Hi Hayley, welcome!

The "pretty" outter pot is called a cache (pronounced: cash) pot. The simple answer to your question is that we empty the cache pot. I'm not trying to sound facetious, so please don't take it that way, but the easiest way to prevent root rot is to make sure your pots are not sitting in water. I don't know of anyone on this forum who has rocks in the bottom of their cache pots, actually, but there may be a few. Another method is to water your plants in the sink and let them drain right there. When you no longer see them dripping with water, you can return them to their cache pots.
As for limestone, I would think that it would add too many minerals into such a small space as a pot, but I am by no means an expert. Hopefully Al and others will chime in with their 2 cents.

Nancy

An appropriate amount of Ca is good, just as an appropriate amount of all other elements essential to growth is good, but too much isn't good. For that reason, I would try to prevent an excessive amount of Ca from getting into the soil, which means it's ok to use under the pot, as long as the effluent that drains into the collection saucer doesn't have a pathway back into the pot.

For agricultural applications, lime particles larger than a 10-mesh (mesh with 100 holes per square inch) size are considered as insoluble and having no effect on soil pH. Half the limestone particles that can pass through a 10-mesh screen but not a 50-mesh WILL impact pH. Particles smaller than 50 mesh neutralize soil acidity in a
relatively short time period.

All that is for agricultural or garden apps. For containers, you need to be MUCH more careful. Realistically, if you mulch with limestone, it's not unreasonable to think that the ratio of limestone to soil could be in the 10:1 ratio range, or even a closer ratio. Agricultural ratios would be infinitesimally wider than 10:1 (millions to one would be closer), and the buffering capacity of container media much, much less than mineral soils, which also figures into the equation in such a way as to show you need to be more careful with limestone around containerized plants because of pH issues.

Also, all limestone isn't created equal. If your limestone is all CaCO3 or very low in Mg, even moderate additions to the soil solution can cause a severe Mg deficiency, as well as deficiencies of other elements necessary for plant health/growth.

Growers should work toward a program that ensures all the elements necessary for normal growth are in the soil and available for uptake, in roughly the same ratio as that at which the plant actually uses the elements, and at a concentration high enough to ensure no deficiencies, yet low enough that it doesn't impair the plant's ability to take up water and the nutrients dissolved in that water. Anything that causes deviation from that scenario is probably going to have only the potential to limit growth and vitality, so topdressing with limestone or using it in any way as a part of the soil (as a drainage layer, e.g.) is probably not a good idea.

Al