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Lime and phosphorus (P) applications are common agricultural management practices. Our aim was to quantify the effects of long-term application practices on root growth and abundance of arbuscular mycorrhizal fungi (AMF) under field conditions. We assessed the effects of lime and P fertilizer applications on barley yield, root growth and AMF abundance in 2016. Treatments were no, low, medium and high liming rate corresponding to application of 0, 4, 8 and 12 Mg lime ha −1 every 5–9 years since 1942 combined with no or yearly application of 15.6 kg P ha −1 since 1944. At harvest, grain yield, root intensity (core-break) and AMF abundance at different soil depths were estimated. Root development was monitored during early growth with minirhizotrons in treatments receiving low, medium and high liming rates and P fertilization. A quadratic model relating grain yield to liming rate estimated yields to peak at 6.4 Mg lime ha −1 with yields of 4.2 and 3.2 Mg grain ha −1 with and without P fertilization, respectively. Low and medium liming rates resulted in greater AMF abundance, especially in the no P treatments. During early growth in P-fertilized treatments, 77% and 65% more roots developed in the soil profile when treated with medium and high liming rate, respectively, compared to low liming rate. We conclude that long-term application of lime in soils receiving yearly P fertilization improved conditions for root growth in soil layers below 30 cm, but at the high liming rate, this did not translate into higher yield.

The natural and cover crop (CC)-induced anisotropy of subsoil pore characteristics is important in assessments of soil compaction but, until recently, has received limited attention. This study aims to quantify the anisotropy of soil pore characteristics and hydraulic conductivity in subsoils subjected to compaction and CC treatments in a split-plot field experiment on temperate sandy loam soils. The main factor was ±compaction and the split-plot factor was ±CC with fodder radish (Raphanus sativus L.). The compacted plots were heavily trafficked for 4 yr (2010–2013). After 4 yr under CC (2013–2016), core samples were collected at 0.3 m. The samples were taken vertically and horizontally to quantify the anisotropy of air-filled porosity (ε a) and air permeability (k a), saturated hydraulic conductivity (k sat), and bulk density (ρ b). The results showed isotropic behavior for ρ b and ε a and significant anisotropic behavior (p a, pore geometry index (PO1) (k a/ε a), ratio of non-Darcian to Darcian k a (R-ratio), and k sat. For parameters with significant anisotropy, higher values occurred vertically than horizontally for all compaction –CC combinations, except for R-ratio. This indicates that pre-existing vertical continuous pores dominated the pore system in the control subsoil. A nonsignificant trend of higher values of k a, PO1, and k sat in the +CC than in the compacted –CC plots suggest that CC could contribute to the formation of vertical biopores. Including an autumn CC in rotation with a summer cereal crop for a longer period may significantly affect the anisotropy of the soil pore and hydraulic properties.

The least limiting water range (LLWR) has been used as a soil structural quality indicator for identifying in-season water dynamics, yet studies focusing on its use for detecting in-season water stresses and their effect on crop response on severely compacted subsoils are scarce. The objectives of this study were, therefore, to examine the in-season water dynamics on a tile-drained soil with compacted subsoil in the light of two different approaches for calculating LLWR (standard LLWR by da Silva et al. [1994] and refined LLWR by Pulido-Moncada & Munkholm [2019]) and to evaluate the crop response to aboveground and belowground conditions. Information on LLWRs was obtained from soil sampling in the most contrasting treatments of a compaction experiment: with and without compaction. In-season water dynamics were measured from 2017 to 2019. The refined LLWR approach defined a wider range of water content nonlimiting for plant growth compared with the da Silva et al. approach. Compaction affected the LLWRs (p

Nitrous oxide (N2O) is the contributor to agricultural greenhouse gas emissions with the highest warming global potential. It is widely recognised that traffic and animal-induced compaction can lead to an increased potential for N2O emissions by decreasing soil oxygen supply. The extent to which the spatial and temporal variability of N2O emissions can be explained by soil compaction is unclear. This review aims to comprehensively discuss soil compaction effects on N2O emissions, and to understand how compaction may promote N2O emission hotspots and hot moments. An impact factor of N2O emissions due to compaction was calculated for each selected study; compaction effects were evaluated separately for croplands, grasslands and forest lands. Topsoil compaction was found to increase N2O emissions by 1.3 to 42 times across sites and land uses. Large impact factors were especially reported for cropland and grassland soils when topsoil compaction—induced by field traffic and/or grazing—is combined with nitrogen input from fertiliser or urine. Little is known about the contribution of subsoil compaction to N2O emissions. Water-filled pore space is the most common water metric used to explain N2O emission variability, but gas diffusivity is a parameter with higher prediction potential. Microbial community composition may be less critical than the soil environment for N2O emissions, and there is a need for comprehensive studies on association between environmental drivers and soil compaction. Lack of knowledge about the interacting factors causing N2O accumulation in compacted soils, at different degrees of compactness and across different spatial scales, limits the identification of high-risk areas and development of efficient mitigation strategies. Soil compaction mitigation strategies that aim to loosen the soil and recover pore system functionality, in combination with other agricultural management practices to regulate N2O emission, should be evaluated for their effectiveness across different agro-climatic conditions and scales.

Wheel tracks can lower topsoil infiltrability and increase water ponding in agricultural fields. A seedbed harrow mounted with two goosefeet tine points, the eradicators, was used to investigate track loosening at different depths on a sandy loam soil as a way of mitigating compaction effects and reducing the risk of pesticide transport to surface waters. Loosening strongly affected air permeability and steady-state infiltration. The agro-ecological system model Daisy was used to simulate the effects of soil structural and hydraulic changes on pesticide leaching to subsurface drain lines over a 332-year period. Measured properties of the topsoil were combined with a representative subsoil and weather series and with realistic management scenarios. The loads of pesticide in the drains for 3 months after loosening were calculated for each year, and the risk was defined as the 90th percentile of the load. We focused on three different herbicides used in sugar beet cultivation in spring: glyphosate, metamitron and phenmedipham. Our simulations showed that for all pesticides loosening could lower the risk by 10% on average for a 3-m working width, and the tracks contribution to the risk by 34%, for all drain spacing and working width settings. Wheeling did not affect the risk but this result was sensitive to the parameterization of the hydraulic conductivity in the compacted soil layer, showing potentially higher risk under certain conditions. These results showed that wheel track loosening is an effective strategy for reducing the risk of surface water contamination from pesticides used in agriculture.

Subsoil 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 (ε a), air permeability (k a) and gas diffusivity (D s/D o) were quantified for samples equilibrated to –100 hPa matric potential. Soil pore structural indicators were estimated from the combination of ε a, k a, and D s/D o. The ratio of non-Darcian to Darcian k a (R) was also used as a pore morphology indicator. For all sites and depths, compaction reduced ε a, D s/D o, k a-Darcy, PO1 (k a-Darcy/ε a) and the effective radius ([(8k a-Darcy)/D s/D o] 0.5) compared to control soil (p s/D o and ε a 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 k a-Darcy (> 32 μm 2), 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.

In inversion tillage systems, the mouldboard plough is fundamental for producing a desirable seedbed. The desired ploughing quality is achieved when the plough layer is inverted homogeneously. This is, however, difficult to obtain in the main-headland intersection zone where the plough is lowered and elevated, as ploughed and unploughed triangles are formed. This results in zones where the soil is inverted twice, which may result in poor residue and weed incorporation and a poor seedbed quality. The design of the three-point linkage-attached mouldboard plough has not changed since the 1950s, but the number of furrows has increased, which has increased the size of the aforementioned triangles. A novel ploughing system was introduced to meet these headland challenges, where each plough section can be lowered and elevated independently. The aim of this study was to evaluate the effects of using a section-controlled mouldboard plough. Two similarly designed, randomized, field plot experiments were conducted on two different soil types (sandy loam and loamy sand) on a stubble field and grass field. The study showed that the section-controlled plough reduced the main-headland overlap area by ~98%. The results of a range of soil physical properties measurements and seedbed quality analyses showed that the section-controlled plough created a homogeneous loosened seedbed quality, improving the incorporation of crop residues and leaving fewer residues on the soil surface. Furthermore, the section-controlled plough showed additional benefits, for example wedge operations and visual line marking.

Reduced use of fossil energy by removing cereal straw for bioenergy potentially threatens soil organic carbon (SOC) storage. Straw incorporation plays a key role for SOC storage in cereal-based cropping systems, but the use of cover crops (CCs) may compensate for straw removal. However, assessing the SOC sequestration potential of management changes requires field experiments with long continued treatments and frequent soil sampling. Based on a field experiment initiated in 1981 on a sandy loam soil at Askov Experimental Station (Denmark), we examined the effect of annual additions of spring barley straw (0, 4, 8 and 12 Mg ha−1) and undersown ryegrass CC on SOC storage in the 0- to 20-cm layer. The effect of straw incorporation and CC on SOC was additive. At steady-state conditions, the SOC stock based on equivalent soil mass (SOC stockFM) increased 3.9, 6.7 and 9.3 Mg C ha−1 after annual incorporation of 4, 8 and 12 Mg straw ha−1, respectively. The ryegrass CC increased SOC stockFM by 3.3 Mg C ha−1 and thus almost compensated for removal of 4 Mg straw ha−1. An asymptotic regression model best described the temporal changes in SOC and showed that the effect of straw incorporation and ryegrass CC on SOC sequestration peaks after 10–15 years when a new equilibrium between input and output of C is reached. Clearly, reliable assessments of SOC sequestration potentials following changes in management require field experiments with frequent soil sampling until reaching steady-state conditions.

There is a need for improved knowledge of the limits to the available water range for root growth in the subsoil. The objective of this study was to recalculate the upper and lower limits of the least limiting water range (LLWR) concept by using respectively the air-filled porosity (ϵa) at which 0.005 of the relative gas diffusivity (Ds/Do) is reached and readily available water (RAW). The refined upper limit estimates the variation in ea related to pore connectivity and the refined lower limit expresses the boundary at which plants suffer physiological water stress. This study was based on soil sampled in compaction trials on two sandy loam soils. Soil samples were taken from plots with no compaction (Control), and compaction with 78 kN (M8) and 58 kN (M6) wheel loads with multiple wheel passes. The soil cores were analyzed for ϵa, Ds/Do, bulk density (rb) and penetration resistance (PR). Heavy farm machinery impact of M8 and M6 led to subsoil compaction up to depth of 0.5 to 0.7 m for the soils under study. The subsoil structure was affected by compaction across depths with the decrease in ϵa (~33-46%) and Ds/Do (~37-61%) and increase in rb (~4-8%) and PR (~40-50%, at -100 hPa at 30-cm depth). The refined LLWR showed a wider water range compared to the original approach. We anticipate that the refined LLWR well reflects the limiting soil physical conditions for root growth for the studied soils, but validation by combined soil physical and plant growth measurements is needed.

Traffic in agricultural fields with very high wheel loads imposes a risk of severe structural damage deep into the subsoil. However, there is a paucity of studies quantifying these effects. This study focuses on heavy traffic-induced changes in soil structure for a sandy loam soil in a temperate region. The treatments included no compaction (Control), compaction with ∼3 Mg (M3) and ∼8 Mg (M8) wheel loads with multiple (4–5) wheel passes, and compaction with a single-pass wheel load of ∼12 Mg (S12). The compaction treatments were replicated four consecutive years. Subsoil structural quality was evaluated visually by the SubVESS method, and soil pore characteristics were quantified for minimally disturbed soil cores sampled at 30, 50, 70 and 90 cm depth two years after the end of the experiment. Our results indicate that M8 significantly affected soil structural properties to >50 cm depth in terms of reduced subsoil structural quality, air-filled pore space, air permeability, gas diffusivity, pore volume and increased bulk density. Results also showed that the degree of compactness was ≥95% for M8 at 30 and 50 cm depth. Even though a pre-existing dense soil matrix was described in the studied soil, results confirmed that high wheel loads may cause significant subsoil compaction at >50 cm depth. Surprisingly, the S12 treatment did not show marked signs of decreasing structural quality at depth. Thus, our results indicate that primarily traffic applying multiple passes with high wheel loads compromises soil structure at depth. The S12 results further suggest the need to investigate the influence of factors other than wheel load and inflation pressure on the risk of subsoil compaction.

Clay is a basic soil constituent that governs many soil properties including pore characteristics, which in turn control a range of crucial soil properties and functions. This study explored the relationships between aggregate strength and soil pore structure characteristics for a range of clay contents along a natural gradient, and investigated the influence of clay content and matric potential on soil workability. Soil samples (100 cm3 soil cores and bulk soil) were sampled from the 5–15 cm depth in four locations in an arable field near Lerbjerg, Denmark, which ranged in clay content from 0.119 to 0.446 kg kg−1. The soil cores were drained to five matric potentials in the range −10 to −1000 hPa to obtain volumetric water content and air-filled porosity (εa). We measured air permeability and gas diffusivity and calculated tortuosity (τ) and pore organization at −100 hPa. Soil aggregates were obtained from the bulk soil and their tensile strength (Y) and specific rupture energy (Esp) were determined at −100, −300, −1000 hPa and at air-dry state. Y and Esp increased with decreasing matric potential and with increasing clay content, which consequently affected soil workability. εa at −100 hPa or air-filled macroporosity were negatively related to Y and Esp, and positively to the range of water contents for tillage (ΔθRANGE). This indicates the importance of air-filled macroporosity for soil crumbling during tillage. The findings of the study suggest that management practices that increase soil macroporosity can potentially decrease aggregate strength and increase ΔθRANGE and hence improve soil workability. We suggest that in fields with highly variable soils operations should be scheduled at periods when the range of water contents for tillage are suitable for the whole field to reduce the risk of soil structural damage induced by tillage.

Vehicle traffic induced soil compaction has negative effects on soil functions and ecosystems which may cause the degradation of farmland. This study investigated the magnitude and distribution of soil stress under the tracked and tired vehicles to explore the penitential of using rubber track instead of tire to reduce the subsoil compaction. The field experiment in this study included three replicates and was conducted on a sandy loam soil. Vertical and horizontal soil stress were measured under the centerlines of the rubber track and tire at a depth of 0.35m by using embedded transducers. The SoilFlex model was applied to simulate vertical and horizontal stress in the soil profile. Unevenly distributed vertical and horizontal stress were observed under the tire and rubber track. The vertical stress was characterized by one peak under the tire and several peaks under each of track wheels and rollers. The horizontal stress exhibited peaks before and after the tire and each of track wheels and rollers. The measured maximum stress was significantly higher under the tire than under the rubber track: that is, vertical and horizontal stress were approximately 3.4 and 2.0 times higher, respectively. This finding indicated that using rubber track maybe an effective method to reduce soil stress when compared with the tire, and was more effective in reducing the vertical stress than horizontal stress. Improving the uniformity of stress distribution under the track is the key to improve the ability of tracked vehicle to mitigate soil compaction.

Highlights • The rate of change in pore size distribution was quantified. • It was faster to degrade than to restore a complex soil structure. • Introducing grassland in degraded soil may induce densification in the short-term., Changes in land use affect the pore size distribution (PSD) of the soil, and hence important soil functions such as gas exchange, water availability and plant growth. The objective of this study was to investigate potentially damaging and restorative soil management practices on soil pore structure. We quantified the rate of change in PSD six years after changes in land use taking advantage of the Highfield land-use change experiment at Rothamsted Research. This experiment includes short-term soil degradation and restoration scenarios established simultaneously within long-term contrasting treatments that had reached steady-state equilibrium. The land-use change scenarios comprised conversion to grassland of previously arable or bare fallow soil, and conversion of grassland to arable and bare fallow soils. In the laboratory, we exposed intact soil cores (100 cm3) to matric potentials ranging from −10 hPa to -1.5 MPa. Based on equivalent soil mass, the plant available water capacity decreased after conversion from grassland, whereas no change was observed after conversion to grassland. Structural void ratio decreased after termination of grassland and introduction of grassland in bare fallow soil, while no change was seen when changing arable to grassland. Consequently, it was faster to degrade than to restore a complex soil structure. The study illustrates that introducing grassland in degraded soil may result in short-term increase in soil density.

Maintaining good soil structural stability is an important element in agricultural sustainability. Incorporation of cereal straw and use of cover crops improve soil structural stability but the long-term individual and interactive effects of these management practices is poorly understood. We examined the impact of four rates of straw incorporation in spring barley (0, 4, 8 and 12 Mg ha-1, annually) combined with a ryegrass cover crop undersown in the barley in spring. Soil was sampled after four decades of treatments in the Askov (Denmark) straw incorporation experiment situated on a sandy loam with 12% clay. We assessed clay dispersibility measured on two macro-aggregate size fractions (ClayDis 1-2 mm and ClayDis 8-16 mm), wet-stability of aggregates, and clay dispersibility of P=0.068), when cover crops were included. The soils receiving 8 and 12 Mg straw ha-1 had a significantly lower ClayDis 1-2 mm and ClayDis 8-16 mm than soil with straw removal. Inclusion of a ryegrass cover crop did not affect ClayDis 1-2 mm, but decreased ClayDis 8-16 mm marginally (P=0.054) and decreased clay dispersibility of field-moist soil. The results suggest, that binding agents from cover crops such as roots increase stabilization of large macro-aggregates, while the increased stability due to straw incorporation was related to SOC irrespective of aggregate size.

This study, based on vegetable production fields, combined soil quality assessed by three approaches (qualitatively by farmers, semi-quantitatively by a researcher and quantitatively by laboratory analyses) with the aim of improving the integration of the different approaches. We interviewed 79 peri-urban vegetable growers in two communities within the Sunyani Municipality, Ghana. Eight of the farmers were selected to participate in the farmer-based assessment of soil quality. Further, visual evaluation of soil quality was conducted by the researcher, followed by laboratory analyses of soil properties to corroborate the farmers’ assessment of good and poor soils in their fields. Results showed that the farmers used locally-defined characteristics to describe the physical, biological and crop performance indicators of soil quality. There was, in general, limited use and understanding of soil chemical properties as indicators of soil quality. The farmers’ perception on soil quality of their fields largely influenced their decision on the type of crops they cultivate, and application regimes of mineral fertilizers. Results from the visual evaluation by the researcher agreed in some respects with the farmers’ assessment of soil quality of the good and poor soils in their respective farms. Laboratory analyses did not show specific trends for the content of chemical properties for neither good nor poor soils. The study highlighted that none of the approaches of soil quality assessment is necessarily superior in and of itself. We emphasized the need for integration to capitalize on the strengths of each approach, enhance mutual learning between farmers and soil scientists, build the capacity of farmers, and improve their decision on soil use for agricultural production.

Assessments of changes in soil organic carbon (SOC) stocks depend heavily on reliable values of SOC content obtained by automated high‐temperature C analysers. However, historical as well as current research often relies on indirect SOC estimates such as loss‐on‐ignition (LOI). In this study, we revisit the conversion of LOI to SOC using soil from two long‐term agricultural field experiments and one arable field with different contents of SOC, clay and particles

An adequate and uniform seeding depth is crucial for the homogeneous development of a crop, as it affects timeof emergence and germination rate. The considerable depth variations observed during seeding operations - evenfor modern seed drills - are mainly caused by variability in soil resistance acting on the drill coulters, whichgenerates unwanted vibrations and, consequently, a non-uniform seed placement. Therefore, a proof-of-conceptdynamic coulter depth control system for a low-cost seed drill was developed and studied in a field experiment.The performance of the active control system was evaluated for the working speeds of 4, 8 and 12 km h−1, bytesting uniformity and accuracy of the coulter depth in relation to the target depth of −30 mm. The evaluationwas based on coulter depth measurements, obtained by coulter position sensors combined with ultrasonic soilsurface sensors. Mean coulter depth offsets of 3.5, 5.3 and 6.3mm to the target were registered for the depthcontrol system, compared to 8.0, 9.1 and 11.0mm without the control system for 4, 8 and 12 km h−1, respectively.However, speed did not affect the coulter depth significantly. The control system optimised coulter depthaccuracy by 15.2% and at 95% confidence interval it corresponded to an absolute reduction in the coulter depthconfidence span of 10.4 mm. The spatial variability, due to variation in soil mechanical properties was found tobe±8 mm, across the blocks for the standard drill and when activating the coulter depth control system thisvariability was reduced to±2 mm. The system with the active control system operated more accurately at anoperational speed of 12 km h−1 than at 4 km h−1 without the activated control system.

Reducing wheel loads has long been the key advice to reduce the risk of subsoil deformation, but this disregards other machinery-soil interactions such as the effects of traction and repeated wheeling. We conducted a field experiment to disentangle the effects of traction (described as drawbar pull) and repeated wheeling on the contact area characteristics and stresses in the upper subsoil. Experimental traffic comprised a tractor (static load 11 Mg) with activated 4 WD towing a trailer (static load 17 Mg or 24 Mg) and took place on a sandy loam with stubble of oats at soil water content near field capacity. Measurements included drawbar pull, contact area and vertical contact stress, and horizontal (in the driving direction) and vertical stresses at ∼0.36 m depth in an undisturbed soil profile. Drawbar pull was significantly higher (9.13 kN compared to 6.46 kN) for the high trailer load, but no differences were observed between the two steering modes. The contact area of the tractor’s rear tyres increased for the high drawbar pull but with no significant differences in length or width. The maximum vertical stress in the tractor rear tyre’s contact area then tended to be lower, despite the increase in the dynamic wheel load (from 3.5 Mg to 4.1 Mg). Whereas high drawbar pull improved the stress distribution in the driving direction, the effect across the tyre was complicated. We found evidence of different tyre-soil interaction for tyres with and without traction. No significant effect of repeated wheeling with a single towed tyre (5.5 Mg) on the contact area characteristics were found. For the towed tyres, horizontal soil stress increased linearly with vertical soil stress, and we suggest that this increase is intrinsic to the soil (at these experimental conditions). Traction does then influence the ratio of horizontal to vertical stress. The results confirm the importance of considering dynamic aspects of field traffic such as traction and dynamic wheel load.

Subsoil compaction is one of the majorcauses of land degradationworldwide and therefore a major threat tofuture crop productivity. The objective of this contribution was to evaluate the effects of compactiontreatments on soil structure based on the numerical visual evaluation of subsoil structure (SubVESS)method and on quantitative measurements of soil pore characteristics. The effect of soil compaction wasevaluated using treatments from a compaction experiment initiated in 2010 at Research Centre Flakkebjerg,Denmark, on a sandy loam soil usingfive levels of compaction. In this study we used i) non-compactedreference, ii) Treatment M3, where soil was subjected to multiple passes (five wheel passes per compactionevent annually) of a tractor-trailer combination with max. wheel load of3 Mg, and iii) M8, with multiplepasses (four wheel passes per compaction event annually) of a tractor-trailer combination with max.wheel load of8 Mg. The tire inflation pressure was generally above the recommended pressure in order tomimic the inflation pressures commonly used in practice. The treatments were applied track-by-track in thespring of 2010–2013 when the soil water content was close tofield capacity. Spring barley (Hordeum vulgareL.) was established every year after a shallow secondary tillage to0.05 m depth to loosen the uppermostlayer. Sampling andfield evaluation were done on May 7, 2014, i.e. after four years of compactiontreatments (2010–2013) and one year of recovery. The soil profiles were evaluated at the same time as soilcores were sampled at 0.3, 0.5 and 0.7 m depth. In the laboratory, we measured water content, totalporosity, air-filled porosity (ea), air permeability (ka) and calculated pore organization indices (PO1 = ka/eaand PO2 = ka/ea2) on the soil cores. We estimated the blocked air-filled porosity and pore continuity indexfrom the relationship between air permeability and air-filled porosity for30 to300 hPa matricpotentials. Assessment using the SubVESS method showed a marked effect of the M8 treatment on soilstructural quality down to0.65 m depth, but the effects of the M3 were not significantly differentfrom the control at any depth. This was confirmed by the laboratory-measured data, which showed thatthe M8 treatment drastically reduced total porosity, air-filled porosity, air permeability, pore sizedistribution, pore tortuosity and continuity, especially at 0.3 and 0.5 m depths.Detailed measurements of the anisotropy of soil pore characteristics at 0.3–0.4 m depth showed that forPO2 (pore size distribution) and blocked air-filled porosity the control soil was significantly anisotropic.Although compaction with the8 Mg wheel load affected the vertically and horizontally-oriented poresdifferently, it did not significantly affect the anisotropy of the different pore characteristics. Our resultsshowed that in general, there was a good agreement between thefield and laboratory methods and thus,the two can be combined to evaluate the effects of compaction in the subsoil. Subsoil compaction is one of the major causes of land degradationworldwide and therefore a major threat to future crop productivity. The objective of this contribution was to evaluate the effects of compaction treatments on soil structure based on the numerical visual evaluation of subsoil structure (SubVESS) method and on quantitative measurements of soil pore characteristics. The effect of soil compaction was evaluated using treatments from a compaction experiment initiated in 2010 at Research Centre Flakkebjerg, Denmark, on a sandy loam soil using five levels of compaction. In this study we used i) non-compacted reference, ii) Treatment M3, where soil was subjected to multiple passes (five wheel passes per compaction event annually) of a tractor-trailer combination with max. wheel load of 3 Mg, and iii) M8, with multiple passes (four wheel passes per compaction event annually) of a tractor-trailer combination with max. wheel load of 8 Mg. The tire inflation pressure was generally above the recommended pressure in order to mimic the inflation pressures commonly used inpractice. The treatments were applied track-by-track in the spring of 2010–2013 when the soil water content was close to field capacity. Spring barley (Hordeum vulgare L.) was established every year after a shallow secondary tillage to 0.05 m depth to loosen the uppermost layer. Sampling and field evaluation were done on May 7, 2014, i.e. after four years of compaction treatments (2010–2013) and one year of recovery. The soil profiles were evaluated at the same time as soil cores were sampled at 0.3, 0.5 and 0.7 m depth. In the laboratory, we measured water content, total porosity, air-filled porosity (ea), air permeability (ka) and calculated pore organization indices (PO1 = ka/ea and PO2 = ka/ea 2) on the soil cores. We estimated the blocked air-filled porosity and pore continuity index from the relationship between air permeability and air-filled porosity for 30 to 300 hPa matric potentials. Assessment using the SubVESS method showed a marked effect of the M8 treatment on soil structural quality down to 0.65 m depth, but the effects of the M3 were not significantly different from the control at any depth. This was confirmed by the laboratory-measured data, which showed that the M8 treatment drastically reduced total porosity, air-filled porosity, air permeability, pore size distribution, pore tortuosity and continuity, especially at 0.3 and 0.5 m depths. Detailed measurements of the anisotropy of soil pore characteristics at 0.3–0.4 m depth showed that for PO2 (pore size distribution) and blocked air-filled porosity the control soil was significantly anisotropic. Although compaction with the 8 Mg wheel load affected the vertically and horizontally-oriented pores differently, it did not significantly affect the anisotropy of the different pore characteristics. Our results showed that in general, there was a good agreement between the field and laboratory methods and thus, the two can be combined to evaluate the effects of compaction in the subsoil.

An even and correct depth placement of seeds is crucial for uniform crop germination and for obtaining the desired agricultural yield. On state-of-the-art seed drills, the coulter down pressure is set manually by static springs or heavy weights, which entails that the coulter’s seeding depth reacts to variations in soil resistance. The aim of the study was to develop and test an instrumentation concept installed on a low-cost, lightweight, three meter wide, single-disc seed drill, for on-the-go measurements of spatial depth distributions of individual coulters under real field conditions. A field experiment was carried out to measure individual coulter depths at three different operational speeds. The targeted seeding depth was −30 mm but shallower mean coulter depths were obtained and the depth decreased slightly – although not significantly – with increasing speed, i.e. to −22.1, −20.9 and −19.0 mm for 4, 8, and 12 km h −1 , respectively. The coulter depths ranged between −60 mm (below the surface) and even above surface at all speeds, but the variation tended to decrease with decreasing speed. However, soil resistance influenced coulter depth as indicated by a significant block effect. The mean coulter depth varied up to ±5 mm between the blocks. In addition, significant depth variations between the individual coulters were found. The mean depths varied between −14.2 and −25.9 mm for the eleven coulters. The mean shallowest coulter depth (−14.2 mm) was measured for the coulter running in the wheel track of the tractor. The power spectral densities (distribution) of the coulter depth oscillation frequencies showed that the majority of oscillations occurred below 0.5 Hz without any natural vibration frequency. The study concluded that the instrumentation concept was functional for on-the-go spatial coulter depth measurements.

Poor seedbed preparation may cause low yields and poor resource utilization. Therefore, novel sensor technology for seedbed quality evaluation is strongly needed to make sure good growing conditions are achieved efficiently. The objective of this study was to quantify the effect of the front leveling tines and the tillage depth of a cultivator on soil surface roughness and aggregate size distribution. Field tests were performed with a seedbed cultivator, using 5 different leveling intensities and 2 cultivation depths. Using a laser range scanner, the soil surface was mapped before, during and after cultivation. These surface maps were analyzed using Granulometry to estimate aggregate size distribution in the seedbed. Mean Weight Diameter (MWD) and Geometric Mean Diameter (GMD) were calculated based on these aggregate size estimates. Additionally, roughness was calculated based on the surface profiles produced by the laser range scanner. The leveling intensity showed a statistically significant effect on the MWD, GMD and roughness, however, the cultivation depth showed no evidence to suggest a significant effect. Finally, roughness calculated during and after cultivation had a good correlation, which shows that it is possible to use the laser range scanner for roughness measurements during the tillage operation.

Conservation agriculture (CA) has been suggested as a means of makingintensification of agriculture sustainable. The purpose of this study was tounderstand and quantify long-term individual and combined effects of keyconservation practices on soil physical properties and topsoil C content.Field experiments were conducted in 11- to 12-yr-old experiments on twoDanish sandy loams at Foulum and Flakkebjerg. Three crop rotations/residuemanagement treatments were compared and tillage was included as a splitplotfactor. The tillage systems were moldboard plowing to a depth of 20 cm(MP), direct drilling (D) and harrowing to a depth of 8 to 10 cm (H). Soilsampling and in-field measurements were performed in autumn 2013 andspring 2014. In the field, soil structure was visually evaluated and penetrationresistance (PR) measured. Soil C, wet stability (clay dispersion and wetaggregate stability), and soil strength were determined in the laboratory. TheMP soil had a uniform soil organic carbon (SOC) content in the 0- to 20-cmdepth of topsoil, whereas H and D resulted in SOC accumulation near thesoil surface. Plowing resulted in the best visually assessed topsoil structureand had the lowest PR. However, H and D in combination with residueretention gave the best structural stability. Residue retention alleviatednegative effects of reduced tillage on PR and improved wet stability in theMP treatment at the Foulum site. Clay and SOC correlated well with soilphysical parameters, confirming their important role in soil structureformation and stabilization. Our study showed benefits of combining key CAelements, although longer-term studies are most likely needed to reveal thefull potential.

Soil tilth is a dynamic and multifaceted concept that refers to the suitability of a soil for planting and growing crops. A soil with good tilth is “usually loose, friable and well granulated”; a condition that can also be described as the soil’s having a good “self-mulching” ability. On the other hand soils with poor tilth are usually dense (compacted), with hard, blocky, or massive structural characteristics. Poor soil tilth is generally associated with compaction, induced by wheel traffic, animal trampling, and/or to natural soil consolidation (i.e., so-called hard-setting behavior). The soil-tilth concept dates back to the early days of arable farming and has been addressed in soil-science literature since the 1920s. Soil tilth is generally associated with soil’s physical properties and processes rather than the more holistic concepts of soil quality and soil health. Improved soil tilth has been associated with deep and intensive tillage, as those practices were traditionally considered the primary method for creating a suitable soil condition for plant growth. Therefore, for millennia there has been a strong focus both in practice and in research on developing tillage tools that create suitable growing conditions for different crops, soil types, and climatic conditions. Deep and intensive tillage may be appropriate for producing a good, short-term tilth, but may also lead to severe long-term degradation of the soil structure. The failure of methods relying on physical manipulation as means of sustaining good tilth has increased the recognition given to the important role that soil biota have in soil-structure formation and stabilization. Soil biology has only received substantial attention in soil science during the last few decades. One result of this is that this knowledge is now being used to optimize soil management through strategies such as more diverse rotations, cover crops, and crop-residue management, with these being applied either as single management components or more preferably as part of an integrated system (i.e., either conservation agriculture or organic farming).Traditionally, farmers have evaluated soil tilth qualitatively in the field. However, a number of quantitative or semi-quantitative procedures for assessing soil tilth has been developed over the last 80 years. These procedures vary from simply determining soil cloddiness to more detailed evaluations whereby soil’s physical properties (e.g., porosity, strength, and aggregate characteristics) are combined with its consistency and organic-matter measurements in soil-tilth indices. Semi-quantitative visual soil-evaluation methods have also been developed for field evaluation of soil tilth, and are now used in many countries worldwide.

Annual removal of crop residues may lead to depletion of soil organic carbon and soil degradation. Gasification biochar (GB), the carbon-rich byproduct of gasification of biomass such as straw and wood chips, may be used for maintaining the soil organic carbon content and counteract soil degradation if applied to soil. This study investigated the effect of straw removal and GB addition on soil biological, chemical and physical properties in a 22-months soil incubation study with a temperate sandy loam soil. Soil application of wood and straw GB (WGB and SGB) resulted in very low CO2 emissions, confirming the stability of the material against microbial degradation. Both GBs increased total organic carbon, cation exchange capacity and pH of the soil. The application of SGB and WGB did not affect aggregate stability, whereas SGB did not affect and WGB decreased clay dispersibility. In contrast, the addition of straw resulted in a high soil respiration rate, and about 80% of the added carbon was respired at the end of the incubation. However, the addition of straw increased aggregate stability and decreased clay dispersibility. Results from Fourier transformed infrared photoacoustic spectroscopy revealed a lower content of O-H and aliphatic C-H together with a higher content of aromatic groups in soils amended with GB compared to soils amended with straw. This suggested that the improvement in aggregate stability in straw treatments could be related to microbial derived aliphatics and simple sugars, and that increased stability against microbial degradation in biochar amended soil was related to highly condensed aromatic groups. Addition of nutrients (N, P and S) together with straw resulted in higher soil respiration compared to the straw treatment, but did not cause differences in other soil processes.Results from this study suggest that GB has a potential for increasing soil carbon sequestration, CEC and pH. However, the straw turnover process plays a vital role for aggregate stability and clay dispersibility. Thus, annual straw removal on loamy soil needs to be considered with care in order to avoid soil degradation and risk of soil compaction or erosion. Annual removal of crop residues may lead to depletion of soil organic carbon and soil degradation. Gasificationbiochar (GB), the carbon-rich byproduct of gasification of biomass such as straw and wood chips, may be usedfor maintaining the soil organic carbon content and counteract soil degradation if applied to soil. This study investigatedthe effect of straw removal and GB addition on soil biological, chemical and physical properties in a22-months soil incubation study with a temperate sandy loam soil.Soil application of wood and strawGB (WGB and SGB) resulted in very lowCO2 emissions, confirming the stabilityof thematerial against microbial degradation. Both GBs increased total organic carbon, cation exchange capacityand pH of the soil. The application of SGB and WGB did not affect aggregate stability, whereas SGB did notaffect and WGB decreased clay dispersibility. In contrast, the addition of straw resulted in a high soil respirationrate, and about 80% of the added carbonwas respired at the end of the incubation. However, the addition of strawincreased aggregate stability and decreased clay dispersibility. Results from Fourier transformed infrared photoacousticspectroscopy revealed a lower content of O–H and aliphatic C–H together with a higher content ofaromatic groups in soils amended with GB compared to soils amended with straw. This suggested that the improvementin aggregate stability in straw treatments could be related to microbial derived aliphatics and simplesugars, and that increased stability against microbial degradation in biochar amended soil was related to highlycondensed aromatic groups. Addition of nutrients (N, P and S) together with straw resulted in higher soil respirationcompared to the straw treatment, but did not cause differences in other soil processes.Results fromthis study suggest that GB has a potential for increasing soil carbon sequestration, CEC and pH. However,the straw turnover process plays a vital role for aggregate stability and clay dispersibility. Thus, annual straw removalon loamy soil needs to be considered with care in order to avoid soil degradation and risk of soil compaction or erosion.

Soil aggregate properties, such as strength, shape and porosity, influence a range of essential soil functions and there is a need for more detailed understanding of the effect of soil management on these aggregate properties. There is also a need for improved knowledge on the link between aggregate and bulk soil properties. The objective of this study was to quantify the long-term effect of rotation and tillage on aggregate shape, strength and pore characteristics, to evaluate the influence of aggregate shape and pore characteristics on aggregate strength and soil friability and to correlate aggregate properties to bulk soil properties. Soil core samples were taken in spring 2010 from the long-term rotation and tillage trial (initiated in 1980) at the University of Guelph, Canada. The rotations included were continuous corn (R1) and a diverse rotation (R6), and the tillage treatments were mouldboard ploughing (MP) and no-tillage (NT). The soil cores were exposed to a drop shatter test and air-dried before separation into different size fractions. Ten aggregates from the4-9.2 mm size fraction per core sample (i.e. 320 in all) were X-ray micro-CT scanned and the size, shape and porosity determined using image analysis. Subsequently, aggregate tensile strength was determined in an indirect tension test. For the aggregates, rotation had a more pronounced than tillage treatment on the different properties. The diverse rotation resulted in higher aggregate total porosity and more rounded aggregates than the continuous corn rotation. Surprisingly, there was no treatment effect on X-ray micro-CT resolvable porosities. Aggregate strength decreased with both total and X-ray micro-CT resolvable porosity even though the correlation was weak. A stronger correlation was found to aggregate sphericity although only around 10% of the variation in tensile strength could be explained by this property. Our study highlights that caution must be taken when trying to predict aggregate strength from general aggregate characteristics. For both bulk soil and aggregates, the R6-MP had highest and R1-NT lowest porosity. However, tillage had strongest effect on bulk soil porosity whereas aggregate total porosity was only affected by rotation. We conclude that the scale of observation is important when evaluating the influence of soil management. A strong correlation was found between aggregate strength and pore characteristics and soil fragmentation in a drop shatter test, i.e. 55% of the variation could be explained. Our study indicates therefore that bulk soil fragmentation behavior can be predicted from aggregate characteristics. Soil aggregate properties, such as strength, shape and porosity, influence a range of essential soil functions and there is a need for more detailed understanding of the effect of soil management on these aggregate properties. There is also a need for improved knowledge on the link between aggregate and bulk soil properties. The objectives of this study were to quantify the long-term effect of rotation and tillage on aggregate shape, strength and pore characteristics, to evaluate the influence of aggregate shape and pore characteristics on aggregate strength and soil friability and to correlate aggregate properties to bulk soil properties. Soil core samples were taken inspring 2010 from the long-term rotation and tillage trial (initiated in 1980) at the University of Guelph, Canada. The rotations includedwere continuous corn (R1) and a diverse rotation (R6), and the tillage treatments were mouldboard ploughing (MP) and no-tillage (NT). The soil coreswere exposed to a drop shatter test and airdried before separation into different size fractions. Ten aggregates fromthe 4–9.2mmsize fraction per core sample (i.e. 320 in all)were X-ray micro-CT scanned. The size, shape and porosity of the aggregateswere determined using image analysis with 40 μm voxel size. Subsequently, aggregate tensile strength was determined in an indirect tension test. Rotation had a more pronounced effect than tillage treatment on the different aggregate properties. The diverse rotation resulted in higher aggregate total porosity and more rounded aggregates than the continuous corn rotation. Surprisingly, therewas no treatment effect on X-ray micro-CT resolvable porosities. Aggregate strength decreased with both total and X-ray micro-CT resolvable porosity even though the correlations were weak. Significant correlation was also found to aggregate sphericity although only around 10% of the variation in tensile strength could be explained by this property. Our study highlights that caution must be taken when trying to predict aggregate strength from general aggregate characteristics. For both bulk soil and aggregates, the R6-MP had highest and R1-NT lowest porosity. Tillage had strongest effect on bulk soil porosity, whereas aggregate total porosity was only affected by rotation. Our results suggest that the scale of observation is importantwhen evaluating the influence of soil management. A strong correlationwas found between aggregate strength and pore characteristics and soil fragmentation in a drop shatter test, i.e. 55% of the variation could be explained. Our study indicates therefore that bulk soil fragmentation behaviour can be predicted fromaggregate characteristics. It needs to be highlighted that our results are based on one long-term experiment on a silt loam soil. The results need to be verified for soils with different soil types, climates and management histories.

As building blocks of soil structure, aggregates shape the physical environment for soil biota, control available surfaces for nutrients, and are key in regulating soil functions including carbon and nutrient cycling, water flow and storage, and soil-atmosphere gas exchange. Abiotic and biotic processes play essential roles in aggregate formation. Consequently, soil management has a strong impact on characteristics and properties of aggregates. In this study, we quantified the impact of compaction and post-compaction management with and without vegetation (permanent grass and bare soil, respectively) on soil aggregate density, tensile strength, fractal dimension, friability and on pore structure within aggregates. Based on theory from material science, we hypothesized that the Weibull modulus, which is a measure tensile strength variability, reveals information on the pore structure within aggregates. Our results show that compaction had a stronger effect on aggregate density and tensile strength than vegetation. Soil friability was highest in the permanent grass control treatment and lowest in the compacted bare soil. Quantification of intra-aggregate pore structure of these contrasting treatments revealed that aggregates from the permanent grass control treatment had a more complex pore structure and longer pores than aggregates from the compacted bare soil. As a result, we show that friability is driven by intra-aggregate pore length rather than aggregate density. Weibull modulus was strongly correlated with intra-aggregate pore structural features (pore-length distribution, number of pore branches and junctions per volume). However, the relationships between Weibull modulus and pore characteristics were treatment specific. The temporal evolution of Weibull modulus could be a helpful metric to better understand how different pore features recover from compaction over time.

Subsoil compaction is a major threat to soil quality. The use of bio-subsoilers has been proposed as a mitigation practice. There is, however, a paucity of knowledge on the effects of potential bio-subsoiling crops in alleviating severely compacted subsoil. X-ray Computed Tomography (CT) was used to assess the changes caused by different crops in the pore network of a severely compacted subsoil. The potential bio-subsoilers, chicory, lucerne, radish and tall fescue, with spring barley as reference, were grown for one year in undisturbed soil columns (Ø = 0.20 m, h = 0.50 m) with soil originating from a heavily compacted soil after mechanical impact. Soil columns were X-ray CT-scanned before and after the experiment. CT-pore soil characteristics were quantified by image analysis. Crop treatments affected the soil porosity differently on the studied soil. Radish and tall fescue did not show a significant impact on CT-derived pore characteristics at any depth. In the compacted layer, the macropore density, the branches number, and the number of pores (for volume sizes of < 100 mm3 and diameter ≤ 1.5 mm) were larger for chicory and lucerne compared to barley (P < 0.05). Chicory and Lucerne appear to contribute to the development of a large number of complex-shaped pores. Differences in the CT-derived pore network indicate that chicory and lucerne are likely to perform better than the other crops when used as bio-subsoilers by creating a larger, more connected and complex pore network. Longer-term growth is needed to obtain a marked loosening effect in the compacted layer.

Most models describing soil water retention imply a uni-modal pore-size distribution (PSD). The uni-modal model presented by van Genuchten (termed vanG) is widely used although double-exponential models (termed Dex) implying a bi-modal PSD may better reflect reality. We tested the ability of vanG and Dex models to represent water retention in sandy top- and subsoils with different texture, in soil with contrasting management (Highfield), and in soil exposed to different tillage (Flakkebjerg). Soils were subjected to matric potentials from –10 hPa to –1.5 MPa. For all soils, the bi-modal Dex model showed a better fit to water retention data than the uni-modal vanG model. Neither of the models worked well for highly sorted soils. The vanG model gave a poorer fit for topsoils than for subsoils because of a more pronounced bi-modality of the PSD in topsoils caused by larger soil organic carbon (SOC) content and tillage. For Highfield soils, the root mean squared error (RMSE) of the vanG fit increased from long-term bare fallow (low C content, intensive tillage) to permanent grass (high C content, no tillage) reflecting a more distinct bi-modality of the PSD for well-structured soils. We conclude that uni-modal models should be used with great caution when describing effects of texture and management on PSD and that bi-modal models may provide a better fit to PSD.

The nitrate (N) present in soil at the end of autumn is prone to leach during winter and spring in temperate climates if not taken up by plants. In Denmark catch crops are used as a regulatory tool to reduce N leaching and therefore a shift from winter cereals to spring cereals with catch crops has occurred. Quantitative data is missing on N leaching of a catch crop compared to a winter cereal in a conventional cereal-based cropping system. The aim of the study was to investigate whether fodder radish ( Raphanus sativus L.) (FR) would be more efficient than winter wheat ( Triticum aestivum L.) (WW) at depleting the soil of mineral nitrogen (N min ) before winter. A secondary aim was to study the agreement between three different root measuring methods: root wash (RW), core break (CB) and minirhizotron (MR). The third aim of the was to correlate the N uptake of FR and WW with RLD. An experiment was made to see if and how root growth was affected by the minirhizotron tube. The experiments were conducted on a Danish sandy loam soil. From September to November the amount of soil N min decreased from 49 kg N ha −1 to 14 kg N ha −1 under FR and increased from 28 kg N ha −1 to 44 kg N ha −1 under WW. A test of correlations between root measuring methods showed that there was only a significant positive correlation for FR between CB and RW ( R 2 = 0.77) and for WW between CB and MR ( R 2 = 0.26). We conclude that FR is more efficient than WW at tightening the N cycle in the autumn by a greater depletion of soil N min . From the comparison of root methods, we conclude that root growth in the subsoil was overestimated for FR by the MR method. This was due to preferential root growth along the MR tubes. The root densities/intensities measured with the three root measuring methods were not directly comparable.

Algorithms are presented which schedule sequential agricultural operations.A location's readiness for the operation is included in the schedule decision.Optimisation algorithms are employed to produce improvements upon initial schedules.A real life scenario is present to show the applicability of the scheduling algorithm.Extensive testing of the optimisation algorithm using numerical examples is also presented. Due to the increasing demands of a growing world population, farmers are tasked with increasing productivity without increasing resource use. This can be achieved by deploying resources in a more efficient manner and by ensuring operations will produce maximum results. However when contemplating the problem of scheduling multiple machines, executing multiple operations at multiple fields, it is a cumbersome task to find an optimal solution without the aid of a decision support tool. A field's readiness, in terms of its trafficability and workability, is an important factor to consider when scheduling operations. Executing operations on an unready field can lead to additional operational costs, such as drying grain harvested at an unready moisture level or removing subsoil compaction caused by machinery driving on unready soils. To ensure a schedule is actionable, the fields' readiness must be accounted for by the scheduling process, further increasing the complexity of the problem to be solved.In this paper, a novel scheduling algorithm is presented which creates individual machine work plans for multiple machines to execute multiple consecutive operations at multiple fields, accounting for the field's readiness for the specified operation. Two optimisation algorithms are utilised to find near optimal solutions of predefined scenarios, a standard Tabu Search and a modified Tabu Search, producing optimised work schedules. The functionality of the optimisation algorithms are tested both for a real world scenario and a large set of numerical examples. The results of the optimisation methods are assessed by their computational time and their relative error compared to the optimal solution, where available. The combination of the presented scheduling algorithm and optimisation algorithms could be used either as a stand-alone tool for farm managers and agricultural contractors as part of their logistic planning, or as a component of an overall Farm Management Information System.

Crop management factors, such as tillage, rotation, and straw retention, need to be long-term to allow conclusions on effects on crop yields, nitrate leaching, and carbon sequestration. In 2002, two field experiments, each including four cash crop rotations, were established on soils with 9 and 15% clay, under temperate, coastal climate conditions. Direct drilling and harrowing to twodifferent depths were compared to plowing with respect to yield, nitrate N leaching, and carbon sequestration. For comparison of yields across rotations, grain and seed dry matter yields for each crop were converted to grain equivalents (GE). Leaching was compared to yields by calculating yield-scaled leaching (YSL, g N kg−1 GE), and N balances were calculated as the N input inmanure minus the N output in products removed from the fields. Direct drilling reduced yields, but no effect on leaching was found. Straw retention did not significantly increase yields, nor did it reduce leaching, while fodder radish (Raphanus sativus L.) as a catch crop was capable of reducing nitrate leaching to a low level. Thus, YSL of winter wheat (Triticum aestivum L.) was higherthan for spring barley (Hordeum vulgare L.) grown after fodder radish due to the efficient catch crop. Soil organic carbon (SOC) did not increase significantly after 7 yr of straw incorporation or noninversion tillage. There was no correlation between N balances calculated for each growing season and N leachingmeasured in the following percolation period.

Exact estimates of soil clay (

Management strategies like organic matter (OM) amendment and mechanical energy inputs are known to influence the soil system. A long-term (13–14-year) field experiment was used to evaluate the effects of these management strategies on soil structural formation, structural stabilization and soil tilth of a sandy loam soil in Denmark. OM was applied as manure and by retention of plant residues (ORG) to be compared with plots dressed only with mineral fertilizer (MIN). The soils were rotovated (ROT), compacted (PAC) or left undisturbed (REF) as split-plot treatments in the main plots with OM management over two years prior to sampling. In two consecutive years, undisturbed soil samples were collected from the 6 to 13 cm soil layer in the field grown with winter wheat to assess soil organic carbon (C) fractions (total organic C, polysaccharide C and microbial biomass C), total organic C and polysaccharide C of 1–2 mm macro-aggregates, bulk density, hyphal length, aggregate stability, clay dispersibility, aggregate tensile strength, direct tensile strength and shear strength. The ease of fragmentation and the torsional shear strength of soil were measured in the field as well. OM application increased all soil organic C fractions. Response patterns of organic C fractions in aggregates were the same patterns as for whole-soil. Polysaccharide C appeared to be an important agent in the aggregation process. The effect of microbial C and fungal hyphae on the aggregation process was not clear. Extensive tillage and traffic produced unfavourable tilth conditions in terms of a greater degree of clay dispersion, lower aggregate stability, higher soil tensile strength and poorer soil fragmentation. OM affected soil reaction to compressive and tensile stresses applied at differing initial bulk densities. The results also indicated the profitability of supplementing the classical laboratory analysis with in situ measurements to better evaluate management effects on soil structure.

Soil structure plays a key role in the ability of soil to fulfil essential soil functions and services in relation to e.g. root growth, gas and water transport and organic matter turnover. However, soils are not a very easy object to study as they are highly complex and opaque to the human eye. Traditionally, they have been studied using invasive or destructive techniques. The advantage of using X-ray computed tomography (CT) in soil morphology is that it enables non-destructive quantification of soil structure in three dimensions (3D). The prime objective of the present study was to characterize soil aggregate properties such as volume, surface area and sphericity based on 3D images. We tested the methods on aggregates from different treatments and quantified changes over time. A total of 32 collections of aggregates, enclosed in mesocosms, were incubated in soil to follow the structural changes over time for different treatments. The aggregates had different origins (tillage and no-till), and the mesocosms were incubated in soil grown with and without plants. The aggregates were not segmented into single aggregates, but considered as an aggregate cluster. To describe the aggregate cluster shape changes, several morphometric parameters were quantified such as aggregate cluster volume, sphericity, and the number of inter-aggregate pores. These parameters were measured from 3D images produced non-destructively by an X-ray CT scanner at three different times: (a) the initial state before incubation, (b) after summer incubation, and (c) after summer and winter incubation.The macroporosity of the aggregate clusters decreased after incubating the samples during summer and during summer and winter. The plant treatment curbed the decrease in porosity over time. The volume and surface area of the aggregate clusters increased with time irrespective of tillage and plant treatments. The sphericity decreased with time. The structure model index (SMI) was not sensitive to effects of time and treatments. This means that with time the aggregate clusters became less round and more elongated, but they kept a rough surface.The 3D CT scanner image analysis based on quantification of morphometric parameters has the potential to provide new fundamental insight into soil aggregate formation and the effect of different treatments despite the limitations of its spatial resolution. In our study, time was the most important factor affecting the changes in aggregate shape and structure. Contrary to what we had expected, there was no significant effect of seven years of different tillage treatments on the measured parameters at any time of measurement. The segmentation method used to separate aggregate from air is of primary importance, particularly when it comes to the quantification of aggregate morphometric parameters.

Soil compaction has increased during recent years due to the traffic with increasingly heavier machinery. We evaluated the effect of soil compaction on soil penetration resistance, rooting depth, light interception, radiation-use efficiency (RUE) and yield of three different cultivars of winter wheat (Triticum aestivum L.). On loamy sand two compaction treatments (PAC-1 and PAC-2) and a no compaction reference treatment (REF) were applied. PAC-1 was intended to affect primarily the subsoil whereas PAC-2 was intended to affect primarily the topsoil. PAC-2 showed the highest and REF the lowest penetration resistance in the topsoil, respectively. In the subsoil both compaction treatments showed higher penetration resistances than REF. In comparison with REF, the compaction treatments decreased the estimated effective rooting depth by ca. 10, 20 and 50 cm in the three winter wheat cultivars tested, equivalent to decreases in the available soil water in the root zone of up to ca. 90 mm. These differences indicate some genetic variation in the ability of cultivars to penetrate compacted soil, although the interaction between compaction treatment and cultivar was not significant. Due to almost sufficient precipitation, the impairment of root penetration resulted in a minor yield decrease of ca. 9% of grain yield in PAC-2 and ca. 8% of total dry matter (DM) in both compaction treatments. The latter was attributed to decreases in interception of light and to efficiency of light energy conversion into biomass. The RUE was positively correlated with an estimated effective rooting depth across cultivars, while DM yield was not. This correlation probably was a result of restrictions on stomatal opening mediated by drought stress and abscisic acid produced in the root system in response to occasional soil drying. Root-sourced signals, triggered in a direct response to soil compaction, may have contributed. Soil compaction has increased during recent years due to the traffic with increasingly heavier machinery. We evaluated the effect of soil compaction on soil penetration resistance, rooting depth, light interception, radiation-use efficiency (RUE) and yield of three different cultivars of winter wheat (Triticum aestivum L.). On loamy sand two compaction treatments (PAC-1 and PAC-2) and a no compaction reference treatment (REF) were applied. PAC-1 was intended to affect primarily the subsoil whereas PAC-2 was intended to affect primarily the topsoil. PAC-2 showed the highest and REF the lowest penetration resistance in the topsoil, respectively. In the subsoil both compaction treatments showed higher penetration resistances than REF. In comparison with REF, the compaction treatments decreased the estimated effective rooting depth by ca. 10, 20 and 50 cm in the three winter wheat cultivars tested, equivalent to decreases in the available soil water in the root zone of up to ca. 90 mm. These differences indicate some genetic variation in the ability of cultivars to penetrate compacted soil, although the interaction between compaction treatment and cultivar was not significant. Due to almost sufficient precipitation, the impairment of root penetration resulted in a minor yield decrease of ca. 9% of grain yield in PAC-2 and ca. 8% of total dry matter (DM) in both compaction treatments. The latter was attributed to decreases in interception of light and to efficiency of light energy conversion into biomass. The RUE was positively correlated with an estimated effective rooting depth across cultivars, while DM yield was not. This correlation probably was a result of restrictions on stomatal opening mediated by drought stress and abscisic acid produced in the root system in response to occasional soil drying. Root-sourced signals, triggered in a direct response to soil compaction, may have contributed.

Catch crops are generally regarded as an efficient tool to reduce nitrate leaching. However, the benefits need to be balanced against potential adverse effects on the main crop yields. The objectives of the study were to study three contrasting catch crops, that is, dyer’s woad (DW) (Isatis tinctoria L.), perennial ryegrass (RG) (Lolium perenne L.) and fodder radish (FR) (Raphanus sativus L.) under three tillage systems. For that, we used a tillage experiment established in 2002 on a Danish sandy loam. The tillage treatments were direct drilling (D), harrowing to 8–10 cm (H) and ploughing (P). Above-ground biomass production and N uptake were measured in the catch crops and the main crop. Catch crop root growth was studied using both minirhizotron and core methods. Soil penetration resistance was recorded to 60 cm depth. Fodder radish and RG produced up to 1800 kg/ha dry matter and DW 900 kg/ha. The nitrogen uptake in November was 55, 37 and 31 kg N/ha for FR, RG and DW, respectively, when averaged across the 2 yr of study. The yield of the spring barley main crop was in general highest where FR was grown as a catch crop. Ploughing tended to result in highest yields although differences were only significant in 2008. The minirhizotron root measurements showed that the crucifers FR and DW achieved better subsoil rooting than RG. In contrast, the soil core data showed no significant difference between FR and RG in subsoil root growth. Our study highlights the need for further studies on subsoil root growth of different catch crops.

Many studies have reported impeded root growth in topsoil under reduced tillage or direct drilling, but few have quantified the effects on the least limiting water range for root growth. This study explored the effects of tillage intensity on critical soil physical conditions for root growth in the topsoil. Samples were taken from a 7-year tillage experiment on a Danish sandy loam at Foulum, Denmark (56°30′ N, 9°35′ E) in 2008. The main crop was spring barley followed by either dyer's woad (Isatis tinctoria L.) or fodder radish (Raphanus sativus L.) cover crops as subtreatment. The tillage treatments were direct drilling (D), harrowing 8–10 cm (H), and ploughing (P) to 20 cm depth. A chisel coulter drill was used in the H and D treatments and a traditional seed drill in the P treatment. Undisturbed soil cores were collected in November 2008 at soil field moisture capacity from the 4–8 and 12–16 cm depths. We estimated the critical aeration limit from either 10% air-filled porosity (εa) or relative gas diffusivity (D/D0) of 0.005 or 0.02 and found a difference between the two methods. The critical limit of soil aeration was best assessed by measuring gas diffusivity directly. Root growth was limited by a high penetration resistance in the D and H soils (below tillage depth). Poor soil aeration did not appear to be a significant limiting factor for root growth for this sandy loam soil, irrespective of tillage treatment. The soil had a high macroporosity and D/D0 exceeded 0.02 at field capacity. Fodder radish resulted in more macropores, higher gas diffusivity and lower pore tortuosity compared to dyer's woad. This was especially important for the H treatment where compaction was a significant problem at the lower depths of the arable layer (10–20 cm depth). Our results suggest that fodder radish could be a promising tool in the amelioration of soil compaction. Highlights We explored root growth conditions in the topsoil as affected by tillage intensity ► For this purpose we applied the least limiting water range concept. ► We found clear differences in estimated critical aeration limits. ► High penetration resistance under reduced tillage was a significant problem. ► Fodder radish positively affected soil aeration parameters. Keywords: Soil tillage; Cover crops; Pore characteristic; Penetration resistance; Gas diffusivity; Least limiting water range

We compared the soil C input potential of a common catch-crop (fodder radish) established in 6-year-old direct-drilled (DD) plots with adjacent conventionally tilled (CT) plots on a Danish sandy loam soil by use of 14C-isotope labelling techniques. Intact monoliths of soil with actively growing fodder radish seedlings were extracted in Autumn of 2008 from DD and CT field plots and labelled with 14CO2 at different time intervals during fodder radish growth. Labelled monoliths were then sampled 6 and 100 days after termination of labelling by clipping above-ground biomass at soil level and separating below-ground components into macro-roots and macro-root-free soil at 0–10, 10–25 and 25–45 cm soil depth. Using fodder radish 14C data and the preceding spring barley biomass yield data we estimated C input from the spring barley-fodder radish cycle in addition to evaluating the effect of the removal of spring barley harvestable straw on soil C input. Potential soil C input under straw removal scenarios with and without an established fodder radish crop was also evaluated. Relative to other depths, over 70% of labelled below-ground C was found in the 0–10 cm soil depth in both DD and CT treatments for each of the two samplings. For both macro-root and macro-root-free soil and in both tillage treatments, labelled C decreased significantly with depth (P

Finding ways of reducing nitrate leaching in Northern Europe has become an extremely important task, especially under the projected climate changes that are expected to exacerbate the problem. To this end, two field experiments were established under temperate coastal climate conditions to evaluate the effect of tillage, straw retainment and cropping sequences, including cover crops, on nitrate leaching. The experiments were established in autumn 2002 on a loamy sand with 92 g clay kg-1 and a sandy loam with 147 g clay kg-1. The tillage treatments were stubble cultivation to 8-10 cm or 3-4 cm, direct drilling, or ploughing to 20 cm. The hypothesis was that (i) decreasing soil tillage intensity would decrease leaching compared to ploughing, (ii) leaving straw in the field would decrease leaching compared to removing straw, and (iii) a spring/winter crop rotation with catch crops would be more efficient in reducing nitrate leaching than a winter crop rotation. Overall, we were not able to confirm the three hypotheses. The effect of soil tillage on leaching might be blurred because the studied crop rotations had a high proportion of winter crops and because catch crops were grown whenever the alternative would have been bare soil in autumn and winter. The spring/winter crop rotation with catch crops was not found to be more efficient in reducing nitrate leaching than the winter crop rotation. In contrast, in a single year the winter crop rotation showed significantly lower leaching than the spring/winter crop rotations, probably due to the spring/winter crop rotation including peas, which may be considered a high-risk crop. Our study highlights that management practices that improve biomass production throughout the year are crucial in order to tighten the nitrogen cycle and thereby reduce nitrate leaching.

Increased awareness of the role of soil structure in defining the physical fertility or quality of soil has led to the need for a simple assessment relevant to the environmental and economic sustainability of soil productivity. A test is required that is usable by farmer, consultant and researcher alike. Here an assessment of soil structure quality (Sq) is described which is based on a visual key linked to criteria chosen to be as objective as possible. The influences of operator, tillage and crop type on Sq value were tested. The test takes 5-15 min per location and enough replicates were obtained for statistical comparison of data sets. The assessments of individual operators were influenced to an extent by differences between fields, making the use of multiple operators desirable. Differences in soil management were revealed by the test and related to differences in soil physical properties (bulk density, penetration resistance and porosity) and crop growth. Indicative thresholds of soil management are suggested. The assessment should be viewed as complementary to conventional laboratory assessments of soil structure. Visual soil structure assessment can indicate to the soil scientist where to sample and what soil measurements are likely to be worthwhile.

Tillage impacts many components and functions of the soil ecosystem. Thus, the prediction of soil structures produced by tillage may be regarded as a major objective in soil science. Brittle fracture is the desired mode of failure in most tillage operations. Mechanistic or phenomenological models based on the probabilistic Weibull "weakest link" theory are commonly applied to model brittle fracture of air-dry aggregates. The overall objective of this study was to develop a Weibull model to describe the strength of different-sized soil aggregates across a wide range of water contents. Rupture energy data were obtained for aggregates sampled in three field experiments. These included two soil compaction experiments (Bygholm I and II, sandy loam) and a long-term tillage and fertilization experiment (Maury, silt loam). Aggregates were subjected to a crushing test after having been adjusted to matric potentials ranging from -10 kPa to -163 MPa (air dry). Water content strongly affected the characteristic rupture energy (Weibull α parameter), and this relationship was successfully modeled with a power law function. In contrast, water content had little or no effect on the spread of aggregate strengths (Weibull β parameter). Based on these results, we proposed a three-parameter Weibull brittle fracture model for the tested sandy loam and silt loam soils that takes account of the effect of water content for a single aggregate size fraction. This model, in which only α depends on water content, explained on average 89% of the total variation in rupture energy. Further research is needed to fully investigate the influence of water content on the rupture energy of different-sized aggregates.

Optimal use of farming machinery is important for efficiency and sustainability. Continuous automated control of the machine settings throughout the tillage operation requires sensory feedback estimating the seedbed quality. In this paper we use a laser range scanner to capture high resolution maps of soil aggregates in a laboratory setting as well as full soil surface maps in a field test. Gaussian curvature is used to estimate the size of single aggregates under controlled circumstances. Additionally, a method is proposed, which cumulates the Gaussian curvature of full soil surface maps to estimate the degree of tillage.

Two case studies on Danish sandy loams investigated long-term effects of fertilization and crop rotation. Case Study 1 compared a diversely cropped organically farmed soil (DFG(1)) with a conventionally farmed soil predominantly growing annual crops (DFA), both receiving animal manure. In Case Study 2, a diversely cropped organically managed soil DFG(2)) receiving animal manure was compared with an almost exclusively cereal cropped conventionally farmed soil receiving no animal manure (CCC). A multi-level experimental strategy was followed to compare integrating field methods with specialized laboratory methods. Ease of tillage was described in the field and characterized using a drop shatter field test and by measuring aggregate tensile strength. Fitness as a seedbed was characterised visually in the field and from the drop shatter test. Impedance to seedling emergence and root penetration was evaluated by measuring bulk soil shear strength. For Case Study 1, soil tilth was better for the DFA soil than for the DFG(1) soil (i.e. lower soil strength and higher ease of fragmentation and friability index for DFA). However, a crumbly structure and a moderate bulk density suggested that the DFG(1) soil was also a favourable medium for plant growth. For Case Study 2, the CCC soil had a lower organic matter content, higher bulk density, higher soil strength, and lower ease of fragmentation in comparison with the DFG(2) soil.

Plough pans have been shown to severely hamper root development, limit rooting depth and reduce crop yields. We evaluated the effect of plough pan compaction on root and yield response for a winter wheat in a field trial conducted in two neighbouring fields (B3 and B4) on a sandy loam. Plots were mechanically loosened to a depth of 35 cm in 1997 (B3 and B4) and again in 1998 (only B4). Perennial grass/clover was grown with limited traffic intensity in 1998-1999 and 1999-2000 for B3 and B4, respectively. The perennial grass/clover was ploughed under in spring 2000 (B3) and spring 2001 (B4) and oats established. After harvest of oats, winter wheat was established. Recompaction treatments were applied to the mechanically loosened plots in 2000 and 2000+2001 for B3 and B4, respectively. On-land ploughing was compared with traditional mouldboard ploughing with the tractor wheels in the furrow. In addition, the loosened plots were either heavy trafficked (10-18 Mg axle load and ~200 kPa inflation pressure) or light trafficked (

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.