A comparison of five instruments for measuring soil strength in cultivated and uncultivated cereal seedbeds

A torsional shear box, shear vane. cone penetrometer, drop-cone penetrometer and pocket penetrometer were used to measure soil strength at several depths less than 150 mm in cultivated and uncultivated seedbeds in a loam and a sandy clay loam. From the shear box results, cohesion was higher and the angle of friction was lower in the sandy clay loam than in the loam. Angle of friction was independent of cultivation but cohesion was higher in uncultivated than in cultivated soil. Despite these differences cone resistance was similar in both soils above 70 mm depth. Vane shear strength and drop-cone penetration, although empirical, indicated strength differences between soils and cultivations similar to those found with the torsional shear box. Vane shear strength, at 42 kPa, was about twice as high as cohesion in the sandy clay loam and, at 33 kPa, over four times as high as cohesion in the loam. These overestimates increased with increasing bulk density. The range of measurement of the pocket penetrometer was inadequate to cover the range of soil strengths encountered. The coefficient of variation within plots for cone resistance decreased from 76 per cent at 10 mm depth to about 22 per cent at 70 mm depth and below, and for vane shear strength it was 33 per cent near the soil surface. The drop-cone penetrometer results were the most variable, reflecting the log-normal distribution of penetrations. The cone penetrometer was the fastest method, followed by the shear vane, drop-cone penetrometer and torsional shear box in that order.     

重庆缙云山典型植被原状土与重塑土抗剪强度研究

为了从土壤力学方面探索防治水土流失的途径与措施,该文采用直剪仪对重庆缙云山5种典型植被的原状土和重塑土抗剪强度进行试验测定。通过比较相同植被同干密度、同含水率的原状土与重塑土在同一垂直荷载下的剪切差异,分析了根(极细根)在土壤－根系复合体(原状土)中的作用。研究结果表明：原状土剪切位移小于重塑土;在相同垂直荷载作用下,同一植被原状土的抗剪强度大于与其同干密度、同含水率的重塑土。原状土的剪切破坏近似于塑性破坏,而重塑土则属于弹性破坏;原状土的抗剪强度与含根量呈正相关,并以此建立关于原状土的抗剪强度回归模型。     

Mechanical weeding effects on soil structure under field carrots (Daucus carota L.) and beans (Vicia faba L.)

The successful production of organic vegetables relies heavily on mechanical weeding, flame weeding and stale seedbeds. These operations involve repeated passes by tractors. Mechanical weeding also involves regular tillage. This combination of repeated tillage and compaction changes soil structure. We studied these structural changes in two fields of organic carrots and one field of beans in eastern Scotland. Structure was described by measuring soil strength with a vane shear tester and a cone penetrometer, by measuring bulk density and by visual assessment. Under beans, vane shear strength below the growing root zone was highly variable and in some areas was high enough to restrict root growth (>50 kPa). The carrots were grown in beds containing crop rows separated by bare soil. The bare soil was regularly weeded mechanically. The structure of this weeded soil in the top 10 cm layer of a loam eventually became disrupted and compacted enough to deter root growth (vane shear strength of 70 kPa). In addition the topsoil and subsoil in the wheel-tracks between the beds became very compact with little distinguishable structure. This compaction extended to the subsoil and persisted into the next cropping season (cone resistance >3 MPa at 35–50 cm depth). Reduced tillage by discing without ploughing was used to incorporate the straw used to protect the carrots overwinter and prepare the soil for the next crop. The resulting topsoil quality was poor leading to anaerobic growing conditions which restricted growth of the following crop and led to losses of the greenhouse gas nitrous oxide. The greatest threat to soil quality posed by mechanical weeding was subsoil compaction by tractor wheeling.     

A recording penetrometer to measure the strength of soil in relation to the stresses exerted by a walking cow

A penetrometer has been designed for field use to simulate the stresses exerted on the ground by the hoof of a walking cow, and to measure the resulting deformation of the soil. The extent of deformation produced by the penetrometer in two intact topsoils under pasture was found to be small and independent of water content, whilst significant correlations between soil strength and water content were obtained using a cone penetrometer and a hand-held shear vane. The results imply that the deep hoofprints associated with poached pasture are not produced immediately upon treading wet soil, but only after a progressive loss of soil strength due to repeated treading. Thus the rate of loss, which is deduced to be a measure of the susceptibility of pasture to poaching, cannot be estimated directly from a single measure of soil strength.     

Tillage effects on shear strength and bulk density of soil aggregates

An experiment was conducted to determine the effect of four tillage systems (moldboard plow, chisel plow, Paraplow and no-till) on soil aggregate shear strength and bulk density. Two soils, a Canisteo clay loam (fine-loamy, mixed (calcareous), mesic, Typic Haplaquoll) and a Haig silt loam (fine, montmorillonitic, mesic, Typic Argiaquoll) were used in this study. Soil samples were collected from the 0.075–0.15-m-depth increment in 1983 and the 0.075–0.15- and 0.225–0.30-m-depth increments in 1985. Shear strength of soil aggregates 0.02–0.03 m in diameter was measured by a fall-cone penetrometer and bulk density of the same aggregates was measured by gamma-ray attenuation. Aggregates were tested at soil water matric potentials (ψ_m) of −0.2, −1.1 and −4.0 kPa in 1983 and at ψ_m of −0.2, −1.1, −4.0 and −7.9 kPa in 1985. Tillage for the 1983 growing season was conducted under very wet conditions, whereas tillage for the 1985 growing season was conducted under much drier conditions. Samples collected in 1983 showed little tillage effect on shear strength or bulk density. In 1985, tillage had an effect on shear strength and bulk density for the Haig soil, but not for the Canisteo soil. Much of the tillage effect on soil aggregate shear strength could be explained by tillage-induced changes in the aggregate bulk density. As bulk density decreased, soil aggregate shear strength decreased.Sampling depth had no effect on soil aggregate shear strength or bulk density. Matric potential had an effect on soil aggregate shear strength and bulk density. As matric potential decreased, both shear strength and bulk density increased.     

Zum Vergleich von Penetrometermessungen,durchgeführt bei unterschiedlichem Wassergehalt

Penetrometer measurements at different soil water contents With use of a large number of field measurements an empirical relation between soil water content and penetrometer resistance was derived. For this purpose the soils were subdivided with respect to their texture and bulk density. The derived relations enable a comparison of penetrometer values derived at different soil water content.     

Two mechanisms for age-hardening of soil

Soil samples from Germany, Israel and USA were moulded at water contents around the lower plastic limit and compacted with uniaxial pressures c . 20-200 kPa. The samples were stored at constant water content. At intervals after moulding, the strengths of sub-samples were measured with a small penetrometer.
The soils showed increases in strength with time. Two types of behaviour were observed and explained by a simplified theory for soil strength. With the German and Israel soils, penetrometer strength increased by the same absolute amount irrespective of compaction pressure. This indicates that new particle-particle bonds were being formed at a rate which was not affected by compaction. With the USA soils, the ratio of penetrometer strength/initial strength increased with time and was independent of compaction pressure. This indicates that existing particle-particle bonds were being reinforced by a cementation mechanism. Evidence is presented which suggests that age-hardening by this second mechanism may be inhibited by soil organic matter.     

Shear strength of surface soil as affected by soil bulk density and soil water content

This paper proposes a new method to measure the soil strength parameters at soil surface in order to explain the processes of soil erosion and sealing formation. To simulate the interlocks between aggregates or particles within top 2 mm of the soil, a piece of sandpaper (30 particles cm⁻²) was stuck on the bottom face of a plastic box of diameter of 6.8 cm with stiffening glue and used as shear media. The soil strength for the soils from sandy loam to clayey loam was measured with penetrometer and the new shear device at soil surface at different bulk density and soil water content. The normal stresses of 2, 5, 8, 10 and 20 hPa were applied for the new shear device. The results indicated that significant effect of bulk density on soil strength was detected in most cases though the difference in bulk density was small, ranging from 0.01 to 0.09 g cm⁻³. It was also indicated that the measurement with the new shear device at soil surface was reproducible. The changes in soil shear strength parameters due to changes in bulk density and soil moisture were explainable with the Mohr–Coulomb’s failure equation and the principles of the effective stress for the unsaturated soils. The implications of the method were later discussed.     

Penetration resistance of gypsiferous horizons

To assess the importance and the possible causes of penetration resistance of horizons with gypsum, 20 horizons in seven irrigated profiles were studied. Gypsum contents ranged from 0 to 900 g kg^?1. Penetrometer tests were performed on undisturbed soil cores by means of a needle penetrometer at different matric potentials. The increase of penetration resistance on drying was caused by changes in the effective stress of the soils, calculated from their soil water characteristic curves. Multiple regression tests showed that besides water content and bulk density, gypsum content was positively correlated with penetration resistance. It seems that in the soils studied the increase of penetration resistance caused by gypsum is due to the growth of gypsum crystals in pre-existing pores, which reduces the volume of regular and continuous voids necessary for root growth.     

Soil mechanical properties as influenced by exchangeable cations

The tensile strengths of dried, undisturbed aggregates and of dried, artificial aggregates and the shear strengths of freshly-moulded, moist soils were investigated as functions of their contents of exchangeable cations. Tensile strength increased with increasing exchangeable sodium and decreased with increasing exchangeable calcium. Shear strength of moist soil, as assessed through the Atterberg limits, increased with increasing exchangeable calcium, potassium and sodium. The influence of exchangeable magnesium was variable in that it tended to increase the tensile strength of the soils when dry, whereas it tended to decrease the shear strength of one of the sets of soils in the moist state. It is concluded that the exchangeable cations which give rise to greater repulsion between, and dispersion of, clay particles in water also give rise to greater strength in the dried soil. It is proposed that this is because greater particle repulsion in the soil water during drying enables particle rearrangements to take place more readily and this results in denser packing arrangements and increased strength.     

Effects of 'Agri-SC' soil conditioner on the erodibility of loamy sand soils in East Shropshire, UK: preliminary results

Abstract. The effect of Agri-SC' soil conditioner on the erodibility of loamy sand soils has been investigated at the Hilton experimental site, Shropshire, since March 1988. Factors measured have included runoff and erosion, soil structure, crust strength, splash susceptibility, aggregate stability, soil micromorphological properties, response to compaction and penetrometer resistance.
Treatment decreased runoff and erosion rates, bulk density, splash erosion, crust strength and penetrometer resistance, and increased pore space and aggregate stability. The effects on crust strength, aggregate stability and bulk density were statistically significant. The results suggest that applications of 'Agri-SC' could have beneficial effects for soil conservation.     

A rapid, inexpensive and quantitative procedure for assessing soil structure with respect to cropping

Abstract. The essential factors dependent on soil structure that influence plant growth are soil/root contact, adequate air and water, and low mechanical impedance. Bulk density, shear strength and texture arc interrelated closely and permit quantification of these factors. A general relationship between clay content and vane shear strength of soil at field capacity and non-limiting bulk density provides a rapid means of quantitatively estimating structure. We propose a procedure utilizing vane shear strength and a tactile assessment of clay content as criteria for judging soil structure in the field.     

A method for prediction of soil penetration resistance

A new equation for predicting penetration resistance of soil is presented. The equation contains two main additive terms: the first is a measure of the degree of compactness of the soil and the second gives the contribution of pore water to the soil strength. It is proposed that these terms are applicable to soils of different texture, at different bulk densities and at different water contents. The equation is calibrated and tested using values of penetrometer resistance measured in the field at a range of locations in Poland. Predictions from the equation are compared with predictions from two other published equations. It is shown that the performance of the proposed equation is superior to the other two, at least for the Polish data set used in this work. On the basis of the assumption that the proposed equation is correct, predictions of penetrometer resistance are made using pedotransfer functions to illustrate typical effects of soil texture, bulk density and water content.     

Changes in penetration resistance of Ultisols from southern China as affected by shearing

Shear stresses and soil properties modified due to stress play an important role during formation of seals in a series of rainfall events and during tillage. The objectives of the study were to evaluate the effects of the penetrometer geometry on the penetration resistance as affected by shearing under different initial soil conditions and to use the information on soil strength to elucidate shearing process. Nine homogenous air-dried soils (<2 mm) were sprayed and stored so as to obtain equilibrium soil water contents. The moist soils were sheared by horizontal displacement of layers of soil particles/aggregates in between hands in one direction. The soil cores were prepared with comparable bulk density before the measurement of maximum penetration resistance (P_max) with a small flat tip and a cone tip penetrometers. At a wide range from 0.05 to 6.2 MPa, P_max was linearly correlated between the small flat tip and the cone tip penetrometers. The conversion ratio was higher under the saturation condition irrespective of the shearing effect. The penetrometer with the small flat tip was more sensitive for the weak soils. Shearing generally increased P_max in most cases, but it decreased P_max for some sandy soils under both saturated and unsaturated conditions and for a clayey soil under the saturated condition. The soil consisting of swelling clay exerted a decrease in P_max. Rearrangement and/or sliding of particles/aggregates and increase in soil suction during shearing were attributed to the increase in P_max. Increase in porosity due to the aggregation during shearing was ascribed to the decrease in P_max. In addition, it was shown that agricultural cultivation resulted in a reduction in soil strength.     

Reference bulk density and critical degree-of-compactness for no-till crop production in subtropical highly weathered soils

The concept of degree of compactness (DC), referred to as field bulk density (BD) as a percentage of a reference bulk density (BD_ref), was developed to characterize compactness of soil frequently disturbed, but for undisturbed soil such as under no-tillage critical degree of compactness values have not been tested. The objective of this study was to compare methods to determine BD_ref and limits of DC and BD for plant growth under no-tillage in subtropical soils. Data from the literature and other databases were used to establish relationships between BD and clay or clay plus silt content, and between DC and macroporosity and yield of crops under no-tillage in subtropical Brazil. Data of BD_ref reached by the soil Proctor test on disturbed soil samples, by uniaxial compression with loads of 200 kPa on disturbed and undisturbed soil samples, and 400, 800 and 1600 kPa on undisturbed soil samples, were used. Also, comparisons were made with critical bulk density based on the least limiting water range (BDc LLWR) and on observed root and/or yield restriction in the field (BDc Rest). Using vertical uniaxial compression with a load of 200 kPa on disturbed or undisturbed samples generates low BD_ref and high DC-values. The standard Proctor test generates higher BD_ref-values, which are similar to those in a uniaxial test with a load of 1600 kPa for soils with low clay content but lower for soils with high clay content. The BDc LLWR does not necessarily restrict root growth or crop yield under no-tillage, since field investigations led to higher BDc Rest-values. A uniaxial load greater than 800 kPa is promising to determine BD_ref for no-tillage soils. The BD_ref is highly correlated to the clay content and thus pedotransfer functions may be established to estimate the former based on the latter. Soil ecological properties are affected before compaction restricts plant growth and yield. The DC is an efficient parameter to identify soil compaction affecting crops. The effect of compaction on ecological properties must also be further considered.     

Effect of trampling on the soils of the St James Walkway, New Zealand

Abstract. The influence of trampling on the soils of the St James Walkway was studied during 1985 by comparing 'on'- and off-track sites. Trampling increased the average soil bulk density by 0.3 g/cm³ at 0–5 cm depth and by 0.1 g/cm³ at 10–15 cm depth. Trampling increased the average soil shear strength by 11 kPa at 0–5 cm depth and by 6 kPa at 5–10 cm depth. All mineral soils were compacted to some extent by trampling. The podzolized high country yellow-brown earths (Dystrochrepts) were the most affected because their organic topsoil was truncated. Their exposed subsoil was however more resistant to further damage than their topsoil. Organic soils (Medihemists) were not compacted but their very low shear strength and high moisture content make them unsuitable for tracks. Untrampled soil bulk density and soil stone content were negatively correlated with the change in bulk density by trampling, and could be used to predict the risk of soil compaction by trampling.     

Soil structural quality, compaction and land management

Soil compaction is a concern worldwide, particularly where compactible soils are used for intensive agriculture in a wet climate. We have investigated the impact of compaction and the associated changes in soil structural qualities on crop production and environmental pollution. The overall objective was to develop soil management systems that provide suitable conditions for crop growth and minimize environmental damage. We ran large-scale field experiments studying the preservation of structural quality in arable and permanent grassland, using management systems such as the control or elimination of field traffic and the application of conservation tillage and zero tillage. We measured bulk density, shear strength, cone resistance, macroporosity, relative diffusivity, air permeability and water infiltrability to identify soil qualities that could be used for selecting suitable soil management. Along with crop yield, we measured environmental impacts, such as the emissions of nitrous oxide from the soil, which require the interaction of soil structure and water content near the soil surface. Soil structure influenced wetness, which affected trafficability, compaction and nitrogen retention. Measurement of properties that affect fluid storage and transport, such as macroporosity, provided soil quality indices that helped in recommending suitable soil management systems. Spatial variation of structure associated with wheel-track locations could be estimated rapidly using a cone penetrometer. Variation was particularly important in determining crop yield consistency. Crop productivity and soil structural qualities were preserved best when field traffic was eliminated. A reduced ground-pressure system successfully minimized compaction in grassland but was less effective in an arable rotation. Unless traffic is eliminated, good timing of operations is the most effective way to preserve soil structural quality.     

Critical state parameters derived from constant cell volume triaxial tests

Critical state parameters were determined in constant cell volume triaxial tests on three remoulded agricultural topsoils, a sandy loam, a clay loam and a clay. Tests were made at a range of water contents. The normal consolidation lines tended to be linear on a semi-logarithmic plot up to a degree of saturation of c. 0.85, above which the soil was incompressible but highly deformable. The slopes of the projected critical state lines were slightly greater than the slopes of the normal consolidation lines for all three soils. For each soil, both lines pivoted about a point as water content increased and, for the two lighter-textured soils, the increases in compactibility tended to be greatest near the plastic limit. For all three soils, strength remained fairly constant with increasing water content until the soil was at around 70-85% of the cone penetrometer plastic limit. Strength then decreased with increasing water content, with the smallest decrease in the sandy loam.     

土壤持水特征测定中质量含水量、吸力和容重三者间定量关系 Ⅱ.原状土壤

用离心机法测定了四种原状土壤不同容重下的土壤水分特征曲线实测点,从实验上获得了四种原状土壤的质量含水量、吸力和容重三者间定量关系曲面。分析了土壤的扰动性对土壤三变量关系曲面的影响,并以实测数据为基础,进一步对我们提出的一种模型进行验证,比较该模型对填装土壤和原状土壤三变量曲面的拟合情况。结果表明:土壤的扰动性使实测的土壤三变量曲面发生较大的变化。填装土和原状土的初始容重差别越大,土壤的扰动性对土壤三变量曲面的影响就越大。两处理模型参数的差异大小能很好地反映土壤扰动性对三变量关系曲面的影响。原状土壤的三变量曲面实验研究和探索有益于将变容重土壤水动力学研究拓展到田间土壤,具有一定的实践意义。     

斥水剂作用下非饱和土壤抗剪强度测定及其变化规律

为获得斥水性土壤抗剪强度的变化规律,采用二甲基二氯硅烷(dimethyldichlorosilane,DMDCS)作为斥水剂,获得了不同斥水程度的改性砂土。在此基础上配制了不同斥水剂体质比和不同含水率的改性砂土及不同亲水黏土质量分数的改性混合土,并采用非饱和土直剪仪开展了不固结不排水剪强度试验。结果表明:1)不同DMDCS体质比下的5种改性砂土斥水等级均为极度。改性混合土的斥水等级受DMDCS和黏土含量的共同影响。相同DMDCS体质比下,随着黏土含量的增加,改性混合土的斥水性能不断减弱;相同黏土含量下,随着DMDCS体质比的增加,改性混合土的斥水性能不断增大。2)不同DMDCS体质比、含水率及黏土含量下的改性土壤抗剪强度均可用摩尔-库仑强度准则描述。DMDCS体质比从0增至1%时,黏聚力从19.6陡降至10.4 kPa,随后缓慢降低,最终趋于稳定。内摩擦角则随着DMDCS体质比的增加缓慢减小,从0时的16.2o降至3%时的11.8o;随着含水率的增加,改性砂土黏聚力逐渐减小,而内摩擦角呈先升后降形态;随着黏土含量的增加,改性混合土黏聚力显著增大,内摩擦角表现为先升后降,变幅不大。纯改性砂土的黏聚力仅为9.3 kPa,而掺入5%的黏土时,其黏聚力骤升至27.2 kPa;当黏土质量分数为50%时,混合土黏聚力为55.1 kPa;内摩擦角最大值为16.2°(黏土质量分数15%),最小值为9.7°(黏土质量分数50%)。该成果可为深入研究斥水性土壤力学性能及工程应用提供参考。