Determining the critical limits of soil quality indicators for paddy fields in different landforms (Case study: Langarud, Guilan Province)

Assessment of soil quality and identification of key indicators with their critical limits are very important for maintaining soil functions and rice productivity. The aim of this research was to determine the minimum data set (MDS) based on terrain attributes and to establish critical limits based on rice yield in Langarud, Guilan Province. Composite soil samples (0-30 cm) were collected from three landforms: mountain, alluvial plain, and coastal area. The MDS in each landform were obtained using auxiliary data extracted from DEM, and the Norm values of soil properties. Finally, the integrated quality index (IQI) was calculated for each landform. Available potassium showed the highest correlation with rice yield in the coastal (R2 = 0.87), alluvial plain (R2 = 0.85), and mountain (R2 = 0.90). The lower and upper limits of the IQI for 40% and 80% relative yield were 0.39 and 0.65 in coastal area, 0.56 and 0.76 in alluvial plain, and 0.41 and 0.73 in mountain, respectively. The highest correlation between the soil quality index (SQI) and relative yield (R2 = 0.87) was obtained for mountain. The mapping showed that the low productivity paddy fields are located in the coastal areas, where the SQI is the lowest. This observation is possibly attributed to the coarser soil texture and the proximity to the Caspian Sea. In contrast, paddy fields in mountain exhibited the highest SQI and yield. Therefore, determining the critical limits for MDS is essential for improving management practices and achieving sustainable productivity in paddy fields.