a, Comparison of resin phosphate concentration and two common extraction procedures for plant-available phosphorus, showing values for 1,184 fresh (field-moist) soil samples at depths of up to 100 cm in lowland tropical forests of Panama. Relationships are shown for Bray-1 phosphate (top) and Mehlich-III phosphate (bottom). For both extractions, phosphate was determined in the extracts by automated molybdate colorimetry. Resin phosphate is strongly correlated to Bray-1 phosphate (Pearson product–moment correlation 0.81, P < 0.0001) and Mehlich-III phosphate (Pearson product–moment correlation 0.87, P < 0.0001). b, Relationship between resin phosphate concentration and total phosphorus (top) and organic phosphorus (bottom). Data are from soils from 83 sites in central Panama, with each value being the mean of multiple individual soil samples at a single site. The relationships are described by the following equations, derived from linear regression of log-transformed data: total phosphorus: y = 342.43 × (x0.3821), R2 = 0.68, P < 0.001; organic phosphorus: y = 106.01 × (x0.4136), R2 = 0.66, P < 0.001.