Since this subject has arisen several times. heres an explanation regarding WHY too many nutrients in a compost kill seedlings.
Take the case of nitrogen. The process by which plants take up nitrogen is called Nitrogen Fixation.
Plants that contribute to nitrogen fixation include the legume family – Fabaceae – with taxa such as clover, soybeans, alfalfa, lupines and peanuts. They contain symbiotic bacteria called Rhizobia within nodules in their root systems, producing nitrogen compounds that help the plant to grow and compete with other plants. When the plant dies, the fixed nitrogen is released, making it available to other plants and this helps to fertilize the soil
The initial chemical process goes N2 + 6 H+ + 6 e− → 2 NH3
i.e. 2 nitrogen atoms plus six positively charged Hydrogen ions plus 6 electrons give 2 molecules of ammonium radical.
The electrons come from the plant - in effect a electric current flows between the plant and the surrounding soil/compost, in exactly the same way an electric current flows between the plates of a lead acid battery. This is why the basic test of the nutrient levels in a compost is the electrical conductivity. And like any other electrical circuit, its possible to cause a current flow so high it 'melts' the wiring or blows the fuse, ie destroys the fixing bacteria on the root nodules. This is what happens when the nutrient levels are so high the electron flow becomes too energetic and destructive. Once the nitrogen fixation system has been destroyed by the current flow, the plant is doomed.