LAUSR

Abstract The precise determination of landforms and their formation processes are key to accomplish detailed soil mapping and better understand of soil genesis. The Upper General River Basin is located at the southeast of Costa Rica on the transition between Cordillera de Talamanca and General River Valley, forming an extensive alluvial fan sequence. Our work aims to determine the soil–geomorphology relationships on these alluvial fans. The methods employed consisted of geomorphological mapping using 1:25,000 aerial photographs to determine five alluvial fans: buried inactive, modeled inactive, early, intermediate, and late. Soil maps were established by combining landforms with soil survey data that allowed to differentiate twelve soil units of Ultisols associated with Oxisols, Inceptisols and Entisols. The most weathered soils were classified as Anionic and were Acrustox found on the oldest landforms located on buried inactive and early alluvial fans. These soils were found associated with Typic Rhodustults and Ustic Haplohumults. Typic Ustifluvents occurred on early alluvial fans and modeled inactive alluvial fans. Similar morphologic and chemical characteristics of the evaluated Oxisols were found for soils previously classified as Ultisols. These findings support the hypothesis that many soil currently classified as Ultisols could be reclassified as Oxisols (if the appropriate mineralogical analyses were conducted). These results put in perspective that the origin of the fans and hence their ages control the consequent soil development. In addition, the study contextualizes the implications of mapping and classifying highly weathered tropical soils for territorial planning, agricultural management and natural resources conservation.