Use of bimodal hydraulic property relationships to characterize soil physical quality

- Bimodal soils have a large-pore structure domain and a small-pore matrix domain.
- van Genuchten soil hydraulic functions (θ-K-h) can be made bimodal.
- Bimodal θ-K-h functions can describe the physical quality (SPQ) of bimodal soils.
- Structure and matrix domain SPQ can differ substantially from bulk soil SPQ.
- Optimizing overall SPQ may require targeted alteration of structure or matrix SPQ.

Soil hydraulic properties have a predominating impact on soil physical quality (SPQ) because they directly or indirectly control air and water storage, infiltration and drainage, nutrient leaching, microbial activity, greenhouse gas generation, and carbon sequestration. The hydraulic properties of many soils are often better described using “bimodal” water content and hydraulic conductivity (θ-K-h) functions, where the θ-K-h of a large-pore “structure domain” is combined with the θ-K-h of a small-pore “matrix domain”. This study uses closed-form bimodal van Genuchten θ-K-h functions to characterize SPQ from the perspective of storage and transmission of water and air in soils containing distinct structure and matrix domains. Consistently good fits were achieved between the soil water content function, θ(h), and water content data from intact soil, repacked diatomite pellets, and repacked soil aggregates (R2 ≥ 0.9854, RMSE ≤ 0.0223 m3 m− 3), but variable fits were attained between the hydraulic conductivity function, K(h), and hydraulic conductivity data. It was found that even though the SPQ of bulk soil may be optimal or near-optimal, the SPQ of the corresponding structure and matrix domains could be limited or poor in one or more categories. The structure domain tended to be water-limited and potentially prone to leaching, while the matrix domain tended to be aeration-limited and potentially prone to greenhouse gas generation. It was concluded that maximizing the economic and environmental performance of field crop production would likely require selective improvement of structure or matrix SPQ, rather than bulk soil SPQ.