Comparison of Soil Taxonomy (2014) and WRB (2015) in Classification of Soils in Iranshahr and Dalghan Regions, Sistan and Baloochestan Province

Introduction:

 Increasing demand for an international classification system as a unique communication tool in soil science has caused development of different systems.  Like many other countries, Soil Taxonomy and WRB are the most popular soil classification systems in Iran. Genetic and morphologic soil properties are used for soil classification in both systems. However, correlation of the two systems and efforts to harmonize them have been a major concern among soil scientists. Comparing Soil Taxonomy and WRB in gypsiferous and calcareous soils of central Iran, Sarmast et al. (13) reported that WRB using various qualifiers is more effective than Soil Taxonomy. Since no study on soils of Iranshahr and Dalghan Regions located in Sistan and Baloochestan Province has performed and/or no reported data is available, the present research was performed to: 1) study morphological, physical, and chemical soil properties in the area, 2) classify soils based on Soil Taxonomy (2014) and WRB (2015) systems, 3) compare the two systems for soil description in Iranshahr and Dalghan regions as a part of Sistan and Baloochestan Province, central Iran.

The study area starts from Iranshahr (590 m asl) in the center of the province and extends to Dalghan (390 m asl) in west. Alluvial fan, pediment, playa, and hill were among different landforms identified using field studies, topography maps (1:50000), and Google Earth image observations. To cover the maximum soil variations in the area, 10 representative pedons were selected, described, and sampled.

Calcic, gypsic, anhydritic, argillic, natric, and salic horizons identified after field work and laboratory analysis. Results of the study showed that addition of Yermic Torrifluvent, Yermic Torriorthent, Calcic Gypsiargid, Gypsic Natrsalid, Natric Gypsisalid, Anhydritic Gypsisalid, Anhydritic Calcisalid subgroups to Soil Taxonomy system from one hand, and addition of anhydrite and aquic (for Solonchak reference soil group) qualifiers to WRB system from the other hand, causes a higher correlation and more harmonization between the two classification systems. Meanwhile, the minimum percentage of calcium carbonate equivalent necessary for calcic horizon identification in coarse textured soils including gravel in Soil Taxonomy is also suggested to be added to WRB system. Besides, requirements of salic horizon in WRB system is recommended to be added to Soil Taxonomy. At the same time, soil names in WRB system provide more information and data about soil properties and characteristics in young soils (such as yermic qualifier showing desert pavement) compared to Soil Taxonomy. Soil Taxonomy is not able to properly classify saline soils of arid regions down to subgroup level which is a weak point for this system. That is why newly added Gypsic Natrsalids is suggested for soils with natric, gypsic, and salic horizons in the upper 100 cm of the soil. On the other hand, the requirements of salic horizon in WRB system (the minimum EC content of 15 dS/m and the EC multiplied by the horizon thickness of more and/or equal to 450) are also suggested for Soil Taxonomy.

Results of the study for both saline and sodic soils show more capability of WRB system compared to Soil Taxonomy to classify soils. From soil management point of view, natric horizon causes more negative effects compared to salic horizon because Na disperses the soil particles and destroys soil structure and sodic soils need more practices to be improved compared to saline soils. Results for gypsiferous soils also show more capability of WRB system compared to Soil Taxonomy because gypsum content which is important for gypsiferous soils management is properly concerned in WRB system. However, lack of anhydritic horizon in WRB seems to be a weak point for this classification system. That is why it is suggested to be added to WRB (13). Since Soil Taxonomy does not use independent abbreviations for anhydritic horizon compared to gypsic horizon, the Ba and Baa abbreviations are also suggested for Soil Taxonomy to be added.