SOC than CT above 5 cm depth. At Hays, NT stored 1.35 Mg ha −1 more SOC than CT above 2.5 cm depth. Moreover, NT management increased mean weight diameter of aggregates (MWDA) by 3 to 4 times for the 0–5-cm depth at Hutchinson and by 1.8 times for the 0–2.5-cm depth at Hays. It also reduced air-dry aggregate tensile strength (TS) for the 0–5-cm depth at Hutchinson and Hays and for the 0–2.5-cm depth at Tribune. The TS ( r = −0.73) and MWDA ( r = 0.81) near the soil surface were more strongly correlated with SOC concentration at Hutchinson than at Hays and Tribune attributed to differences in precipitation input. Results suggested NT impacts on increasing SOC pool and improving soil structural properties decreased with a decrease in precipitation input. Changes in soil properties were larger at Hutchinson (880 mm of precipitation) than at Hays and Tribune (≤580 mm). While NT management did not increase SOC pool over CT for the whole soil profile, the greater near-surface accumulation of SOC in NT than in CT was critical to the improvement in soil structural properties. Overall, differences in precipitation input among soils appeared to be the dominant factor influencing NT impacts on soil-profile distribution of SOC and soil structural properties in this region.

Improvement of a testing apparatus for dynamometry: procedures for penetrometry and influence of strain rate to quantify the tensile strength of soil aggregates

Soil penetration resistance (PR) and the tensile strength of aggregates (TS) are commonly used to characterize the physical and structural conditions of agricultural soils. This study aimed to assess the functionality of a dynamometry apparatus by linear speed and position control automation of its mobile base to measure PR and TS. The proposed equipment was used for PR measurement in undisturbed samples of a clayey "Nitossolo Vermelho eutroférrico" (Kandiudalfic Eutrudox) under rubber trees sampled in two positions (within and between rows). These samples were also used to measure the volumetric soil water content and bulk density, and determine the soil resistance to penetration curve (SRPC). The TS was measured in a sandy loam "Latossolo Vermelho distrófico" (LVd) - Typic Haplustox - and in a very clayey "Nitossolo Vermelho distroférrico" (NVdf) - Typic Paleudalf - under different uses: LVd under "annual crops" and "native forest", NVdf under "annual crops" and "eucalyptus plantation" (&gt; 30 years old). To measure TS, different strain rates were applied using two dynamometry testing devices: a reference machine (0.03 mm s-1), which has been widely used in other studies, and the proposed equipment (1.55 mm s-1). The determination coefficient values of the SRPC were high (R² &gt; 0.9), regardless of the sampling position. Mean TS values in LVd and NVdf obtained with the proposed equipment did not differ (p &gt; 0.05) from those of the reference testing apparatus, regardless of land use and soil type. Results indicate that PR and TS can be measured faster and accurately by the proposed procedure.

Desenvolvimento de um consolidômetro pneumático: modelagem da compactação, penetrometria e resistência tênsil de agregados de solo

A compactação é um dos processos responsáveis pela degradação física do solo que pode ocasionar a perda da sustentabilidade da produção agrícola. Assim, é fundamental dispor de estratégias instrumentais para quantificar as propriedades físicas que são influenciadas pela compactação e utilizadas para avaliar a qualidade do solo. Os objetivos deste trabalho foram: desenvolver um consolidômetro com propulsão pneumática de baixo custo; avaliar sua funcionalidade mediante estudo do comportamento compressivo de um Latossolo Vermelho distrófico de textura argilo-arenosa sob plantio direto; e avaliar o potencial de utilização da propulsão pneumática para determinações de resistência do solo à penetração (RP) e de resistência tênsil de agregados (RT). A avaliação do comportamento compressivo do solo foi realizada mediante curvas de compressão, utilizando amostras indeformadas, obtidas das posições de amostragem relativas à linha e à entrelinha da cultura de aveia-preta. Nessas amostras, foram realizados ensaios de compressão uniaxial em condição de teor de água correspondente ao potencial mátrico de -10 kPa, sendo determinados o índice de compressão (IC) e a pressão de preconsolidação (σp), bem como suas relações com outras propriedades físicas do solo. A RP foi determinada em amostras indeformadas de um Argissolo Vermelho-Amarelo distrófico de textura franco-arenosa sob citros. Para as determinações da RT foram utilizados agregados de dois solos: um Argissolo Acinzentado distrófico arênico coeso e um Argissolo Amarelo distrófico arênico. Os resultados mostram que a densidade do solo, antes do ensaio de compressão uniaxial, foi maior (p &lt; 0,05) para a entrelinha da cultura de aveia-preta. A curva de compressão foi sensível às alterações estruturais do solo entre as posições de amostragem; a σp e o IC indicaram, respectivamente, maior capacidade de suporte de carga e menor suscetibilidade à compactação (p &lt; 0,05) para a entrelinha da cultura de aveia-preta. A utilização de propulsão pneumática não influenciou os resultados da RP e da RT. Isso permite concluir que a curva de compressão do solo, a RP e a RT podem ser determinadas utilizando o equipamento desenvolvido neste estudo.

Controlled Traffic Impacts on Physical and Hydraulic Properties in an Intensively Cropped No‐Till Soil

Changes in soil properties with wheel traffic involving intensive cropping systems under no‐till (NT) are not well understood. We assessed the 8‐yr impacts of intensive cropping systems and wheel traffic on soil physical and hydraulic properties and their relationships under NT on a Ladysmith silty clay loam (a fine, smectitic, mesic Udertic Argiustoll) near Hesston, KS. Winter wheat (Triticum aestivum L.) (W), grain sorghum [Sorghum bicolor (L.) Moench] (S), double‐crop grain sorghum (*S), soybean [Glycine max (L.) Merr.] (B), and double‐crop soybean (*B) arranged in S‐S‐S, W*S‐S‐B, W‐S‐B, and W*B‐S‐B rotations were studied. In doubled crops, sorghum or soybean was planted immediately after wheat harvest. Cropping systems had less of an effect on soil properties than wheel traffic. Wheel traffic increased bulk density (ρb) from 1.16 ± 0.06 (mean ± SD) to 1.38 ± 0.03 Mg m−3, cone index (CI) from 1.78 ± 0.29 to 3.10 ± 0.15 MPa, shear strength (SHEAR) from 23 ± 2.2 to 61 ± 5.2 kPa, and aggregate tensile strength from 377 ± 80 to 955 ± 148 kPa over nontrafficked rows in the 0‐ to 7.5‐cm depth. Wheel compaction reduced cumulative infiltration by 40 to 120 times except in S‐S‐S. It also reduced the logarithm of the saturated hydraulic conductivity (log Ksat), soil water retention at 0 kPa, plant‐available water, effective porosity (ϕe), and the volume of &gt;50‐μm pores. An increase in ρb, CI, and SHEAR linearly reduced ϕe (r &gt; −0.74), which, in turn, reduced cumulative infiltration and log Ksat (r &gt; 0.74). Data strongly support the need for using controlled traffic to reduce the adverse impacts of wheel traffic on soil physical quality.