Silicon (Si) is an essential element for the growth and development of rice plants, playing a crucial role in their overall health and productivity. This study aimed to measure earthworm's impact on Si dynamics in northern Vietnam's paddy fields. The properties of earthworm casts from 23 different sites were compared to the surrounding topsoil. The results showed that the casts were enriched in biogenic silica (herein phytoliths) and plant-available Si (measured through acid acetic extraction, SiAC) compared to the reference topsoil. Also, casts had a higher sand content, while their carbon content was similar to the reference soils. This suggests a possible preference for sand particles by earthworms (e.g., for grinding plant material within their gizzards) and/or the consumption of soil from another layer enriched in sand content. The influence of earthworms on Si dynamics was found to be dependent on the soil's environmental properties. In soils with low fertility (characterized by a higher proportion of sand and lower concentrations of C and oxides), earthworms increased the concentration of SiAC. However, beyond a certain level, the effect of earthworms on Si availability became neutral. While this study highlights the critical role of earthworms in paddy fields, further research is needed to understand how earthworms enhance the concentration in SiAC in the topsoil, and the consequences to rice growth and resistance to environmental hazards.