Soil test--yes. And if it were mine, I'd probably do 2. One soil test of just the affected area (septic field), and a 2nd one of the rest of the yard. If the results are drastically different, this might give a clue that something was done differently in the septic field (ie, some other soil brought in, etc.).

Forgive me if you already know this, but remember that a soil test is only as accurate as the sample given. Don't just take one shovel full of soil--you want 8-12 samples from around the yard (or in this case, around the septic field--and another dozen fm the rest of the yard.) Take a "slice" with the shovel, cut that into a square, long slice--say 2" by 2" across, and 6 or 8 inches deep. Toss each "square slice" into a bucket. Mix them up, and then fill your sample container from that.

Aside from the soil test, 2 thoughts:

Are you watering the grass seed? I'm assuming so since you said you got good germination in the fall & the grass was still alive early spring. Remember to "taper off" the water once you're a few weeks post-germination. That is, transition from frequent, shallow watering (i.e., "Please sprout and stay alive, you beautiful baby plants!) to deeper, less frequent watering (i.e., "get those roots down deep!").

Finally--and forgive me if this is a stupid train of thought as I'm on sewer--do you know how far down the septic field starts? It's basically just a french drain kind of thing in a gravel bed, right? Is it possible that the gravel starts only a couple of inches below the surface? That would easily fry some tender grass once the first heat arrives.

Hope that helps at least a little!

--JC

Soil test is always a good idea. When you send away your soil test also check the box for the soluble salts and organic matter tests follow the recommendations of the soil test. Septic drain fields often contain a lot of salts (which make it hard for the plant to take up the water and nutrients it needs) also your organic matter on that sandy soil is probably pretty low, nothing to hold water in the root zone. My advice would be to sod the area to tall fescue, because of its drought and salt tolerance, in the early fall. After you strip existing sod apply any soil amendments the soil test indicated. Lay sod and water. Let the sod start to root down then give the sod a pound per thousand sq. ft. of nitrogen fert plus whatever else the soil test recommends. The next mid spring use a preemergent herbicide to keep the crabgrass out, plus another shot of nitrogen at a pound per thousand sq. ft. You will probably have over seed a few spots the the next fall because tall fescue sod generally has a lot of kentucky bluegrass in it. The kentucky bluegrass is not as drought or salt tolerant as the tall fescue. A couple of years of over seeding in the fall you should not have many problems. Also, when you mow the area, in the summer make sure to mow early in the morning before 11 am. People Kill a fair amount of grass mowing their yard at two o'clock in the afternoon when the grass is under drought and or heat stress. If sod is not an option, in the early fall aerify the area (you only need to affect the top inch or two) apply your soil amendments, nutrients (as recommended by soil test), seed, and water. Keep the seed wet, after seed germinates give it .5 lbs of quick release nitrogen per 1000 sq. ft. and a couple weeks after that give it another .5 lbs. The next spring use your preemergent herbicide, as well as, a pound of nitrogen per 1000 sq. ft. in a slow release form. Remember that this area is going to dry out faster than the rest of the lawn.