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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 1.5 MPa, it is assumed that plants wilt at a pore water suction of 1.5 MPa. 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 = 1.5 MPa 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 ≤ 1.5 MPa, and that h can be as small as 0.2 or 0.3 MPa. Implications for estimation of plant water availability in soils, for plant breeding, and for soil microbial activity are discussed. Additional keywords: hydraulic cut-off, permanent wilting point, pore water suction, pressure cell extractors, soil water retention characteristics., Introduction In response to the apparent increasing problems of water shortage and drought in many parts of the world, we reexamine two of the key factors that contribute to plant [...]

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 Additional keywords: Atterberg consistency limits, clay, liquid limit, pedo-transfer function, plasticity index., Introduction The plastic limits of soil define a range of soil moisture content in which soil has a plastic consistency. The limits are termed the lower plastic limit, often just [...]

Soil water content or potential can be adjusted in the laboratory by equilibrating the soil at different relative humidities. These relative humidities can be produced by the use of different saturated salt solutions or by heating the soil to different temperatures in a convection oven. Usually, the conditions for equilibrating the soil are expressed as relative humidities if salt solutions are used or as the temperature of drying if a convection oven is used. This dichotomy does not allow all the results to be compared or plotted on a common axis. In this paper, the Magnus--Tetens and the Arden Buck equations are used in combination with the Kelvin equation to show how the results from both types of equilibration can be presented on the same scale (e.g., water potential or pF). Two new equations show how the results from oven drying depend not only on the oven temperature, but also on the temperature and relative humidity of the air in the laboratory. For example, drying soil in an oven in one set of atmospheric conditions can produce a significantly different water potential than in different atmospheric conditions. The new equations enable the soil water potentials (or pF values) produced by oven drying to be calculated for all combinations of oven temperature and atmospheric conditions. Predictions from the equations are evaluated by comparing water retention data obtained by oven drying and by equilibration over saturated salt solutions, k is shown that the mean absolute error in gravimetric water content is 2.3 g [kg.sup.-1]. Equations are presented for the mean thickness of water films adsorbed on the surfaces of soil particles under different drying conditions. The amount of water adsorbed on particle surfaces is predicted to be zero at pF6.6.