Thanks for all the soil/drainage advice last night, folks. I talked to some knowledgeable folks at my local nurseries today. They suggested amending the greenhouse soil with “chunky” compost. The large organic material serves to aid drainage and has the benefit of adding nutrients. I’m still a little concerned about salt and mineral buildup from the local water supply but I think I can rig an in-line pump up to my rain barrels which should serve my watering needs about 80% of the time.