Thanks @Morpheauspa! I was hoping you would reply. I've been able to make adjustments this far, based on a lot of your responses to others

Yes, I was pleased to find the Logan Labs test confirmed, the changes I could visually see in my lawn. The front/sides look good, but the back yard is fighting me... It get's a couple hours of direct sun, and then bright filtered sunlight the rest of the day. I'm fairly certain it gets plenty of sun to support a healthy lawn. I water the whole lawn the same, in-ground sprinkler system. I aim for 1" per week, adjusted for rain patterns.

I basically do everything the same for the front lawn and back in terms of maintenance, fertilizer, and watering. I went a bit heavier in the back with the Mag-i-cal and gypsum to help bring up those lower levels, but other than that it's been treated the same.

The problem I am seeing is some type of compaction or possibly Hydrotropic. I've tried the soap, and core aerate every fall, but I'm still having an issue. It seems like the soil isn't holding water correctly. I overseeded/seeded the area several times, and each year when the temps rise, this is the first area to go dormant, but it doesn't recover, it literally dies, indicating the roots are shallow. Here are some pictures from Fall 2014. The front lawn looks even better now, but the back still has this problem area. Also worth noting, this area has a ton of Plantain weed, confirming that the area is compact despite my efforts

Backyard

Front Yard

Tigher soils with shallow-rooted grasses can be caused by a few things, but in this case your Logan test gives us the answer--overly-high magnesium.

The gypsum will help as it raises the calcium levels (to a bit over the top, actually), and dispels some magnesium from the soil profile. The potassium won't hurt, although it's essentially neutral on soil compaction.

That one area is a point where aeration might actually be of some temporary assistance. It won't last long, but it'll definitely get some air down there for the roots to expand. Over time, and feeding organically, the worms will move in and do the job for you.

The back problem area does look a little more heavily shaded from the photos (the fallen leaf color is darker). That can also be a problem as the difference between just enough sunlight and not enough sunlight is rather unobvious to the human eye. As a general rule, most northern grasses require 4 to 6 hours of sun per day minimum and simply won't perform well under that.

It might be a great spot for a shade garden, or simply a mulched area, or possibly red fescue (which tolerates far less sun than other grasses).

That's what I thought too about the magnesium, so I figured I would see an improvement this year, as I have improved the ratio. The other thing that doesn't make sense to me is that the Ca:Mg ratio is actually worse in the front 1:3 vs. 1.4 in the back...not statistically different, but at least as good, yet the problem persists in the back. I will re-evaluate next year, after running through your suggested program above.

I will be removing two oak trees and a maple with a large canopy next spring as well, so that should significantly increase the sun expose for the back yard. That is actually the south side of my house, so once those two trees are removed it should allow well over 4 to 6hrs of direct sun. I will follow-up next year and let you know if these changes solved the backyard issues!

Thanks again for the feedback

Do you end up watering less frequently in the back under the shade?

The removal of three trees is absolutely going to help, and that could also be causing the issues with the compacted soil. Having nothing much growing in it (other than tree roots), there's no real soil life to de-compact it for you.

Your magnesium levels are kind of unreasonable in both sections (again, this doesn't cause the lawn any major problems), and soils tend to be rugged individuals in how they react to it. That one section's decided to be tight doesn't shock me in the slightest.

Shampoo will certainly help, but don't overdo. Once a month is just fine, and by spring you should only need two or three a season.

@DC, I run all zones for the same length of time, same schedule. Occasionally will supplement the front lawn to prevent it from going dormant in the height of the summer, but that's it. I'm pretty frugal with the water, and don't water until the blades stop bouncing back up after stepping on them.. basically keeping it right at the edge of going dormant.

@Morph, no doubt, large oak's and maples have huge root systems and can put a monopoly on the water that flows into the soil. The trees I will be cutting are a good 20-30ft away from the lawn, but are also 75' tall, so the root systems could probably reach into the lawn.

Yes, not happy about the Mg levels...but that was self inflicted before I knew any better... Hopefully the gypsum will help flush that faster. I'll start up the shampoo again, did it a couple times, didn't see any changes, so quit, but I will try again