Comparison of spatial interpolation techniques for mapping soil pH and salinity in agricultural coastal areas, northern Iran

Abstract

This study attempted to characterize the spatial distributions of soil pH and electrical conductivity (EC_e) of coastal fields in the Miyandoroud region, northern Iran, for three soil layer depths by assessing spatial variability and comparing different interpolation techniques such as inverse distance weighting (IDW), ordinary kriging (OK), and conditional simulations (CS). Three soil composite samples were collected from 0–50, 50–100, and 100–150 cm depths at 105 sampling sites. At all three soil depths, pH and EC_e were best fitted by exponential and spherical models, respectively. Nugget effects were higher for soil EC_e data sets compared with soil pH at all three soil depths showing soil EC_e had a spatial variability in small distances. The prediction accuracy of the interpolation methods indicated that the minimum error for all data sets was achieved with the OK method, except for pH at 50–100 cm depth, and the CS technique revealed the largest error. The effect of different numbers of simulations (100, 500 and 1000) in the CS interpolation method resulted not in a realistic mapping for the soil EC_e and pH. Considering the high importance of irrigated agriculture in the Caspian Sea coastal areas, more subsoil salinity build-up and groundwater salinity monitoring plans are needed as a prerequisite for sustainable agricultural production systems of the future.

Keywords:

• soil pH and salinity
• spatial variability
• geostatistical techniques
• Caspian Sea coastal areas

Acknowledgments

The projects’ field work and laboratory analysis were provided by the Abgostraan-Sabzdasght consulting engineering company under permission of Agricultural Organization (Jahad-Keshavarzi) of Mazandaran province, Sari, Iran. The authors would like to express their sincere appreciation and gratitude to Mr Jafari, director of Abgostraan-Sabzdasght consulting engineering company, and the Agricultural Organization (Jahad-Keshavarzi) of Mazandaran province for the necessary cooperation. The authors are grateful to two anonymous reviewers and editor-in-chief, Dr Heidrun Beschow, for their critical comments and suggestions that greatly improved the quality of the manuscript.