Root proliferation into the Oa and Oe soil horizons in tropical forests is often substantial and allows direct cycling of nutrients from the organic matter; this was thought to be an adaptation to the low nutrient supply in infertile soils. In this study, we show that experimentally increased litter inputs promote root proliferation into the Oi and Oe horizons in a relatively fertile soil, suggesting that it is a response to a more readily available nutrient source rather than an adaptation to nutrient shortage, and the absence of root mats on fertile tropical soils is simply a consequence of the lack of persistent organic horizons due to high decomposition rates.