In a petri dish, solubility and stability of the drug can be significantly different.
British Dictionary definitions for solubility
noun (pl) -ties
the ability of a substance to dissolve; the quality of being soluble
a measure of this ability for a particular substance in a particular solvent, equal to the quantity of substance dissolving in a fixed quantity of solvent to form a saturated solution under specified temperature and pressure. It is expressed in grams per cubic decametre, grams per hundred grams of solvent, moles per mole, etc