Pinewood biomass in Portugal can be considered a major source of biochar for soil physical, chemical, and biological edaphic amendment. This work intended to evaluate the aptitude of lab produced biochar for upgrading soil moisture dynamics’ relationships considering mixtures of biochar with silica-based sand. The methodology used focused on the carbonization of pine biomass with inert atmosphere at 300 °C, 400 °C, 500 °C and 600 °C, followed by a chemical proximate and thermogravimetric analysis, scanning electron microscopy, Fourier Transform Infrared analysis, numerical modeling, and characterization of biochar porosity by gas adsorption (Brunauer–Emmett–Teller) and mercury porosimetry. The results showed the increased amounts of soil water retention and plant available water, evaluated through pF curves, due to biochar application. The thermogravimetric analysis mass loss patterns and FTIR transmittance, reflected major structural modifications in carbonized products by comparison with raw biomass. Mercury porosimetry showed that biochar pores between 392 and 250 μm and 32 μm and 6 μm gave the highest pore volume for water retention with a major increase from carbonization, by comparison with physical activation. The used methodologies allowed us to conclude that the carbonaceous feedstock can potentiate the improvement of soil water relations aiming at agricultural land use.