i live in central Florida and in the back yard is sandy soil , i need grass to grow . So i tilled the yard and got many bags of cow manure and compost to add organics to the soil even a little starter fertilizer. I am using centerpide seed , because in this kind of sandy it is a very tough grass like Augustine.
So how should i lay all this down ?
Should i just lay the seed first and spread all the organics on top ? OR - just mix everything together lay it down, and mix it into the soil ? Or can you suggest a better way ?

Grass may not be your best option. What are your goals and constraints for your backyard?

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Spread the organics out, till them in, spread grass seed and fertilizer, spread a top-dressing (small-particle compost), roll to press the seed down and water well.