Solubility: The Basics
Something is soluble if it dissolves in a particular liquid: picture stirring sugar into hot tea (soluble) versus cold water (not). Plant compounds that we use for medicine have to dissolve in the water/alcohol/honey/oil/etc. for medicine to be a thing.
All plants contain both water-soluble and alcohol-soluble constituents. The question is how the medicinal compounds that you want extract best. The plants have a preference for the ideal balance to get the most well rounded medicine; make water-preferential or alcohol-preferential medicine based on what results you want (i.e. burdock tincture versus burdock vinegar).
Most plant medicine is best with a balance of both: dandelion, nettles, holy basil. Some really extract best in one or the other: slippery elm, marshmallow, echinacea. Remember that vinegar extracts water-soluble compounds (and in the case of minerals, is better than water at it) and oil extracts alcohol-soluble compounds.
How to Tell
Intuition, observation, and practice!
Where it’s growing, how it’s growing, when it flourishes: intuition from experience. Does is grow on the banks of a river? On a wind-swept prairie? Does it bloom only when it’s very hot and sunny?
What worked last time? How does that season compare to this one? Is there a drought, or a late snow storm, or did the river jump its banks last fall and deposit fertile silt in your garden?
Taste, smell, & feel
Mucilage, berberine, tannin, resin: plants often directly tell you how to make medicine with them. Gooey demulcent mucilage is water-soluble, but sharp tingling berberine is alcohol-soluble.
The goop at the bottom of a tincture is the plant matter that didn’t dissolve in the tincture. Think about the goop at the bottom of a burdock or elecampane tincture–that’s the stuff that can’t dissolve in alcohol. Actually, it’s mostly fiber, which does for plants what bones and fat do for people (structure & food storage). Burdock and elecampane are both examples of herbs that tincture best at lower alcohol concentrations so their water-soluble compounds can play too. If there’s way more precipitate than there should be, it often means your tincture had too much or too little alcohol in it. That said, most high alcohol tinctures, especially of roots, will have precipitate, since plant matter always contains water-soluble constituents (see up, second paragraph).
Look it up in a good herbal reference
I recommend the works of Michael Moore and Lisa Ganora
Energetics of Solubility
These are generalizations meant to give you a place to start, not definitive truths for all plants. I’m developing a graphic for this but it’s not ready for the internet yet!
Plants that have very water-soluble medicine are often extremes on the wet-dry continuum:
Marshmallow & slippery elm; willow & witch hazel
Plants that do well with high alcohol tinctures are usually more in the middle of wet-dry, but on an extreme of hot or cold:
Echinacea & cayenne; california poppy & skullcap
Remember, most plants make good medicine in both water- and alcohol-based preparations, but the individual effects still follow these basic guidelines. For example:
Burdock tea: water-soluble medicine, benefits are dramatic on the wet-dry continuum
Burdock tincture: alcohol-soluble medicine, benefits are dramatic on the hot-cold continuum