Characterization of soil structure and stability using modulus of rupture - exchangeable sodium percentage relationships

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Abstract
Measurements of the modulus of rupture (MOR) and in particular of the way in which it changes for a given soil with change in exchangeable sodium percentage (ESP) from zero to 20, have been used to characterize the structural stability of a range of soils from the Western Australian wheat-belt. For convenience soils were categorized as hard-setting or non-hard-setting by an arbitrary choice of an MOR value of the natural soil of 60 kPa as the lower limit of the hard-setting behaviour. The most striking differences between the two soil categories are the higher soil strength at zero ESP (baseline MOR) and rapid increase in MOR with increasing ESP (sodium sensitivity) generally characteristic of the hard-setting soils compared with the non-hard-setting soils. The MOR values quantitatively evaluate the tendency of the soils to slake on wetting, and the sensitivity of the MOR to increasing ESP (as a result of increasing double layer swelling forces) illustrates the extent and permanency of the stabilizing bonds in the soil matrix. The presence of exchangeable magnesium on the exchange sites has been shown to increase the MOR values compared with those for exchangeable calcium at all ESP values in line with previous evidence of the deleterious effects of this cation on soil structure. Although more detailed studies are required, the value of this approach is illustrated by its apparent ability to differentiate clearly between the effects of different management techniques (e.g. continuous cultivation as against continuous cropping), and even between short term effects arising within 1/1 rotations. The results from the 'paired' sites where soils, similar in most respects and having undergone identical management, exhibit significant differences in sodium sensitivity of their MOR, suggest that the strength and longevity of structural bonds may be related to the nature of the exchangeable cations present (in particular, whether sodium or calcium dominate) at the time of incorporation of organic matter in the soil.