Water availability to soybean crop as a function of the least limiting water range and evapotranspiration1

ABSTRACT Irrigation management aimed at optimal production has been based only on the water factor. However, in addition to the water potential of the soil, factors such as soil penetration resistance and soil O2 diffusion rate also affect plant growth and interfere with water absorption, even if moisture is within the available water range. This study aimed at quantifying the least limiting water range and demonstrating its potential in soil and water management in irrigated agriculture. In order to determine the least limiting water range, soil water retention curves and soil resistance to penetration were determined from undisturbed soil samples. The sequential water balance and the reference, crop and real evapotranspiration were determined for a soybean crop season. Soil aeration was the least limiting water range upper limit for soils with bulk density greater than 1.33 Mg m-3, whereas soil resistance to penetration was the lower limit for bulk density higher than 1.43 Mg m-3. The bulk density of the soil studied was 1.35 Mg m-3, indicating 0.37 m3 m-3 of water availability, based on the least limiting water range, which is sufficient to supply the crop evapotranspiration. Irrigation management based on the least limiting water range is more efficient and complete than that based only on available water.摘要 以实现最优生产为目标的灌溉管理，以往仅以水分因子作为依据。然而，除土壤水势外，土壤穿透阻力（soil penetration resistance）、土壤氧气扩散速率等因素同样会影响植物生长并干扰水分吸收，即便土壤湿度处于有效水分范围之内。本研究旨在量化最小限制水分范围（least limiting water range），并阐明其在灌溉农业水土管理中的应用潜力。为确定最小限制水分范围，研究通过原状土样（undisturbed soil samples）测定了土壤水分特征曲线（soil water retention curves）与土壤穿透阻力。本研究针对大豆生育期开展了序列水量平衡计算，并确定了参考蒸散量、作物蒸散量与实际蒸散量。对于容重（bulk density）大于1.33 Mg·m⁻³的土壤，土壤通气性为最小限制水分范围的上限；而当容重超过1.43 Mg·m⁻³时，土壤穿透阻力则为其下限。供试土壤容重为1.35 Mg·m⁻³，基于最小限制水分范围计算得出其有效水分含量为0.37 m³·m⁻³，足以满足作物蒸散耗水需求。相较于仅以有效水分为依据的灌溉管理方案，基于最小限制水分范围的灌溉管理更为高效与全面。