Rewriting PTR salinity, alkalinity, chemical toxicity and drainage based on the wrb-standard
Review of available information in the SGDBE
In the SGDBE, the attribute WR gives information about the water regime of the STU following 4 classes. This attribute gives information about the duration and the depth where the soil is saturated by water during the year. The relation between the values of this attribute and the drainage classes used for CGMS is given in Table 4-21. The class 4 of WR is corresponding to the VP (very poorly drained) class used in CGMS. But for the other classes of WR, there are several CGMS classes that correspond. For these WR classes, it is necessary to use other information to choose which CGMS class corresponds. To do so, the information from the soil name (FAO85- FULL, FAO90-FULL and WRB-FULL) and the agricultural limitations (AGLIM1, AGLIM2) will be used. These attributes will also be used when WR is not given (that is the case for all the data coming from the Digital Soil Map of the World).
The current CGMS rule for drainage class on the part of the SGDBE where soil names were given in the FAO-UNESCO 1974 legend is evaluated. Note that not each individual record is compared separately. First, all records of the CGMS rule have been applied. Thus, for example, not all Gleysols will receive value ‘P’; only the STUs that have not been assigned ‘W’ or ‘VP’ because of a drained or flooded phase.
For each occurrence of the rule, the values taken by the WR attribute (except when WR = 0) were counted (Table 4-22). Some occurrences show a good correlation with WR:
- with a good estimation (in bold): for Gleysols, gleyic soils, Podzoluvisols, Arenosols and coarse-textured soils;
- with a bad estimation (in light grey): for Histosols, Placic Podzols, Plinthic Acrisols, Stagnic soils, Humic Podzols, Albic Luvisols, Orthic Greyzems and Vertisols.
The occurrences using the phase are generally dispatched in several WR classes. This result is not surprising because phases are generally not well defined. Moreover, the use of phases was very heterogeneous from one country to another.
Some soil types have more or less a high variability in their water regime:
- for Planosols, the water regime is mainly between 2 and 3;
- for Fluvisols, the water regime is mainly 1, but could be 2 or 3.
In the FAO-UNESCO 1974 legend, the water regime of soils is described by:
- hydromorphic properties;
- aridic moisture regime;
- presence of an albic E horizon overlying a slowly permeable horizon;
- presence of a sulphuric horizon or sulphidic materials (see 4.4).
The different soil names for which one or more of these characteristics are present, or are not allowed, are listed in Table 4-23.
Hydromorphic properties mark soils showing one or more of the following properties:
- saturation by groundwater;
- occurrence of a Histic H horizon;
- dominant hues that are neutral N, or bluer than 10Y;
- saturation with water at some periods of the year, or artificially drained, with evidence of
reduction processes or of reduction and segregation of iron reflected by different criteria
like for example:
- in Podzols, the presence of a duripan or a thin iron pan;
- in soils having an argillic horizon, iron-manganese concretions larger than 2 mm;
- in Ferralsols, plinthite that forms a continuous phase within 30 cm.
For some soil groups, the presence of hydromorphic properties was not defined because it was not possible to separate them at the scale of the Soil Map of the World. It is the case for Fluvisols, Regosols, Luvic Kastanozems, Luvic Chernozems and Nitosols. In the Soil Map of the European Communities (CEC, 1985), several subdivisions were added:
- showing hydromorphic properties:
- within 50 cm from the soil surface: Gleyo-eutric, Gleyo-calcaric and Gleyo-dystric Fluvisols,
- related to surface water stagnation during most of the year: Stagno-eutric, Stagno-Calcaric Stagno-dystric Gleysols, Stagno-gleyic Cambisols, Stagno-gleyic Luvisols, Stagnogleyic Podzoluvisols, Stagno-gleyic Podzols.
- having a thin iron pan: Placi-dystric Histosols.
If the soil name can give information about the presence of hydromorphic properties and their depth of appearance, it gives no information about the duration of the saturation by water periods.
In the Digital Soil Map of the World, there is a rule for drainage estimation based on FAO soil name, topsoil texture, slope and phases (Table 4-24). The estimation gives a repartition in % to each drainage class that enables calculation of areas.
The FAO rule for drainage class on the part of the SGDBE where soil names were given in the FAO-UNESCO 1974 legend was evaluated. For each occurrence of the rule, the values taken by the WR attribute (except when WR = 0) were counted (Table 4-25). All the occurrences for which we have WR values show a good correlation with WR:
• with a good estimation (in bold)
• with a bad estimation (in light grey): An estimation is considered as bad if the dominant FAO drainage class doesn’t correspond to the WR value, or if there are two classes having equal percentage, even if one corresponds to the WR value.
Topsoil texture, slope and phase don’t show an impact on WR values.
In the FAO-UNESCO 1990 legend, the water regime of soils is described by:
- gleyic properties;
- stagnic properties;
- presence of an E horizon abruptly overlying a slowly permeable horizon;
- presence of a sulphuric horizon or sulphidic materials (see 4.4);
- well drained;
- imperfect to very poor drainage;
- very poor drainage or undrained.
The different soil names for which one or more of these characteristics are present or are not allowed, are listed in Table 4-26.
Gleyic and stagnic properties refer to soil material which is saturated with water at some periods of the year, or throughout the year, in most years, and which show evidence of reduction processes or of reduction and segregation of iron. Gleyic properties are related to saturation by groundwater. Stagnic properties are related to saturation by surface water within 50 cm.
In WRB the water regime is described by:
- Gleysols: soils having gleyic properties within 50 cm from the soil surface;
- Planosols: soils having an eluvial horizon, the lower boundary of which is marked, within 100 cm from the soil surface, by an abrupt textural change associated with stagnic properies above that boundary;
- Histosols: soils having:
- a folic horizon which one of its characteristics is to have water saturation for less than one month in most years;
- or a histic horizon which one of its characteristics is to have water saturation for at least one month in most years;
- Gelistagnic soils present a temporary water saturation at the surface caused by a frozen
- Gleyic soils are soils having gleyic properties within 100 cm from the soil surface. Two specifiers
can be used:
- endogleyic: soils having gleyic properties between 50 and 100 cm from the soil surface;
- epigleyic: soils having gleyic properties within 50 cm from the soil surface.
- Oxyaquic Cryosols: Cryosols saturated with water during the thawing period and lacking redoximorphic features within 100 cm from the soil surface;
- Planic soils are soils having an eluvial horizon abruptly overlying a slowly permeable horizon within 100 cm from the soil surface;
- Rheic Histosols: Histosols having a water regime conditioned by surface water;
- Stagnic soils are soils having stagnic properties within 50 cm from the soil surface;
- Endostagnic soils are soils having stagnic properties between 50 and 100 cm from the soil surface.
Gleyic properties refer to soil materials which are, at least temporarily, completely saturated with groundwater for a period that allows reducing conditions to occur and show a gleyic colour pattern. Stagnic properties refer to soil materials which are completely saturated with surface water for a period long enough to allow reducing conditions to occur and show a stagnic colour pattern.
Attributes AGLIM1 and AGLIM2
Several phases are related with the water regime of the soils: drained, fragipan, flooded, phreatic. The drained phase has no definition, and becomes ‘excessively drained’ in the Instructions guide for version 4.0. When looking at the STUs having a dominant phase drained, all the values for WR are represented; the values 1, 2 and 3 are representing each of them around 30% of these STUs, the value 4 representing 8%. There is a low correlation between the phase drained and the water regime showing its bad quality. It is proposed to ignore this value in the rule.
The fragipan phase is defined in the FAO-UNESCO 1974 legend. It marks soils which have the upper level of the fragipan occurring within 100 cm from the soil surface. A fragipan is a loamy (uncommonly a sandy) subsurface horizon which has a high bulk density relative to the horizons above it. It is slowly to very slowly permeable.
The flooded phase has no definition.
The phreatic phase is defined in the FAO-UNESCO 1974 legend. It marks soils which have a groundwater table between 3 and 5 m from the soil surface. At this depth the presence of a groundwater is not normally reflected in the morphology of the solum; however, its presence is important for the water regime of the soil.
The petrocalcic phase marks soils in which the upper part of a petrocalcic horizon occurs within 100 cm from the soil surface (FAO-UNESCO, 1974). The petrocalcic horizon is a continuous cemented or indurated calcic horizon. The hydraulic conductivity is moderately slow to very slow.
When the current CGMS rule for drainage was evaluated, it appears that phases have often a high variability for the WR attribute. In this case it is proposed not to use them for estimating the drainage class.