Abstract
A phenomenon causing instability of soil structure and associated hydraulic properties in recently tilled soils is aggregate fragmentation induced by wetting and drying cycles. We analyzed data from three experiments in Puerto Rico, the UK and China measuring fragmentation and resulting evolution of aggregate size distributions during successive wetting and drying cycles in heavy textured soils. Aggregate distributions were represented as the cumulative fraction F of aggregates passing through successively larger sieve sizes X. To a good approximation, all distributions exhibited similarity in that the aggregate diameter X(F) corresponding to F in a given test distribution was always a characteristic multiple α¯ of X(F) in a fixed reference distribution, where α¯ for a distribution was calculated as its mean weight aggregate diameter (MWD) divided by the MWD of the reference distribution. In most cases, α¯ for a given soil varied inversely with the square of the number of wetting and drying cycles. For different soils of similar initial aggregate sizes, α¯ for a given wet–dry cycle decreased with increasing activity coefficient, reflecting the enhancing effect of soil shrink–swell potential on fragmentation. Results highlight usefulness of the van Bavel mean weight diameter as a natural scaling parameter for characterizing aggregate distributions.
Highlights
Tillage of agricultural soils produces a loose, unstable structure that gradually settles back to a more stable state, resulting in highly transient soil mechanical and hydraulic properties [1,2]
Another cause is generalized shear failure or “slumping” of moisture-weakened soil aggregates [3], or localized shearing or “sintering” at inter-aggregate contact points induced by capillary forces [5]
Our study examined three experiments describing fragmentation of soil aggregates under successive wetting and drying cycles
Summary
Tillage of agricultural soils produces a loose, unstable structure that gradually settles back to a more stable state, resulting in highly transient soil mechanical and hydraulic properties [1,2]. A major cause of structural instability in tilled soils appears to be tensile failure of soil aggregates, caused by intra-aggregate air compression by infiltrating water [6,7], and differential soil swelling across the advancing wetting front [4,8]. Another cause is generalized shear failure or “slumping” of moisture-weakened soil aggregates [3], or localized shearing or “sintering” at inter-aggregate contact points induced by capillary forces [5]. The low degree of confinement facilitates aggregate swelling and shear deformation, transforms inter-aggregate contact forces into intense deviatoric stresses [3,5], and allows formation of tensile stresses in the material between opposite contact points [9]
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