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.     

Effects of additions of organic matter on the penetration resistance of glacial till for the entire water tension range

Hardsetting soil properties are undesirable in agricultural soils because they hamper crop production by limiting seedling emergence and root growth via increased mechanical soil resistance at low moisture contents. The objective of this study was to determine the effect of additions of organic matter on the penetration resistance of a hardsetting soil for the entire water tension range. Investigations were carried out on Saalian glacial till, which is used as a reclamation substrate in post-lignite-mining reclamation. Proportions of 0%, 1%, 2%, 3% and 4% by mass of organic matter (OM) were used. The remoulded samples were saturated under a constant load of 2.4 kPa to achieve bulk densities equivalent to a soil depth of 15–20 cm via water-induced consolidation. Subsequently, the mixtures were adjusted to water tensions between 10⁰ and 10⁷ hPa and penetrated using a small cone penetrometer. Compared to 0% OM, the addition of 1% OM led to a very small but significant (P < 0.01) increase in the bulk density, while between 1% and 4% OM bulk density was seen to decrease in a linear fashion. At moisture contents greater than field capacity, penetration resistance values were consistent with the observed changes in bulk density, leading to an increase in the samples containing 0–1% OM to critical values for root-growth and a decrease for samples containing 2% and more organic matter reaching to values non-critical for roots. At moisture contents smaller than field capacity, penetration resistance values were inversely related to the bulk density, supporting the concept that the type of organic matter added contributed to soil cohesion. Modeling the relation between water tension and penetration resistance using a sigmoidal equation showed a high consistency between the observed data and the model.     

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 tractor traffic on spatial variability of soil strength and water content in grass covered and cultivated sloping vineyard

Frequent machinery traffic on sloping vineyard influences spatial distribution of soil physical properties. Our objective was to assess the effects of crawler tractor traffic across the slope (20%) on spatial distribution of soil strength and water content of silt loam soil under controlled grass cover and conventionally cultivated vineyard. The experiment was situated on hillside vineyard (NW, Italy) arranged with rows crosswise the slope. The grass covered treatment included periodical mowing and chopping of herbs and the cultivated treatment—autumn ploughing (18 cm) and spring and summer rotary-hoeing in the vineyard inter-rows (2.7 m). A crawler tractor (2.82 Mg) was used at the same locations across the slope for all tillage and chemical operations. The measurements of soil bulk density, penetration resistance and volumetric water content were done in autumn (after vintage) within the sloping inter-row. The results were analyzed using classic statistics and geostatistics with and without trend. The highest variability was obtained for penetration resistance (CV 56.6%) and the lowest for bulk density (9.6%). In most cases, the semivariograms of the soil parameters were well described by spherical models. The semivariance parameters of all properties measured were influenced by trend. Three-dimensional (3D) maps well identified areas with the highest soil strength in lower crawler ruts being positioned in the upper side of vine row and successively lower strength in upper ruts situated on other side of the same row and inter-rut area. Higher strength in lower than upper ruts was induced by tractor's tilt and resulting higher ground contact pressure. Soil water content in both treatments was the lowest below the upper rut and increased in inter-rut and lower rut areas. The differences in the soil properties between the places within the inter-row were more pronounced in grass covered than in cultivated soil.     

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.     

The effects of wheel-induced soil compaction on anchorage strength and resistance to root lodging of winter barley (Hordeum vulgare L.)

Lodging is the permanent displacement of cereal stems from the vertical. Cereal plants growing in the edge rows next to both wheel tracks (‘tramlines’) and the gaps between experimental plots (‘inter-plot spaces’), which are traversed by farm vehicles during planting operations and agrochemical application, are less prone to lodge than plants growing elsewhere in fields and plots. Previous research has attributed this phenomenon to an increase in the stem strength of edge row plants, and hence their resistance to stem lodging, resulting from reduced competition between edge row plants for resources. However, this explanation gives no consideration to the anchorage strength of edge row plants, and hence their resistance to root lodging. Differences in soil and plant characteristics between the edge and centre rows of plots of winter barley (Hordeum vulgare L.) were examined on sand, silt and clay dominated soil types. Edge rows next to tramlines were investigated on the silt and clay soil types, whereas edge rows next to inter-plot spaces were investigated on the sand soil type. Edge row plants next to both tramlines and inter-plot spaces had 58.8% greater anchorage strength and hence resistance to root lodging than centre row plants. This was attributed to (1) greater soil compaction in the edge rows resulting from wheel traffic in the tramlines and inter-plot spaces, which increased the strength of the soil matrix surrounding the roots, and (2) greater plant root growth in the edge rows resulting from reduced competition. Bulk density, root plate spread and structural rooting depth were 19, 22, and 12% greater, respectively, in the edge rows of all soil types. The results suggest that in order to reduce lodging risk, energies should be directed towards identifying agricultural practices that optimise soil compaction in the seedbed without causing significant limitations to root growth.     

Effect of deep ploughing on the water status of highly and less compacted soils for coconut (Cocos nucifera L.) production in Sri Lanka

Soil compaction limits soil water availability which adversely affects coconut production in Sri Lanka. Field experiments were conducted in coconut (Cocos nucifera L.) plantations with highly and less compacted soils in the intermediate climatic zone of Sri Lanka. Soil physical properties of sixteen major soil series planted with coconut were evaluated to select the most suitable soil series to investigate the effect of deep ploughing on soil water conservation. Soil compaction and soil water retention with respect to deep ploughing were monitored during the dry and rainy seasons using cone penetrometer and neutron scattering techniques, respectively. Evaluation of soil physical properties showed that the range of mean values of bulk density (BD) and soil penetration resistance (SPR) in the surface soil (0–10 cm depth) of major soil series in coconut lands was from 1.38 ± 0.02 to 1.57 ± 0.07 g/cm³ and 55 ± 10 to 315 ± 16.4 N/cm² respectively. The total available water fraction increased with clay content of soil as a result of high micropores. However, due to soil compaction, ability of soils to conserve water and to remain aerated was low for those series. Deep ploughing during the rainy and dry periods in highly compacted soils (BD > 1.5 g/cm³ and SPR > 250 N/cm²) greatly increased conserved soil water in the profile, while in less compacted soils (BD < 1.5 g/cm³ and SPR < 250 N/cm²) conserved water content was adversely affected. Soil water retention in bare soils of both highly and less compacted soil series was higher than that of live grass-covered soil. Amount of water conserved in ploughed Andigama series with respect to bare soils and grass-covered treatments during the severe dry period was 10.4 and 16.9 cm/m, while water storage reduction in the same treatments with ploughed Madampe series was 6.55 and 5.45 cm/m respectively. In addition, deep ploughing even in the effective root zone with live grass-covered highly compacted soils around coconut tree was favorable for soil water retention compared to that of live grass-covered less compacted soils.     

Vulnerability of Bavarian silty loam soil to compaction under heavy wheel traffic: impacts of tillage method and soil water content

Soil compaction caused by traffic of heavy vehicles and machinery has become a problem of world-wide concern. The aims of this study were to evaluate and compare the changes in bulk density, soil strength, porosity, saturated hydraulic conductivity and air permeability during sugar beet (Beta vulgaris L.) harvesting on a typical Bavarian soil (Regosol) as well as to assess the most appropriate variable factors that fit with the effective controlling of subsequent compaction. The field experiments, measurements and laboratory testing were carried out in Freising, Germany. Two tillage systems (conventional plough tillage and reduced chisel tillage) were used in the experiments. The soil water contents were adjusted to 0.17 g g⁻¹ (w₁), 0.27 g g⁻¹ (w₂) and 0.35 g g⁻¹ (w₃).Taking the increase in bulk density, the decrease in air permeability and reduction of wide coarse pore size porosity (−6 kPa) into account, it seems that CT (ploughing to a depth of 0.25 m followed by two passes of rotary harrow to a depth 0.05 m) of plots were compacted to a depth of at least 0.25 m and at most 0.40 m in high soil water (w₃) conditions. The trends were similar for “CT w₁” (low soil water content) plots. However, it seems that “CT w₁” plots were less affected than “CT w₃” plots with regard to bulk density increases under partial load. In contrast, diminishments of wide coarse pores (−6 kPa) and narrow (tight) coarse pores (−30 kPa) were significantly higher in “CT w₁” plots down to 0.4 m. Among CT plots, the best physical properties were obtained at medium soil water (w₂) content. No significant increase in bulk density and no significant decrease in coarse pore size porosity and total porosity below 0.2 m were observed at medium soil water content. The soil water content seemed to be the most decisive factor.It is likely that, CS (chiselling to a depth of 0.13 m followed by two passes of rotary harrow to a depth 0.05 m) plots were less affected by traffic treatments than CT plots. Considering the proportion of coarse pore size porosity (structural porosity) and total porosity, no compaction effects below 0.3 m were found. Medium soil water content (w₂) provides better soil conditions after traffic with regard to wide coarse pore size porosity (−6 kPa), air permeability (at 6 and 30 kPa water suction), total porosity and bulk density. Proportion of wide coarse pores, air permeability and bulk density seems to be suitable parameters to detect soil compaction under the conditions tested.     

Effects of long-term organic and mineral fertilizers on bulk density and penetration resistance in semi-arid Mediterranean soil conditions

Soil aggregation is of great importance in agriculture due to its positive effect on soil physical properties, plant growth and the environment. A long-term (1996-2008) field experiment was performed to investigate the role of mycorrhizal inoculation and organic fertilizers on some of soil properties of Mediterranean soils (Typic Xerofluvent, Menzilat clay-loam soil). We applied a rotation with winter wheat (Triticum aestivum L.) and maize (Zea mays L.) as a second crop during the periods of 1996 and 2008. The study consisted of five experimental treatments; control, mineral fertilizer (300-60-150 kg N-P-K ha⁻¹), manure at 25 t ha⁻¹, compost at 25 t ha⁻¹ and mycorrhiza-inoculated compost at 10 t ha⁻¹ with three replicates. The highest organic matter content both at 0-15 cm and 15-30 cm soil depths were obtained with manure application, whereas mineral fertilizer application had no effect on organic matter accumulation. Manure, compost and mycorrhizal inoculation + compost application had 69%, 32% and 24% higher organic matter contents at 0-30 cm depth as compared to the control application. Organic applications had varying and important effects on aggregation indexes of soils. The greatest mean weight diameters (MWD) at 15-30 cm depth were obtained with manure, mycorrhiza-inoculated compost and compost applications, respectively. The decline in organic matter content of soils in control plots lead disintegration of aggregates demonstrated on significantly lower MWD values. The compost application resulted in occurring the lowest bulk densities at 0-15 and 15-30 cm soil depths, whereas the highest bulk density values were obtained with mineral fertilizer application. Measurements obtained in 2008 indicated that manure and compost applications did not cause any further increase in MWD at manure and compost receiving plots indicated reaching a steady state. However, compost with mycorrhizae application continued to significant increase (P < 0.05) in MWD values of soils. Organic applications significantly lowered the soil bulk density and penetration resistance. The lowest penetration resistance (PR) at 0-50 cm soil depth was obtained with mycorrhizal inoculated compost, and the highest PR was with control and mineral fertilizer applications. The results clearly revealed that mycorrhiza application along with organic fertilizers resulted in decreased bulk density and penetration resistance associated with an increase in organic matter and greater aggregate stability, indicated an improvement in soil structure.     

The effect of soil compaction on mole plough draught

Conventional and zero traffic systems were mole ploughed and effects on soil physical properties were compared. Draught of the plough operating at 550 mm depth was measured while it was winched across plots having a 5-year history of different traffic regimes. Results showed that the draught was reduced by about 18% on non-trafficked compared with conventionally-trafficked soil.

Cone resistance measurements, 1 month before and 3 months after mole ploughing, confirmed that the non-trafficked soil had significantly less strength to a depth of about 400 mm. Bulk density measured at 75 and 175 mm depth 1 month before mole ploughing indicated a similar trend, but clod and bulk densities at 125 mm and 350 mm depth 3 months later, failed to show any consistent differences between treatments.     

Quantifying seedbed condition using soil physical properties

Soil physical condition following tillage influences crop yield, but the desired condition cannot be adequately evaluated with current techniques. This study was conducted to determine a soil condition index (SCI) that could be used to select the type of implement needed to achieve an optimal seedbed with minimum energy input. Effects of bulk density, moisture content, and penetration resistance resulting from three tillage systems (no-till, chisel plow and moldboard plow), on the growth of corn (Zea mays L.) were studied. The experiment was conducted in Boone County, Ames, IA, on soils that are mostly Aquic Hapludolls, Typic Haplaquolls and Typic Hapludolls with slopes ranging from 0 to 5%. The results are from the 2000 season, which had normal weather conditions and yield levels for the Iowa state. The average corn grain yield at this site was 9.36 Mg/ha. At the V2 corn growth stage, the average dry biomass was 1.34 g per plant. The soil physical properties were normalized with respect to reference values and combined via multiple regression analysis against corn biomass at V2 stage into the SCI. Mean SCI values for the no-till, chisel and moldboard plow treatments were 0.86, 0.76, and 0.73, respectively, all with a standard error of 0.0127. The lower the SCI, the more optimum the soil physical conditions. An analysis of variance showed significant differences among mean SCI for each treatment (p-value=0.001). The use of the SCI could improve the tillage decision-making process in environments similar the one studied.     

Spatial variability of soil water content and mechanical resistance of Brazilian ferralsol

The spatial variability of mechanical resistance to penetration (PR) and gravimetric moisture (GM) was studied at a depth of 0–0.40 m, in a ferralsol cropped with corn, and under conventional tillage in Ilha Solteira, Brazil (latitude 20°17′S, and longitude 52°25′W). The purpose of this study was to analyse and to try explaining the spatial variability of the mentioned soil physical properties using geostatistics. Soil data was collected at points arranged on the nodes of a mesh with 97 points. Geostatistics was used to analyse the spatial variability of PR and GM at four depths: 0–0.1, 0.1–0.2, 0.2–0.3 and 0.3–0.4 m. PR showed a higher variability of data, with coefficients of variation of 52.39, 30.54, 16.91, and 15.18%, from the surface layers to the deepest layers. The values of the coefficients of variation for GM were lower: 9.99, 5.13, 5.59, and 5.69%. Correlation between GM and PR for the same soil layers was low. Penetration resistance showed spatial structure only in the 0.30–0.40 m layer, while gravimetric moisture showed spatial structure at all depths except for 0–0.10 m. All the models of fitted semivariograms were spherical and exponential, with ranges of 10–80 m. Data for the variable ‘GM’ in the 0.20–0.30 and 0.30–0.40 m layers revealed a trend in data attributed to the occurrence of subsurface water flow.     

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.     

Assessment of earthworm burrowing efficiency in compacted soil with a combination of morphological and soil physical measurements

Summary Column experiments were carried out to quantify the effect of earthworms on compacted soil. The earthworms (Lumbricus terrestris) were able to burrow into soil which was artificially compacted to a pore volume as low as 40%; they may also penetrate an artificial plough pan deep in the soil. The effect of the burrowing activity of Lumbricus terrestris was quantified by measuring hydraulic conductivities and infiltration rates through the whole soil column (19 cm wide, 40 cm long). Morphological parameters, mainly the vertically projected burrow depth, were correlated with the saturated hydraulic conductivity. The amount of casts deposited by Lumbricus terrestris on the soil surface increased with the degree of soil compaction. The bulk density of casts was always less than that of the original soil.     

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.     

A comparison between seasonal changes in soil water storage and penetration resistance under conventional and conservation tillage systems in Aragón

Low and extremely variable precipitations limit dryland crop production in the semi-arid areas of Aragón (NE Spain). These areas are also affected by high annual rates of topsoil losses by both wind and water erosion. A long-term experiment to determine the feasibility of conservation tillage in the main winter barley production areas of Aragón was initiated in 1989 at four locations, three on loam to silt loam soils (Xerollic Calciorthid) and one on a silty clay loam (Fluventic Ustochrept), receiving between 300 and 600 mm of average annual rainfall. In this study, we compared, under both continuous cropping and cereal-fallow rotation, the effects of conventional tillage (mouldboard plough) and two conservation tillage systems, reduced tillage (chisel plough) and no-tillage, on soil water content and penetration resistance during the first two growing seasons. Whereas reduced and conventionally tilled treatments generally had similar soil water content during the experimental period, the effects of no-tillage were inconsistent. No-tilled plots had from 26% less to 17% more stored soil water (0–80 cm) than conventional tilled plots at the beginning of the growing season. In contrast to the conventional and reduced tillage treatments, penetration resistances were between 2 and 4 MPa after sowing in most of the plough layer (0–40 cm) under no-tillage at all sites. Fallow efficiencies in moisture storage in the cereal-fallow rotation, when compared with the continuous cropping system, ranged from −8.7 to 12%. The highest efficiencies were recorded when the rainfall in the months close to primary tillage exceeded 100 mm. Since this event is very unlikely, long fallowing (9–10 months) appears to be an inefficient practice for water conservation under both conventional and conservation management. Our results suggest that, up to now, only reduced tillage could replace conventional tillage without adverse effects on soil water content and penetration resistance in the dryland cereal-growing areas of Aragón.     

Variability in topsoil texture and carbon content within soil map units and its implications in predicting soil water content for optimum workability

The ability to predict the timing of optimum soil workability depends on knowledge of the extent and structure of variability in main physical characteristics of the soil. Our objectives were to quantify the variability in texture and carbon content within soil map units in a small agriculture-dominated catchment in South-east Norway and to assess implications of variability in texture and carbon content on land management operations, using the predicted maximum water content for optimum workability as an example. Information from three different sources were used: a soil map (1:5000), a large sample grid (100 m spacing, 270 ha extent), and a small sample grid (10 m spacing, 2.25 ha extent). Readily available information on texture and organic matter content from the soil map was found to be of limited use for soil management due to broad textural classes together with deviations from the mapped main textural classes. There were significant differences in clay, silt and sand content between the different soil textural classes on the soil map. Statistical distributions within soil map units were generally either positively or negatively skewed and the coefficient of variation was intermediate, 15–50%. Most of the variation in both grids was spatially correlated. The large grid was dominated by a patchy structure, whilst the small grid showed a systematic trend with a gradual transition indicating fuzzy boundaries between map units in this catchment. The effective range for texture was 16 times larger in the large grid. Implications of variability in texture and carbon content on land management operations were assessed for the maximum water content for optimum workability (Wopt), predicted using pedotransfer functions. Wopt was usually in the same range as the water content at–100 kPa matric potential, indicating that considerable evaporation in addition to drainage is required for obtaining workable conditions in the field. The time required for obtaining the water content was estimated to about 5 days, which is longer than an average length of periods without precipitation in the area, median 3.7 days. Wopt predicted from the soil map deviated strongly from Wopt predicted from the sample grids. Comparing estimates of Wopt from the large grid with measurements in the small grid showed differences corresponding to ±2–3 days of evaporation.     

Effect of dairy slurry application on soil compaction and corn (Zea mays L.) yield in Southern Wisconsin

As stocking rates on Wisconsin dairy farms continue to increase, one possible nutrient management solution is to haul slurry to nearby grain farmer's fields. Although the nutrient and soil building benefits of manure are well known, many grain farmers are hesitant to apply manure on their fields due to potential soil compaction. Studies were initiated to evaluate the effects of tanker-applied slurry on soil compaction and corn (Zea mays L.) yield. An on-station trial was established to address the issues of compaction caused by manure tankers, repeated traffic associated with field headlands, and the possible ameliorating effect of manure itself on corn yield. In addition, 15 replicated on-farm trials were established to evaluate the impact of single pass manure applications on soil compaction and yield. These predominately fall applications were conducted when the host farmer felt that the soil would support tanker traffic. Due to its portability and instrument sensitivity, compactness was evaluated with a data-logging hand held penetrometer.Results from the on-station trial indicate that multiple passes did increase compactness above single-pass traffic and the check. The slurry itself did not attenuate the effect of traffic on soil compaction, nor on yield. Despite yield reductions estimated from in-track samples in both years of 6% (one-pass traffic) and 22% (six-pass traffic) in this study, whole plot corn yields were not reduced due to compaction. The on-farm trials indicated that manure application technique does affect compaction patterns; with broadcast application resulting in less area trafficked by the tanker than injection application, and therefore less area compacted. The narrower gauge truck tires used at some sites led to significantly higher penetrometer readings compared to the control, but this was not the case at sites with wider tractor tires. As in the on-station work, although compaction led to higher penetrometer readings, whole plot corn yields in compacted plots were not adversely affected compared to the control. These results suggest that, in the first year after slurry application, on predominantly prairie derived soils; well-timed applications of dairy slurry do not cause extensive soil compaction nor a reduction in corn yields. This study did not look at the potential residual effects that may positively (>soil organic matter) or negatively (residual soil compaction) impact subsequent crops.     

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.     

Influence of the water regime on denitrification and aerobic respiration in soil

Summary The influence of soil moisture on denitrification and aerobic respiration was studied in a mull rendzina soil. N₂O formation did not occur below –30 kPa matric water potential (_m), above 0.28 air-filled porosity (a) and below 0.55 fractional water saturation (_v/PV volumetric water content/total pore volume). Half maximum rates of N₂O production and O₂ consumption were obtained between _m = –1.2 and –12 kPa,a = 0.05 and 0.23, and _v/PV = 0.63 and 0.92. No oxygen consumption was measured at _v/PC 1.17. O₂ uptake and denitrification occurred simultaneously arounda = 0.10 (at _m = –10 kPa and _v/PV = 0.81) at mean rates of 3.5 µl O₂ and 0.3 µl N₂ h^–1g^–1 soil. Undisturbed, field-moist soil saturated with nitrate solution showed constant consumption and production rates, respectively, of 0.6 µl O and 0.22 µl N₂O h^–1g^–1 soil, whereas the rates of air-dried remoistened soil were at least 10 times these values. The highest rates obtained in remoistened soil amended with glucose and nitrate were 130 µl O₂ and 27 µl N₂O h^–1g^–1 soil.