Volumetric Air Content Research Articles

Accelerated carbonation of non-plastic soils mixed with hydrated lime: index properties influencing binder formation rate and mechanical improvement

Chemical stabilization—the mixing of additives like cement, lime, or fly-ash with soil to improve its mechanical properties—conventionally relies on hydration reactions to generate a binder. Accelerated soil carbonation is a nascent alternative method, whereby carbon dioxide is intentionally introduced in soil mixed with alkali additives to generate a carbonate binder and sequester carbon dioxide. Non-plastic sand and silt specimens mixed with hydrated lime were carbonated for varying amounts of time at different water contents and densities to evaluate the index properties influencing the rate of carbonation and degree of mechanical improvement. It was demonstrated that volumetric air and water contents primarily govern the rate of binder formation and that mechanical properties are influenced by the carbonate binder content and density. Under optimum conditions, soil specimens could be fully carbonated within 3 to 24 hours and unconfined compressive strengths as great as 3 to 4 MPa were achieved. The degree of strength improvement is comparable to cement-stabilized materials with similar dependence on soil type, density, and binder content. If techniques are developed that enable carbonation at scale, the sequestration of carbon dioxide would offset emissions associated with production of chemical additives used for chemical stabilization.

The fall cone test in unsaturated soil and tailings pastes

The fall cone test (FCT) is usually performed in saturated paste-like geomaterials to determine their undrained shear strength or liquid limit. Here the test is used to determine strength when a geomaterial is unsaturated and paste-like in its consistency. A mechanics-based interpretation is used, where the pore air pressure, pore water pressure and suction which prevail around the cone tip post-test are incorporated in strength and bearing capacity equations. The pore air and water pressures combine to form an apparent cohesion that dominates cone penetration resistance and can be determined from a FCT result. When the volumetric air content (va/v) is less than 0·15, samples become unvented and effectively sealed, and the FCT induces volumetric compression in the sample. This causes the apparent cohesion post-test to be smaller than the initial value. The ratio between the two is proportional to va/v for three tailings in a unique way. The initial apparent cohesion ranged from 0·6 kPa to 28 kPa. The post-test value was smaller by an amount depending on va/v. When va/v > 0·15, the pore air becomes connected to the atmosphere and very different pressure changes and volume changes occur around a penetrating cone, for which an alternative interpretation method is needed.

Numerical modeling of water–heat–vapor–salt transport in unsaturated soil under evaporation

Evaporation-induced soil salinization will threaten the durability of engineering buildings, and it is crucial to study the coupling effect of water–heat–vapor–salt (WHVS) for anticorrosion of porous materials. To analyze the coupled system, a modified and developed WHVS transport model is proposed under the conditions that the film water transport theory, surface tension affected by salt, the filling effect of precipitated salt on soil pores, the residual volumetric air content, the effect of salt on heat transport and storage and the effect of vapor flux and thermal radiation on thermal boundary are all taken into account. The predictions are validated and analyzed by the results obtained from evaporation experiments under different boundary conditions. The data of silica sand are from the literatures ([16,17,57]), and the data of silty clay are from our laboratory test. The results indicate that the calculated water content, solute concentration, temperature and evaporation rate are in good agreement with the experimental data, especially for silty clay. For silica sand, there are some deviations in concentration and temperature, and the overall agreement is satisfied. The consistency between the simulated and measured data shows the effectiveness of proposed model and that the model can be applied successfully in soil science and groundwater hydrology and can be extended to related disciplines.

Petrophysical Joint Inversion Applied to Alpine Permafrost Field Sites to Image Subsurface Ice, Water, Air, and Rock Contents

Quantification of ground ice is crucial for understanding permafrost systems and modelling their ongoing degradation. The volumetric ice content is however rarely estimated in permafrost studies, as it is particularly difficult to retrieve. Standard borehole temperature monitoring is unable to provide any ice content estimation, whereas non invasive geophysical techniques such as refraction seismic and electrical resistivity measurements can yield information to assess the subsurface ice distribution. Electrical and seismic data are hereby complementary sensitive to the phase change. A petrophysical joint inversion was recently developed to determine volumetric water, air, ice and rock contents from electrical and seismic data using a petrophysical model, but was so far only tested on synthetic data and one proof-of-concept field example. In order to evaluate its applicability on different types of permafrost materials and landforms (bedrock, rock glacier, talus slope), we apply this petrophysical joint inversion scheme to five profiles located in the northwestern Alps. The electrical mixing rule (Archie's second law) was hereby identified as a source of model uncertainty, as it applies only when the electrolytic conduction is the dominating process. We therefore investigate and compare four petrophysical models linking the electrical resistivity with the ground constituents: Archie's law, Archie's law with an additional surface conduction factor, a model considering only surface conduction, and the geometric mean model. In most cases, the three first resistivity relations yield largely comparable results, whose reliability is discussed. The geometric mean model better resolve high ice content, as it is less influenced by the ice-rock ambiguity. We perform a systematic analysis of the regularisation parameters and then evaluate our results with validation data including thaw depths and ice contents derived from borehole measurements. Geophysical surveys have generally a lower resolution than borehole data, but have the advantage of providing spatio-temporal information in 2D or 3D. The joint inversion results are in relatively good agreement with the validation data for all sites from ice-poor to ice-rich conditions, when choosing the most adequate resistivity model and porosity initial value. Additional forcing constraints (e.g. porosity range constraint) based on site knowledge can improve the model parameter estimation.

Water and Air Relations in Propagation Substrates

Greenhouse propagation of unrooted plant cuttings is characterized by short container cell height and high irrigation frequency. These conditions can result in high moisture level and low air content in soilless container substrates (“substrates”), causing delayed growth of adventitious roots and favoring root disease. The objective of this study was to quantify and compare substrate water and air relations for three propagation substrates (peat, rockwool, and phenolic foam) that varied widely in physical characteristics using four methods: 1) evaporation method with a tensiometer, 2) frozen column method, 3) gravimetric analysis, and 4) X-ray computed tomography (CT) analysis. Moisture retention curves based on evaporation (1) and the frozen column (2) resulted in differences for peat, but similar curves for rockwool and foam. The frozen column method was simple and low cost, but was constrained by column height for peat, which had a higher water potential compared with the other two substrates. Substrate porosity analysis at container capacity by gravimetric or CT methods were similar for volumetric water and air content (VWC and VAC) in rockwool and foam, but differed for peat for VWC and VAC. Gravimetric analysis was simple, rapid, and low cost for whole-cell analysis, but CT further quantified spatial water and air relations within the cell and allowed visualization of complex water and air relations in an image. All substrates had high water content at container capacity ranging from 67% to 91% VWC with 5% to 11% VAC in the short propagation cells, emphasizing the need for careful irrigation management.

Quantification of Poinsettia Root Growth by Image Scans or X-ray Computed Tomography Scans in Three Propagation Substrates at Varied Moisture Levels

ABSTRACTThe objective was to refine protocols to quantify rooting of plant cuttings and the water/air microenvironment of substrates using x-ray computed tomography (CT). Poinsettia (Euphorbia pulcherrima) cuttings were propagated in three substrates (peat, rockwool, and phenolic foam) at varied moisture levels. In Experiment 1, adventitious rooting occurred in cuttings grown in rockwool or foam from 12% to 86% volumetric water content (VWC) and 12% to 80% volumetric air content (VAC). The highest root growth occurred in rockwool at 59% VWC and 33% VAC. There was an advantage to quantifying root growth by CT in rockwool and foam, in contrast to peat, because of clear differences in material density between root and substrate during image processing. In Experiment 2, root growth was quantified in peat by two-dimensional image scans under similar growth chamber environments, with rapid root growth from 52% to 63% VWC and 16% to 26% VAC. In Experiment 3, CT was used to quantify the substrate microenvironment at 0.5 cm slices to further describe the environment at the base of the plant cutting. Rapid rooting occurred in microenvironments above 56% VWC and 14% VAC, whereas the low 3% VAC in foam at high moisture may limit root growth.Abbreviations: volumetric water content (VWC), volumetric air content (VAC), volumetric solid content (VSC)

Lorenz Curve and Gini Coefficient Reveal Hot Spots and Hot Moments for Nitrous Oxide Emissions

AbstractIdentifying hot spots and hot moments of nitrous oxide (N2O) emissions in the landscape is critical for monitoring and mitigating the emission of this potent greenhouse gas. We propose a novel use of the Lorenz curve and Gini coefficient (G) to improve the estimation of the mean as well as the spatial and temporal variation of N2O emissions from a bioenergy landscape. The analyses indicate that the G was better correlated (R2 = 0.72, P < 0.001) with daily N2O emissions than the coefficient of variation and skewness. A hot moment for N2O emissions occurred after a storm event, with a heterogeneous spatial distribution of N2O emissions (G = 0.65); in contrast, CO2 emissions remained spatially uniform throughout the same period (G = 0.36). Volumetric soil air content below 0.03 m3 m−3 occurred more frequently in the wetter footslope positions and created N2O hot spots, with a high temporal inequality during the growing season (G = 0.75). In contrast, well‐drained shoulder positions were cold spots, with uniformly distributed and low N2O emissions (G = 0.44). The spatial N2O inequality mirrored the landscape wetness generated by rain events, while biogeochemical equality prevailed in the landscape. The Lorenz curve and G are tools to standardize the spatial and temporal variation of N2O emissions across diverse landscapes and management scenarios. These two inequality indicators, in association with spatial maps, can help delineate the critical spatial mosaics and temporal windows of N2O emissions and guide landscape‐scale monitoring and mitigation strategies to reduce N2O emissions.

Influence of stress and water content on air permeability of intact loess

Air permeability is one of the key parameters in performing consolidation analysis of unsaturated soil. Few data are available regarding the air permeability for intact loess under different applied stresses. The work reported in this paper evaluated the air permeability at isotropic stress using data obtained from laboratory investigations for intact loess. Air permeability tests were performed on specimens of intact loess at various water contents and isotropic stresses. Stress remarkably affected air permeability as a function of both liquid saturation and volumetric air content. There were all unique relationships not only between relative air permeability and liquid saturation as well as relative volumetric air content, but also between air permeability and modified air saturation for different stresses. The relative air permeability against liquid saturation could be well described by the Parker–van Genuchten–Mulaem (P-vG-M) model in terms of total liquid saturation. The relationship between relative air permeability and relative volumetric air content could be more conveniently expressed by the reference point power law (RPL) model, using the reference point values at natural water content rather than that at air-dried water content. Compared with the other two models, the proposed modified air saturation power law (MAPL) model was more appropriate to predict the air permeability.

Thermal conductivity of geosynthetic clay liners

The thermal conductivities of powdered and granular bentonite based needle punched geosynthetic clay liners (GCLs) were investigated at different gravimetric water contents under 25, 50, 75, and 100 kPa vertical stresses. Both types of GCLs exhibited an increase in thermal conductivity with increasing vertical stress at all water contents. The effect of vertical stresses was more pronounced for the specimens hydrated at lower gravimetric water contents and this was attributed to their high initial volumetric air content. The variability of water distribution in partially hydrated GCLs has been identified as a factor that may affect their thermal conductivity. The forms of bentonites (i.e., powder or granular) affected their thermal conductivities; however, this effect was less apparent at higher gravimetric water contents due to the reduced air content and gel formation in the bentonites. Finally, the GCL thermal conductivity calculated from the measured thermal conductivities of its various constituents (i.e geotextile and bentonite) components differed from the measured values. This was attributed to the nonuniform water distribution across the GCL specimen and change in material properties when components of GCL were disassembled.

Exploring the relationship between soil mesofauna, soil structure and N2O emissions

Agricultural soils are a large source of nitrous oxide (N2O) emissions. Soil mesofaunal species can accelerate, delay, increase or decrease N2O emissions. However, it is still unknown whether the soil fauna affect N2O emissions through trophic interactions or through their effect on soil structure. We explored the role of these two pathways in a 70 day microcosm experiment with a sandy loam subsoil with hay mixed in. Enchytraeids, fungivorous mites and predatory mites were added to the soil in a full factorial design to test for both single species effects as well as interactions between species. We measured N2O and CO2 fluxes and we analysed soil structural parameters using X-ray micro tomography. After 35 days of incubation, enchytraeid presence significantly increased the volumetric air content of the soil (0.049–0.067 cm3 cm−3, P = 0.010) as well as the abundance of pores with sizes similar to their body width. At the same time N2O emissions, NO3− and DOC concentrations were significantly higher when enchytraeids were present (3.6 to 8.4 mg N2O-N m−2, P < 0.001; 6.0 to 17.1 mg NO3−-N kg−1 soil, P < 0.001; 121.1 to 135.6 mg C kg−1 soil, and P = 0.017, respectively). Neither fungivorous mites nor predatory mites nor their interactions had a significant effect on soil structure or N2O emissions. Enchytraeids accelerated peak N2O emissions (P = 0.001), but did not increase cumulative N2O emissions on day 70. Structural equation modelling confirmed that enchytraeids enhanced nitrogen mineralisation directly and also indirectly by creating a higher volumetric air content, and thereby increased N2O emissions. We conclude that the soil structure pathway was important in driving N2O emissions, and that soil ecosystem engineers such as enchytraeids disproportionately affected N2O emissions as compared to other soil fauna.

A two-phase model to simulate the 1-D shock wave propagation in porous metal foam

A two-phase model is proposed to simulate the dynamic behavior of porous metal foam. The model is applied to known experimental data describing equations of state for different aluminum foams. To analyze impact and shock problems in compressible media with analytical, semi-analytical and numerical methods, the corresponding Riemann problem solution is required. Its solution for a metal foam medium has been obtained. The possible wave configurations have been analyzed and the corresponding equations for the contact pressure and velocity evaluation have been derived. The effect of the volumetric air content in a metal foam medium on the Riemann problem solution has been investigated. The proposed equation of state has been successfully used to simulate shock waves propagation in the Alporas foam. The comparisons of the present model predictions with numerical results computed with LS-Dyna as well as with experimental data show very good correspondence and demonstrate the power of the present approach.

Effects of Tillage, Rotation and Cover Crop on the Physical Properties of a Silt-Loam Soil

Abstract Soil and crop management practices can affect the physical properties and have a direct impact on soil sustainability and crop performance. The objective of this study was to investigate how soil physical properties were affected by three years of tillage, cover crop and crop rotation treatments in a corn and soybean field. The study was conducted on a Waldron siltyloam soil at Lincoln University of Missouri. Soil physical properties studied were soil bulk density, volumetric and gravimetric water contents, volumetric air content, total pore space, air-filled and water-filled pore space, gas diffusion coefficient and pore tortuosity factor. Results showed significant interactions (p&lt;0.05) between cover crop and crop rotation for bulk density, gravimetric and total pore space in 2013. In addition, cover crop also significantly interacted (p&lt;0.05) with tillage for bulk density and total pore space. All soil physical properties studied were significantly affected by the depth of sampling (p&lt;0.0001), except for bulk density, the pore tortuosity factor and total pore space in 2012, and gravimetric and volumetric in 2013. Overall, soil physical properties were significantly affected by the treatments, with the effects changing from one year to another. Addition of a cover crop improved soil physical properties better in rotation than in monoculture.

Tolerance of young seedlings of different tree species and a cereal to poor soil aeration

Poor soil aeration restricts the establishment of plants, yet some species used in revegetation programs are more tolerant of waterlogging than others. This study evaluated the sensitivity to poor soil aeration of young seedlings of Acacia salicina, Eucalyptus camaldulensis, E. leucoxylon, and E. kochii. A reference cereal crop, barley (Hordeum vulgare var. Keel), was also included. Seedlings were grown for 21 days in a loamy sand under a range of controlled volumetric air contents of the soil (εair, 0.000–0.200 m3 m–3), then the diameter and total length of roots, plus the dry masses of shoots and roots, were measured, and water use and water-use efficiency (WUE) calculated. All plant species had some sensitivity to poor soil aeration but their tolerance varied widely. Seedlings of E. camaldulensis were highly tolerant of waterlogging, whereas seedlings of E. kochii were highly sensitive. Seedlings of barley and A. salicina showed moderate tolerance and had significantly greater WUE than any of the eucalypts. Among the more sensitive species, no single aeration state caused a consistent reduction in all plant parameters; some species responded to any restriction in aeration by reducing WUE; others showed declining WUE only when aeration fell below 0.15 m3 m–3. No single early-growth parameter adequately described a species’ sensitivity to waterlogging; some were simple (e.g. shoot mass) and others more integrated (e.g. WUE). Differences in growth rate and water use by the different species under low εair suggest that land managers and revegetation agencies have scope to select superior species for improving the establishment of trees on waterlogged soils. Furthermore, the wide variation in tolerance to poor soil aeration among the tree seedlings compared with barley suggests considerable scope for genetic improvement of cereals by using material from native trees.

Variability of Soil Physical Properties in a Clay-Loam Soil and Its Implication on Soil Management Practices

We assessed the spatial variability of soil physical properties in a clay-loam soil cropped to corn and soybean. The study was conducted at Lincoln University in Jefferson City, Missouri. Soil samples were taken at four depths: 0–10 cm, 10–20, 20–40, and 40–60 cm and were oven dried at 105°C for 72 hours. Bulk density (BDY), volumetric (VWC) and gravimetric (GWC) water contents, volumetric air content (VAC), total pore space (TPS), air-filled (AFPS) and water-filled (WFPS) pore space, the relative gas diffusion coefficient (DIFF), and the pore tortuosity factor (TORT) were calculated. Results showed that, in comparison to depth 1, means for AFPS, Diff, TPS, and VAC decreased in Depth 2. Opposingly, BDY, Tort, VWC, and WFPS increased in depth 2. Semivariogram analysis showed that GWC, VWC, BDY, and TPS in depth 2 fitted to an exponential variogram model. The range of spatial variability (A0) for BDY, TPS, VAC, WFPS, AFPS, DIFF, and TORT was the same (25.77 m) in depths 1 and 4, suggesting that these soil properties can be sampled together at the same distance. The analysis also showed the presence of a strong (≤25%) to weak (&gt;75%) spatial dependence for soil physical properties.

Limited denitrification in glacial deposit aquifers having thick unsaturated zones (Long Island, USA)

The goal of this study was to demonstrate how the extent of denitrification, which is indirectly related to dissolved organ carbon and directly related to oxygen concentrations, can also be linked to unsaturated-zone thickness, a mappable aquifer property. Groundwater from public supply and monitoring wells in Northport on Long Island, New York state (USA), were analyzed for denitrification reaction progress using dissolved N2/Ar concentrations by membrane inlet mass spectrometry. This technique allows for discernment of small amounts of excess N2, attributable to denitrification. Results show an average 15 % of total nitrogen in the system was denitrified, significantly lower than model predictions of 35 % denitrification. The minimal denitrification is due to low dissolved organic carbon (29.3–41.1 μmol L−1) and high dissolved oxygen concentrations (58–100 % oxygen saturation) in glacial sediments with minimal solid-phase electron donors to drive denitrification. A mechanism is proposed that combines two known processes for aquifer re-aeration in unconsolidated sands with thick (>10 m) unsaturated zones. First, advective flux provides 50 % freshening of pore space oxygen in the upper 2 m due to barometric pressure changes. Then, oxygen diffusion across the water-table boundary occurs due to high volumetric air content in the unsaturated-zone catchment area.

Acoustic method for measuring volumetric air content and air conductivity of sandy soil and its application to the evaluation of air phase structure

Acoustic method for measuring volumetric air content and air conductivity of sandy soil and its application to the evaluation of air phase structure

Arsenic speciation and turnover in intact organic soil mesocosms during experimental drought and rewetting

Wetlands are significant sources and sinks for arsenic (As), yet the geochemical conditions and processes causing a release of dissolved arsenic and its association with the solid phase of wetland soils are poorly known. Here we present experiments in which arsenic speciation was determined in peatland mesocosms in high spatiotemporal resolution over 10 months. The experiment included a drought/rewetting treatment, a permanently wet, and a defoliated treatment. Soil water content was determined by the TDR technique, and arsenic, iron and sulfate turnover from mass balancing stocks and fluxes in the peat, and solid phase contents by sequential extractions. Arsenic content ranged from 5 to 25 mg kg −1 and dissolved concentrations from 10 to 300 μg L −1, mainly in form of As(III), and secondarily of As(V) and dimethylated arsenic (DMA). Total arsenic was mainly associated with amorphous iron hydroxides ( R 2 > 0.95, α < 0.01) and deeper into the peat with an unidentified residual fraction. Arsenic release was linked to ferrous iron release and primarily occurred in the intensely rooted uppermost soil. Volumetric air contents of 2–13 % during drought eliminated DMA from the porewater and suppressed its release after rewetting for >30 d. Dissolved As(III) was oxidized and immobilized as As(V) at rates of up to 0.015 mmol m −3 d −1. Rewetting mobilized As(III) at rates of up to 0.018 mmol m −3 d −1 within days. Concurrently, Fe(II) was released at depth integrated rates of up 20 mmol m −3 d −1. The redox half systems of arsenic, iron, and sulfur were in persistent disequilibrium, with H 2S being a thermodynamically viable reductant for As(V) to As(III). The study suggests that rewetting can lead to a rapid release of arsenic in iron-rich peatlands and that methylation is of lesser importance than co-release with iron reduction, which was largely driven by root activity.

Air permeability of compost as related to bulk density and volumetric air content

Compost air permeability controls air flow through compost during composting or when using compost as biofilter material. Air permeability is therefore an important characteristic of compost. The relationships between air permeability (k(a)) in compost and compost dry bulk density (rho b), gravimetric water content (omega), and volumetric air content (epsilon) was investigated for two types of composts. The composts used were produced from a digested sewage sludge-straw mixture and from garden waste and measurements were conducted on sieved and repacked 100 cm3 compost samples. Results showed a linear relation between log(k(a)) and rho b at constant values of omega for both composts, indicating an exponential relationship between k(a) and rho b. The slopes of these relationships generally became more negative with increasing rho b. The results further showed a linear relationship between log(k(a)) and log(epsilon) for both composts as also often observed for soils. It was observed that the log(k(a)) and log(epsilon) relationships for the garden waste compost all intercepted at the same location despite having very different slopes. This means that it is possible to predict the entire k(a)-epsilon relationship using only one measurement of corresponding (k(a), epsilon) for garden waste. It was not possible to determine whether this was also the case for the sewage sludge compost due to difficulties in sample preparation at low and high water content.

Triaxial constant stress and constant strain rate tests on ice-rich permafrost samples

A range of mechanical compression tests were conducted on frozen soil samples at temperatures close to the melting point of ice to determine both the creep and the strength properties under triaxial stress conditions. The samples were obtained from two rock glaciers in the Swiss Alps. For comparison, additional samples were prepared artificially for testing. The results showed that the minimum creep strain rate increases exponentially with increasing temperature and applied deviatoric stress. The applied strain rate is the main parameter influencing resistance. However, test results show that the micromechanical behaviour is similar for all tests and independent of the applied loading condition. Measurements of the volumetric strain showed that the sample composition has a major influence, so samples with high air void ratios resulted in continuous contraction (i.e., reduction of the sample volume), whereas samples with low volumetric air and ice contents demonstrated dilatant behaviour.Key words: permafrost, frozen soil, creep, strength, triaxial testing, self-healing.

GAS COMPOSITION AND OXYGEN SUPPLY IN THE ROOT ENVIRONMENT OF SUBSTRATES IN CLOSED HYDROPONIC SYSTEMS

The objective of this study was to get more information about the root zone, mainly the gas composition in substrates, which are used in closed hydroponic systems. For analyses of carbon dioxide (CO2), ethylene (C2H4) and oxygen (O2) different methods were proved and used. For analyses of carbon dioxide and ethylene, a gas sampling system was used to get gas samples from the root zone. CO2 gas samples of 20 ml were analyzed with an IR sensor by air diffusion into a measuring cuvette. C2H4 gas samples of 3 ml were analyzed using gas chromatography (GC). Gas sample cells were inserted into the substrates at 2 or 3 heights. Dissolved oxygen was determined with a membrane covered galvanic sensor, in the air, in the in the nutrient solution, and in the drain solution. To analyze the root zone, a hydroponic system with chrysanthemum cv. 'Snow' on 3 substrates was performed. The following substrates were used: Sawagrow (polyester fiber), polyester fleece (thin layer of fleece), and UC-mix (peat substrate). Gas sampling at 2 or 3 heights from root zone did function well, results ranged from 350 ppm to 7700 ppm CO2, and from 0.0088 ppm to 0.1147 ppm of C2H4. The organic substrates UC-mix contains more CO2, from top to the bottom the CO2 level increased. The CO2 level is influenced by microbial and root respiration. The determined CO2 concentration is no limiting growth factor. Results of C2H4 shown an influence by the substrates and a gradient from the top to the bottom. But highest level was determined in the middle and at the bottom of the substrates. The time course shows during the day pattern according the photoperiod. The trend of C2H4 level was almost constant during the experiment. The dissolved O2 level in solution was about 57-82% and in drain solution 78%-99%. O2 level of solution and drain can be an indicator for oxygen deficiency, but it is no direct measurement in root zone or roots. The O2 content in air between 10.5% to 20.4%. During the experiment the O2 concentration decreased; for CO2 the opposite trend was found. INTRODUCTION Plant growth in closed hydroponic systems, with restricted root environment, and consequently, low buffering capacity is more related to water, nutrient solution, and oxygen supply. Water and nutrients are supplied with the nutrient solution, which is regulated by the composition of the solution and irrigation control. Available oxygen is mainly determined by the layout of the hydroponic system and the physical substrate properties. The oxygen diffusion rates into the water depends directly on volumetric air content, the partial oxygen pressure, and temperature. Within the hydroponic systems there is an oxygen gradient for design flow techniques and flow rates (Vestergaard, 1984; Bunt, 1991; Baas et al. 2000; Wever et al., 2000). In addition, the substrate parameters change at the end of each crop, due to decomposition of organic substrates or root and increasing root mass (Wever and van Leeuwen, 1995). Oxygen deficiency may be a limiting factor for plant growth. Oxygen deficiency can be tolerated by roots for a short time, even for several days (Buwalda, 1991; Gislerod et al., 1997; Yosida et al., 1997). Proc. IS on Growing Media Eds.: Alsanius, Jensen & Asp Acta Hort 644, ISHS 2004