Evaluation of pedotransfer and measurement approaches to avoid soil compaction

On-farm approaches are needed to help farmers avoid soil compaction. It is the purpose of this paper to document the experience of using the Horn and Fleige [Horn, R., Fleige, H., 2003. A method for assessing the impact of load on mechanical stability and on physical properties of soils. Soil Till. Res. 73, 89–99] procedures to develop improved guidance to help farmers avoid compaction in agricultural operations in the Commonwealth of Pennsylvania, USA. A soil characterization database for the Commonwealth of Pennsylvania, USA, was used to provide input to the Horn and Fleige [Horn, R., Fleige, H., 2003. A method for assessing the impact of load on mechanical stability and on physical properties of soils. Soil Till. Res. 73, 89–99] approach to estimate the pre-consolidation stress and the maximum depth of compaction for 29 agricultural soils in Pennsylvania. The Horn and Fleige [Horn, R., Fleige, H., 2003. A method for assessing the impact of load on mechanical stability and on physical properties of soils. Soil Till. Res. 73, 89–99] approach was tentatively validated using previously measured pre-consolidation stress or penetration resistance values measured on five of the 29 soils. The estimated maximum depth of compaction indicated that an 89-kN (10-ton) axle load was excessive in almost all cases for soils at matric potentials of −33 and −6 kPa for both tillage and no-till management. A 53-kN (6-ton) axle load was acceptable for most cases when tillage was planned to a 0.20-m depth, but was excessive in most cases for no-till management at a matric potential of −6 kPa while mostly acceptable for no-till management at a matric potential of −33 kPa. Penetration resistance measurements are recommended to decide when a load is excessive.     

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

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

Crop responses to applied soil compaction and to compaction repair treatments

Crop responses to annual compaction treatments (applied to whole plots) and management treatments to ameliorate compacted soil were determined in a field experiment on a Vertisol. Initially, all treatments except a control were compacted with a 10 Mg axle load on wet soil (26% gravimetric water content compared with a plastic limit of 22%). Annually applied axle loads of 10 and 6 Mg on wet soil (25–32% soil water) tended to reduce seedling emergence, grain yield (wheat, sorghum and maize), soil water storage and crop water use efficiency (WUE). Annual applications of an axle load of 6 Mg on dry soil (<22% soil water) had little effect on crop performance. Mean reductions in the yield of five crops (three wheat, one sorghum and one maize) in comparison with the uncompacted control were 23% or 0.79 Mg ha⁻¹ (10 Mg on wet soil), 13% or 0.44 Mg ha⁻¹ (6 Mg on wet soil) and 1% or 0.03 Mg ha⁻¹ (6 Mg on dry soil). Maize grown in the fifth year of treatment application was most affected by compaction of wet soil, its WUE being reduced from 14.3 to 9.7 kg ha⁻¹ mm⁻¹ in response to an axle load of 10 Mg. Reduced WUE was associated with delayed soil water extraction at depth. A 3-year pasture ley was the most successful amelioration treatment. A wheat and a maize crop grown after the ley outyielded the control by 0.33 and 0.90 Mg ha⁻¹, respectively. So the pasture not only ameliorated the initial compaction damage, with respect to crop performance, but resulted in improvements in two subsequent crops.     

农田土壤压实过程及模型研究进展

农田土壤受到农业机械田间作业的影响发生压实板结,造成土壤孔隙率降低,容重和紧实度增大,限制水分入渗和根系生长,影响作物产量。随着我国农业机械化水平不断提高,土壤压实对农业可持续发展的影响引起了广泛的关注。本文通过文献调研,总结了土壤压实过程的国内外研究进展,对土壤压缩行为、压缩曲线与预固结压力的计算方法进行了梳理,综述了土壤压实机理和压实模型的发展历程和未来动向,可为推进农田土壤压实研究提供参考。     

Evaluating the performance of a soil compaction sensor

One of the most significant soil parameters affecting root growth is soil compaction. It is therefore important to be able to determine the presence of compacted layers, their depth, thickness and spatial location without the necessity of digging a large number of holes in the field with either a spade or backhoe. Previous investigations have identified soil compaction by different methods such as: using ground penetrating radar, acoustic systems, vertical and horizontal penetrometers and instrumented wings mounted on the faces of tines. Linking the output from these sensors to global positioning systems would give an indication of the spatial patent variation. The aim of this study was to evaluate the performance of a soil compaction profile sensor in both controlled laboratory and field conditions. The sensor consisted of a series of instrumented flaps; a flap is defined as the sensing element which comprises one half of a pointed leading edge to the leg of a tine to which strain gauges are placed on the rear face of the flap. Studies measured the effect of compaction on the changes in the soil resistance acting upon a flap face in a soil bin laboratory and under field conditions. The results indicated that the sensor was sensitive to differences in soil strength at different depths in soils. A technique was developed to identify the soil compaction resulting from different tyre inflation pressures and loads. The soil compaction profile sensor was tested on a number of fields in south‐eastern England to determine the changes in soil strength below the wheelings of a pea harvester operating at different tyre inflation pressures.     

SoilFlex: A model for prediction of soil stresses and soil compaction due to agricultural field traffic including a synthesis of analytical approaches

Soil compaction is one of the most important factors responsible for soil physical degradation. Soil compaction models are important tools for controlling traffic-induced soil compaction in agriculture. A two-dimensional model for calculation of soil stresses and soil compaction due to agricultural field traffic is presented. It is written as a spreadsheet that is easy to use and therefore intended for use not only by experts in soil mechanics, but also by e.g. agricultural advisers. The model allows for a realistic prediction of the contact area and the stress distribution in the contact area from readily available tyre parameters. It is possible to simulate the passage of several machines, including e.g. tractors with dual wheels and trailers with tandem wheels. The model is based on analytical equations for stress propagation in soil. The load is applied incrementally, thus keeping the strains small for each increment. Several stress–strain relationships describing the compressive behaviour of agricultural soils are incorporated. Mechanical properties of soil can be estimated by means of pedo-transfer functions. The model includes two options for calculation of vertical displacement and rut depth, either from volumetric strains only or from both volumetric and shear strains. We show in examples that the model provides satisfactory predictions of stress propagation and changes in bulk density. However, computation results of soil deformation strongly depend on soil mechanical properties that are labour-intensive to measure and difficult to estimate and thus not readily available. Therefore, prediction of deformation might not be easily handled in practice. The model presented is called SoilFlex, because it is a soil compaction model that is flexible in terms of the model inputs, the constitutive equations describing the stress–strain relationships and the model outputs.     

Impact of soil compaction upon soil water balance and maize yield estimated by the SIMWASER model

The capability of the soil water balance model SIMWASER to predict the impact of soil compaction upon the yield of maize (Zea mays L.) is tested, using the results of a field experiment on the influence of soil compaction by wheel pressure upon soil structure, water regime and plant growth. The experimental site was located on an Eutric Cambisol with loamy silt soil texture at an elevation of 260 m in the northern, semi-humid sub-alpine zone of Austria. Within the experimental field a 7 m wide strip was compacted by a tractor driven trailer just before planting maize in May 1988. Compression effects due to trailer traffic resulted in distinct differences of physical and mechanical soil parameters in comparison with the uncompressed experimental plots down to a depth of about 30 cm: bulk density and penetration resistance at field capacity were increased from 1.45 to 1.85 g/cm³, and from 0.8 to 1.5 MPa, respectively, while air-filled pore space as well as infiltration rate were appreciable lowered from about 0.08–0.02 cm³/cm³ and from 50 to 0.5 cm per day, respectively. The overall effect was a clear depression of the dry matter grain yield from 7184 kg/ha of the non-compacted plot to 5272 kg/ha in the compacted field strip. The deterministic and functional model SIMWASER simulates the water balance and the crop yield for any number of crop rotations and years, provided that daily weather records (air temperature, humidity of air, global radiation, wind and precipitation) are available. Crop growth and soil water regime are coupled together by the physiological processes of transpiration and assimilation, which take place at the same time through the stomata of the plant leaves and are both reacting in the same direction to changes in the soil water availability within the rooting zone. The water availability during rainless seasons depends on the hydraulic properties of the soil profile within the rooting depth and on rooting density. Rooting depth and density are affected by both the type of the crop and the penetration resistance of the soil, which depends on the soil moisture status and may be strongly increased by soil compaction. The model SIMWASER was able to simulate these effects as shown by the calculated grain yields, which amounted in the non-compacted plot to 7512 and to 5558 kg dry matter/ha in the compacted plot.     

Subsoil compaction by heavy sugarbeet harvesters in southern Sweden: III. Risk assessment using a soil water model

Due to its persistence, subsoil compaction should be avoided, which can be done by setting stress limits depending on the strength of the soil. Such limits must take into account soil moisture status at the time of traffic. The objective of the work presented here was to measure soil water changes during the growing period, use the data to calibrate a soil water model and simulate the soil susceptibility to compaction using meteorological data for a 25-year period. Measurements of soil water content were made in sugarbeet (Beta vulgaris L.) from sowing until harvest in 1997 on two sites classified as Eutric Cambisols in southern Sweden. Sampling was carried out at 2-week intervals in 0.1 m layers down to 1 m depth, together with measurements of root growth and crop development. Precompression stress of the soil at 0.3, 0.5 and 0.7 m depth was determined from uniaxial compression tests at water tensions of 6, 30, 60 and 150 kPa and adjusted as a logarithmic function of the soil water tension. Soil water content was simulated by the SOIL model for the years 1963–1988. Risk calculations were made for a wheel load of 8 t and a ground pressure of 220 kPa, corresponding to a fully loaded six-row sugarbeet harvester. Subsoil compaction was expected to occur when the major principal stress was higher than the precompression stress. The subsoil water content was very low in late summer, but increased during the autumn. At the end of August, there was practically no plant available water down to 1 m depth. There was in general good agreement between measured and simulated values of soil water content for the subsoil, but not for the topsoil. In the 25-year simulations, the compaction risk at 50 cm depth was estimated to increase from around 25% to nearly 100% between September and late November, which is the period when the sugarbeet are harvested. The types of simulation presented here may be a very useful tool for practical agriculture as well as for society, in giving recommendations as to how subsoil compaction should be avoided.     

Estimating soil resilience to compaction by measuring changes in surface and subsurface levels

Abstract. In view of the increasing wheel loads of agricultural vehicles, the question arises as to whether soil can recover from the mechanical impact of traffic. The damage to soil quality depends also on the soils resilience. This paper presents a new approach to monitoring vertical soil movement in situ . We assessed the effects of trafficking the soil with excavators and sugarbeet harvesters by monitoring surface and subsurface levels. The caterpillar loads of the crawlers varied from 13 to 19 t, the wheel loads of the sugarbeet harvesters from 6 to 11 t. Classical geodetic levelling was used to record soil surface movement and a hydrostatic settlement meter measured deformation at three depths within the soil profile. The results of three field tests prove the importance of wheel load and soil moisture for soil compaction. Trafficking very dry soil with an excavator did not cause significant plastic deformation in 30 cm depth. Conversely, trafficking wet soil with a sugarbeet harvester led to soil sinkage of 1 to 2 cm even at 60 cm depth. Increased wheel load in subsequent passes led to greater subsidence than during the first pass. Settlement decreased from the soil surface to deeper layers, but it remained throughout the monitoring period of up to 12 days. No soil recovery from plastic deformation was recorded within this time. The measuring system has the potential for long-term monitoring of the mechanical recovery of the soil. Additionally, it can contribute to the validation of mechanical impact models, which are based on soil stresses.     

Technical solutions to reduce the risk of subsoil compaction: effects of dual wheels, tandem wheels and tyre inflation pressure on stress propagation in soil

The use of heavy machinery is increasing in agriculture, which induces increased risks of subsoil compaction. Hence, there is a need for technical solutions that reduce the compaction risk at high total machine loads. Three field experiments were performed in order to study the effects of dual wheels, tandem wheels and tyre inflation pressure on stress propagation in soil. Vertical soil stress was measured at three different depths by installing probes into the soil horizontally from a dug pit. In one experiment, also the stress distribution below the tyre was measured. Beneath the dual wheels, vertical stresses at 0.15 and 0.3 m depth were lower between the two wheels than under the centre of each wheel, despite the gap between the wheels being small (0.1 m). At 0.5 m depth, vertical stress beneath the wheels was the same as between the two wheels. The stress interaction from the two wheels was weak, even in the subsoil. Accordingly, measured stresses at 0.3, 0.5 and 0.7 m depth were highest under the centre of each axle centre line of tandem wheels, and much lower between the axles. For a wheel load of 86 kN, tyre inflation pressure significantly affected stress at 0.3 m depth, but not at greater depths. Stress directly below the tyre, measured at 0.1 m depth, was unevenly distributed, both in driving direction and perpendicular to driving direction, and maximum stress was considerably higher than tyre inflation pressure. Calculations of vertical stress based on Boussinesq's equation for elastic materials agreed well with measurements. A parabolic or linear contact stress distribution (stress declines from the centre to the edge of the contact area) was a better approximation of the contact stress than a uniform stress distribution. The results demonstrate that stress in the soil at different depths is a function of the stress on the surface and the contact area, which in turn are functions of wheel load, wheel arrangement, tyre inflation pressure, contact stress distribution and soil conditions. Soil stress and soil compaction are a function of neither axle load nor total vehicle load. This is of great importance for practical purposes. Reducing wheel load, e.g. by using dual or tandem wheels, also allows tyre inflation pressure to be reduced. This reduces the risk of subsoil compaction.     

Alleviation of soil compaction: requirements, equipment and techniques

The nature of soil disturbance required to alleviate soil compaction in a range of agricultural and land restoration situations is identified. Implement geometry and adjustments required to achieve the desired brittle or tensile deformation of compacted soil are discussed. Field operating procedures to achieve the required degrees of soil fissuring, loosening or soil unit rearrangement using the power units and equipment available are described. A new progressive loosening technique is identified for use within deep, extremely compacted soil profiles. Emphasis is given to the importance of making visual field checks across the loosened soil zone at an early stage, to check the desired disturbance is being achieved. Care must be taken during subsequent trafficking operations, to minimize the risk of recompaction.     

Effect of the number of tractor passes on soil rut depth and compaction in two tillage regimes

The initially high level of soil compaction in some direct sowing systems might suggest that the impact of subsequent traffic would be minimal, but data have not been consistent. In the other hand on freshly tilled soils, traffic causes significant increments in soil compaction. The aim of this paper was to quantify the interaction of the soil cone index and rut depth induced by traffic of two different weight tractors in two tillage regimes: (a) soil with 10 years under direct sowing system and (b) soil historically worked in conventional tillage system. Treatments included five different traffic frequencies (0, 1, 3, 5 and 10 passes repeatedly on the same track). The work was performed in the South of the Rolling Pampa region, Buenos Aires State, Argentina at 34°55′S, 57°57′W. Variables measured were (1) cone index in the 0–600 mm depth profile and (2) rut depth. Tyre sizes and rut depth/tyre width ratio are particularly important respect to compaction produced in the soil for different number of passes. Until five passes of tractor (2WD), ground pressure is responsible of the topsoil compaction. Until five passes the tyre with low rut depth/tyre width ratio reduced topsoil compaction. Finally, the farmer should pay attention to the axle load, the tyre size and the soil water content at the traffic moment.     

Soil compaction and stress propagation after different wheeling intensities on a silt soil in South-East Norway

The objective of this study was to evaluate the effect of wheeling with two different wheel loads (1.7 and 2.8?Mg) and contrasting wheeling intensities (1x and 10x) on the bearing capacity of a Stagnosol derived from silty alluvial deposits. Soil strength was assessed by laboratory measurements of the precompression stress in topsoil (20?cm) and subsoil (40 and 60?cm) samples. Stress propagation, as well as elastic and plastic deformation during wheeling were measured in the field with combined stress state (SST) and displacement transducers (DTS). We also present results from soil physical analyses (bulk density, air capacity, saturated hydraulic conductivity) and barley yields from the first two years after the compaction. Although the wheel loads used were comparatively small, typical for the machinery used in Norway, the results show that both increased wheel load and wheeling intensity had negative effects on soil physical parameters especially in the topsoil but with similar tendencies also in the subsoil. Stress propagation was detected down to 60?cm depth (SST). The first wheeling was most harmful, but all wheelings led to accumulative plastic soil deformation (DTS). Under the workable conditions in this trial, increased wheeling with a small machine was more harmful to soil structure than a single wheeling with a heavier machine. However, the yields in the first two years after the compaction did not show any negative effect of the compaction.     

The efficacy of three techniques to alleviate soil compaction at a restored sand and gravel quarry

Reinstated soil at restored sites often suffers from severe compaction which can significantly impede root development. Several methods, such as ripping and complete cultivation, are available to alleviate compaction that may occur as a result of soil reinstatement. This paper examines the effectiveness of the industry standard industrial ripper and a prototype modern ripper, the Mega‐Lift, in comparison with the recommended best practice method of complete cultivation. An investigation of the penetration resistance of the soil at a restored sand and gravel quarry was carried out using a cone penetrometer and a ‘lifting driving tool’ (dropping weight penetrometer) 3 years following cultivation. All the cultivation treatments reduced soil compaction to some degree compared with the untreated control. However, the penetration resistance values suggest that rooting would be restricted at relatively shallow depths in the plots cultivated using the industrial and Mega‐Lift ripper; penetration resistance exceeded 2 MPa within the first 0.33 m. Complete cultivation maintained penetration resistance values of less than 2 MPa within the depth limit of the penetrometer of 0.42 m. In addition, the results from the ‘lifting driving tool’ indicate that soils treated using complete cultivation remained significantly looser than those treated with the ripper to a depth of at least 0.80 m. The results demonstrate that complete cultivation remains the most effective method of alleviating soil compaction on restored sites, although it is recognized that its relatively high cost may restrict the uptake of the technique.     

Structural improvement in a grassland soil after changes to wheel-traffic systems to avoid soil compaction

Abstract. We report on a study aimed at assessing improvements in soil structure that developed when a reduced ground-pressure traffic system was introduced onto grassland previously compacted by conventional machinery traffic, and when a zero traffic system was introduced on land previously under a reduced ground-pressure system. Increases in the volume, average size, and number of macropores, measured by image analysis, together with decreases in vane shear strength indicated structural improvement in soil under the substitute traffic systems relative to the same soil in the original systems. A smaller content of organic matter in the soil of the substitute systems than in the original systems was attributed to improved aeration and greater earthworm activity in the former.     

Remediation of subsoil compaction and compaction effects on corn N availability by deep tillage and application of poultry manure in a sandy-textured soil

Subsoil compaction may reduce the availability and uptake of water and plant nutrients thereby lowering crop yields. Among the management options for remediating subsoil compaction are deep tillage and the selection of crop rotations with deep-rooted crops, but little is known of the effects of applications of organic amendments on subsoil compaction. The objectives of this study were to determine the effects of subsoil compaction on corn yield and N availability in a sandy-textured soil and to evaluate the use of deep tillage and surface applications of poultry manure to remediate subsoil compaction. A field experiment planted to corn (Zea mays L.) was conducted from 2000 to 2001 on a Reelfoot fine sandy loam (fine-silty, mixed thermic Aquic Argiudolls) formed in silty alluvium located in southeast Missouri near the Mississippi River. Treatments were arranged in a factorial design with three levels of subsoil compaction and subsoiling and four rates (averaging 0, 6, 11 and 18 Mg ha⁻¹) of poultry manure. Subsoil tillage to a depth of 30 cm had multiple effects, including overcoming a natural or tillage-induced dense layer or pan and increasing volumetric soil water content and crop N uptake, especially in the 2001 cropping year with low early season precipitation. N recovery efficiency (NRE) was significantly higher in the subsoil treatment compared to the highest compaction treatment in 2001. No significant interactions between manure rates and compaction and subsoiling treatments were observed for corn grain and silage yields, N uptake and NRE. Average increases in corn grain yields over all manure rates due to subsoil tillage of compacted soil were 2002 kg ha⁻¹ in 2000 and 3504 kg ha⁻¹ in 2001. Application of poultry manure had a consistent positive effect on increasing grain yields and N uptake in 2000 and 2001 but did not significantly alter measured soil physical properties. The results of this study suggest that deep tillage and applications of organic amendments are management tools that may overcome restrictions in both N and soil water availability due to subsoil compaction in sandy-textured soils.     

Interaction of soil compaction, phosphorus and zinc on clover growth and accumulation of phosphorus

Soil compaction generally reduces crop performance because of degraded soil physical and biological properties, and possibly inappropriate soil nutritional status. The effects of varying compaction, and phosphorus (P) and zinc (Zn) supplies on the growth of Berseem or Egyptian clover (Trifolium alexandrimum), and accumulation of P and Zn in shoots and roots were investigated in a pot experiment using a surface layer of a Typic Torrifluvent (USDA), Calcaric Fluvisols (FAO) soil. Plants were treated with three soil compaction levels, three rates of P and three rates of Zn in a factorial combination. Phosphorus accumulation in shoots did not change up to bulk densities of 1.65 Mg m⁻³ and declined at bulk density of 1.80 Mg m⁻³. Increasing the levels of Zn and P resulted in a significant increase in shoot dry mass (from 0.3 to 0.8 g pot⁻¹), and root length (from 11.4 to 32.5 m pot⁻¹). Shoot and root growth were reduced by soil compaction particularly at low P and Zn application rates. Shoot dry mass was reduced from 0.8 to 0.3 g pot⁻¹, and root length from 43 to 5 m pot⁻¹ at bulk densities of 1.4 and 1.8 Mg m⁻³, respectively. However, the accumulation of P (from 0.06 to 0.15 g kg⁻¹) and Zn per unit length of roots (from 0.8 to 1.8 μg pot⁻¹) increased as soil compaction increased. As the Zn supply increased, Zn accumulation per unit length of roots, and total Zn accumulation increased. Severe compaction reduced P and Zn accumulation in shoots and also decreased shoot dry mass, and root length compared to lower soil compaction levels. The present study suggests that Zn and P supply can moderate the adverse effect of soil compaction on clover performance.     

Influence of soil compaction on carbon and nitrogen mineralization of soil organic matter and crop residues

 We studied the influence of soil compaction in a loamy sand soil on C and N mineralization and nitrification of soil organic matter and added crop residues. Samples of unamended soil, and soil amended with leek residues, at six bulk densities ranging from 1.2 to 1.6 Mg m^–3 and 75% field capacity, were incubated. In the unamended soil, bulk density within the range studied did not influence any measure of microbial activity significantly. A small (but insignificant) decrease in nitrification rate at the highest bulk density was the only evidence for possible effects of compaction on microbial activity. In the amended soil the amounts of mineralized N at the end of the incubation were equal at all bulk densities, but first-order N mineralization rates tended to increase with increasing compaction, although the increase was not significant. Nitrification in the amended soils was more affected by compaction, and NO₃ ^–-N contents after 3 weeks of incubation at bulk densities of 1.5 and 1.6 Mg m^–3 were significantly lower (by about 8% and 16% of total added N, respectively), than those of the less compacted treatments. The C mineralization rate was strongly depressed at a bulk density of 1.6 Mg m^–3, compared with the other treatments. The depression of C mineralization in compacted soils can lead to higher organic matter accumulation. Since N mineralization was not affected by compaction (within the range used here) the accumulated organic matter would have had higher C : N ratios than in the uncompacted soils, and hence would have been of a lower quality. In general, increasing soil compaction in this soil, starting at a bulk density of 1.5 Mg m^–3, will affect some microbially driven processes. Received: 10 June 1999     

Effect of soil compaction on photosynthesis and carbon partitioning within a maize–soil system

Soil compaction is known to affect plant growth. However, most of the information regarding the effects of this factor on carbon partitioning has been obtained on young plants while little is known about the evolution of these effects with plant age. The objective of this work was to investigate how soil compaction affects carbon assimilation, photosynthate partitioning and morphology of maize plants during vegetative growth up to tassel initiation. A pressure was applied on moist soil to obtain a bulk density of 1.45 g cm⁻³ (compacted soil (CS) treatment) while the loose soil (LS) treatment (bulk density of 1.30 g cm⁻³) was obtained by gentle vibration of soil columns. Plants were grown in a growth chamber for 3–6 weeks and carbon partitioning in the plant–soil system was evaluated using ¹⁴C pulse-labelling techniques. Soil compaction greatly hampered root elongation and delayed leaf appearance rate, thereby decreasing plant height, shoot and root dry weights and leaf area. The increase in soil bulk density decreased carbon assimilation rate especially in early growth stages. The main effect of soil compaction on assimilate partitioning occurred on carbon exudation, which increased considerably to the detriment of root carbon. Furthermore, soil microbial biomass greatly increased in CS. Two hypotheses were formulated. The first was that increasing soil resistance to root penetration induced a sink limitation in roots and this increased carbon release into the soil and resulted in a root feedback that regulated carbon assimilation rate. The second hypothesis relies on soil–plant water relations since, due to compaction, the pore size distribution has to be considered. In a compacted soil, the peak of the pore size distribution curve is shifted towards the small pore size. The volume of small pores increases and the unsaturated conductivity decreases substantially, when compared to non-compacted soil. Due to small hydraulic conductivity, the inflow into the roots is well below optimum and the plant closes stomata thus reducing carbon assimilation rate. The effects of soil compaction persisted with plant age although the difference between the two treatments, in terms of percentage, decreased at advanced growth stages, especially in the case of root parameters.     

Field compaction at different soil-water status: effects on pore size distribution and soil water characteristics of a Rhodic Ferralsol in Western Cuba

The level of compaction induced on cultivated fields through trafficking is strongly influenced by the prevailing soil-water status and, depending on the attendant soil degradation, vital soil hydraulic processes could be affected. Therefore, understanding the relationship between field soil-water status and the corresponding level of induced compaction for a given load is considered an imperative step toward a better control of the occurrence of traffic-induced field soil compaction. Pore size distribution, a fundamental and highly degradable soil property, was measured in a Rhodic Ferralsol, the most productive and extensively distributed soil in Western Cuba, to study the effects of three levels of soil compaction on soil water characteristic parameters. Soil bulk density and cone penetration index were used to measure compaction levels established by seven passes of a 10 Mg tractor at three soil-water statuses corresponding to the plastic (Fs), friable (Fc) and relatively dry soil (Ds) consistency states. Pore size distribution calculated from soil water characteristic curves was classified into three pore size categories on the basis of their hydraulic functioning: >50 μm (f_>50 μm), 50–0.5 μm (f_{50–0.5 μm}) and <0.5 μm (f_<0.5 μm). The greatest compaction levels were attained in the Fs and Fc soil water treatments, and a significant contribution to compaction was attributed to the existing soil water states under which the soil compaction was accomplished. Average cone index (CI) values in the range of 2.93–3.70 MPa reflected the accumulation of f_<0.5 μm pores, and incurred severe reductions in the volume of f_>50 μm pores in the Fs and Fc treatments, while an average CI value of 1.69 MPa indicated increments in the volume of f_{50–0.5 μm} in the Ds treatment. Despite the differential effects of soil compaction on the distribution of the different pore size categories, soil total porosity (f_Total) was not effective in reflecting treatment effects. Soil water desorption at the soil water potentials evaluated (0.0 to −15,000 cm H₂O) was adversely affected in the f_<0.5 μm dominated treatments; strong soil water retention was observed with the predominance of f_<0.5 μm, as was confirmed by the high water content at plant wilting point. Based on these findings, the use of field capacity water content as the upper limit of plant available soil water was therefore considered inappropriate for compacted soils.