Soil-Scientific Characteristic Values 1990
01.06.1 Types of Soil
The soil type of a particular soil is determined by the grain size composition of its mineral components. Coarse soil (grain diameter > 2 mm) und der fine soil (grain diameter < 2 mm) types are distinguished. In addition, in very wet locations, peat is formed by the accumulation of incompletely decomposed plant material, which overlays the mineral soils.
Fine Soil Types
Fine soil types are formed from certain proportions of the grain fractions clay, silt and sand. The main soil types are subdivided into clay, silt, loam and sand, with loam representing a grain mixture of sand, silt and clay. Soil type is an important identification value for the derivation of such ecological qualities as nutrient and pollutant retention capacity, hydrologic budget and retention capacity, and filtration and buffering capacity for pollutants.
Coarse Soil Types
All mineral components of the soil >2 mm in diameter are described as coarse soil types, or the soil skeleton. The proportion of coarse soil has an effect on water permeability, air and nutrient balance, and the capacity to bind nutrients and pollutants. The higher the share of coarse soil, the more permeable a soil is, due to the large pores, while the capacity to bind and the nutrient level depend on the type of fine soil.
Types of Peat
Peat is formed in a water-saturated environment from the accumulation of incompletely decomposed plant material. It is characterized by a high water-retention capacity and a very high cation exchange capacity. Various types of peat can be distinguished, according to the type of plant remains and the formation conditions. Bog peat is rich in alkalines and nutrients, and in many cases, even in carbonates. Transition-mire peats include plant remains from both low and high-nutrient locations.
The fine, coarse and peat soil types, each differentiated between topsoil and subsoil, were determined for each soil association. The data were essentially taken from the profile sections by Grenzius (1987). Some values have been supplemented by expert evaluations.
The mapped fine soil types are summarized in Table 1. Since the types of soil are in many cases different in the topsoil and the subsoil, respectively, due to the material of which the soil was originally formed, to the soil development and to its use, they have been examined separately. In addition, soil types occurring frequently within a soil association are identified as the main soil type, and distinguished from the more rarely occurring soil types, known as subsidiary soil types.
Those soil associations which have largely the same fine soil types for the topsoil and for the subsoil were combined to a soil type group. The assignment of soil type groups has thus been done merely for the sake of a readable map with an easily comprehensible number of legend units. For details or further calculations, more precisely differentiated data are available. Soil associations occur which consist of the same soil types, both in the topsoil and in the subsoil. However, the majority of soil associations differ in terms of soil types between the topsoil and the subsoil.
The combination of the types of soil of the topsoil with those of the subsoil resulted in 14 soil type groups of fine soil (< 2 mm), which are represented by the legend units of the map.
However, the soil associations of a soil type group may differ within this group with regard to peat or stone content (soil skeleton, coarse soil >2 mm) of the topsoil and subsoil, so that these have been represented by additional designations.
The coarse soil types in the Berlin soils are compiled in Table 2 zusammengestellt. Their occurrence in the topsoil and the subsoil, respectively, is distinguished.
The types of peat occurring in Berlin are compiled in Table 3 zusammengestellt. For the representation of their ecological qualities and the ascertainment of their characteristic values, a distinction is made between peat occurring in the topsoil and the subsoil, respectively. If several peat types occur in a soil or a soil association, only the characteristic type of peat is taken into account (characteristic peat type).
01.06.4 Utilizable Capabillary Capazity pf the Effective Root Zone
An assessment of the hydrologic budget via the utilizable capillary capacity in the effective root zone (nFKWe) yields a differentiated analysis of the water available to plants at any location. The different rooting depths and root zones are taken into account, in accordance with soil type and use. Thus, forests and groves have a considerably greater root zone than, e.g. garden uses. In sandy soils, the effective root zone is lower than in loamy soils. In loamy soils, precipitation water is retained longer than in sandy soils, so that it is advantageous for plant roots, in terms of the water and nutrient balance, to develop a larger root zone than in sandy substrata. In boggy soils, the effective root zone only extends down to the zones affected by groundwater, so that only the top 20-30 cm usually serve as a root zone. The reason for the shallow root zone is the lack of air in the permanently water-saturated zones. Therefore, with the exception of some specialist plants, roots are confined to the upper zones, which conduct both sufficient air and water.
The additional water supply to the plants from the capillary rise of the groundwater during the vegetation period, which decisively influences the nFKWe at low land-parcel intervals, was not taken into account in the present investigation.
The ascertainment of the nFKWe for soil associations in dependence on actual land use was carried out by the soil science branch of the TU Berlin in the context of an expert report (Plath-Dreetz/ Wessolek/ Renger 1989).
First, the effective root zones for Berlin locations appropriate to the respective uses were taken from Table 1 Based on the depth of the effective root zones, the usable capillary capacities ascertained for each zone for the sample profiles documented by Grenzius (1987) were added up to form the nFKWe. Appropriate correction factors for organic substances were taken into account. Since different soil types appear within a soil association, a range is derived which can be described by the minimum and maximum value of the nFKWe per soil association. In addition, the typical nFKWe value for the respective soil association, which is represented in the map, is determined depending on use.
The results were compiled in five stages (Tab. 2):
01.06.5 Humus Quantities
The organic fraction of soils consists of the transformed remains of dead plants and animals. The humus is formed by mulch and humin materials. The high sorption capacity of the humin materials, the high share of nutrients available to plants, and favorable qualities for the hydrologic budget characterize many soil functions. The humus content of mineral soils is determined by soil genesis and use. Such uses as horticulture with introduction of compost, or intensive pasturing favor humus enrichment, while other uses show a considerably lower organic-substance content (cf. Tab. 1).
Wet vegetation locations, e.g. flood-plain soils and mires, have high biomass production but low humus reduction. The enriched organic substance is present in the form of peats of varying degrees of decomposition. Fens and bogs have organic substance contents of 15 – 80%, depending on their use and the degree of decomposition of the peat. The prerequisite for a high of organic substance content is permanent wetness in the topsoil and near-natural utilization, such as an extensive pasturing.
The humus quantity represents the quantity of organic substance present at a location for a defined soil lot, depending on soil type and land use. The amount of humus is primarily an indicator of the nitrogen stock and the easily mobilizable nitrogen proportion. But other important nutrients such as potassium, calcium, magnesium and phosphorus are also released and made available to plants by means of the decomposition and humification of organic substances. In addition to the availability of nutrients, the amount of humus functions as a nutrient and water reservoir, and is able to bind pollutants to a high degree. The humus quantity of a soil depends on the humus content and the thickness of the humus zone. This differs according to soil type and use. Thus, for example, damp boggy locations with high biomass production and low decomposition have a high humus quantity, and sandy dry soils with low vegetation coverage have a low humus quantity.
The average humus content of mineral soils depending on soil type and use was taken from investigations by Grenzius (1987) and soil analyses performed under the heavy-metal investigation program (1986, 1987). These data were initially evaluated by Fahrenhorst et al. (1990) and the average humus content ascertained for the characteristic soil type of the various soil associations at different uses An expansion of the database using various specific mappings was carried out in 1993 (Aey 1993). A rough orientation, purely by use, is compiled in Table 1.
The humus contents of peats formed at wet locations are not taken into account for mineral soils; their contents and thicknesses are listed separately in the investigation of humus quantity.
Humus quantity was ascertained from humus content of the humus layer, taking into account peat quantity [mass %] and the effective retention density and thickness of the organic zones.
Humus quantity ascertained for the various locations was broken down into five stages, according to Table 2.
01.06.6 Mean pH Values as an Average of Topsoil an Subsoil
The pH value (soil reaction) influences the chemical, physical and biological qualities of the soil. It affects the availability of nutrients and pollutants, and provides information about the ability of the soil to neutralize acids or bases. It is important for the filtration and buffering capacities of soils. Thus, at low pH values, no acids can be neutralized in the soil, the heavy-metal connections increasingly dissolve and the available nutrients are largely washed out.
The pH values were derived from existing documents for the soil associations, taking land use into account. The data were essentially taken from the profile sections in Grenzius (1987). Some values have been supplemented by expert assessments, in most cases using a great variety of different soil-scientific reports. If there were no measurements, the values were assessed using data of comparable uses or comparable soil associations. In addition to the representative values (typical pH values) for the topsoil and subsoil, the respective maximum and minimum values were also determined.
However, since only one characteristic pH value per lot can be processed for generalized soil function assessments, but pH values are stated separately for the topsoil and the subsoil, it was necessary to combine these two values. The arithmetic average was thus calculated from the typical pH values for the topsoil and subsoil. This simplification permits general statements regarding the soil reaction of the locations, e.g. whether a location is more alkaline or more acidic. However, for soils with very different pH values in the topsoil and subsoil, this averaging often yield values which do not always accurately represent the ecological qualities at the location.
The gradation of the mean pH values, as averages of the topsoil and subsoil values, has been carried out according to the Soil-Scientific Mapping Directive (Bodenkundliche Kartieranleitung) (1994) in the stages 1 – 13, from extremely alkaline to extremely acidic (cf. Tab. 1). This gradation permits the soil reaction to be differentiated according to its alkalinity or acidity.
01.06.8 Alkaline Saturation of the Topsoil
The alkaline saturation (Bs) corresponds to the share of alkaline-bound cations in the cation-exchange capacity (KAKpot). These are primarily the basic cations calcium (Ca), magnesium (Mg), potassium (K) and sodium (Na). At a neutral pH value, the soil solution is completely saturated with alkaline cations. At low pH values, acidifying hydrogen (H) and aluminum (Al) ions dominate the soil solution. The alkaline cations serve as important nutrients and as buffers and neutralizers for acidic immissions into the soil; the H-ions have an acidifying effect, while free aluminum ions are toxic for plants. The alkaline saturation of the topsoil is important for plant growth, since this area is the main root zone, except at certain forest and grove locations.
The fewer alkaline cations are available, the lower the pH value is. Alkaline saturation can therefore be derived depending on pH value (CaCl2).
Ascertainment is accomplished using the pH value typical of the location of the topsoil, and the alkaline saturation according to Table 1. Then a linear interpolation between these pH stages of this table is carried out.
The gradation of alkaline saturation is carried out according to the Soil-Scientific Mapping Directive (1994), using the stages 1 -5 (very alkaline-poor – very alkaline-rich), as in Table 2.
01.06.9 Mean Effective Cation Exchange Capacity
The effective cation exchange capacity (KAKeff) represents the quantity of cations bound to soil colloids, taking into consideration the strongly pH-value-dependent charge of the organic substances. The interchangeable cations are bound to clay minerals and humus colloids. In neutral to weakly acidic soils, calcium (Ca), magnesium (mg), potassium (K) and sodium (Na) dominate the sorption complex; in acidic soils, e.g. pine and heath locations, aluminum (Al), hydrogen (H) and iron (Fe) predominate. The binding capacity of organic substances is considerably higher than that of the clay minerals. The strength of the bond with the organic substance is pH-dependent, while the bond with clay minerals is independent of the pH value. Thus, the binding capacity of the humus drops with the pH value. Clay and humus-rich soils with neutral soil reaction can therefore bind considerably more nutrients and pollutants, and prevent a washout of these substances into the groundwater, than can sandy, humus-poor locations. Effective cation exchange capacity is therefore useful for describing the nutrient and pollutant binding potentials of soils.
In calculating the KAKeff, a simple method was used which was on the one hand to show the characteristic value of the respective soil association, while on the other also taking into account the strong deviations from the typical value within the respective soil association.
The KAKeff of the soil associations is derived from the main soil type of the topsoils and subsoils, as in Tab. 1. The topsoil is assumed to have a depth of 0 – 1 dm; the subsoil 3 – 15 dm. In order to ascertain unusual features of a soil association, the subsidiary soil type with the greatest difference in clay content to the main soil type is also taken into account. This permits deviations from the main soil type to be considered; however, this method lends very great weight to the subsidiary soil type, so that the exchange capacities of the soil associations are in some cases estimated too high or too low. The exchange capacity of the humus, corrected by a pH-dependent factor, is added to the averaged cation exchange capacity of the main and subsidiary soil types. Since both the humus contents and the thickness of the humus layer may differ, depending on soil genesis and use, and since these are also incorporated into the calculation of the KAK, different use-specific values are ascertained for each soil association.
The values ascertained have been assigned to the stages 1 -5, from very slight – very great, according to the Soil-Scientific Mapping Directive (1994).
01.06.10 Water Permeability (kf)
Water permeability (saturated water conductivity, kf value) indicates the permeability of soils. It depends on the soil type and storage density of the soil. Loose soils with high sand contents therefore have a considerably higher permeability than do clay-rich soils consisting of till. Water permeability is important for the evaluation of storage wetness, filtration qualities, erosion vulnerability and drainage effectiveness of soils. The speed of water permeability is given in m/s or in cm/d. The data on speed of water movement apply only to completely water-saturated soil, in which all pore spaces are filled with water. As a rule, terrestrial soils display non-saturated water conditions, with only a portion of the pores filled with water; at such non-saturated conditions, water movement is considerably slower. In addition, a large portion of the available water is taken up by the plants and is not available for flow. Since measurement of non-saturated water conductivity (ku) is very expensive and complicated, so that no accessible data are available in the Soil-Scientific Mapping Directive (1994), the attested values for saturated water conductivity are used in scientific practice as a rough measure.
The influence of the coarse soil was not taken into account.
The kf value for the main soil types of the topsoil and the subsoil according to Table 1 was taken. The kf value for the topsoil and subsoil is the average of the topsoil kf and the subsoil kf. The kf values listed in the Table depending on soil type are based on an effective storage density of Ld3, which corresponds to the average for Berlin soils.
For representation on the map, the results of water permeability have been categorized in six stages, from very low to extremely high (1 -6), as shown in Table 2.