Yes, burying fertilizer traps biomass CO2 and then they can use that as carbon credit equivalent to claim CO² neutrality.
Of course, there’s a reason why fertilizer is an inexpensive source of fixated carbon biomass and this means all fertilizer will increase in price by the amount value of it’s CO2 carbon credit equivalent

Then maybe the buried fertilizer will become so valuable that it can be dug out and sold as fertilizer again.

I don’t see any problems with this plan !