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You can replace one watering a month with the 2 tablespoons per gallon solution or use it as a foliar spray.

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/epsom-salt-gardening.htm

What normally happens is that the soil dries out, and then the water beads slowly release water to the roots, and soil.

I use them in some of my own containers for extended periods away from the house. It can increase the time it takes to draw all of the moisture from the root zone. Just don't overwater. It will take a very long time to dry out from that.

One other thing to note-

Microbes in soil break these and other plastics down faster than most people realize, depending on the type. Sodium polyacrylate will break down into a gel. Soils will get very thick over time. It will be necessary to re-compost this when it happens, and it will be time to change the soil out.

I would not opt for that method. Since it, already, carries an elevated risk to soil infections, any of those to the root zone could ruin the crop. With that being said, if you can really watch your watering and moisture levels then you may be able to get crops like carrots to grow. Because of the way that you would treat potatoes, this cardboard method would not work.

With all said and done, it couldn't hurt to try for an experiment. Just be sure that you have some in the regular fashion, also, just in case. A moisture meter will be a good idea to keep handy to measure the moisture in the soil under the cardboard.

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-moisture-in-plants.htm

You will need to amend it with something to do so. Perlite, Vermiculite, (small amounts of) sand, coco coir, peat moss, dolomitic lime, and compost can be added each year, or biennially to keep the soil at its best.

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/how-to-improve-clay-soil.htm

One other thing to consider is earthworms. They are superior at loosening compacted soils.

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/how-to-attract-earthworms.htm

It will improve the texture of your soil but it won't affect the pH level. Used coffee grounds have a neutral pH.

By adding organic material to the soil, it will attract earthworms, not kill them. Here is more:

https://www.gardeningknowhow.com/composting/ingredients/coffee-grounds-gardening.htm

Probably the easiest thing to do is to have topsoil brought in and build up a layer of soil several inches thick on top of the rocky soil. This article gives some ideas for plants that do well in rocky soils.

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/working-in-rocky-soil.htm