Despite having one of the world's highest deforestation rates, the tropical forest of Rondônia state in the southwest Amazon is virtually unknown in terms of nutrient dynamics. To fill this knowledge gap, the distribution of nutrients in a humid tropical forest located in the Ecological Reserve of Samuel, Rondônia state, was addressed. A total of 474 trees with diameter at breast height (DBH) >10 cm were mapped at the Ecological Reserve of Samuel, a 20,000-ha area bordering the Samuel Hydroelectric Reservoir, which is located 50 km south of Porto Velho, Rondônia state, Amazon region, Brazil. These individuals belonged to ca 220 species distributed in 41 families. From this total, leaves of 354 trees were collected for chemical compositional analysis; boles of 96 trees and fine branches of 75 trees also were collected. Soil samples were gathered for fertility analysis at two soil depth intervals: 0–50 and 50–100 cm. In general, soils were acidic and very poor in terms of elemental contents. For plants, the highest concentrations of P, K, and Mg were found in leaves and fine branches, and the highest concentration of Ca was in the litter. The lowest nutrient concentration was observed in boles. The nutrient concentrations of leaves collected at Samuel were similar to that found in other terra firme forests of the Amazon region and other tropical forests growing on poor soils. Comparisons of nutrient concentrations among families, genera, and species revealed that only N concentrations were distinct among botanical taxa; the variability found in concentrations of other nutrients were larger and did not reveal any significant differences. Like aboveground biomass, the total nutrient stocks of trees was concentrated in a few large individuals. This fact reveals important implications about the impacts caused by selective logging. Although few trees are cut per hectare in this activity, the effects on the ecosystem structure, due to the loss of nutrients, may be underestimated.