In coastal China, there is an urgent need to increase the land area for agricultural production and urban development, where there is a rapid growing population. One solution is land reclamation from coastal tidelands, but soil salinization is problematic. As such, it is very important to characterize and map the within-field variability of soil salinity in space and time. Some proximal sensors such as the EM38 allow for the rapid and cost-effective in situ collection of high-resolution data. In this study, we used the EM38 to study spatiotemporal variability of soil salinity in a coastal paddy field. Geostatistical methods were used to determine the horizontal spatiotemporal variability of soil salinity over three consecutive years. The study found that the distribution of salinity was heterogeneous and the leaching of salts was more significant in the edges of the study field. By inverting the EM38 data using a Quasi-3D inversion algorithm, the vertical spatiotemporal variability of soil salinity was determined and the leaching of salts over time was easily identified. We concluded that the methodology of this study can be used as guidance for researchers interested in understanding soil salinity development as well as land managers aiming for effective soil salinity monitoring and management practices.