Investigation of water retention functions of artificial soil-like substrates for a range of mixing ratios of two components

Urban greening is politically fostered as an adaptation strategy to climate change. Therefore, the demand for fertile planting substrates increases. Such substrates are usually mixed from mined geogenic resources but should rather be produced from recycled materials. Furthermore, their hydraulic properties should be designed according to their application, e.g., by optimizing the mixing ratio of their components. Therefore, this study introduces an approach to investigate the water retention curves (WRC) of soil-like substrates as a function of the mixing ratio of two recycled components: exemplarily for green waste compost (GWC) and ground bricks (GB) in the fraction of sand. Seven mixing ratios for GWC and GB, 0/100, 18/82, 28/72, 37/63, 47/53, 68/32, and 100/0 have been packed to mixture-specific densities using a newly constructed packing device. The packing density resulted from applying six strokes with a constant momentum of 5.62 × 10−3 N s m−2 that was chosen according to the German green roof guideline. Thus, a standardized compaction was assured. The WRCs were measured using the simplified evaporation method in five replicates for each of the seven mixtures. A set of water retention models was parameterized and analyzed in regard to their suitability to represent the full range of binary mixtures. The newly constructed packing device enables to pack cylinders reproducibly. The densities in the cylinders for the mixtures varied from 0.64 g cm−3 (GWC/GB = 100/0) to 1.35 g cm−3 (GWC/GB = 0/100) with a coefficient of variation less than 1.3%. The simplified evaporation method delivered homogeneous results for all five replicates of the investigated mixtures. The WRC of the seven mixtures is the result of a complex combination of the pore systems of GWC and GB. The multi-modal water retention models of Peters, Durner, and Iden are principally suitable to describe soil-like substrates that are rich in organic matter. The models PDI (van Genuchten) and PDI (Fredlund–Xing) best described the WRCs for the full range of mixing ratios according to the quality criterion RMSE. The study delivers a template how to prepare and analyze soil-like substrates regarding their WRCs using the simplified evaporation method. Complemented by total porosity and measurements at pF > 4, it is a suitable method to gain high-resolution WRCs of soil-like substrates. Available water retention models are capable to describe the hydraulic behavior of binary mixtures over the full mixing ratio. Therefore, it would be possible to model the WRC of binary mixtures as a function of their mixing ratio.