This study investigates the biological control of terrestrial silica cycling and export fluxes to watersheds, using the distinct signatures of weathering and biogenic silica cycling in germanium/silicon (Ge/Si) ratios. The research focuses on basaltic soils across the Hawaiian islands, where silica fluxes to stream waters are analyzed to assess the relative contributions of weathering and biogenic silica cycling. The results suggest that most of the silica released to Hawaiian stream water has passed through the biogenic silica pool, while direct mineral-water reactions account for a smaller fraction of the stream silica flux. This indicates that biological processes play a significant role in controlling silica cycling and export in these systems. Germanium/silicon ratios have been used to trace silica sources in rivers and the oceans. In streams unaffected by pollution or hydrothermal inputs, Ge/Si ratios are lower than those in the silicate bedrock they drain. Secondary minerals, such as clays, which are higher in Ge/Si, are the major complementary reservoirs. The Murnane, Stallard, Froelich (MSF) model explains the dissolved Ge–Si relationships in rivers as a mixture of silica derived from the weathering of primary minerals and silica derived from the weathering of secondary clays. According to this model, the low-Ge/Si, high-[Si] component is produced during the incongruent dissolution of primary minerals, while the high-Ge/Si, low-[Si] component arises from the dissolution of secondary clays. The study tested the predictions of the MSF model by measuring [Ge] and [Si] in soil waters from seven depth profiles along a chronosequence of basaltic soils across the Hawaiian islands. The results show that young soils retain primary minerals and volcanic glass, have experienced little Si loss, and have Ge/Si ratios close to that of fresh basalt. Older soils have experienced significant Si loss and have lower Ge/Si ratios. The data suggest that biogenic silica cycling plays a major role in controlling stream Si export, with the majority of dissolved silica coming from the biogenic silica pool. This highlights the importance of biological processes in regulating silica cycling and export in terrestrial ecosystems.This study investigates the biological control of terrestrial silica cycling and export fluxes to watersheds, using the distinct signatures of weathering and biogenic silica cycling in germanium/silicon (Ge/Si) ratios. The research focuses on basaltic soils across the Hawaiian islands, where silica fluxes to stream waters are analyzed to assess the relative contributions of weathering and biogenic silica cycling. The results suggest that most of the silica released to Hawaiian stream water has passed through the biogenic silica pool, while direct mineral-water reactions account for a smaller fraction of the stream silica flux. This indicates that biological processes play a significant role in controlling silica cycling and export in these systems. Germanium/silicon ratios have been used to trace silica sources in rivers and the oceans. In streams unaffected by pollution or hydrothermal inputs, Ge/Si ratios are lower than those in the silicate bedrock they drain. Secondary minerals, such as clays, which are higher in Ge/Si, are the major complementary reservoirs. The Murnane, Stallard, Froelich (MSF) model explains the dissolved Ge–Si relationships in rivers as a mixture of silica derived from the weathering of primary minerals and silica derived from the weathering of secondary clays. According to this model, the low-Ge/Si, high-[Si] component is produced during the incongruent dissolution of primary minerals, while the high-Ge/Si, low-[Si] component arises from the dissolution of secondary clays. The study tested the predictions of the MSF model by measuring [Ge] and [Si] in soil waters from seven depth profiles along a chronosequence of basaltic soils across the Hawaiian islands. The results show that young soils retain primary minerals and volcanic glass, have experienced little Si loss, and have Ge/Si ratios close to that of fresh basalt. Older soils have experienced significant Si loss and have lower Ge/Si ratios. The data suggest that biogenic silica cycling plays a major role in controlling stream Si export, with the majority of dissolved silica coming from the biogenic silica pool. This highlights the importance of biological processes in regulating silica cycling and export in terrestrial ecosystems.