Can you post a couple pictures? The best time to photograph a lawn is when a cloud goes overhead. It would be good to see from far enough away to see the trees and then a close up of the dry spot in the middle. Did you have Marathon in Calabasas before? If not where? And what was your watering regimen (how often and for how long)? Can we assume you are watering this lawn the same way? When were the last two times you fertilized, or have you fertilized yet? Hopefully your close up picture will help someone here to identify your current grass type. Different grasses can be eradicated by different means. Sometimes it's as simple as stopping watering. I'm not sure you will ever be able to apply enough water for those trees. That is a lot of timber to keep moist. The tool which professional landscapers use to prep a garden is a box blade pulled behind a tractor. This is what it looks like in action. The ripper bars you see on the box blade (like a rake sitting above the soil in the blade) can be used to remove most small roots, but there is a limit. When you get ready to get rid of the trees, you might want to consider major stump removal, not grinding, and root removal throughout the rest of the yard. For those two jobs they might bring in a small bulldozer or other heavy equipment.

I will gladly come down and help you establish a lawn if I can stay at your house free of charge!!!! I don't need much space, but you will have to feed me :). The first thing you need to consider is water. Establishing a lawn, from either seed or sod, will require a lot of fresh water, especially with your sandy soil and hot windy climate. Will this be a problem for you? If it is a problem, or too expensive, then it will make a lawn somewhat difficult. It might be a good idea to become friends with a local golf course superintendent, as they will have plenty of information as well as sources of things like fertilzers and amendments. In terms of what type of grass I think you should consider paspalum. Paspalum is a salt tolerant grass that spreads well and does best when cut short, like a golf course fairway. It can be grown from seed or sod. Are there any sod farms on your island? If not, sod might be very expensive. Again, a golf course superintendent might be your best source. When establishing a lawn, the best results generally happen when you start from scratch. This requires killing whatever grass/weeds you have with a herbicide, and seeding a few weeks later. P.S. Your English is very good, much better than at least one member on this forum.

Sometimes you need to know the exact weed before you can kill it. In this case, whatever it is, it should be treatable with a broadleaf weed killer like Ortho Weed-B-Gone. WBG will leave your grass alone and only kill the broadleaf plants. Watch your overspray because most of the plants in your yard are broadleaf. You only need to mist the leaves of the weeds. WBG is not a soil drench. Some weeds take a little time to die. The Weed-B-Gone works by hyperactivating the growth hormone in the plant. It dies by exhausting it's energy reserves. So it will look healthy for several days...and then it's dead. Unfortunately it is too late to seed for this year in your area. Once the weeds are gone, you will be left with bare soil which will attract more weeds. This is especially true if you seed in the spring. You might try dormant seeding to help 'densify' the turf. With dormant seeding you seed in the winter hoping that the seed will sprout early before the weed seeds are sprouting.

Everything you read about watering 3x per day is very consistent. It has always been my standing suggestion, but I see dogwind's point. This late in the season you can probably get away with less water. You might go to once a day and see how the soil dries out. What you don't want is for the interface between the sod and soil to dry out. You can put corn meal down on the same day as the grass if you want. John Dromgoole's nursery has corn meal. Be sure you get corn meal and not corn gluten meal. He has both. CGM is just a lot more expensive and is not known for helping with fungal issues. Sorry I don't know his nursery's name but if you Google his name it will come up. Also use Google Maps to find feed stores near you. Then call them all to find out what they have. I apply corn meal by hand like I was feeding chickens. It does not go through a spreader very well. If I had an old plastic colander (thrift store??) with big enough holes I would use that to shake it out a little more evenly. I just did that with corn gluten meal a few minutes ago and it worked well. CGM is finer ground than corn meal and went through the colander very well. Mow when the grass is tall enough to mow. At this time of year it might not get tall enough to mow until April. You don't ever have to aerate the lawn. If you water correctly the soil will be very soft after you water and become very hard before you water again. In that sense it is just like a sponge (hard when dry and soft when wet). Your stump area will be a problem for at least 3 years. The ground up wood will suck up nitrogen causing the areas to be yellow. Furthermore the one you back filled with compost mix will settle for years requiring you to level it. You can add sand any time but only 1/4 inch at a time. Search the Internet for the "Organic Lawn Care FAQ." It is all over the place including here at the Organic Gardening forum FAQs. You will probably also find some books on organic lawn care. I don't like them. They are all based on the Rodale philosophy that compost is all you need. Not true. You need real food like pounds and pounds of corn, soy, wheat, cottonseed, used coffee grounds, etc. to feed the soil microbes. The minimum application rate for ground grains is 10 pounds per 1,000 square feet. The max I've seen used without issue is 80 pounds per 1,000. Beyond that it starts to smell or smother the underlying grass. Organic lawn care should NOT smell bad. I fertilize on the federal holidays because I can remember them. With organics you can fertilize any day or every day. I picked those and fertilize relatively lightly from 10 to 15 pounds per 1,000. I know (crazy) people on other forums who fertilize up to 200 pounds per month. Their lawns look amazing. I start on Washington's Birthday, then Memorial Day, 4th of July, Labor Day, and Thanksgiving. If you fertilize now, do it again on Thanksgiving. Being in Austin you are a little cooler so you could probably wait a week or so after Washington's birthday but why not just do it on the days that are easy to remember. Anything else? After you get off the initial watering plan, then water deeply but infrequently. I water for 3-5 hours all at one time. This time of year I water once every 2 or 3 weeks. In the hottest heat of summer I water once a week but I might water longer depending on the humidity, wind, shade, soil conditions, etc. Deep and infrequent watering is the single most important thing anyone can do for their lawn. Mowing high (for St Augustine) is the second most important. Fertilizing is the third.