According to the World Reference Base for Soil Resources (F.A.O, 1998), eight main groups are identified in the study area (Figure 22). The description of the some soil profile, one for each group, is visible in the Annex 2.
They are present in the northern part of the study area. All the soils with a plinthic diagnostic horizon are here grouped. This plinthic horizon is a subsurface horizon which constitute an iron-rich, humus poor mixture of kaolinitic clay with quartz and other components and which irreversibly changes to a hardpan or to irregular aggregates on exposure to repeated wetting and drying with free access of oxygen.
They are rich in ferro-manganic concretions (nodules), ranging from 15% to more than 80% of the total soil volume. Their diameter ranges from a few millimetres to more than 2cm in the plinthic horizon. Their structure is generally sub-angular blocky or massive. Their colour is generally yellowish brown to dark brown. They are also characterised by an abrupt textural change.
The presence of this kind of soil is typical of Morocco but also of the Mediterranean basin. In fact this kind of soil is characteristic of tropical areas with a humid climate. However the Moroccan climate, as mentioned before, does not present these characteristics. This induced us to suppose that the formation of these soils is quite old. Ghanem in his Ph.D. thesis (1981) estimates their age as previous to the Villafranchian.
The strong lessivage they sustained, coupled with the strong precipitation of the pluvial period, led to waterlogged conditions and so to the succession of anaerobiosis and aerobiosis conditions.
Figure 22 - Soil map
These processes, repeated in time, caused a strong manganese and iron sesquioxide release and accumulation, and successively their hardening like concretions.
For the lower level units, two qualifiers have been recognised: endoeutric, abruptic and haplicThe soil sub-unit and the corresponding profiles included in this group are:
They are located in the eastern and southern part of the study area. They are characterised by the presence of the argic diagnostic horizon with a CEC greater than 24cmol/kg throughout all the profile. This argic horizon presents a high percent of clay skins on ped surfaces and pores (more than 5%, required limit for an argic horizon). This argic horizon is saturated with exchange bases.
These soils are deep (all more than 60cm) and have a strong abrupt textural change. The argic horizon occurs at depth ranging from 20 to 40cm and can reach a thickness of 60cm. They are slightly acid on the surface to acid in depth. The CaCO3 is totally absent in the profile. The CEC is around 40cmol/kg.
They are rich in iron (free iron), which can be concentrated in nodules, but with a much lower content and size than those of plinthosols. The colour of the Bt horizon is generally from red to dark red.
To obtain an argic horizon (depth more than 35cm) and manganese and iron sesquioxide concentrations, a strong climatic contrast must have occurred. So these soils are supposed to be very old but not so old as plinthosols.
For the lower level units, four qualifiers have been recognised: leptic, rhodic, chromic and cutanic.
The soil sub-unit and the corresponding profiles included in this group are:
They are located in the eastern scarp part of the study area on schist substratum but also on the quartzite outcrop (skhour). The common point shared between these soils is the presence of a cambic diagnostic horizon. This horizon is characterised by an angular to sub-angular blocky structure and also by a CEC greater than 16cmol/kg.
They have a neutral pH but are sometimes acid in depth. It is important to note that clay increase in depth is mainly due to the rock nature, the clay-rich schist, and not to a clay illuviation. The clay mineralogy is illitic in the case of cambisols developed on schist and, kaolinitic in the case of skhour.
They are not too deep (around 30cm) however the friable nature of the schist allows a pedogenesis and soil development until 2m like pockets. The rock fragments are very abundant (more than 40%).
They are very poor in iron and manganese sesquioxide nodules. They are all situated on steep slopes and thus are submitted to a high erosion risk and are always rejuvenated by erosion phenomena.
For the lower level units, five qualifiers have been recognised: leptic, mollic, ferralic, skeletic and chromic.
The soil sub-unit and the corresponding profiles included in this group are:
Soils are grouped here according to the presence of a vertic diagnostic horizon. This vertic horizon is characterised by a heavy texture (more than 30% of clay) and the presence of polished, shiny ped surfaces (slickensides). The clay mineralogy is of montmorillonite kind and the exchange capacity is totally saturated by exchangeable bases. Their pH is from neutral to basic and increases from the surface to the depth according to CaCO3 presence.
The profile is slightly to totally decarbonated therefore causing a calcic or hypercalcic horizon formation in the depth. In the small depressions they become salted and even sodic in depth. Basing on soil colour there are two main kinds of vertisols: a dark gray to black first group (pellic vertisols) and a red to black red second one (chromic vertisols).
The vertical genesis is mainly due to the clayification (neoformation) process in saturated environment and to the shrinking and swelling of these neoformed clays. These processes, governed by the alternation of humid and arid periods, led to the vertic horizon formation. According to Ghanem (1981) soils are autochthonous and their formation dates back to the Neolithic, for the youngest one, or to an older period (old quaternary).
For the lower level units, four qualifiers have been recognised: calcic, pellic, chromic and eutric.
The soil sub-unit and the corresponding profiles included in this group are:
They are quite spread in the study area. Soils are here grouped according to the presence of a mollic diagnostic horizon and to the concentration of secondary carbonates in the profile.
This horizon is generally rich in organic matter (>0.6% of organic carbon), well-structured (structure almost granular or sub-angular-blocky), and saturated with exchangeable bases. They are dark brown to black in colour.
Two main sub-units were found:
The first ones are characterised by low depth, balanced texture and high CaCO3 content, the second ones by heavy texture, depth of more than 1m, and the scarce presence of CaCO3.
These soils are younger than vertisols and could be considered as a preliminary step for their formation.
For the lower level units, four qualifiers have been recognised: vertic, calcic, siltic, and chromic.
The soil sub-unit and the corresponding profiles included in this group are:
They are located in the depressions in the vertisols area. They are waterlogged in the winter period and in summer they are under the water table influence. They are characterised by the presence of natric diagnostic horizon. This horizon is characterised by a prismatic to massive structure, much higher than 15 exchange sodium percent (ESP) value. The texture is very heavy and the colour is black.
For the lower level units, they present an intergrade with vertisols reference soil group
The soil sub-unit and the corresponding profiles included in this group is:
These soils are limited in depth by continuous hard rock within 25cm from the soil surface.
For the lower level units, four qualifiers have been recognised: lithic and mollic.
The soil sub-unit and the corresponding profiles included in this group are:
According to F.A.O classification this group is the last of soil groups. Only two profiles are classified in this group: