growing medium

it depends what she wants to grow in it
how tall they are
is there connection with the soil
or are they like pots

easy question
but with a lot of possibilities

Roland

Our types of low-growing plants suitable for such a situation planted in spaces between some of the stones. She's an experienced grower. She is asking for fine-tuning recipe opinions regarding the balancing of drainage/richness/particle size options.

If planting where structural integrity of the steps themselves is important, I would use the best mix recommend for that, and plant what might might survive in that environment.  But on the flat parts, one can play with the soil mix more. However, it is standard practice never to use more than an inch of just sand under any paving materials, because sand doesn't pack and continually shifts under the pavers/paving stone.  A concept to keep in mind - the soil mix needs to be stable.  Also remember that strongly delineated soil layering is not conducive to normal root growth.  Always integrate any differing soil aggregates.   

I have a short walkway (6ft) that I placed some flat limestone rock as crazy pavement.  Really, I only did it because it was heavy clay and was often too muddy to walk through.  But I am a bit surprised at the plant survivors between these limestone rocks, knowing the really austere conditions of wet mud to dry, cracked, hot conditions it vacillates through in full sun:
I had planted:
---- various thymes
---- Veronica liwanensis
---- Veronica oltensis
---- Veronica pectinata
---- Antennaria neglecta var. gaspensis
---- Valeriana montana

Of course, the thymes were the easiest, but I was happy that Veronica oltensis, V. pectinata and Valeriana montana survived well, too.  (These, of course do not flower as well as they might, but I was elated that they survived at all.)  The Antennaria and V. liwanensis just limp along.

Thanks, Rick. However, I seem to have gotten the question wrong. You replied to what I thought was the issue, but the step-owner says:"The answer to the forum question was generalized to what media gets put under the steps.  At least that is how I read it.  I want to know what to fill up the open pockets with in order to plant my plants into the steps."
I replied that I think they should be the same, +-, to encourage roots to delve deep. I don't think it's a good idea to scoop out a shallow planting pocket of special soil for each plant.

Jeremy wrote:

I replied that I think they should be the same, +-, to encourage roots to delve deep. I don't think it's a good idea to scoop out a shallow planting pocket of special soil for each plant.

That's exactly what I was getting at when I said: ".remember that strongly delineated soil layering is not conducive to normal root growth.  Always integrate any differing soil aggregates."

Drainage is the most important thing.  Obviously that's not a problem, or else you'd have an unstable base.  A very lean mix would be my recommendation too.  Plants might grow better and faster in a richer mix and even stay within the natural growth parameters of the species, but... I don't know about you, but I hate it when plants continuously overgrow over the stepping stones and get squashed by feet, causing an ugly mess that's might be not be so easy to clean up. 

Something else I did with my walk is leave the stepping stones raised considerably above the medium - half to three-quarter inch, so plants are protected from foot travel and tend to stay in their only little area better.