The Effect of Different Classification Levels and Topography on Potassium Status of Sepidan Soils, Fars Province

The soil characteristics of each region are affected by the topography factor. This factor has direct and indirect effects on the physicochemical properties of soil and even nutrient concentrations. Potassium is one of the most important elements in plant nutrition and is the second major nutrient absorbed by plants. Potassium exists in the soil in four forms, including soluble, exchangeable, non-exchangeable and structural. Many studies conducted on different potassium forms in relation with mineralogy, soil characteristics and classification, climate, and especially topography, but investigating the effect of different physiographic units and different levels of classification had been neglected. Therefore, this study was aimed to investigate the effect of different levels of classification and topography (physiographic units) on the potassium status of Sepidan soils in Fars province.

In order to investigate the effect of topography on the potassium status of Sepidan soils in Fars province, toposequence was applied in Sepidan region of Fars province. After digging and describing the selected profiles in each identified physiographic unit, samples were taken and physical and chemical properties of soils were measured by conventional methods. The different forms of potassium were measured by summary method; and using SPSS16 statistical software and LSD statistical test, the significance of differences in the amount of different forms of potassium were determined at different classification levels and different physiographic units.

Investigation of studied pedons showed that soils were classified as Entisols, Inceptisols and Alfisols based on 2014 taxonomy classification system. Three suborders, four great groups and subgroups were identified. In the study area, soluble, exchangeable, non-exchangeable, structural and total potassium had a mean of 0.96, 279, 510, 3792 and 4583 mg kg-1, respectively. The mean of soluble, exchangeable, non-exchangeable, structural forms and total potassium related to different physiographic units were reported 0.73, 421, 1360, 1405, 15833 mg kg-1 in the colluvial-alluvial fan, 1.21 , 236, 565, 2803 and 3603 in piedmont plain, 0.76, 352, 436, 2071, 2860 mg kg-1 in flood plain, 0.73, 270, 372, 2249, 2893 mg kg-1 in river terrace, 0.91, 321, 656, 2541, 3520 mg kg-1 in alluvial plain and 1.17, 172, 88, 2292, 2554 mg kg-1, respectively. Total potassium and also different forms of potassium except soluble potassium in different physiographic units were significantly different from each other (P<0.05). In relation to the effect of different classification levels, there were significant differences (P<0.05) between exchange and non-exchange forms in all the studied levels.

The present study showed that topography in the form of different physiographic units by controlling the amount of humidity, received sunlight and also the speed of weathering lead to differences in physicochemical properties and classification of soil. The set of these factors led to observe significant differences in the amount of different forms of potassium in different physiographic units. Due to the lack of application of very significant differences in different classification levels up to the subgroup level, it is suggested that for having better monitoring of the potassium element to obtain sustainable agriculture, at least the classification of soils at the family level should be performed so that the differences are better be understood. Also, it is worth mentioning that the type of parent material, climate, topography and other environmental conditions can affect the accuracy of the study in different areas, which should not be neglected when choosing the accuracy of the study.