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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]

Abstract: Soil water content during tillage can have a large impact on soil properties and tillage outcome. Measurement of soil relief in relation to fixed elevation points provides a non-destructive method of monitoring loosening/compacting processes during the year. The main objective of this study was to determine the effect of soil water content during primary tillage on soil physical properties. The treatments included mouldboard and chisel ploughing of a clay soil on three occasions in the autumn, with gradually increasing water content (0.76, 0.91 and 1.01×plastic limit). Soil surface height was measured by laser within a 0.64m2 area from fixed steel plates after each tillage occasion, and before and after seedbed preparation in the following spring. The measurements of surface height were compared with measurements of other soil physical properties, such as bulk density, saturated hydraulic conductivity and seedbed properties. Tillage at the lowest water content (0.76×plastic limit) produced the greatest proportion of small aggregates, and generally the most favourable soil conditions for crop growth. Soil loosening, as measured by increase in soil height during primary tillage, was highest for mouldboard ploughing and for tillage at the lowest water content. Differences between tillage treatments decreased with time, but were still significant after sowing in the spring. Natural consolidation during winter was smaller than the compaction during seedbed preparation in the spring. No significant differences in bulk density were found between treatments, and thus soil surface height was a more sensitive parameter than bulk density determined by core sampling to detect differences between treatments. Late tillage under wet conditions caused a greater roughness of the soil surface and the seedbed base, which was also found in the traditional seedbed investigation. The effect of tillage time on seedbed properties also resulted in a lower number of emerged plants in later tillage treatments. The laser measurements were effective for studying changes in soil structure over time. The results emphasize the need to determine changes in soil physical properties for different tillage systems over time in order to model soil processes. [Copyright &y& Elsevier]

Abstract: The objective of the present study was to measure the specific draught (force per cross-sectional area of worked soil) and energy use for soil fragmentation for different tillage implements and soil conditions. Draught was calculated from measurements of fuel consumption and speed during tillage with a mouldboard plough and a chisel plough set to working depths of 13, 17 and 21cm, and a disc harrow. Tillage was carried out at three different water contents (“Wet”, “Moist” and “Dry”) on two sites. The average working depth was calculated from weighing the loose soil within a 0.25-m2 frame. Specific area of the soil was determined by sieving. Soil strength was measured in situ using a shear vane and a penetrometer. Average working depth was much less than the set working depth for the chisel plough. Specific draught was generally the lowest for the mouldboard plough and the highest for the chisel plough, and increased with decreasing soil water content. The specific draught was strongly correlated to soil cohesion, but not to penetration resistance. The proportion of coarse aggregates after tillage was the highest for the mouldboard plough and the lowest for the moist soil. The energy use for soil fragmentation was in most cases the lowest for the disc harrow, while there were small differences between the chisel and the mouldboard ploughs. The results show that the mouldboard plough is energy efficient for loosening soil, while the disc harrow is energy efficient for soil fragmentation during primary tillage. Tillage at an intermediate water content, close to the plastic limit, gave the largest proportion of small aggregates and consequently the lowest energy use for soil fragmentation. [Copyright &y& Elsevier]

Due to its persistence, subsoil compaction should be avoided, which can be done by setting stress limits depending on the strength of the soil. Such limits must take into account soil moisture status at the time of traffic. The objective of the work presented here was to measure soil water changes during the growing period, use the data to calibrate a soil water model and simulate the soil susceptibility to compaction using meteorological data for a 25-year period. Measurements of soil water content were made in sugarbeet (Beta vulgaris L.) from sowing until harvest in 1997 on two sites classified as Eutric Cambisols in southern Sweden. Sampling was carried out at 2-week intervals in 0.1 m layers down to 1 m depth, together with measurements of root growth and crop development. Precompression stress of the soil at 0.3, 0.5 and 0.7 m depth was determined from uniaxial compression tests at water tensions of 6, 30, 60 and 150 kPa and adjusted as a logarithmic function of the soil water tension. Soil water content was simulated by the SOIL model for the years 1963–1988. Risk calculations were made for a wheel load of 8 t and a ground pressure of 220 kPa, corresponding to a fully loaded six-row sugarbeet harvester. Subsoil compaction was expected to occur when the major principal stress was higher than the precompression stress. The subsoil water content was very low in late summer, but increased during the autumn. At the end of August, there was practically no plant available water down to 1 m depth. There was in general good agreement between measured and simulated values of soil water content for the subsoil, but not for the topsoil. In the 25-year simulations, the compaction risk at 50 cm depth was estimated to increase from around 25% to nearly 100% between September and late November, which is the period when the sugarbeet are harvested. The types of simulation presented here may be a very useful tool for practical agriculture as well as for society, in giving recommendations as to how subsoil compaction should be avoided. [Copyright &y& Elsevier]

Abstract: Soil disturbance by tillage and traffic often adversely affects aggregate stability, leading to colloid and particle mobilisation, which may result in crusting, low water infiltration rate and sediment transport. Hence, minimising soil tillage is an important step in conserving soil quality. This study investigated readily dispersible clay (RDC) and particle transport as affected by shallow tillage or mouldboard ploughing in five Swedish long-term reduced tillage experiments that had been underway for 15–31 years at the time of soil sampling. The soils of the experimental sites are Eutric and Dystric Cambisols. RDC of soil samples, collected in 0–10cm and 12–17cm depth, from mouldboard ploughed (MP) and shallow tillage (ST) plots was compared by measuring turbidity. Particle transport was also determined in undisturbed soil columns (20cm in diameter and 20cm high) from these treatments. RDC and particle transport were significantly lower for ST than for MP at three sites where clay content was above 30%. Particle transport in two soils (clay content above 40%) increased with irrigation events, indicating that preferential transport dominated in unsaturated soil columns, but matrix flow and, consequently, particle dispersion increased as the soil approached saturation. Despite more organic C accumulation in the upper 0–10cm of ST than in the 12–17cm soil layer, clay dispersion was lower in the latter. Particulate P (Total P−dissolved P) was well-correlated to turbidity, suggesting that analysis costs often can be cut by only measuring turbidity. Shallow tillage generally produced positive environmental effects without negative effects on crop yield, especially on soils with high clay content. [Copyright &y& Elsevier]

Abstract: Soil compaction is one of the most important factors responsible for soil physical degradation. Soil compaction models are important tools for controlling traffic-induced soil compaction in agriculture. A two-dimensional model for calculation of soil stresses and soil compaction due to agricultural field traffic is presented. It is written as a spreadsheet that is easy to use and therefore intended for use not only by experts in soil mechanics, but also by e.g. agricultural advisers. The model allows for a realistic prediction of the contact area and the stress distribution in the contact area from readily available tyre parameters. It is possible to simulate the passage of several machines, including e.g. tractors with dual wheels and trailers with tandem wheels. The model is based on analytical equations for stress propagation in soil. The load is applied incrementally, thus keeping the strains small for each increment. Several stress–strain relationships describing the compressive behaviour of agricultural soils are incorporated. Mechanical properties of soil can be estimated by means of pedo-transfer functions. The model includes two options for calculation of vertical displacement and rut depth, either from volumetric strains only or from both volumetric and shear strains. We show in examples that the model provides satisfactory predictions of stress propagation and changes in bulk density. However, computation results of soil deformation strongly depend on soil mechanical properties that are labour-intensive to measure and difficult to estimate and thus not readily available. Therefore, prediction of deformation might not be easily handled in practice. The model presented is called SoilFlex, because it is a soil compaction model that is flexible in terms of the model inputs, the constitutive equations describing the stress–strain relationships and the model outputs. [Copyright &y& Elsevier]