Geoscience Reference
In-Depth Information
approach that now defi nes the highest level or category in the system. A total of 32
reference soil groups, known as RSGs, are now formulated. The fi rst edition of
WRB (1998) was comprised of 30 RSGs. Today, the number has been extended to
32. The nomenclature retains terms used traditionally and usually related to current
languages. The lower-level units are expressed by exactly defi ned prefi x and suffi x
qualifi ers that are related to secondary soil-forming processes.
The RSGs are sequenced into ten sets according to dominant identifi ers. We now
elucidate each set.
1. Organic soils with a dominant presence of organic materials at various degrees
of humifi cation. The infl uence of abundant organic materials accounts for spe-
cifi c soil features that separate these soils from mineral soils.
Histosols are the one and only RSG belonging to this fi rst set. Its name was
derived from the Greek
histos
meaning tissue. These soils have more than about
15 % of organic carbon in the top 40 cm even though, according to WRB, the top
horizon may be of much greater thickness. According to the US Taxonomy, the
content of organic matter is more than 25 % and typically increases with time. Due
to a high content of semi-decomposed organic materials, the bulk density is very
low. Let us remind ourselves that wood fl oats in water only because of its low den-
sity. The soil is poorly drained since organic matter holds water very well and the
products of humifi cation contribute to high values of contact angle and hydropho-
bicity. Hence, the wettability of a Histosol is low. Once it becomes dry, it accepts
water very slowly and wets with diffi culty. For example, the rain infi ltration rate
after a dry summer is practically zero with raindrops easily rolling across its surface
causing a high runoff without measurably wetting the surface mixture of soil organic
matter and the more or less decomposed organic remnants. Very low hydraulic con-
ductivity values further contribute to waterlogging of the once water-saturated
Histosol and subsequent anaerobic processes together with prevailing acidic reac-
tion. If they are used in agronomy, which is not very frequent, they are usually man-
aged as a pasture. The majority of Histosols occur in boreal and subarctic regions
and in lowlands of mountainous areas. If Histosols are drained and brought into
agricultural production, they should be regularly limed in order to reduce the acidity
and in order to bring various plant nutrients into chemical forms more acceptable by
cultivated plants. But generally, it is more advantageous to introduce plantation
cropping and eventually forestry instead of annual cropping. Histosols are known in
some countries in their national classifi cation systems as, e.g., Organosols, Moore
(Niedermoor, Hochmoor), Peat Soils, Muck Soils, or Felshumusböden. The US
Taxonomy retains the identifi cation of Histosols as the name of a soil order term.
2. Since WRB considers human activity as a soil-forming factor, it separates a spe-
cial set of soils where human activity dominates over other factors, some of
which do not even have a chance to play a peripheral subordinate role. The sys-
tem recognizes two RSGs in this set:
Anthrosols
and
Technosols
.
Anthrosols are soils which have been profoundly modifi ed by human activities,
and their name was derived from the Greek
anthropos
meaning human being.
