St. Augustine advice needed.

Hey guys,
I haven't posted in a while, but I could use some advice.
I moved into my new house a couple years ago, and sodded the backyard with St. Augustine myself. Before I layed it, I rented a tiller and tilled the whole thing, and pulled any weeds from the soil. The builder had put a layer of topsoil already, so it was decent dirt.
First year - the grass was beautiful and lush. Winter came and it browned as usual. The next spring it took a while for it to green up, and we had patches that never did. So we raked out the dead spots and used some organic food and it filled in eventually.
This year, the grass has started to green up, sort of. But the problem is that it is INFESTED with weeds. Neither of our neighbors put grass in their yards, and we have a wild green belt behind us.
I've got crabgrass everywhere, and another type of grass, thin bladed, long Y shaped seed heads that grows at least 5 times faster than the St. Augustine. It's too small and spread out to pull by hand.
The St. Augustine is the patchiest it has been yet.

I've tried to keep my garden organic, so I have only used natural fertilizers. We water it regularly. The same grass in the front yard is the envy of the block.
I don't know what I'm doing wrong, and how to fix it. I surely can't afford to tear it up and start over.
Any advice would be hugely appreciated.
-James

I don't think you're doing anything wrong, the problem is the wild area behind your house--it's basically a never-ending supply of weed seeds (I can sympathize since I have an area like that behind my house too) It won't help with the weeds that are there now, but if you can get rid of them this year by hand-pulling then next year you could try putting down a pre-emergent herbicide (corn gluten is an organic one) and that should help with the weeds (if most of them are annual weeds, the pre-emergent should take care of most of them if you apply at the right time and follow the directions for how often you should reapply). However, if the St. Augustine is so thin that you need to reseed, then it would prevent grass seed from germinating too.

For the weeds that are there now, there are weed killers made especially for lawns but as far as I know none of them are organic (the only organic weed killers I can think of are broad-spectrum herbicides that will kill everything). So if you want to stay organic, I think you're looking at hand pulling. Either that, or if you know most of them are annual weeds, if you can mow them down to minimize the ones that go to seed, and then use the pre-emergent they shouldn't come back next year.

The lawn weed killers will get rid of broadleaf weeds and crabgrass, but they probably won't take care of the thin bladed grass with y-shaped seedheads (which sounds to me like it could be Bermuda grass). Pre-emergent won't help control the Bermuda either since it spreads by runners too. I have no idea how to control Bermuda when it's mixed in with your lawn besides trying to pull it up by hand--there are herbicides that just kill grass so if you have Bermuda in your garden beds you can use those, but when it's mixed in with your lawn there's nothing that will specifically control Bermuda without also killing your St. Augustine.