hPa Matric Potential Research Articles

Soil characteristics and root growth in a catena across and outside the wheel tracks for different slurry application systems

Compaction of subsoils is a steady increasing challenge as induced by the increase in weight as well as power of field machinery. We performed a field experiment with two different machinery systems used in slurry application. A tractor-trailer combination (TT) was compared to a tricycle-like self-propelled machine with traction on all three wheels (SP). The maximum wheel load was 88 and 114 kN for the TT and SP treatments, respectively. The wheels on the TT machinery were equipped with 710–800 mm wide tires inflated to 300 kPa, while the width and inflation pressure of the most heavily loaded tire on the SP machine were 1050 mm and 250 kPa. The machines were driven across the soil in three replicate plots in a randomized block experiment. Traffic took place at a soil water content close to field capacity. Immediately following the experimental traffic, cores of undisturbed soil were sampled at 30, 50 and 70 cm depths in a catena across the width of the wheel tracks. The cores were drained to − 100 hPa matric potential prior to measurement of air permeability. For both machinery systems, we measured significant differences in soil properties across and outside the width of the wheel tracks. The volume of pores > 30 µm, ε100, and/or the air permeability, ka, indicated a loosening of the soil at or closely outside the edge of the tire for both the SP and the TT treatment. The bulk density, ρb, was higher and ε100 lower for TT than SP at 30 and 50 cm depth for soil under the tire, at the tire edge and also some decimetres from the tire edge. Accordingly, ka was also lowest for TT, and the permeability normalized to a unit volume of ε100 indicated that the effect on pore permeability was due to the increased ε100 rather than to differences in tortuosity. The rooting depth of a spring barley crop was lower beneath and at the edge of the wheeled tracks than outside. This may reflect the effect of experimental traffic on soil properties although it cannot be excluded that the effect on the topsoil while establishing the barley crop has played a role as well. We consider that the results reflect the effects of repeated wheeling and/or consistent differences in traction and horizontal stresses. We encourage further studies of wheeling effects including the soil close to and outside the wheel tracks.

Characterisation of soil pore structure anisotropy caused by the growth of bio-subsoilers

Deep-rooting cover crops (tap-rooted plants and grasses) are suggested to enhance soil properties when used as bio-subsoilers in compacted soils. However, there is limited knowledge the impact of bio-subsoilers on anisotropy of the soil pore system of compacted zones. The objective of this study is to evaluate the impact of different bio-subsoiler species on the anisotropy of the soil pore characteristics of a compacted sandy loam subsoil. The experimental treatments consisted of spring barley as control and chicory, lucerne and tall fescue as bio-subsoilers. The soil samples were taken in both vertical and horizontal directions at 0.3–0.4 m depth. Air permeability (ka) and air-filled porosity (εa) were quantified at −10, −30, −50, −100 and −300 hPa matric potential, and X-ray computed tomography (CT) scans were obtained for samples drained at −100 hPa. Results showed no significant treatment effect (p < 0.05) on ka, the ratio of the non-Darcian to the Darcian air permeability (R), or the PO1 index (ka/εa) in either vertical or horizontal directions. However, the factor of anisotropy of ka, PO1 and R-ratio was significantly affected by chicory and lucerne (p < 0.05), indicating the creation of more vertically than horizontally oriented pores. The results from measured parameters were supported by CT image analysis as macropores connecting from end-to-end of the soil cores were mainly captured for vertically oriented samples by the CT scanner. Lucerne had significantly larger proportion of CT-macroporosity (∼50%) and CT-total branch length (∼40%) for vertically connected pore network as compared to spring barley (∼20% for both parameters). This study suggests that the impact of growing bio-subsoilers on the anisotropy of the soil pore system of the studied compacted subsoil is species-dependent.

Effects of bio-subsoilers on subsoil pore-system functionality: Case study with intact soil columns

Field traffic-induced subsoil compaction poses an escalating problem in relation to key soil functions because of the use of increasingly heavier machinery. Natural recovery from subsoil compaction is slow and therefore there is a need for measures such as bio-subsoilers to stimulate the recovery. This study thus investigated the influence of potential bio-subsoilers on pore functionality and morphology of a sandy loam soil, with an emphasis on the transition between the topsoil and compacted subsoil and on the compacted subsoil per se, in an intact soil column experiment. The control crop was spring barley, and potential bio-subsoilers included chicory, lucerne, fodder radish, tall fescue and kernza. The intact soil columns (Ø = 0.20 m, h = 0.50 m) were X-ray CT-scanned prior to and two years after crop establishment. Undisturbed subsamples (246 cm3) were collected one and two years after crop establishment in five replicates at 0.10, 0.25 and 0.35-m depth from the soil columns. The laboratory measurements and derived indices showed that after one-year non-restrictive air-filled porosity (εa = 0.13 m3 m−3) and relative gas diffusivity (Ds/Do = 0.0059) was reached by lucerne in the Transition layer (0.25 m). In the second year, lucerne also showed a significantly higher εa (0.09 m3 m−3), and a non-significantly higher Ds/Do (0.0029) compared to barley in the Compacted layer (0.30 m). Pore geometry indices derived from the relationship between εa, Ds/Do and air permeability indicated the formation of less tortuous and more connected small pores (at −100 hPa matric potential) throughout the Transition and Compacted layers by chicory, lucerne and tall fescue. A significantly higher CT-number of pores < 1 mm3 was found for lucerne and a higher CT-volume of pores < 1.5 mm in diameter for lucerne and chicory in the Compacted layer. Root number density was significantly higher for chicory, lucerne and tall fescue compared to barley from the first year, and also for kernza from the second year. Lucerne had the greatest subsoil compaction mitigation potential in our experiment in intact columns. Further studies in real field situations are needed.

Residual effects of compaction on the subsoil pore system—A functional perspective

AbstractSubsoil compaction caused by heavy traffic affects the soil pore system, resulting in long‐term damage to soil functions. The study contrasted two treatments from compaction experiments conducted at three different sites in Denmark: non‐trafficked control soil and soil subjected to four annual traffic events (2010–2013) with a wheel load of 58 to 78 kN. A cover crop of fodder radish (Raphanus sativus L.) was grown in half of the initial experimental plots after completion of the compaction treatments (2013 and onwards). In the spring of 2017, undisturbed soil cores were sampled at 0.3 and 0.5 m depth. The air‐filled porosity (εa), air permeability (ka) and gas diffusivity (Ds/Do) were quantified for samples equilibrated to –100 hPa matric potential. Soil pore structural indicators were estimated from the combination of εa, ka, and Ds/Do. The ratio of non‐Darcian to Darcian ka (R) was also used as a pore morphology indicator. For all sites and depths, compaction reduced εa, Ds/Do, ka‐Darcy, PO1 (ka‐Darcy/εa) and the effective radius ([(8ka‐Darcy)/Ds/Do]0.5) compared to control soil (p &lt; 0.05). The Buckingham‐X variable relating Ds/Do and εa tended to be smaller for compacted soil, significantly for one of the sites. Compacted soils were also characterised by a significantly smaller R‐ratio at high levels of ka‐Darcy (&gt; 32 μm2), but also by having a tendency for the R‐ratio to decrease rapidly with increasing pore air velocity compared to the control. The results reflect a compaction‐induced reduction in the number of marginal pores connected to large arterial pores, promoting a simple pore system formed by continuous vertical pores. The compaction effect was not affected by the cover crop. Neither natural recovery nor fodder radish‐induced mitigation of soil compaction was evident for the studied soils.

Subsoil shear strength – Measurements and prediction models based on readily available soil properties

Traffic-induced long-term damage to agricultural subsoils is a serious threat in modern, mechanized agriculture. Soil failure due to shear stresses is often not considered in soil compaction models but probably contributes significantly to soil structure deterioration. We measured shear strength for a total of 720 undisturbed soil cores collected at nine different locations in Denmark. Soil clay content ranged from 0.025 to 0.375 kg kg−1. We sampled at soil depths 0.3, 0.5 and 0.8 m and drained the soil cores to either -50, -100 or −300 hPa matric potential prior to shear tests. We used a shear annulus device to apply shear stress to the soil cores. The normal load, NL, in tests was either 30, 60, 90, 120, 150 or 180 kPa. Soil shear strength, τ, was estimated as the peak (maximum) shear stress at soil failure. Soil cohesion and angle of internal friction was estimated from linear regression of τ and NL. Multiple regression indicated that soil cohesion was well predicted by soil organic matter, clay content, the initial (preload) suction stress, σw, and soil bulk density. σw was calculated from water saturation and the matric potential. It was superior to quantity expressions of soil water (volumetric water content and water ratio) in explaining the trends in soil cohesion. Quantity of soil water should not be used in prediction models across soil types. The angle of internal friction correlated poorly to soil properties. Soil shear strength at a given NL could be well described by a model combining the above soil properties with the NL. This pedotransfer function predicted reasonably well the measured shear strength from two independent data sets. More studies are needed to evaluate a range of methodological aspects and for inclusion of more clay-holding soils. We encourage the inclusion of soil shear failure prediction in soil compaction models and suggest a specific procedure for this.

Influence of Static and Cyclic Loading on Mechanical and Hydraulic Properties of Soils with Different Textures and Matric Potentials

Mechanical, hydraulic, chemical or biological processes on various scales and intensities can alone or as coupled processes cause soil deformation. To quantify the effect of mechanical and hydraulic stresses on changes in soil functions, vertical displacement and pore water pressure were measured on repacked soils with three different textures (sand, silt loam and clay loam) at two matric potential values (−60 and −300 hPa). Two types of loading (static and cyclic loading) and three compaction levels (50, 100, and 200 kPa) were analyzed. In the case of static loading, the time‐dependent vertical displacement curves for all treatments under all compaction levels are similar in shape but different in scale. However, different situations of time‐dependent changes in pore water pressure were found due to different internal soil strength. An exponential change in pore water pressure as a function of vertical displacement was observed for treatments at −300 hPa matric potential during static loading regardless of vertical holes. In the case of cyclic loading, vertical displacement increased during loading but an elastic rebound was observed during unloading. Pore water pressure changed differently for different treatments. During cyclic loading, there was a linear relationship between the change in pore water pressure and vertical displacement for all treatments. The alternation of soil strength due to compaction is reflected by the effective stress. In our experiment, we found that, as the stress application increased, the effective stress decreased for wetter soils (−60 hPa), and increased for drier soils (−300 hPa). Hence, future soil deformation models need to include changes in pore water pressure as an important stress variable for unsaturated soils. Meanwhile, we found that the soil perforation technique may alleviate the negative effect of soil compaction by improving soils physical conditions.

Anisotropy of Transport Properties of a Remolded, Compacted Andisol

The transport properties of soil gas and water are known to be anisotropic in undisturbed field samples. However, little is known about the same in remolded samples that resemble the field condition after plowing. Moreover, the effect of added fresh organic matter (OM) on the anisotropy has attracted little attention. In this study, rice (Oryza sativa L.) husks and rice straw were added to a Japanese Andisol. These samples and those with no added fresh OM were remolded and compacted at 225 kPa. The saturated hydraulic conductivity (Kₛ), relative gas diffusivity [(Dₚ/D₀)₁₀₀], and air permeability (kₐ₁₀₀) at −100 hPa matric potential were measured in the vertical and horizontal directions. These values were then used to calculate the specific hydraulic conductivity (SKₛ), specific gas diffusivity (SD₁₀₀), and specific air permeability (Sₖₐ₁₀₀). The vertical (Dₚ/D₀)₁₀₀, kₐ₁₀₀, and Kₛ values were significantly higher than the horizontal values, suggesting anisotropic gas and water flows. Larger pores were arranged anisotropically based on the higher SD₁₀₀, Sₖₐ₁₀₀, and SKₛ in the vertical than in the horizontal direction. The rice husks improved the horizontal gas flow, and the rice straw impeded the vertical gas flow. Both of these OMs aligned horizontally and improved the horizontal water flow. The results of this study suggest that the anisotropy of the remolded, compacted samples may be different from that of the intact samples. Thus, further studies on a range of soil types using both disturbed and undisturbed samples need to be conducted to test the generality of this result.

Gas Diffusion, Non-Darcy Air Permeability, and Computed Tomography Images of a Clay Subsoil Affected by Compaction

Soil productivity and other soil functions are dependent on processes in the untilled subsoil. Undisturbed soil cores were collected at the 0.3- to 0.4-m depth from a heavy clay soil in Finland subjected to a single heavy traffic event by agricultural machinery three decades before sampling. Untrafficked control plots were used as a reference. Computed tomography (CT) scanning was performed on soil cores at a field-sampled field capacity water content. Gas diffusion and air permeability were measured when the soil cores were drained to −1000 hPa matric potential (air permeability also at −100 and −300 hPa). The air-filled pore space was measured with an air pycnometer and also calculated from mass balance and CT data. Gas diffusion and air permeability were also measured on a straight model tube and on autoclaved aerated concrete. The compaction treatment had not influenced soil total porosity but had significantly lowered the volume fraction of air-filled macropores at the matric potentials investigated. The compacted soil displayed significantly lower air permeability, while gas diffusivity was not affected. Our analyses indicate that this was due to a compaction-induced reduction in the cross-sectional area of vertical, arterial macropores and in the volume of marginal pores branching from these vertical pores. We observed non-Darcian air flow during air permeability measurements and thus suggest the use of a nonlinear regression technique based on measurements at several pressure differences to arrive at true Darcian air permeability. The tests on artificial materials supported the conclusions that the dominating pores in this clayey subsoil are nearly straight, vertical macropores.

Clay Dispersibility and Soil Friability—Testing the Soil Clay‐to‐Carbon Saturation Concept

Soil organic carbon (OC) influences clay dispersibility, which affects soil tilth conditions and the risk of vertical migration of clay colloids. No universal lower threshold of OC has been identified for satisfactory stabilization of soil structure. We tested the concept of clay saturation with OC as a predictor of clay dispersibility and soil friability. Soil was sampled 3 yr in a field varying in clay content (∼100 to ∼220 g kg−1 soil) and grown with different crop rotations. Clay dispersibility was measured after end‐over‐end shaking of field‐moist soil and 1‐ to 2‐mm sized aggregates either air‐dried or rewetted to −100 hPa matric potential. Tensile strength of 1‐ to 2‐, 2‐ to 4‐, 4‐ to 8‐, and 8‐ to 16‐mm air‐dried aggregates was calculated from their compressive strength, and soil friability estimated from the strength–volume relation. Crop rotation characteristics gave only minor effects on clay dispersibility and no detectable effects on soil friability. Dispersed clay correlated to soil content of clay, but the correlation increased if subtracting a fraction assumed protected by OC. This trend was less convincing for soil tensile strength and friability. Increased clay dispersibility and reduced soil friability for 1 yr of measurements could be ascribed to wet conditions for potato (Solanum tuberosum L.) harvest and tillage the preceding year. Literature data indicate soils' content of clay and silt (Fines20) to be a better predictor of specific surface area than clay. We conclude that a clay/OC ratio of 10 and a Fines20/OC ratio of 20 may serve as corresponding thresholds for clay dispersibility, the latter probably best reflecting organo‐mineral interactions of importance to the soil physical properties.

Mitigation of subsoil recompaction by light traffic and on-land ploughingI. Soil response

Abstract Mechanically loosened subsoil has been shown to be prone to recompaction. We addressed a sandy loam that had been mechanically loosened by a subsoiler to a depth of 35 cm in 1997 and again in 1998. Perennial grass/clover was grown with limited traffic intensity in 1999 and 2000. A recompaction experiment was conducted in 2001 and 2002 when the soil was grown with oat and winter wheat, respectively. Using the formerly loosened plots, on-land ploughing was compared with traditional mouldboard ploughing with the tractor wheels in the furrow. In addition, the loosened plots were either light-trafficked ( 30 μm and air permeability at −100 hPa matric potential. The results showed that on-land ploughing mitigated recompaction of the upper part of the formerly loosened subsoil. In contrast, only small differences in recompaction between heavy and light traffic were observed. The mitigation of subsoil recompaction was needed for the loosened soil to provide an upper subsoil with similar—not better—pore characteristics than the non-loosened soil in the conventional treatment. The structural conditions in the plough pan improved for the conventional treatment from 1998 to 2002 as indicated by an almost doubling in air permeability. This was interpreted as being related to the growing of grass/clover ley in 1999 and 2000 combined with a shift from traditional tillage and traffic to on-land ploughing and light traffic when growing cereals in 2001 and 2002. Results on root growth and crop yield are reported in an adjoining paper.

Mitigation of subsoil recompaction by light traffic and on-land ploughing: I. Soil response

Mechanically loosened subsoil has been shown to be prone to recompaction. We addressed a sandy loam that had been mechanically loosened by a subsoiler to a depth of 35 cm in 1997 and again in 1998. Perennial grass/clover was grown with limited traffic intensity in 1999 and 2000. A recompaction experiment was conducted in 2001 and 2002 when the soil was grown with oat and winter wheat, respectively. Using the formerly loosened plots, on-land ploughing was compared with traditional mouldboard ploughing with the tractor wheels in the furrow. In addition, the loosened plots were either light-trafficked (<6 Mg axle load and <100 kPa inflation pressure) or heavy-trafficked (10–18 Mg axle load and ∼200 kPa inflation pressure), respectively. Finally, the soil loosened by non-inversion deep tillage was referenced with a conventional ploughing–harrowing tillage system that never received the subsoil treatment. The conventional treatment was also grown with the grass/clover in 1999 and 2000. On-land ploughing and light traffic was applied in 2001 and 2002 instead of traditional ploughing and traffic for the conventional treatment. Penetration resistance and bulk density was recorded in the field. Undisturbed soil cores were taken in 1998, 1999 and 2002 from the 7–14, 18–27 and 25–30 cm layer and used for measuring total porosity, pores >30 μm and air permeability at −100 hPa matric potential. The results showed that on-land ploughing mitigated recompaction of the upper part of the formerly loosened subsoil. In contrast, only small differences in recompaction between heavy and light traffic were observed. The mitigation of subsoil recompaction was needed for the loosened soil to provide an upper subsoil with similar—not better—pore characteristics than the non-loosened soil in the conventional treatment. The structural conditions in the plough pan improved for the conventional treatment from 1998 to 2002 as indicated by an almost doubling in air permeability. This was interpreted as being related to the growing of grass/clover ley in 1999 and 2000 combined with a shift from traditional tillage and traffic to on-land ploughing and light traffic when growing cereals in 2001 and 2002. Results on root growth and crop yield are reported in an adjoining paper.

Tensile strength of soil cores in relation to aggregate strength, soil fragmentation and pore characteristics

Tensile failure of soil is desirable in tillage. Soil tensile strength estimates most often are obtained from compression tests of dry aggregates. As tillage is performed under moist conditions, it would be highly relevant to measure tensile strength at high water contents. Plough-layer soil was sampled in a compacted soil (PAC) and in a non-compacted reference soil (REF). Tensile strength was measured in a new direct tension test using undisturbed soil cores (4.45 cm in diameter and 5.00 cm in height) adjusted to either −50 or −100 hPa matric potential. The air-filled pore space, ε a, was determined from water retention measurements. Air permeability, K a, was determined at −30, −100 and −300 hPa matric potentials and from these measurements an index of pore organization (PO= K a/ ε a) was calculated. Soil behaviour in the field was evaluated at approximately −300 hPa matric potential by measuring soil strength using the torsional shear box method and soil fragmentation using a simple soil drop test. The direct tensile strength results showed that the PAC soil had significantly higher tensile strength than the REF soil (e.g. 3.2 and 2.0 kPa, respectively at −100 hPa matric potential). This finding was in accordance with the aggregate tensile strength results and also agreed well with soil fragmentation in the field (i.e. geometric mean diameter (GMD) equal to 38.7 and 14.2 mm, respectively, for PAC and REF samples dropped from 75 cm height). The tensile strengths of the soil cores were close to the predicted values determined from the aggregate tensile strength results. The energy input in the soil drop test (i.e. approximately 8.9 J kg −1 dry soil) was low in comparison with the energy input in tillage but high compared with the specific rupture energy of soil aggregates (e.g. 0.4 and 0.3 J kg −1, respectively for PAC and REF aggregates adjusted to −100 hPa matric potential). The relatively poor fragmentation in the soil drop test indicated that a substantial amount of the energy input was stored as volumetric strain energy and/or lost to processes such as plastic deformation. The tensile strength of soil cores was negatively correlated to the macroporosity of the soil, whereas the ease of soil fragmentation was positively correlated to PO.