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Daniela Hohenstern
Institute of Forest Ecosystems

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Contact persons for



pH values for all land-use classes

Project

 (c)

The pH values of the soils of forests and agricultural land in Germany

Background and Objective

For the mapping of salt-affected soils and the estimation of salinization potential (Omuto et al., 2020a, 2020b), the Global Soil Partnership (FAO, 2020) of the Food and Agriculture Organization of the United Nations (FAO) required grid maps of soil properties as a basis for calculation in 2020. The maps of soil pH produced for this purpose by the Thünen Institute were further developed and published in Thünen a la Carte 2021.

Approach

Soil pH values collected during BZE II-Forest and BZE-LW were used to calculate pH values on a 1 km² grid using a machine learning method. The machine learning method was selected based on a recommendation from FAO (Quantile Regression Forest model, Meinshausen, 2006). Two soil pH maps were created: one for soil depth 0-30 cm and one for soil depth 30-100 cm. For both maps, this machine learning method was applied to 17 independent variables each, with predictor selection based on FAO guidance (Omuto et al., 2020a).

 

Results

The mean modeled pH in the topsoil is about 0.3 lower than in the subsoil. Only in about 25 percent of the cases is the pH value in the topsoil equal to or higher than at a depth of 30 to 100 cm.
The agriculturally used soils on the northern edge of the low mountain ranges are characterized by high pH values. This is particularly evident in the Central German loess region of the eastern and northern Harz foreland and the Börden areas as well as the Thuringian Basin. Furthermore, high pH values are common in the loess zones along the major rivers such as the Rhine and Danube, in the Kraichgau or in the Lower Bavarian Hills. The alluvial loess soils in the northern Upper Rhine lowlands form one of the largest areas with high pH values.
Agriculturally used sites with low pH are predominantly found in northwestern Lower Saxony and western Schleswig-Holstein, where they serve as pasture land. Further pasture on areas with neutral to slightly acidic pH values are mainly found in the low mountain ranges of western Germany, the Bavarian Forest and the foothills of the Alps.
Forest soils are often characterized by low pH values. This mainly affects soils under pine forests in northeastern Germany and parts of northern Germany from nutrient-poor unconsolidated sediments and the spruce forests of the low mountain regions from crystalline parent rock.
Forests on alkaline sites are common on the calcareous soils of the Franconian and Swabian Alb and the Alps. Furthermore, the topsoils of coastal areas in northern Germany are often more basic due to the influence of salt water. However, if pine forest adjoins the coastal dunes, the pH values are also low there.

Thünen-Contact


Involved Thünen-Partners


Funding Body

  • Federal Ministry of Food und Agriculture (BMEL)
    (national, öffentlich)

Duration

1.2021 - 10.2021