Answer: When I first read your question, I assumed that you meant planting directly into a large compost pile. But then I thought perhaps you meant backfilling planting holes with compost and putting the plants in. I'll try to answer both of those possibilities, and if I missed the point, please write back with more detail.
Depending on the ingredients that went into the compost, the NPK (nitrogen, phosphorous, potassium) levels can vary considerably from about .5-.5-.5 to 4-4-4 and pH levels could be acidic or alkaline. If you had plenty of variety in the compost ingredients, you might get away with planting directly in it. But although compost is a fantastic soil amendment, it's usually lacking in some nutrients or minerals that are found in soil, which could show up as deficiencies in your plants.
If you used compost as a backfill, plant roots could eventually grow beyond it to reach the soil. It's an interesting question that you pose, and I did not find a definitive answer in my references, nor has anyone ever asked me this in all the compost questions I've fielded! My personal experience shows me that plants thrive in soil that is well mixed with compost. If I were you, I would be tempted to transplant into soil mixed with compost. But I would leave a few plants in the compost as an experiment to see what happens. Another possibility is to have the compost tested, just like a soil test, to see what its characteristics are. Finally, many plants develop some yellowing from transplant shock. I suspect that may be the case with your pansies since they've only been in the ground a week. But a wide range of problems show up as yellow leaves, including nitrogen and iron deficiences. I hope that I haven't created more questions than I've answered. If you leave the plants in the compost, write back and let us know how they fared. Good luck!
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