Bryant: I read on German websites and webshops that sell sprouting soils that these soils are less fertile than normal garden soil! Just the other way round! And there are also instructions on other DIY websites, how to make those soils and it's instructed hat you take plain garden soil from the garden center (not additionally fertilized) or your own garden soil and mix it with sand so that you'll create "poor" soil for your seeds to sprout in! I am suprised to read that yours is so very different then...?
So in fact we are now talking about two topics here:
1. Why don't masses of weeds on top of a well fertilized mulch system, with plenty of access to daylight, come up like crazy?
2. Is it perhaps because the over fertilized "sprouting soil" - meaning here the top layer of this well fertilized mulch system - is too strong for the young seedlings? If this is the case, is that why sprouting soils need to be weaker, poorer than normal garden soil? What makes sprouting soil so different and better for sprouts and seedlings than normal garden soil anyway?