Gully erosion and woody plant encroachment are frequently observed in grasslands worldwide. Soil aggregate stability is one of the drivers of gully erosion and needs to be studied, especially in the context of both tree encroachment and gully erosion. This study compared the effects of tree encroachment [paperbark thorn; Vachellia sieberiana (DC.) Kyal. & Boatwr. previously called Acacia sieberiana DC.] and soil surface horizon properties on the aggregate stability (as measured by mean weight diameter, MWD) in a degraded semihumid grassland in the KwaZulu‐Natal province of South Africa. Soil properties from under trees (UT) and open areas (OA) of both an Entisol within deep gullies (locally called dongas) and Alfisol from grasslands outside the gullies (grassland). The MWD values were higher in the grassland (3.06 ± 0.21 mm) than in the dongas (1.84 ± 1.03 mm). In the grassland, the MWD was significantly higher in the OA than UT, associated with more fine roots in the OA. In the donga, soil properties UT were not significantly different than those in OA. The soil aggregate stability showed strong local variability that depends on the age of the donga and the nature of the material that composes the pediments. Illite was associated with higher MWD values. The presence of interstratified illite–vermiculite located in active donga increases the shrink–swell potential of the aggregates, exacerbating their dispersion reflected in the low MWD values measured, especially when C and root contents were also low. These results suggest that the local soil clay mineralogy controls the orientation and extension of gullies rather than soil chemistry, topography, or the size of the drainage area.

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Sugarcane (Saccharum officinarum L.), an intensive, long-term, monoculture, economical crop in South Africa, is known to degrade soil characteristics. Soil structure, a key indicator of soil health and biomass production potential, is manageable by agricultural practices. This study aims to evaluate the effect of crop residue management practices (mulching, burning with residues scattered or removed) and mineral fertilization (with or without) on soil structure by analyzing soil shrinkage curves (SSCs) and other soil physical and chemical properties in a long-term sugarcane trial established in 1939. The SSC provides descriptive structural soil data by differentiating and characterizing two pore systems (plasma and structural pores). By analyzing the SSC of 24 plots (four replicates each treatment), residue management and fertilization practices were found to have statistically and physically significant effects on hydrostructural variables. Partial redundancy analysis showed that residue management practices had a slightly higher effect (19% of total variance) on hydrostructural variables compared with fertilization (12%). The main hydrostructural variables representing the management effect were total soil shrinkage, specific volume, and swelling capacity of the plasma, which were higher in mulched and/or unfertilized plots, indicating that soil was less compact, and shrinkage was more intense, including at the plasma level. The stronger structural dynamics and aggregate stability of the soil were explained by the behavior of the primary aggregates (peds), which were more porous and reactive during the drying process. This study highlights the importance of mulching and limited fertilization to maintain soil structure in the long term while still ensuring yield production.