Summary 1. The leaf litter of tropical forests serves as a carbon sink, slows soil erosion and leaching, and is home to a large fraction of the forest's biodiversity. Standing leaf litter reflects the balance of litterfall and decomposition; both rates may be limited by element availability. We propose a mechanism for this regulation that assumes phosphorus limits metabolism in tropical soils, and that phosphorus limitation is more pronounced in faster growing organisms. 2. Leaf litter depth varied 16-fold (1·4-22·4 cm) across 28 forest stands in Panama and Peru and was deeper on sand vs. clay soils. Of five elements tested (N, P, K, Mg and Ca), the concentration of P in decomposing litter best predicted litter depth ( r 2 = 0·76, C : P 1·90 ). This relationship broke down in the most impoverished sandy soils. 3. These data are consistent with the hypothesis that the weathering of tropical soils limits the metabolism of microbes first and trees second, with decomposition and litterfall co-limiting litter depth in ecosystems with the least available phosphorus. This has implications for the dynamics of weathering: nutrient leaching may be regulated through negative feedback if deeper litter buffers soil from rainfall.