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).