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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.

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.

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.

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.

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