First and foremost, I'm not trying to nitpick - I just really like dirt. It's embarrassing kinda.

Secondly, being a good soil scientist doesn't necessarily mean you have a good understanding of plant requirements. Plants are their own can of worms, and their needs vary by species.

Third, the soil carbon and fertility cycle is a terrifying place. There are so many facets of organic matter in the soil it will make your head spin. In some cases OM can be physically protected by other, more recalcitrant OM.

Building up OM can yield more humus and more cation exchange capacity, which does play a role in fertility for secondary and micronutrients.

I really don't know much about fungi holding onto minerals, but in general, the organic nutrients (like P tied up in ATP) only get released when organisms die, so in that capacity fungi and other biota play a buffering role