Evaluation of a combined penetrometer for simultaneous measurement of penetration resistance and soil water content

A combined penetrometer is an appropriate tool to measure the soil cone resistance and the water‐content profile. As a relatively new technique, a combined capacitance‐penetrometer for the simultaneous measurement of cone index and soil water content was developed at the Department of Agricultural Engineering of Bonn University in 2002. The objective of this study was the evaluation of the effectiveness and applicability of the innovated penetrometer with a focus on three aspects: (1) A capacitance sensor with two electrode configurations was calibrated for silt loam, sandy loam, and sand. The calibration results show that both electrode configurations have sufficient water‐content sensitivity, but soil‐specific calibrations seem necessary. (2) Under laboratory conditions, the dynamic resolution and response of the capacitance‐penetrometer were validated, and its radius of influence was determined. (3) The field measurement results demonstrate that this measurement technique can be used to improve the interpretation quality of soil cone index data.     

On-the-go measurement of soil water content and mechanical resistance by a combined horizontal penetrometer

A combined horizontal penetrometer was designed for the on-the-go and simultaneous measurement of soil water content and mechanical resistance. The maximum sampling rate for both sensors was 10 Hz and the maximum operating depth was 20 cm. For the water-content sensor, its measurement principle depends on the electric field of the fringe-capacitance. In order to evaluate the applicability of this combined penetrometer, four experiments in the field were carried out. These experiments included: (1) soil water content profiles test; (2) soil compaction measurement test; (3) effect of the operating velocity on the water content and resistant force measurement; (4) effect of operating depth on the force measurement. The experimental results show that the combined horizontal penetrometer is a practical tool since it can provide more useful information of soil physical properties.     

A NEW HAND-HELD RECORDING PENETROMETER FOR SOIL STUDIES

This paper describes and evaluates a new solid-state recording hand-held cone penetrometer developed for in-situ soil studies. Force is measured by a strain-gauged transducer, and depth by an accurate optical system. Data for up to 20 penetrations per plot are stored and processed within the instrument in the field. The new penetrometer has been evaluated in field and soil tank experiments in comparison with three widely contrasting existing penetrometers ranging from a simple hand-held penetrometer to a trailer-mounted power-driven instrument. The results indicated that the simple hand-held penetrometer gave appreciably different cone resistance profiles from the other instruments. In particular, spurious ‘treatment effects’ obtained from the tillage experiment were shown to be due to inadequate instrument performance. The new solid-state recording hand-held penetrometer gave results comparable with those obtained from the more complex trailer-mounted instrument and another hand-held recording penetrometer.     

Determination of soil surface strength with a needle-type penetrometer

Soil crusting was characterized by means of direct measurement of penetration resistance with a needle type penetrometer at intervals of 0.1 mm over a depth of 15 mm in intact soil samples, treated with soil stabilizers or not, as a function of soil water content and bulk density. Relationships were established between the penetration resistance of the needle and standard cones of 60° angle and 1 cm² or 26.4 mm² base area. The effect of water content was stronger in dense than in loose soil.The effect of soil surface strength on the emergence of salsify (Scorzonera hispanica) was monitored in a field experiment on a loamy sand of which the natural crust was stabilized with soil conditioners. The penetration resistance was affected by the nature of the soil stabilizers as some created more or less hydrophobic and spongy crusts. Seedling emergence was negatively correlated with the penetration resistance and positively with the water content of the crust.     

Investigating soil physical properties and yield response in a grassland field using a dual‐sensor penetrometer and EM38

Precision‐farming applications are mainly based on site‐specific information of soil properties at the field scale. For this purpose, a number of novel sensor techniques have been developed but not intensively tested under different field conditions. This study presents a combined application of a self‐developed dual‐sensor vertical penetrometer (DVP) for measuring volumetric soil water content (VSWC) and cone index (CI), and an EM38 for soil apparent electrical conductivity (EC_a) in a pasture (1.4 ha). To verify the feasibility of the DVP for interpreting the depth‐specific information in the field, not only the soil physical properties and their geographical coordinates were measured, but also geo‐referenced yield data were collected. We found that the yield pattern was quite similar to the soil water‐content pattern of each layer (layer‐1: 5–15 cm; layer‐2: 15–25 cm, layer‐3: 25–35 cm) and EC_a pattern. Using the map‐based comparisons in conjunction with the statistical analyses, the effect of each measured soil physical property (VSWC, CI, and EC_a) on the yield was investigated. The regression between the yield and VSWC at each layer fitted a quadratic equation (R² = 0.515 at 5–15 cm; R² = 0.623, at 15–25 cm; R² = 0.406 at 25–35 cm). The negative correlation between yield and CI at each layer fitted a linear model with R² ≥ 0.510.     

Correction of cone index for soil water content differences in a coastal plain soil

Soil penetration resistance (cone index) varies with water content. The field variation of water content could mask treatment differences. The correction of cone index data to a single water content would help prevent this. We used equations from TableCurve^™ software and from the literature to correct cone indices for differences in soil water contents. Data were taken from two field experiments where cotton (Gossypium hirsutum L.) was grown using conventional and conservation tillage without irrigation, and beans (Phaseolus vulgaris L.) were grown using conventional tillage with microirrigation. Boundary conditions based on hard, dry and soft, wet soils were imposed on the equations. Equations fit the data with coefficients of determination ranging from 0.55 to 0.92 and error mean squares from 1.37 to 6.35. After correction, cone index dependence on water content was reduced. A single-equation correction did not always fit the data across all treatments. Separate corrections, based on treatment, might be required. When corrections required multiple equations, differences may be real or may be a manifestation of the correction differences. In this case, the correction may not be feasible (unless some future work can coordinate different equations and assure a uniform correction).     

Influence of failure mode induced by a horizontally operated single-tip penetrometer on measured soil resistance

Excessive soil compaction has negative effects for agriculture and the environment. Measurement of soil strength is a common indirect measure of soil compactness. In the context of precision farming, on-the-go soil mechanical resistance measurements using single- and multiple-tip horizontal sensors have been developed. It has been reported that there was a significant relationship between soil mechanical resistance values measured with both vertically operated cone penetrometer and horizontally operated sensors only for relatively deep layers. It was hypothesized that the differences in horizontally measured soil resistance in different soil layers could be explained by different failure modes. The objective of this research was to develop a horizontal soil mechanical resistance sensor and to observe the failure mode in front of it while penetrating soil at three different depths. A single-tip horizontal penetrometer was equipped with a 30° prismatic tip and had a base area of 324 mm². The prismatic tip was mounted horizontally to an S-shaped load cell housed inside a shank. A data-logging system was also developed to record measurements with 10 Hz sampling rate. The sensor was tested in a field with silty clay loam soil at three depths of 20, 25 and 30 cm. Cone index (CI) values were obtained with 1 cm depth increments and 1 m horizontal intervals along each transect for comparison using a standard cone penetrometer. The results showed that average horizontal soil mechanical resistance index (HRI) values for both depths of 20 and 25 cm were similar due to the brittle failure mode in both cases. However, when the tip was operated below the critical depth of the sensor, the value of HRI at 30 cm depth increased three times when compared with 20 or 25 cm depth values. This was due to change in failure mode from brittle to compressive mode below the critical depth. There was a significant relationship (R² = 0.75) between HRI and CI for the 30-cm depth, whereas for shallower depths the relation was not significant. It can be concluded that the correlation between measurements obtained with the vertically and horizontally operated penetrometers would be significant as long as both produced the same soil failure mode.     

Soil strength and crop emergence in direct drilled and ploughed cereal seedbeds in seven field experiments

Cone resistance and vane shear strength were measured in the top 50–100 mm of seven soils. Bulk density and water content were also measured in the same layer by coring. At each site cone resistance and vane shear strength tended to decrease with increasing water content and decreasing bulk density down to 1300 kg m-3, but at lower densities they were not related to bulk density. Cone resistance and vane shear strength tended to decrease with increasing coarseness of texture but also depended on soil structure and organic matter content. Plant populations, mainly spring barley, were reduced in soils with cone resistances and vane shear strengths greater than 2500 kPa and 65 kPa respectively. Such high strengths in undisturbed soils were associated with wheeling during harvesting, were apparently independent of soil type and, at the only site of measurement, apparently decreased during weathering in the subsequent season. In contrast to the cone penetrometer, the vane shear tester has minimal shaft friction in undisturbed soils and is more sensitive to soil differences and less sensitive to water content differences than the penetrometer. It is apparently more suitable for indexing the suitability of undisturbed soils for the penetration of direct drill coulters and for subsequent plant establishment.     

Drop-cone penetration in situ and on minimally disturbed soil cores

Drop-cone penetration was measured both in situ and on minimally disturbed cores at a selection of soil matric potentials for four soils. The drop-cone penetration/water content relation was linear in situ and, if results for very low water contents were excluded, for the cores. The slope of the line for the field data was much greater than for the core data and this raises doubts about the value of other than in situ measurements of soil strength for predicting field behaviour. Vane shear strength is negatively correlated with in situ drop-cone penetration. The drop-cone test is a useful strength test for small volumes of soil, but care must be taken in selecting the mass of the cone as this affects the volume of soil which will influence the test results.     

Structural characteristics of clay-dominated soils of a marsh and a palaeosol in a crossed diagram

Although the shrink‐swell phenomenon of clays has been thoroughly studied, the in situ relation of the shrinkage curve to the structure profile is rarely presented from the shrinkage limit to the liquid limit. We studied the consolidated structure of clay‐dominated (<2 μm) soils formed on ‘pseudo‐liquid’ marsh sediments in the ‘Marais de l’Ouest’ (France). The profiles were studied in a grassland field and in a sunflower field from unsaturated surface soils down to deeper, saturated, levels characterized by a very large water content (100% by weight). The consolidation states were quantified recording cone resistance (Qd) profiles using a dynamic penetrometer in successive seasons. These Qd profiles were compared with the associated wet density and gravimetric water‐content profiles. Two consolidation depths were evident, the surface soil and a 130‐cm deep palaeosol. The seasonal Qd profiles demonstrate the partial irreversibility of the consolidation peaks associated with the surface soil and with the palaeosol. The shrinkage properties were established through drying curves of undisturbed test samples. In the void ratio (e)‐water content (W) and water content (W)‐saturation index (Sr) diagrams, the profiles as a whole exhibit only one clay soil behaviour from their pseudo‐liquid to plastic to solid states. Each Qd profile is represented by a hyperbolic curve in the e/Qd diagram. Represented in a (e – W – Qd —Sr) crossed diagram, the vertical evolution of the successive profiles shows the soil structure behaviour from the initial pseudo‐liquid sediment to the consolidated soil. A simple cone resistance recording associated with gravimetric water‐content profiles, characterizes the evolution of structural layers of soils for the seasonal drying‐wetting cycles, for the over‐consolidation associated with the palaeosol, and also for the effect of ploughing.     

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.     

Penetrometer measurements for screening soil physical variability

Spatial variability of soil physical conditions can easily be assessed by means of a penetrometer. This is demonstrated at a reforestation site near Hannover on a Podzol-Cambisol (FAO). Reafforestation was performed after deep ploughing to a depth of 80–100 cm. Penetration resistance was measured as kg cm⁻² with a dial penetrometer. The data showed recompaction after deep ploughing. Furthermore, it became evident that measuring penetration resistance is a simple and inexpensive way to obtain a preliminary impression of the area to be examined. Based on these results, locations with minimum, maximum and average values can be chosen for further investigations, depending on the aim of the investigation. For screening purposes it is not important whether penetration resistance is expressed in physical units or other values, such as the number of blows per unit depth with a falling-weight penetrometer. In these circumstances the low specificity of penetration resistance is not a disadvantage, but enables any change in soil physical conditions to be detected.     

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.     

Effects of seedbed structure and water content at sowing on the development of soil surface crusting under rainfall

This study was carried out to observe the dynamics of crust formation on the soil surface under field conditions and analyse the effects of seedbed structure and water content on soil surface crusting. Seedbed sensitivity to crusting was also estimated in the laboratory by stability tests on aggregates. We observed 57 plots during the sowings of spring and autumn crops in fields in Northern France (Estrees-Mons, 50°N latitude, 3°E longitude). The soil is an Orthic Luvisol according to the FAO classification (0.17–0.25 g g⁻¹ clay and 0.02 g g⁻¹ organic matter on average). Visual assessments in situ were performed and photographs taken of crust stages on delimited areas, each 5 mm of cumulated rainfall since sowing. In 2004–2005, the seedbeds were characterised by their distribution of aggregate sizes and tests of aggregate stabilities of surface samples kept with their water content at sowing. A penetrometer was used to measure crust resistance and estimate its thickness. These data were analysed to detect the cumulative rainfall values needed for the initiation and development of the successive stages of crusts. A fully developed structural crust (stage F1) required 13, 22, 27 mm cumulated rainfall respectively for seedbeds with proportions of clods over 2 cm ranging from 0 to 0.15 (fine seedbed), 0.15 to 0.30 (medium seedbed), >0.30 g g⁻¹ (coarse seedbed). Aggregate stability measured on samples kept at sowing water content was low for soil with low water content (<0.17 g g⁻¹) but increased sharply for water contents over 0.17 g g⁻¹. Stage F1 was reached more rapidly (only 11 mm versus 19 mm cumulated rainfall) only for fine seedbeds with less than 0.15 g g⁻¹ of clods over 2 cm and with a low water content at sowing, The stage of 50% of soil surface covered with sedimentary crusts was reached for 85 mm for fine seedbed versus 120 mm for medium seedbed. The mean penetrometer resistance of dry crusts was 0.55 ± 0.43 MPa for stage F1 and 3.54 ± 0.83 MPa for a sedimentary stage; mean penetrometer resistance increased continuously with cumulated rainfall and was much lower for wet crusts. These quantitative data gathered under field conditions constitute the first step towards the prediction of soil surface crusting. The cumulative rainfalls were used in order to estimate the risk of occurrence of structural and sedimentary crusts forming during crop emergence with several types of seedbeds.     

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.     

Soybean seedling root growth as influenced by soil texture,matric suction and bulk density

Abstract

Laboratory experiments were conducted under controlled conditions to determine the effect of five matric suctions (0.05, 0.10, 0.30, 1.00 and 3.00 bars) and three bulk densities (1.10, 1.30 and 1.50 g.cm^?3) on the moisture content, penetrometer resistance and soybean (Glycine max L.) root growth in six different soil textural groups (sand, silt, clay and their combinations).

The different textural groups were compacted in PVC pipes 4.4 cm ID and 10 cm long and placed in pressure cells to obtain the desired matric suction. After equilibrium five pregerminated soybean seedlings were fixed on the soil surface. At the end of 48 hours root elongation was measured.

There was an increase in root growth in all the textural groups at all the bulk densities when the matric suction was increased from 0.05 to 0.30 bar. There was however a gradual decrease in root growth as the matric suction increased from 0.30 to 3.0 bars. The reduction in root growth at low and high matric suctions was related to moisture content, change in soil resistance and redox status of the soil system.

The measured penetrometer resistance values were directly related to the level of compaction, soil matric suction and also were dependent upon the texture. Close relationships were recorded between redox potentials and soil matric suction.     

THE PLASTIC LIMIT, AS DETERMINED BY THE DROP-CONE TEST, IN RELATION TO THE MECHANICAL BEHAVIOUR OF SOIL

The variation with water content is examined of soil cohesion, soil-metal friction, susceptibility to soil compaction, implement draught and the slope and intercept of the virgin compression line of critical state soil mechanics theory. For a given soil, all these relations are shown to exhibit turning points at a similar water content which corresponds to the cone penetrometer plastic limit, as determined with a drop-cone penetrometer, rather than the Casagrande plastic limit. The cone penetrometer plastic limit, which can also be more reliably determined than the Casagrande plastic limit, is the better indicator of soil behaviour in the field and of the water content at which the soil changes from the brittle to the plastic state.     

Use of an instrumented disc coulter for mapping soil mechanical resistance

A soil mechanical resistance sensor with a large-diameter disc coulter was developed to delineate areas of differing soil strength across agricultural fields. The instrumented disc coulter consisted of a 76.2 cm disc with two depth-measuring sensors (rotary potentiometer and ultrasonic proximity sensor) along with a global positioning system (GPS) receiver to georeference operating depth measurements. The consistency and repeatability of the system response were evaluated by making six passes across long-term tillage comparison plots with different degrees of soil disturbance, including: 20 cm plowing, 15 cm disking, 30 cm chiseling, and no-till in several combinations. At the time of testing, standard soil cone penetrometer measurements were taken. The relationship between the average cone index in the 0–30 cm soil profile (CI_0–30 cm) and the disc operating depth was evaluated. In addition, the cumulative energy density of the given depth of penetration defined as specific cone penetration energy (J m⁻² or N cm⁻¹) for each tillage plot was calculated using the cone index profiles. The average measured depth in each tillage plot was compared to the average predicted depth (d_ci) of a fixed specific cone penetration energy (P_ci). Static calibration tests on the depth sensors showed excellent linearity with coefficients of determination (R²) greater than 0.99. The results showed that, on the average, the changes in the depth measured with the rotary potentiometer were 44 and 68% of the changes in the depth measured with the ultrasonic proximity sensor while the disc coulter was passing across, or along, the tillage plots. This difference was primarily due to the sinkage of the tractor wheels. The depth measured with the ultrasonic sensor had significant correlation with both CI_0–30 cm and d_ci. This was partially due to the fact that a significantly high correlation (R² = 0.97) between the CI_0–30 cm and d_ci was observed, which was not expected and originated from the type of soil profiles present. The instrumented disc coulter is a low soil disturbance system and could be used as an inexpensive and simple sensor to obtain information about the mechanical condition of the soil for spot tillage or other management decisions.     

Spatial variability of Southeastern U.S. Coastal Plain soil physical properties: Implications for site-specific management

Our objectives were to describe the field-scale horizontal and vertical spatial variability of soil physical properties and their relations to soil map units in typical southeastern USA coastal plain soils, and to identify the soil properties, or clusters of properties, that defined most of the variability within the field. The study was conducted on a 12-ha field in Kinston, NC. A 1:2400 scale soil survey had delineated three soil map units in the field: Norfolk loamy sand, Goldsboro loamy sand, and Lynchburg sandy loam. These are representative of millions of hectares of farmland in the Coastal Plain of the southeastern USA. Sixty soil cores were taken to ∼ 1-m depth, sectioned into five depth increments, and analyzed for: soil texture as percentage sand, silt, and clay; soil water content (SWC) at − 33 and − 1500 kPa; plant available water (PAW); saturated hydraulic conductivity (Ksat); bulk density (BD); and total porosity. A penetrometer was used to measure cone index (CI) at each sample location. Variography, two mixed-model analyses, and principal components analysis were conducted. Results indicated that soil physical properties could be divided into two categories. The first category described the majority of the within-field variability and included particle size distribution (soil texture), SWC, PAW, and CI. These characteristics showed horizontal spatial structure that was captured by soil map units and especially by the division between sandy loams and finer loam soils. The second class of variables included BD, total porosity, and Ksat. These properties were not spatially correlated in the field and were unrelated to soil map unit. These findings support the hypothesis that coastal plain soil map units that delineate boundaries between sandy loams versus finer loam soils may be useful for developing management zones for site-specific crop management.     

Hydraulic energy indices reveal spatial dependence in a subtropical soil under maize crop in Southern Brazil

Soil physical quality (SPQ) assessment is an important part in the evaluation of soil use, management, and conservation. It can be assessed using several physical properties, hydraulic indices, and functions. Soils from tropical and temperate regions represent different physical behaviors, and the quantification of their physical properties is important to support soil evaluation and modelling. The objective of this study was to evaluate the SPQ in a subtropical field under maize crop cultivation according to its physical properties, hydraulic indices, and functions in an attempt to infer the spatial variability and to determine the behavior of soil physical structure across a spatial domain. Commonly used soil key physical variables, such as texture, bulk density, total porosity, saturated hydraulic conductivity, and organic carbon content, were measured in a regular grid with a soil sampling density of 30 points per hectare, covering an area of 0.5 ha. Saturated hydraulic conductivity varied strongly between subsamples and in the field, suggesting the heterogeneity of the soil structure regarding water drainage. The physical variables were combined with other indicators, which were based on the soil water retention curve and the pore size distribution (PSD) function. Correlation analysis was performed to verify the relationship between the measured and calculated variables, and some strong linear correlations were revealed, such as between aeration energy index and microporosity (r = 0.608) and water retention energy index with microporosity (r = 0.532) and with bulk density (r = 0.541). For most sampled locations, the shape and location parameters of PSD showed results outside of the optimum ranges, whereas the hydraulic energy indices and cumulative hydraulic energy functions presented values that were similar to those found for some tropical soils described in the literature. The spatial variability of these indices was described using semivariograms and kriged maps, indicating the variability of the SPQ in this field.