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Aggregate stability, one of the main factors controlling soil erodibility, varies over time. Knowledge of its variation would help to identify better soil management practices. In 10 soils from central Greece devoted to cereal cropping, seasonal wet aggregate stability (WAS) fluctuations were investigated over a period of two successive years. The wet-sieving technique of airdried aggregates was used for WAS determinations, according to a test resulting in an instability index calculation. Over the first year, when typical Mediterranean climatic conditions dominated, WAS varied according to a nearly cyclic pattern, from a low in winter or early-spring months to a high in summer months. The instability index varied from a high between 123-152% of annual average to a low between 58-83% of annual average. Total monthly rainfall (TR) and mean monthly air temperature (MAT) strongly correlated with seasonal WAS. Their ratio (TR/MAT, ombrothermic ratio) has been proved to be a good predictor of structural stability throughout the year, for most of the soils studied. Possible mechanisms deteriorating aggregation seemed to be raindrop impact, repeated soil drying and wetting and repeated soil freezing and thawing, while possible mechanisms promoting aggregation seemed to be soil drying and warming and biological activity. Over the second year, severe climatic inconsistencies complicated the seasonal pattern of WAS response. From January to March, WAS unexpectedly increased, obtaining its maximum value for most of the soils in March, then it varied inconsistently until October and thereafter decreased, obtaining its minimum value in December for all soils. The instability index varied from a high between 130-196% of annual average to a low between 61-83% of annual average. Uneven seasonal distribution of climatic characteristics and extreme events decisively modified the typical for Mediterranean conditions seasonal WAS variation pattern.

Aggregate stability is a fundamental property influencing soil erodibility and hydraulic characteristics. Knowledge of soil components controlling aggregate stability is very important to soil structure conservation. The objective of this study, which was carried out in surface soils from central Greece, was to relate wet aggregate stability to selected soil properties, with emphasis on excessive free carbonate content. The wet‐sieving technique of air‐dried aggregates was used for structural stability evaluation, according to a test that calculates an instability index. The soils studied were developed on Tertiary marly parent material and ranged in calcium carbonate content from 5 to 641 g kg−1. From the texture analysis before and after removal of carbonates, it was concluded that carbonates mainly contributed to total silt and sand fractions, that is to the mechanical fractions which, as a rule, negatively affect aggregate stability. The results of the correlation analysis showed that aggregate stability was positively affected by aluminosilicate clay content, cation exchange capacity (CEC) and Al‐containing sesquioxides. Clay fraction from carbonates and total sand and silt negatively affected aggregate stability. CEC has been proved a very significant determinant of aggregate stability, which in a hyperbolic form relationship with instability index explained 78·9 per cent of aggregate stability variation. Copyright © 2011 John Wiley & Sons, Ltd.

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