SUMMARY Intrinsic permeability to air of macropore space (ka) is related to macroporosity (ɛ) and organization of macropore space (O). Organization is defined as ka/ɛ. The use of ka for estimating saturated hydraulic conductivity (Ka) is also considered. The relationship between Log (O) and ɛ (Oɛ characteristic) can be used to describe changes to the macropore space of clay soils by amelioration and compaction. The effects of dominant macropore shape can also be identified and calculated as an empirical index of the efficiency of the pore organization E (E=log (O)/ɛ). Intrinsic permeability can then be related to E in a E:ka characteristic. Intrinsic permeability is the parameter most sensitive to structural change and E is mainly influenced by the dominant shapes of the macropores. Thus, the E:ka characteristic is suggested as a basis for studying differences in macropore space as may occur in response to external and internal stresses upon the soil and different systems of soil management, for example increases of packing pores by cultivation or of fissures by gypsum application and loss of packing pores by compaction. Empirical data indicate that Ks of the B horizons of Australian red‐brown earths can be estimated from ka of macropore space at a standard potential.

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Sodic soils occur extensively on the agricultural lands of Australia. The poor crop productivity of sodic soils is often associated with their low infiltration rates and restricted drainage. This is caused by low macroporosity and macropore instability, due to the presence of sodium on the clay surfaces. To achieve long-term improvements, tillage techniques to increase macroporosity have to be combined with chemical and biological techniques to improve macropore stability. Macropore stability is improved by addition of chemical ameliorants and organic matter. Maintenance of macroporosity also requires protection of the tilled soil from recompaction during flood irrigation, raindrop impact and trafficking. Adverse effects of sodicity of surface soil layers can be corrected by incorporating gypsum and by using conservation farming practices to add organic matter and to protect the surface from mechanical disturbance and raindrop impact. Subsoil sodicity can be corrected by combining deep ripping with chemical ameliorant additions, but the beneficial effects are often quickly lost under flood irrigation and trafficking. Longer term increases in crop production can be achieved by providing surface and subsurface drainage, bed farming and gypsum-slotting. Advantages and disadvantages of these techniques, their application in dryland and irrigated cropping and the areas for future research are discussed. In soils such as Vertisols with high shrink-swell potential, strong-rooted crops such as safflower could be used for biological soil loosening, through deep soil profile drying. The effectiveness of soil ameliorative techniques can be evaluated by assessing the soil factors limiting crop growth during a growing season in a non-ameliorated soil, and the subsequent changes in these soil factors due to the ameliorative practices. A technique which can be used in field studies to monitor these changes through the cropping season, based on the concept of the 'non-limiting soil water range' for crop growth, is described. Irrigation management of sodic soils and re-use of saline drainage waters require an understanding of the changes in soil hydraulic properties with changes in water quality parameters. The 'equivalent salt solution' concept can be used to predict such changes in soil hydraulic properties. These predicted values could then be used in existing water flow models for assessing water and salt flow through irrigated sodic clay soils.

Effects of soil solution cation concentrations and ratios on hydraulic properties must be understood in order to model soil water flow in reactive soils or develop guidelines for sustainable land application of wastewater. We examined effects of different ratios and concentrations of the cations Ca2+, Mg2+, Na+, and K+, using hydraulic conductivity measurements in repacked soil cores, as an indicator of soil structural stability. We examined widely used indices—sodium, potassium, and monovalent cation absorption ratios (SAR, PAR, MCAR)—which assume that the flocculating effects of Ca2+ and Mg2+ are the same, and the dispersive effects of Na+ and K+ are the same. Our laboratory measurements indicate that at any given values of MCAR, the reductions in soil hydraulic conductivity with decrease in electrolyte concentration are not identical for different cation combinations in solution. The hydraulic conductivity curves showed a marked lateral shifting for both the surface and subsurface soils from a winery wastewater application site. This indicates that MCAR is inadequate as a soil stability parameter in soil solutions containing a mixture of Na+, K+, Ca2+, and Mg2+. We employed an unpublished equation that was proposed by P. Rengasamy as a modified index of soil stability for mixed cation combinations, using calculated relative flocculating powers of different cations (‘CROSS’, cation ratio of structural stability). Our observation of lateral shift in hydraulic conductivity measurements at any value of MCAR appears to relate to changes in CROSS values for all cation combinations tested, except for K–Mg solutions, for which a more generalised CROSS equation with modified parameters seems more suitable for calculating the CROSS value. Appropriate modified parameters for use in this generalised CROSS equation were determined empirically, using the experimental data. We derived a combination of threshold electrolyte concentration and CROSS values required to maintain high hydraulic conductivity for the soils at a winery wastewater application site. The potential use of this relationship in developing management practices for sustainable wastewater management at the site is discussed. Further research on the applicability of CROSS and generalised CROSS equations for other soils in the presence of different mixed cation combinations is needed.

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