## 2 Materials and Methods  
### Soil Property Analysis and Spectral Measurements  
The soil samples were collected from 21 to 22 April 2023 from saltwater paddy fields in Jeongnam-myeon, Hwaseong-si, Gyeonggi-do in the Republic of Korea (Figure 1). The humidity and ambient air temperature were 36% and 16.7 \({}^{\circ}\)C, respectively, and the soil textures were silty clay loam and silty loam. A total of 120 soil samples were collected at a depth of 15 cm using a soil sampler ([PERSON], Eijkelkamp, The Netherlands) (Figure 2a), after removing 3 cm from the topsoil to discard debris. Five soil samples were collected with mixed soil properties to minimize the error in soil sample composition. Except for some soil samples for spectroscopic analysis, the remaining soil sample properties were analyzed at the Soil Verification Center of the Korea Agriculture Technology Promotion Agency. The soil properties pH, EC, soil organic matter (SOM), TN, total organic carbon (TOC), and clay were analyzed according to the soil chemical analysis manual of the Korea Agriculture Technology Promotion Agency [20].  
Figure 1: Location of soil sampling point in Hwaseong-si, Gyeonggi-do, Republic of Korea. All fields were divided into 30 sections. Each number was a soil sampling number.  
Table 1 represents the mean and standard error of the soil samples analyzed, which are as follows: pH = 7.48 \(\pm\) 0.37; EC = 7.48 \(\pm\) 0.37 dS/m; SOM = 32.09 \(\pm\) 5.36 g/kg; TN = 0.13 \(\pm\) 0.02 g/kg; TOC = 1.86 \(\pm\) 0.31 g/kg; and clay = 30.66 \(\pm\) 0.03%. Most of the soil properties showed less than 15% difference from the mean.  
[PERSON] et al. (2018) [21] compared the estimation models of the moist and dry soil. Most of the models of dry soil provided better estimates of TOC and TN than those of moist soil. Within the VIS-NIR spectrum [17; 22; 23; 24], the estimated soil moisture content was affected. Spectra were collected for each soil sample before drying (wet soil) and after drying (dry soil). The dry soil samples were dried in an oven desiccator at 103 \({}^{\circ}\)C for 24 h in accordance with ASAE Standard S358 2 (DEC93) [25], as shown in Figure 2b, and were sieved to 2 mm. Spectra for the pre-dried and post-dried soils were collected utilizing ASD Field Spec PRO4 spectrometry (ASD Inc., Boulder, CO, USA), as shown in Figure 2c. The spectra were collected on 8-9 May 2023, during the peak solar period from 1100 to 1500 h, when the ambient air temperature was 19.5 \({}^{\circ}\)C and the average solar radiation was 944.0 W/m\({}^{2}\). A white, 100% reflective, standardized Spectralon(r) (Labsphere, North Sutton, NH, USA, 2013) reflector was used, which was calibrated and optimized to minimize the scattering and noise caused by sunlight. The spectra were collected at 1 nm intervals, and reflectance was measured in the 350-2500 nm range. Each spectrum was measured using 10 replicates per soil sample point to reduce error and then averaged [26].  
\begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline
**Soil** & \multicolumn{3}{c}{**All Fields**} & \multicolumn{3}{c}{**Field 1**} & \multicolumn{3}{c}{**Field 2**} & \multicolumn{3}{c}{**Field 3**} & \multicolumn{3}{c}{**Field 4**} \\ \cline{2-11}
**Sample** & **Mean** & **SD** & **Mean** & **SD** & **Mean** & **SD** & **Mean** & **SD** & **Mean** & **SD** \\
**Data** & **(_n_ = 120)** & **(_n_ = 120)** & **(_n_ = 30)** & **(_n_ = 30)** & **