## 2 Materials and Methods
### Study Area  
This study was conducted in 12 soybean fields in Kaizu City, Gifu, Japan. The soybean fields were planted between the 2018 and 2020 growing seasons from July-November for each year. The farmers in this location implemented crop rotation with three crops (paddy rice-winter wheat-soybean) over two years, which became a challenging task to find exactly the same fields cultivating soybean in multiple years sequentially. Thus, fields in different locations for each year were observed for the analysis. A local leading soybean cultivar, 'Fukuyutaka', was grown in all observation fields. According to a previous study [38], the optimum planting date of the 'Fukuyutaka' cultivar was approximately 10 July. However, the sowing dates of the observed fields were varied due to the climate conditions, as machinery cannot be used for seeding several days after heavy rain. The differences in sowing dates and soil conditions during the seeding caused unstable seedling establishment rate and plant density variability in fields. Thus, selected fields sown from the optimum to late sowing dates in the three years were surveyed to cover high variations of plant density. The detailed locations map and sowing dates for the research fields are presented in Figure 2 and Table 1.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline
**Field Name** & **Area** **(ha)** & **Latitude** & **Longitude** & **Sowing Dates** & **UAV Imagery** & **The Number of** **Aquisition Date** & **PlanetScope Imagery a** \\ \hline Field 1 & 0.9 & 35\({}^{\circ}\)11\({}^{\prime}\)13.2\({}^{\prime\prime}\) & 136\({}^{\circ}\)40\({}^{\prime}\)4.8\({}^{\prime\prime}\) & Jul 15, 2018 & 6 August 2018 (22 DAS) & 12 \\ Field 2 & 1.16 & 35\({}^{\circ}\)11\({}^{\prime}\)31.2\({}^{\prime\prime}\) & 136\({}^{\circ}\)40\({}^{\prime}\)1.2\({}^{\prime\prime}\) & Jul 15, 2018 & 6 August 2018 (22 DAS) & 12 \\ Field 3 & 1.1 & 35\({}^{\circ}\)11\({}^{\prime}\)20.4\({}^{\prime\prime}\) & 136\({}^{\circ}\)39\({}^{\prime}\)54.0\({}^{\prime\prime}\) & Jul 17, 2019 & 29 July 2019 (12 DAS) & 13 \\ Field 4 & 1.19 & 35\({}^{\circ}\)11\({}^{\prime}\)20.4\({}^{\prime\prime}\) & 136\({}^{\circ}\)39\({}^{\prime}\)54.0\({}^{\prime\prime}\) & Jul 17, 2019 & 29 July 2019 (12 DAS) & 13 \\ Field 5 & 1.11 & 35\({}^{\circ}\)11\({}^{\prime}\)16.8\({}^{\prime\prime}\) & 136\({}^{\circ}\)39\({}^{\prime}\)54.0\({}^{\prime\prime}\) & Jul 10, 2019 & 30 July 2019 (20 DAS) & 11 \\ Field 6 & 0.94 & 35\({}^{\circ}\)11\({}^{\prime}\)16.8\({}^{\prime\prime}\) & 136\({}^{\circ}\)39\({}^{\prime}\)54.0\({}^{\prime\prime}\) & Jul 10, 2019 & 30 July 2019 (20 DAS) & 11 \\ Field 7 & 0.88 & 35\({}^{\circ}\)11\({}^{\prime}\)2.4\({}^{\prime\prime}\) & 136\({}^{\circ}\)38\({}^{\prime}\)16.8\({}^{\prime\prime}\) & Jul 25, 2019 & 5 August 2019 (11 DAS) & 10 \\ Field 8 & 0.95 & 35\({}^{\circ}\)11\({}^{\prime}\)2.4\({}^{\prime\prime}\) & 136\({}^{\circ}\)38\({}^{\prime}\)9.6\({}^{\prime\prime}\) & Aug 125, 2019 & 5 August 2020 (11 DAS) & 10 \\ Field 9 & 1.87 & 35\({}^{\circ}\)11\({}^{\prime}\)16.8\({}^{\prime\prime}\) & 136\({}^{\circ}\)39\({}^{\prime}\)43.2\({}^{\prime\prime}\) & Aug 2, 2020 & 19 August 2020 (17 DAS) & 8 \\ Field 10 & 0.73 & 35\({}^{\circ}\)14\({}^{\prime}\)42.0\({}^{\prime\prime}\) & 136\({}^{\circ}\)39\({}^{\prime}\)50.4\({}^{\prime\prime}\) & Jul 21, 2020 & 4 August 2020 (14 DAS) & 6 \\ Field 11 & 0.61 & 35\({}^{\circ}\)14\({}^{\prime}\)27.6\({}^{\prime\prime}\) & 136\({}^{\circ}\)40\({}^{\prime}\)1.2\({}^{\prime\prime}\) & Aug 5, 2020 & 18 August 2020 (13 DAS) & 8 \\ Field 12 & 0.76 & 35\({}^{\circ}\)10\({}^{\prime}\)4.8\({}^{\prime\prime}\) & 136\({}^{\circ}\)39\({}^{\prime}\)36.0\({}^{\prime\prime}\) & Aug 4, 2020 & 18 August 2020 (14 DAS) & 8 \\ \hline \end{tabular}  
* The duration of the PlanetScope imagery acquisition was from 1 to 60 days after sowing.  
\end{table}
Table 1: Locations, sowing dates, UAV imagery acquisition dates, and the number of satellite (PlanetScope) imagery used for all observation fields.  
Figure 1: Flow chart of the research.