Cacti in pots are not the same as the plants in open ground in habitat. There they have a large amount of soil to tap into with their extensive root system to make use of what nutrients are available. With the limited soil in a pot if we imitate habitat soils for more adult plants the nutrients would soon be exhausted, therefore we need to plant in slightly more nutrient rich potting mixtures or use weak fertilisation occasionally for more mature plants.
However cactus seedlings in habitat with their limited root system would be in nutrient poor soil, therefore obviously you need to duplicate that condition in cultivation and not fertilise too quickly since the potting soil will probably contain all they need for the first six months or so. My dad used to say when raising seedlings "you don't feed babies beefsteaks", meaning the nourishment an adult plant can take could upset or even kill a small seedling.
A quote from the following link:-
"It is a misconception that cacti actually live in true deserts where the ground is all sand. Most live in a sandy environment with very little soil, but enough for some limited nutrients to be found. Cacti are usually found in semi-desert regions and dry grasslands. One of the benefits of living in an arid region is that the cacti have no competition for the limited amount of nutrients in the ground because most other plants cannot survive in the harsh environment."
http://www.scienceiq.com/facts/cactisurvive.cfm" onclick="window.open(this.href);return false;
Too much fertiliser can kill a plant because plants take in water by osmosis where a stronger solution in the plant cells draws in a weaker solution from the soil. If you reverse this situation by making the soil water solution over strong by excess fertilisation, because fertilisers are soluble salts in one form or another, you get reverse osmosis or plasmolysis which actually draws water out of the plant and would eventually kill it:-
"A cell placed in a solution more concentrated than itself (a hypertonic solution) will shrink due to loss of water. It may eventually die of dehydration. You can observe this effect with a carrot placed in salty water. Within a few hours the carrot becomes limp and soft because its cells have shrivelled.
By contrast, a cell placed in a solution more dilute than itself (a hypotonic solution) will expand as water enters it."
http://www.scienceclarified.com/Oi-Ph/Osmosis.html" onclick="window.open(this.href);return false;
http://www.ask.com/science/plasmolysis- ... e50af04605" onclick="window.open(this.href);return false;
Nothing to do with drying up, everything to do with rotting, algae, water moulds, damping off, and all the other names people have for nasties that thrive in a humid environment that a cactus has no defence against. And if you're really unlucky, fungus gnats 
