What Soil Classification Should Tell Us about the Soil System.

Soil classification condenses our knowledge of a soil to a name. On the landscape level, soil classification provides the names of map units. A map unit may comprise several soil names, e.g. of a dominant, a codominant and some associated soils. But the map unit requires generalization, and each of the soil names in the map unit is shorter than of its corresponding pedons. The World Reference Base for Soil Resources (WRB) has set up rules that certain principal qualifiers have to be used at certain scale levels to give the most important characteristics of the soils of the map unit. But what about the interrelationships between the soils of different map units in a landscape? WRB allows adding any qualifier as optional qualifier. Even though a qualifier may generally be less important for the individual soil (and therefore is not required to be added to the name of the map unit), it may be very important for understanding the landscape (so it may be added as optional qualifier). In a loess sequence, the upper soils may be eroded and need the Nudiargic qualifier and the lower soils show accumulation and need the Colluvic qualifier. In a landscape with stagnant water, the upper soils may have lost iron in the topsoil and need the Albic qualifier, whereas the lower soils show iron accumulation and may need the Chromic qualifier. The distribution of the following qualifiers may indicate in which map units the higher and lower crop yields are expected: textural qualifiers, Dystric/Eutric, Sideralic (low CEC), Humic, Leptic (shallowness), Skeletic, (Hypo-)Salic. With the addition of optional qualifiers, the names of the map units can be adjusted in such a way that many information about spatial distribution and interrelationships of soil properties can be provided.