Modeling within field variation of the compaction layer in a paddy rice field using a proximal soil sensing system

A key characteristic of flooded paddy fields is the plough pan. This is a sub-soil layer of greater compaction and bulk density, which restricts water losses through percolation. However, the thickness of this compacted layer can be inconsistent, with consequences such as variable percolation and leaching losses of nutrients, which therefore requires precision management of soil water. Our objective was to evaluate a methodology to model the thickness of the compacted soil layer using a non-invasive electromagnetic induction sensor (EM38-MK2). A 2.7ha alluvial non-saline paddy rice field was measured with a proximal soil sensing system using the EM38-MK2 and the apparent electrical conductivity (ECa) of the wet paddy soil was recorded at a high-resolution (1.0x0.5m). Soil bulk density (n=10) was measured using undisturbed soil cores, which covered locations with large and small ECa values. At the same locations (within 1m(2)) the depth of the different soil layers was determined by penetrometer. Then a fitting procedure was used to model the ECa - depth response functions of the EM38-MK2, which involved solving a system of non-linear equations and a R-2 value of 0.89 was found. These predictions were evaluated using independent observations (n=18) where a Pearson correlation coefficient of 0.87 with an RMSEE value of 0.03m was found. The ECa measurements allowed the detail estimation of the compacted layer thickness. The link between water percolation losses and thickness of the compacted layer was confirmed by independent observations with an inverse relationship having a Pearson correlation coefficient of 0.89. This rapid, non-invasive and cost-effective technique offers new opportunities to measure differences in the thickness of compacted layers in water-saturated soils. This has potential for site-specific soil management in paddy rice fields.