Soil-Scientific Characteristic Values 2020

Methodology

01.06.1 Soil Textures

Description

The type of soil, or soil texture is determined by the grain size composition of its mineral components. Coarse soil (grain diameter > 2 mm) and fine soil (grain diameter < 2 mm) types are distinguished. In addition, in very wet locations, peat is formed by the accumulation of partially decomposed plant material, which may overlay mineral soils.

Fine Soil Textures
Fine soil textures are formed from specific proportions of clay, silt, and sand fractions. The main soil types are subdivided into clay, silt, loam, and sand, with loam comprising a mixture of roughly equal parts sand, silt, and clay. Soil texture is an important characteristic value for the derivation of such ecological properties as nutrient and pollutant storage capacity, water balance and storage capacity, as well as filtering and buffering capacity regarding pollutants.

Coarse Soil Textures
Mineral components of the soil with a diameter < 2 mm are referred to as coarse soil textures, or the ‘soil skeleton’. The proportion of coarse soil has an effect on water permeability, the air and nutrient balance, and the capacity to bind nutrients and pollutants. The greater the proportion of coarse soil, the more permeable the soil becomes due to larger pores, while the nutrient-binding capacity and nutrient levels depend on the type of fine soil.

Peat Textures
Peat is formed in a water-saturated environment from the accumulation of partially decomposed plant material. It exhibits a high water storage capacity and a very high cation exchange capacity (KAK). Various peat textures exist, distinguished by the type of plant remains and the conditions under which they formed. Low-moor bog peat is rich in alkaline substances and nutrients, and sometimes even carbonates. Transitional bog peats contain plant remains from both low and high-nutrient locations.

Methodology

The soil textures of fine soil, coarse soil and peat, each differentiated by topsoil (depth: 0 to 10 cm) and subsoil (depth: 90 to 100 cm), were determined for each soil association. The data was primarily taken from profile sections by Grenzius (1987), with some values supplemented by expert evaluations.

The mapped fine soil textures are summarised in Table 1. Given that the soil textures in the topsoil and subsoil may differ due to the parent material of soil formation, soil development, and land use, they are analysed separately. In addition, soil textures that are frequently encountered within a soil association are identified as the main soil texture, while those occurring less frequently are distinguished as subsidiary soil textures.

Tab. 1: Soil textures and their occurrence in Berlin

Combining the soil textures of the topsoil with those of the subsoil resulted in 14 soil texture groups of fine soil (< 2 mm), which make up the units in the map legend. Soil texture groups were assigned solely to produce a map that is easily understood with a manageable number of legend units. For more detailed information or further calculations, more differentiated data is available. While some soil associations display the same soil textures in the topsoil and the subsoil, the majority differ between the two horizons.
Within a soil texture group, soil associations may vary in terms of peat or stone content (soil skeleton, coarse soil > 2 mm) in their topsoil and subsoil, however. These variations have hence been labelled separately.

Table 2 displays coarse soil textures characteristic of Berlin soils, distinguishing between their occurrence in the topsoil and the subsoil.

Tab. 2: Designations of coarse soil textures occurring in Berlin soils

Table 3 provides an overview of the peat textures found in Berlin. To represent their ecological properties and establish their characteristic values, a distinction is made between peat found in the topsoil and that found in the subsoil.

Tab. 3: Designations of peat textures occurring in Berlin soils

01.06.2 Available Water Capacity of Soils for Shallow-Rooted Plants (nFK30)

Description

The Available Water Capacity (nFK) is the quantity of water, measured in l/m² or mm, that soil can retain, and which is available for plants. This water fraction is held in the soil pores against the force of gravity and is accessible to plants. The nFK depends on soil texture, humus content, bulk density, and stone content. Fine soil can store significantly more water over longer periods of time compared to coarse soil. In coarse soil, precipitation seeps away more quickly and becomes unavailable to plants. High levels of humus and peat increase water storage.

Methodology

The nFK values of soil associations were determined following the procedure outlined in the Bodenkundliche Kartieranleitung KA6 (2024) (KA6 2024 Soil-Scientific Mapping Guidelines). This involved considering factors such as fine and coarse soil textures (cf. Tab. 1 and 2), the proportion of coarse soil (Tab. 2), and the humus content (Tab. 3). A distinction was made between a shallow-root zone (0 to 30 cm) and a deep-root zone (0 to 150 cm). Additionally, the minimum nFK for both root zones were calculated based on the soil texture of the association with the lowest nFK. The map illustrates the average nFK of the shallow-root zone, calculated using the equations provided below:

eq. 1 nFK_{Flachwurzelzone} = nFK_Oberboden * 0.1 + nFK_Unterboden * 0.2
eq. 2 nFK_Oberboden = nFK_Hb * 0.7 + nFK_Nb * 0.3) * (1 – Sg_Oberboden/100) + H_real * 0.1
eq. 3 nFK_Unterboden = nFK_Hb * 0.7 + nFK_Nb * 0.3) *( 1 – Sg_Unterboden/100) + H_real * H_dm – 0.1)

• where nFK_Oberboden = nFK of the topsoil per decimetre depending on soil texture, peat content, and coarse soil content according to KA6 in mm/dm
• where nFK_Unterboden = nFK of the subsoil per decimetre depending on soil texture, peat content, and coarse soil content according to KA6 in mm/dm
• where nFK_Hb = nFK of the main soil texture per decimetre depending on soil texture according to KA6 in mm/dm
• where nFK_Nb = nFK of the subsidiary soil texture per decimetre depending on soil texture according to KA6 in mm/dm
• where Sg_Oberboden = maximum coarse soil content in vol% in the topsoil depending on the coarse soil texture according to KA6
• where Sg_Unterboden = maximum coarse soil content in vol% in the subsoil depending on the coarse soil texture according to KA6
• where H_real = nFK supplement depending on the soil’s humus content according to KA6 in vol%
• where H_dm = thickness of the humus layer in decimetres

Tab. 1: Available water capacity depending on soil texture at LD3 bulk density, according to the KA6 2024 Soil-Scientific Mapping Guidelines

Tab. 2: Proportion of coarse soil (in vol%) for selected coarse soil textures, according to the KA6 2024 Soil-Scientific Mapping Guidelines

Tab. 3: Supplements to available water capacity (nFK) based on humus content, according to the KA6 2024 Soil-Scientific Mapping Guidelines, averaged

Tab. 4: Available water capacity for the shallow and deep-root zones (in mm) and their evaluation

01.06.4 Available Water Capacity of Soils in the Effective Root Zone (nFKWe)

Description

Assessing the water balance through the available water capacity in the effective root zone (nFK_We) offers a nuanced understanding of water available to plants at a given location. This evaluation takes into consideration varying rooting depths and root zones based on soil texture and land use. For instance, forests and groves have a considerably larger root zone compared to areas used as farmland. In sandy soils, the effective root zone is shallower than in loamy soils. The latter therefore retain water longer than sandy soils. In loamy soils, plant roots benefit from a slightly larger root zone in terms of the water and nutrient balance, compared to sandy substrates. In boggy soils, the effective root zone only extends as far as the zones affected by groundwater; typically limited to the top 20 to 30 cm. The reason for the shallow root zone is the lack of air in the permanently water-saturated horizons. Therefore, with the exception of some specialist plants, roots are confined to the upper horizons, which offer both sufficient air and water.

Please note, this assessment does not factor in the additional water supply to plants from capillary rise of groundwater, which significantly impacts the nFK_We during periods of close proximity to the water table in the growing season.

Methodology

The calculation of the nFK_We is based on nFK values determined per decimetre for both the topsoil and subsoil, considering soil texture, humus content, and coarse soil fraction. This process follows the KA6 2024 Soil-Scientific Mapping Guidelines. Converting nFk to nFK_We involves summing the nFK values from both layers, while taking into account the thickness of the effective root zone. For Berlin locations, Table 1 (Plath/Dreetz, 1988) provides information on the effective root zone, in dependence of land use. The equation for calculating nFK_We is as follows:

eq. 1 nFKWe [mm] = nFK_Oberboden [mm/dm] * 0.1 [dm] + nFK_Unterboden [mm/dm] * (We [dm] – 0.1 [dm])

• where nFK_Oberboden = nFK of the topsoil per decimetre depending on soil texture, peat content, and coarse soil fraction according to KA6
• where nFK_Unterboden = nFK of the subsoil per decimetre depending on soil texture, peat content, and coarse soil fraction according to KA6
• where We = thickness of the effective root zone according to Plath/Dreetz (1988) depending on land use in decimetres

Tab. 1: Depths of the effective root zone (in cm), by soil texture and land use, modified

As is the case for the available water capacity, the results are grouped into six levels for shallow-rooted and deep-rooted plants (cf. Table 2).

Tab. 2: Available water capacity levels in the effective root zone

01.06.5 Humus Content of Soils

Description

Humus comprises the entirety of organic matter from dead plants and animals in the soil. It consists of mulch and humic materials. The high sorption capacity of humic materials, the high share of nutrients available to plants, and favourable qualities for the water balance have a significant influence on many soil functions. The humus content of mineral soils is determined by soil formation and land use. Activities such as horticulture, involving compost application, or intensive grazing promote humus enrichment. In contrast, other land uses often result in lower levels of organic material (cf. Tab. 1).

Wet locations, such as floodplain soils and bogs, have a high biomass production but a slow decay rate of humus. The enriched organic substance is present in the form of peat with varying degrees of decomposition. Half-bogs and low-moor bogs contain 15 % to 80 % of organic material, influenced by factors such as land use and the degree of peat decomposition. The prerequisite for a high organic matter content is sustained moisture in the topsoil which impedes or inhibits mineralisation, and near-natural land use, such as extensive pasturing.

The humus content represents the amount of dead organic matter present within a defined area of soil, depending on soil type and land use. The amount of humus is primarily an indicator of the nitrogen stock and the share of easily mobilisable nitrogen. Furthermore, decomposition and humification processes release other important nutrients such as potassium, calcium, magnesium and phosphorus, increasing their availability to plants. In addition to the supply and storage of nutrients, humus also facilitates a higher water and pollutant storage capacity. The humus content depends on humus content, horizon thickness, soil texture and land use. For example, wet, boggy areas with high biomass production and slow decomposition typically exhibit high humus levels. Conversely, sandy dry soils with sparse vegetation often have lower levels of humus.

Methodology

The expected average humus content of mineral soils, depending on their soil texture and land use, was derived from investigations by Grenzius (1987) and soil analyses performed under the Heavy-Metal Investigation Programme (1986, 1987). This data was initially analysed by Fahrenhorst et al. (1990) who also ascertained the average humus content for the characteristic soil texture of each soil association under different land uses. In 1993, the database was expanded through the inclusion of various individual mappings (Aey 1993). Subsequently, Kaufmann-Boll (et al. 2023) revised the input data, drawing on investigations conducted within the NatKoS and UEP projects. This involved a relative increase in the existing values for land uses and scenarios that were prominent in the NatKoS project. Table 1 provides a rough overview, solely dependent on land use.

Tab. 1: Average humus content by land use

The humus levels of peat that forms in wet locations are not considered if the soil in question is a mineral soil. Instead, humus levels are accounted for separately, based on their level and depth, when calculating the overall humus content. This quantity is derived from the humus content of the humus layer, considering the proportion of peat (in mass%), effective storage density, and thickness of the organic horizons. The resulting humus content for each location are then categorised into six levels, as outlined in Table 2.

Tab. 2: Humus content levels, according to typical amounts of Berlin soils

01.06.6 Organic Carbon Stock of Soils

Description

Dead organic matter (humus) in the soil consists of approximately 50 % organic carbon and is of fundamental importance to the hydrological and nutritional balance of soils. As a result of the concentration, storage and mineralisation of organic matter, and therefore organic carbon, soils play a major role in the global carbon cycle.

Soils constitute the largest form of terrestrial carbon storage and, besides oceans, the largest form of carbon storage of the world (IPCC 2000). A major influence on the carbon dynamic in soils is land use. Soils in urban environments are subjected to very high land use pressure and suffer significant anthropogenic impact. On the one hand, this results in higher organic carbon stock than in natural systems, for instance due to horticultural activities. On the other hand, the destruction of natural soil functions leads to higher levels of mineralisation of humus and therefore an increased release of carbon dioxide (CO₂) into the atmosphere. This is of particular climatic importance, especially long-term, as the accumulation of humus and therefore the climate-effective binding of carbon in soils takes place over long periods of time.

Soils as carbon sinks play a special role in the global carbon cycle. Carbon sinks are also present in urban environments. Hydromorphic soils such as bogs are notable in this context. Bog soils can potentially store up to ten times more carbon than other ecosystems (Batjes 1996). Due to changes in the water balance following amelioration measures, most bog soils emit CO₂ and CH₄ (methane). For this reason, the protection and conservation of bog soils are of particular importance to local, regional and global climate protection. With a total area of just about 7 , accounting for about 65  of the organic carbon storage in the low-moor bog soils and transitional bog soils of Berlin, it is evident that bog soils have an important role to play as carbon sinks. Allotment gardens and other sites with long-term pedogenesis including cemeteries, old forest sites and park facilities are also valuable carbon sinks, functioning as long-term carbon reserves.

As a result of their function as carbon sinks, soils play a very important role in climate protection, which should be taken into account during planning and authorisation processes (Dahlmann et al. 2012). It thus makes sense to protect soils containing a high amount of carbon from land use with negative impacts, such as construction of buildings on previously pervious soils. It is also advantageous to re-cultivate existing structures, especially bog soils. The soil buffer within the organic carbon balance is therefore also considered to assess the filter and buffer function of soils (cf. Map 01.12.3).

The calculation based on this map shows that a total of 7.03 million tonnes of carbon is stored in Berlin’s soils. This equals 25.8 million tonnes of CO₂.

Berlin’s total CO₂ emissions amounted to approximately 14.6 million tonnes in 2020 (Statistical Office for Berlin Brandeburg 2022). Therefore, the amount of carbon stored in the soil exceeds the total carbon emissions from primary energy consumption in Berlin for the entire year of 2020.

Methodology

The organic carbon stock in Berlin’s soils was calculated based on the humus content (in kg/m²) provided by the Berlin Soil Database (cf. Map 01.06.05 Humus Content). Based on the results of the research project Berlin’s Peatlands and Climate Change (Klingenfuß et al. 2015), the calculation of organic carbon reserves from humus content levels in 2015 was initially conducted following the KA5 2005 Soil-Scientific Mapping Guidelines. In this updated version, the calculation has been aligned with the KA6 2024 Soil-Scientific Mapping Guidelines, standardising it to the conversion factor of 2. This factor applies to all soil associations, whether they contain peat or not.

To calculate the organic carbon stock for all of Berlin, the carbon contents were multiplied by the area of the Berlin block structure.

It is important to note, however, that the determined organic carbon stock of Berlin’s soils are estimates and may sometimes be rather inaccurate, due to methodological limitations. This is the case as the humus contents presented in the Berlin block structure are based on the Soil Associations Map which only functions as a concept map in some cases (cf. Map 01.01). In addition, the humus content, the thickness of the mineral top soil horizons and the peat layers containing humus, as well as the bulk density have been estimated in some cases. By integrating the results of the research project Berlin’s Peatlands and Climate Change (Klingenfuß et al. 2015) in 2014, along with the outcomes of the NatKoS and UEP projects conducted within the NatKEV project framework in 2022/23 (Kaufmann-Boll et al. 2023), substantial improvements have been made to the data regarding the location, dimensions, peat thickness, bulk density, and humus to carbon ratio in bog soils. Nevertheless, Map 01.06.6 Organic Carbon Stock can only approximate the actual situation. The determined organic carbon stock is divided into six levels, according to Table 1.

Tab. 1: Organic carbon stock levels

01.06.7 pH Values of Topsoil

Description

The pH value (the negative logarithm to the base 10 of the hydrogen ion concentration) influences the chemical, physical and biological properties of the soil (soil reaction). It affects the availability of nutrients and pollutants and provides information about the ability of the soil to neutralise acids or bases. It is important for the filtering and buffering capacities of soils. At low pH values, the soil cannot neutralise acids, heavy-metal compounds dissolve more easily and available nutrients are largely washed out.

Methodology

The pH values were derived from existing documents for the soil associations, taking land use into account. The data was largely extracted from the profile sections in Grenzius (1987). Some values were supplemented by experts, often based on a wide range of soil-scientific reports. If no measurements were available, the values were estimated using data of comparable land uses or comparable soil associations. Furthermore, alongside the typical pH values for both the topsoil and the subsoil, the corresponding maximum and minimum values were also determined.

On the map, only the pH values of the topsoil are shown, since they are more important for determining soil functions (cf. Map 01.12) than the pH values of the subsoil; pH values of the topsoil also display greater differences, largely influenced by land use.

The pH levels were determined according to the KA6 2024 Soil-Scientific Mapping Guidelines, ranging from 1 to 13. This facilitates a differentiation of the soil reaction according to its alkalinity or acidity, ranging from extremely alkaline to extremely acidic (cf. Tab. 1).

Tab. 1: pH levels

01.06.8 Sum of Exchangeable Base Cations in the Topsoil (S Value)

Description

Exchangeable cations present in the soil are usually categorised into acidic and base cations. Acidic cations primarily consist of hydrogen ions (H⁺ ions). Those cations triggering hydrolysis upon release into the soil solution, such as aluminium (Al³⁺), are also considered acidic. Their combined total is referred to as the H value. Base cations primarily include calcium ions (Ca²⁺), potassium ions (K⁺), magnesium ions (Mg²⁺) and sodium ions (Na₄⁺). In agricultural soil (after fertilisation), ammonium ions (NH⁺) are also present, although calcium ions (Ca²⁺) typically predominate, accounting for more than 80 %. The S-value represents the sum of cations that have base properties. Its concentration is measured in cmol_c/kg, and its quantity is expressed in mol_c/m². The percentage of the S value relative to the total exchangeable cations is known as the base saturation.

The S value thus describes the quantity of the cations provided by the soil, which is crucial for plant nutrition, and serves as an important indicator of soil fertility.

Methodology

The amount of exchangeable base ions (S value) in the topsoil (here, 0 to 30 cm) can be calculated by multiplying the effective cation exchange capacity (KAK_eff) by the base saturation (BS), also taking into account bulk density and the proportion of coarse soil.

The procedure for calculating the effective cation exchange capacity is illustrated in Map 01.06.09. The base saturation may be derived from the pH value (measured in calcium chloride, CaCl₂).

It is determined based on the typical pH value of the topsoil at the site in question (cf. Map 01.06.07) according to Table 1. Linear interpolation is used between the pH levels listed in this table.

Tab. 1: Relationship between base saturation (BS, in %) and pH value (CaCl2) of mineral soil horizons in Berlin

S values are categorised into levels 1 to 10 (extremely low to very high), as shown in Table 2.

Tab. 2: S value levels

The classification of low values is carried out in very narrow increments to accurately distinguish the subtle variations in nutrient-poor soils necessary for assessing their role as ‘habitats for rare and near-natural plant communities’ (cf. Map 01.12.1).

01.06.9 Mean Effective Cation Exchange Capacity of Soils (KAK_eff)

Description

The effective cation exchange capacity (KAK_eff) represents the quantity of cations bound to soil colloids, taking into consideration the charge of the organic substances, strongly dependent on the pH value. The exchangeable cations are bound to clay minerals and humus colloids. In neutral to slightly acidic soils, calcium (Ca²⁺), magnesium (Mg²⁺), potassium (K⁺) and sodium (Na⁺) dominate the sorption complex; in acidic soils, e.g. pine and heath locations, aluminium (Al³⁺), hydrogen (H+) and iron (Fe²⁺/³⁺) predominate. The binding capacity of organic substances is considerably higher than that of clay minerals. The strength of the bond formed with organic substances is pH-dependent, while the strength of the bond formed with clayey minerals is not pH-dependent. The binding capacity of humus thus drops with the pH value. Clayey and humus-rich soils with a neutral pH can therefore bind considerably more nutrients and pollutants, and prevent a washout of these substances into the groundwater, compared to sandy, humus-poor locations. The effective cation exchange capacity is therefore useful for describing the nutrient and pollutant binding potential of soils.

Methodology

The KAK_eff of the soil associations is derived from the main soil texture of the topsoil and subsoil (Tab. 1). The topsoil is assumed to have a depth of 0 to 30 cm, and the subsoil a depth of 30 to 150 cm. The exchange capacity of the humus (Tab. 3), adjusted by a pH-dependent factor (Tab. 2), is added to the KAK of the main soil texture. Both the humus contents and the thickness of the humus layer may vary, depending on soil formation and land use, which are also considered in calculating

Tab. 1: Average KAK values of the soil textures

Tab. 2: pH factors for determining the effective KAK of the humus fraction

Tab. 3: Relationship between humus content and potential KAK expanded to include humus content ranging from 30 % to 100 %

The results were categorised into six levels, ranging from very low to very high, to be displayed on the map in accordance with the KA6 2024 Soil-Scientific Mapping Guidelines (Tab. 4).

Tab. 4: Effective cation exchange capacity levels, modified

01.06.10 Saturated Water Permeability of Soils (kf)

Description

Saturated water permeability (also known as saturated hydraulic conductivity, kf) indicates the ability of completely water-saturated soils to transmit water. It is influenced by the soil texture and bulk density. Loose soils with high sand content therefore have a considerably higher permeability than clay-rich soils, such as boulder marl. Saturated water permeability is crucial for assessing soil moisture, filtering properties, susceptibility to erosion and drainage efficiency. Saturated water permeability is measured in m/s or in cm/d.

Typically, terrestrial soils exhibit unsaturated water conditions, with only a portion of the pores filled with water. Under unsaturated conditions, water movement is considerably slower. In addition, a large portion of available water is absorbed by plants and is no longer available for movement. Due to the complexity and labour-intensive nature of measuring the unsaturated hydraulic conductivity (ku), the KA5 2005 Soil-Scientific Mapping Guidelines do not contain any data in this regard. Therefore, verified values of saturated hydraulic conductivity are commonly used in scientific practice as a rough estimate.

Please note, the influence of coarse soil was not considered in this analysis.

Methodology

The kf values for the main soil textures of the topsoil (0 to 10 cm depth) and subsoil (90 to 100 cm depth) were extracted from Table 1. The combined kf for the topsoil and subsoil is the harmonic mean of the topsoil kf and the subsoil kf. These kf values, listed in the table according to soil texture, are based on an effective bulk density of LD3, which represents the average for soils in Berlin.

Tab. 1: Water permeability in saturated soil (kf) depending on soil type at a mean effective bulk density of LD3, supplemented by moderately decomposed peat (Z3) with medium substance volume (SV3).

For the representation on the map, the saturated water permeability results were categorised into six levels, ranging from very low to extremely high (1 to 6), as shown in Table 2.

Tab. 2: Water permeability levels in water-saturated soil, expanded to include Level 6