Soil science
H.R. Owliaie
Abstract
Introduction
Soil classification is the systematic categorization of soils based on distinguishing soil characteristics, aiding in the comprehension of soil properties through soil surveys, and establishing suitable strategies for effective soil utilization and management. One of the main reasons ...
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Introduction
Soil classification is the systematic categorization of soils based on distinguishing soil characteristics, aiding in the comprehension of soil properties through soil surveys, and establishing suitable strategies for effective soil utilization and management. One of the main reasons for creating soil classification systems is to identify the differences in important soil characteristics for management purposes. Globally, Soil Taxonomy (ST) and the World Reference Base for soil resources (WRB) are widely used for soil classification. However, these two classification systems have varying criteria which can pose difficulties when exchanging classification results. After years of intensive worldwide testing and data collection, new versions of the ST and WRB systems have been released. In its current state, ST has a strong hierarchy with six categorical levels: order, suborder, great group, subgroup, family, and series (Soil Survey Staff, 2022), while the WRB has a flat hierarchy with only two categorical levels: reference soil groups and soil units (IUSS Working Group WRB, 2022). Several scientists have endeavored to evaluate the merits and demerits of these soil classification systems and offer recommendations for their enhancement. The arid and semi-arid regions located in the western and southwestern parts of Kohgiluyeh and Boyerahmad Province, distinguished by their considerable diversity in parent materials, topography, climate, and land use, present an excellent opportunity for scrutinizing and contrasting the effectiveness of soil classification systems. Remarkably, no prior research has delved into this subject in this specific geographical area. Consequently, this research aims to compare the effectiveness of the ST and WRB systems in characterizing soil attributes. Furthermore, it seeks to analyze the alterations that these two systems have undergone during an eight-year period, spanning from 2014 to 2022.
Materials and Methods
This study was conducted in the western and southwestern regions of Kohgiluyeh and Boyerahmad Province, specifically in the divisions of Gachsaran, Basht, Choram and Kohgiluyeh. A total of 26 soil profiles were excavated, described, and sampled based on aerial photos, satellite images, topographical and geological maps, as well as field observations. These profiles were selected following the soil description guide provided by the Department of Soil Conservation of the US Department of Agriculture. Subsequently, after reviewing the preliminary results and aligning with the research objectives, 12 representative soil profiles were chosen for further analysis. Soil samples were collected from all genetic horizons and transferred to the laboratory. After air-drying, the samples were passed through a two-millimeter sieve and the routine physical and chemical analyses were conducted, including soil texture, pH, electrical conductivity (EC), calcium carbonate equivalent (CCE), organic carbon, cation exchange capacity (CEC), and gypsum analyses. For mineralogical studies, soil clay minerals were separated and identified using standard methods. Additionally, soil thin sections were prepared from intact soil samples of selected soil horizons and examined under a polarizing microscope. Finally, the soil profiles were classified based on the criteria outlined in Soil Taxonomy (2022) and WRB (2022).
Results and Discussion
Soil Taxonomy and WRB, as the two most popular classification systems, aim to encompass as manysoil characteristics as possible. According to the ST classification, the soils were classified into four orders: Entisols, Inceptisols, Alfisols, and Mollisols. In the WRB system, they were grouped into seven reference soil groups: Regosols, Flovisols, Luvisols, Cambisols, Kastanosems, Gypsysols and Glysols. The results revealed that WRB was significantly more effective in describing the characteristics of the studied soils. One of the key advantages of this two-level system is its flexibility, allowing for the inclusion of additional principal and supplementary qualifiers to cover all essential soil characteristics. Moreover, in many cases, WRB exhibits better prioritization compared to ST. For example, the presence of gypsic, combic, calcic horizons, as well as fluvic and gley properties, can allocate the soil to the reference groups of Gypsisols, Cambisols, Calsisols, Fluvisols, and Gleysols, respectively. However, a limitation of the WRB system is the absence of mineralogical information in soil classification. Enhancing this classification system's quality and making it more appealing to planners could be achieved by incorporating suitable mineralogical attributes for the reference groups or criteria that express soil fertility conditions with relatively straightforward measurements. In addition, it is proposed to add three subgroups to ST: Gypsic Haplustalfs, Fluventic Gypsiustepts and Cambic Haplustolls. Similarly, following the WRB model, it is recommended to introduce a qualifier in ST to indicate the presence of lithological discontinuity. Regarding the WRB system, suggestions include adding qualifiers such as "Cutanic" to gypsisols with clay films, "hypercalcic" to reference groups of Kastanozems and Luvisols with a calcic horizon comprising more than 50% of calcium carbonate, and "aridic" for better expression of soil characteristics with Aridic-Ustic moisture regimes.
Conclusion
The results of this research demonstrated that WRB is more effective in describing the conditions and characteristics of the studied soils. The WRB system, through its diverse set of qualifiers, is capable of representing field conditions more efficiently. However, it is suggested that the surveyors have the freedom to select an appropriate qualifier from the list provided by WRB without limitation, which can enhance its success in practical applications. Furthermore, it is recommended that both classification systems be used to categorize soils, not only to evaluate their efficiency for the soils in other regions but also to gain a comprehensive understanding of their suitability for different contexts.
Moghbeli Z.; S. Sanjari; E. Adhami
Abstract
Introduction: In sustainable agriculture, it is essential to know soil various characteristics for increasing the soil productivity. The relationship between soil and geomorphology in arid and semi-arid regions has been considered by many researchers. Faryab plain is located in arid region of Kerman ...
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Introduction: In sustainable agriculture, it is essential to know soil various characteristics for increasing the soil productivity. The relationship between soil and geomorphology in arid and semi-arid regions has been considered by many researchers. Faryab plain is located in arid region of Kerman Province and has diversity in geomorphic positions and parent materials. No previous study has been conducted in this region. Therefore, the objectives of the present research were 1) to study the genesis and development of soils related to different geomorphic surfaces in Faryab region, 2) to study the physicochemical properties, clay mineralogy and micromorphology of soils, and 3) to classify the soils according to Soil Taxonomy (ST) (2014) and World Reference Base (WRB) (2015) systems and compare them.
Materials and Methods: Faryab region with a mean elevation of 630 m above sea level is located in Kerman province, south-eastern of Iran. Mean annual rainfall and temperature of the area are 160 mm and 23.8 oC, respectively. Soil temperature and moisture regimes of the area are thermic and aridic, respectively. From geological point of view, the studied area is a part of west and south west zones and Flysch zone of east of Iran. Ten representative pedons on different geomorphic units including hill, alluvil-colluvial fan, alluvial plain, and lowland were selected, sampled, and described. Routine physicochemical analyses, clay mineralogy, and micromorphological observations performed on soil samples. Soil pH, texture, electrical conductivity, calcium carbonate, Na, Ca, Mg, cation exchangeable capacity and gypsum were identified. Eight samples were selected for clay mineralogy investigations. Four slides including Mg saturated, Mg saturated treated with ethylene glycol, K saturated, and K saturated heated up to 550 oC were analyzed. A Brucker X-Ray diffractometer at 40 kV and 30 mA was used for XRD analyses. Undisturbed soil samples from some representative pedons were selected for micromorphological observations. A vestapol resin with stearic acid and cobalt as hardener was used for soil impregnation. A Lite petrographic microscope was used for micromorphology investigations.
Results and Discussion: The results of the present study indicated that the soils with more evolution were located on the geomorphic surfaces of the lowland and alluvial plain and the soils with lower development on the hill and alluvil-colluvial fan. The most important pedogenic processes of the soils were the eluviation of salt, gypsum, calcium carbonate as well as clay, and the formation of calcic, gypsic, petrogypsic and natric horizons. The soils of the region were classified using ST as Aridisols with three suborders of Argids, Calcids and Gypsids and classified according to the WRB as three soil reference groups of Solonetz, Gypsisols and Calcisolos. A new subgroup of Calcic Natrigypsids is suggested for inclusion to ST for the soils with aridic soil moisture regime and three horizons of gypsic, calcic and natric. The WRB system, due to its flexibility in the use of principle and supplementary qualifiers, prepare a better qualification than ST for the soils of the region. According to mineralogical results, the observed minerals consisted of illite, palygorskite, chlorite, smectite, kaolinite, vermiculite and quartz. The highest amount of palygorskite was observed in the gypsic horizons of hill and alluvil-colluvial fan. By moving to the central part of the plain (lowland), the amount of palygorskite was greatly reduced and the amount of smectite was increased. Two origins of inheritance and transformation (illite and palygorskite) are suggested for the occurrence of smectite in the soils. Due to the lack of the conditions for the formation of kaolinite, illite and chlorite, these minerals are inherited from parent materials. SEM observations suggested a pedogenic pathway for the occurrence of large amounts of palygorskite in the soils of the region. Calcareous and gypsiferous media seems to prepare a favorite environment for the pedogenic formation and stabilizing of this mineral in the studied soils. Coating and infilling of gypsum and calcite crystals in voids and channels, clay coating along chanels as well as Fe and Mn oxide nodules were among the common pedofeatures observed in the thin sections of the studied soils. Occurrence of variable habits of gypsum crystals in different geomorphic surfaces suggested a dynamic soil environment. Larger lenticular gypsum crystals were found in the soils with lighter texture located on more stable geomorphic surfaces.
Conclusion: Different geomorphic situations in the region affected the development and evolution, physicochemical properties, clay mineralogy, micromorphology and soil classification and caused the differences in these characteristics in the Faryab region.
Farsila Mahmoudian; Ali reza Karimi; Amir Lakzian
Abstract
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Mohammad Ghasemzadeh Ganjehie; Ali reza Karimi; Ali Zeinadini; Reza Khorasani
Abstract
Introduction: Playa is one of the most important landscapes in arid regions which covers about 1% of the world's total land area. Study of playas is important from different points of view especially pedology, sedimentology, mineralogy, environmental geology, groundwater and surface water chemistry. ...
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Introduction: Playa is one of the most important landscapes in arid regions which covers about 1% of the world's total land area. Study of playas is important from different points of view especially pedology, sedimentology, mineralogy, environmental geology, groundwater and surface water chemistry. More than 60 playas have been identified in Iran. Considering the fact that playas and surrounding landforms are important archive of landscape evolution and paleoenvironmental variations, it seems that less attention has been paid to them so far. Soils are known as indicators of the landscapes evolution. Previous studies in arid regions of Iran imply different periods of deposition and soil formation in playa and alluvial fans or pediments. Bajestan playa is one of the known playa in northeastern Iran, and the largest clay flat exists in this playa. There is no information on the soils and their evolution in Bajestan playa. The objective of this study were to 1) identify the soils in different landforms along a transect from alluvial fan to clay in Bajestan playa 2) determine the morphological, micromorphological and mineralogical characteristics of these soils 3) determine the periods of soil and landform evolution and 4) comparison of soils evolution of the study area to other arid regions of Iran.
Material and Methods: The study area of approximately 20000 hectares is located in southeastern of KhorasanRazavi province. The climate of the study area is hot and dry with mean annual temperature and rainfall of 17.3 °C and 193 mm, respectively. Soil moisture regime is aridic with subdivisions of weak aridic and soil temperature regime is thermic. Firstly, landforms and geomorphic surfaces of the study area were recognized based on Google Earth images interpretations and field observations. Four main landforms were recognized in the study area. The landforms from north to the south of the study area were alluvial fan, intermediate alluvial fan- clay flat, pediment and clay flat. Considering the diversity of geomorphic units, 11 soil profiles were described and diffrenet soil layers and horizons were sampled. Undisturbed soil samples were taken micromorphological studies. Some horizons were selected for clay mineralogy analysis. The mineralogy of clay fraction was determined using X-ray diffraction method.
Results and discution: All studied soils except the profiles in the pediment were classified in the Aridisols order. There were two geomorphic surfaces in alluvial fans. In the first geomorphic surface a soil with the Bk horizon buried a soil with red Btk horizon. In the second geomorphic surface, it seems that the erosion has been removed the overlying soil. The Bk horizon showed the maximum soil development in the clay flat and intermediate alluvial fan-clay flat landforms. Clay coating on sand in thin section was the evidence of clay illuviation in Btk horizon. Carbonate nodules associated with clay coating are the compound pedofeature in Btk horizon. These evidences reflect polygenetic nature of the soils and different period of climate change and soil formation. Smectite, mica, chlorite and palygorskite are the clay minerals in the studied soils. Similar to soils in arid regions of Iran, palygorskite was found in Bk, Bt and Bz horizons. The existence of Bk horizon in overlying soils, buried Btk horizon, removal of surface horizon in alluvial fan are the evidences of regressive and progressive of pedogenic processes in the study area. Btk horizon represents a warm and wetter and Bk horizon indicates a relatively wetter period in comparison to present time.
Conclusion: Btk was the most developed horizon in the study area that occurred as buried paleosol in alluvial fan. Bk, Bw, By and Bz were the common horizon in other landforms. Clay coating and red color of Btk horizon might seem as indicators of hot and humid conditions in the past, during the argillic horizon formation. Covered carbonate nodules with clay coating can also be mentioned as sign of a hot and wet period which is suitable for clay illuviation and weathering after a period of carbonate accumulation. The buried Btk horizon under alluvial layers in the alluvial fan indicates that after apedogenic period, alluvial processes have been responsible in burying this horizon. Bk horizon in overlying soil of all landforms represents a less intense period of soil formation. The dominant clay minerals in the study area were Illiite, cholorite, kaolinite, and palygorskite. The sequence of Bk and Btk horizons in this research and the occurrence of these soils in central, eastern and northeastern Iran imply the similar pedogenetic conditions in arid regions of Iran.
