Also, I'll be renting a slice seeder. If I use it over my existing lawn, does it really tear it up? Should I just use it on the side with just soil? Aerate both sides, right?

Thanks again, great ideas here.

What is the problem with the existing grass? What do you mean 'needs some attention?'

Generally what makes a lawn look uneven after repairs is using different seed for the repair from the grass you already have growing.

What kind of grass do you have?
How often and how long do you water it?
How high/low do you mow it?
When do you fertilize and with what?
Is shade an issue with either side of the lawn?

My grass is a typical New England mix of fescue, bluegrass, ryegrass, etc. I water it every other morning for 20 minutes and it gets a little more than 1" per week. I mow it at the highest possible setting, but because I'm on a slope, some spots are lower than others.
I recently changed to Milorganite using it last about a month ago. The existing lawn is in partial shade.

I've been doing a lot of reading and researching about soil lately and you're the first one to suggest NOT aerating or adding compost or peat moss. I understand that you may have a system that works for you, but I don't think that it works for everyone. Your advice seem to be left of center for someone like me looking for general suggestions. I appreciate your advice, but can you dumb it down a bit, or maybe someone else here could respond?

Thanks!

The system I use and talk about is one I learned from reading this forum since 2001 and from other forums where I both participate and/or moderate. I try to continue the lessons I have learned here for those of y'all coming along since the people I consider to be the real gurus left. A couple of them are still around. Look for TexasWeed, Billl, MorpheusPA, bpgreen, and Andy(with some numbers after his name). Some of the other old coots have been banned from these forums, so you won't see them. Newer gurus include rdaystrom, nearandwest, and a couple others I will offend because I cannot remember them right at the moment. These people bring a lot of personal experience to the forums that none of us can possibly have gathered individually. We do not always agree but we all respect each others experience and suggestions...well except for me and rdaystrom - we barely tolerate each other, snicker, snicker.

Here is what I think I see.

Filling in the low spot was an important part of any success you have had this year. Unfortunately you seeded in the spring. Spring seeding always leads to crabgrass. We tell people this every spring. I don't remember if you were here in the spring or not, but we would have said the same thing. Spring seeded grass plants have trouble during a normal summer. This summer has been horrendous. Crabgrass; however, is a heat loving grass. This is not a major surprise to those of use who read here. It is also not a reason to fret too much. The solution is not that hard.

Mowing high is another leg in your success. Tall grass makes it harder for weeds to get started.

You are watering too frequently. Established lawn grass can almost always go for a full week before needing water (as rdaystrom will be the first to tell you, this year is an exception to that rule). Now while the grass can go a week, grass seeds must be kept moist in order to germinate. That goes for weed seeds, too. Crabgrass seed needs to be kept moist for a few days to germinate. When you water every other day, that is just about perfect for crabgrass seed. If there has been no other lesson learned from the Internet grass forums it is that infrequent watering is the best method. Infrequent means watering once a month during the cool months and gradually changing to once a week in the hottest part of summer. The amount that YOU water will depend on your soil type (sand versus clay), soil condition (organic matter), grass type, grass condition, mowing height and frequency, shade from trees, shade from clouds, humidity, temperature, and wind. I realize that is a lot of factors to hit you with but there they are.

When you water less often the surface of the soil will completely dry out between watering. That dryness at the surface is what prevents weeds from germinating. Keep an eye on the grass to tell you when to water. If the grass dries up faster than a week, then water immediately and water longer the next time. Avoid runoff by stopping the instant you see runoff and let the soil soak up what water is there. Then return to water again after 15-30 minutes.

Nutsedge is a swamp grass. It indicates clearly that your soil is too moist. There are many reasons for nutgrass to get enough water, but usually it is due to poor drainage. Couple that with too much moisture and you get swampy conditions that promote the nutgrass. If you have low spots in the yard or spots where the soil does not drain well, that is where the nutgrass will start.

Weeds love to move into thin grass. Dense grass will keep the weeds out without doing much of anything else. Of the grasses you have, the Kentucky bluegrass is the only one that will become dense by itself. But it will not thrive in the shade. Shady areas are where you will only have fescue and rye growing. Fescue must be reseeded in the fall to keep it dense enough for weed control.

Why do I suggest not rototilling?
Because you can never, ever, get the rototiller to carve out a level surface at the bottom of the dust pile it creates. When you level it on top and leave the bottom uneven, as the fluffy surface settles, it settles unevenly leaving you with a bumpy ride for the lawn mower.

Why do I suggest not rototilling in compost and peat moss?
Because compost and peat moss has not decomposed yet. It is the hardest parts of the original plants that went into the compost to decompose. When you bury the compost you cut off the supply of air to the compost. It cannot decompose without nitrogen. Normally it gets it from the air but if you cut off the air, it takes it from the soil. As you apply fertilizer all the nitrogen goes to decomposing the compost. Gardens with buried compost always seem to stagnate for a year or two before really coming in. In my opinion, compost goes on top of the soil, not buried into the soil. [I like to look at the way Mother Nature gardens. She has no mechanism for burying compost. Her method of planting seeds is to let cattle, deer, goats, and other grazing or browsing animals to knock the seeds on the ground and trample them down at the surface. Compost does not get buried.]

Why do I suggest not aerating?
Because it doesn't do anything to help germination. I am not hard over on not aerating. I am not sold on the process, either. For you I think it is a waste of time and money. Again, on the scale we're talking about, Mother Nature only has one aerator that I am aware of and that is the dung beetle. Dung beetles can punch about 1,000 holes per square yard and make them 5-10 feet deep. You can't do anything like that with a machine. On a micro scale, the natural fungi in the soil will soften your soil for you. In this case soil moisture is critical but you are still applying too much moisture for the beneficial soil fungi.

I realize I am the only one suggesting to not rototill compost and aerate. I think for the homeowner you don't have to and they are even detrimental. Obviously I can't come through the screen and stop you from doing it. All I can do is explain what I think are good reasons. Also I should point out that your opinion is exactly the same as mine was back in 2001 when I first found this forum. In fact I have argued for rototilling, composting, and aerating. Yet I have been convinced otherwise from the experience of others. One of my experiences from reading the forums has been to unlearn a lot of what I thought I knew about lawn care. Some of the gurus I mentioned above will have different opinions. Be sure to read anything they have to say on the topic and decide for yourself.