The drop in soil pH is not due to organic matter since OM does not significantly affect soil pH. Did the test lab suggest what you might do to get that soils pH closer to the more optimal 6.2 to 6.8, although it does look like the major nutrients are readily avaialable enough for the plants you are growing.
What is the humus levels?

Without venturing too far into the realm of soil chemistry, decay of organic matter acidifies soil in the short term by increasing available hydrogen+ ions. When you already have acidic soil, the effect can be quite dramatic. (Long term, additional organic matter buffers pH, but this plot is very much in the "short term" phase.)

Given the very adequate levels of magnesium and calcium in the soil, the strongly acid pH doesn't appear to be indicative of soil depletion. Having 8" of straw decay into the soil in less than 3 months is a lot of extra hydrogen.

They gave the typical advice to apply lime to tie up that excess hydrogen.

They didn't offer a humus test, but I am pretty sure it's fairly low. By touch and sight I'd say it has improved a lot. It's been "lawn" for 50 years... a lawn that doesn't even have a high percentage of grass in it, and most of the plants in it have fairly shallow root systems.

I am hoping the fall root crops will help start to bust up those dense lower layers.

The only thing I question is the change from clay to loam. No amount of OM can change a soil's texture (structure - yes). Only adding sand and/or silt can change texture.

I don't know how they determined soil type, DrH. Probably by "eyeball" method. I have a lot of small rocks in the soil that look like sandstone to me but should be limestone. It's possible that the initial tilling and soil disturbance eroded these rocks enough to contribute some silt.

Techically, according to the USDA soil survey in 2006, my soil is "Baxter cherty silt loam, eroded, rolling."

Typical profile:
0 to 12 inches: Gravelly silt loam
12 to 48 inches: Gravelly silty clay loam
48 to 60 inches: Gravelly clay

I don't seem to have the upper layer they mention, though.

Testing the humus level in your soil is farily simple. A 1 quart clear jar with staight sides and a tight cap is all that is needd. Sample the soil and mix it together very well and then put enough of ot that soil into that 1 quart jar to make 4 inches of that soil (mine is marked in 1 inch increments with a felt marker) and then fill the jar with water. Put the lid on, tightly, and then shake the jar vigorously. Let that then sit for at least 24 hours and look at how the various substances in the soil settle out. Sand, the largest and heaviest soil particle will settle on the bottom, silt next, and then clay. Organic matter will be on top.
Loam is a specific soil type that has about 45 percent sand, 25 percent silt, 25 percent clay, and about 5 percent organic matter, adding organic matter to a mostly clay soil is not going to make that soil into loam.
All of the research that I have seen on adding organic matter to soil tells me that the organic matter does not make a signifanct change in the soils pH. Adding minerals such as lime, Magnesium, Sulfur, Ammonium Sulfate, etc. will change soil pH over time, but not organic matter.

I agree, mulch over time is hugely powerful.

The pH drop may be caused by the rapid OM decay and the fact that newly cultivated soil has leached base cations. Did you experience above normal rainfall?

I wonder what your base saturation percentages were and what they are now? Gaining the proper base saturation levels for your soil type is a worthy project, as well as appropriate amendment to adjust the pH.

Acidity is normally caused over years by rain and leaching, acidic parent material, OM decay, and harvest of crops.

Keep records of your soil tests and note the effects of your amendments. Use the same testing lab but consider broadening the analysis to include base percentages.

How did your crops perform?

We are actually behind in rainfall for the year by about 10", but we got a LOT of rain in the spring not long after the soil was turned. (Enough to put Nashville under 24 feet of water.) A typical rainfall here is pouring buckets for 15 minutes and then the sun comes back out. We rarely get those long, soaking rainfalls.

Acidic soil is normal for this region. 5.3 is lower than normal, though.

My crops did/are doing fantastic this year; much better than I expected for a throw-them-in-the-ground and see-what-happens experimental year. Squashes and cukes were especially productive, and the one eggplant that survived the flea beetles is setting an astonishing amount of fruit. Good taste, too, especially from the tomatoes and peppers, although my serrano peppers were not as hot as they should be.

The lab only offers one panel -- what I got. If I want more, I have to find a different lab.