Size of Sand Grains Controls Pore Structure and Water Dynamics: Implications for Water Retention and Hydraulic Conductivity

Sand grain size strongly influences the physical and hydraulic behaviour of sandy soils, particularly water retention, pore distribution, and water movement under unsaturated conditions. This study evaluated the effect of five sand grain-size classes, ranging from very coarse to very fine, on pore distribution, aeration, water retention, and unsaturated hydraulic conductivity. Quartz sand samples with different particle sizes were saturated and subjected to matric tensions ranging from 10 to 15,000 hPa. Very fine sand (0.053–0.106 mm) showed the highest field capacity (0.38 m3 m−3) and available water content (0.30 m3 m−3), which were associated with a predominance of pores between 0.2 and 3 μm in diameter. In contrast, coarser sand fractions were dominated by macropores (>50 μm) and exhibited lower water retention. Permanent wilting point values remained low and similar among grain-size classes (≈0.02 m3 m−3). Under unsaturated conditions (matric tensions > 100 hPa), very fine sand exhibited hydraulic conductivity values up to ten times greater than those of coarser fractions. Overall, decreasing sand particle size increased water retention and plant-available water while reducing macroporosity and aeration capacity. These findings demonstrate that sand grain-size distribution plays a major role in regulating water dynamics in sandy soils and may support the development of more efficient irrigation and soil management strategies to improve water conservation and plant water availability in drought-prone environments.