A model has been presented to illustrate the way in which the influence of exchangeable Na on the fundamental processes of dispersion and flocculation on Na-Ca systems affects the various soil physical properties in the field. Most cultivated soils slake (breakdown into microaggregates) when subjected to rapid wetting, giving rise to a surface seal and a reduction in infiltration rate. However, slaking alone may not neccessarily reduce the soil's productivity, e.g. surface aggregates of the highly productive self-mulching black earths slake even when in the virgin state. If dispersion follows slaking, in most cases it will lead to poor physical properties which may manifest as poor drainage, surface crusting. hardsetting and poor trafficability or workability of the soil and eventually lead to reduced crop yields.It is the dispersion phase that is affected by the presence of excessive sodium on the exchange complex of the soil, and this may have a profound effect on the soil's physical properties and behaviour.This paper reviews the possible mechanisms by which excessive sodicity may manifest in undesirable soil physical behaviour. It also attempts to relate observations made in the laboratory on pure Na-Ca-clay systems to the behaviour of the soil in the field.The effect of sodium on the dispersive behaviour of a soil is discussed in relation to its hydraulic conductivity and the processes of infiltration, redistribution and evaporation of water which in turn affects the subsoil water storage in a soil profile. The presence of sodium is also discussed in relation to changes in soil strength characteristics, the soils workability and ease of tillage and ultimately the soil's productivity. Data are presented which show that the validity of a threshold ESP and the exclusive use of ESP as a measure of sodicity are open to question.
Light-textured soils (<35% clay) make up more than 80%, by area, of cropping soils in Australia. Many have inherent soil physical problems, e.g. hardsetting, sodicity and low organic carbon levels. Maintenance and improvement of soil organic carbon levels are crucial to preserving the soil structure and physical fertility of these soils.A review of field trials on conservation tillage (3–19 years duration) on these soils in southern Australia revealed that significantly higher soil organic carbon levels compared with conventional tillage were found only in the wetter areas (>500 mm) and the differences were restricted to the top 2.5–10.0 cm. The average magnitude of the difference was lower than that reported in the USA. The lack of a positive response to conservation tillage is probably a reflection of a number of factors, namely low crop yield (due to low rainfall), partial removal of stubble by grazing and the high decomposition rate (due to the high temperature). There is evidence suggesting that under continuous cropping in the drier areas, the soil organic carbon level continues to decline, even under conservation tillage.Better soil structure and soil physical properties, namely macro-porosity, aggregate stability and higher infiltration have been reported under conservation tillage when compared with conventional tillage. However, little information on long-term changes of these properties under conservation tillage is available. As many of these soil qualities are associated directly or indirectly with soil organic carbon levels, the lack of significant increase in the latter suggests that many of these improvements may not be sustainable in the longer term, particularly in the drier areas. Continuous monitoring of long-term changes in the soil organic carbon and soil quality under conservation tillage in different agro-ecological zones is needed.
The energy applied to a soil-water suspension by an ultrasonic probe was determined for seven vertisol soils using calorimetric techniques. The rate of energy consumed by aggregate dispersion during sonification was calculated as the difference between the energy components measured before and after complete dispersion. Dispersive energy consumption was found to vary significantly during sonification and significant differences (P
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