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Sampling along a precipitation gradient in tropical South America extending from ca. 0.8 to 2.0 m a⁻¹, savanna soils had consistently lower exchangeable cation concentrations and higher C / N ratios than nearby forest plots. These soil differences were also reflected in canopy averaged leaf traits with savanna trees typically having higher leaf mass per unit area but lower mass-based nitrogen (N_m) and potassium (K_m). Both N_m and K_m also increased with declining mean annual precipitation (<i>P</i>_A), but most area-based leaf traits such as leaf photosynthetic capacity showed no systematic variation with <i>P</i>_A or vegetation type. Despite this invariance, when taken in conjunction with other measures such as mean canopy height, area-based soil exchangeable potassium content, [K]_sa , proved to be an excellent predictor of several photosynthetic properties (including ¹³C isotope discrimination). Moreover, when considered in a multivariate context with <i>P</i>_A and soil plant available water storage capacity (&theta;_P) as covariates, [K]_sa also proved to be an excellent predictor of stand-level canopy area, providing drastically improved fits as compared to models considering just <i>P</i>_A and/or &theta;_P. Neither calcium, nor magnesium, nor soil pH could substitute for potassium when tested as alternative model predictors (&Delta;AIC > 10). Nor for any model could simple soil texture metrics such as sand or clay content substitute for either [K]_sa or &theta;_P. Taken in conjunction with recent work in Africa and the forests of the Amazon Basin, this suggests – in combination with some newly conceptualised interacting effects of <i>P</i>_A and &theta;_P also presented here – a critical role for potassium as a modulator of tropical vegetation structure and function.