There are insufficient data supporting the enormous potential of biochar in highly weathered tropical soils. This glasshouse study assessed rice-husk biochar (RHB) effects on soil organic carbon, aggregate stability and nitrogen fertility of sandy-loam Ultisols which were evaluated using spinach (<em>Celosia argentea</em>) growth. Five RHB rates 0, 5, 10, 20, and 40 g per two-kg-soil (0, 7.5, 15, 30 and 60 t ha⁻¹, respectively) were studied under 0, 4, 8, and 12 weeks of incubation (WOI). Batched potting of treatments enabled sowing on one date. Treatment effects on soil quality were assessed at sowing and spinach growth six weeks later. Soil organic carbon generally increased with RHB rate, with the greatest increments (37%) in maximum rate relative to no-biochar control for 8 WOI. Aggregate stability also generally increased with RHB rate, the range being 7.21%-17.21% for 8 WOI, beyond which it decreased in 10 and 20 but not 40 g pot^–1. Total nitrogen was always highest in maximum rate, increasing with rate only for 8 WOI. Treatment affected plant height more clearly than leaf count. Optimum rates were 5 or 10 g pot^–1 for 8 and 4 WOI, respectively (plant height) and 10 g pot^–1 for 8 WOI (leaf count). Soil organic carbon influenced soil aggregate stability (R² = 0.505) which in turn was quadratically related to plant height (R² = 0.517), indicating stability threshold for spinach. Adding RHB at 40 g pot^–1 (≈ 60 t ha⁻¹) to coarse-textured tropical soils is suggested to sustain its soil aggregating effect beyond the growth phase of short-cycle leafy vegetables which require a lower rate (10 g pot^–1) 8 weeks before sowing. The observed role of soil aggregate stability in spinach growth rather than the overall effects of RHB should guide further search for edapho-agronomic optimum rate of RHB.

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