Bulletin of Geography. Physical Geography Series, No. 14 (2018): 63-70 http://dx.doi.org/10.2478/bgeo-2018-0005 Challenges of Soil Taxonomy and WRB in classifying soils: some examples from Iranian soils ISSN 2080-7686 Mohammad Hassan Salehi Shahrekord University, Iran * Correspondence: College of Agriculture, Shahrekord University, Shahrekord, Iran. E-mail: [email protected] Abstract. The two most widely used soil classifications are the Soil Taxonomy (ST) and the World Ref- erence Base for Soil Resources (WRB). The purpose of this paper is to clarify the differences and the similarities between ST and WRB in their current state, with some examples for representative soils in arid and semi-arid regions of Iran. Four representative pedons were classified and soil units from WRB were compared to those obtained by using ST at the family level. WRB could show the status Key words: of soils polluted by heavy metals through the Toxic qualifier and its subqualifiers. On the other hand, Arid and semi-arid regions, ST could indicate the status of shallow soils in our studied soils but it was not able to show gleyic poorly drained soils, conditions and the existence of a salic horizon because of the differences in its criteria compared to polluted soils, those of WRB. Special effort should be made to quantify various anthropogenic activities in upcom- Soil Taxonomy, ing editions of both classification systems. WRB Introduction ogists with different experiences to classify soils in much the same way. The two most widely used modern soil classifi- Soil classification systems generally aim to establish cation schemes are American Soil Taxonomy, or ST a taxonomy based on breaking the soil continuum (Soil Survey Staff 2014) and the World Reference into more or less homogeneous groups (Guo et al. Base for Soil Resources, or WRB (IUSS Working 2003). Furthermore, most modern soil classification Group WRB 2015). After years of intensive world- systems are developed to complement and support wide testing and data collection, new versions of soil survey activities (Ahrens et al. 2003). Classifi- the ST and WRB systems have been released. In cation systems are conceptual frameworks that ena- its current state, ST has a strong hierarchy with six ble the assimilation of information and the delivery categorical levels, i.e., order, suborder, great group, of information to a user (Blum and Laker 2003). subgroup, family, and series (Soil Survey Staff 2014), Since the earliest days of soil science, attempts have whereas the WRB has a flat hierarchy with only two been made to develop a universal soil classification categorical levels, i.e., reference soil groups and soil system. Most early soil classification systems were units (IUSS Working Group WRB 2015). Rossit- based on the recognition of soil-forming process- er (2001) stated that the reference soil group level es, whereas modern systems classify soils based on of WRB is an intermediate in the conceptual lev- quantitative characteristics defined as diagnostic ho- el between ST orders and suborders, while the sec- rizons, properties, and materials. This allows pedol- ond-level subdivisions, i.e., soil units, which are Bulletin of Geography. Physical Geography Series 2018. This is an Open Access article distributed under the terms of the Creative Commons Attribution- -NonCommercial 4.0 International License (http://creativecommons.org/licenses/by-nc/4.0/), permitting all non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 Nicolaus Copernicus University. All rights reserved. © 2018 De Gruyter Open (on-line). Challenges of Soil Taxonomy and WRB in classifying soils: some examples from Iranian soils M.H. Salehi defined by combinations of qualifiers, are similar to way for the possibility of comparing differing tax- ST great groups (one qualifier) or subgroups (mul- onomies and could open the way for a more com- tiple qualifiers). prehensive classification method. Some scientists have tried to compare the advan- In recent decades, the role of humans in soil tages and disadvantages of these soil classification formation has become a matter of great concern systems and give comments for their improvement among soil scientists. Human influence is now con- (e.g., Deckers et al. 2003; Esfandiarpor et al. 2013). sidered as a soil-forming factor, and anthropogeni- Toomanian et al. (2003) focused on gypsiferous soils sation is recognised as a soil-forming process that in Central Iran and argued that the WRB seems to consists of a collection of geomorphic and pedo- be the most appropriate system for the classification logical processes resulting from human activities. of these soils. Dazzi et al. (2009) proposed a new di- Industrial developments, mines and their activities agnostic horizon for Anthrosols in WRB as a “ge- and intensive agriculture have all led to soil chang- omiscic” horizon which can be succinctly defined es in urban areas. The importance of human impact as a horizon that develops when a layer of at least on soil properties is considered in soil classification 30 cm thick and of different kinds of earthy mate- systems and both soil classifications have experi- rials is added to the soil using earthmoving equip- enced important enhancements which allow urban ment. Charzyński et al. (2013) proposed that new and industrial soils to be described and mapped. qualifiers of Edific, Nekric, Misceric, Artefactic, Ra- For instance, the new reference soil group of Tech- dioactivic and new specifier of Technic be added to nosols was introduced from the 2006 edition of the Technosols in WRB. They also recommended that WRB onwards (IUSS Working Group WRB 2007). Salic and Sodic qualifiers should be available for Ahrens and Engel (1999) reported that although a Technosols. Hulisz et al. (2010) suggested that two few categories in the ST distinguish anthropogenic new qualifiers (Anthrosalic and Anthrosodic) be soils, two soil reference groups, i.e., Anthrosols and used with the Technosols Reference Soil Group of Technosols, were distinguished in the WRB system WRB. Charzyński et al. (2018) studied the soils of at a higher level and it is also possible to account gardens in Toruń and stated that none of the stud- for different anthropogenic elements in other refer- ied mineral surface horizons meets the criteria for ence groups. On the other hand, from the 2010 edi- hortic horizon according to WRB 2015, due to the tion onwards, the Soil Taxonomy tried to consider phosphorus content being too low. They suggest- human effects in different ways, mainly by defini- ed that the research on classification issues of gar- tion of a Master horizon, M, the horizon suffix, u, den soils should be continued on a larger scale to and recently (Soil Survey Staff 2014) by introduc- evaluate whether WRB criteria are not too strict in ing “Human-altered and human-transported mate- taking into account only the features of the most rial classes” at family level. typical, few-decades-old garden soils. Most soils around cities are intensively used and Esfandiarpour-Borugeni et al. (2018) mentioned heavily influenced by humans. Processes in these that the presence of lithologic discontinuity in taxon soils often differ greatly from those in rural soils, name was totally neglected by the ST system but the with features such as contaminant loads, parent ma- WRB system showed this property with the Rap- terials and chemical composition (Rossiter 2007). tic qualifier. They recommended the “Raptic” sub- Soil classification should provide a method for plan- group for all taxa in the ST system. ning agricultural output, allowing the application of Láng et al. (2016) and Michéli et al. (2016) test- new management techniques and supporting the use ed the distance calculation for comparing the great of environmentally sound land-use practices (Shi group of the ST and concluded that it was useful et al. 2010). The concept of soil security serves to in determining differences between soil taxa and make explicit the connections between soil and oth- improving classification definitions. Hughes et al. er global existential challenges. Therefore, a better (2017) compared the USDA Soil Taxonomy and understanding of the connections between the earth the Australian Soil Classification System using their and its inhabitants is needed. Indeed, it is necessary taxa centroids calculated via principal component to consider geologic, geographic, and climatic con- analysis and concluded that this method opened the tributions to public health (Catherine and Skinner 64 Citation: Bulletin of Geography. Physical Geography Series 2018, 14, http://dx.doi.org/10.2478/bgeo-2018-0005 M.H. Salehi Challenges of Soil Taxonomy and WRB in classifying soils: some examples from Iranian soils 2007). Therefore, soil classification systems should The representative pedons were selected and de- also explain the soil’s pollutants and also their ef- scribed according to the “Field Book for Describ- fects on human health. However, one of the impor- ing and Sampling Soils” (Schoeneberger et al. 2002). tant missions of soil classifications is still to identify Then, soil samples from different genetic horizons important properties which have an effect on man- of all pedons were taken, and were analyzed to de- agement purposes and health issues. The purpose of termine: particle size distribution by hydrometer this study was to compare the efficiency of current method); calcium carbonate equivalent (CCE) by editions of the American Soil Taxonomy and WRB HCl treatment or titrimetric method; cation ex- soil classification systems in describing the manage- change capacity (CEC) by NH4Ac method (at pH ment properties of some representative soils in arid 7.0); organic matter (OM) by Walkley–Black meth- and semi-arid regions of Iran. od; gypsum percentage by acetone method; and percentage of rock fragments (RF) (by volume). Saturated paste extracts were prepared to evaluate Materials and Methods soil salinity.