SOIL DEVELOPMENT ON SHALES OF THE SAN SEBASTIAN FORMATION, PUERTO RICO

The Susquehanna Shale Hills Observatory (Penn State) links a latitudinal transect of shale soils in different climates from Wales to Puerto Rico. The aim of the Puerto Rico research is to serve as the southernmost anchor for analysis of the development of shale weathering profiles in a humid tropical climate. An on-site weather station provides local comparison with the climate station at nearby Lares. A small stream in a ravine has shown low concentration and removal of cations from the study area. Field work at the site near the Juncal community includes two 2-meter pits and two trenches (6-meter and 4-meter), distributed within the study area. Physical descriptions of these soil profiles show that red clay dominates the soil formation, matching previous descriptions that the area soils are Ultisols. The soil profile is more than 6-meter deep, and chemical analysis of the soil samples vs. shale bedrock showed a loss in cations such as K₂O (0.65%), and CaO (0.35%) with increases in Al₂O₃ (17.67%) and Fe₂O₃T (10.14%). From bottom to top in the soil profile cations are higher (K₂O~0.92% and CaO~16.41%) and sesquioxides are less abundant (Al₂O₃~9.27% and Fe₂O₃T~4.31%). The chemical compositions of the soils in the Juncal area behave similarly to three of the U.S. transect sites (Alabama, Tennessee and Virginia) that also show high accumulation of sesquioxides (~15 % to 20% for Al₂O₃ and ~7%-10% for Fe₂O₃T) and low percentage of cations (~0.1% up to >10% for CaO and 0.5% up to 4.2% for K₂O). Climate and location appear to be the key factors to understand the soils in this study area, where weathering processes develop faster and soil profiles tend to be deeper and thicker than farther north.