Humus

Meomye

What exactly do I need to purchase if a plant label reads 'humus rich soil preferred' and I intend to plant in a container?  

Borderline

I would use a loam based soil if it's a longterm plant like shrubs. John Innes No2 mixed with one third multi-purpose compost and continue to top dress with compost. I think it's important to always incorporate grit into the mix. The soil can be very heavy and easily compacted by rain and watering.