Availability of resources from composted materials is exceedingly slow--what's going to be quickly available has already become available and either washed out or bound up.

I'd consider this an excellent long-term solution for maintenance of P and K in the soil, but not a good one if you require P for seeding (I'm not sure of your soil test P level, offhand).

So if you do require it, I'd add synthetic P as well. I almost always recommend balancing the soil initially with synthetics and then taking over maintenance with organics. With all the P I added last year organically, I counted none of it. Which is good, as it didn't show on my soil test--but the synthetic did.