Posted: 17/02/2016 at 15:03
This question arises from time to time on the sweet pea thread.
Basically, all seeds need a compost with a low nutrient content (as all seed compost have)......this is because richer composts would result in quick, lush growth that would be susceptible to pests and disease. The idea is to germinate your seed in low nutrient seed compost, before potting-on using slightly richer compost, commensurate with the plants development, until they are ready for their final quarters.
I think what you understand as 'good quality all purpose soil' is in fact just sifted multipurpose compost.......soil and compost are quite different.
I would just add that sweet peas and other largish seed do not need a covering of sifted compost.....just smaller seed such as lettuce.
Hope that clarifies matters slightly.