Use of bimodal hydraulic property relationships to characterize soil physical quality


Reynolds, W.D. (2017). Use of bimodal hydraulic property relationships to characterize soil physical quality. Geoderma, [online] 294 38-49.

Plain language summary

The physical quality of a soil is determined largely by its ability to store and transmit water and air – a good quality soil can store large quantities of water and air required for crop growth, rapidly drain excess water away from crop roots, and minimize both leaching losses of crop nutrients and emissions of soil-derived greenhouse gases. The storage and transmission of crop-available soil water and air are controlled by the soil’s hydraulic properties, namely its water retention curve (WRC) and hydraulic conductivity function (HCF). Hence, a soil’s physical quality is determined largely by the soil’s hydraulic properties.

Most agricultural soils contain a “structure domain” of large pores (e.g. abandoned root channels, worm holes, cracks, etc.), and a “matrix domain” of small pores (e.g. voids between individual soil grains and granules). The two domains have separate and highly distinctive WRC and HCF relationships, which give the soil an overall “bimodal” or “two-peaked” hydraulic property signature - one peak resulting from the soil structure domain and one peak resulting from the soil matrix domain.

The purpose of this study was to develop and test a theoretical framework for using measured bimodal WRC and HCF relationships to characterize the physical quality of agricultural soils. It was found, using several soil materials, that the structure and matrix domains can have differing abilities to store and transmit water and air, and thereby different physical qualities. The structure domain can be susceptible to nutrient leaching as well as limited in its ability to store crop-available water, while the matrix domain can be prone to greenhouse gas generation and limited in its ability to store crop-available air. As a result, enhancing the physical quality of any particular soil (and thereby its economic and environmental performance) may require selective or targeted improvements to the WRC and HCF relationships of the soil’s structure and/or matrix domains.


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.

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