The enhancement of water vapor diffusion in soil resulting from a temperature gradient was determined using a transient state thermal conductivity measurement. The method of Parikh et al. (1979) was adapted for this study to allow measurement of the thermal conductivity as a function of temperature, water content, and pressure. The data allowed separation of thermal conductivity from thermally induced latent heat transport. Both the mechanistic enhancement factor η of Philip and de Vries and the phenomenological enhancement factor β of Cary are calculated from the slope of the relationship between thermal conductivity and reciprocal relative pressure. The enhancement factor, η approaches 1 for dry soil and increases rapidly to values around 10 for a sand and a silt loam at water contents above half saturation and temperatures above 22°C. At high water content and 3.5°C, η increased to 15. The inflection point water content for the η vs. saturation curve was about twice as high in silt loam as in sand. Measurements conform well to the pattern predicted by a theoretical examination of enhancement factor behavior. Experimental results were also compared with two recent quantitative models of β from Jury and Letey (1979) and Cary (1979). These model predictions did not agree with measured values, nor did they conform to the theoretical limits of β derived in this paper except at intermediate saturation and temperature values.

A sigmoidal, non-hysteretic two-part retentivity function having only two constants in addition to porosity was developed for use in soil-water flow simulation models. Values of the constants, shapes of the retentivity curves, and soil textural properties were related by fitting the retentivity function to retentivity data generated using regression equations. Two sets of regression equations, which relate water content at specific matricpotentials to soil texture and bulk density, were used, one for British soils and one for South African soils. Hydrologically inhomogenous soils may be modelled by varying the values of the retentivity constants through the profile to reflect changing soil properties. I N T R O D U C T I O NSoil-water retentivity equations relating water content 8 to matric potential w are useful for rnodelling and planning purposes, especially if soil properties can be reflected by choosing appropriate values for the parameters in the equation.Investigations into the variability of soil hydrological properties (Nielsen et al., 1973; Cameron, 1978) have shown that many measurements are required in order to obtain reliable estimates of retentivity and hydraulic conductivity. Most models necessarily neglect important but poorly defined facets of soil water behaviour, such as hysteresis, vapour diffusion, swelling, temperature and solute effects. For these reasons, it is often acceptable to estimate retentivity and conductivity relationships, ensuring that predominant soil profile features are reflected in the estimated data. A method of describing a wide range of retentivity behaviours is to relate the constants, in a general retentivity equation, to soil type. For practical purposes the retentivity equation should not have more than two or three constants otherwise it becomes too difficult to relate them to soil properties.

Poor establishment of seedlings is a problem in semi‐arid cropping regions. High drying rates that are characteristic of these regions can result in the early onset of high soil strengths and high soil temperatures, which are detrimental to seedlings. In this study, we examined the response of preemergent shoot growth of maize (Zea mays L.) to differences in the drying rate of a hardsetting red‐brown earth (fine, mixed, thermic Typic Paleustalf or Calcic Luvisol). We examined the impact of high soil strength on the constituent shoot parts (coleoptile and first internode), and used a temperature‐based shoot‐growth model to separate the effects of soil temperature and soil strength on preemergent shoot growth.Preemergent shoot growth was sensitive to small differences in drying rate. Rapid drying resulted in no emergence, moderate drying in 52% emergence, and slow drying in 78% emergence. Soil strength impeded shoot elongation at a cone index of 1.1 MPa and, at 2 MPa, growth and emergence ceased. Differences in soil strength development reflected not just the total amount of water removed from the profile, but also the distribution of water remaining within the profile. Rapid drying quickly reduced soil water content near the surface and led to the immediate development of high soil strength there.Reduction in shoot elongation at high soil strengths was largely due to the sensitivity of the first internode. The coleoptile elongation rate showed no response to treatment differences in soil strength although twisting, buckling, and rupture was more prevalent at high soil strengths.

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