While not tested as such, a gelling agent may be added to this solution to form a solid medium. As this is a prepared solution, the agar would be added directly to the medium.
Please see the link below, step 7, to review the general phycology media preparation protocol: https://www.sigmaaldrich.com/US/en/technical-documents/protocol/cell-culture-and-cell-culture-analysis/plant-tissue-culture/phycology-media