For further reading please go to:

•  http://www.fao.org/DOCREP/003/Y1899E/Y1899E00.HTM
  
• Eswaran et al., 2003 (chapter 19 by Deckers/Driessen/Nachtergaele/Spaargaren/Berding)
  

Table of contents:

1. Concepts behind WRB
   
2. Elements of WRB
   a. introduction
   b. Subdivision of the 30 reference soil groups
   
   • diagnostic horizons (basic identifiers, 1st level)
     
   • diagnostic properties and materials (qualifier, 2nd level)
     

• Since its endorsement by the International Union of Soil Sciences (IUSS) in 1998, the WRB has established itself as a comprehensive soil correlation system. The structure, concepts, and definitions of the WRB are strongly influenced by the legend of the FAO-UNESCO 1/5,000,000 Soil Map of the World, which in turn borrowed the diagnostic horizons and properties approach from USDA Soil Taxonomy (see Eswaran et al., 2003; World Soil Resources Report No. 94, 2001).
  
• It is considered as an international reference and correlation system to create and refine a common and global language for soil classification.
  
• The WRB aims to serve as a framework through which ongoing soil classification throughout the world can be harmonized.
  
• The ultimate objective is:
  a. to reach international agreement on the major soil groups to be recognized at a global scale.
  b. To reach international agreement on the criteria and methodology to be applied for defining and separating them.
  
• Such an agreement is needed to:
  a. facilitate the exchange of information and experience.
  b. To provide a common scientific language.
  c. To use standardized criteria and characteristics (= qualifiers).
  d. To strengthen the applications of soil science.
  e. And to enhance the communication with other disciplines.
  

a. Introduction

• The subdivision of the Reference Soil Groups occurs through:
  a. Vertical combination of horizons
  b. Lateral arrangement of the horizons or absence of horizons
  

In this context, horizons are named diagnostic when they possess a minimum degree of soil development . This is defined by:

• visibility
  
• Prominence
  
• measurability
  
• Importance and relevance for soil formation and soil usage (-> reference to bio-climatological factors)
  

b. Subdivision of the Reference Soil Groups http://www.fao.org/DOCREP/003/Y1899E/Y1899E00.HTM

• the 30 Reference Soil Groups (1st level) (Table 1, Annex 1) are aggregated in 10 sets and are differentiated and grouped based on:
  
  1. diagnostic horizons as used in soil classification (as basic identifiers) (Table 2, Annex 2). Diagnostic horizons are defined through (combinations of)
     a. diagnostic soil properties (Table 3, Annex 2)
     b. and/or diagnostic soil materials (Table 4, Annex 2)
     
  2. Within each Reference Soil Group 3 to 9 soil units are distinguished based on:
     
     • Soil qualifiers (Annex 3)
       

1. Diagnostic horizons (Table 2, Annex 2):

• Diagnostic horizons are characterized through 1 or several soil properties that:
  
  • Occur above or in a particular soil depth
    
  • With a definite prominence, intensity
    
  • Vertical: a few decimeters
    
  • Lateral: 1 to several kilometers
    
  • Boundaries are diffuse, gradual, clear or abrupt
    

1. a. Diagnostic soil properties (Table 3, Annex 2):

• Soil properties comprise an assemblage of several soil characteristics and reflect present or past mechanism of soil formation.
  

Soil characteristics are:

• Sole parameters (laboratory, field, microscopy)
  
• Characteristics such as texture, soil depth, voids, mottles, cutans, nodules
  
• Analytical determination like pH, cation exchange capacity, soluble salts
  Example: vertic properties
  
  • Comprises: heavy texture, smectic mineralogy, slickensides, gilgai, hard consistence when dry and sticky consistence when wet, shrinking when dry and swelling when wet.
    

1. b. Diagnostic material (Table 4, Annex 2) reflect:

• original parent material
  
• Pedogenetic processes have not yet been so active that they left a significant mark
  
• Comprise anthropogenic, calcaric, fluvic, gypsic, organic, sulfidic, tephric
  

Three to nine soil units (2nd level) within each Reference Soil Group are distinguished based on one or more:

2. soil qualifiers ( Annex 3):

• Soil qualifier originate from already existing criteria such as diagnostic horizons, properties and materials but may include additional (new) elements.
  
• Not used for distinction of soil units:
  
  1. Characteristics like climate, parent material, vegetation, slope, erosion
     
  2. Soil-water related variables like groundwater table, run-off or drainage
     
• Qualifier names can be used in combination with depth, intensity and so on
  example:
  
  • epi-dystric Luvisol: epi indicates shallow and dystric is a label for low base saturation (< 50 %)
    
• When more than 2 qualifiers are used to classify the soil unit, they are put in brackets behind Reference Soil Group
  example:
  
  • Calcic-chromic Vertisol (mazic): the Vertisol has, additionally to calcic and chromic criteria, a very hard surface horizon = mazic
    
• The nomination of the soil units cannot overlap or be in contrast to other soil units or other Reference Soil Group Definitions
  examples:
  
  1. Dystric-petric Calcisol: dystric signals low base saturation (< 50 %) -> this is not compatible with Calcisol having secondary CaCO₃-enrichment, i.e. high Ca and thus high base saturation
     
  2. Eutric-petric Calcisol: the prefix eutric does not give additional information beyond Calcisol; eutric indicates a base saturation above 50 % -> this overlaps with the information that can be inferred from Calcisol.