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Soil bulk density was investigated as a function of soil contents of clay and organic matter in arable agricultural soils at a range of locations. The contents of clay and organic matter were used in an algorithmic procedure to calculate the amounts of clay-organic complex in the soils. Values of soil bulk density as a function of soil organic matter content were used to estimate the amount of pore space occupied by unit amount of complex. These estimations show that the effective density of the clay-organic matter complex is very low with a mean value of 0.17 ± 0.04 g ml-1 in arable soils. This value is much smaller than the soil bulk density and smaller than any of the other components of the soil considered separately (with the exception of the gas content). This low value suggests that the clay-soil complex has an extremely porous and open structure. When the complex is considered as a separate phase in soil, it can account for the observed reduction of bulk density with increasing content of organic matter. [ABSTRACT FROM AUTHOR]

A method for the experimental determination of the amount of clay dispersed from soil into water is described. The method was evaluated using soil samples from agricultural fields in 18 locations in Poland. Soil particle size distributions, contents of organic matter and exchangeable cations were measured by standard methods. Sub-samples were placed in distilled water and were subjected to four different energy inputs obtained by different numbers of inversions (end-over-end movements). The amounts of clay that dispersed into suspension were measured by light scattering (turbidimetry). An empirical equation was developed that provided an approximate fit to the experimental data for turbidity as a function of number of inversions. It is suggested that extrapolation of the fitted equation to zero inversions enables the amount of spontaneously-dispersed clay to be estimated. This method introduces the possibility of replacing the existing subjective, qualitative method of determining spontaneously-dispersed clay with a quantitative, objective method. Even though the dispersed clay is measured under saturated conditions, soil samples retain a ‘memory’ of the water contents at which they have been stored. [ABSTRACT FROM AUTHOR]

Abstract: Factors that affect water retention in soil are discussed in terms of thermodynamic and hydraulic equilibria. It is assumed that the Groenevelt and Grant (2004) water retention equation describes soil water that is at or close to thermodynamic equilibrium. It is shown that immiscible displacement, as occurs in a pressure cell apparatus in which air displaces water, can leave residual soil water that is not in thermodynamic equilibrium. We describe this residual water as being in hydraulic equilibrium. As hydraulic equilibrium is approached, the outflow of water becomes at first very slow and then essentially ceases. An empirical equation for water retention in bi-modal soils is used for soils that exhibit residual water content. It is found that residual water is that which remains after the connected (drainable) textural pore space has emptied by Darcian convective flow. The point at which Darcian flow ceases is called hydraulic cut-off. The water remaining after hydraulic cut-off is the residual water that moves much more slowly mainly through vapour-phase diffusion. The suction of residual pore water can be significantly smaller than the air pressure that has been applied in a pressure cell apparatus. Models for predicting the amount and pressure of residual water are presented and tested using experimental data for 14 soils from France and Poland. Some consequences for soil water retention studies and for the status of soil water in the field are discussed. [Copyright &y& Elsevier]

Two standard tests for soil stability in water involve assessment of the readily dispersible clay (RDC) content of predried soil samples. It is shown that this predrying limits the applicability of the tests to very dry soils. Ways to avoid this limitation are developed. Undisturbed soil samples were collected from the 10-to 15-cm depth of arabic topsoils at Villamblain and Faux Perche and from the 0- to 5-cm depth from plowed and direct-drilled plots at Boigneville. These soils in north-central France were collected from the field when moist (near field capacity). A range of water contents and corresponding pore water potentials (or stictions h) was produced by slow drying in air. For moist soils at field capacity (h = 100 hPa) the amount of RDC was large. Readily dispersible clay decreased progressively with increasing antecedent suction, h. For soil samples dried to beyond h > 10 MPa, the RDC was reduced to small values. With such intensely dried soils, rewetting for up to 2 wk did not reverse the effect on RDC. At h = 4500 hPa, re-equilibration to h = 100 hPa reversed the effect of this moderate drying on RDC. Two standard conditions (moist and dry) are proposed for measurement of clay dispersion from soil into water. The moist condition is relevant to soil at the end of along, wet period (e.g., at the end of the winter in temperate climates), whereas the dry condition is relevant to dry surface crusts and dry topsoils. We conjecture that the soil "resets" during long, wet periods to values of stability and RDC at equilibrium with field capacity. Experimental procedures used to determine the content of RDC in moist and dry soils are described. [ABSTRACT FROM AUTHOR]

Soil water content or potential can be adjusted in the laboratory by equilibrating the soil at different relative humidities. These relative humidities can be produced by the use of different saturated salt solutions or by heating the soil to different temperatures in a convection oven. Usually, the conditions for equilibrating the soil are expressed as relative humidities if salt solutions are used or as the temperature of drying if a convection oven is used. This dichotomy does not allow all the results to be compared or plotted on a common axis. In this paper, the Magnus-Tetens and the Arden Buck equations are used in combination with the Kelvin equation to show how the results from both types of equilibration can be presented on the same scale (e.g., water potential or pF). Two new equations show how the results from oven drying depend not only on the oven temperature, but also on the temperature and relative humidity of the air in the laboratory. For example, drying soil in an oven in one set of atmospheric conditions can produce a significantly different water potential than in different atmospheric conditions. The new equations enable the soil water potentials (or pF values) produced by oven drying to be calculated for all combinations of oven temperature and atmospheric conditions. Predictions from the equations are evaluated by comparing water retention data obtained by oven drying and by equilibration over saturated salt solutions. It is shown that the mean absolute error in gravimetric water content is 2.3 g kg-1. Equations are presented for the mean thickness of water films adsorbed on the surfaces of soil particles under different drying conditions. The amount of water adsorbed on particle surfaces is predicted to be zero at pF66. [ABSTRACT FROM AUTHOR]

The aim of this study was to quantify soil properties, microbial biodiversity and crop yield under two tillage systems used for winter wheat production in monoculture. The study was conducted in the period 2013-2016, on a long-term field experiment on a silt loam at the Krasne Research Station near Rzeszów, Poland. Traditional tillage involved soil inversion whereas reduced tillage was a non-inversion system. The following soil properties: chemical (soil organic carbon, pH, available P, K, Mg), physical (soil bulk density, water content, stability in water), and biological (the diversity of diatoms) were measured on samples collected throughout the growing season and at harvest. Soil organic carbon content, water content and bulk density in the 0-5 and 5-10 cm layers were greater in reduced tillage than in traditional tillage. Under reduced tillage the amount of readily dispersible clay was reduced giving increased soil stability in water. Soil under reduced tillage had greater diversity of diatoms (139 taxa) than that under traditional tillage (102 taxa). Wheat yields were positively correlated with precipitation, soil water content and soil organic carbon, and negatively correlated with readily dispersible clay. [ABSTRACT FROM AUTHOR]

The effects of different tillage systems on the properties and microbial diversity of an agricultural soil was investigated. In doing so, soil physical, chemical and biological properties were analysed in 2013-2015, on a long-term field experiment on a loamy sand at the IUNG-PIB Experimental Station in Grabów, Poland. Winter wheat was grown under two tillage treatments: conventional tillage using a mouldboard plough and traditional soil tillage equipment, and reduced tillage based on soil crushing-loosening equipment and a rigid-tine cultivator. Chopped wheat straw was used as a mulch on both treatments. Reduced tillage resulted in increased water content throughout the whole soil profile, in comparison with conventional tillage. Under reduced tillage, the content of readily dispersible clay was also reduced, and, therefore, soil stability was increased in the toplayers, compared with conventional tillage. In addition, the beneficial effects of reduced tillage were reflected in higher soil microbial activity as measured with dehydrogenases and hydrolysis of fluorescein diacetate, compared with conventional tillage. Moreover, the polimerase chain reaction - denaturing gradient gel electrophoresis analysis showed that soil under reduced tillage had greater diversity of microbial communities, compared with conventionally-tilled soil. Finally, reduced tillage increased organic matter content, stability in water and microbial diversity in the top layer of the soil. [ABSTRACT FROM AUTHOR]

Complexes formed between clay and soil organic matter are important for carbon sequestration and for soil physical quality. Here, we use samples of loessial soil from South-East Poland to explore the phenomenon of complexing in loess. Soil samples were collected from a single catchment 8 years after the introduction of strip tillage and their compositions were characterized by traditional methods. Complexing was characterized in terms of the content of non-complexed clay which was estimated in two ways: firstly, by measurement of the content of readily-dispersible clay (which was assumed to be the non-complexed clay); and secondly, by calculation using algorithms that had been developed and evaluated previously. The calculations were based on the concept that, at carbon saturation, the clay/organic carbon mass ratio is equal to n. The calculations were done with a range of values of n. It was assumed that the correct value of n was that which gave the greatest coefficient of correlation between the measured values of clay dispersion and the predicted values of non-complexed clay. For the loess used, the optimum value was n = 14. [ABSTRACT FROM AUTHOR]

The aim of this study was to compare the effects of different farming systems (organic, integrated, conventional and monoculture) on some soil properties as: bulk density, contents of readily-dispersible clay, organic matter and particulate organic matter, and enzymatic activity measured in terms of the intensity of fluorescein diacetate hydrolysis. Soil under permanent grass was used as a control. The study was conducted on the 20 years lasting field experiment. Samples of Haplic Luvisol soil were collected twice a year on fields under winter wheat from the layers of 0-5, 5-10, 15-20, and 30-35 cm. Within arable soils the soil under organic farming contained the greatest amount of organic matter, which influenced strongly the readily-dispersible clay content, especially in the layer of 5-20 cm. The readily-dispersible clay content in soil under organic farming was 3 times lower, as compared to the conventional and monoculture farming. The highest contents of particulate organic matter 6.2 and 3.5 mg g−1 air dry soil, on average were measured in the 0-5 cm layer of control soil and soil under organic farming, respectively. Also, soil under organic farming and control soil from the depth of 0-5 cm showed 2-2.5 times greater activity of microorganisms in fluorescein diacetate hydrolysis than soil under conventional and monoculture farming. Increase of concentration of organic matter in soil under organic farming decreased soil bulk density. Statistical analysis showed significant correlations between studied parameters of soil quality and confirmed their effectiveness as indicators of disturbances in soil environment. [ABSTRACT FROM AUTHOR]

Abstract: Tillage experiments were carried out in order to study the effect of water content on the aggregate size distribution produced by tillage, and to investigate the relationship between the soil structures produced by tillage and Dexter''s index of soil physical quality, S. Tillage with a mouldboard plough was done on four different soils over a range of naturally occurring water contents. The aggregate size distribution and the specific surface area produced by tillage were obtained by sieving. We define the optimum water content for tillage, θ OPT, as the water content at which the specific surface area of the aggregates produced is maximum. This is consistent with the water content at which the amount of small aggregates produced is greatest and the proportion of clods produced is smallest. For the four investigated soils, θ OPT was found to be close to the water content at the inflection point of the water retention curve, and in the vicinity of 0.8θ PL (where θ PL is the lower plastic limit). At water contents either lower or higher than θ OPT, the specific surface area produced was smaller. The specific surface area produced at θ OPT was found to be strongly correlated with the index of soil physical quality, S. The specific surface area produced is larger the greater S, i.e. the better the soil physical quality. Consistently, the proportion of small aggregates produced at θ OPT is larger and the proportion of clods produced at θ OPT smaller, the greater S. No clods (>50mm) are produced on soils with good physical quality. [Copyright &y& Elsevier]

Highlights • Amount of readily-dispersible clay increased with deformation level of loess soil. • Clay dispersibility was positively correlated with soil bulk density. • Amount of readily-dispersible clay was greater in subsoil than in plough layer. Abstract Clay dispersibility is a good indicator of the vulnerability of soil to disperse and therefore of the associated risks of soil water erosion and crusting and subsequent losses of agricultural chemicals in overland flow. The aim of this study was to determine the effect of soil deformation of an erodible soil derived from loess (Haplic Luvisol) on the content of dispersible clay. The soil deformation levels were: non-compacted (NC, 0 tractor pass), moderately compacted (MC, 3 passes), and strongly compacted (SC, 5 passes). The amount of readily-dispersible clay (RDC) was determined using a Hach 2100AN ratio turbidimeter, by measurement of the turbidity of a suspension of soil in water. All the measurements were done with soil from the plough (5–20 cm) and subsoil (25–35 cm) layers. The amount of RDC increased with increasing deformation level and depth. In the plough layer this increase was from 0.786 under NC to 1.216 g 100 g−1 under SC. Corresponding values in the subsoil layer were 1.227 and 1.874 g 100 g−1. Positive correlations were found between readily-dispersible clay and bulk density in both the plough (R2 =0.685) and the subsoil layers (R2 =0.332). The results for clay dispersion are discussed in relation to the different contents of organic carbon in the plough and subsoil layers and the potential mechanisms of soil vulnerability to soil erosion, crusting and deformation. [ABSTRACT FROM AUTHOR]

Abstract: 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 (300s). Remolded and saturated soil samples were used to focus on the impact of chemical changes due to repeated fertilization. The pre-compression stress values Pc 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 Pc values of highly acid soils were much greater than other treatments (greater than 150kPa compared with 30kPa). Nevertheless their Pc values decreased down to 40kPa, 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. [Copyright &y& Elsevier]

Abstract: Compaction of tilled layers under the single effect of rainfall or irrigation was called slumping. Slumping affects strongly root development and plant biomass production. It has been observed in different soil types, but sandy soils appear particularly prone to this physical degradation. Our objectives in this study were (i) to measure in the field the changes in soil structure and water status simultaneously, (ii) to study the effects of rainfall and management practices on slumping, and (iii) to propose a conceptual model for sandy soil slumping. An experimental site was selected in Northeast Thailand and we studied the effect of tillage depth and initial water content on slumping dynamic. Plots (9m×15m each) were tilled at (i) two depths (20 and 40cm, called S and D respectively) in dry conditions, (ii) at 20cm depth in dry or wet conditions (called Y and W respectively). These plots were submitted to natural rainfall for 20 or 61 days to get different total rainfall amounts (114 and 212mm respectively). In addition, smaller plots (0.24m2 each) were used for experimental flooding irrigation (similar to measured rainfalls, i.e. 100 and 200mm). Soil bulk density, soil surface elevation, soil water content and matric potential were measured. A decrease in soil elevation was observed in all treatments. In the absence of erosion it was interpreted as a loss of porosity which resulted from slumping. Bulk density increased in all layers of the tilled profile (from 1.38 to 1.57gcm−3). In the surface layer (0–5cm) this increase was systematically higher compared to deeper layer. No significant difference in final bulk density was found between the S and W treatments, and between the Y and W treatments. Bulk density increased more rapidly in the Y and W treatments than in the S and D treatments, even though the cumulative rainfall was lower. After the flooding experiments, bulk density was higher than after natural rainfall despite similar amounts of water added to the soil. [Copyright &y& Elsevier]

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

Abstract: No-tillage has been adopted worldwide as a strategy to reduce the negative effects of soil erosion in conventional tillage. However, no-tillage can lead to over-compaction of the soil, which can have detrimental effects on soil structure and on crop yield. The objective of this study was to evaluate the soil physical functionality of a Rhodic Ferrasol under long-term no-tillage. The following physical properties were measured: dry bulk density (Db), total porosity (Pt) and macroporosity as well as saturated hydraulic conductivity (K sat) and intrinsic air permeability (K a). Physical quality of the soil was also evaluated by the S index at each depth increment. The study was carried out in a Rhodic Ferrasol under a 14-year old, no-tillage system in Ponta Grossa, Paraná, Brazil. Undisturbed soil samples were collected at depths of 0–0.10, 0.20–0.30, 0.40–0.50, and 0.60–0.70m. The soil properties such as dry bulk density, total porosity and macroporosity showed the presence of a denser layer at a depth of 0.20–0.30m. The Db and Pt values in this layer were significantly different from the other layers. However, the values of 1.23Mgm−3 for Db, 0.52m3 m−3 for Pt, and 0.17m3 m−3 for macroporosity were all still adequate for the growth and development of roots at this layer. The values of K sat and K a were higher at the surface layer than deeper layers. The lowest values were around 80 and 70% in comparison with surface, respectively to the depths of 0.40–0.50, and 0.60–0.70m K sat and K a values were 7.79×10−5, 2.76×10−5, 1.58×10−5, and 1.91×10−5 ms−1, and K a values were 3.70×10−11, 2.04×10−11, 7.69×10−12, and 1.10×10−11 m2 for the depths of 0–0.10, 0.20–0.30, 0.40–0.50, and 0.60–0.70m, respectively. This demonstrated that better conditions for the exchange of gases and water existed in the layers above 0.30m. The S index indicated that the soil had good structure in all layers studied, with values above 0.035, but that a small increase in Db in the 0.20–0.30m layer could lead to deterioration of the soil structural quality. The long-term use of no-tillage resulted in increases of organic carbon and pore connectivity in the surface layer, providing functionality in the soil. [Copyright &y& Elsevier]