نوع مقاله : مقاله پژوهشی
1 دانشکده جغرافیا و علوم محیطی، دانشگاه حکیم سبزواری
2 گروه جغرافیای طبیعی، دانشگاه شهید بهشتی
عنوان مقاله [English]
Soil is one of the most important elements of the universe, substrate production; food security and self-sufficiency are directly related to soil conditions. Soils are formed on geomorphologic surfaces. These surfaces have certain age, aspects and slopes and also specific position. Time dependent soil formation generally occurs in regions with certain age landforms such as river terraces debris fans or slide masses and mud flows. This may leads to soil diversity in different geomorphological environments. Therefore geomorphological studies, due to different physical and human factors, could have an important role in identification of agricultural prone areas and planning for soil conservation and preventing from soil losses. This research uses a comprehensive procedure and soil lithological and geomorphological data in order to assess the effects of geomorphology on rate of soil forming process in Joghatay County.
In this study, after preparing the required maps of physical characteristics, layers were incorporated and then locations of sampling were determined. These points were located in different elevation, different slopes, certain plan and profile curvature and lithological features. Partial Least Square Regression method was used to analysis and determining the important factors in soil forming process in studied area. PLS path modeling is primarily used to develop theories in exploratory research. It does this by focusing on explaining the variance in the dependent variables when examining the model.
Results and discussion
Using 9-unit model of catena, 7 units were identified in studied area and from these units soil samples were prepared.
Most of samples in unit 4 have large to medium grain size. Except in sample 5, 6 (because of convex slops). In unit 5 amount of sand in comparison with silt and clay is higher. Also, soil depth in this unit is low and minimum soil depth is l0 cm. Gravel size in this unit is medium. In the middle of this unit formation of some geomorphic factures such as alluvial fans is common. Hydrometric analysis of this unit samples (3, 4, 8, 12, 14) show that in sample 3 that is located in upper part of the fan, amount of sand is more than silt and clay. Results from sieve analysis show that the percent of coarse grain size in sample 3 is more than sample 2 and 4. The reason is that in an alluvial fan from higher elevation to flatter areas, coarser grain sizes changed to finer grain sizes. Sample 4 is located between 2 alluvial fans. In this area the percent of sand is more than two others. Samples 12, 8, 14 have large amount of sands since they are located in upper part of alluvial fan. In location of sample 14, the slope has a special from. The plan curvature in this point is convex and profile curvature is concave therefore there is a high convergence in this location, therefore gravels are not enable to persist here. In units 6, 7 that are concave parts of slope and receive large amounts of sediments and solutions in overland or underground flows and deposited in foot slope. In sample 2 that is located in lower part of alluvial fan, sand has the first rank in soil components (84%) but the amount of silt and clay is less than sample 3. Sample 1 that is located in flattest part of the area has a fine grain soil and the least amount of sand and gravels are in minimum ranges. Also, locating of sample 1 in the end part of alluvial fan and near the Jovein river, resulted in increasing soil depth.
Part 2 of investigation, begins with preparing of a model to investigate the role of each soil forming factor on soil formation and development in study area. This model as mentioned before is in the form of structural equations and PLS procedure. In this case, latent variables are divided in 2 forms reflective and formative variables. Fitting of this model was performed in 3 parts. 1- Fit of measurement model 2- Fit of structural model 3- Fit of general model. In designed model, four models related to hydrological, topographical and geomorphological, geological and soil formation variables were created. The results are shown in table 1. The results show that all of models have proper design and are suitable for our study.
Each unit of catena in slope extent is affected by special process and drainage condition and show different soil properties, therefore, slopes are the best areas for soil geomorphological studies. In unstable areas such as slope shoulder, weathered materials transfer to down slope and deposit in more stable area. In studied area slope gradient and aspect are of important factors in soil formation. Especially southward slopes have an important effect on soils. Elevation is another factor that shows direct effects on soil formation that leads to very thick soil layers in flat areas and very thin layer in high gradient slopes and slope shoulders. Also hydrological factors such as soil moisture and order of river are effective factors on soil genesis and development. Results from PLS analysis show that among all investigated factors, slope is the most important one that has critical effect on soil formation. Regarding these results it is clear that geomorphology could have an important role in soil conservation planning because in each unit of slope concerning its influence on soil formation we can present suitable strategies for soil conservation and maintenance.