What material is permeable for plant roots, but "impermeable" for fungi & bacteria?
I might have found a solution! Kuijken et al. (2015) first germinate seeds in a sterile eppendorf filled with agar and nutrients. They then cut off the bottom of the eppendorf and stick it through the lid of a sterile plastic jar and pad it with sterile cotton. So I guess they are using the agar as a material that is permeable to the roots and &quot;impermeable&quot; to bacteria and fungi.
Kuijken, R. C., Snel, J. F., Heddes, M. M., Bouwmeester, H. J., &amp; Marcelis, L. F. (2015). The importance of a sterile rhizosphere when phenotyping for root exudation. Plant and Soil, 387(1-2), 131-142.