Low levels of extractable phosphorus (1.6 ? 0.14 ,tg/g) were observed in soils which apparently developed from an old lava flow at La Selva, and progressively higher levels were found in soils derived from another lava flow (2.6 ? 0.15 ,ug/g), old alluvial material (3.9 ? 0.31), and recently deposited alluvium (36.6 ? 3.4). No significant differences in nitrogen mineralization were observed among these soils. Seedlings of Phytolacca rivinoides accumulated more biomass when grown in old alluvial soils than in soils in derived from lava flow. Studies of population-, physiological-, and ecosystem-level processes could benefit from considering such differences. MANY STUDIES OF POPULATION BIOLOGY and community ecology have been carried out at the La Selva Biological Station in the Atlantic lowlands of Costa Rica (10026'N, 83?58'W). In part because of this background, La Selva was identified by a committee of the National Academy of Sciences as one of four priority tropical forest sites for ecosystem-level research, the only fertile Neotropical lowland rainforest site so chosen (NAS 1980). La Selva has already been the site for a number of studies on element cycling and other ecosystem-level processes. Bourgeois et al. (1972) described and characterized soils derived from recent alluvial deposits, older alluvium, and residual basalt at La Selva; Gessel et al. (1980) provided an overview of nutrient circulation; and Jordan (1985) compared ecosystem characteristics at La Selva with those at other lowland tropical sites. McColl (1970) and Pringle et al. (1986) measured streamwater chemistry, and Johnson et al. (1975, 1977, 1979, 1983) summarized the processes controlling ion leaching through soils. Vitousek and Denslow (1986) evaluated water and nutrient availability within and outside treefall gaps at La Selva, and a number of recent studies have reported the effects of disturbance and succession on aspects of water and nutrient cycling (Huston 1982, Raich 1983, Luvall 1984, Robertson 1984, Sollins et al. 1984, Werner 1984, Parker 1985). In the course of studying nutrient availability in residual basalt-derived soils within treefall gaps at La Selva (Vitousek & Denslow 1986), we noted strong differences in soil color and phosphorus concentrations. These differ' Received 11 September 1985, revision accepted 22 March 1986. ences were associated with what Alvarado (1982) identified as two distinct lava flows. Many of the studies cited above do not specify where within the La Selva Biological Station they were carried out, and it seemed possible that certain results might be valid only for certain soils, and more importantly, that certain areas of the La Selva Biological Station might not be reasonably characterized as having fertile soils. Accordingly, we surveyed nitrogen and phosphorus availability in surface soils at La Selva. This survey is not intended as a substitute for a thorough characterization of the soils, but we believe it can be used to suggest which soils have greater or lesser availabilities of frequently limiting nutrients. Such information can be useful in interpreting population-level and physiologicalas well as ecosystem-level studies. For example, Cox (1981) showed that females of the dioecious tree Trophis involucrata at La Selva were found on higher-P soils than were males; our results could be used to determine the extent to which habitat differences versus soil modification contributed to this pattern.