Seeds contain stored nutrients and will need few additional nutrients to germinate and start to grow , so compost straight from the bin or wormery will be too rich to be used as a seed compost. It will also be of the wrong texture
and may not provide adequate drainage.
While Leafmould has a low nutrient level and could be used as initial seed compost the seedlings would need to be pricked out and transplanted almost immediately after they have germinated to provide
sufficient nutrients for future growth.
It is therefore better to make a seed compost containing just sufficient nutrients to provide for germination and initial growth. Then, as the seedlings grow, and need more nutrients transplant
them into larger pot containing a potting compost with a richer mix of nutrients. Potting onto larger pots, may be required several times, increasing the size of the pot gradually, when the roots nearly fill the existing pot.
addition to providing nutrients the compost should provide drainage, so that the soil does not become waterlogged and cause the seedling to rot, while retaining sufficient moisture for the plant to grow. It will need to contain air spaces so that soil microbes
and the roots will have oxygen while being of an even consistency free from lumps. This favours a light or fine textured mix but it must be sufficiently firm to retain and support the seedling as it grows to a size suitable for potting on. It should
also retain its volume in the pot and be free from pest and disease.
Not much to ask.