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ARS Home » Midwest Area » Wooster, Ohio » Application Technology Research » Research » Research Project #438305

Research Project: Improving Methods and Models to Characterize Hydraulic Performance of Soilless Substrates

Location: Application Technology Research

Project Number: 5082-21000-001-29-S
Project Type: Non-Assistance Cooperative Agreement

Start Date: Jul 1, 2020
End Date: Jun 30, 2021

Objective:
Develop enhanced techniques for characterizing physical and hydraulic properties of soilless substrates for containerized nursery production, specifically to: 1) Characterize temporal evolution in substrate physical and hydraulic properties using a combination of experimentation and modeling; and 2) Analyze and model the role of preferential flow and non-uniform water retention in soilless substrates.

Approach:
Two soilless substrate types will be utilized: a 9:1 pine (Pinus taedea L.) bark:sand blend, and a 3:1 Sphagnum peat moss:perlite mix. A layered system will also be included, with the 9:1 pine substrate on top of the 3:1 peat moss:perlite mix. The substrates will be hand-packed into 40 cm tall by 20 cm diameter PVC columns to a consistent and representative bulk density. The layered substrates will each be packed to equal thicknesses (20 cm each),with four replicates per substrate. In the initial study the focus will be on substrate evolution in the absence of growing plants. The bottom of the columns will be sealed by a rubber cap that includes a drain line for removing any water that ponds along the lower surface. The top surface will be left uncovered to permit evaporation. Once per week substrates will be wetted to container capacity by hand-watering from above. Column weights will be recorded before and after watering using a field scale, and the amounts of added and drained water to/from each column will also be measured. The experiment will be repeated for twelve weeks. Each week, prior to water application, a tension infiltrometer will be used to measure surface infiltration rates under three pressure heads (h = -20 cm, -10 cm, and – 2 cm). The tension infiltrometer includes a 20-cm diameter supply disk, which by having the same diameter as the columns will constrain water to infiltrate as one-dimensional vertical flow. Moving to progressively higher pressure heads will activate larger pores and permit estimation of unsaturated hydraulic conductivity and water retention parameters. If necessary, we fine-textured sand will be used to hydraulically connect the infiltrometer and substrates. Should the substrates become water repellent, infiltration tests will also be conducted using positive (ponded) surface pressure heads. Volumetric samples will be collected using 5-cm diameter by 5-cm tall stainless steel rings. Samples will be collected at the beginning of the experiment (using separately packed substrates) and at the end of the experiment (from the packed columns). The final samples will be collected every ~10 cm in the vertical direction (2 replicates per depth = 10 samples per column). Samples will be saturated and measured for saturated hydraulic conductivity and water retention. At each water content, water repellency will be estimated from contact angles. Data used to build and parameterize a commercially-available one-dimensional model for simulating water, heat, and solute movement in variably saturated media, and if necessary, the corresponding 3D model. Depending on the observed infiltration patterns, single versus dual-permeability models will be tested. This comparison will reveal possible tradeoffs between greater model complexity and performance.