Knowledge of plant nutrient limitations within natural and agricultural ecosystems is important for a full understanding of the ecology of plant populations and communities. While broad generalizations have been made for wet tropical forests there are few studies that directly address the question of nutrient limitations. The lack of research, in part, is due to the time and expense of traditional long-term fertilization experiments that examine ecosystem productivity under different nutrient regimes. To identify nutrient limitations within a lowland tropical rainforest we utilized an alternative method, the root ingrowth-core technique, which uses nutrient enriched substrates implanted into the forest soil. Over time root growth into the enriched substrates is greatest in those containing limiting nutrients. In addition, we analyzed intact soil cores from the forest floor for nutrients and root length density to see if natural nutrient variations were sufficient to elicit changes in root proliferation. We found that root ingrowth cores as well as soil cores showed greater root length in cores richer in phosphorus and nitrogen. Comparisons between root ingrowth core treatments revealed no differences in root width. However, overall root width was greater in the ingrowth cores than in the forest soil. While most authors suggest that phosphorus and nutrient cations (e.g., K, Mg, and Ca) tend to be limiting in lowland tropical forests, our study suggests that both phosphorus and nitrogen but not potassium are in limited supply within the forest studied.