Regeneration Of Soil Structure Research Articles

Nicodrilus nocturnus and Allolobophora icterica drill compacted soils but do not decrease their bulk density – A laboratory experiment using two contrasted soils at two different compaction levels

Earthworms are known to play an important role in soil processes, especially in the regeneration of soil structure. However, quantitative studies about their role on soil physical properties are still scarce. In this study the effects of two earthworm species (Nicodrilus nocturnus as anecic, Allolobophora icterica as endogeic) following three treatments (N. nocturnus only, A. icterica only and both species with 80% weight of N. nocturnus and 20% of A. icterica) on soil specific volumes and pore properties are evaluated in mesocosms (30 cm height and 15 cm diameter) for a loamy Anthrosol and a silt loam Luvisol. The soils were repacked to bulk density observed in the field (1,15 and 1,25 g cm−3 respectively) and to compacted bulk density (1,4 and 1,5 g cm−3 respectively). Except earthworm-free controls, introduced earthworm biomass was close to 500 g.m−2. The experiment lasted 23 weeks, under constant temperature and soil matrix potential, and earthworms were fed with hay. The impact of earthworms on soil porosities and specific volumes was assessed using (i) computed tomography on mesocosm and (ii) shrinkage analysis on undisturbed cubic samples (150 cm3). Anecic surface cast bulk density was determined after wax coating.At mesocosm scale, the specific volume of compacted soils increased significantly with the anecic and mixed earthworm treatments (+1.9% for the Anthrosol and +2.6% for the Luvisol), while no change was observed with endogeics regardless of the initial level of compaction or the soil type. After subtracting the burrow volumes, the remaining soil matrix specific volume showed significant decrease with earthworms in case of loose soils, particularly with endogeics with 5.6% decrease of the specific soil matrix volume, while the compacted soil matrix was not decompacted. At undisturbed cubic sample scale, shrinkage analysis confirmed these observations with earthworms decreasing the larger structural pores and promoting a more rigid plasma. Anecic surface casts showed intermediate bulk density (0.82 cm3 g−1 for the Anthrosol and 0.73 cm3 g−1 for the Luvisol) between compacted (0.73 cm3 g−1 for the Anthrosol and 0.67 cm3 g−1 for the Luvisol) and loose (0.88 cm3 g−1 for the Anthrosol and 0.81 cm3 g−1 for the Luvisol) soil matrices. We concluded that the decompaction effect of earthworms was due to the opening of burrows at mesocosm soil scale, while the matrix volume was i) either compacted in case of loose soil especially with endogeics at the expense of the >150 µm equivalent radius structural pores or ii) unchanged in case of compacted soil. Our results support the conclusion that earthworms alone cannot regenerate the matrix of compacted soils and even compact the soil matrix in case of loose soils.

Black Alder (Alnus glutinosa (L.) Gaertn.) on Compacted Skid Trails: A Trade-off between Greenhouse Gas Fluxes and Soil Structure Recovery?

The compaction of forest soils can deteriorate soil aeration, leading to decreased CH4 uptake and increased N2O efflux. Black alder (Alnus glutinosa) may accelerate soil structure regeneration as it can grow roots under anaerobic soil conditions. However, symbiotic nitrogen fixation by alder can have undesirable side-effects on greenhouse gas (GHG) fluxes. In this study, we evaluated the possible trade-off between alder-mediated structure recovery and GHG emissions. We compared two directly adjacent 15-year old beech (Fagus sylvatica) and alder stands (loamy texture, pH 5–6), including old planted skid trails. The last soil trafficking on the skid trails took place in 1999. GHG fluxes were measured over one year. Undisturbed plots with beech had a moderately higher total porosity and were lower in soil moisture and soil organic carbon than undisturbed alder plots. No differences in mineral nitrogen were found. N2O emissions in the undisturbed beech stand were 0.4 kg ha−1 y−1 and 3.1 kg ha−1 y−1 in the undisturbed alder stand. CH4 uptake was 4.0 kg ha−1 y−1 and 1.5 kg ha−1 y−1 under beech and alder, respectively. On the beech planted skid trail, topsoil compaction was still evident by reduced macro porosity and soil aeration; on the alder planted skid trail, soil structure of the uppermost soil layer was completely recovered. Skid trail N2O fluxes under beech were five times higher and CH4 oxidation was 0.6 times lower compared to the adjacent undisturbed beech stand. Under alder, no skid-trail-effects on GHG fluxes were evident. Multiple regression modelling revealed that N2O and CH4 emissions were mainly governed by soil aeration and soil temperature. Compared to beech, alder considerably increased net fluxes of GHG on undisturbed plots. However, for skid trails we suggest that black alder improves soil structure without deterioration of the stand’s greenhouse gas balance, when planted only on the compacted areas.

Planting of alder trees at the edge of skid trails helps to stabilize forest topsoil structure against damage caused by heavy forestry machines

The regeneration and soil-mechanical reinforcement effect by the rooting system of grey alder (Alnus incana) were tested in an annually driven forest skid trail on a silty clay loam soil located in Southern Germany. One side of the skid trail was bordered by an eight-year-old alder plantation, on the other side, there were no trees. Soil physical properties (relative apparent gas diffusion coefficient (Ds/D0), bulk density and the coarse porosity) were measured in order to detect the effects on soil structure as well as on fine root densities, taken as a biological target variable.Values of soil physical parameters linked to the soil aeration status did not show significant differences between planted and unplanted wheel track; however, low values of bulk density measured on the laterally planted wheel track indicated significant improvements in the topsoil compared to the unplanted one. Also, values for fine root density were higher in these plots. Strong relationships between Ds/D0, bulk density and the coarse porosity with fine root densities were also found, which give evidence of their mutual effects, either amplifying each other or reciprocal. In the top soil, root density positively affects soil gas permeability, while in the deeper horizons fewer macropores are found, which are partially blocked by fine roots for gas exchange.In summary, the wheel track which is laterally planted with alder trees showed signs of an initial regeneration of soil structure as well as a reinforcement against further soil damage caused by annual wheeling.

Geometric characterization of soil structure through unconventional analytical tools

Soil structure is essential for soil function and environmental quality. Therefore, adequate evaluation of management systems is essential to ensure the preservation and regeneration of soil structure. Techniques such as X-ray microtomography and automated soil particle size analysis based on gamma-ray attenuation provide rapid quantification of soil physical properties and allow for the evaluation of soil structure. The aim of this study is to determine soil physical parameters using unconventional analytical tools to characterize and evaluate the geometric characteristics of soils with different structures at high accuracy. Soil samples extracted from the UNESP experimental farm (Brazil) were separated into two different management groups according to their textural similarity. The following results were obtained from the analysis: 1) Microtomography can be used to extract soil porosity, including soil microporosity. 2) The shape factor indicates the geometric organization of the soil structure, which is relevant and valid for evaluating the degree of soil degradation. 3) The fractal dimension is related to the soil particle and aggregate size distribution but was not sufficient for describing the soil geometric properties when used alone. 4) Coupling the fractal dimension with the lacunarity provides an important tool for estimating the soil structure. Finally, the application of the physical parameters from the unconventional method provided new insights for understanding soil geometric structure, which can support future investigations of flow phenomena in the soil.

Initial recovery of compacted soil—planting and technical treatments decrease CO2 concentrations in soil and promote root growth

The combination of technical treatments and planting of alder trees in a compacted forest soil improves the circulation of air and water through the pore system. This leads to decreases in CO 2 concentrations and increases in root growth in the soil. Both are indicative of an initial recovery of soil structure. The compaction of forest soils, caused by forest machinery, has as a principal consequence: the destruction of soil structure and thus the reduction of the soil aeration status. Thus, the gas exchange between soil and atmosphere is reduced and the depth propagation of roots is limited, resulting in the shortage of water and nutrient supplies for trees. This research aimed at detecting the first stages of recovery of soil structure in a compacted forest soil, which was treated with a combination of techniques (i.e., planting tree species, mulching, addition of lime), which could presumably accelerate the regeneration of soil structure. The variation of CO2 concentrations and the dynamics of root growth were repeatedly measured. Linear mixed models were developed in order to test the effects of the treatments and the planting of trees on soil aeration, as well as to identify the influence of the different environmental effects on CO2 concentration in soil. The planting of root-active trees showed significant effects on decreases in CO2 concentrations. However, during the short-term observation, some negative effects occurred especially for the mulched sites. Nevertheless, all applied technical treatments promoted an improved soil aeration and a higher root growth compared to untreated sites which points to an initial enhanced recovery of soil structure. Pronounced seasonal and interannual variations of soil respiration were highly influenced by soil temperature and soil water content variations. An initial regeneration of soil structure is indicated by distinct changes of the soil aeration status. This regeneration is partially enhanced by the applied treatments. The quantitative potential of the regeneration techniques needs a longer observation period for mid- and long-term soil recoveries.

Meta‐analysis of field‐saturated hydraulic conductivity recovery following wildland fire: Applications for hydrologic model parameterization and resilience assessment

AbstractHydrologic recovery after wildfire is critical for restoring the ecosystem services of protecting of human lives and infrastructure from hazards and delivering water supply of sufficient quality and quantity. Recovery of soil‐hydraulic properties, such as field‐saturated hydraulic conductivity (Kfs), is a key factor for assessing the duration of watershed‐scale flash flood and debris flow risks after wildfire. Despite the crucial role of Kfs in parameterizing numerical hydrologic models to predict the magnitude of postwildfire run‐off and erosion, existing quantitative relations to predict Kfs recovery with time since wildfire are lacking. Here, we conduct meta‐analyses of 5 datasets from the literature that measure or estimate Kfs with time since wildfire for longer than 3‐year duration. The meta‐analyses focus on fitting 2 quantitative relations (linear and non‐linear logistic) to explain trends in Kfs temporal recovery. The 2 relations adequately described temporal recovery except for 1 site where macropore flow dominated infiltration and Kfs recovery. This work also suggests that Kfs can have low hydrologic resistance (large postfire changes), and moderate to high hydrologic stability (recovery time relative to disturbance recurrence interval) and resilience (recovery of hydrologic function and provision of ecosystem services). Future Kfs relations could more explicitly incorporate processes such as soil‐water repellency, ground cover and soil structure regeneration, macropore recovery, and vegetation regrowth.

Effects of rice straw incorporation and tillage depth on soil puddlability and mechanical properties during rice growth period

Soil puddlability, a measure of susceptibility of soil to puddling, affects regeneration of soil structure during the rice growth period, but the intrinsic controlling factors are unclear. It was hypothesized that rice straw incorporation and tillage depth influence soil puddlability and soil mechanical and hydraulic properties during the rice growth period as they influence soil organic carbon (SOC) in the plough layer. Using a three-year experiment, the objectives of this study were (1) to determine the effects of rice straw incorporation and tillage depth on root growth traits, sinkage resistance after puddling, and tensile strength, shrinkage capacity, water retention curve and penetration resistance during the rice growth period, and (2) to determine whether mechanical properties from puddling tillage, through the rice growth period, to harvest are correlated with SOC concentration. Rice straw was incorporated partially (C1) and fully (C2) to different depths of plough layer by shallow tillage (ST) and deep tillage (DT). Compared with no rice straw incorporation (C0), rice straw incorporation decreased the sinkage resistance after puddling (C2<C1<C0) and increased SOC concentration and aggregate stability. Shallow tillage increased the penetration resistance more than deep tillage irrespective of rice straw incorporation. The difference in tensile strength and total porosity were small among the rice incorporation treatments at the 1st drainage (C2≥C1≥C0), but became more profound with increasing number of drainage because the tensile strength and total porosity decreased in C0, increased in C2, particularly under ST. The root weight was lower, and the root length was longer in C1 than in C2 particularly at the deeper depth of the plough layer under ST, suggesting the impedance of root growth due to higher sinkage and penetration resistances. SOC concentration was weakly correlated with the sinkage and penetration resistances and the tensile strength after the 1st drainage, but not with the tensile strength after the 2nd and 3rd drainages. In conclusion, rice straw incorporation is important to maintain SOC and then improve soil puddlability and the recovery of pore structure during the rice growth period.

Recovery of forest soil from compaction in skid tracks planted with black alder (Alnus glutinosa (L.) Gaertn.)

Although soil compaction has become a common problem in forests nowadays, little is still known on the possibilities to promote the structural recovery of compacted forest soils. Here we examined how planting black alder into the ruts of skid lanes with and without application of compost would affect the regeneration of soil structure in a gleyic Cambisol on the Swiss Plateau. Seven years after planting, soil samples taken at three different depths next to the trees were analyzed for bulk density, total and coarse porosity, air permeability, precompression stress and root mass density and compared to samples from unplanted skid lanes and planted but non-trafficked reference plots. All four physical parameters showed strong compaction effects in the skid lanes. Coarse porosity was reduced down to 0.5m depth, and air permeability even down to 0.7m depth. In combination with compost application, the planting of black alders resulted in a significant regeneration of structure, although recovery was still far from complete compared to the untrafficked reference soil. Without compost, there was no significant treatment effect compared to the unplanted skid lanes, but a clear trend, as the differences in coarse porosity and precompression stress between the two alder treatments with and without compost were not significant either. Air permeability showed a significant effect of the alder treatment with compost that extended down to 0.7m depth, but also reached values of the same magnitude as in the non-trafficked reference soil. Air permeability furthermore showed a very similar pattern of treatment effects to rooting density in the skid lane soils. The correlation between these two parameters increased with depth. These results suggest that the roots of the planted alder trees initiated structure recovery by generating air-conducting porosity and thereby promoted other biotic structure-formation processes. Our study highlights the fact that tree root growth may significantly contribute to soil structure recovery after compaction.

Enhancing the regeneration of compacted forest soils by planting black alder in skid lane tracks

Soil compaction due to the use of heavy machinery for timber harvesting has become a widespread problem in forestry. However, only few studies deal with the regeneration of compacted forest soils. In the present study, we examined the potential of accelerating soil regeneration by planting black alder trees (Alnus glutinosa (L.) Gaertn.) in skid lane tracks. In 2003, seedlings were planted into the rut beds of severely compacted skid lanes in two Swiss forest sites. In addition, some of the ruts were filled with compost. In 2009 and 2010, we assessed the success of these measures by analysing physical parameters of soil structure (bulk density, total and coarse porosity and air permeability), root densities and tree growth. Tree growth was exceptionally strong on the skid lanes. Total and coarse soil porosity and air permeability showed significant increase in planted skid lanes as compared to untreated control subplots, approaching values found for untrafficked soil in the immediate vicinity. All soil physical parameters were closely correlated to root mass density. Compost application enhanced tree growth and soil structure regeneration on one site, but had a retarding effect on the other site. Planting black alders has great potential as an environmentally friendly measure to accelerate the structural regeneration of compacted forest soils in temperate humid climates.

Water Retention, Gas Transport, and Pore Network Complexity during Short-Term Regeneration of Soil Structure

Soil structure maintains prime importance in determining the ability of soils to carry out essential ecosystem functions and services. This study quantified the newly formed structure of 22-mo field-incubated physically disturbed (2-mm sieved) samples of varying clay mineralogy (illite, kaolinite, and smectite) amended with organic material (7.5 Mg ha−1). The newly formed structure was compared with that of sieved, repacked (SR) and natural intact samples described in previous studies. Assessment and comparison of structural complexity and organization was done using water retention (pore size distribution), soil gas diffusivity, air permeability, and derived pore network complexity parameters. Significant decreases in bulk density and increases in pores >100 μm were observed for incubated samples compared with SR samples. For the soils studied, the proportion of pores >100 μm increased in the order: smectite < illite < kaolinite, with no effect of organic amendment. Soil structural complexity, quantified by soil gas diffusivity, air permeability, and derived pore network indices, was greater for incubated than SR samples. For illitic soils, incubated samples had lower water content and higher air-filled porosity and air permeability than natural intact samples at a matric potential of −10 kPa. Despite this, soil pore organization was similar for both natural and incubated soils, but pore network complexity increased in the order: SR < incubated < natural soils. Finally, the air permeability percolation threshold corresponding to the physically based diffusion threshold increased with structural complexity (SR = 0.02 μm2; incubated = 0.20 μm2; natural = 0.70 μm2). Thus, critical reexamination is needed of the often-used 1.0-μm2 percolation threshold for convective air transport when analyzing pore network complexity. Lack of a clear effect of organic amendment for incubated samples suggests using higher application rates in future studies.

Ability of the “profil cultural” method to assess the soil structure of untilled layers

In France, agronomists have studied the effects of cropping systems on soil structure, using a field method based on a morphological description of soil structure. The “profil cultural” method (Manichon and Gautronneau, 1987) has been set up to perform a field diagnostic of the effects of tillage and compaction on soil structure. However, this method was developed and mainly used in conventional tillage systems, with ploughing. As several forms of reduced, minimum and no tillage systems are expanding in many parts of the world, it is necessary to re evaluate the ability of this method to describe an interpret soil macrostructure in unploughed situations. In unploughed fields, soil structure dynamics of untilled layers is mainly driven by compaction and regeneration by natural agents. Amongst them the role of climate and earthworms is essential and it is of major importance to appreciate the natural processes of soil structure regeneration. The “profil cultural” method is based on a visual observation of the macroscopic structure of different zones in the top part of the soil profile. Therefore, the method must be able to take into account the change in soil properties due to the presence of cracks and macropores. The objective of this presentation is to review the ability of the morphological method to describe the structure of untilled layers. Complementary indicators of biological macropores and cracking are proposed to complete the previous description.

Effect of regenerated soil structure on unsaturated transport of Escherichia coli and bromide

Summary This study was conducted to assess the role of the regeneration of soil structure by physical weathering of soils in columns on the transport of Nalidixic acid-resistant Escherichia coli strain ( E. coli NAR) and also Bromide (Br). Leaching experiments were carried out at steady-state flow conditions, under a suction head of 5 cm, through columns of repacked and weathered clay loam (CL) and sandy loam (SL) soils. A pulse-type boundary condition was used and breakthrough curves for bacteria and Br were obtained at three depths during a 24 h leaching experiment. Observed data were analyzed employing the mobile–immobile water (MIM) models for repacked or weathered soil columns using HYDRUS-1D code. Both models described well the effluent concentrations. Calibration of the models to match the observed breakthrough curves of Br and E. coli NAR resulted in a smaller value of dispersivity for the later. For bacteria, the fitted attachment and straining coefficients increased with the clay content of the soil. Earlier peaks in the breakthrough curves were observed for the weathered soil columns. This was attributed to increased pore connectivity associated with weathering. In contrast the balanced and symmetrical shape of the breakthrough curves from the repacked soil columns indicates that leaching occurred mainly through matrix flow.

Ability of the “profil cultural” method to assess the soil structure of untilled layers

In France, agronomists have studied the effects of cropping systems on soil structure, using a field method based on a morphological description of soil structure. The “profil cultural” method (Manichon and Gautronneau, 1987) has been set up to perform a field diagnostic of the effects of tillage and compaction on soil structure. However, this method was developed and mainly used in conventional tillage systems, with ploughing. As several forms of reduced, minimum and no tillage systems are expanding in many parts of the world, it is necessary to re evaluate the ability of this method to describe an interpret soil macrostructure in unploughed situations. In unploughed fields, soil structure dynamics of untilled layers is mainly driven by compaction and regeneration by natural agents. Amongst them the role of climate and earthworms is essential and it is of major importance to appreciate the natural processes of soil structure regeneration. The “profil cultural” method is based on a visual observation of the macroscopic structure of different zones in the top part of the soil profile. Therefore, the method must be able to take into account the change in soil properties due to the presence of cracks and macropores. The objective of this presentation is to review the ability of the morphological method to describe the structure of untilled layers. Complementary indicators of biological macropores and cracking are proposed to complete the previous description.

Changes in soil structure and hydraulic properties in regenerating rain forest

Abstract. The structure of soils in areas of rain forest cleared for pasture is often compacted due to trampling by animals. When pasture is abandoned, regeneration of forest cover may be achieved from natural seed sources nearby.However, the regeneration of soil structure is also important for local hydrology and successful establishment of some plant species. In this study we investigated changes in soil structure and hydraulic properties in a series of plots on volcanic soils in the San Luis Valley, Costa Rica. The plots were current pasture, 15‐ and 20‐year‐old regenerating forest, and primary rain forest.Infiltration rate increased with increasing forest age and the water release characteristic reverted gradually from one with greater water retention at all matric potentials in the pasture plot towards that found in the primary forest. Compaction and low porosity were features of both the current pasture and 15‐year‐old regenerating forest in comparison to the primary forest.

Turnover of organic matter in differently textured soils: I. Physical characteristics of structurally disturbed and intact soils

Soil type effects on organic matter turnover are most often ascribed directly to differences in soil clay content. Since soil texture determines the physical characteristics of soil, aggregation and water holding capacity may be more relevant to address in the search for controls of organic matter turnover. Most studies of microbial processes in soils are based on structurally disturbed soil, where the abiotic conditions for the microbial activity may be quite different from those in intact soils. In this study, basic physical characteristics were determined for structurally disturbed and intact soil samples from differently textured soils. Bulk soil was retrieved from 0–20 cm depth at six locations along a textural gradient in an arable field on Weichselian morainic deposits in Denmark. The samples (NA1 to NA6) ranged in clay from 11 to 45% and in silt from 7 to 15%. Clay and silt-sized organomineral complexes were isolated from NA2 soil by ultrasonic dispersion and sedimentation in water. The clay and silt fractions were added individually and in varying proportions to NA1 soil, providing three clay-amended (CL2, CL4 and CL6) and three silt-amended (SI2, SI4 and SI6) soils. All 12 soils were crushed in air dry state to <2 mm, mixed, re-moistened and exposed for 17 months to freeze/thaw and dry/wet cycles as well as tillage to induce regeneration of soil structure. Intact soil cores were then equilibrated at four water matric potentials on ceramic plates (−30, −100, −500 and −1500 hPa) and analyzed for volumetric water and air content, and air diffusivity and permeability. Cores of undisturbed, but recently tilled topsoil from each sampling location in the field (RE1 to RE6), were included as reference samples for the experimentally manipulated (disturbed) soils. The CEC of the soils was closely related to clay content. For the clay-amended soils, CEC also correlated to organic matter content. Cores of disturbed and undisturbed soils with <20% clay were similar in bulk density. At higher clay contents, disturbed soils were less dense than undisturbed ones. All SI soils and the NA1 soil showed similar pore space distribution, while clay-amended (CL) soils resembled their corresponding NA soils. In contrast to undisturbed RE soils, the disturbed NA, CL and SI soils had a much greater volume of large pores (>100 μm). Air diffusivity and permeability measurements showed disturbed soils to have a less continuous and more tortuous pore system than undisturbed reference samples. Water-filled pore space at a critical level of air diffusion potential was significantly higher for undisturbed than for disturbed samples, especially in soils high in clay. Drop cone measurements showed disturbed soils to be structurally weaker than undisturbed ones. Intact and structurally disturbed soils were found to differ significantly in physical properties even after 17 months of soil structure regeneration. Water-filled pore space seems to reflect the potential of available water and aeration status to regulate aerobic microbial activity of structurally disturbed soil, but not of intact field soil.

Quantitative analysis of earthworm burrow systems with respect to biological soil-structure regeneration after soil compaction

On arable land, tilled with conventional tillage (CT) and conservation tillage (CS) respectively, plots were compacted by wheeling them 6 times with a 5 Mg wheel load in spring 1995. Immediately after compaction, undisturbed soil monoliths were excavated from the compacted and uncompacted plots. The monoliths were defaunated and inoculated with either Lumbricus terrestris or Aporrectodea caliginosa. One monolith from each plot remained uninoculated as a control. After 6 months the monoliths were defaunated again and then scanned with X-ray helical computed tomography. The data were transformed, the void systems inside the monoliths were reconstructed and visualised, and the parameters total void length, total void volume, tortuosity and continuity were quantified. The parameters' values were generally lower in the controls than in the inoculated monoliths. Differences in burrow construction could be explained by the different life strategies of the two earthworm species. Changes in burrow morphology due to tillage system and soil compaction were minor. Only the continuity of the burrow systems clearly changed: decreasing for L. terrestris and increasing for A. caliginosa. This can be explained by a change in the earthworms' burrowing activity to minimise energy expenditure in compacted soil. By extrapolating field data, we concluded that earthworms have great potential for biologically regenerating the soil structure after a single compaction event. Due to higher earthworm abundances in soil managed by CS the regeneration of the soil structure is assumed to be better in these plots than those tilled by CT.

Impact of soil management systems on soil structure and physical properties in a clay loam soil, and the simulated effects on water deficits, soil aeration and workability

The effects of different soil management systems on soil structure and associated physical properties in a Dutch clay loam soil were evaluated using a combination of simulation modelling, field investigations and laboratory experiments. The investigations were focused on field recognition of identical soils only differing in structure type caused by soil management systems. Two soil structure types were recognised: a type developed in permanent grassland and a type in young arable land, which had been used for intensive vegetable growing since 1979. The types were characterised by examining the micro- and macro-morphology and soil hydraulic properties, such as soil structure, rooting patterns, pore shape and size distribution, bypass flow and water retention and unsaturated hydraulic conductivity characteristics. Field recognition of structure types and subsequent morphological and physical characterisation was used as an alternative to the application of a machine-soil submodel. Significantly different properties of both soil structure types were functionally interpreted by using a simulation model for the soil-plant subsystem. Morphological and hydraulic analyses were essential for the definition of proper boundary conditions and input parameters. The land qualities water deficits, workability and soil aeration were calculated for a potato crop under Dutch climatological conditions. Expressions for temporal variability were obtained by using a 30 year record of weather data. The soil structure of permanent grassland proved to be more favourable than that of young arable land. This resulted in a 10% decrease in the average annual water deficit, 40% more workable days in the planting phase and 5–10% more days with a well-aerated soil. The impact of these differences on the potato tuber yield interpreted in terms of the effect of soil structure degradation, was estimated to be at least 5% in 20 out of 100 years under Dutch climatic conditions. The differences calculated also equalled the maximum possible effect of soil structure regeneration.

Use of simulation to assess the effects of different tillage practices on land qualities of a sandy loam soil

Four different soil-structure types could reproducibly be recognized by soil surveyors working on Dutch sandy loam soils with different management practices. The 4 structure types were soils with a primary ploughpan, a loosened ploughpan, a secondary ploughpan and a grassland structure. These types had significantly different soil-physical properties and rooting depths. Soil-water regimes for a 30-year period were stimulated to obtain quantitative information on the influence of soil structure, as an expression of different soil-management practices, on land qualities such as moisture deficit, aeration status and workability. These land qualities were not significantly different for the 3 structure types of arable land when considering the entire soil profile. However, differences in moisture content and aeration status were significant when undisturbed and disturbed ploughpans were compared. Deep ploughing to disrupt the primary ploughpan should therefore not be encouraged in these soil as long as common field operations remain unchanged. The same land qualities for grassland indicated what may be attained by soil-structure regeneration. The probability of having a workable day in spring or autumn is increased by an estimated 20%, and the number of days with adequate aeration by 10%, in soil-structure types of arable land (A, B and C) as compared with grassland soil structure (D). Moisture deficits in the growing season should be about 50% less. In this study, simulation was not used to simulate soil-structure formation as a function of soil management, but soil structure was used as an input for the models, reflecting different management practices. To that end, the various structure types were characterized by physical measurements.

PHYSICAL PROPERTIES OF SOME PRINCE EDWARD ISLAND SOILS IN RELATION TO THEIR TILLAGE REQUIREMENT AND SUITABILITY FOR DIRECT DRILLING

Successful reduced tillage systems depend on suitable soil types and the maintenance of optimum soil structure. Studies were conducted to assess the tillage requirement of some well-drained Podzolic and Luvisolic soils in Prince Edward Island and their suitability for direct drilling. Relatively high amounts of silt and fine sand indicated that the soils were structurally unstable with a propensity for compaction. Compactability was inversely related (r2 = 0.90) to concentrations of soil organic carbon over a range of 0.2–2.0%. In the Ap horizons, where organic carbon exceeded 1.6%, the level of macroporosity (equivalent pore diam. &gt; 50 μm) was generally greater than 10%. The B horizon and lower level of the Ap horizon, generally, had low porosity and organic matter, although pore continuity was adequate. Macroporosity was closely related to soil bulk density (r2 = 0.81). Penetrometer resistance down the soil profile indicated a relatively shallow rooting depth potential of 25–40 cm. A 3-yr direct drilling study with spring cereals illustrated that some degree of soil loosening is required to maintain optimum soil structure. Soil limitations for direct drilling were mainly low resistance to compaction combined with low potential for regeneration of soil structure. The possible implications of the soil physical properties are discussed in relation to the use of direct drilling within the short-term rotational mixed farming systems of Prince Edward Island. Key words: Soil structure, macropores, Podzolic soils, Luvisolic soils

Porosity aspects of the regeneration of soil structure after compaction

Porosity aspects of the regeneration of soil structure after compaction