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Dexter A.R.,Institute of Soil Science and Plant Cultivation IUNG PIB | Czyz E.A.,Institute of Soil Science and Plant Cultivation IUNG PIB | Czyz E.A.,University of Rzeszow | Richard G.,French National Institute for Agricultural Research
Geoderma | Year: 2012

Factors that affect water retention in soil are discussed in terms of thermodynamic and hydraulic equilibria. It is assumed that the Groenevelt and Grant (2004) water retention equation describes soil water that is at or close to thermodynamic equilibrium. It is shown that immiscible displacement, as occurs in a pressure cell apparatus in which air displaces water, can leave residual soil water that is not in thermodynamic equilibrium. We describe this residual water as being in hydraulic equilibrium. As hydraulic equilibrium is approached, the outflow of water becomes at first very slow and then essentially ceases. An empirical equation for water retention in bi-modal soils is used for soils that exhibit residual water content. It is found that residual water is that which remains after the connected (drainable) textural pore space has emptied by Darcian convective flow. The point at which Darcian flow ceases is called hydraulic cut-off. The water remaining after hydraulic cut-off is the residual water that moves much more slowly mainly through vapour-phase diffusion. The suction of residual pore water can be significantly smaller than the air pressure that has been applied in a pressure cell apparatus. Models for predicting the amount and pressure of residual water are presented and tested using experimental data for 14 soils from France and Poland. Some consequences for soil water retention studies and for the status of soil water in the field are discussed. © 2012 Elsevier B.V.

Dexter A.R.,French National Institute for Agricultural Research | Dexter A.R.,Institute of Soil Science and Plant Cultivation IUNG PIB | Richard G.,French National Institute for Agricultural Research | Czyz E.A.,French National Institute for Agricultural Research | And 3 more authors.
Soil Science | Year: 2010

The temperature dependence of the matric potential of soil water was investigated as part of a study of the effects of climate change on soil physical properties. Undisturbed samples of topsoils were collected in cylinders. The pore water suction near the center of each cylinder was measured using two miniature tensiometers. The temperature of the samples was varied in the range of 10°C to 40°C. Three different phenomena were observed. The first was a slow drift to increased pore water suctions with time. This was attributed to the rearrangement of soil particles and has been described as the age-hardening or thixotropic effect. The second was the appearance of pressure transients when the temperature was changed stepwise. These were such that a step increase in temperature produced a rapid reduction of pore water suction that decayed during a period of hours. This was attributed to pressure changes of gas bubbles entrapped within water-filled pores-these pressure changes being transmitted to the pore water. The third (or TISSI) effect was a linear increase in pore water suction with time that started at temperatures around 40°C. This increased rate of change persisted for at least three further days after the temperature was again reduced to 20°C. Tests with sand showed none of the above effects. Several hypotheses were tested in attempts to explain the third phenomenon. These included tests to determine if some soil components were dissolving at higher temperatures. However, neither electrical conductivity nor optical absorbance showed any effects at temperatures up to 45°C and times of up to 7 days. It is conjectured that the TISSI effect is associated with the formation of organic micelles, although this needs further research to confirm it. © 2010 by Lippincott Williams & Wilkins.

Hallett P.D.,University of Aberdeen | Baumgartl T.,University of Queensland | Seville J.P.K.,University of Surrey | Horn R.,University of Kiel | Dexter A.R.,Institute of Soil Science and Plant Cultivation IUNG PIB
Vadose Zone Journal | Year: 2014

With a tensiometer <1 mm in diameter, we demonstrated a direct impact of strain rate induced changes to capillary pressure on the tensile strength of soil. This work is relevant to understanding soil cultivation, crack formation in soil caused by desiccation, and the selection of strain rates for static testing of soils. Microtensiometer probes with a tip radius <1 mm were used for direct measurement of the change in pore water pressure caused by tensile loading at different strain rates in soils. These probes responded rapidly to changes in pore water pressure during testing and demonstrated that the applied tensile stress was transmitted almost entirely through the pore water, as would be expected. Above a strain rate of 1% min-1, viscous effects became significant, leading to a significant increase in the fracture stress. The results are described using an extended version of the Kelvin-Voigt model of rheological behavior. At low strain rates, capillary forces dominate the fracture stress. Above the critical strain rate, the viscosity of the soil also contributes to the fracture stress. © Soil Science Society of America 5585 Guilford Rd., Madison, WI 53711 USA. All rights reserved.

Li L.,Gansu Agricultural University | Chan K.Y.,Investment and Industry NSW | Niu Y.,Gansu Agricultural University | Li G.,Charles Sturt University | And 3 more authors.
Soil and Tillage Research | Year: 2011

The common soil physical quality indicators are related to each other because they all reflect soil structure, but to measure all of these parameters would be very time consuming. Therefore, it is desirable to obtain one simple index for overall assessment. The soil physical parameter, S, may serve this purpose but the theory needs to be validated on soils affected by different management practices. Therefore, in this paper, soils from a long-term tillage/stubble experiment which commenced in 1979 in New South Wales, Australia were analysed for treatment effects on aspects of soil physical quality and on the S value. The treatments ranged from direct-drilling, stubble-retention through to multiple-cultivation stubble-burnt and also included a wheel track vs. non-wheel track comparison. Results showed that after 26. years of different tillage and stubble management practices, significant changes in soil physical qualities were detectable at 0.05. m depth. The direct-drilling soil had lower bulk density, higher soil organic carbon and improved water-stable aggregation (>0.25. mm aggregates) and friability. The S values obtained from the soil water retention curves were well correlated with the other soil physical quality indicators determined in this research and covered the range of S values from 0.030 to 0.046 associated with soil physical qualities in the poor and good categories. It is concluded that S value can be used as an overall index of soil physical quality for soils under different tillage, stubble and field traffic treatments. © 2011 Elsevier B.V.

Asgarzadeh H.,Bu - Ali Sina University | Mosaddeghi M.R.,Isfahan University of Technology | Mahboubi A.A.,Bu - Ali Sina University | Nosrati A.,Bu - Ali Sina University | Dexter A.R.,Institute of Soil Science and Plant Cultivation IUNG PIB
Geoderma | Year: 2011

Different approaches have been proposed for quantification of soil water availability for plants but mostly they do not fully describe how water is released from the soil to be absorbed by the plant roots. A new concept of integral energy (EI) was suggested by Minasny and McBratney (Minasny, B., McBratney, A.B. 2003. Integral energy as a measure of soil-water availability. Plant and Soil 249, 253-262) to quantify the energy required for plants to take up a unit mass of soil water over a defined water content range. This study was conducted to explore the EI concept in association with other new approaches for soil water availability including the least limiting water range (LLWR) and the integral water capacity (IWC) besides conventional plant available water (PAW). We also examined the relationship between EI and Dexter's index of soil physical quality (S-value). Twelve agricultural soils were selected from different regions in Hamadan province, western Iran. Soil water retention and penetration resistance, Q, were measured on undisturbed samples taken from the 5-10cm layer. The PAW, LLWR and IWC were calculated with two matric suctions (h) of 100 and 330hPa for field capacity (FC), and then the EI values were calculated for PAW, LLWR and IWC. There were significant differences (P<0.01) between the EI values calculated for PAW100, PAW330, LLWR100, LLWR330 and IWC. The highest (319.0Jkg-1) and the lowest (160.7Jkg-1) means of EI were found for the EI(IWC) and EI(PAW330), respectively. The EI values calculated for PAW100, LLWR100 and LLWR330 were 225.6, 177.9 and 254.1Jkg-1, respectively. The mean value of EI(PAW330) was almost twice as large as the mean of EI(IWC) showing that IWC is mostly located at lower h values when compared with PAW330. Significant relationships were obtained between EI(IWC) and h at Q=1.5MPa, and EI(LLWR100) or EI(LLWR330) and h at Q=2MPa indicating strong dependency of EI on soil strength in the dry range. We did not find significant relationships between EI(PAW100) or EI(PAW330) and bulk density (pb) or relative pb (pb-rel). However, EI(LLWR100) or EI(LLWR330) was negatively and significantly affected by pb and pb-rel. Both EI(PAW100) and EI(PAW330) increased with increasing clay content showing that a plant must use more energy to absorb a unit mass of PAW from a clay soil than from a sandy soil. High negative correlations were found between EI(PAW100) or EI(PAW330) and the shape parameter (n) of the van Genuchten function showing that soils with steep water retention curves (coarse-textured or well-structured) will have lower EI(PAW). Negative and significant relations between EI(PAW100) or EI(PAW330) and S were obtained showing the possibility of using S to predict the energy that must be used by plants to take up a unit mass of water in the PAW range. Our findings show that EI can be used as an index of soil physical quality in addition to the PAW, LLWR, IWC and S approaches. © 2011 Elsevier B.V.

Czyz E.A.,Institute of Soil Science and Plant Cultivation IUNG PIB | Czyz E.A.,University of Rzeszow | Dexter A.R.,Institute of Soil Science and Plant Cultivation IUNG PIB
Soil Research | Year: 2012

In this paper, plant wilting is re-analysed and re-interpreted on the basis of previously published work. Wilting is considered only in terms of the stress caused by the matric suction of the soil water. Other factors that can induce wilting, such as salinity and plant pathogens, are not considered. It is found that there is confusion around the subject for two main reasons. First, it is usually assumed that the matric suction of the pore water that exists in soil samples when they are removed from pressure plate extractors is equal to the air pressure that was applied. Second (and this is a special case of the first reason), because the soil water content when most plants wilt is very close to that remaining in soil samples on pressure plates operating with an air pressure of 1.5MPa, it is assumed that plants wilt at a pore water suction of 1.5MPa. These assumptions are examined here, and it is shown that neither of them is true. Published results are used for the wilting condition. The recently described double-exponential (DE) equation for soil water retention is used for cases where the water is non-equilibrated because of hydraulic cut-off. The non-equilibrated condition is appropriate for plant roots because they, like pressure plate extractors, extract water from soil by immiscible displacement. The DE equation is used to illustrate the conditions under which plant wilting can be either a plant or a soil property. It is shown how this approach can be used to estimate the pore water suction at which plants would wilt because the soil is no longer able to supply water to their roots. It is demonstrated that the commonly used, but often erroneous, value for the wilting-point suction of h≤1.5MPa is a consequence of the fact that this is the largest value of air pressure used in pressure cell extractors. It is therefore neither a plant nor a soil property, but is an artefact of the experimental procedure. The use of the DE equation for soil water retention shows that we know only that h ≤1.5MPa, and that h can be as small as 0.2 or 0.3MPa. Implications for estimation of plant water availability in soils, for plant breeding, and for soil microbial activity are discussed. © 2012 CSIRO.

Czyz E.A.,Institute of Soil Science and Plant Cultivation IUNG PIB | Czyz E.A.,University of Rzeszow | Dexter A.R.,Institute of Soil Science and Plant Cultivation IUNG PIB
International Agrophysics | Year: 2013

It has been shown that the water remaining in soil when plants wilt due to soil limitations and the residual water content as observed when soils are de-watered in pressure cell apparatus are essentially the same. Both are produced by immiscible displacement of water by air, and this leads to the water remaining in soil not being in thermodynamic equilibrium. Water removal by immiscible displacement ceases when hydraulic cut-off is reached. The point of hydraulic cut-off may be calculated by fitting waterretention data to equations for both the non-equilibrium case and the equilibrium case, and then solving these simultaneously. This has been done forwater retention data for 52 soil horizons in Poland. These results are used to obtain a pedotransfer function for the permanent wilting point due to soil limitations and the results are presented for the different soil texture classes. The pore water suction when wilting occurs is estimated to be 1.0 MPa. The methods and findings in this paper are used to explain a range of published results on plant wilting. © 2013 Institute of Agrophysics, Polish Academy of Sciences.

Czyz E.A.,University of Rzeszow | Czyz E.A.,Institute of Soil Science and Plant Cultivation IUNG PIB | Dexter A.R.,Institute of Soil Science and Plant Cultivation IUNG PIB
International Agrophysics | Year: 2015

A method for the experimental determination of the amount of clay dispersed from soil into water is described. The method was evaluated using soil samples from agricultural fields in 18 locations in Poland. Soil particle size distributions, contents of organic matter and exchangeable cations were measured by standard methods. Sub-samples were placed in distilled water and were subjected to four different energy inputs obtained by different numbers of inversions (end-over-end movements). The amounts of clay that dispersed into suspension were measured by light scattering (turbidimetry). An empirical equation was developed that provided an approximate fit to the experimental data for turbidity as a function of number of inversions. It is suggested that extrapolation of the fitted equation to zero inversions enables the amount of spontaneously-dispersed clay to be estimated. This method introduces the possibility of replacing the existing subjective, qualitative method of determining spontaneously-dispersed clay with a quantitative, objective method. Even though the dispersed clay is measured under saturated conditions, soil samples retain a 'memory' of the water contents at which they have been stored. © 2015 Institute of Agrophysics, Polish Academy of Sciences.

Dexter A.R.,Institute of Soil Science and Plant Cultivation IUNG PIB | Czyz E.A.,Institute of Soil Science and Plant Cultivation IUNG PIB | Czyz E.A.,University of Rzeszow
International Agrophysics | Year: 2011

The soils from England, France and Poland having a range of organic matter contents were studied. The total contents of clay and organic matter are used to calculate the amounts of non-complexed clay and organic matter in the soils. Measurements of soil water retention are used to calculate the values of an index of soil physical quality, S*. It is shown that soils with larger contents of non-complexed clay have smaller values of S* whereas soils with larger contents of non-complexed organic matter have larger values of S*.Wethen use these values of S* in an existing model based on results from tillage experiments in the field to predict the amount of soil crumbling produced by mouldboard ploughing. It is predicted that soils with larger contents of organic matter will crumble into smaller aggregates (and fewer clods) when tilled. A new measure of non-complexed material, NCM, is introduced that enables different soils that are poor and rich in organic matter to be considered on the same scale. © 2011 Institute of Agrophysics, Polish Academy of Sciences.

Keller T.,Agroscope Reckenholz Tnikon Research Station ART | Keller T.,Swedish University of Agricultural Sciences | Dexter A.R.,Institute of Soil Science and Plant Cultivation IUNG PIB
Soil Research | Year: 2012

The plastic limits (lower plastic limit, PL; and liquid limit, LL) are important soil properties that can yield information on soil mechanical behaviour. The objective of this paper is to study the plastic limits of agricultural soils as functions of soil texture and organic matter (OM) content. The plastic limits were highly related to the clay content. The LL was more strongly correlated with clay than was PL, but the reasons are unclear. Interestingly, PL was virtually unaffected by clay content for soils with clay contents below ∼35%. The OM had a strong effect on the plastic limits. This effect was clearly demonstrated when analysing soils of similar texture with a range of OM. We present equations (pedotransfer functions) for estimation of PL, LL, and plasticity index (PI) from soil texture and OM. Finally, we predict that the clay content must be <10% for soils without OM to be plastic; however, soils with 10% clay can be plastic if OM is present. More research is needed to investigate OM effects on soil consistency. © CSIRO 2012.

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