Describe the feature you have in mind.

Allowing Liquid Fertilizer to waterlog blocks such as stairs/fences/slabs.

Does this solve a particular problem?

It would be just a nice small detail. I find the texture quite nice and allowing it to waterlog blocks increases its potential for aesthetic industrial agriculture building.

This would be an incredible effort, as the entire minecraft codebase is coded in a way which only allows water to be used when waterlogging blocks.
Nice suggestion but sadly way out of scope