You can go either way but you'll nave better luck with the mix. There are some minerals in soil that are not present in the humus alone but more important is that the soil will retain water for a much longer time than humus alone. Humus can also compact at the top and water will just run off instead of soaking through. And as the humus rots it will lose volume so your planting medium will shrink. A good mix would be about 3/4 soil and 1/4 humus. If you have some compost, that is humus that has completly broken down, replace up to a 1/4 of the soil with it. Be sure and MIX it, don't put it in layers or you'll get stratification, a layer may form that blocks water and roots. As time goes on the humus in your beds will break on down and you can add more of it each year along with some more compost. The best thing you can do with the humus is just pile it up somewhere out of the way and let it compost. No real work envolved in composting, every spring I get 2 to 4 wheel barrows from last falls leaves I have just piled up in a corner of the chain link fence. Half rotted humus makes a good mulch if it is fine enuff to let water through, just pile it a couple of inches high all around your vegies once they get growing. Just a warning, don't use pressure treated lumber to build your retaining walls, the anti-mold chemicals are also anti-plant and will leach into the soil.If you are gonna grow in just humus, don't forget the chick peas. Sorry, I can't give you any info on growing pita bread.

Notmrjohn mentioned something very important. Treated lumber is very toxic and contains arsenic. These old railroad ties are terrible. Older playground equipment made of treated lumber is probably the toxic type. In two years, this treated lumber will be phazed out because of its toxicity. Next to a vegetable garden, it could be bad news. Actually, there are some rock powders and soil amendments which would be great for your garden. Top soil is not always great. In fact, it often is "played out and no longer fertile" farmland. The farmer gets more for the dirt than he could for anything which might happen to grow. Compost is always the top priority. You can bring in lots (usually about $28 a cubic yard). Build about 8 inches or so of compost. If you can blend this in with a tiller with the soil there and amendments, it will help set you up. Use molasses to kick off the microbe activity.