Precompression Stress Values Research Articles

Structure dependent changes in pore water pressure due to stress application and consequences on the effective stress

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.

Scale-dependent soil strengthening processes – What do we need to know and where to head for a sustainable environment?

Due to the need of securing food production for a growing world population, modern agriculture enhances the risk of land degradation. Thus, knowledge of biological, chemical and physical soil properties and processes under in situ conditions at all scales are needed to give a precise quantification of relevant soil functions. Scenarios for political discussions must be included. Parameters and their boundary conditions need to be defined. These parameters include the interparticle forces on the micro-scale, which are determined by the zeta potential and rheological properties. They are affected by matric potential and organic carbon. In addition, fertilizer application (amount and composition) and micro (aggregate) stability, which includes the processes of repulsion and dispersion, affect these forces. The latter are strongly related to the ionic composition and concentration. Strengthening processes can be also documented on the meso- and macroscale. On all scales, we need to regard tensorial properties and functions, as they also serve as a feedback mechanism for anthropogenic effects including changes in fluxes, accessibility of particle or pore surfaces for filter and buffer processes, or plant growth. Land use affects the physical soil functions, which may result in yield decline, enhanced gas emission, enhanced soil erosion, or more frequent flooding, if the rigidity of the pore system is not maintained. Soil degradation due to inadequate management mainly occurs when the internal soil strength (i.e., the precompression stress range, as its transitions from the recompression to the virgin compression range) is exceeded. The degradation can occur with any stress whether it is static or dynamic or mechanical or hydraulic. In the field, it can be verified by visible deformation (as a result of too much traffic) and, by means of critical physical values, the consequences on soil functions can be estimated using Compaction Verification Tools (CVT) that document harmful (subsoil) compaction.Thus, to avoid irreversible soil degradation because of subsoil compaction, precompression stress values (or ranges), in combination with hydraulic conductivity and air capacity values, are suggested to be the basis for values used in the German soil-protection law.

The need for lime in dependence on clay content in arable crop production in Germany

Liming is a well-established practice to ameliorate acidic soils worldwide. However, the role of liming in soil structure formation and its relation to the nitrogen cycling is only little explored. We hypothesized that improved physical and mechanical soil properties, which are directly linked to soil structure, lead to lower nitrogen losses in an agricultural system. Water holding capacity, total pore volume, bulk density, hydraulic conductivity, shear stress and pre-compression stress were quantified in the topsoil of three surveys located in Germany on different soil types: Eutric Gleyic Cambisol (Loamic) (Magdeburg, Saxony-Anhalt), Haplic Luvisol (Neubrandenburg, Mecklenburg-Western Pomerania) and Eutric Cambisol (Loamic) (Puch, Bavaria). The field experiments were arranged in a Randomized Complete Block Design and treated with two different rates of lime (calcium carbonate). Furthermore, three rates of nitrogen in form of urea or calcium ammonium nitrate were applied every year. Liming significantly increased the pH, plant available water capacity, total pore volume and decreased bulk density in the soils six months after application, but twelve months after lime application the soil structure collapsed again causes by ploughing and chiseling. We, therefore, assume a positive influence of lime application on soil physical parameters, if tillage is excluded at least initially. On the other hand, our results from the shear stress parameters and the pre-compression stress values indicate that their changes need time and we assume that it may take several years to develop a sustainable structure system.

Computed tomography and soil physical measurements of compaction behaviour under strip tillage, mulch tillage and no tillage

In recent years, there has been an increased application of conservation-oriented tillage techniques, where instead of being turned the soil is only loosened or not tilled at all. Strip tillage, a special form of conservation tillage, results in small-scale structural differences, since tillage is performed only within the seed row, while the soil between seed rows is not tilled. However, tillage always impacts upon physical soil properties and processes.A combined application of conventional soil mechanical methods and X-ray computed tomography (X-ray CT) is employed here in order to investigate small-scale structural differences in a chernozem (texture 0–30 cm: silt loam) located in central Germany under strip tillage (within and between seed rows) compared to no tillage and mulch tillage. Apart from recording changes over time (years: 2012, 2014, 2015) to dry bulk density and saturated conductivity at soil depths 2–8 and 12–18 cm, stress-strain tests were conducted to map mechanical behaviour for a load range of 5–550kPa at a soil depth of 12–18cm (year 2015). Mechanical precompression stress was determined from the stress-dry bulk density curves. In addition, computed tomography scans were created followed by quantitative image analysis of the morphometric parameters mean macropore diameter, macroporosity, connectivity and anisotropy of the same soil samples.For strip tillage between seed rows and no tillage, a significant increase in dry bulk density was observed over time compared to strip tillage within the seed row and mulch tillage. This was more pronounced at a soil depth of 2–8cm than at 12–18cm. Despite higher dry bulk density, strip tillage between the seed row displayed also an increasing saturated conductivity compared to strip tillage within the seed row and mulch tillage. The computed tomography scans showed that the macropores became more compressed and soil aggregates were pushed together as mechanical stress increased, with the aggregate arrangement being transformed down into a coherent soil mass. The soil mechanical and morphometric parameters supported each other in terms of what they revealed about the mechanical properties of the soil structures. For instance, in the strip tillage between seed rows and no tillage treatments, the lack of soil tillage not only resulted in higher dry bulk densities, but also higher aggregate densities, mechanical precompression stress values, mean macropore diameters as well as lower macroporosity and connectivity values compared to mulch tillage and strip tillage within the seed row. The computed tomography parameters are therefore highly suitable for providing Supplementary information about the compaction process. Overall, this study showed that strip tillage combines the advantages of no tillage and a deeper, soil conservation-oriented primary tillage because, on a small scale, it creates two distinct soil structures which are beneficial in terms of optimal plant growth as well as mechanical resistance by driving over the soil.

Uniaxial compression behaviour and soil physical quality of topsoils under conventional and conservation tillage

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.

Hydro-Physical Properties of a Typic Hapludult under the Effect of Rice Husk Ash

The combustion of rice husk generates a partially burnt mixture called rice husk ash (RHA) that can be used as a source of nutrients to crops and as a conditioner of soil physical properties. The objective of this study was to evaluate the effect of RHA levels on the hydro-physical properties of a Typic Hapludult. The experimental design was composed of random blocks with four replications, which comprised plots of 24 m2 and treatments with increasing RHA rates: 0, 40, 80 and 120 Mg ha-1. Undisturbed soil samples were collected in the soil layers of 0.00-0.10 and 0.10-0.20 m after nine months of RHA application, using steel cylinders (0.03 m of height and 0.047 m of diameter). These samples were used to determine soil bulk density (Bd), total soil porosity (TP), soil macroporosity (Ma), soil microporosity (Mi) and the available water capacity (AWC). Disturbed soil samples were collected to determine the stability of soil aggregates in water, mean weight diameter of water stable aggregates (MWD), and soil particle size distribution. The results show that, as the RHA rate increased in the soil, Bd values decreased and TP, Ma and MWD values increased. No effect of RHA was found on Mi and AWC values. The effects of RHA on the S parameter (Dexter, 2004), precompression stress and compression index (Dias Junior and Pierce, 1995) values are consistent those shown for density and total porosity. Rice husk ash was shown to be an efficient residue to improve soil physical properties, mainly at rates between 40 and 80 Mg ha-1. Rice husk ash reduces bulk density and increases total porosity, macroporosity and soil aggregation, but does not affect microporosity, field capacity, permanent wilting point, and available water capacity of the soil. The effect of rice husk ash on the S parameter, precompression stress and index compressibility coefficient values are consistent with those observed for the bulk density and total porosity.

Quantifying the effect of soil physical properties on the compressive characteristics of two arable soils using uniaxial compression tests

The compaction of cultivated soils by agricultural machines considerably affects both the structure and physical properties of soil, and thus can have major impacts on crop production and the environment. In this study, we have used experimentally determined soil compression curves to constrain two important soil compression properties for black and brown soil from northeast China: the compression index and pre-compression stress condition. We have investigated the impact of variable initial water contents (0.14, 0.16, 0.18, 0.20 and 0.22g/g) and initial bulk densities (1.20, 1.30, 1.45 and 1.60Mg/m3) on these compressive properties by performing uniaxial compression tests on both types of remolded soils. It was determined that the pre-compression stress values were higher for black soil than for brown soil, whereas the opposite was true for the compression indexes, suggesting that black soil is less vulnerable to soil compaction than brown soil. The effect of initial water content, bulk density and the mutual interaction between them significantly influenced the compression properties of both soils (P<0.01). The pre-compression stress condition of both arable soils showed a positive correlation with bulk density and a negative correlation with water content. By contrast, the compression index for both soils showed a prominent negative correlation with bulk density. The compression index of brown soil showed an insignificant dependence on water content, but showed a positive correlation for black soil. Consequently, cultivation operations should be avoided under low bulk densities and high water contents on the premise that the crops could grow well in order to reduce the risk of soil compaction.

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.

Compressive response of some agricultural soils influenced by the mineralogy and moisture

Abstract 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.

Impact of lime and mineral fertilizers on mechanical strength for various soil pHs

Application of lime and fertilizers might impact soil properties by affecting their physico-chemical parameters. The focus of the Versailles 42-plots experiment that was started in 1929 has always been to determine in fallow conditions the impact of mineral fertilization over time on the physico-chemical properties of a loamy soil. This paper's aim is to assess the impact of long-term mineral fertilization on mechanical strength and soil stability. Soil samples of similar texture and bulk density were taken from the 42-plots site, with pH ranging from 3.5 to 8.2. Aggregate stability followed the procedure of Le Bissonnais (1996). Compression tests were performed in drained conditions and short loading time (300 s). Remolded and saturated soil samples were used to focus on the impact of chemical changes due to repeated fertilization. The pre-compression stress values P c were derived from compression curves using various model fits according the shape of curve. Chemical discrepancies among soils provided from pH, organic matter and diversity of treatments. The decrease of pH reduced the cationic exchange capacity (CEC) value due to the protonation of variable charges and retention of positively charged hydroxyl-Al polymers. Exchangeable aluminum represented up to 90% of the CEC in acid soils as calcium ions represented from 80 to 98% of CEC with increasing pH of neutral and alkaline soils. Compression curves showed different global shapes according pH values and the contrasted behavior of acid soils when compared to other ones. The P c values of highly acid soils were much greater than other treatments (greater than 150 kPa compared with 30 kPa). Nevertheless their P c values decreased down to 40 kPa, when soils were air-dried before compression. Meanwhile the deformation rate decreased when the pH dropped. This lower deformation cannot be explained by kinetic aspects and was not sensitive to drying. So the hypothesis of a set of yield stress avoiding drainage of the water phase and limiting soil compression was proposed. Aggregate stability tests revealed the unstable nature of all plots especially with the highly acid soils. The size distributions of fragment were very similar for all three tests with a narrow distribution in elementary particle class suggesting the predominance of dispersion in fragmentation process. But we found opposite results for this soil by comparing mechanical strength and water stability of aggregates. This contrast raised the question of the crucial role of the air-drying step on aggregate stability measurements. A small increase of aggregate stability was observed in neutral and alkaline soils due to calcium bridges when compared to reference plot. Such effect was not observed in compression behavior of remolded soils. Future studies will require the use of structured soil samples; in particular for neutral and alkaline soils to gather evidence of the effects of calcium bridges on soil stability and mechanical strength. Our results showed the need to consider interfacial processes in the soil stability and compression behavior and the great interest of the 42-plots experiments at INRA Versailles to relate chemical and physical soil properties.

Grazing Effects on Compressibility of Kastanozems in Inner Mongolian Steppe Ecosystem

In Inner Mongolia, animal trampling is one of the main factors causing soil degradation manifested by altered mechanical strength or changes in water and gas fluxes. Soil samples were collected at two depths (4–8 and 18–22 cm) on the Stipa grandis steppe ecosystem in Inner Mongolia from two treatments characterized by different grazing intensities: ungrazed since 1979 (UG) and continuously grazed (CG). The following mechanical soil properties were determined under static and repeated loading conditions: precompression stress, Pc; coefficient of cyclic compressibility, cn, and compression index, Cc Air conductivity measurements were used to quantify the changes in soil functions due to application of repeated loading. The CG site showed significantly higher precompression stress values (111 kPa) than the UG site (64 kPa) at the first soil depth. The highest cn values were found in the topsoil of the UG site, while the CG site had significantly lower cn values. Repeated loading caused higher soil deformation compared to the static loading test. It was also found that the strain of soil samples from the UG site was higher than the CG site. We found a good fit between cn and precompression stress. The Cc values of the cyclically loaded samples were significantly lower at the CG site than the statically loaded ones. The air conductivity of the UG site remained constant for a wider stress range of repeatedly applied stress compared with the CG site, which reflects higher stability of the soil pore network at the UG site.

Variance of mechanical precompression stress in graphic estimations using the Casagrande method and derived mathematical models

Mechanical precompression stress is a yardstick for the strength and compressibility of soils. The default method for the estimation of precompression stress is the graphic method according to Casagrande. It involves a subjective perception by the engineer who not only determines the point of the highest curvature visually, but decides also which points are to be used for generating the virgin compression line. In order to avoid such subjective approaches, mathematical models for the determination of precompression stress have been developed emanating from the Casagrande method. These models estimate the smallest radius of the curvature based on the minimum of the second numerical derivative. The paper has the aim to quantify the variance of subjectivity implied by the person executing the graphic method, the variance of different model approaches and the accuracy of the latter in handling the graphic values. Additionally we wanted to investigate the effect of different parameters on the ordinate of the diagram and the effect of the first load step on the precompression stress. To understand these relationships, stress/bulk density functions and stress/void ratio functions measured on 13 sites were analysed by five experienced but independent engineers and by use of three mathematical models. The mean errors of precompression stress estimations by the different testers were 0.01–0.12 and by the models 0.10–0.87 on a logarithmic scale. Expressed in kPa, increasing mean errors were observed with rising precompression stress, due to delogarithmization. For the graphical determination, they reached approx. 10–20 kPa at precompression stress levels of 60–150 kPa in typical subsoils; this means 15% on average. The handling of graphically obtained values by help of mathematical models disclosed considerable deviations between them. In the logarithmic variant, the mean absolute errors varied from 0.09 (9 kPa) to 0.40 (30 kPa) and the determination coefficients from 0.71 to 0.96. Another influence on the level of precompression stress has been observed when different variables were plotted on the ordinate of the graph. The graphically obtained values of precompression stress and those shown in the dry bulk density graph exceed the values calculated on the basis of the void ratio by the factor 1.2–1.5. Furthermore, it can be stated that in soil-compression tests with an initial load of 25 kPa higher precompression stress values were obtained than with lower initial loads (5 kPa), if the precompression values were low.

Soil testing of dynamic deformation processes of arable soils

Soil compaction is an ongoing issue of concern in agricultural land use. Measuring and modeling stress–strain relationships for various loading conditions and field traffic intensities is therefore important for assessing and predicting soil deformation and estimating related changes in soil functions such as aeration and hydraulic conductivity. A well-known and widely used parameter that quantifies soil stability is the pre-compression stress ( P c). Pre-compression stress values are determined from compression curves measured in confined compression tests under static loading conditions. It reflects the stress history of the soil and separates the re-compression stress range (below the pre-compression stress), where soil deformation is assumed to be fully elastic, from the virgin compression stress range (above the pre-compression stress), where soil deformation is considered plastic. Particularly, in agricultural land use, however, soil deformation is caused by dynamic rather than static loading processes where in situ stress paths diverge from the conditions applied during standard static soil testing. While the pre-compression stress is a useful indicator for the soil's bearing capacity it neglects plastic soil responses in the re-compression stress range which may become important, especially concerning subsoil compaction, when long-term intensive field traffic inducing multiple reloading events is considered. On the other hand if volume deformation is estimated from compressibility indices derived under static loading conditions it may potentially overestimate the true compression since loading times are much shorter during real wheeling events than during consolidation tests. We developed a new programmable pneumatic multistep oedometer (PMO) in order to quantify soil deformation under dynamic loading conditions. The oedometer allows defining virtually any stress path and hence provides a flexible method for dynamic soil testing. To demonstrate the potential applications of dynamic soil testing accounting for typical loading conditions on arable soils we conducted confined compression tests under various stress paths. Examples from cyclic loading tests (100 loading cycles) and the simulation of stress paths measured in situ with stress-state transducers (SST) during a wheeling experiment are shown. Results suggest that dynamic soil loading leads to cumulative soil deformation with significant effects on soil functions shown by a reduction of air conductivity coefficients. Soil compaction studies concerning agricultural field traffic should account for the time-dependency and non-static nature of load application to obtain more realistic predictions of possible changes in soil functions.

Effect of Long-term Wastewater Application on Physical Soil Properties

To evaluate the long-term effect of wastewater application on soil physical properties, two treatment sites, close to Taupo and Levin, New Zealand, and non-irrigated control sites were compared. The soil at Taupo was a silt loam and has received wastewater application for 12 years. The soil at Levin is a sand, and has been wastewater irrigated for 22 years. The disposal blocks at both sites had a higher pH, a higher organic matter (OM) content, a lower bulk density, and thus higher total porosity, but a lower macroporosity than the control sites. The disposal block at the Levin site showed a higher hydrophobicity than the control block, which coincided with the higher soil carbon. In contrast, the Taupo soil showed a higher hydrophobicity at the control site, the site with the lower OM content. Long-term wastewater irrigation resulted in a higher aggregate stability, and changes of the total porosity following stress application were lower, suggesting higher internal soil strength. The hydraulic conductivity close to saturation was also higher in the disposal blocks. The soil mechanical strength, as determined by the precompression stress value was, however, slightly lower in the disposal blocks and not correlated with the aggregate stability.

Determination of pre-compression stress of a variously grazed steppe soil under static and cyclic loading

In many land use systems all over the world soil deformation is a major problem due to increasing land use intensity. On arable soils machine traffic is continuously intensified with respect to load and wheeling frequency leading to (sub-)soil compaction and deeper soil degradation concerning hydraulic or pneumatic functions. Altered soil functions, in particular reduced hydraulic conductivities and impeded aeration, may decrease crop growth and productivity as well as the filtering and buffering capacity of soils. Prevented gas exchange and longer lasting anoxia in soils due to the reduced pore continuity and pore functioning also affects global change processes. In order to evaluate potential risks for irreversible soil deformation, it is necessary to quantify their mechanical stability. A commonly applied method is the determination of the pre-compression stress, commonly under static loading conditions in oedometer tests. The determination of pre-compression stresses under static loading may not quite resemble the conditions encountered in the field where soils are loaded repeatedly with a sequence of short intermittent loading–unloading–reloading events. Such dynamic loading conditions are encountered, e.g. at multiple wheel passes or in grassland soils due to animal trampling. In this study we present a comparison of a standard (static loading) and a modified (cyclic/dynamic loading) oedometer test using data of a Calcic Chernozem from the Inner Mongolian steppe under various grazing intensities. Static loading lasted for 10 min per loading step, while the dynamic/cyclic loading was carried out by 30 s loading and following 30 s unloading (=1 cycle) for in total 20 cycles. Differences between statically and cyclically determined pre-compression stresses at an identical time of loading show lower values for the statically determined pre-compression stress values compared to those determined cyclically. Among the dynamically determined pre-compression stresses, the values decrease with increasing number of loading steps and loading time, respectively. This is particularly true for the ungrazed sites. Thus, it could also be proofed that increased grazing intensities lead to structure deformation and increased sensitivity to wind- and water erosion followed by severe land degradation of grassland soils, particularly in semi-arid areas. Furthermore, hydraulic effects, e.g. positive pore water pressure due to intense shearing and kneading processes induced by grazing animals can enhance this structural deterioration. Thus, dynamic or cyclic loading results in an intense soil deformation which also causes serious changes in ecological and soil physical properties like hydraulic conductivity or gas flux.

Determination of precompression stress from uniaxial compression tests

Soil compaction has been studied for many years due to its implications for crop yield. In addition, precompression stress is important tool in evaluating soil load capacity and its implications for soil structure. This study evaluated whether precompression stress fitted by different methods gave rise to different results when a wide range of soils was tested. Soil cores were collected from different depths at 13 sites in Sweden, Denmark and Brazil and subjected to uniaxial compression tests. Soil precompression stress was determined using five different fitting methods: (1) Casagrande by polynomial fit; (2) Casagrande by van Genuchten equation fit; (3) intercept of the virgin compression line (VCL, straight portion of stress–strain curve) and regression with the first three points of the curve; (4) intercept of VCL and regression with the first two points of the curve; (5) intersection of VCL with the x-axis at zero strain. Method 1 produced the highest precompression stress values, while method 5 differed from other methods for all sites. Methods 1 and 3 did not significantly differ for most sites. Mean precompression stress was 50–130 kPa for the different methods. Polynomial and sigmoidal equations showed a good fit for the compaction curve in our dataset ( R 2 = 0.99), but Casagrande method parameters, e.g. point of maximum curvature, its tangent and slope of VCL, were different, resulting in different precompression stress values. Choice of method proved to have a significant influence on precompression stress values. Although methods 1 and 2 are both Casagrande-based, the results frequently differed. For the stress range tested (up to 1000 kPa), the van Genuchten equation was less suitable than polynomial fitting, since the compaction curve was usually not sigmoidal and the inflection point was sometimes outside observed values. Thus, the method used to fit precompression stress must be carefully chosen to avoid overestimates and underestimates. This restricts use of the absolute precompression stress value as a limit for soil stress in order to avoid soil compaction.

Impact of modern forest vehicles on soil physical properties

Ecological as well as economical consequences of mechanized harvesting procedures are of great importance in forestry, not only because of an intense increase in machine mass during tree cutting and transportation, but also because of a drastic increase in the stress application due to more pronounced vibration energy created from the harvesting machines themselves. Working procedures of typical forest harvesting vehicles were analysed in southern Black Forest area, 30 km east of Freiburg/Breisgau, Germany. The field site location was situated on top of a hill at an altitude of 1000 m above sea level (a.s.l.). The parent material was acid gneiss. The soils are classified as Cambisols, Spodo-Dystric Cambisols (according to WRB, 1998). The actual soil stresses and displacements in soil profiles were analysed to give an overview on the range of possible effects of harvesting/forwarding systems with respect to the changes in soil physical properties of the forest soils. Soil stresses were determined by stress state transducer systems (SST) and displacement transducer system (DTS) at two depths: 20 cm and 40 cm. Complete harvesting and trunk logging processes with a total duration of 9 min were observed with a resolution of 20 readings per second. The maximum vertical stresses for all experiments exceeded always 200 kPa and reached at the 20 cm depth for some vehicles and sequences of harvesting operations 500 kPa or more. Furthermore, it could be detected that the loading procedure itself doubled the short term mean normal stress as well as the octahedral shear forces and resulted in a more pronounced shear deformation and a corresponding homogenisation of the soil material. To evaluate the impact of soil stresses on soil structure, the internal soil strength was determined by the precompression stress method. The precompression stress values of forest soils at the field sites ranged from 20 to 50 kPa at the 20 cm depth up to 25–60 kPa at the 40 cm depth at a pore water pressure of −60 hPa. Generally, the data obtained for the measured soil stresses and the natural bearing capacity prove that sustainable wheeling is impossible, irrespective of the vehicle type and the working process. Top and subsoil compaction, an increase in precompression stress values in the various soil horizons, deep rut depth and vertical and horizontal soil displacement associated with shearing effects take place and affect the mechanical strength of forest soils. In order to sustain the present “unwheeled” situation, no increase in mass but a decrease of the machine mass is required to prevent or at least to keep the compacted forest soils small. This can be achieved amongst others by a permanent skid trail system.

Soil compressibility and penetrability of an Oxisol from southern Brazil, as affected by long-term tillage systems

The precompression stress value defines the transition from the reloading curve to the virgin compression line in the stress–strain curve, which can be used to quantify the highest load or the most intense predrying previously applied to the soil. Thus, in soils with well-defined structured soil horizons, each layer can be characterized by such mechanical strength. Penetration resistance measurements, on the other hand, can be used to determine total soil strength profiles in the field. The effect of long-term tillage systems on physical and mechanical properties was determined in undisturbed and remolded samples collected at 5 and 15 cm depth, 6 months after applying no-till (NT), chisel plow (CP), and conventional tillage (CT) treatments, along with the application of mineral fertilizer and poultry litter. The compressibility tests were performed under confined conditions, with normal loads varying from 10 to 400 kPa after a defined predrying to −6 or −30 kPa. Penetration resistance was determined in the field, after seeding, in three positions: seeding row (SR), untrafficked interrow (UI), and recently trafficked interrow (TI). No-till system showed greater soil resistance to deformation than tilled treatments, as determined by the higher precompression stress and lower coefficient of compressibility. When original soil structure was destroyed (remolded samples), smaller differences were found. The application of extra organic matter (poultry litter) resulted in a reduction of precompression stress in undisturbed samples. Penetration resistance profiles showed greater differences among tillage treatments in the upper layer of the untrafficked interrow, where NT system showed the higher values. Smaller differences were found in the seeding row (with lower values) and in recently trafficked interrow (with higher values), showing that even traffic with a light tractor after soil tillage reduced drastically the effect of previous tillage by loosening up the soil. On the other hand, the tool used to cut the soil and to open the furrow for seeding, incorporated in the direct seeding machine, was sufficient to realleviate surface soil compaction.

Soil assessment of apple orchards under conventional and organic management

To determine the effect of wheel traffic and two different management practices on soil compaction and its consequences on physical and chemical soil properties, we measured penetration resistance, water infiltration, bulk density, macroporosity, chemical mobility, air permeability, and soil strength in a conventional orchard (integrated fruit-production program) with bare (sprayed with herbicides) rows and an organic apple orchard with grassed rows. Resistance measurements were taken both within the tree row and the wheel track, down to a depth of 0.35 to 0.40 m. The results indicate that compaction is greater in the wheel tracks under both management methods. Compaction in the wheel track was higher under organic than conventional management. Organic management resulted in a higher macroporosity in both the row and the wheel-track than conventional management. The ‘close-to-saturation’ infiltration rate was significantly greater within the row of the organic orchard (0.06 m/h) compared with the row of the conventional orchard (0.02 m/h), and compared with the wheel tracks (0.01 m/h). The precompression stress value in the top 100 mm, a measure of the soil strength, was low on all sites. The chemical mobilities were 57 and 50% in the organic orchard, and 86 and 93% in the conventional orchard, respectively, for wheel track and row. Apart from the compaction in the wheel track of the organic orchard, physical and chemical soil characteristics were in a better condition compared with the conventional orchard.

Tillage effects on soil strength and solute transport

To study the effect of different soil tillage practices and the consequences of soil deformation on the functioning of the pore system, we performed unsaturated leaching experiments (by applying a suction of −10 kPa) on undisturbed soil columns from the Ap-horizon of a luvisol derived from glacial till (agricultural site at Hohenschulen, North Germany). We compared two different tillage practices (conventionally tilled to 30 cm depth, and conservational chiselled to a depth of 8 cm-Horsch system) with respect to soil strength, pore connectivity and their effect on the fate of surface-applied fertilisers. The soil strength was measured by determining the precompression stress value (PCV). The conventionally tilled topsoil had a PCV of 21 kPa at a pore water potential of −6 kPa, while the conservation treatment resulted in a slightly higher PCV of 28 kPa, suggesting a slowly increasing soil strength induced by the formation of aggregates under reduced tillage practice. The leaching experiments were modelled using the convection dispersion equation (CDE) and a modified version of the mobile–immobile approach (MIM), which included three water fractions: mobile, immobile and totally immobile water. From the CDE mobile water fractions ( θ m) ranging from 47 to 67% were found, and θ m was slightly higher in the ploughed seedbed compared to the conservation-tilled one. This could be due to higher aggregation in the latter one. Dispersivities were relatively large, ranging from 44 to 360 mm, but no difference was found for the treatments. The MIM could simulate the drop in concentration when leaching was interrupted, but overall did not improve the simulation, despite the larger number of fitting parameters. Compacting the soil with loads of 70 kPa prior to the leaching experiment did not affect solute transport in the conservational tilled soil. In the conventional-tilled soil, however, the dispersivity decreased and the mobile water content increased compared to the non-compacted soil, suggesting that the former one is less prone to deformation by mechanical loads.