More pics...

i would get a soil test ordered from Logan Labs right away. best $25 you will ever spend. post results here and get detailed responses on any deficiencies. a healthy, thick lawn is your best defense against weeds. that said, you need to knock them down. i found herbicides at a local farm/garden place that sells concentrated herbicide for a lot cheaper than buy the stuff from home depot etc. Since you have st augustine, atrizine is what you need. i can buy the highly concentrated stuff fro about $20 a quart here, you just have to look around. you need to use a surfactant and time the application for it to be most effective. mow your grass, wait at least 2 days and then apply during mid morning on a sunny day..ideally it wont rain for 2 days before or after. and no mowing for at least 2 days after application.

Here is a pretty good guide to St. Augustine

http://publications.tamu.edu/TURF_LANDSCAPE/PUB_turf_Maintaining%20St.%20Augustine%20Grass%20Lawns.pdf

I agree with Dan that atrazine is a great herbicide to use with St. Augustine. There are temperature restrictions (I believe 90 degrees) so make sure the temps are not too hot when you spray. Use a quality sprayer and surfactant and make sure it won't rain (or irrigate) for 2-3 days for best results. Follow all directions carefully, it can harm ornamentals and trees if not used carefully. It takes a couple of weeks to work so don't keep spraying if the weeds don't instantly die. Atrazine will take care of most of the broadleaf weeds. It won't take care of crabgrass or nutsedge. If you have any of those I have other specialty herbicides I can recommend.

Here is a cheap atrazine I have used that is found in big box stores.

https://www.amazon.com/Spectracide-HG-95684-Augustine-Centipede-Concentrate/dp/B0025U4GGC

I don't use the rts container, but put it in a sprayer and use 4.3 oz per gallon to treat 500 sq ft.

Going forward you can prevent most weeds from popping up by using a pre-emergent herbicide. I like prodiamine. It seems expensive, but it will last for years and price per app is very cheap.

https://www.amazon.com/Prodiamine-Generic-Barricade-5lbs-ali8056/dp/B004GTQBEK/ref=sr_1_1?s=lawn-garden&ie=UTF8&qid=1465829496&sr=1-1&keywords=prodiamine 

Once the weeds have died, you need to reassess how much lawn you have left. You may need to add a few squares of sod in areas that are totally bare. If it is just thin in spots, not completely bare, then fertilizer + time is all you need to get the St. Augustine to spread. It may take a whole season or more to fill in if it is very thin.

Fertilizer 3-4 times a year with slow release chemical fertilizer, or 3-5 times with organic fertilizer. If you are interested in organic options, do some searches on the forum. There is a lot of information out there. I like Milorganite, soybean meal and alfalfa pellets.

Here's an easy way to apply a weed killer to St Augustine.

St Aug will spread about 2 inches per week when temps are in the 70s and low 80s. If you have any patience, just let it spread.

Mow at the mower's highest setting every week or two. Stretching out the mowing cycle puts more shade on the other plants (weeds) and will help shade them out. Most broadleaf weeds will shade out the St Aug but the St Aug will shade out most grassy weeds.

Water deeply once a week in the summer to keep most weeds. Even nutsedge will go away when you do this. More on watering below.

Fertilize in May, September, and November if you're using chemical fertilizers. You can fertilize any time with organics like Milorganite, soybean meal, alfalfa pellets (rabbit chow), corn meal, or corn gluten meal. Application rate for these feed type fertilizers is 15 pounds per 1,000 square feet. You can apply as often as your budget allows - seriously. You can apply every week if you want to without any fear of hurting anything. But if you're using Scott's or some other chemical fert, stick to the 3x per year avoiding early spring and summer.

Watering: Deep and infrequent is the mantra for watering. This is for all turf grass all over the place. Deep means 1 inch all at one time. Put some cat food or tuna cans around the yard, and time how long it takes your sprinkler(s) to fill all the cans. Memorize that time. That will be the time you water from now on. My hose, sprinkler and water pressure takes 8 full hours to fill the cans. Your time will likely be less. I like gentle watering. As for watering frequency, that depends on the daytime air temperature. With temps in the 90s, deep water once per week. With temps in the 80s, deep water once every 2 weeks. With temps in the 70s, deep water once every 3 weeks. With temps below 70, deep water once a month. Note that you have to keep up with quickly changing temps in the spring and fall. This deep and infrequent schedule works in Phoenix and in Vermont, so it should work for you. The reason for deep and infrequent is to grow deeper, more drought resistant roots and to allow the soil to dry completely at the surface for several days before watering again. If it rains, reset your calendar to account for the rainfall.

Great info, thanks All. I have sent off my soil samples to Logan's Labs. Results here.

I guess that is crab grass in there as well. What can I add to the atrazine to treat this but leave my desired St. Augustine grass?

Unfortunately, I will need to wait until it cools down in a month or two to perform the atrazine treatment. I will definitely be picking some up.

Thanks again!

Crabgrass will die at the first frost. Unfortunately that is about
when the St Aug goes dormant, so no gain there. You can kill it easily
in St Aug (but no other grass) using baking soda. Here's how:

1. Crabgrass is hydrophobic so this method needs a "sticker" to make it work. Spray the crabgrass with a mix of water (32 ounce sprayer) with a teaspoon of shampoo and one of molasses. Spray and see if the water beads up or sticks to the crabgrass. If it sticks, you're good.
2. Put some baking soda into a stocking or sock. This does not ruin the stocking or sock.

3. Hover the sock over the wet crabgrass and squeeze (or pat, or beat) the
   sock to release the baking soda. The sock is a very fine sieve so only
   the fine particles of baking soda are released. Don't be shy with the
   baking soda. You don't need to cover the soil, but you do want a pretty
   good dusting on the weed. This also works on orchard grass. I may
   work on other grasses, but St Augustine is not bothered much by other
   grassy weeds, so it hasn't been tested. Within a few hours you should
   see the crabgrass wilting. Within a day or two it should be completely
   black and dead. If you do this now then the St Aug will spread to fill
   when the temps fall back into the lower 80s in Sept or Oct.

Don't
do this on a windy day and don't inhale the baking soda. Oh and
don't get any soda dust in your eyes. It burns like acid. If this
happens I don't have to tell you to rinse your eyes out. You'll be
running for the hose. Ask me how I know ;-)

It's often too hot this late in summer for chemical herbicides. They will burn up your lawn. Sometimes even the ones that claim not to actually do so. So be careful with the directions.

If you have large areas of weeds with no desirable grass, you can try 20% vinegar. I paid about $25 for a gallon. Put it in a sprayer with 1T of dish soap and 2T of molasses. I found this recipe on the forums. The best part is that it works best in full sun.

This will help clear areas that are weed only and keep the weeds from spreading.

I took some photos of some smooth crabgrass I sprayed.

July 31, 2016

1 hour later

Another hour later

August 1, 130pm, just before a second spritz

August 2, 530 pm

August 3, 700pm

I will be doing the baking soda method at some point for various kinds of weeds growing low through my St. Augustine.

Here is a video dchall posted on another forum. pferrell314 shows the results of the baking soda on crab grass and how it stresses but does not kill the St. Augustine.