2.2 Collection and analyses of soil samples
Soil samples were collected in February 2017, when soybean was ready to be harvested. The total twenty-six fields with soybean yields > 4.20 Mg ha-1 were selected to represent the high-productivity environments. These selected plots were those whose average productivity of the last three crops was higher relative to the national average yield (3.08 Mg ha-1; Conab, 2018
Soil samples from depths of 0-0.10, 0.10-0.20, and 0.20-0.40 m were collected for the measurement of soil physical properties. These soil depths were designated Layer (L) 1, L2 and L3, respectively. These soil samples were obtained from locations in the plot that could be considered representative of the plot. The plot harvest map was used to locate the pit for sample collection according to the described productivity. Undisturbed soil samples were obtained using a volumetric core of approximately 0.1 dm-3 (50-mm height and 50-mm diameter).The indices of soil physical properties such as penetration resistance (PR, MPa), soil bulk density (BD, Mg m-3), total porosity (TP, m3 m-3) and soil water retention curve (SWRC) were estimated for all collected samples. Additionally, bulk samples were also obtained to determine the clay (g kg-1) and sand (g kg-1) contents and the particle density (PD, Mg m-3).