Representing air as <i>imaginary water</i> : Analysis of soil water and soil aeration corequisites for plant growth

Abstract Plant‐available water and adequate soil aeration are two fundamental requirements for successful plant growth. These prerequisites have generally been assessed independently in relation to plant growth, with limited focus on their complementary and competing behavior in a soil–water matrix. In this study we introduce a corequisite index adopted from a complex number representation by linking soil–water (the real component) and soil–air (the imaginary component) as the orthogonal counterparts in an Argand diagram. The new corequisite index constitutes a soil–water component defined based on field capacity and permanent wilting point, and a soil–air component defined based on critical soil–gas diffusivity. To calibrate model parameters, the soil–water characteristics were measured in vadose soil profiles (0‐ to 60‐cm depth) from 48 replicate sites. Results revealed that the corequisite index, with its magnitude (0.5–1) and corequisite angle (0–30°) in the given range, provided the best combined soil water and aeration status for the selected soil. The majority of the selected soils were affected by insufficient aeration (gas diffusivity &lt; 0.01) when at field capacity (drained to −10 kPa), requiring the soils to drain further (−50 to −100 kPa) to satisfy the corequisites. The derived soil aeration parameters showed promising relationships with measurable soil physical properties. We further recommend adopting a 15% volumetric soil air content as a general threshold for minimum soil aeration in the absence of measured soil–gas diffusivity data.