# Field Soil Description and Soil Classification

Overview of existing standards

# German Soil Survey Guidelines

The first edition (1965) was a nation-wide accepted guideline in administration and research. Still in use in some federal states are also the editions (KA4, 1994 and KA3, 1982). The administrative soil surveys of the federal states ( Bundesländer) often use adaptations and modifications of this guideline. A shortened and simplified version of the German Soil Survey Guidelines has been extracted from KA5 [1] for soil conservation purposes (Arbeitshilfe Boden; short: AHB); the Federal Soil Protection and Contaminated Sites Ordinance in Germany refers nonetheless to KA4.

The major part is about describing site, soil profile, layers and horizons, naming horizons and soil’s allocation in the German soil system (“Bodensystematik”) as well as a classification of the soil material (substrate classification; ”Substratsystematik”). A large number of parameters, often with partly extensive code lists, and a formalized way to record auxiliary information (form, degree of expression, share, and size) are provided. It is thus one of the most detailed guidelines for soil description in the world. However, a strict data model is not included.

A new edition is awaited (“KA6”, possibly in 2023) with improvements in the soil system and substrate classification, horizon notation, code lists and in the mapping part. The simplification of soil description (AHB) shall be included in the new edition.

# World Reference Base for Soil Resources

The WRB [2] was developed by a working group with the participation of IUSS, FAO, and ISRIC. It provides a standard taxonomic soil classification system which allows the accommodation of national systems. It is designed to serve as a common denominator for communication at the international level and as a correlation between existing classification systems. The WRB classification system contains two hierarchical levels: On the level of “Reference Soil Group” (RSG) 32 units are differentiated. On the second level the RSG is described more precisely by the use of “Qualifiers”. For every RSG a list with corresponding principal and supplementary Qualifiers is defined. Classification is done by using both levels via diagnostic horizons, materials and features.

# DIN 19682-1ff

# Soil Quality - Field Investigation

The DIN 19682 series describes in 9 parts determination and measurement techniques for soil properties which are applicable in the field. Those parts that address description of soil properties (color (1), particle size distribution and soil type (2), soil moisture (5), soil structure (10) and decomposition level of peat soils (12)) are adopted from the German Soil Survey Guidelines. The other parts contain analytical methods for field-measurements of the following parameters: water infiltration rate (7), water and air permeability (8 & 9), carbonates, sulfides, pH-value and iron(II)-ions (13). This German standard is often used in international soil studies and can be a helpful supplement for general soil descriptions in the field by ISO 25177:2019.

# Guidelines Soil Assessment

Soil assessment (“Bodenschätzung”) is the basis for the taxation of arable land in Germany. Soil mapping according to these guidelines is required by law and organized by the German fiscal authorities. Mapping is realized by the description of soil profiles with 1 m depth at intervals of 50 m, resulting in large scale soil information. The guidelines provide a soil taxation framework for field assessments (“Bewertungsrahmen”) which combines soil type, soil condition/development and the parent material of which the soil is composed. Soil investigation which is following this standard, results in a soil value (“Bodenwertzahl”). It expresses the relative net income that, under normal and proper management, is determined only by the profitability of the soil. It is the basis for taxation of every plot used as crop- or grassland.

# FAO Guidelines for Soil Description

The FAO guidelines provide a complete procedure for soil description and for collecting fiel data necessary for classification according to second edition of the World Reference Bas for Soil Resources (WRB, 2006 [4]). Notes for classification purposes are added to each chapte and explain the relevance of the described feature for classification according to the WRB.

# ISO 25177:2019

# Soil Quality – Field Soil Description

ISO 25177 traditionally was based on the FAO Guidelines, but was removed from it during the last editions. A current revision process aims at amalgamating the soil scientific soil description with the geotechnical soil description (ISO 14688). This international standard provides rules for describing soil in the field and its environmental context at a given site. Sites may be natural, near natural, urban or industrial. Soil observations can be made on various levels (project site, plot, layer, horizon, specific soil constituents). ISO 25177 can also be used to describe non-soil layers, e.g. artificial material and coarse material. The standard addresses various research fields, e.g. soil science, geotechnical examinations, and investigation on soil contamination. Depending on the specific objective, ISO 25177 can be used in combination with other standards that provide guidelines or requirements for specific aspects of soil observation and measurements. Several parameters are mentioned that have to be observed for soil investigation and partially corresponding code lists are provided (Appendix, Table 10).

# DIN 4220:2020-11

# Pedologic Site Assessment - Designation, Classification and Deduction of Soil Parameters

This German standard provides guidelines with several code lists, based on the German Soil Survey Guidelines, for pedologic site assessment in various fields, e.g. agriculture, forestry or water economy. It defines rules for procedures of soil survey in the field, classification and deduction of pedologic parameters.

# DIN 19706 and DIN 19708 Soil erosion risk

National standards provide guidelines and determination keys to quantify the risk of soil erosion by water (DIN 19708:2022) and wind (DIN 19706:2013). The DIN 19706 corresponds to the international “Universal Soil Loss Equation” USLE and its modifications.

# DIN 19707:2004 [withdrawn]

# Classification of the nutrient supply conditions of soil

This national standard is used to classify nutrient supply conditions of agricultural used soils. The classification is based on nitrogen, phosphorous, and potassium and can be used to determine fertilizing requirements and defines target values for different crops and soil texture types.

# Conflicts and solutions (transformation and derivation tools)

# National

The Soil Survey Guidelines in its 5th edition (KA5 [1]) is currently the valid basis for soil description in Germany. However, some administrative soil surveys of the federal states use modifications of these guidelines. Even prior editions of 1994 (KA4) and 1982 (KA3) are still applied. Harmonization of data acquired according to different editions of KA can be realized by the use of a transformation tool developed by the Federal Institute for Geosciences and Natural Resources BGR (see Chapter Data Standards for Soil- and Agricultural Research). DIN 4220 is based on KA5, but deviates in some detail and in particular regarding some site assessment parameters. DIN 4220 is often applied in the context of geotechnical investigation. KA5 has a more expanded scope of application and community of users than DIN 4220. Thus, DIN 4220 as a national standard for soil description in the field has less importance than the German Soil Survey Guidelines.

The substrate classification of soil assessment deviates clearly from that given in the German Soil Survey Guidelines. Mineral soil types are determined by the fractions of sand, loam and clay. Silt is not represented as a grain fraction, contrary to other established substrate classification systems, so that comparison with other soil data is difficult. However, soil assessment data are valuable for various soil scientific issues regarding their high spatial resolution ( scale 1:5000) and comprehensive availability for agricultural area (see Transformation tool for Soil Assessment data, Chapter 2.3).

The German version of ISO 25177 is not intended to compete with the German Soil Survey Guidelines or with its deducted standards (e.g. DIN 4220) nor to replace them. It rather should facilitate soil survey work abroad, especially in Europe. In principle, soil description is feasible according to ISO 25177 also for soils occurring in Germany. But for most national purposes the application of the German Soil Survey Guidelines is recommended or even mandatory, according to national law.

Table 1: Selected national standards in the field of soil description and classification

# International

There are significant differences between WRB and German soil classification. Main criteria for the latter are type and vertical order of genetic horizons in soil profiles. In contrast, the WRB system uses diagnostic horizons, features and materials that are described independently from each other. The WRB nomenclature of soil types is based on 32 Reference Soil Groups (RSG) with principal and supplementary qualifiers. A simple translation of the German soil name into the WRB name is not feasible for a wide range of soil types due to the deviant approaches. Moreover, WRB uses analytical parameters for classification, which are either not available for many soil profiles in Germany or are analyzed with different methods than those intended by WRB.