1 Department of Soil Science Engineering, University of Tehran, P. O. Box: 4111, Karaj 31587-77871, I. R. Iran. 2 Department of Chemistry and Biochemistry, University of Agriculture, Faisalabad 38040, Pakistan. 3 Department of GIS and RS, Islamic Azad University, Yazd Branch, Yazd, I. R. Iran.
The geopedological approach was used for soil mapping from Qazvin province, Iran. A semi-detailed soil survey was performed to select the representative units and the soil profiles were classified according to soil taxonomy and physic-chemical characteristics.By combining the soil and geoform layers, the digital soil map was developed using Arc GIS 9.3 environment and soil was divided into three map units (A, B and C). The soil map unit A showed similar soil properties despite three geoform units were existed. Soil map unit B having single geoform unit also revealed same soil characteristics, whereas unit C soil showed two separate soil classes even in the presence of single geoform unit which was due to the difference in particle size distribution. From the results, it is concluded that the position of landscape profile has a significant effect on determination of soil map units and geopedological approach can be used successfully to evaluate the variation in soil characteristics.
Keywords: Geoform unit, Geopedological approach, Soil forming factor, Soil mapping
The soil-geomorphology study plays an important role in mapping and classification of soil. In general, the soil physio–chemical characteristics are considered to be alike in same region due to similar environmental conditions. In such circumstances, the geomorphology technique can be used to study the variation in soil properties. The geopedological approach was first established by (Schaetzl and Anderson 2005) aiming to emphasize on the relationship between soil and different parts of landscapes. Author divided land into four levels; landscape, relief, geology (lithology) and landform according to a hierarchical structure. In this approach landform as an element geoform is located in the lowest level havinghigh degree of homogeneity. All levels in this approach are dependent on scale and each level is separated in an identified scale. Therefore, the above-mentioned system has a direct relationship between perceptual levels of phenomena and map units which is highly compatible with the concept of comprehensive classification of soils. In this sense, (Daniels and Hammer 1992) conducted the soil survey for the investigation of relationship between soil and geomorphology and introduced the concept of smallest map unit with highest level of homogeneity (landform, lithology and soil).
A soil map unit is a spatial and cartographic display that represents polypedon and is the structure of soil map which can be defined and introduced through taxonomic units. Pedon as a soil unit is described, classified and sampled by pedologists (Baver and Gardner 1973). Despite polypedon which is a pure classification unit and a single concept, a soil map is a real, visible and independent entity. Thus, map units mostly contain impurities from other soils (Moameni 1999). Several studies have been conducted using this approach to study geology worldwide (Esfandiarpour et al. 2009, Rossiter 2000). In Iran, few studies have been conducted by (Alijani and Sarmadian 2013; Daniels and Hammer 1992; Gholizadeh and Moameni 2002; Hengl and Rossiter 2003) using soil mapping approaches with the purpose of uniform soil unit separation in different regions. The aim of this study was to employ hierarchical technique of geopedological approach to assess the variation among soil units, soil occurrence in different parts of landscapes and the role of geoform units in determining the soil map units.
Materials and Methods
In present study, the eastern part of Qazvin province, Iran was selected. This area covers 16630 hectares and is located at 36° 1′ to 36° 9′ north latitudes and 50° 21′ to 50° 14′ east longitudes (Figure. 1). The average annual rainfall is 257 mm and area has dry xeric and weak aridic soil moistures with thermic temperature regime. Total area has four types of landscapes; hill, peneplain, piedmont and plain. The piedmont and plain units cover most part of the area. Wheat, barley and maize are the major crop systems of the area.
Preparing of geoform map
Aerial photos are the fundamental basis of geomorphological studies and can be used for the identification of surface roughness and geomorphology (Zinc 1998). Aerial photo of the defined area were provided with 1:40000 scale and obtained from National Mapping Agency. The interpretation of aerial photo was done by considering expert’s opinion, the hierarchical structure of geopedology approach and the location of land latitudes based on geomorphology, geology, morphology and density maps. Finally, geomorphology units were classified into four levels (landscape, relief, lithology and landform). Then, the defined land for reference map units on aerial photos were removed from the study area using reference points and the examination of the carried boundaries precision was performed. After ensuring the accuracy of the initial zone boundaries, the geoform was prepared and used for soil map using geopedology approach.
Field work and laboratory analysis
After the preparation of geoform map, the locations of sampling points were determined and 61 profiles were excavated in the study area (Figure. 2). All selected profiles were classified according to soil taxonomy 2010 (SSS 2010). The soil samples were collected from all points and subjected to physico-chemical tests in laboratory using standard methods (Sparks et al. 1996).
Figure 1: Location of the study area
Figure 2: Profiles position in the study area
Preparing soil map
The soil map was prepared from soil information and geoform map using geopedological approach with 1:40000 scale. The analysis of obtaineddata was performed using Arc-GIS software (version 9.3) and finally, three units were selected (A, B and C) to investigate the variations among soil map units. Table 1 shows the geoform classification system (Zink 1989).
Table 1. The geoform classification system (Zink, 1989)
Figure 3: The Geoform of study area
Results and Discussion
Land under investigation was divided into four landscapes, seven relief, thirteen lithology and nine landform units. The four landscapes include hilly area, peneplain, piedmont and plain. Individually, the hilly area, peneplain, piedmont and plain units cover 1.62%, 9.22%, 43.63% and 45.53% of total area of the landscape. Overall, total area was divided into seven units and 32 geoforms and positions of geoforms can be seen in (Figure. 3).
Table 2. Some features of the selected units to investigate changes in the map units
Loamy over fragmental,mixed,active,thermic,Fluventic Haploxerepts
Loamy skeletal over clayey, mixed, active, thermic, Typic Calcixerepts
Fine, mixed, active, thermic,Typic Calcixerepts
After selection of geoform, the soil classes were divided into families (Table 3). Finally, by combining soil information with geoform map, 24 map units were selected through digitization using Arc-GIS software (Figure. 4). For understanding of the variations in soil properties, three units were separated and named A, B, and C on soil map and geoform (Table 2).
The results of physico-chemical characteristics are shown in (Table 3). The profiles number 62, 72, and 73 were in area A, which are located in Pe214, Pe444 and Pe224 geoform units. The differences in relief types and geo–morphology lead to different distributions in the geoform units. Due to similarity in geoform units, it can be stated that soils formation in the first stage is based on five factors of soil forming (time, relief, parent material, climate and organisms). This indicates that geomorphologic processes are not always the final determinants of soil occurrence on landscapes and in some areas, they may assist the topographer in determining map units as an accessory factor like any other feature of land such as vegetation distribution, management and land use. Considering that landform is the most homogeneous stage in classification (Table 1), all three units of landform have Midslope drainage landform which suggests that landform unit plays a significant role in the final determination of soil map unit. It can be concluded that soil variation in this area has a slower and a more gradual variation rate as compared to landscape variations. Similar observation was recorded in unit 10. Similar to present investigation, (Esfandiarpour et al. 2009) conducted a semi-detailed soil survey in Yugoslavia, Branja region. Author isolated 21 classes in landform unit and declared that the approach based on landform separation and geopedological approach of interpretation is highly efficient in soil unit’s isolation. This type of isolation based on fine, mixed, mesic, vertichaploxerepts, in two units of geoform (Pi311 and Pi411) is also reported by (Baver et al. 1973) and (Rossiter 2000). Another type of soil was considered in two different positions of landform (summit and shoulder of the slope). A soil with Fine, mixed, semi-active, thermic, Typic Paleudults was observed in different position of Midslope and foot slope.
Table 3. Some physical and chemical characteristics of the studied profiles
Position of selected unit
Figure 4: The soil map of study area with geopedological approach
As highlighted in Table 1, the profiles 48 and 58 are located in B area which is a part of Pi212 geoform unit. This geoform unit has landscape piedmont, glacis relief unit and upper landform slope. Geological formation of this unit is based on the geological map of the area with younger terraces of an alluvial sedimentary origin. These are constantly influenced by the processes of erosion and sediment due to the temporary movement of rivers in the area. Classification of this soil up to the family level is loamy over fragmental, mixed, active, thermic, fluventic Haploxerepts and irregular variation in organic carbon from 0 to 150 cm depth was observed. The variation in organic carbon may also be due to the geomorphologic processes as mentioned above in soil type A. Results indicated that similar geoform unit contains identical soil which were observed in majority of soil map units and ~83% of the prepared map units showed same conditions. This similarity was observed in units 1–6, 8−14, 16–19 and 22–24. Overall, an extremely close relationship and interaction in soil was observed with the landscapes that are formed and evolved on surfaces. In accordance to present investigation, (Olsen 1954) argued that the variation in soil is a function of location or geomorphic unit and the pattern and extent of variations depend on the position of soil in landscape, soil type and soil other features. In this sense, (Schaetzl and Anderson 2005) also reported that geopedological approach can be used to achieve homogeneous and monotonous limits in soil mapping. Using this approach author selected monotonous map units and performed dispersion pattern of soil in a area with minimal costs through generalizing the result of the study in a geoform unit with respect to other similar units in the study area. Similar trend was observed in most of soil map units in present investigation. As can be seen in Table 3, area C has two profile numbers (33 and 38) in PL234 geoform unit. This unit is located in plain landscape, flat relief and Midslope drainage landform. The soil of profile number 33 was fine, mixed, active, thermic, Typic Calcixerepts type, whereas it was loamy skeletal over clay, mixed, thermic, Typic Calcixerepts soil in profile number 38. The subgroup of both soil type was Typic Calcixerepts and only based on the difference in particle size distribution; two soil classes were observed. Profile number 38 has a layer with more than 35% of rock fragment and less than 30% of clay in the soft part of soil, whereas profile number 33 contains 35–60% clay in the soft part of soil. So, the soil was included in association unit due to difference in distribution percentage and similar pattern was observed in units 7, 15, 20, and 21 also. The trend observed in present study were in line with previous studies e.g. (Shaw et al. 2004) studied coastal plain of North Caroline, America and reported that difference in soil properties was due to the difference in parent material, however, the other characteristics (geophormic unit, slope and orientation gradient) of the area were identical. According to (Alijani and Sarmadian 2013), the soil geoform must be identical with respect to landscape, relief, parent material and landform. The difference in soil of a geoform may result from type and position of slope and land use or vegetation as one of the constituent and distinctive factors of soil (Rossiter 2000). In identical geoform surfaces, the difference in soil properties was due to the difference in slope position, land use and parent material. In addition to the geomorphology, one of the reasons to justify the presence of these soil types in a soil map unit may be the influence of identical applications and management practices on both soil types.
The variation among soil mapping unit was investigated using geopedological approach. The results showed using this technique the difference in soil characteristics can be determined. Based on geoform units, it was observed that soil characteristics may not be the only determinant factor in soil map unit, however, the management practices and the applications also have a significant influence on map units. Moreover, the dependence of various processes to the scale and inseparability in study scale, landscape and soil variation and even the inability of geopedological approach to separate homogeneous units may also affect the data consistency.
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