Your pine straw will not decompose fast enpough to affect a beet crop this fall. And anyway, it is my understanding that any mulch, as it decays, becomes neutral in pH.

BTW, adding lots of compost to your soil, or turning under your mulch at the end of the growing season will help your sandy soil retain water. You can never add too much organic matter, esp. in the South where things break down quickly.
Also, even in my soil (municiple compost in raised beds), whenever I plant carrots, beets, etc., I cover the row or area with burlap to help shade the soil as well as keep it moist, and water it each morning to hasten germination. The burlap lets light, air, and water throgh and you can start checking in a week or so to see if the seeds have sprouted. When seedlings appear, I remove the burlap. At that point I then use row cover suspended over the area to break up rain as it falls so as not to beat down the little plants nor crust the soil. Works well for me.

You are right. Fresh pine straw is better than grass clipping as mulch. It will no decompose this soon.
since you have sandy soil, sprinkle a layer of peat moss before mulching. This may help moisture retention better.

I am working on improving the soil, but that's a multi-year, even multi-decade, project.

I wouldn't use peat moss for anything in this climate. When its dry its waterproof and since our usual summer weather is 90+ degrees with 90+ humidity and a slight chance of scattered thunderstorms it WILL get dry -- which would then cause the water to run off my garden instead of soak in.

I'll keep the burlap in mind to save if I ever happen to buy something in a burlap bag. But I don't have any and can't make the investment right now. It definitely sounds better than the idea of laying a board on the row -- considering the price of lumber I don't see where exposing $$$ wood to termites to raise carrots that are not so expensive to purchase made any sense. :-D

Burlap is ideal b/c not too expensive, loose weave etc. This is a case where good-enough can do though - ie, 2nd hand sheets from your local thrift shop. A couple bucks should get you enough sheets for a bed/row or two, and leave you enough $$ for the bales of straw to mulch with so you don't have to wait decades...

I've never seen burlap at a thrift shop, though you never know what you might come across. I will be watching for it now that I've been told about it. :-)

Pine straw is free for the raking out from under my own trees and free for the taking if I get to people's roadside piles before the town truck does.

I don't till pine straw into the soil (except incidentally when its mixed in the grass clippings I get from the neighbor). I till in grass clippings, old leaves, and finished compost. One neighbor has already given the OK to horse manure and I hope to secure agreement from the other as well.

The pine straw is either raked off before tilling or burned in place to provide the ash (This area is a fire ecology so that's natural).

Improving any soil takes years before you reach a maintenance stage. Improving pure quartz sand in a hot, moist climate takes longer.

I wish to correct one small error. Most all decomposing organic mater is acidic. Especially pine straw which is why it's so good for acid loving azaleas.
She's right though that it won't decompose fast enough to affect your crops this year. It would improve your sandy soil to turn it under after the season is over. Just add a little powdered dolomitic limestone to neutralize the acid. Dolomitic provides magnesium as well as calcium.