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In this study, we explored the effects of microbial activity on the evaporation of water from cores of a sandy soil under laboratory conditions. We applied treatments to stimulate microbial activity by adding different amounts of synthetic analogue root exudates. For comparison, we used soil samples without synthetic root exudates as control and samples treated with mercuric chloride to suppress microbial activity. Our results suggest that increasing microbial activity reduces the rate of evaporation from soil. Estimated diffusivities in soil with the largest amounts of added root exudates were one third of those estimated in samples where microbial activity was suppressed by adding mercuric chloride. We discuss the effect of our results with respect to water uptake by roots. Highlights: We explored effects of microbial activity on the evaporation of water from cores of a sandy soil. We found the effect of microbial activity on water release characteristic was small. Increasing microbial activity reduced evaporation from soil, while microbial suppression increased it. Effect of microbial activity on root water uptake was estimated to be equivalent to a change in soil structure. [ABSTRACT FROM AUTHOR]

We explore the effect of microbial activity stimulated by root exudates on the penetrometer resistance of soil and its elastic modulus. This is important because it is a measure of the mechanical strength of soil and it correlates closely with the rate of elongation of roots. A sandy soil was incubated with a synthetic root exudate at different temperatures, for different lengths of time and with selective suppression of either fungi or bacteria. The shape of the temperature response of penetrometer resistance in soil incubated with synthetic exudate was typical of a poikilothermic temperature response. Both penetrometer resistance and small strain shear modulus had maximum values between 25 and 30°C. At temperatures of 20°C and less, there was little effect of incubation with synthetic root exudate on the small strain shear modulus, although penetrometer resistance did increase with temperature over this range (4-20°C). This suggests that in this temperature range the increase in penetrometer resistance was related to a greater resistance to plastic deformation. At higher temperatures (> 25°C) penetrometer resistance decreased. Analysis of the DNA sequence data showed that at 25°C the number of Streptomyces ( Gram-positive bacteria) increased, but selective suppression of either fungi or bacteria suggested that fungi have the greater role with respect to penetrometer resistance. Highlights Effect of microbial activity stimulated by synthetic root exudates on the mechanical properties., We compared penetrometer measurements and estimates of elastic modulus with microbial community., Penetrometer resistance of soil showed a poikilothermic temperature response., Penetrometer resistance might be affected more by fungi than bacteria. [ABSTRACT FROM AUTHOR]

Soil compaction is a serious issue that affects the growth of crop roots Penetrometer resistance is often used as an indicator of the resistance of soil to root elongation. We developed a simple function to estimate penetrometer resistance from the soil compression characteristic. Five soils with contrasting textures and organic matter contents were used in this work. Air-dry soil samples were saturated on a tension table and then drained to different matric potentials not less than -30 kPa. The equilibrated soil were compacted in a uniaxial compression device to give precompression stresses in the range of 30 to 1000 kPa. The penetrometer resistance of the soils was then measured with a 2-mm-diameter 60° cone penetrometer. Soil compression characteristics varied with soil texture, organic matter content, and initial soil water content. Penetrometer resistance values increased with decreasing void ratio and could be explained by the precompression stres and the slope of the compression characteristic, using a simple equation with parameters independent of soil type. Our equation explained 84% of the variance in penetrometer resistance. [ABSTRACT FROM AUTHOR]

The velocities of shear waves in three different soils were measured as a function of consolidation using a Bishop and Wesley triaxial cell. We considered the soil deforming plastically on the normal consolidation line as well as elastic deformation on the rebound curve. In both cases, the soils were saturated and subjected to an isotropic stress environment. The relationships between the velocity of a shear wave and effective stress were similar for the sand and silt soils, but for clay soils the velocity of a shear wave was smaller, showing a soil type dependency. Additionally there was no unique relationship between shear wave velocity and effective stress for soils during plastic deformation (on the normal consolidation line) and elastic deformation (on the rebound elastic curve). By combining empirical relationships between shear wave velocity as a function of void ratio and effective stress with the normal consolidation curve, we were able to collapse the data from three soils onto a common curve irrespective of whether the soil was subjected to elastic or plastic deformations. [ABSTRACT FROM AUTHOR]

Measurements have been made of the effect of compaction on water retention, saturated hydraulic conductivity, and porosity of two English soils: North Wyke (NW) grassland clay topsoil and Broadbalk silty topsoil, fertilized inorganically (PKMg) or with farmyard manure (FYM). As expected, the FYM topsoil had greater porosity and greater water retention than PKMg topsoil, and the NW clay topsoil retained more water at each matric potential than the silty topsoils. Compaction had a clear effect on water retention at matric potentials wetter than −10 kPa for the PKMg and FYM soils, corresponding to voids greater than 30 μm cylindrical diameter, whereas smaller voids appeared to be unaffected. The Pore-Cor void network model has been improved by including a Euler beta distribution to describe the sizes of the narrow interconnections, termed throats. The model revealed a change from bimodal to unimodal throat size distributions on compaction, as well as a reduction in sizes overall. It also matched the water retention curves more closely than van Genuchten fits and correctly predicted changes in saturated hydraulic conductivity better than those predicted by a prior statistical approach. However, the changes in hydraulic conductivity were masked by the stochastic variability of the model. Also, an artifact of the model, namely its inability to pack small features close together, caused incorrect increases in pore sizes on compaction. These deficiencies in the model demonstrate the need for an explicitly dual porous network model to account for the effects of compaction in soil. [ABSTRACT FROM AUTHOR]

In this study, we explored the effect of the roots of different forage grasses on soil hydraulic properties at the plot scale. To achieve this, we set up a field experiment in which six different grass cultivars were grown on replicated field plots at North Wyke, UK. We used tension infiltration measurements to assess soil hydraulic properties and structure. These measurements were made over two consecutive seasons. Measurements of shrinkage, water repellence, and the water release characteristic on soil samples taken from the North Wyke site were also made. We also wanted to compare the effects of different grasses on soil structure with the effects of differences in soil management; we therefore made tension infiltration measurements on fallow soil, permanent grassland, and arabic land on a long. term experiment at Rothamsted, Harpenden, UK. Our data showed that the saturated hydraulic conductivity of the capillary matrix of the soil sown with grass depended on the grass species. Grass species affected the characteristic pore size estimated from tension infiltration data. At the Rothamsted site, we were able to infer that the development of macroporc structure can be ranked grassland > arabic> fallow (from the greatest to the least amount of maeropores). In the North Wyke site, all the grass plots showed evidence of a macropore structure, consistent with the grassland site at Rothamsted, but there did not appear to be any variation between grass species. We concluded that changes to soil structure were probably due to physical rearrangement of soil particles by shrinkage. [ABSTRACT FROM AUTHOR]

Water-filled tensiometers are widely used to measure the matric potential of so ii water. It is often assumed that, because these give a direct reading, they are accurate. With a series of laboratory tests with model laboratory systems of increasing complexity we show that the output of water-filled tensiometers can, particularly in drying soils, be in serious error. Specifically, we demonstrated that water-filled tensiometers can indicate a steady matric potential, typically between -60 and -90 kPa, when the soil is much drier. We demonstrate the use of water-filled tensiometers that can measure matric potentials smaller than -100 kPa in the laboratory and in the field. The physics of the failure of water-filled tensiometers is discussed. When the matric potential was greater than -60 kPa, in laboratory and field tests water-filled and porous matrix sensors were in good agreement. In the field environment the porous matrix sensor was useful because it allowed early detection of the failure of water-filled tensiometers. In dry soils (matric potential < -60 kPa) the porous matrix sensor was more reliable and accurate than the water-filled tensiomctcr. [ABSTRACT FROM AUTHOR]

We describe the use of a capillary rise method to measure the wettability of 10 samples of agricultural soil from Rothamsted long-term experimental sites. The samples have very similar clay contents, but organic carbon (C) contents range from 11.5 to 31.2 g kg−1. Their wetting rates were interpreted by an improved method of data analysis, consistent with the Washburn equation, and showed an increase in the effective contact angle between the water meniscus and the soil with increasing C content. This corresponds to a decrease in wettability with increasing C content, and accords with other results reported in the literature. By contrast with water, we found that capillary rise for n-hexane into soil did not depend on the soil’s bulk density or C content. A priori calculations of the expected wetting rates from fluid properties and an effective hydraulic radius estimated by other methods gave magnitudes and trends that agreed with our experimental data. The results show that estimates of effective hydraulic radius can provide a useful approximation for characterizing soil wetting, but that further modelling should be carried out. [ABSTRACT FROM AUTHOR]

Soil compaction has deleterious effects on soil physical properties, which can affect plant growth, but some soils are inherently resilient, whereby they may recover following removal of the stress. We explored aspects of soil physical resilience in a field-based experiment. We subjected three soils of different texture, sown with winter wheat or remaining fallow, to a compaction event. We then monitored soil strength, as a key soil physical property, over the following 16 months. We were also interested in the associated interactions with crop growth and the microbial community. Compaction had a considerable and sustained effect in a sandy loam and a sandy clay loam soil, resulting in an increase in strength and decreased crop yields. By contrast compaction had little effect on a clay soil, perhaps due initially to the buoyancy effect of pore water pressure. Fallow clay soil did have a legacy of the compaction event at depth, however, suggesting that it was the actions of the crop, and rooting in particular, that maintained smaller strengths in the cropped clay soil rather than other physical processes. Compaction generally did not affect microbial communities, presumably because they occupy pores smaller than those affected by compaction. That the clay soil was able to supply the growing crop with sufficient water whilst remaining weak enough for root penetration was a key finding. The clay soil was therefore deemed to be much more resilient to the compaction stress than the sandy loam and sandy clay loam soils. [ABSTRACT FROM AUTHOR]

This article discusses the increasing demands that are being placed on watersheds due to socio-economic and climate related influences. The result is that it is becoming more difficult for watersheds to supply an adequate amount of clean water to the populations that they have traditionally served. The article also mentions that increasing agricultural demands are depleting the resources that watersheds supply. The article discusses the growing use of vegetation to regulate watersheds and the hydrological cycle. It notes that research has shown the valuable role that plants and their roots systems play in this process and that continuing genetic research is being conducted on various grasses to develop a solution to some of the problems watersheds are facing.

The matric potential of soil water is probably the most useful assessment of soil water status. However, the water-filled tensiometer (the benchmark instrument for measuring matric potential) typically only operates in the range 0 to −85 kPa. In this paper, we report the development of a porous-matrix sensor to measure matric potential in the approximate range −50 to −300 kPa. The sensor uses a dielectric probe to measure the water content of a ceramic material with known water retention characteristics. The calculation of matric potential takes into account hysteresis through the application of an appropriate model to measured wetting and drying loops. It is important that this model uses closed, rather than open, scanning loops. The calibrated sensors were tested in the field and the output compared with data from water-filled tensiometers and dielectric measurements of soil water content. These comparisons indicated that conventional tensiometers gave stable but false readings of matric potential when soil dried to matric potentials more negative than −80 kPa. The porous-matrix sensors appeared to give reliable readings of matric potential in soil down to −300 kPa and also responded appropriately to repeated wetting and drying. This porous-matrix sensor has considerable potential to help understand plant responses to drying soil. [ABSTRACT FROM AUTHOR]

The challenge that faces agriculture at the start of the 21st Century is to provide security of food production in a sustainable way. Achieving this task is difficult enough, but against a background of climate change, it becomes a moving target. However, one certainty is that soil factors that limit crop growth must be taken into account as new strategies for crop management are developed. To achieve this, it is necessary to measure the physical and nutritional status of the root environment in the field. Before considering measurement methods, our understanding of how the plant interacts with its soil environment is reviewed, so that it is clear what needs to be measured. Soil strength due to soil drying is identified as an important stress that limits agricultural productivity. The scope to measure soil factors that directly affect plant growth is reviewed. While in situ sensors are better developed, progress in the development of remote sensors of soil properties are also reviewed. A robust approach is needed to interpret soil measurements at the field scale and here geostatistics has much to offer. The present review takes a forward look and explores how our understanding of plant responses to soil conditions, the newly emerging sensing technologies and geostatistical tools can be drawn together to develop robust tools for soil and crop management. This is not intended to be an exhaustive review. Instead, the authors focus on those aspects that they consider to be most important and where the greatest progress is being made. [ABSTRACT FROM PUBLISHER]

The restoration of the physical structure of soil by plant roots depends on the ability of roots to penetrate strong soil. The sensitivity of root growth to mechanical impedance can be considered as a possible screen for species suitable as 'biological subsoilers'. The ability of roots to penetrate hard wax layers in sand cores has recently been identified as a useful tool for identifying rice varieties that are good at penetrating strong soil, and this method provides the possibility of a rapid laboratory screen which can be used to identify other species with the same characteristic. However, comparing plants with very different root anatomy may be more difficult than comparing the performance of root systems that are essentially similar, except for the number and length of roots, as is the case with rice. In this paper, we compare the penetration of roots of barley, chicory, lupin, red clover and lucerne through soft and hard wax layers installed at 50 mm depth in sand columns. Barley had the lowest fraction of roots that penetrated the hard wax layer, although it had a higher absolute number of roots penetrating the hard wax than the other species. The numbers of chicory, lupin, red clover and lucerne roots penetrating the hard wax layer were similar, although lucerne had the highest number. The proportion of roots penetrating the hard layer as a proportion of those penetrating the soft layer was highest for lucerne. Comparison with previous data for barley and lucerne supports the use of the wax layer screen for identifying species with good root penetration. [ABSTRACT FROM AUTHOR]

The physical characteristics of the soil at the root–soil interface are crucial because they determine both physical aspects of root function such as water and nutrient uptake and the microbial activity that is most relevant to root growth. Because of this we have studied how root activity modifies the structure and water retention characteristic of soil adjacent to the root for maize, wheat and barley. These plants were grown in pots for a 6-week growth period, then the soil adjacent to the root (rhizosphere soil) and bulk soil aggregates were harvested. These soil aggregates were then saturated and equilibrated at matric potentials between−600 kPa and saturation, and the water retention characteristics were measured. From subsamples of these aggregates, thin sections were made and the porosity and pore-size distributions were studied with image analysis. Both image analysis and estimates of aggregated density showed that the rhizosphere soil and bulk soil had similar porosities. Growing different plants had a small but significant effect on the porosity of the soil aggregates. Image analysis showed that for all the plant species the structure of the rhizosphere soil was different to that of the bulk soil. The rhizosphere soil contained more larger pores. For maize and barley, water retention characteristics indicated that the rhizosphere soil tended to be drier at a given matric potential than bulk soil. This effect was particularly marked at greater matric potentials. The difference between the water retention characteristics of the bulk and rhizosphere soil for wheat was small. We compare the water retention characteristics with the data on pore-size distribution from image analysis. We suggest that differences in wetting angle and pore connectivity might partly explain the differences in water retention characteristic that we observed. The impact of differences between the water retention properties of the rhizosphere and bulk soil is discussed in terms of the likely impact on root growth. [ABSTRACT FROM AUTHOR]

The water retention characteristic provides the traditional data set for the derivation of a soil's pore-size distribution. However, the technique employed to achieve this requires that assumptions be made about the way pores interconnect. We explore an alternative approach based on stray field nuclear magnetic resonance (STRAFI-NMR) to probe the water-filled pores of both saturated and unsaturated soils, which does not require information relating to pore connectivity. We report the relative size distributions of water-occupied pores in saturated and unsaturated samples of two sets of glass beads of known particle size, two sands, and three soils (a silty loam, a sandy loam and a loamy sand), using measurements of the NMRT1 proton relaxation time of water. TheT1 values are linearly related to pore size and consequently measuredT1 distributions provide a measure of the pore-size distribution. For both the sands and the glass beads at saturation theT1 distributions are unimodal, and the samples with small particle sizes show a shift to smallT1 values indicating smaller voids relative to the samples with larger particles. Different matric potentials were used to reveal how the water-occupied pore-size distribution changes during drainage. These changes are inconsistent with, and demonstrate the inadequacies of, the commonly employed parallel-capillary tube model of a soil pore space. We find that not all pores of the same size drain at the same matric potential. Further, we observe that theT1 distribution is shifted to smaller values beyond the distribution at saturation. This shift is explained by a change in the weighted average of the relaxation rates as the proportion of water in the centre of water-filled pores decreases. This is evidence for the presence of pendular structures resulting from incomplete drainage of pores. For the soils the results are similar except that at saturation theT1 distributions are bimodal or asymmetrical, indicative of inter-aggregate and intra-aggregate pore spaces. We conclude that the NMR method provides a characterization of the water-filled pore space which complements that derived from the water retention characteristic and which can provide insight into the way pore connectivity impacts on drainage. [ABSTRACT FROM AUTHOR]

The development of a method using wax layers to simulate the effect of strong soil crusts on seedling emergence is described. Wax layers of different strengths were prepared by melting together white soft paraffin and paraffin wax in different proportions. The wax discs were placed above seeds planted in wet, but well-aerated sand in controlled environments. The effect of adding charcoal to the wax discs to prevent transmission of light was also tested. When wax layer penetrometer pressure was 0.2-0.25 MPa, light transmission greatly decreased the final emergence of carrot, but not onion seedlings. Penetrometer pressures above 0.25 MPa greatly decreased emergence through black wax layers in both carrot and onion. The presence of 2-4 mm stones immediately below wax layers (to simulate aggregates) decreased the emergence of onion shoots through wax layers with penetrometer pressures of 0.25 MPa and above. The emergence of carrot and onion seedlings from wax layers was compared to emergence in the field from different soil types and aggregate sizes. In both laboratory and field experiments, carrot gave better emergence than onion when emergence was relatively poor. Results were consistent with the hypothesis that mechanical impedance is a major factor in poor crop emergence in temperate conditions. [ABSTRACT FROM AUTHOR]

Both temperature and soil moisture vary greatly in the surface layers of the soil through which seedlings grow following germination. The work presented studied the impact of these environmental variables on post‐germination carrot growth to nominal seedling emergence. The rapid pre‐crook downward growth of both the hypocotyl and root was consistent with their requirement for establishment in soil drying from the surface. At all temperatures, both hypocotyl and root growth rates decreased as water stress increased and there was a very distinct temperature optimum that tended to occur at lower temperatures as water stress increased. A model based on the thermodynamics of reversible protein denaturation was adapted to include the effects of water potential in order to describe these growth rate responses. In general, the percentage of seedlings that reached the crook stage (start of upward hypocotyl growth) decreased at the extremes of the temperature range used and was progressively reduced by increasing water stress. A model was developed to describe this response based on the idea that each seedling within a population has lower and upper temperature thresholds and a water potential threshold which define the conditions within which it is able to grow. This threshold modelling approach which applies growth rates within a distribution of temperature and water potential thresholds could be used to simulate seedling growth by dividing time into suitable units. [ABSTRACT FROM PUBLISHER]

In recent years the use of porous material sensors for matric potential, which were originally intended for soil drier than -100 kPa, has been extended to wet soils. In these wetter soils, unpredictable behaviour of the sensors has been reported. We have studied the design of porous material sensors of matric potential in soil and propose a hypothesis to explain this unpredictability, and suggest recommendations for a design of sensor which will behave more reliably. The development of an experimental porous material sensor of matric potential based on this design is described. It operates between 0 and -60 kPa, and both the drying and wetting moisture characteristics were measured. In this sensor the porous material was a ceramic and its water content was measured with a dielectric water content sensor. We tested a simple closed-form hysteresis model to convert the measured water content of the porous material into matric potential under laboratory conditions. This was shown to give better results than using a calibration based on the drying moisture characteristic curve, where the predicted matric potentials were too small. The use of the experimental sensors in the field environment is described. Both types of sensor were installed using the same procedure. As far as we are aware the experimental sensor described in this paper is the first porous material sensor of matric potential that can be installed in the same way as a conventional tensiometer. Both conventional tensiometers and the experimental porous material sensors gave similar estimates of matric potential. [ABSTRACT FROM AUTHOR]

Roots of 3-d-old pea seedlings ( Pisum sativum L.) were mechanically impeded using a sand core apparatus, which allowed mechanical impedance to be varied independently of aeration and water status. Turgor of root cortical cells was then measured using a pressure probe. In seedlings grown in sand cores for 1 d, impedance had little effect on turgor, but in seedlings grown in the sand cores for 2 d, impedance increased turgor by 0.18 MPa in the apical 6 mm. [ABSTRACT FROM AUTHOR]

X-ray sedimentation is potentially attractive for estimating the particle-size distribution of soil, but has been thought to overestimate clay fractions because of the iron they contain. We have examined X-ray sedimentation for estimating the particle-size distribution of fully dispersed and of aggregated clay. We obtained good agreement between the pipette and X-ray sedimentation methods using two different kaolinite samples with a small iron concentration. We also studied montmorillonite saturated with Li, Na, K, Mg or Ca to investigate possible effects of hindered settling. For the Ca montmorillonite, we obtained good agreement between X-ray sedimentation and the pipette method at concentrations up to 40 g l-1. We also found that X-ray sedimentation could be used at concentrations less than those recommended by the manufacturer. Hindered settling was observed at concentrations > 20 g l-1 for Li- and Na-saturated montmorillonite. Our data suggest that conducting experiments at different mass concentrations is a sensitive test to detect such problems. Finally we used a fractal model to investigate the possible errors that might result from the size dependence of aggregate density and show that the error caused by assuming a single value for particle density of solids is small. [ABSTRACT FROM AUTHOR]

In this paper we describe an experimental approach which allows turgor (P) in an impeded root to be measured without the need to remove the root the impeding environment. The maximum axial growth pressure (σmax) generated by completely impeded pea (Pisum sativum L.) roots was measured using a novel apparatus incorporating a force transducer. The apparatus was designed so that it was possible to gain access to the impeded root with the microcapillary of a pressure probe and so obtain in situ measurements of P. Turgor in cells in the apical region of impeded roots was 0.78 MPa, compared with 0.55 MPa in unimpeded roots. In impeded roots, σmax was 0.52 MPa, showing that the pressure component resulting from cell wall tension (W, where W = P - σ) decreased from 0.55 to 0.26 MPa as the roots became impeded. When impeded roots were removed from the apparatus, there was no decrease in P over the following 90 min. Impedance did not cause P to change in the non-elongating part of the roots further from the apex. [ABSTRACT FROM AUTHOR]

SUMMARY A procedure is developed which allows centrifugal photosedimentation to be used for analysing samples, such as clays, containing a broad range of particle sizes. The implicit assumptions that have to be made are discussed and the calculation of the particle size distribution from photosedimentation data is described. In particular, it is important to be certain that any light scattering theory, used to correct for the variation in light scattering with particle size, accurately represents the light scattering behaviour of clays. Appropriate theories of light scattering that can be used to correct photo sedimentation data for the variation in light scattering with particle size are identified. For kaolinite, over the size range 100 nm−1 μm, the anomalous diffraction theory for oblate ellipsoids was found to be a suitable theory for correcting photosedimentation data. [ABSTRACT FROM AUTHOR]

SUMMARY Time-domain reflectometry (TDR) is becoming an increasingly popular technique for measuring soil water content. It is based on measuring the dielectric constant of soil from the propagation velocity of a pulse travelling along an electromagnetic transmission line embedded in the soil. Although various designs of transmission line have been proposed, there have been no attempts to contrast these designs to enable the experimenter to select an appropriate system. A second area which needs consideration is the choice of calibration function, as the use of an empirical calibration with no physical basis persists. In this paper, transmission line design (balanced and unbalanced) is discussed and a simple linear calibration function based on refractive index is developed. [ABSTRACT FROM AUTHOR]

SUMMARY Plate-like particles can sediment in an oriented manner or completely randomly. The hydrodynamic conditions that lead to an oriented sedimentation are reviewed. The tendency for plate-like clay particles to adopt oriented sedimentation in high centrifugal fields is demonstrated. Maximum centrifugal fields that still permit random sedimentation are suggested on the basis of experimental observation. [ABSTRACT FROM AUTHOR]

Investigates whether the addition of organic materials can form stable aggregates in a degraded soil. Implications of soil organic matter content for soil management; Treatments of soil with added organic material; Assessment of soil stability.

"Agar and agarose gels were evaluated as systems to mechanically impede roots of rice (Oryza sativa L.). Two-layer gels were used so thatseedlings established in a layer of weak gel (0.35% weight/volume) and then grew downwards to encounter a treatment gel of up to 5.0% (w/v). Agarose gels were stronger than agar gels of the same concentration, reaching a maximum penetrometer resistance of 1.2 MPa at a concentration of 5.0%, compared to 0.3 MPa with agar. The 5.0% agar gel stimulated elongation of the seminal axis by 40% in seedlings of varietyTN1 (compared with elongation in the 0.2% gel), but decreased it by 15% in the variety Lac 23. Although increasing agarose concentration decreased seminal axis elongation in both varieties, the seminal axisdid not reach the lower layer of treatment gel when the concentration of the treatment gel was greater than 2.0%. The decreased root elongation was therefore a non-mechanical inhibition. In experiments conducted using a different batch of agarose, these inhibitory effects were not seen and strong agarose gels stimulated seminal axis elongation. It was concluded that the agar and agarose gel systems studied were unsuitable for studying the effect of mechanical impedance on the elongation of rice roots and that great care should be taken in interpreting the results of experiments using gels as a growth medium. [ABSTRACT FROM AUTHOR]

There is evidence that microbial populations play an important role in altering soil pore geometry, but a full understanding of how this affects subsequent soil behaviour and function is still unclear. In particular the role of microorganisms in soil structural evolution and its consequence for pore morphological development is lacking. Using a combination of bio-chemical measurements and X-ray Computed Tomography (CT) imaging, a temporal comparison of microscale soil structural development in contrasting soil environments was made. The aim was to quantify the effect of microbial activity in the absence of other features likely to cause soil deformation (e.g. earthworms, roots etc.) on soil structural development in wet soils, defined by changes in the soil porous architecture i.e. pore connectivity, pore shape and pore volume during a 24 week period. Three contrasting soil textures were examined and changes compared between field soil, sterilised soil and a glucose enhanced soil treatment. Our results indicate that soil biota can significantly alter their microhabitat by changing soil pore geometry and connectivity, primarily through localised gaseous release. This demonstrates the ability of microorganisms to modify soil structure, and may help reveal the scope by which the microbial-rich rhizosphere can locally influence water and nutrient delivery to plant roots. [ABSTRACT FROM AUTHOR]