Technical solutions to reduce the risk of subsoil compaction: effects of dual wheels, tandem wheels and tyre inflation pressure on stress propagation in soil

The use of heavy machinery is increasing in agriculture, which induces increased risks of subsoil compaction. Hence, there is a need for technical solutions that reduce the compaction risk at high total machine loads. Three field experiments were performed in order to study the effects of dual wheels, tandem wheels and tyre inflation pressure on stress propagation in soil. Vertical soil stress was measured at three different depths by installing probes into the soil horizontally from a dug pit. In one experiment, also the stress distribution below the tyre was measured. Beneath the dual wheels, vertical stresses at 0.15 and 0.3 m depth were lower between the two wheels than under the centre of each wheel, despite the gap between the wheels being small (0.1 m). At 0.5 m depth, vertical stress beneath the wheels was the same as between the two wheels. The stress interaction from the two wheels was weak, even in the subsoil. Accordingly, measured stresses at 0.3, 0.5 and 0.7 m depth were highest under the centre of each axle centre line of tandem wheels, and much lower between the axles. For a wheel load of 86 kN, tyre inflation pressure significantly affected stress at 0.3 m depth, but not at greater depths. Stress directly below the tyre, measured at 0.1 m depth, was unevenly distributed, both in driving direction and perpendicular to driving direction, and maximum stress was considerably higher than tyre inflation pressure. Calculations of vertical stress based on Boussinesq's equation for elastic materials agreed well with measurements. A parabolic or linear contact stress distribution (stress declines from the centre to the edge of the contact area) was a better approximation of the contact stress than a uniform stress distribution. The results demonstrate that stress in the soil at different depths is a function of the stress on the surface and the contact area, which in turn are functions of wheel load, wheel arrangement, tyre inflation pressure, contact stress distribution and soil conditions. Soil stress and soil compaction are a function of neither axle load nor total vehicle load. This is of great importance for practical purposes. Reducing wheel load, e.g. by using dual or tandem wheels, also allows tyre inflation pressure to be reduced. This reduces the risk of subsoil compaction.     

A METHOD OF PREDICTING BULK DENSITY CHANGES IN FIELD SOILS RESULTING FROM COMPACTION BY AGRICULTURAL TRAFFIC

A simplified soil mechanical model was constructed to predict compaction beneath agricultural wheels when running on soils of certain characteristics. Soil strength functions were developed from in situ measurements of field soils and some laboratory measurements. Soil strain was measured by surface sinkage and changes of dry bulk density by gamma-ray transmission methods. Soil stresses were measured by deformable spherical transducers and compared to predicted stresses using equations developed by Söhne. A method of analysis was devised to identify a form of the virgin compression line from field data. Changes of the slope and intercept of this line were monitored over a range of moisture contents for two soils and used in the prediction model. The prediction model was tested against compaction measured during independent experiments at different sites. Good prediction was found for soils of initial dry bulk density greater than 1.1 g cm^?3 and cone resistance greater than 500 kPa, using a 30°, 12.9mm diameter cone. On looser and weaker soils the predicted compaction was often less than measured values. Using the model for simulation of compaction beneath a range of wheels revealed that contact pressure alone can be a misleading guide to compaction. Increases of bulk density below 10cm are considerably influenced by wheel load. The most effective way of reducing compaction requires the use of both a minimum load and a maximum contact area.     

Compaction of virgin soil by mechanized agriculture in a semiarid environment

Soil compaction was assessed in terms of soil strength as measured with a penetrometer. Penetrometer resistance was measured on virgin soil and on the same soil after one and after five passes of a 7,610 kg tractor. Also, comparative studies were made of strength profiles of soils in arable fields and in adjacent areas of virgin soil. The strength of virgin soil was increased by wheel traffic and agricultural operations in all cases. The increase in soil strength was significant down to 0.3 m, which is considerably greater than the normal depth of tillage in the area (0.05 m). Reduction in the coefficient of variation of penetrometer strengths after the passage of wheels was taken as evidence for associated losses of soil structure. Virgin soils provide important reference sites for assessing the impact of agriculture in an area.     

A new approach for modelling vertical stress distribution at the soil/tyre interface to predict the compaction of cultivated soils by using the PLAXIS code

Soil compaction by agricultural machines can have adverse effects on crop production and the environment. Different models based on the Finite Element Method have been proposed to calculate soil compaction intensity as a function of vehicle and soil properties. One problem when modelling soil compaction due to traffic is the estimation of vertical stress distribution at the soil surface, as the vertical stress is inhomogeneous (non-uniform) and depends on soil and tyre properties. However, uniform stress distribution at the soil/tyre interface is used to predict the compaction of cultivated soils in most FEM compaction models. We propose a new approach to numerically model vertical stress distribution perpendicular to the driving direction at the soil/tyre interface, employing the FEM models of PLAXIS code. The approach consists of a beam (characterised by its geometric dimensions and flexural rigidity) introduced at the soil surface and loaded with a uniform stress with the aim to simulate the action of a wheel at the soil surface. Different shapes of stress distribution are then obtained numerically at the soil surface by varying the flexural rigidity of the beam and the mechanical parameters of the soil. PLAXIS simulations show that the soil type (soil texture) modifies the shape of the stress distribution at the edges of the contact interface: a parabolic form is obtained for sand, whereas a U-shaped is obtained for clay. The flexural rigidity of the beam changes the shape of distribution which varies from a homogenous (uniform) to an inhomogeneous distribution (parabolic or U-shaped distribution). These results agree with the measurements of stress distributions for different soils in the literature. We compared simulations of bulk density using PLAXIS to measurement data from compaction tests on a loamy soil. The results show that simulations are improved when using a U-shaped vertical stress distribution which replaces a homogenous one. Therefore, the use of a beam (cylinder) with various flexural rigidities at the soil surface can be used to generate the appropriate distribution of vertical stress for soil compaction modelling during traffic.     

Anthropogenic effects on soil quality of ancient agricultural systems of the American Southwest

Soil studies of ancient agricultural fields contribute to research on long-term human–environmental relationships and land use sustainability. This kind of research is especially applicable in desert landscapes of the American Southwest because: (1) soil formation is slow enough that cultivation effects persist for centuries to millennia; (2) many ancient fields in valley margins have remained uncultivated since they were abandoned, so long-term soil properties reflect ancient agricultural use; and (3) agricultural features (e.g., terraces, rock alignments and rock piles, and irrigation canals) provide clues for identifying and sampling ancient cultivated and uncultivated soils. Surficial remnants of these field systems persist and remain intact in many cases. Soil studies of ancient and modern American Indian agricultural systems across the Southwest indicate that soil changes are highly variable, ranging from degradation (e.g., organic matter/nutrient decline, compaction), to minimal net change, to enhanced soil quality. Soil changes caused by cultivation can be inferred by comparing soils in agricultural fields relative to reference uncultivated areas in similar landscape settings (that is, space-for-time substitution). Soil response trajectories vary for a number of reasons, such as variability in initial ecosystem conditions, diversity in agricultural methods, variability in the mix of crops and cropping intensity, and environmental sensitivity to alteration (varying resistance and resilience). Studies of rock mulch soils indicate enhanced fertility, with elevated organic carbon, nitrogen, and available phosphorus levels, increased infiltration rates and moisture retention, and no evidence of compaction. By contrast, cultivation effects vary widely for terraced soils. Although numerous studies have focused on irrigation canals, irrigated soils have received far less attention. Soil studies of irrigation systems along the Gila and Santa Cruz rivers of Arizona now underway will help fill this research gap.     

Influence of soil water on stress–strain behaviour of a compacting soil in semi-arid Kenya

Depending on the top and subsoil textures, semi-arid soils exhibit cohesive and frictional properties that are associated with the relatively high soil strength, bulk density and penetration resistance. The objective of this study was to gain the knowledge of mechanical properties of the compacting chromic luvisols in order to improve the design of tillage tools. Therefore, we applied critical state soil mechanics to study the stress–strain behaviour of the luvisols using triaxial tests under laboratory conditions. Field investigations involved random collection of undisturbed soil samples which were subjected to triaxial testing first by isotropic consolidation and compression and then triaxial shearing. Plots of deviatoric stress against axial strain were made to determine the soil shear strengths at the critical states over different soil water levels and the two soil depths of 0–20 cm for the plough and 20–40 cm for the hard pan layers, respectively. An exponential model used to fit the deviatoric stress–axial strain test data accurately predicted the trends. Soil water significantly influenced the shear strength, cohesion (c′) and internal angle of friction (′) and hence the mechanical behaviour of the luvisols. The regression equations developed showed that c′ and ′ have quadratic relationships with soil water. The very high clay bonding strength in the subsoil (hard pan) layer resulted in high shear strength, bulk density and penetration resistance values for this soil layer. The increase in shear strength with decreasing water content affected the deviatoric stress–axial strain relationships between the upper and lower plastic limits of the sandy soil. Thus, as the soil dried, the soil ceased to behave in the plastic (ductile flow) manner and thus began to break apart and crumble. The crumbling was indicative of brittle failure. The transition stage from an increase to a decrease in c′ and ′ values with soil water occurred in the soil water content range of 6–10%. Knowledge of stress–strain behaviour of compacting soils is of practical significance in the design of appropriate tillage tools for the specific soil type.     

轮式和履带式车辆行走对农田土壤的压实作用分析

由履带式行走机构代替轮胎被认为是减缓大型农业车辆对土壤压实的有效手段之一。与轮胎相比,履带具有更大的接地面积,能够有效减小车辆对土壤的平均压力。然而履带与土壤接触面间的应力分布极不均匀,应力主要集中在各承重轮下方,履带减缓土壤压实的能力是目前有待研究的问题。该研究通过在土壤内埋设压力传感器,测试比较了相近载质量的轮胎和履带式车辆作用下,0.15和0.35 m深度土壤内的最大垂直及水平应力,同时研究了车辆行驶速度对土壤内垂直及水平应力大小的影响。基于土壤压实分析模型计算了轮胎和履带压实的0.1~0.7m深度土壤内的最大垂直及水平应力分布。通过对0.15和0.35 m深度的土样进行室内测试,比较了轮胎和履带式车辆压实对土壤透气率、先期固结压力及干容重大小的影响。结果表明,履带相比较于轮胎,能够减小土壤内的垂直及水平应力,但垂直应力的减小量比水平应力大;轮胎对0.15和0.35m深度土壤作用的平均最大垂直应力分别约为履带的2.2及2.0倍,而平均最大水平应力仅分别约为履带的1.2及1.1倍。轮胎作用下的最大垂直及水平应力在表层土壤内明显大于履带,但两者的应力差值随着土壤深度的增加逐渐减小,分别在0.7和0.4 m深度时无明显差别。轮胎和履带压实作用下,0.15和0.35 m深度土壤内的垂直及水平应力均随车辆行驶速度的增加而减小,履带作用下的应力减小速度大于轮胎。履带作用下0.15和0.35 m深度内土壤的透气率均明显小于轮胎,但土壤的先期固结压力及干容重无显著区别。研究结果为可为农业车辆行走机构的选择及使用提供参考。     

Evaluation of pedotransfer and measurement approaches to avoid soil compaction

On-farm approaches are needed to help farmers avoid soil compaction. It is the purpose of this paper to document the experience of using the Horn and Fleige [Horn, R., Fleige, H., 2003. A method for assessing the impact of load on mechanical stability and on physical properties of soils. Soil Till. Res. 73, 89–99] procedures to develop improved guidance to help farmers avoid compaction in agricultural operations in the Commonwealth of Pennsylvania, USA. A soil characterization database for the Commonwealth of Pennsylvania, USA, was used to provide input to the Horn and Fleige [Horn, R., Fleige, H., 2003. A method for assessing the impact of load on mechanical stability and on physical properties of soils. Soil Till. Res. 73, 89–99] approach to estimate the pre-consolidation stress and the maximum depth of compaction for 29 agricultural soils in Pennsylvania. The Horn and Fleige [Horn, R., Fleige, H., 2003. A method for assessing the impact of load on mechanical stability and on physical properties of soils. Soil Till. Res. 73, 89–99] approach was tentatively validated using previously measured pre-consolidation stress or penetration resistance values measured on five of the 29 soils. The estimated maximum depth of compaction indicated that an 89-kN (10-ton) axle load was excessive in almost all cases for soils at matric potentials of −33 and −6 kPa for both tillage and no-till management. A 53-kN (6-ton) axle load was acceptable for most cases when tillage was planned to a 0.20-m depth, but was excessive in most cases for no-till management at a matric potential of −6 kPa while mostly acceptable for no-till management at a matric potential of −33 kPa. Penetration resistance measurements are recommended to decide when a load is excessive.     

A simplified method for estimating soil compaction

We describe a simplified model that allows users to explore some of the main aspects of soil compaction. It is intended for use by non-experts, such as students, and is written as an easy-to-use spreadsheet. It estimates soil bulk density under the centre-line of a wheel track from readily available tyre details. The model uses an analytical method to estimate the propagation of stress in the soil. It contains compactibility data for contrasting soils and it accounts for both rebound and recompression realistically. We present examples that show the potential of the model in selecting tyres and wheel systems to minimise compaction.     

Effects of traffic on soil aeration, bulk density and growth of spring barley

This paper reports the results of field experiments on several different soils to quantify the effects of different numbers of passes of vehicular traffic on soil aeration status (measured in terms of oxygen diffusion rate, ODR and redox potential, Eh), soil bulk density and development of spring barley. In a further series of field experiments, the effects of single and dual wheels were compared and the effectiveness of a soil loosener operating behind the wheels was evaluated. Additionally, some microplot experiments are reported in which a range of known values of soil bulk density were produced and the effects on soil aeration and development of spring barley were evaluated. It is shown that repeated wheeling, even by a tractor of only about 2 tonnes weight, can produce soil conditions in which aeration can be limiting for crop growth. The use of dual wheels resulted in lower values of soil bulk density and associated greater soil aeration. The loosener alleviated the compaction produced by wheels and also improved soil aeration. For a sandy loam soil, greatest root growth and crop yield occurred at a bulk density of 1.43 Mg m⁻³. Soil aeration as a component of soil physical quality is discussed.     

初始含水量和容重对黑土压缩特性的影响

东北黑土区农业机械化水平高,农机作业压实导致的土壤结构和物理性状退化问题日益严重,压缩特性是定量分析土壤压实过程的有效手段,但目前黑土压缩特性随初始含水量和初始容重的变化规律尚不明确。为了解初始含水量和初始容重对黑土压缩特性的影响程度及其变化关系,该研究以重塑黑土为对象,设0.15、0.20、0.25、0.30、0.35、0.40 g/g共6个初始含水量水平,设1.00、1.10、1.20、1.30、1.45、1.60 g/cm³共6个初始容重水平,使用固结仪进行单轴压缩试验测定土壤压缩曲线,分析初始含水量和容重对压缩特性影响。结果表明,土壤初始含水量、容重及两者交互作用均极显著影响重塑黑土压缩特性(P<0.001),据此建立了预测压缩特性的土壤传递函数。黑土的预固结压力为10.42～1 106.17 kPa,与初始含水量显著线性正相关、与初始容重显著线性负相关(P<0.05);压缩指数为0.311～0.852,与初始含水量和容重呈二元多项式方程的关系,随初始容重的增大而降低,在中等含水量时最大;回弹指数为0.007～0.321,与初始含水量正相关,与...     

超声波土壤含水量检测装置的模型建立与验证

为探究利用超声波脉冲速度检测土壤体积含水量的可行性,以广东省红壤、赤红壤、水稻土为研究对象,设计了一种超声波土壤含水量检测装置,并利用ZBL-U510型非金属超声波检测仪在3种不同温度环境下(10、20、30℃)对不同含水量的土壤样本进行声速测定,构建了土壤体积含水量与超声波差值声速的温度效应数学模型。结果表明:超声波在水稻土中的传播速度比红壤、赤红壤快,且温度对超声波声速随土壤体积含水量变化节律的影响不同。20℃环境下超声波在土壤中的传播速度最快,10℃其次,30℃最慢。采用Richards模型表征土壤体积含水量与超声波差值声速关系的预测误差在3%左右,采用分段结构温度效应模型的预测误差在5%以内,证明该文提出的超声波脉冲速度-土壤体积含水量的温度效应模型可用于动态温度条件下的土壤含水量预测。该研究可为超声波技术在土壤水分检测领域的应用研究提供参考。     

农田土壤压实过程及模型研究进展

农田土壤受到农业机械田间作业的影响发生压实板结,造成土壤孔隙率降低,容重和紧实度增大,限制水分入渗和根系生长,影响作物产量。随着我国农业机械化水平不断提高,土壤压实对农业可持续发展的影响引起了广泛的关注。本文通过文献调研,总结了土壤压实过程的国内外研究进展,对土壤压缩行为、压缩曲线与预固结压力的计算方法进行了梳理,综述了土壤压实机理和压实模型的发展历程和未来动向,可为推进农田土壤压实研究提供参考。     

Mechanisms of soil vulnerability to compaction of homogenized and recompacted Ultisols

Soil compaction is a main cause of soil degradation in the world and the information of soil compaction in subtropical China is limited. Three main Ultisols (quaternary red clay, sandstone and granite) in subtropical China were homogenized to pass through 2 mm sieve and recompacted into soil cores at two bulk densities (1.25 and 1.45 g cm⁻³). The soil cores were equilibrated at different matric potential values (−3, −6 and −30 kPa) before subjected to multi-step compaction tests. Objectives of this study were to determine how different initial soil conditions and loading time intervals influence pre-compression stress and to evaluate an easy measure to determine soil vulnerability to compaction. It became evident that the soil strength indicator, pre-compression stress, was affected by soil texture, initial soil bulk density and matric potential. The coarser the soil texture, the lower the bulk density and the higher the matric potential, the lower was the pre-compression stress. The pre-compression stress decreased exponentially with increasing initial soil water content. Soil water content and air permeability decreased after compaction. The amount of water loss was affected not only by soil texture, bulk density and initial water content but also by loading time interval. These results indicate soil pore structure and hydraulic conductivity changed during compactions. The applied stress corresponding to the highest changes of pore water pressure during compaction had a significant linear relationship with the pre-compression stress (R=0.88, P<0.001). The correlation was ascribed to that the changes in pore water pressure describe the dynamics of the interactive effects of soil pore characters and soil water movement during compaction. The results suggested the evaluation of soil vulnerability to compaction have to consider the initial soil condition and an easy method to measure the changes in pore water pressure can be applied to compare soil strength and soil vulnerability to compaction.     

一种改进的土壤压实模型及试验研究

土壤压实现象普遍存在于农业生态系统中。土壤的压实效应不但会给农业生产带来不良的影响，还可能增加地表径流的产生，从而加快地表水的污染。 为了更好地研究压实土壤中的水分、溶质运移以及压实效应对农业生产及生态与环境的影响，该文在原有土壤压实模型的基础上提出了一种两参数改进模型，并以4种原状土壤为例，用离心机法对改进模型进行验证。研究结果表明：改进模型能够较好地模拟土壤的压实过程，且拟合效果好于L模型；虽然改进模型的物理意义和模型精度与Assouline的三参数模型相当，但是参数少、形式简单是改进模型的优势。同时，改进模型的提出对研究土壤水分特征曲线测定过程中的容重变化特性具有重要的参考价值。     

Using arbuscular mycorrhiza to alleviate the stress of soil compaction on wheat (Triticum aestivum L.) growth

Since large areas of agricultural fields in the world become compacted every year, much effort has been made to reduce the adverse effects of soil compaction on plant growth. Mechanical methods to control soil compaction may be laborious and expensive; however, biological methods such as using arbuscular mycorrhiza (AM) may be more useful, economically and environmentally. The objectives of this study were: (1) to evaluate the effects of soil compaction on wheat (Triticum aestivum L.) growth, and (2) to evaluate if using AM of different origin can reduce the stressful effects of soil compaction on wheat growth. Unsterilized and sterilized soils, different levels of compaction and three species of arbuscular mycorrhiza were applied in four replicates. The experiments were conducted in the Soil and Water Research Institute, Karaj, Iran. Soil physical and chemical properties were determined. The AM increased wheat growth in both soils at different levels of soil compaction in both experiments. For root, shoot (P=0.1) and grain (P=0.05) dry weights increases were significant. AM enhanced root growth more than shoot growth under compaction (AM resulted in significant increase in root/shoot ratios, P=0.1). Due to its unique characteristics, AM may reduce the stressful effects of soil compaction on wheat growth, though its effectiveness may decrease with increasing compaction.     

农田土壤机械压实研究进展与展望

土壤机械压实是威胁全球农业可持续发展的重要因素之一。从农田土壤压实的检测、危害、缓解和预防四个方面系统介绍当前国内外土壤压实的最新研究进展与不足。指出检测方法的创新和突破是实现田间尺度下压实土壤空间分布检测的关键;压实土壤危害的研究多集中在耕层土壤,但忽视了深层土壤压实危害及其在应对气候变化中可发挥的生态服务功能;提倡采用轮作轮耕等合理田间管理措施缓解压实土壤;深层土壤压实具有存在时间久和恢复难度大的特征,因此重点应以预防为主,但当前对土壤压实预防重视不足且预防技术体系尚不成熟。鉴于我国农业机械化正处在快速发展期,采取有效预防措施是避免重蹈发达国家土壤压实退化的有效手段。     

Compaction characteristics of towed wheels on clay loam in a soil bin

Wheel induced soil compaction is an ongoing concern in mechanized agriculture. This experimental study was performed with the aim to evaluate whether soil compaction is related to stresses induced by towed wheels. Soil bin studies were conducted and soil compaction variables were measured under two towed tires, with different tread patterns, commonly used in Turkey. Tests were carried out at three tire loads (3.5, 5.5 and 7.5 kN) and two forward velocities (0.8 and 1.4 m/s) on a clay loam. To determine soil compaction, surface sinkage, subsurface layer deformation, compaction index, penetration resistance and bulk density were measured. With increasing vertical load, average contact pressure of tires increased from 39.3 to 68.5 kPa. In different trials, surface sinkage, compaction index, penetration resistance and bulk density varied from 46 to 86 mm, 0.18 to 0.48, 1472 to 2530 kPa and 1.31 to 1.70 Mg m⁻³, respectively. The soil contact projected area of tire 2 was approximately 10% greater than tire 1. The greater contact surface reduced the compaction at the soil surface and subsurface, but the tire load was still the dominant factor in the 0–20 cm depth range used in this study. According to the experimental results, decreasing contact duration with increasing forward velocity decreased soil compaction. Tire load and type affected soil deformation characteristics stronger than forward velocity.     

Soil compaction in cropping systems: A review of the nature, causes and possible solutions

Soil compaction is one of the major problems facing modern agriculture. Overuse of machinery, intensive cropping, short crop rotations, intensive grazing and inappropriate soil management leads to compaction. Soil compaction occurs in a wide range of soils and climates. It is exacerbated by low soil organic matter content and use of tillage or grazing at high soil moisture content. Soil compaction increases soil strength and decreases soil physical fertility through decreasing storage and supply of water and nutrients, which leads to additional fertiliser requirement and increasing production cost. A detrimental sequence then occurs of reduced plant growth leading to lower inputs of fresh organic matter to the soil, reduced nutrient recycling and mineralisation, reduced activities of micro-organisms, and increased wear and tear on cultivation machinery. This paper reviews the work related to soil compaction, concentrating on research that has been published in the last 15 years. We discuss the nature and causes of soil compaction and the possible solutions suggested in the literature. Several approaches have been suggested to address the soil compaction problem, which should be applied according to the soil, environment and farming system.

The following practical techniques have emerged on how to avoid, delay or prevent soil compaction: (a) reducing pressure on soil either by decreasing axle load and/or increasing the contact area of wheels with the soil; (b) working soil and allowing grazing at optimal soil moisture; (c) reducing the number of passes by farm machinery and the intensity and frequency of grazing; (d) confining traffic to certain areas of the field (controlled traffic); (e) increasing soil organic matter through retention of crop and pasture residues; (f) removing soil compaction by deep ripping in the presence of an aggregating agent; (g) crop rotations that include plants with deep, strong taproots; (h) maintenance of an appropriate base saturation ratio and complete nutrition to meet crop requirements to help the soil/crop system to resist harmful external stresses.