Theory :
Your sunshine mix is an inert medium, not a living soil. To best utilize dry amendments, especially stuff like Dr Earth that’s made stuff like feather meal and kelp instead of chelated nitrates and phosphates, you really need a living microbiome to do that work of breaking it down for the plant. Something like sunshine mix doesn’t have a bunch of organic material, inoculants, or colonized micro life in it to help do that work. The plant itself can’t break down organic material, and it can’t eat anything unless it’s “ready to eat” and freely available. We either use chemically-chelated synthetic nutrients that are readily available, or build a living soil and have the microbes break down more complex organic foods for the plant over time. It’s generally recommended to add something like compost or worm castings to a ”plain“ mix if you want to feed the organic route. This supplies that early boost in micro life and adds a buffer of organic material. Its also advised to let that soil ”cook” - letting the microbes feast and break down all that food before planting. A good colony of micro-life will also be able to eat through additional organic feedings like topdressings, and the microbes will self-adjust the ph into their desired range.