The Reference Soil Groups set #6 of the WRB reference system accommodates mineral soils conditioned by wet tropical climate and comprises the red and yellow soils of the tropical and subtropical regions. High soil temperatures and (at times) ample moisture promote rock weathering and rapid decay of soil organic matter. The Reference Soil Groups in this set have in common that a long history of dissolution and transport of weathering products has produced deep and genetically mature soils. The set includes 5 Reference Soil Groups (Plinthosols are excluded in the online version). For each soil special explanatory notes are compiled in sub-chapters named Excursus, and include characteristic chemical, physical or environmental (landscape, climate…) features of the respective soil:

• Deeply weathered FERRALSOLS that have a very low cation exchange capacity and are virtually devoid of weatherable minerals
  
  • Excursus: process of ferralitization
    
  • Excursus: sesquioxides
    
  • Excursus: cation exchange capacity
    
  • Excursus: point of zero net charge (PZNC)
    
  • Excursus: stone lines
    
• ALISOLS with high cation exchange capacity and much exchangeable aluminum
  
  • Excursus: clay minerals
    
  • Excursus: clay eluviation
    
  • Excursus: saprolite
    
  • Excursus: aluminium toxicity
    
• deep NITISOLS in relatively rich parent material and marked by shiny, nutty structure elements
  
  • Excursus: process of ferralitization
    
  • Excursus: clay eluviation
    
  • Excursus: termites
    
• strongly leached, red and yellow ACRISOLS on acid parent rock, with a clay accumulation horizon, low cation exchange capacity and low base saturation
  
  • Excursus: cation exchange capacity
    
  • Excursus : clay eluviation
    
  • Excursus : process of ferralitization
    
  • Excursus: aluminum toxicity
    
• LIXISOLS with a low cation exchange capacity but high base saturation percentage
  
  • Excursus: clay eluviation
    
  • Excursus: stone line